Subnanoporous hydrophobic thin films for ultrahigh-efficiency seawater and brines desalination using membrane distillation

Domain: chemistry
Published: 2026-01-10
Editorial outcome: major_revision

Abstract
Fabricating breakthrough materials capable of desalinating seawater and brine with high selectivity and low energy consumption is crucial for addressing global water and energy challenges. We report here the desalination capability of ultra-thin polymeric films with subnanometer pores synthesized through the polymerization of fluorinated trichlorosilane monomers and diamine-based monomers. The combination of subnanometer pore size, submicron thickness, and superhydrophobicity facilitates efficient liquid-to-vapor phase change in the membrane distillation process, enabling effective desalination performance. The thin-films demonstrate high salt rejection (99.8%), complete boron rejection, and water fluxes of 40 L.m−2.h−1 (1.88 kWh.m−3, WRRsp 0.32%) and 238 L.m−2.h−1 (20.65 kWh.m−3, WRRsp 3.87%) for seawater at 25 °C and 60 °C, respectively. For the desalination of real seawater reverse osmosis brine at 25 °C, the thin-films maintain 12 L.m−2.h−1 (4.4 kWh.m−3, WRRsp 0.09%) under comparable conditions. Their polymeric nature, chlorine resistance, and low energy requirements, indicate a potential for scalable and sustainable desalination systems.

Introduction
Thin films have significantly contributed to membrane-based separation technologies in various fields, particularly seawater desalination1, organic solvent separation2, and crude oil separation3. They are typically fabricated by interfacial polymerization (IP)4, molecular layer-by-layer5, 3D-printing6, and chemical vapor deposition7 techniques. The IP strategy, based on the Schotten-Baumann reaction, is commonly employed for producing polymeric thin films, where the film grows at the interface of two immiscible phases via irreversible polymerization of two highly reactive multifunctional monomers8–10. The result is a self-terminated film growth with an overall thickness of a few hundred nm and a rough surface morphology11. In the early 1980’s, Cadotte et al. developed the first thin film utilizing the IP technique, layering the thin film onto an ultrafiltration support to make a composite membrane (TFC). This led to a breakthrough with the development of membranes for reverse osmosis (RO), a desalination technology that dominates today’s industry12. In a typical synthesis, aqueous solutions of organic amines react with organic solvents containing acyl chlorides, resulting in the formation of a thin and dense polyamide (PA) film. There have been several attempts to incorporate new features into the PA thin film in an effort to improve their performance13–17. Apart from its application in desalination, TFC has also been employed in the fractionation of crude oil or for molecular separation in organic liquids, commonly referred to as organic solvent RO18,19. Hydrophobic thin films produced by either capping thin films with a hydrophobic group or directly incorporating the hydrophobic group into the monomers during the reaction, demonstrated remarkably fast liquid transport20,21. On one hand, the hydrophobic vesicles facilitate efficient paths for the transport of nonpolar liquids across the thin film3. On the other hand, the electrostatically negative fluorous inner surface of the hydrophobic nanochannels breaks water clusters, thus allowing more water to pass22. Also, the hydrophobic thin films perform excellently in the evaporation-based desalination process, which relies on the transport of water vapor through the hydrophobic nanopores23–25. In previous work, we also delivered evidence to support the hypothesis that a thin film of sub-micron thickness has promise for significantly improving the efficiency of desalination26. Therefore, the development of a superhydrophobic thin film with high permselectivity could be a breakthrough for sustainable desalination of highly saline brines. To address this challenge, herein we report the design of scalable superhydrophobic polymeric thin films with submicron thicknesses. The thin film grows in a minute at the organic/aqueous interface owing to the reaction between fluorinated trichlorosilane molecules and an m-phenylenediamine (MPD) monomer. The thickness of the thin films can be readily adjusted by controlling the fabrication conditions (i.e., reagent concentration and reaction time). As-synthesized superhydrophobic thin films are layered on a hydrophilic porous material for structural support and additional enhancement of water evaporation27, taking advantage of the effect of a hydrophilicity gradient in the composite membrane design25. Using the membrane distillation (MD) process, the resulting thin films efficiently desalinate highly challenging solutions i.e., real RO desalination brine, real seawater, scale-forming ions, and monovalent ions, with the lowest energy consumption reported to date. We demonstrate, using a multiscale model, that the experimentally observed evaporation fluxes can be predicted over a wider range of operating conditions using the Kelvin equation with curvature-dependent water surface tension at the pore level28, providing an important connection between the molecular behavior of water and the tangible performance of the designed membrane.

Results
Synthesis of polymeric thin films

We synthesized scalable polymeric thin films with various thicknesses (60.6 to 238.5 nm) for desalination using a straightforward, scalable, and controllable IP technique. This IP involves the reaction between 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTCS) and MPD at the interface of two immiscible solutions, forming a superhydrophobic thin film (Fig. 1a and Fig. S1). The freshly prepared FTCS/hexane (organic phase) solution was slowly poured into the MPD/deionized (DI) water (aqueous solution). Upon pouring the FTCS/hexane solution, the head (–SiCl3) of FTCS gets closer to the water surface, whereas its hydrophobic tail remains in the organic phase. Once the FTCS/hexane solution spreads, the head group undergoes rapid hydrolysis and aminolysis reactions with water and MPD, respectively, followed by a condensation reaction (Fig. 1a–c), forming a free-standing superhydrophobic thin film with subnanometer pores at the aqueous–organic interface (Fig. S1a), that was then transferred onto a porous support. Further, we explored the effect of the IP reaction time and FTCS concentration on the thickness, physicochemical properties, and performance of the synthesized thin films (Supplementary Notes 1 and 2). For briefness, the thin-film samples are referred to as TFx–y, where x is the FTCS concentration (wt/vol%) and y is the IP reaction time (min), respectively (e.g., TF0.1–0.5; Table S1). The scalability is demonstrated by fabricating a free-standing thin film with an area of 324.0 cm2 (18 cm × 18 cm) (Fig. S1b), without compromising its mechanical strength. As such, owing to its robust mechanical integrity, the scaled-up free-standing thin film was safely transferred to the surface of a porous support (Fig. S1c). To support scalability, the thin film was also synthesized using the support-IP method29.

Scanning electron microscopy (SEM) observations of the thin films transferred onto the substrate (substrate properties in Table S2) demonstrate the integrity of the thin films (Fig. 1d–h). The morphology of the thin films varied with the FTCS concentration and IP reaction time (Fig. S2). For a short IP reaction time (0.5 min), the thin film exhibited a smooth surface with small nodular features. As the reaction time increased (1–3 min), more prominent leaf-like structures developed, creating a distinct ridge-and-valley topography (Fig. 1d–g and Fig. S2). Increasing FTCS concentration further intensified these surface features, resulting in a denser distribution of leaf-like formations (Fig. S2i–l). The two-dimensional (2D) atomic force microscopy (AFM) images (Fig. 2a and Fig. S3) were consistent with the SEM observations. Cross-sectional SEM images of the thin films on 60 µm-thick substrates revealed film thicknesses ranging from 60.6 nm to 238.5 nm, demonstrating a direct correlation with longer IP reaction times and higher FTCS concentrations (Fig. 2b–f; and Fig. S4 and Table S3). AFM profiling of free-standing thin films, transferred onto silicon wafers, further confirmed that thicknesses remained below 250 nm, with a clear trend of increasing thickness with longer IP reaction times and elevated FTCS concentrations (Fig. 2g, h, Fig. S5). Additionally, surface roughness values varied with the IP reaction time or FTCS concentration (Fig. S6 and Table S3).

Structural insights provided by grazing incidence wide-angle X-ray scattering (GI-WAXS) provide further understanding of molecular orientation and arrangement within the films (Figs. S7 and S8). The in-plane GI-WAXS profiles of films TF0.1–1, TF0.2–1, and TF0.3–1 exhibit consistent peaks around Q|| = 1.27 Å−1, corresponding to a d-spacing of approximately 0.495 nm30–32. This uniform in-plane feature suggests a consistent molecular arrangement, such as π-π stacking of aromatic groups perpendicular to the supporting substrate, an arrangement likely established during the IP process30,31,33. In contrast, the out-of-plane GI-WAXS profiles exhibited distinct peak positions for each sample (Q⊥ = 0.36 Å−1, 0.55 Å−1, and 0.90 Å−1 for TF0.1–1, TF0.2–1, and TF0.3–1, respectively), corresponding to d-spacing of 1.748 nm, 1.142 nm, and 0.698 nm, respectively (Fig. 2i, j and Figs. S7 and S8). These variations reflect differences in molecular packing density along the out-of-plane direction, likely influenced by polymerization kinetics33–39. This anisotropic arrangement, characterized by consistent in-plane alignment and variable out-of-plane packing, suggests that thickness and cross-linking density may influence transport properties, as thicker films with higher structural density could resist vapor transport due to increased structural complexity.

The estimation of the thin film porosity based on the BET method yielded a value of 4.94% (Supplementary Notes 2.9 and 3.6). The thermal conductivity measurements of the polymeric thin films showed a value of 0.07 ± 0.02 W.m−1.K−1 (Fig. S9), highlighting their strong thermal insulation properties, a crucial factor for efficient desalination. The low thermal conductivity, coupled with controlled structural anisotropy, enables these films to minimize heat loss while allowing selective vapor transport. The chemical structure of the polymeric thin films was validated using Fourier-transform infrared (FTIR) and Raman spectroscopy (Supplementary Note 3.2). The band at 880 cm−1 is assigned to the Si–N stretching mode40–43, confirming that FTCS reacted with MPD via an aminolysis reaction (Fig. 1b and Fig. S10), forming a continuous thin film (Fig. 1b). Additionally, the bands at 1068 cm−1 and 1024 cm−1 are assigned to the Si–O stretching mode of Si–O–Si (Fig. S10), resulting from hydrolysis and condensation of FTCS, respectively. We characterized the degree of cross-linking in the polymeric thin films by observing the relative intensities of Si–N and Si–O–Si peaks, normalized to an internal C–C reference (Supplementary Note 3.2). The increasing peak ratios, correlating with higher FTCS concentrations and extended reaction time, indicate more cross-linking as revealed in the growth of normalized Si–N and Si–O–Si peaks, suggesting a dynamic interplay between aminolysis and condensation in forming the film structure (Fig. S10 and Table S4). Furthermore, Raman spectroscopy supported the FTIR results (Figs. S10 and S11). Complementing the FTIR findings, Energy Dispersive X-ray (EDX) analysis was conducted to further evaluate the chemical composition and stoichiometry. The analysis of the Si to N ratios (elements unique to FTCS and MPD, respectively) confirmed that the FTCS concentration plays a pivotal role in the film composition. The results from these analytical techniques demonstrate the relationship between monomer concentration and the resulting structure of the polymeric thin film (Table S5). Based on these results, the reaction mechanism can be explained as follows: when the FTCS/hexane solution is added to the MPD/DI water solution, highly active –SiCl3 groups are hydrolyzed by water, forming –Si–OH, whereas others react with MPD to form –Si–NH and release HCl as a byproduct, as illustrated in Fig. S12.

Fluorine-based chains on the thin-film surface could produce low surface energy, imparting superhydrophobicity with water contact angle (WCA) values > 160°. Moreover, varying the IP reaction time or FTCS concentration had a negligible effect on the wettability of the thin films (Fig. S14a and Table S2). Importantly, the thin films exhibited superhydrophobicity towards salty water (30 g.L−1 of NaCl) over a wide range of pH values (1.5–12.0) and toward a surfactant-stabilized oil-in-water emulsion (Fig. S14b). As a comparison, the commercial hydrophobic membranes (with different porosities of 39.6% ± 4.7% (Commercial#1) and 48.3% ± 4.3% (Commercial#2) exhibited WCA values < 135°, smaller than those of the fabricated thin films (Supplementary Note 1.1 and Table S2).

Desalination performance

Before performing desalination experiments, the as-synthesized thin films were transferred onto a commercial hydrophilic substrate. The performance of the thin films was evaluated for the desalination of saline water with different concentrations in term of water-vapor flux and salt rejection using membrane distillation (MD) process. These membranes were sandwiched between the feed (saline solution) and distillate (DI water) streams, with the thin film facing the feed solution (Fig. S15). To provide a slight driving force across the membrane, we set the transmembrane temperature difference at a very low value of 5 °C, using a low feed temperature of 25 °C and a distillate temperature of 20 °C.

The vapor flux was notably influenced by the concentration of FTCS, decreasing with higher concentrations, while the impact of IP reaction time was less pronounced (Fig. 3a). As such, despite increased roughness, which typically enhances flux by increasing surface area, a decline in vapor flux was observed with higher FTCS concentrations or extended reaction times (Table S3). This decline can be attributed to a corresponding increase in film thickness (Fig. 3b) and tighter molecular packing, as evidenced by GI-WAXS out-of-plane profiles (Figs. S7 and S8). Specifically, the d-spacing for TF0.1–1 was approximately 2.5 times greater than for TF0.3–1, reflecting increased compact molecular packing34–36. The decrease in d-spacing and increase in film thickness indicated that higher concentrations promote additional polymer growth and denser molecular packing along the out-of-plane direction (Figs. S7 and S8 and Table S3). The tighter molecular packing and increased cross-linking density observed in thicker films are likely to contribute to increased transport resistance, reducing flux. For example, at a 1-minute reaction time, increasing the FTCS concentration from 0.1% to 0.3% resulted in a substantial thickness increase from 97.5 nm to 238.5 nm (Fig. 3b and Table S3). These structural and morphological variations underscore the trade-off between surface roughness, thickness, and molecular packing, explaining why the thinner, smoother membrane (TF0.1-1) achieved the best performance.

The best-performing thin film, TF0.1–1 (0.1 wt.% FTCS, 1 min reaction time), achieved a vapor flux of 40 ± 3 L.m−2.h−1 and salt rejection of ∼99.9% for simulated seawater (30 g.L−1 NaCl), significantly outperforming commercial membranes (Commercial#1 and Commercial#2), which showed almost zero flux under the same conditions (Fig. 3c). The obtained flux is more than double the RO flux reported for treating seawater with a hydraulic pressure of 55 bar44. Furthermore, the TF0.1–1 efficiently removed both monovalent (K+, Na+, and Cl−) and divalent (Mg2+ and SO4 2−) ions from water with a high separation efficiency of 99.9% (Fig. 3d). However, the flux slightly decreased when divalent ions were used, which could be attributed to the magnesium and sulfate ion scaling and different water activities associated with multivalent ions45. Additionally, the excellent desalination performance of TF0.1–1 was sustained at different NaCl concentrations (Fig. 3e), even when the concentration of the salt in the feed was increased to 70 g.L−1 (brine; 31 ± 3 L.m−2.h−1), which cannot be achieved using RO. Interestingly, the as-prepared thin film (TF0.1-1) exhibits excellent performance towards real seawater (Red Sea salinity = 40.98 g/L, Table S6) and real desalination brine (discharge from SWRO commercial plant = 74.4 g/L, Table S6) with average permeate fluxes of 14 ± 1 and 12 ± 1 L.m−2.h−1 and high salt rejections of 99.3 ± 0.3 and 99.0 ± 0.6%, respectively (Fig. 3f) at a feed temperature of 25 °C and atmospheric pressure. Importantly, while our group reported the evidence of boron volatility (ppb level) in MD starting at about 60 °C, this thin film acts as a full barrier to boron (100% rejection in all cases)46. Furthermore, after the long-term desalination of real feeds, the SEM analysis (Fig. S16a, b) of the used thin films revealed that the morphology remains stable even at high cross-flow velocities, demonstrating its superior mechanical integrity. Moreover, the films showed high resistance to chlorine (Fig. S16c), which further enhances stability. This excellent long-term stability in treating real seawater and real SWRO brine (Fig. 3f) and the fabrication of large samples of 324.0 cm2 (Fig. S1b) demonstrate the possibility of scaling up the composite membrane manufacturing process.

We investigated all synthesized thin films over a wide range of feed temperatures (25 °C–60 °C), covering waste heat generated from industrial plants as low-grade heat sources. For all thin films, the vapor flux increased with an increase in feed temperature due to an increase in driving force (vapor-pressure difference; Fig. 4a and Figs. S17 and S18). In addition, Fig. 4a reveals that the TF0.1–1 membrane provided an ultrahigh vapor flux (238.2 ± 17.5 L.m−2.h−1), six times higher than that of the commercial membranes (41 ± 1 and 38 ± 2 L.m−2.h−1) at a transmembrane temperature difference of 40 °C (feed at 60 °C and distillate at 20 °C). Moreover, the flux exhibited by the TF0.1–1 membrane is higher than that reported for other commercial membranes in the literature (Table S7). Additionally, this desalination flux (at a 60 °C feed temperature) is among the highest values reported in the literature for polymeric and inorganic membranes (Fig. 4b). A TF0.1–1 had higher fluxes than other advanced materials with subnanometer pores, including graphene (226.6 L.m−2.h−1 at 70 °C under −800 mbar vacuum)23, nanoporous carbon (221.6 L.m−2.h−1 at 90 °C and under −825 mbar, VMD mode)24, and COFs (220.0 L.m−2.h−1 at 65 °C and under −160 mbar, VMD mode)25. However, the performance of these materials was assessed under vacuum and higher temperatures, requiring additional energy (Fig. 4b and Table S8). Additionally, all thin films exhibited high salt rejection (>99.0%) due to their superhydrophobicity, which prevented liquid penetration (Fig. S18).

Desalination mechanism

The high vapor flux produced by the thin films could be attributed to several factors. First, vapor-pressure enhancement due to the curved meniscus at the entrance of the superhydrophobic subnanometer pores of the thin film results in a substantial increase in the evaporation driving force. This effect is facilitated by the thin film's low thickness and the hydrophobicity gradient provided by the hydrophilic support. By performing additional experiments, we confirmed the hypothesis that the finely tuned properties of the thin film, such as the lower thickness and a lesser degree of cross-linking, represent the main contribution to the high vapor flux. The original hydrophilic substrate and modified hydrophobic substrate (pore size = 0.2 µm) were evaluated in the same desalination system at a high feed temperature of 60 °C. As presented in Fig. S19, the hydrophilic support demonstrated no salt rejection due to its hydrophilicity, as expected. In addition, the modified hydrophobic support produced a very low vapor flux value of 16.8 ± 2 L.m−2.h−1 and 99.99% salt rejection (feed at 60 °C and distillate at 20 °C). However, the membrane did not produce any flux at 25 °C inlet feed. The poor performance of the hydrophobic support confirms that the fabricated thin film is the key to the experimentally observed high desalination performance. Recent molecular dynamics simulations and experimental studies have indicated that the water evaporation flux in nanopores is orders of magnitude higher than that of bulk water evaporation. In addition, the evaporation rate is significantly increased by reducing the confinement characteristic size47,48, which could be useful in designing efficient materials for low-energy desalination processes. Molecular dynamics simulations, conducted at 27 °C and low pressure (≤ 1 atm), revealed that water exists in a vapor-like state in hydrophobic subnanometer carbon nanotubes (0.8–1.0 nm)49 and between hydrophobic nanoscale sheets (d ≤ 0.5 nm)50, owing to the peculiar behavior of water under confinement effects51,52.

We further performed an analytical and numerical investigation of transmembrane mass transfer to provide a physical basis for the high vapor fluxes observed using the as-prepared thin films under ambient operating conditions (detailed explanations are provided in Supplementary Note 6). Because the developed thin films have a submicron thickness and subnanometer pores, we used the Beskok and Karniadakis53 (BK) unified model to determine the mass transfer across these thin films. This unified model is based on the solution of momentum equations using a second-order slip boundary condition to account for the gas slippage and rarefaction effects observed in narrow channels of microstructures54. This approach has been successfully used to describe gas transport in porous media55,56 and nanopores57,58. The transmembrane mass-transfer model was embedded in a computational fluid dynamics (CFD) code to account for the entire desalination system used in these experiments, including feed and distillate channels. This implementation enables the prediction of momentum, heat, and mass transfer in both channels, coupled in a conjugate approach with heat and mass transfer across the thin film59,60. The heat transfer across the membrane includes the conduction across the membrane-air-water vapor composite structure as well as the latent heat transferred with the vapor convection from the feed to the permeate side of the membrane. Additionally, the heat extracted from the feed and added to the permeate as water evaporates and condensates, respectively, is taken into account in the calculations (see equations in Supplementary Note 6). Computations were performed to estimate the theoretical water-vapor flux, in which the effect of vapor-pressure enhancement (water evaporation in hydrophobic nanopores) was considered using the Kelvin equation, which remains valid in nanometer confinements (Fig. S20)61,62. It is worth noting that the Knudsen-Molecular-Poiseuille Transition (KMPT) model, which is a widely used model to describe mass transfer in MD, was not able to reproduce the permeate flux evolution with temperature (see Fig. S21). Given the subnanometer pore size of the prepared membrane, the KMPT model reduces to diffusion, which is less sensitive to temperature changes compared to Poiseuille flow63. On the contrary, the BK model, which retains features of the Poiseuille flow while compensating for the damping of the small pore radius with the rarefaction coefficient, is able to reflect the flux changes with temperature at high Knudsen numbers.

The theoretical results revealed that the vapor flux enhanced by subnanometer pores is considerably higher than the vapor flux produced by common microporous membranes (Fig. 4b, c). The vapor-pressure enhancement occurs primarily on the feed side due to the submicron thickness and subnanometer pores. Indeed, as the moderate flow pressure pushes water at the feed–thin film interface, an increase in local vapor pressure occurs, whereas it remains almost unchanged at the meniscus of the substrate pores because of the larger pore size (~0.2 µm; Fig. 4c).

Moreover, the submicron thickness and the very low thermal conductivity of the thin-film layer play an important role in enhancing the vapor flux by maximizing the driving force across the membrane64. The simulation results reported in Fig. 3b indicate the drastic effect of the thin-film thickness on the vapor fluxes for a feed temperature of 25 °C and a distillate temperature of 20 °C. If the thin-film thickness is increased from 120 nm to 1 µm, which is still considerably thinner than traditional membranes, the water flux decreases by a factor of eight, due to an increase of the mass transfer resistance. The permeate fluxes obtained using the BK model for different thicknesses of the thin films are similar to those in the experimental results (Fig. 3b). Interestingly, in Fig. 3b, there is no apparent optimum in the range of film thicknesses that were fabricated, as the permeate flux keeps increasing with thinner films. Therefore, with an ultrafast vapor transport, a film thickness two orders of magnitude thinner and pore size two to three orders of magnitude smaller than those of common MD membranes, the optimum film thickness below which conduction losses start penalizing the processes has shifted below 60 nm. This is further consolidated with Fig. 4a which demonstrates that the results using the BK model with varying temperatures are aligned with the experimental results (corresponding to the properties of TF0.1–1). The unified model can reproduce the trend of the permeate flux variations, probably due to its calibration at a high Knudsen number using both experiments and simulations53 and its embedding in a fully coupled approach, thus ensuring proper heat and mass transfer from the feed to the permeate domains.

Energy saving

Energy consumption remains a major contributor to the total cost of desalination, making energy efficiency a critical metric for evaluating the membrane performance. We evaluated the specific energy consumption (SEC; kWh.m⁻3) of the fabricated thin-film membrane together with the water recovery ratio (WRRsp)65 in a single pass DCMD configuration (Fig. S22). This comprehensive metric, which accounts for both thermal (SECth) and electrical (SECelec) energy inputs required for the production of distilled water66, includes energy for heating, cooling, and pumping, making it a reliable indicator of system efficiency (see detailed calculations in Supplementary Note 7)67–74.

To validate energy savings under scalable conditions, we conducted strongly coupled multiscale CFD-membrane modeling, which revealed negligible temperature polarization at a feed temperature of 25 °C, enabling consistent operation across varying module lengths and flow rates (Fig. S23a). Under these ambient conditions, the TF0.1–1 membrane achieved a total SEC as low as 1.88 kWh.m−3 with a corresponding single-pass recovery (WRRsp) of 0.32% (Fig. 4d and Supplementary Note 7.2). In this ambient operation, where the feed enters at room temperature without any heating (Fig. S22a), the maximum single-pass water recovery ratio is constrained by the first law of thermodynamics to 0.76% (ΔT=5 oC) (Supplementary Note 7.2). However, since MD is not significantly limited by increasing feed concentration, higher recoveries can be obtained under recirculating operation with auxiliary or waste-heat input using a heat exchanger. When the feed temperature was increased to 35 oC, the SEC increased to 9.37 kWh.m−3 and WRRsp rose to 1.12%. At 40 oC, the SEC reached 12.90 kWh.m−3 with a WRRsp of 1.60%; at 50 oC SEC further increased to 16.65 kWh.m−3 and WRRsp to 2.68%; and at the highest feed temperature of 60 oC, SEC peaked at 20.65 kWh.m−3 with a WRRsp of 3.87%. One can note the one order of magnitude increase in SEC as the temperature is increased from 25 oC to 35 oC, primarily attributed to the substantial thermal energy required to elevate the feed temperature, which intensifies with increasing operating temperatures.

At 60 °C, where the feed was preheated through a heat exchanger followed by external heating (Fig. S22b), the SEC increased more than tenfold compared to ambient operation, reflecting the substantially larger thermal input required and the stronger polarization effects (Fig. S23b). In this configuration, the maximum single-pass recovery is thermodynamically limited at 6.02%. Unlike the ambient case, where higher recoveries can be achieved through recirculation with low-grade heat input, operating at 60 °C inherently requires significant external thermal input, which sharply increases SEC. These findings emphasize the intrinsic advantage of the thin-film membrane in maintaining exceptionally low SEC under near-ambient operation, while providing a clear framework for distinguishing between ambient no-heating operation and high-temperature operation with external thermal input.

Given the membrane low SEC at ambient conditions, we further evaluated SEC across different feed compositions at 25 °C using both synthetic and real feed seawaters (Fig. 4d and Supplementary Note 7). The TF0.1–1 membrane consistently maintains low SEC values across all tested compositions, achieving 2.51 kWh.m⁻3 for 70 g.L⁻1 NaCl (WRRsp of 0.22%), 3.47 kWh.m⁻3 for real untreated seawater (WRRsp of 0.11%), and 4.43 kWh.m⁻3 for real SWRO brine (WRRsp of 0.09%) (Fig. 4d and Supplementary Note 7). Importantly, even when treating challenging high-salinity feeds like SWRO brine, conditions under which RO is generally impractical due to high osmotic pressures, the TF0.1–1 membrane sustains low SECs. The slightly higher SEC observed for real brine (compared to NaCl solutions) is likely due to reduced water flux at elevated salinities, caused by a lower vapor pressure and increased solution complexity. Moreover, the efficacy of MD as an evaporation-based desalination process is commonly assessed via its Gained Output Ratio (GOR). In this context, the fabricated thin film demonstrated an exceptional GOR of 21 (Supplementary Note 7). Additionally, an exergy analysis revealed a low exergy destruction (3.67 kWh.m−3), two orders of magnitude lower than that of a commercial PTFE membrane (>870 kWh.m−3) when operated at the lowest temperature of 25 °C (see detailed analysis in Supplementary Note 9).

For such moderate feed temperatures, if a free heating resource (i.e., waste heat or solar energy) is available, the thermal energy required by the system becomes negligible, and the recovery ratio can be increased beyond the single-pass thermodynamic limit by employing recirculation or multi-effect operation. For example, the discharge (30–40 g.L−1) from the dump condensers of thermal plants at 60 °C or brine discharge (60–70 g.L−1) from thermal desalination processes at approximately 40 °C can be used as feed, significantly reducing the total SEC by offsetting heating demands. In such cases, only the energy needed for pumping or circulating water through the system remains (SECelec), which ranges between 0.1 and 0.4 kWh.m−3 for pilot-scale air gap MD75 and 1.95 kWh.m−3 for a large-scale MD plant76. This low SEC is significantly lower than single-pass DCMD with heat recovery at a 60 °C feed temperature, which exhibits a minimum of 7.7 kWh.m−377.

These results indicate that the fabricated thin films have a high potential for sustainable, energy-efficient desalination in the future. The energy requirements could be reduced to mere fluid recirculation if available heat sources are used, such as low-grade renewable sources (i.e., geothermal and solar energy sources), waste heat, or latent heat from rejected brine. Moreover, using multiple stages, where the latent heat of evaporation is recovered by heating up the inlet feed during vapor condensation in each stage, decreases the SEC value further and drives the system even closer to the thermodynamic limit of the process. This approach enables desalination at low SEC values (Fig. 4d) while maintaining sustainable productivity using a scalable, simple, and low-cost membrane. This performance is combined with the simplicity of the thin-film synthesis process, representing a breakthrough in desalination. This enhanced evaporation process can address water scarcity in arid lands, where solar energy or ambient temperature can be used to operate a system sustainably and obtain sufficient fresh water.

While the thin films demonstrated excellent SEC and desalination performance compared to commercial membranes, it is important to acknowledge that the current fabrication approach, based on a free-interface IP method, may incur higher production costs than those associated with conventional hydrophobic PTFE and SWRO membrane manufacturing. To address scalability and cost concerns, we propose adopting the in-situ IP (support-IP) method, which aligns with the established manufacturing processes used for polyamide (PA) layers in TFC-RO membranes, potentially reducing costs and facilitating large-scale production (see Supplementary Note 8). This method could take advantage of existing industrial infrastructure, enabling a more economical pathway for mass production. As illustrated in Figs. S24 and S25, the free-interface IP method produced a smoother and thinner nanofilm morphology than the support-IP method, likely due to the high heat capacity of water, which dissipates the exothermic heat generated during the IP reaction. In desalination tests (Fig. S26), a slight decrease in flux was observed for thin film fabricated using the support-IP process compared to those made by free-interface IP. Although additional optimization is necessary to ensure cost efficiency at both pilot and industrial scales, we believe this strategy holds promise for the broader economic feasibility of these membranes. Alongside cost and scalability considerations, it is crucial to carefully evaluate the environmental impact of the IP method. Although the large-scale fabrication of the membrane could leverage existing RO membrane manufacturing infrastructure, the conventional IP still involves organic solvents and potentially hazardous chemicals, posing sustainability and environmental safety concerns if not properly managed. Even though industrial-scale RO membrane manufacturing facilities currently adhere to stringent environmental and safety regulations, including solvent recovery systems and rigorous waste disposal protocols, there is an urgent need to pursue eco-friendly alternatives with the development of green solvents and new polymerization techniques aligned with global sustainability objectives.

Furthermore, while commercial PTFE membranes generally offer lower initial fabrication costs, they exhibit significant limitations in terms of productivity at lower operating temperatures, and low-temperature differences such as 5 °C, which restricts their energy efficiency. In contrast, our thin films retain high flux and thus low SEC at minimal temperature differences, offering a more sustainable solution with reduced energy demands. Although further work is required to fully optimize the scalability of this IP-based process, we believe that adopting an in-situ IP approach and aligning the production with TFC-RO infrastructure could significantly reduce costs and environmental impact while preserving the thin films’ high desalination efficiency. This approach, alongside continued design optimization, could enhance the commercial viability of these membranes for sustainable desalination applications.

Methods
Fabrication of polymeric thin films

For the synthesis of polymeric thin films, free-standing nanoporous hydrophobic polymeric thin films were initially fabricated using the IP technique. In a typical procedure, various amounts (Table S1) of 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTCS) were dissolved into 30 mL of hexane (Table S2). Similarly, an amount of m-phenylenediamine (MPD) were dissolved in deionized water to make 3 wt./vol.% of MPD/aqueous solution. Then, polymeric thin films were fabricated by carefully layering the hexane solutions FTCS onto the aqueous solution of MPD at room temperature (25 °C), for a selected time (see Table S1 for concentrations and reaction time details for the twelve thin films). Subsequently, the fabricated thin films were removed from the interface and transferred to water, where the low-density thin film remains floating on the surface. The TFC membranes were prepared by simply transferring the floating thins film onto the substrates, followed by curing at 120 °C for 3 min (Fig. 1 and Fig. S1).

Characterization

The surface and cross-sectional morphologies of the supported thin films were investigated using scanning electron microscopy (SEM, Nova Nano 630 and FEI Teneo VS). Atomic force microscopy (AFM, Dimension Icon SPM, model Dimension 3100, Veec) was used to measure the topography (2D and 3D) and roughness of the films. The wettability of the thin films was measured using a CAM 200 goniometer (KSV Instruments). Water contact angle (WCA) measurements were performed for at least 10 random positions, and the average value is reported in the results. An attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectrophotometer (PerkinElmer Spectrum 100S) was used to detect the functional groups present on the surface of the fabricated polymeric thin films. Raman spectroscopy (20-mW power) was used to demonstrate the binding properties of the thin films further. Raman signals were collected using a 473-nm laser system with 50× objective operated at a power of 20 mW. X-ray diffraction spectra (XRD) patterns were obtained using a Bruker D2 PHASER system with CuKα radiation (λ = 1.54178 Å), operating at 40 mA and 40 kV with a step of 0.02°·s−1. Additional details are presented in Supplementary Note 2.

Desalination under ambient conditions

The desalination capabilities of the as-prepared thin films were evaluated at ambient conditions (25 °C) using a variety of saline solutions, including highly brackish water (10 g.L−1), seawater (30 g.L−1), brine (70 g.L−1), and solutions containing various salt ions (MgCl2, KCl, and Na2SO4). Although most MD studies typically operate at higher transmembrane temperature differences, often utilizing feed temperatures around 60 °C and permeate temperatures around 20 °C (a 40 °C difference), we purposely selected a lower temperature difference of 5 °C. Indeed, while a larger temperature gradient generally enhances water flux, it also significantly increases energy demand, which remains a key obstacle to the broader adoption of MD. The prepared membrane demonstrates strong desalination performance under a minimal temperature gradient, thus offering a more energy-efficient alternative.

Desalination tests were conducted in a custom-designed experimental setup (Supplementary Note 4). The desalination performance of these polymeric thin film was quantified by measuring the water-vapor flux (J, L.m−2.h−1) and percentage of salt rejection (SR, %). Water-vapor flux was calculated by dividing the rate of change of the permeate mass by the active membrane area (2 cm2) and water density (Eq. (S1)). Salt rejection was calculated from the conductivity of the feed and permeate as presented in Eq. (S2). Each experiment was replicated at least three times to ensure reproducibility, and the average flux values were reported. All reported measurements were recorded only after the system had reached a stable operating condition.

In addition, the desalination performance of the thin films was tested using actual samples of seawater from the Red Sea and brine from a commercial seawater reverse osmosis (SWRO) plant, under ambient conditions (Table S6). The chlorine resistance of the thin films was evaluated by introducing 200 ppm of NaOCl to the feed side during the desalination process; stability and flux measurements were subsequently monitored over a period of three hours. Furthermore, the desalination capabilities under varying thin-film fabrication conditions, specifically the IP reaction time and reactant concentration, were examined. This aspect of the study involved nine different membranes, all assessed at room temperature (25 °C) for their ability to desalinate saline solutions. The performance of these membranes at varied feed temperatures was also investigated to determine the thermal effects on desalination efficiency. Further details can be found in the Supplementary Note 4.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information files. All the raw data relevant to the study are available from the corresponding author upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "1) Authors defined their membranes (and testing equipment) suitable for “evaporation-based membrane desalination process”, where “These membranes were sandwiched between the feed (saline solution) and coolant (DI water) streams, with the thin film facing the feed solution” and “To provide a slight driving force across the membrane, we set the transmembrane temperature difference at a very low value of 5 °C, using a low feed temperature of 25 °C and a coolant temperature of 20 °C”. What Authors described in such a bizarre definition (unless misunderstood by this reviewer), and according to the equipment depicted in Fig. S12, is normally defined as direct contact membrane distillation (DCMD) and the “coolant” is normally said distillate. I understand the Authors submitted their work to a generalist journal like NC, however this is the normal and consolidated nomenclature used in the membrane world, so they are invited to follow the proper terminology assessed within almost half a century of published research papers (or to provide justification if this is not the case).\n2) Authors reported the highest calculated transmembrane flux (238.2 LMH) amongst the whole published data, with only a temperature difference of 40 °C between feed and distillate. This would potentially make an exceptional innovation and a revolution in the desalination industry (se also discussion on the energy consumption), towards the solution of the water desalination problem. However, they made such calculation based on weight data from a system of 2 cm2 of active membrane area. Using such minuscule equipment size, means to use reduced feed and distillate volumes, small tube diameters and small-scale sensors and balance to measure the increase in the distillate weight to calculate transmembrane flux as the water is evaporated from the feed towards the distillate side. Even if data from TF membranes is compared to “commercial” samples, in such small system the indeterminateness of a single droplet of water (e.g., coming from the condensation of air humidity on the shell of the tubes in the distillate pipe) could make a big difference, affecting the calculated transmembrane flux from few units to few thousands of LMH. Since Authors affirmed that they can easily scale up the membrane manufacture protocol up to produce surface area of 324 cm2, why they have not used larger membrane size to confirm what they obtained at so small scale? I would be convinced by data when using handleably membrane modules of larger area to provide smaller errors.\n3) Most importantly, Authors calculated the specific thermal energy consumption (SECthe) for their process at the feed temperature of 25 °C, being in the range between 1.8 and 2.36 kWh·m−3 for 35 and 70 g.L-1 NaCl solutions, respectively, and 3.1 and 4.0 kWh·m−3 to desalinate real seawater (Red Sea) and SWRO desalination brine, respectively. If compared with state-of-the-art large-scale RO systems for seawater desalination, for which the SEC value typically varies between 2.5 and 5.2 kWh·m−3 with energy recovery, the proposed numbers would be amazingly significant, also considering that RO is not practical to desalinate brines or other hypersaline solutions. However, such astonishing numbers are calculated (if well understood by this reviewer) by using eq. S10, where Authors included the difference between Tf,i, the inlet temperature to membrane module, and Tf, the temperature of the seawater in the feed tank (…. in a 2 cm2 membrane active area equipment) that is kept at constant temperature. Based on scholastic considerations, to calculate the amount of heat released from the feed and taken from the distillate (due to water evaporation + conduction through the membrane + waste heat), the numerator of Eq. S10 should include the difference between the feed inlet and outlet temperature to membrane module, instead, as normally is for such types of calculations. I’m also very curious to know what the calculated SECth would be when using 40 °C of driving force with 238 LMH transmembrane flux and 35 g L-1 NaCl as feed.\n4) Authors used the Beskok and Karniadakis (BK) unified model to describe the enhanced mass transfer across these thin films. Based on their CFD simulations implementing the BK model, theoretical results revealed that due to the submicron thickness and the nanoscale pores, the vapor flux enhancement was considerably higher than the vapor flux produced by microporous membranes. However, based on Eq. S3&S4, transmembrane BK flux is directly proportional to the square of the pore radius. Then they assumed the vapor-pressure enhancement due to the curved meniscus at the entrance of the superhydrophobic nanosized pores of the thin film, resulting in a substantial increase in the evaporation driving force according to the Kelvin’s equation. However, how is influencing the said above flux vs. pore radius proportionality compared to the meniscus effect, and what about the influence of membrane surface porosity, tortuosity and wetting aptitude (hydrophobicity) in determining the observed amazing mass transfer effects? In this respect, how can this result be compared to mass transfer mechanisms described e.g., by the Dusty Gas model which is conventionally used to describe membrane distillation mechanism and for which the transmembrane flux increases with the pore size?\nThe simulation results also reported the drastic effect of the thin-film thickness on the vapor fluxes when the thickness is increased from 120 nm to 1 μm with the water flux decreasing by a factor of eight, due to an increase of the mass transfer resistance. However, what about heat conduction through the membrane as the thickness is decreased, and its effect on the temperature polarization and the net driving force of the process that are still usual issues in DCMD operation?\n5) How pore sizes of TF membranes have been measured and which are their values?\n6) Considering that Authors defined as “free-standing” their films, why they needed to use the AAO support for their membranes since the support, even if hydrophilic and with large nominal pore size, introduces somehow additional resistance to mass transfer?\n7) Caption to Figure 4a-4c: which is the used feed?\n8) Even if concentration polarization has a reduced influence in MD operations compared to pressure-driven membrane processes, operation at such high transmembrane flux and relatively low temperature (25 °C) would generate locally at the feed/membrane interface, a rather high solute concentration that is expected to induce, first, a reduction in water activity and then salts nucleation due to high supersaturation. This would affect in both cases transmembrane flux by means of reduced vapour pressure/driving force for water evaporation and then membrane permeability due crystals deposition on/in the membrane surface (this effect would be even more pronounced due to solution evaporation in confined pores, thus clogging the pore). How authors comment on this point considering the performances of their membrane for long time operation (24 h) in the absence of any anti-scaling feature?", "reviewer_2": "Comment#1: The authors fabricated the thin films by IP at a support-free organic/water interface, followed by transferring the film onto a substrate to make a supported composite membrane relevant to applications. This free-interface method was developed to ensure that the formation of the film is not affected by the uncontrollable and localized diffusion of the monomers which may result in inhomogeneity of the film1,2. In my understanding, the goal of this method is to either achieve a uniformly-structured and ultra-thin film (i.e., <20nm), or controlled polymerization of certain molecules. However, I did not see a clear reason, or advantage, of using this method in this work, as the thickness of the films (~90nm) was still in the range of those produced by supported IP. Since the free-interface IP method severely harms the scalability of such membranes, I would like to ask the authors to elaborate more on and provide justification for their choice of fabrication method, possibly by providing a comparison between the membrane made from free-interface and support IP procedures in terms of their surface morphology and separation performance.\nComment#2: The thin films fabricated in this work showed excellent vapor flux when performing membrane distillation at temperature differences ranging from 5 to 40°C. The performance of the thin films was superior to that of commercial membranes and recently published membranes. I have a small comment on the logistics of the membrane testing and comparison: most of the comparison was done at a transmembrane temperature difference of 5°C, which is not as common in literatures3–5. I would like to ask the authors to explain the choice of this condition: is it a commonly used condition in MD processes when treating certain types of feed solutions, or was this condition chosen to show the maximum difference between the membrane synthesized in this work and the commercial membranes?\nComment#3: The authors did a great job explaining the possible reasons for the outstanding performance of their membranes using simulation and fitting experimental data with transport models. In the desalination mechanisms section, the authors hypothesized that the ultra-fast vapor transport can be attributed to the “tiny” pores of the thin films and in the SI the authors claimed to pores to be <1 nm. Although it is a claim that I would agree on by looking at the SEM images and performance data, I’m wondering if the author can show more evidence of the pore size to support the claim of sub-nanometer pores. In addition, the author also hypothesized that the degree of crosslinking and the density also have effects on the vapor transport, but I did not find any quantitative measurements to confirm this hypothesis. I would like to ask the authors to provide more evidence (preferably numbers) to support their discussion.\nComment#4: When considering the economics of a membrane system, the cost of membrane fabrication and scale-up is as important as the cost to operate the membrane modules. Although the thin films fabricated in this work showed great SEC compared to commercial membrane, their fabrication cost (i.e., money and time spent on an unconventional IP process) will be much higher than that of the commercial SWRO and hydrophobic PTFE membrane. I would like to ask the author to acknowledge this disadvantages and provide a deeper discussion on the possible way to reduce these costs.\nComment#5: The figures in the manuscript of high quality and are well explained by the captions. However, there are some inconsistencies among the figures, especially the plots. In Figure 3b, the y-axis label is missing (although from my understanding y-axis is not needed); in Figure 3e, the y-axis is colored in purple; In most of the plots in Figure 3&4, the widths and colors of the gridlines are not consistent and some of the words in them are too small and not readable. I would like to ask the authors to give the plots a serious revision to improve the overall quality to make it more readable and consistent.\nComment#6: The authors examined the change in membrane surface morphology with different IP recipes and showed that the size of leaf-like and ridge-and-valley features increased when the IP time increases. Since the vapor transport mechanism is heavily dependent on size and number of the sub-nanometer pores/voids in the membrane, I would expect to see a greatly improved performance when the surface roughness (i.e., accessible surface area) increases 6. However, the membrane with the best performance (TF0.1-1) is the one with relatively smooth surface. I would like to ask the authors to provide further discussion on this result, probably on the trade-off between surface area and membrane thickness or other factors.\nComment#7: Though the hydrophobic FTCS membranes in this work were used for membrane distillation, as shown by the direct-contact MD testing mode and the vapor-pressure-driven transport mechanism, the authors did not clearly reference membrane distillation until the later sections of the paper. Directly specifying the end use of these membranes as well as mentioning the direct-contact testing mode in either the methods section or Supporting Information would allow readers to draw comparisons between similar studies.\nComment#8: While the authors provided performance comparisons to two commercial membranes in this work, they do not specify the brand(s), structure (supported vs. unsupported), or composition of the membranes. The only information was in the SI stating that they are PTFE membranes with a pore size of 0.22 µm. I think including more information on these membranes, or potentially extend the comparison to more commercially available membrane (either comparing to the values reported in literatures7 or performing the same test on more membranes), would make it easier for readers to judge whether these commercial membranes are suitable comparisons for the FTCS membranes studied in this paper.", "reviewer_3": "Methodology: The synthesis of thin films using interfacial polymerization with fluorinated trichlorosilanes and diamine-based monomers is well-executed. The authors present a comprehensive material characterization of the thin films, utilizing a wide range of techniques such as SEM, AFM, Raman spectroscopy, and FTIR to establish their chemical and morphological properties. However, the membrane distillation characterization of the new membranes is limited (see major comments below). The scalability of the membranes is well-supported by the successful fabrication of large-area films (up to 324 cm²) without compromising mechanical strength.\nNovelty and Impact: Membrane distillation membranes are typically ultrafiltration membranes made using phase inversion. The novelty of the work lies in the development of ultrathin membrane distillation membranes using interfacial polymerization, which is widely used for reverse osmosis or nanofiltration membrane manufacture. This approach represents a valuable contribution to the field of membrane science. However, for a broader journal such as Nature Communications, significant changes are required to strengthen the manuscript before it can be reconsidered for publication.\nThe changes must address the following Major Comments:\n1. Energy Efficiency Calculations:\nAll discussion of the energy efficiency calculations needs to be revised to:\na) Explicitly clarify when only electrical energy consumption has been included in energy efficiency calculations.\nThe manuscript should make it explicitly clear in both the abstract and main text when only electrical energy consumption is being considered. This distinction is crucial because it prevents misunderstanding about the total energy requirements of the process.\nb) Include thermal energy consumption more clearly in the main manuscript and not only in the Supplementary Information.\nThermal energy or heat consumption is critical to the driving force for mass transfer in thermal desalination processes like membrane distillation and cannot be omitted from the main manuscript text. The authors currently emphasize low specific energy consumption (SEC) values but focus primarily on electrical energy consumption (parasitic losses). A comprehensive energy analysis that includes both thermal and electrical energy inputs is essential for accurately assessing the process's overall energy efficiency and comparing it with other desalination technologies.\n2. Use of Exergy Efficiency:\nGiven that cheap low-temperature heat is key for the calculations presented, the authors should use exergy efficiency rather than just energy efficiency to gauge process performance. Exergy efficiency, taking into account the quality of the heat supplied, would provide readers with a more accurate description of how efficient the various membrane distillation processes are with these new membranes compared to the existing literature.\nBy employing exergy analysis, the authors can account for the actual useful work potential of the thermal energy sources utilized, especially when low-grade heat is considered. This approach provides a more meaningful metric for evaluating the thermodynamic efficiency of the process and facilitates a fair comparison with other desalination technologies that might use higher-grade energy sources.\n3. Membrane Thermal Conductivity:\nThe authors need to measure the thermal conductivity of the membrane and report this in the manuscript. Thermal conductivity is a critical performance parameter in membrane distillation membranes, as it significantly impacts the actual energy efficiency of the process.\nA membrane that is thin but has high thermal conductivity can lead to large entropy generation from uncontrolled transmembrane heat transfer, reducing the process's overall thermal efficiency. Additionally, the modeling detailed in Section S6 does not consider heat transfer, despite latent heat transfer during evaporation and condensation being critical in membrane distillation. Incorporating thermal conductivity measurements and heat transfer modeling will provide a more accurate assessment of the membrane's performance and its impact on energy consumption.\n4. Membrane Thickness:\nMembrane thickness is a critical parameter in membrane distillation, as the authors’ research groups and others have shown. Previous studies have demonstrated that reducing membrane thickness beyond a certain point can have diminishing returns or adverse effects, such as increased heat transfer leading to reduced thermal efficiency and reduced exergy efficiency (the authors have shown that potential mechanical instability is not a major issue here).\nThe authors should discuss the implications of their membrane's ultrathin structure on both mass transfer and heat transfer. While thinner membranes can reduce mass transfer resistance and potentially increase flux, they can also lead to higher heat losses due to increased conductive heat transfer. An analysis of the trade-offs between membrane thickness and thermal efficiency is necessary to understand the membrane's optimal design parameters.\n5. Membrane Porosity:\nUnderstanding membrane porosity is vital in membrane distillation membranes because it gives insight into how membrane permeance and thermal conductivity are impacted by (1) the membrane material and (2) the membrane architecture.\nPorosity influences both the mass transfer (permeate flux) and the thermal conduction through the membrane. High porosity generally enhances mass transfer but may also increase thermal conduction if not properly managed. The authors should measure and report the membrane's porosity, and discuss how it affects the membrane's performance, including any trade-offs between flux and energy efficiency.\n6. Membrane Pore Diameter:\nMembrane pore diameter is an important parameter for membrane distillation membranes because it provides key insight into liquid entry pressures (LEP), which affect operating pressure and pressure drop limitations.\nThe authors should measure the pore size distribution of their membranes and report the average pore diameter. Understanding the pore size is crucial for ensuring that the membrane can prevent liquid intrusion while allowing vapor transport. This information will also help assess the membrane's suitability for various operating conditions and its stability during long-term operation.\n7. Revision of the Title:\nThe authors need to revise the title to more accurately reflect the content and focus of the manuscript. There is no mention of either the chemistry being used or that the desalination is being performed via membrane distillation. Most readers seeing the current title will think of the fabrication of thin-film composite reverse osmosis membranes, which are already highly scalable.\nMaking it clear that these are membrane distillation membranes is vital because thin-film composite reverse osmosis membranes are not new. Furthermore, using terms like “ultrahigh-efficiency brine desalination” is potentially misleading because the authors have not fully accounted for thermal energy in their calculations. The process itself is not more energy efficient compared to reverse osmosis when considering all energy inputs.\n8. Discussion of technoeconomics:\nA large part of the value described in the manuscript is based on “free” low-grade heat or low-temperature heat. If this argument is central to the paper the authors need to have a discussion about costs and not just energy efficiency. As the authors note water desalination or brine concentration costs are only partly dependent on energy consumption. Typically, it is possible to make a separations process more energy efficient by reducing driving forces and increasing membrane or adsorbent area. Of course in practical applications, membrane area or separations footprints are limited by cost."}, "author_response": "Responses to reviewers’ comments: Manuscript NCOMMS-24-48277-T\nDear Chief Editor and reviewers,\nThe authors would like to thank you for your thoughtful consideration and time in reviewing the\nmanuscript. Your comprehensive review improved the overall quality of the manuscript. All\ncomments and suggestions have been incorporated in the revised version, and individual responses\nto comments are provided below. The changes made in the manuscript and SI document are\nhighlighted.\nReviewer #1 (Remarks to the Author)\nHere, Authors developed an interfacial polymerization (IP) protocol where 1H,1H,2H,2H-\nperfluorododecyltrichlorosilane (FTCS) and m-phenylenediamine (MPD) monomer reacted at the\ninterface of two immiscible solutions (hexane/water solvents, respectively), forming a\nsuperhydrophobic (water contact angle from ~145° to ~160°) free-standing polymeric thin\nfilms with sub-nanosized pores. The so prepared films presented variable thicknesses (from ~60\nto ~240\nnm) and surface morphology (dense, nodulus, leaf-like, ridge-and-valley) which could be\ncontrolled by adjusting the IP reaction time and the FTCS concentration.\nThe so prepared membranes were then transferred onto a hydrophilic porous support (anodized\naluminum oxide with nominal pore size of 0.2 µm) and used for desalination of salts solutions,\nreal seawater and real RO brine by an “evaporation-based membrane desalination process”.\nBased on experiments, Authors reported that the thin film fabricated at 0.1 wt.% of FTCS for 1\nmin (TF0.1–1) exhibited the best desalination performance for 30 g·L−1 NaCl solution with a\nvapor flux of 40 LMH and salt rejection ⁓99.9%, whereas commercial membranes produced\nalmost zero flux under similar operating conditions. Furthermore, when the concentration of the\nsalt (NaCl) in the feed was increased to 70 g·L−1, simulating the brine of SWRO, they observed\na flux of 31 LMH. With real seawater (Red Sea salinity = 40.98 g/L) and real desalination brine\n(discharge from SWRO commercial plant = 74.4 g/L) the TF0.1-1 membranes exhibited fluxes of\n14 and 12 LMH and salt rejections of 99.3% and 99.0 %, respectively. Finally, TF0.1–1 membrane\nprovided a flux of 238.2 LMH at a transmembrane temperature difference of 40 °C (feed at 60 °C\nand “coolant” at 20 °C), making their performance, based on Authors’ discussion, the highest\nvalues reported in the literature for both polymeric and inorganic membranes.\nFinally, Authors reported that SEM analysis confirmed the long-term stability in treating real\nseawater and real SWRO brine, with resistance to chlorine, demonstrating its superior mechanical\nintegrity and the ability to fabricate large samples of size 324 cm2 (18x18 cm2).\nComments:\n1) Authors defined their membranes (and testing equipment) suitable for “evaporation-based\n1\nmembrane desalination process”, where “These membranes were sandwiched between the feed\n(saline solution) and coolant (DI water) streams, with the thin film facing the feed solution” and\n“To provide a slight driving force across the membrane, we set the transmembrane temperature\ndifference at a very low value of 5 °C, using a low feed temperature of 25 °C and a coolant\ntemperature of 20 °C”. What Authors described in such a bizarre definition (unless misunderstood\nby this reviewer), and according to the equipment depicted in Fig. S12, is normally defined as\ndirect contact membrane distillation (DCMD) and the “coolant” is normally said distillate. I\nunderstand the Authors submitted their work to a generalist journal like NC, however this is the\nnormal and consolidated nomenclature used in the membrane world, so they are invited to follow\nthe proper terminology assessed within almost half a century of published research papers (or to\nprovide justification if this is not the case).\nResponse: We would like to thank the reviewer for the invaluable comment, which we believe is\ncritical for future research to be undertaken either by neophytes or experts in the field. Indeed, it\nis common knowledge that membrane distillation (MD) is an evaporation-based process that relies\non species relative volatility for separation. The driving force is provided as a pressure differential\nbetween the feed and permeate sides of the membrane, which depending on the process variant,\ncould be sustained with streams set at different temperatures or by applying vacuum at the\npermeate side. The membranes used in MD are commonly polymeric, made e.g. PTFE or PVDF,\nwith thicknesses in the order of tens of microns and an average pore size ranging between 0.1 and\n1.0 microns. Since the early nineties, attempts to explain the transmembrane transport mechanism\nof vapor from the feed to the permeate side used a panoply of models which evolved from a\ncombination of Knudsen diffusion and Poiseuille flow, to the dusty gas model and the widely used\nKnudsen-Molecular-Poiseuille transition model. Additionally, after decades of intensive research,\nit is established that the operating conditions in MD for sustainable production using existing\nmembranes require a feed heating in the range of 60 °C or applying vacuum as low as 80 mbar1.\nComparing all these features to our system including membrane and equipment, we found out that\nwe only share the MD experimental set-up and the nature of the separation process, which is\nbelieved to remain volatility based. As such, the new membrane exhibits superhydrophobicity and,\nhas a thickness two orders of magnitude thinner than that of commercial MD membranes with an\naverage pore size two to three orders of magnitude smaller. The membrane can sustain productivity\nwith a feed stream at room temperature (25 °C) giving fluxes that cannot be explained with state-\nof-the-art models used for MD. As demonstrated in the manuscript (including in Fig. 4c) and\ndiscussed above, the typical models used for MD could not match our results due to the different\nmembrane characteristics, while considering water vapor enhancement due to curvature with the\nunified transport model led to an excellent agreement. Therefore, without deviating from MD per\nse, which belongs to the family of evaporation-based processes, we believe that using a process\nname having a higher level of abstraction would be more appropriate. However, we understand\nfrom the reviewer’s comment, which remains of the utmost importance to us, that this decision is\nvery likely to mislead readers. Therefore, to address the reviewer’s concern, for whom the process\nname should be based on the equipment we used, thus MD, we replaced “evaporation-based” by\n“membrane distillation (MD)” in the manuscript, namely in the experimental and desalination\nperformance sections, except in two sentences (page 2 and page 16) where the term may include\nother processes as well. We also replaced “coolant” by “distillate” as suggested by the reviewer\nin both manuscript and SI document. Also, we modified the title to specify the hydrophobic nature\nof the membrane and added MD in keywords.\n1. Zhao K, Heinzl W, Wenzel M, Büttner S, Bollen F, Lange G, et al. Experimental study of the\nmemsys vacuum-multi-effect-membrane-distillation (V-MEMD) module. Desalination 2013, 323:\n150-160.\n2) Authors reported the highest calculated transmembrane flux (238.2 LMH) amongst the whole\npublished data, with only a temperature difference of 40 °C between feed and distillate. This would\npotentially make an exceptional innovation and a revolution in the desalination industry (se also\ndiscussion on the energy consumption), towards the solution of the water desalination problem.\nHowever, they made such calculation based on weight data from a system of 2 cm2 of active\nmembrane area. Using such minuscule equipment size, means to use reduced feed and distillate\nvolumes, small tube diameters and small-scale sensors and balance to measure the increase in the\ndistillate weight to calculate transmembrane flux as the water is evaporated from the feed towards\nthe distillate side. Even if data from TF membranes is compared to “commercial” samples, in such\nsmall system the indeterminateness of a single droplet of water (e.g., coming from the\ncondensation of air humidity on the shell of the tubes in the distillate pipe) could make a big\ndifference, affecting the calculated transmembrane flux from few units to few thousands of LMH.\nSince Authors affirmed that they can easily scale up the membrane manufacture protocol up to\nproduce surface area of 324 cm2, why they have not used larger membrane size to confirm what\nthey obtained at so small scale? I would be convinced by data when using handle ably membrane\nmodules of larger area to provide smaller errors.\nResponse: We fully understand the reviewer’s concern regarding the potential impact of using\nsmall membrane areas on flux measurement accuracy. The justification for the fabrication of a\n2 cm2 membrane as a starting point is twofold. The area needs to be large enough for an accurate\nestimation of the permeate flux and small enough to be tested in a cell with minimized polarization\neffects to capture the intrinsic properties of the membrane. As shown in Fig. S15, we utilized a\nrelatively large setup with feed and permeate volumes of 1.5 L each, equipped with a high-\nprecision balance capable of measuring up to 3.2 kg with two-digit sensitivity. This setup\nminimized any variations in weight measurements, thus ensuring the accuracy of our membrane\nflux calculations. Additionally, each experiment ran for at least 2 hours and was repeated a\nminimum of 5 times to minimize experimental errors. As such, in a single experiment the system\nproduces between 16 mL (40 LMH) and 95 mL (238 LMH) of distillate, which is sufficient for an\naccurate flux measurement. It is also worth noting that the size of our coupon is in the range of\nmembrane area reported in earlier studies such as that of Chen et al [Nature Nanotech 13, 345–\n350 (2018)] who used a 1.26 cm² membrane. The area used in our investigation was even larger\nand revealed itself well-suited for laboratory-scale experiments in terms of practicability and\nproductivity. In response to the reviewer’s comment, we prepared additional eight TFC\nmembranes (four by free-interface IP and four by support-IP), by scaling-up the fabrication process\n3\nto an area of 100 cm² for all substrates except AAO, which was limited to a 5 cm diameter due to\nmaterial availability (see Fig. S24 and S25). Furthermore, although not reported in this study, we\nconfirmed the scalability of these thin films by fabricating TFC membranes with a total area of\n2100 cm² (shown in the digital photo of Fig. 1, below). The current limitations in the scale-up\nprocess to lift the concept to a technology readiness level of 2-3 concern the availability of the\nspecific equipment for membrane testing. The high performance of the membrane exacerbates\ntemperature polarization, thus requiring specific module design, which is under development.\nFigure 1. Large-scale TFC membrane fabricated on a polymeric substrate with an area of 70 cm\nx 30 cm.\n3) Most importantly, Authors calculated the specific thermal energy consumption (SEC ) for their\nthe\nprocess at the feed temperature of 25 °C, being in the range between 1.8 and 2.36 kWh.m−3 for 35\nand 70 g.L-1 NaCl solutions, respectively, and 3.1 and 4.0 kWh·m−3 to desalinate real seawater\n(Red Sea) and SWRO desalination brine, respectively. If compared with state-of-the-art large-\nscale RO systems for seawater desalination, for which the SEC value typically varies between 2.5\nand 5.2 kWh·m−3 with energy recovery, the proposed numbers would be amazingly significant,\nalso considering that RO is not practical to desalinate brines or other hypersaline solutions.\nHowever, such astonishing numbers are calculated (if well understood by this reviewer) by using\neq. S10, where Authors included the difference between Tf,i, the inlet temperature to membrane\nmodule, and Tf, the temperature of the seawater in the feed tank (…. in a 2 cm2 membrane active\narea equipment) that is kept at constant temperature. Based on scholastic considerations, to\ncalculate the amount of heat released from the feed and taken from the distillate (due to water\nevaporation + conduction through the membrane + waste heat), the numerator of Eq. S10 should\ninclude the difference between the feed inlet and outlet temperature to membrane module, instead,\nas normally is for such types of calculations. I’m also very curious to know what the calculated\nSECth would be when using 40 °C of driving force with 238 LMH transmembrane flux and 35 g\nL-1 NaCl as feed.\nResponse: We thank the reviewer for this insightful observation and appreciate the opportunity to\nclarify our methodology for calculating the specific thermal energy consumption (SEC ) of our\nth\nsystem. We believe that the way we presented the calculations and named the respective\n4\ntemperatures is misleading, while it reduces exactly to the methodology the reviewer is pointing\nto, for a closed system. In the SCE calculations, given the small temperature difference required\nth\nbetween the feed and permeate, with both streams being at near ambient, we assumed that one\nstream is already available in a single pass system. Therefore, the process requires either heating\nthe feed from 20°C to 25°C if the distillate is available at 20°C or cooling the distillate to 20°C if\nthe feed is available at 25°C. In our calculations of the original manuscript, we assumed that both\nthe feed and distillate streams are available at 20°C, and the energy required for the process to run\nis brought by heating the feed from 20°C (feed tank temperature denoted T in the original\nf\nmanuscript) to 25°C (Module inlet temperature denoted T in the original manuscript), which\nf,i\nexplains the use of (𝑇 −𝑇 ). This is equivalent to assuming that the feed is available at 25°C\n𝑓,𝑖 𝑓\n(module inlet - T ) and the temperature at module outlet is 20°C (T ), which will require the use\nf,i f,o\nof (𝑇 −𝑇 ) as suggested by the reviewer. Therefore, we fully agree with the reviewer that, in\n𝑓,𝑖 𝑓,𝑜\na closed (batch) system, the numerator of Eq. S10 (now updated as Eq. S16 in the revised SI\ndocument) needs to account for the heat loss in the module to set the heater requirements, and the\ntemperature difference between the feed inlet and outlet temperatures need to be considered to\naccurately reflect the heat balance. However, instead of a closed loop, our calculations were\nperformed for a single-pass MD configuration where the feed solution flows through the\nmembrane module once and exits as brine without recycling. Therefore, energy recovery from the\nbrine is not incorporated in low-temperature scenarios (e.g., 25°C). To address the reviewer’s\nconcern and enhance clarity, we have added a detailed schematic of this single-pass configuration\nin the SI document (Fig. S22) with feed available at 25°C. At lower feed temperatures near\nambient, where brine recirculation may not be energy- or cost-effective, the single-pass setup\nprovides an efficient, adaptable approach with minimized energy requirements (See Section S7\nand Fig. S22a).\nImportantly, we appreciate the reviewer’s suggestion to consider the SEC at elevated feed\ntemperatures. Clearly, at higher temperatures, integrating a heat exchanger to preheat the feed with\nthe exiting brine can significantly reduce thermal energy requirements. Therefore, we have\nrecalculated the SEC for this scenario, incorporating a heat exchanger with an efficiency of 60%.\nThese updated SEC values, along with corresponding assumptions and schematics, are now\nincluded in the SI document (Section S7 and Figs. S22-S23 on pages 37-42), providing a clearer\ndepiction of the system’s energy dynamics at different operating temperatures. Energy section is\nalso revised in the manuscript, see pages 16-18.\nAdditionally, we acknowledge that optimizing the SEC becomes even more critical at pilot and\nindustrial scales. Indeed, at these scales, factors such as ambient climate, availability of waste\nheat, and integration with existing technologies (e.g., coupling with RO for brine treatment) will\nstrongly impact energy efficiency. We are currently conducting energy analyses for our future\npilot-scale system to further explore these factors in an industrial environment. We believe that\nthis ongoing work will provide deeper insights into SEC optimization under varying conditions.\n5\n4) Authors used the Beskok and Karniadakis (BK) unified model to describe the enhanced mass\ntransfer across these thin films. Based on their CFD simulations implementing the BK model,\ntheoretical results revealed that due to the submicron thickness and the nanoscale pores, the vapor\nflux enhancement was considerably higher than the vapor flux produced by microporous\nmembranes. However, based on Eq. S3&S4, transmembrane BK flux is directly proportional to\nthe square of the pore radius. Then they assumed the vapor-pressure enhancement due to the curved\nmeniscus at the entrance of the superhydrophobic nanosized pores of the thin film, resulting in a\nsubstantial increase in the evaporation driving force according to the Kelvin’s equation. However,\nhow is influencing the said above flux vs. pore radius proportionality compared to the meniscus\neffect, and what about the influence of membrane surface porosity, tortuosity and wetting aptitude\n(hydrophobicity) in determining the observed amazing mass transfer effects? In this respect, how\ncan this result be compared to mass transfer mechanisms described e.g., by the Dusty Gas model\nwhich is conventionally used to describe membrane distillation mechanism and for which the\ntransmembrane flux increases with the pore size?\nThe simulation results also reported the drastic effect of the thin-film thickness on the vapor fluxes\nwhen the thickness is increased from 120 nm to 1 μm with the water flux decreasing by a factor of\neight, due to an increase of the mass transfer resistance. However, what about heat conduction\nthrough the membrane as the thickness is decreased, and its effect on the temperature polarization\nand the net driving force of the process that are still usual issues in DCMD operation?\nResponse: Thank you for raising these very important points. Indeed, the translation of physical\nobservations into tractable mathematical models represents a route to understanding the underlying\nphenomena that result in the high permeate fluxes exhibited by the new membrane. Nevertheless,\nas no approach can claim universality, we would like to share our philosophy with the reviewer\nand the approach undertaken in this investigation. Several modes can potentially contribute to\ntransport in porous media, namely Knudsen diffusion, molecular diffusion and viscous flow.\nRegarding transmembrane vapor transport in MD, much effort has focused on finding the parallel\nversus in series combination of these transport modes to explain experimental observations1,2. As\nmentioned by the reviewer, further progress led to the introduction of more elaborate approaches\nsuch as the dusty gas model (DGM), which is based on Maxwell-Stephan diffusion equation3.\nDGM, which finds its developments in catalysis leads to an additive form of vapor diffusion modes\nthrough stagnant air in pores. Yet, DGM has been proved unable to predict simple diffusion with\na stagnant component4, a fact ignored in MD developments before the shortcoming acknowledged\nwith the publication of the widely used Knudsen-Molecular-Poiseuille-transition (KMPT) model.\nThese mechanistic approaches have known limits, yet still offer a mathematical form that can\nexplain observations within specific ranges of parameters and operating conditions. This is\nacceptable, since model universality cannot be achieved. The general approach is to minimize the\nerror between experimental and calculated values by adjusting unknown parameters. The\nparameter that is commonly used for calibration is tortuosity, of which determination has seen no\nconsensus yet whether experimentally or theoretically5. Adopting this approach, the KMPT model\nwas first implemented during this investigation with the vapor-pressure enhancement caused by\n6\nmeniscus curvature. As can be seen in Fig. S21, the model is unable to explain the trend of the\npermeate flux evolution with temperature. Based on these results, we questioned the ability of the\nKMPT model to explain the underlying transport mode for the new membrane. As such, the choice\nof Beskok-Karniadakis (BK) model, which is based on an extension of the validity of the Navier-\nStokes equations, is not fortuitous. The model is built using a second order slip condition and the\ninsertion of a rarefaction coefficient thoroughly calibrated using both experiments and direct\nsimulation Monte Carlo to cover a large practical range of Knudsen numbers. The major difference\nwith the KMPT model is that the BK model retains the features of a Poiseuille flow, which is\ninherent to the model’s nature as derived from momentum equation, while the KMPT model\nreduces to diffusion as the pore radius gets smaller in which case the dependence on process\ntemperature becomes small6. The reducing effect of the square of the radius as it becomes smaller\nin BK is compensated by the increase of the rarefaction coefficient as the Knudsen number\nbecomes higher and the model reflects the dependence of the experimental flux with temperature.\nAdditional explanations were added to the manuscript in the “Desalination mechanism”\nsection, page 14.\nThe effect of the vapor-enhancement as a function of pore radius based on Kelvin equation is now\nprovided in Fig. S20 (SI, page 35) for different temperatures and a contact angle of 160°.\nThe enhancement ranges between 8 and 5 at 20 °C and 60 °C, respectively, for a pore radius of 0.495\nnm (new data added to the manuscript, pages 6-7 and SI pages 12-13), which explains the flux\ndifferences between the new membrane and the commercial membranes. As for the effect of the\nmembrane porosity (4.94%; new data added to the manuscript, page 7 and SI pages 7 and 23)\nand tortuosity of which value typically ranges between 1.2-2.2, respectively, their variation could\nnot explain the significant increase of the permeate fluxes compared to common MD membranes.\nRegarding the hydrophobicity, we believe that the new thin film being superhydrophobic favors\nthe meniscus deformation without wetting, leading to the enhancement of the local vapor pressure.\nRegarding the reviewer’s concern about the effect of the membrane thickness and how it is related\nto heat conduction, the net driving force and polarization, it is very important to first emphasize\nthat the current multiscale model is fully coupled. Momentum, heat and mass transfer are solved\nin both feed and permeate channels in a conjugate approach including heat convection, heat\nconduction and mass transfer across the porous membrane. The heat convection part is embedded\nvia an equivalent thermal conductivity, which includes the heat transfer that accompanies the\nevaporation process. The associated cooling of the feed and heating of the permeate are also taken\ninto account as bulk source terms at the vicinity of the membrane7. As such, the simulation results\nshowing the significant effect of film thickness on permeate fluxes include all these interrelated\nphenomena. It is established from the literature that the evaporation efficiency is a tradeoff\nbetween less resistance to mass transfer and increased conduction losses when thickness is\ndecreased. However, the literature pertaining to these interrelated phenomena dealt with common\nMD membranes. In this investigation, the thin films are two orders of magnitude thinner with a\npore size between two and three orders of magnitude smaller than those of membranes that are\ntypically used in MD. Therefore, we believe that a translation of literature results related to the\n7\nrange of optimal thicknesses may not be valid, particularly that theoretical approaches are often\nbased on the use of either the dusty gas model (DGM) or the Knudsen-Molecular-Poiseuille\nTransition (KMPT) model. In our work, the KMPT model could not explain the effect of\ntemperature on the experimental fluxes, which incited us to use the Beskok-Karniadakis (BK)\nunified model. The latter, which was derived from momentum equations with a second order\nboundary slip condition, incorporates a correction factor for very high Knudsen numbers calibrated\nusing direct simulation Monte Carlo and experiments, which was able to capture the experimental\ntrend. Importantly, the calculations of the permeate flux as a function of the membrane thickness\nwere in good agreement with the experimental values as reported in Fig.3b of the manuscript.\nFrom the same figure, no optimum thickness is visible as it is likely to require the fabrication of\nthinner films, which was not possible due to the short reaction time and film fragility below the\nrange considered in the investigation. Therefore, we came to conclude that the optimum thickness\nfor the new thin films might have shifted towards smaller thickness values below 60 nm.\nTo address the reviewer’s concern, a statement is added to the “Desalination mechanism” of the\nmanuscript to emphasize on the shift of the thickness optimum towards thicknesses smaller than\n60 nm compared to orders of magnitude higher values for common MD membranes.\nModifications were brought to the manuscript accordingly on page 14.\nReferences\n1. Schofield RW, Fane AG, Fell CJD. Heat and mass transfer in membrane distillation. Journal\nof Membrane Science 1987, 33(3): 299-313.\n2. Schofield RW, Fane AG, Fell CJD. Gas and vapour transport through microporous\nmembranes. I. Knudsen-Poiseuille transition. Journal of Membrane Science 1990, 53(1): 159-\n171.\n3. Lawson KW, Lloyd DR. Membrane distillation. Journal of Membrane Science 1997, 124(1):\n1-25.\n4. Kerkhof PJAM. A modified Maxwell-Stefan model for transport through inert membranes: the\nbinary friction model. The Chemical Engineering Journal and the Biochemical Engineering\nJournal 1996, 64(3): 319-343.\n5. Fu J, Thomas HR, Li C. Tortuosity of porous media: Image analysis and physical simulation.\nEarth-Science Reviews 2021, 212: 103439.\n6. Ding Z, Ma R, Fane AG. A new model for mass transfer in direct contact membrane\ndistillation. Desalination 2003, 151(3): 217-227.\n7. Soukane S, Nawaz MS, Obaid M, Gudideni V, Al-Qahtani A, Ghaffour N. Advanced 3D\nmultiscale modeling of forward osmosis-membrane distillation integrated designs. Desalination\n2024, 571: 117089.\n8\n5) How pore sizes of TF membranes have been measured and which are their values?\nResponse: We would like to thank the reviewer for the insightful comment. To address the\nreviewer’s request, we employed grazing-incidence wide-angle X-ray scattering (GI-WAXS) to\nanalyze the in-plane structure of the TF membranes and determine the pore size. The GI-WAXS\nin-plane profiles for the three variants (TFC0.1-1, TFC0.1-2, and TFC0.1-3) demonstrate\nconsistent peaks at 1.27 Å-1 which corresponds to a pore size of 0.495 nm. The peaks remained\nconsistent across different samples suggesting a similar pore size throughout the films. For a\ncomprehensive description of the GI-WAXS method, please refer to the SI document, Section\n2.7. The results of this analysis are discussed in detail within the main manuscript (Page 6-7),\nFig. 2i, and the SI document (Page 12-13).\n6) Considering that Authors defined as “free-standing” their films, why they needed to use the\nAAO support for their membranes since the support, even if hydrophilic and with large nominal\npore size, introduces somehow additional resistance to mass transfer?\nResponse: We agree that the use of a support introduces additional resistance to mass transfer.\nHowever, handling the thin film without support is impractical, as it is extremely challenging to\nextract it from the organic-aqueous interface in a free-standing form. To address this, we\nspecifically selected an AAO support to minimize the impact on mass transfer, owing to its large\nstraight pores. Providing this mechanical support is essential to maintain the structural integrity of\nthe thin film, particularly during desalination. This approach ensures the film remains intact and\nfacilitates easier handling in practical applications. This is similar to the structure of reverse\nosmosis (RO) membranes, where an ultrafiltration (UF) membrane provides mechanical support\nto the active polyamide (PA) desalination layer.\n7) Caption to Figure 4a-4c: which is the used feed?.\nResponse: Thank you for the scrutiny. We utilized a feed solution of NaCl at a concentration of\n35 g/L and a flow rate of 500 mL/min. We have clarified these details in the figure caption (Page\n15).\n8) Even if concentration polarization has a reduced influence in MD operations compared to\npressure-driven membrane processes, operation at such high transmembrane flux and relatively\nlow temperature (25 °C) would generate locally at the feed/membrane interface, a rather high\nsolute concentration that is expected to induce, first, a reduction in water activity and then salts\nnucleation due to high supersaturation. This would affect in both cases transmembrane flux by\nmeans of reduced vapour pressure/driving force for water evaporation and then membrane\npermeability due crystals deposition on/in the membrane surface (this effect would be even more\npronounced due to solution evaporation in confined pores, thus clogging the pore). How authors\n9\ncomment on this point considering the performances of their membrane for long time operation\n(24 h) in the absence of any anti-scaling feature?\nResponse: We totally agree that concentration polarization and potential salt nucleation are critical\nconcerns, especially during long-term operations. With the goal to capture the membrane intrinsic\nability to separate water from the feed, we have chosen in our experiments a membrane area large\nenough to produce amounts of water that can be measured with accuracy while requiring a small\ndesalination cell to minimize the flow path along the membrane. Consistently, the selected flow\nrate (0.5 L/min) leads to high crossflow velocity (~30 cm/s), which mitigates both concentration\nand temperature polarization, ensuring more stable flux over time. Consequently, this approach\nlimited the accumulation of solutes near the feed/membrane interface and reduced the risk of\nlocalized crystallization.\nHowever, we acknowledge that the reviewer’s comment is particularly relevant for large-scale MD\napplications, where concentration polarization and scaling could become more pronounced due to\nincreased membrane area and longer flow paths. Indeed, in industrial-scale settings, localized\nsupersaturation at the membrane’s vicinity could lead to salt deposition, potentially clogging the\npores and reducing membrane permeability over time. To address these concerns, the\nincorporation of anti-scaling strategies is essential. Potential solutions include the addition of scale\ninhibitors, periodic flushing of the membrane system to remove salt deposits and optimizing\nmodule design to maintain high crossflow velocities while minimizing dead zones. These measures\ncan help limit crystallization risks and support sustained performance during long-term operations.\nIt is also worth noting that these challenges are not unique to MD but are shared with pressure-\ndriven processes such as RO. In fact, some researchers argue that current RO membrane\ntechnology may have reached a performance plateau, where further flux enhancement may not\nyield significant economic advantages. Nonetheless, ongoing research in RO continues to explore\nadvanced materials like carbon nanotubes and graphene membranes to enhance flux, despite\nsimilar challenges of concentration polarization and scaling. Therefore, developing effective\nsolutions for concentration polarization and scaling is imperative for both MD and RO as we\ncontinue advancing towards ultrahigh-flux membranes.\nObaid et. al. present a class of polyamide thin films with ultrahigh flux in membrane distillation,\noutstanding salt (>99.8%) and boron rejection (100%), and low energy consumption. These films\nwere fabricated via a rapid interfacial polymerization (IP) process, and owing to their super-\nhydrophobicity, sub-nanometer pore sizes and ultra low thickness (as low as <100nm), these thin\nfilms showed superb vapor flux during membrane distillation with transmembrane temperature\ndifferences ranging from 5 to 40°C. These performance metrics are superior to most commercial\nmembranes, as well as other hydrophobic membranes reported previously. In my opinion, this\nwork does not show novelty in terms of membrane material design and fabrication. But on the\nother hand, the resultant thin films exhibit excellent desalination relevant properties that are\nunprecedented, and which provide unique advantages compared to existing membranes and at the\nsame time showed good potential towards scale-up and commercialization. I suggest the following\nrevisions and clarifications for the authors’ consideration.\nResponse: We sincerely thank the reviewer for their invaluable feedback and thoughtful\nassessment of our work. We have carefully addressed all the reviewer’s points in the revised\nmanuscript, including all the requested specific revisions and clarifications.\nComment#1: The authors fabricated the thin films by IP at a support-free organic/water interface,\nfollowed by transferring the film onto a substrate to make a supported composite membrane\nrelevant to applications. This free-interface method was developed to ensure that the formation of\nthe film is not affected by the uncontrollable and localized diffusion of the monomers which may\nresult in inhomogeneity of the film1,2. In my understanding, the goal of this method is to either\nachieve a uniformly-structured and ultra-thin film (i.e., <20nm), or controlled polymerization of\ncertain molecules. However, I did not see a clear reason, or advantage, of using this method in this\nwork, as the thickness of the films (~90nm) was still in the range of those produced by supported\nIP. Since the free-interface IP method severely harms the scalability of such membranes, I would\nlike to ask the authors to elaborate more on and provide justification for their choice of fabrication\nmethod, possibly by providing a comparison between the membrane made from free-interface and\nsupport IP procedures in terms of their surface morphology and separation performance.\nResponse: We acknowledge that the support-free organic/water interface (free-interface IP)\nmethod is typically employed to produce ultra-thin and smooth films, benefiting from the high\nheat capacity of water to dissipate the exothermic heat generated during IP, thereby promoting\nuniform film formation. However, as mentioned by the reviewer, the choice of the free interface\nmethod is to ensure that the film formation is not affected by uncontrolled mass transfer, which\nwe believe represents a natural choice as a starting point for the analysis of a new category of thin\nfilms. Nevertheless, as the reviewer’s concern is critical, we extended the study by conducting\nadditional experiments comparing a new set of membranes fabricated by the free-interface IP\nmethod to another new set fabricated by the support-IP (in-situ IP) method. Specifically, we\nfabricated TFC membranes using both approaches on four different substrates, namely a\n11\ncommercial PES substrate with a pore size of 0.2 µm, an AAO substrate, and two lab-prepared\nelectrospun nanofiber substrates (PVDF and PAN). These substrates were selected to provide a\nrange of pore structures and wettability, for a better comparison of the effect of the two methods\non final film morphology. Thus, we prepared a total of eight TFC membranes (four using free-\ninterface IP and four using support-IP), scaling the fabrication area to 100 cm² for all substrates\nexcept AAO, which was limited to a 5 cm diameter due to material unavailability. Details of these\nexperiments are included in a new section of the SI document (Section S8; Pages 42 – 45).\nIn terms of morphology, SEM analysis (Figs. S24 and S25, pages 43-44) demonstrates that\nmembranes produced by the support-IP method exhibit a relatively rougher and inhomogeneous\nsurface structure. This increased roughness is likely due to confinement effects during IP and the\nlimited ability to dissipate heat generated by the exothermic reaction. In contrast, the free-interface\nIP method yielded a smoother film morphology, likely due to water’s thermal dissipation\nproperties, which mitigates localized heating effects and promotes more uniform film formation.\nIn order to evaluate the practical impact of these fabrication methods, we assessed the desalination\nperformance of membranes produced by both techniques (Fig. S26, page 45). Specifically, we\ncompared the desalination flux and salt rejection of membranes fabricated by free-interface IP and\nsupport-IP on the AAO substrate (Section S8, Fig. S26). Although results show that membranes\nprepared by the free-interface IP method give higher permeate fluxes and a better rejection, the\nperformance of the support-IP membranes is noticeable and remains outstandingly high for a 5 °C\ntemperature difference as a driving force. Regarding the thickness, the films produced by free-\ninterface IP were thicker than ultra-thin films typically achievable with this method (such as\npolyamide (PA) films). This may be due to the differences in reaction kinetics and the heat\ngenerated in this system, which may differ from those observed during PA films fabrication. We\nacknowledge that further studies focusing on the specific reaction kinetics and heat generation in\nthis system would be beneficial to elucidate these differences. Based on these findings, we totally\nagree with the reviewer that for scalability and large-scale applications the support-IP method is\nmore suitable, despite a slight reduction in flux and rejection compared to free-interface IP. We\nhave clarified this in the revised manuscript (Pages 17-18) and SI document (Section S8)\nwith emphasis on the need for a full optimization of the fabrication process to enable translation to an\nindustrial scale.\nComment#2: The thin films fabricated in this work showed excellent vapor flux when performing\nmembrane distillation at temperature differences ranging from 5 to 40°C. The performance of the\nthin films was superior to that of commercial membranes and recently published membranes. I\nhave a small comment on the logistics of the membrane testing and comparison: most of the\ncomparison was done at a transmembrane temperature difference of 5°C, which is not as common\nin literatures 3–5. I would like to ask the authors to explain the choice of this condition: is it a\ncommonly used condition in MD processes when treating certain types of feed solutions, or was\n12\nthis condition chosen to show the maximum difference between the membrane synthesized in this\nwork and the commercial membranes?.\nResponse: We would like to thank the reviewer for this thoughtful observation. We totally agree\nwith the reviewer that most MD studies typically operate at higher transmembrane temperature\ndifferences, often utilizing feed temperatures around 60 °C and permeate temperatures around 20\n°C (a 40 °C difference). However, our selection of a lower temperature difference of 5 °C was\nintentional and based on two key considerations. Firstly, while higher transmembrane temperature\ndifferences generally increase water flux, they also lead to significantly higher energy demands,\nwhich remains a primary barrier to the broader adoption of MD technology. For instance, studies\nconducted at a 40 °C temperature difference report high flux rates, but the associated energy\nconsumption can exceed 1000 kWh/m³ 1,2,3. By demonstrating strong performance at a lower\ntemperature gradient, the new thin films offer a more energy-efficient alternative. In our study,\nMD performance at feed temperatures as low as 25 °C results in a drastic reduction of the energy\nrequired to run the process, thus enhancing the sustainability and feasibility of MD for large-scale\ndesalination. Secondly, one of the primary strengths of the new thin films is their ability to sustain\nproductivity even at minimal temperature differences. This feature is particularly valuable for\napplications in regions with limited access to electricity or heating infrastructure, where large\ntemperature gradients may not be attainable. For example, in desertic regions with ambient\ntemperatures often above 25 °C, our membrane’s ability to operate effectively at small temperature\ndifferences could enable freshwater production using ambient conditions and minimal external\nenergy input. This makes MD more accessible for off-grid or low-energy applications, where water\nscarcity is often most critical. Furthermore, at large scale application, a multi-stage or multi-effect\nconcept similar to MSF or MED concepts is necessary to attain high efficiency4-6.\nReferences\n1. Chen X, Zhu Y-B, Yu H, Liu JZ, Easton CD, Wang Z, et al. Ultrafast water evaporation\nthrough graphene membranes with subnanometer pores for desalination. Journal of Membrane\nScience 2021, 621: 118934.\n2. Jantaporn W, Ali A, Aimar P. Specific energy requirement of direct contact membrane\ndistillation. Chemical Engineering Research and Design 2017, 128: 15-26.\n3. Khayet M. Solar desalination by membrane distillation: Dispersion in energy consumption\nanalysis and water production costs (a review). Desalination 2013, 308: 89-101.\n4. J.-G. Lee, W.-S. Kim, J.-S. Choi, N. Ghaffour, Y.-D. Kim, A novel multi-stage direct contact\nmembrane distillation module: Design, experimental and theoretical approaches, Water Research\n107 (2016) 47-56.\n5. J.A. Andrés-Mañas, I. Requena, G. Zaragoza, Characterization of the use of vacuum\nenhancement in commercial pilot-scale air gap membrane distillation modules with different\ndesigns, Desalination 528 (2022) 115490.\n13\n6. L. Francis, N. Ghaffour, A. Al-Saadi, S.P. Nunes, G. L. Amy, Performance evaluation of the\nDCMD desalination process under bench scale and large scale module operating conditions,\nJournal of Membrane Science 455 (2014) 103–112.\nComment#3: The authors did a great job explaining the possible reasons for the outstanding\nperformance of their membranes using simulation and fitting experimental data with transport\nmodels. In the desalination mechanisms section, the authors hypothesized that the ultra-fast vapor\ntransport can be attributed to the “tiny” pores of the thin films and in the SI the authors claimed to\npores to be <1 nm. Although it is a claim that I would agree on by looking at the SEM images and\nperformance data, I’m wondering if the author can show more evidence of the pore size to support\nthe claim of sub-nanometer pores. In addition, the author also hypothesized that the degree of\ncrosslinking and the density also have effects on the vapor transport, but I did not find any\nquantitative measurements to confirm this hypothesis. I would like to ask the authors to provide\nmore evidence (preferably numbers) to support their discussion.\nResponse: To address the reviewer’s concern and further validate our claim of sub-nanometer\npores in the thin films, we proceeded with additional film characterization. As such, we employed\ngrazing-incidence wide-angle X-ray scattering (GI-WAXS) to analyze the in-plane structure of the\nTF membranes. The GI-WAXS in-plane profiles for the three variants (TFC0.1-1, TFC0.2-1, and\nTFC0.3-1) demonstrate consistent peaks at Q = 1.27 Å−1, which corresponds to a pore size of\n||\n0.495 nm. A detailed description of the GI-WAXS methodology is provided in the SI document\n(Section 2.7, pages 6-7), and the results are discussed in the main manuscript (Pages 6-7, Fig.\n2i-j are added) and Supplementary Information (Pages 12-14).\nRegarding the degree of crosslinking and its impact on vapor transport, the out-of-plane GI-WAXS\nanalysis provided further insight. The out-of-plane d-spacing decreased with increasing FTCS\nconcentration, from 1.748 nm (FTC0.1-1) to 1.142 nm (FTC0.2-1) and 0.698 nm (FTC0.3-1),\nindicating denser packing with increasing the FTCS concentration. This reduction in d-spacing\nsuggests enhanced crosslinking and tighter molecular packing, driven by factors such as film\nthickness and reactant concentration. The observed out-of-plane arrangements likely play a\nsignificant role in influencing transport properties. Specifically, the denser out-of-plane packing\nseen with higher concentration could influence vapor transport efficiency, as discussed in the main\nmanuscript (Pages 6-7 and 9) and the SI document (Pages 12 and 13).\nComment#4: When considering the economics of a membrane system, the cost of membrane\nfabrication and scale-up is as important as the cost to operate the membrane modules. Although\nthe thin films fabricated in this work showed great SEC compared to commercial membrane, their\nfabrication cost (i.e., money and time spent on an unconventional IP process) will be much higher\nthan that of the commercial SWRO and hydrophobic PTFE membrane. I would like to ask the\nauthor to acknowledge this disadvantages and provide a deeper discussion on the possible way to\nreduce these costs.\ncommercialization of membrane systems. As the reviewer rightly noted, our current fabrication\nmethod, which employs an unconventional IP process, may indeed be more costly and lengthier\nthan established manufacturing methods for commercial hydrophobic PTFE and SWRO\nmembranes. We have incorporated the reviewer’s valuable insights into the revised manuscript,\ndiscussing potential strategies to mitigate fabrication and operational costs without compromising\nperformance (Manuscript Pages 17-18).\nFollowing the reviewer’s suggestion in the first comment, we have explored the feasibility of a\nsupport-IP (in-situ IP) method to fabricate the TFC membranes. This method aligns more closely\nwith the established fabrication processes used for PA layers in commercial TFC-RO membranes,\nwhich could simplify the scale-up process and reduce fabrication costs. Using this method could\nallow us to utilize existing infrastructure and equipment designed for TFC-RO membrane\nproduction, which could facilitate cost-effective, large-scale manufacturing. While further\nresearch is needed to fully optimize this fabrication process and confirm its cost-effectiveness on\npilot and industrial scales, we believe this approach holds significant potential for reducing\nproduction costs while maintaining membrane performance.\nRegarding PTFE membranes, it is also important to consider that, while these have lower initial\nfabrication costs, they face notable performance limitations, particularly at low temperatures.\nOperating MD at higher temperature differences may increase flux; however, it also introduces\nscalability challenges (see previous comments). Specifically, as membrane area increases,\ntemperature gradients across the membrane are reduced due to polarization, which diminishes the\neffective driving force and increases the specific energy consumption. These factors contribute to\nhigher operational costs for large-scale applications of PTFE membranes in MD.\nComment#5: The figures in the manuscript of high quality and are well explained by the captions.\nHowever, there are some inconsistencies among the figures, especially the plots. In Figure 3b, the\ny-axis label is missing (although from my understanding y-axis is not needed); in Figure 3e, the y-\naxis is colored in purple; In most of the plots in Figure 3&4, the widths and colors of the gridlines\nare not consistent and some of the words in them are too small and not readable. I would like to\nask the authors to give the plots a serious revision to improve the overall quality to make it more\nreadable and consistent.\nResponse: We have thoroughly revised and improved the figures in both the manuscript and the\nSI document. We appreciate the reviewer’s guidance in improving the presentation quality and\nbelieve these changes significantly enhance the clarity of the figures.\nComment#6: The authors examined the change in membrane surface morphology with different\nIP recipes and showed that the size of leaf-like and ridge-and-valley features increased when the\nIP time increases. Since the vapor transport mechanism is heavily dependent on size and number\n15\nof the sub-nanometer pores/voids in the membrane, I would expect to see a greatly improved\nperformance when the surface roughness (i.e., accessible surface area) increases 6. However, the\nmembrane with the best performance (TF0.1-1) is the one with relatively smooth surface. I would\nlike to ask the authors to provide further discussion on this result, probably on the trade-off\nbetween surface area and membrane thickness or other factors.\nResponse: We fully agree that in pressure-driven processes, such as RO, increased surface\nroughness, featuring more leaf-like and ridge-and-valley structures, can enhance flux by providing\na greater accessible surface area and more voids for water passage, as the applied pressure directly\ndrives water through the membrane. However, in our process, rather than a direct hydraulic force,\nthe driving force is a temperature difference between the feed and permeate sides generating a\nvapor pressure gradient across the membrane. In this context, a more complex surface structure\n(leaf-like) with additional closed voids1 can trap air and increase the vapor path, thus reducing the\neffective driving force and increasing resistance to vapor transport.\nAs the reviewer noted, there is also a trade-off between surface roughness and membrane\nthickness. While increased surface roughness theoretically enhances flux by increasing surface\narea, the corresponding increase in thickness reduces overall flux by introducing additional\ntransport resistance. Additionally, as shown by the out-of-plane GI-WAXS profiles (representing\nreciprocal space perpendicular to the thin film surface), each film exhibited unique peak positions\nat Q⊥ = 0.36 Å−1, 0.55 Å−1, and 0.90 Å−1, corresponding to d-spacing of 1.748 nm, 1.142 nm,\nand 0.698 nm, respectively (Figs. S7 and S8). These decreases in d-spacing correlate with\nincreased film thickness (Table S3) and suggest that increasing the FTCS concentration likely\nfacilitate additional polymer growth and denser molecular packing.\nWe have added a detailed discussion in the manuscript to emphasize the balance between surface\narea and membrane thickness, as well as the insights from the out-of-plane GI-WAXS results,\nand how these factors collectively influence vapor transport efficiency in MD (Page 9 and 10).\nReferences\n1. Pacheco F, Sougrat R, Reinhard M, Leckie JO, Pinnau I. 3D visualization of the internal\nnanostructure of polyamide thin films in RO membranes. Journal of Membrane Science 2016,\n501: 33-44.\nComment#7: Though the hydrophobic FTCS membranes in this work were used for membrane\ndistillation, as shown by the direct-contact MD testing mode and the vapor-pressure-driven\ntransport mechanism, the authors did not clearly reference membrane distillation until the later\nsections of the paper. Directly specifying the end use of these membranes as well as mentioning\nthe direct-contact testing mode in either the methods section or Supporting Information would\nallow readers to draw comparisons between similar studies.\nreference membrane distillation (MD) in the original manuscript given the many dissimilarities\nwhen considering the MD process as a whole, including materials, equipment, setup and operating\nconditions. Following our actual understanding, we acknowledge that the separation process\nremains based on species relative volatility. However, besides using a direct contact MD setup,\nwe successfully operated in a lower range of temperature differences that were never successfully\nreached in MD before. More importantly, the new membrane exhibits characteristics that deviate\nfrom common MD membranes, with active layers two order of magnitude thinner and pore size\ntwo to three orders of magnitude smaller. Additionally, the transfer mechanism, which is thought\nto be enhanced by the meniscus curvature owing to superhydrophobicity and small pore size,\ndid not follow the commonly used Knudsen-Molecular-Poiseuille transition model. In view of all these\ndifferences, we believe that casting the process name into a wider family of “evaporation-based”,\nto which MD belongs, would be more appropriate. However, from the reviewer’s comment, we\nnow understand that this decision is more likely to be misleading and may bring confusion to\nreaders. To address the reviewer’s concern and since an MD setup is used to test the membranes,\nwe now clearly refer to MD in both manuscript and SI document as well as to direct-contact MD\nin Fig. S15 showing the experimental setup.\nComment#8: While the authors provided performance comparisons to two commercial membranes\nin this work, they do not specify the brand(s), structure (supported vs. unsupported), or\ncomposition of the membranes. The only information was in the SI stating that they are PTFE\nmembranes with a pore size of 0.22 µm. I think including more information on these membranes,\nor potentially extend the comparison to more commercially available membrane (either comparing\nto the values reported in literatures7 or performing the same test on more membranes), would make\nit easier for readers to judge whether these commercial membranes are suitable comparisons for\nthe FTCS membranes studied in this paper.\nReferences\n1. Park, S.-J. et al. Fabrication of polyamide thin film composite reverse osmosis membranes via\nsupport-free interfacial polymerization. J Memb Sci 526, 52–59 (2017).\n2. Zhou, Y. et al. Support-free interfacial polymerized polyamide membrane on a macroporous\nsubstrate to reduce internal concentration polarization and increase water flux in forward osmosis.\nJ Memb Sci 689, 122165 (2024).\n3. Zhang, X., Koirala, R., Date, A. & Jegatheesan, V. Modelling and simulation of flux prediction\nand salinity variation in direct contact membrane distillation for seawater desalination and brine\ntreatment. Desalination 540, 116021 (2022).\n4. Lawal, D. U., Antar, M. A., Ismaila, K. G., Khalifa, A. & Alawad, S. M. Hybrid multi-stage\nflash (MSF) and membrane distillation (MD) desalination system for energy saving and brine\nminimization. Desalination 548, 116231 (2023).\n5. Ugarte, P. et al. Air-Gap Membrane Distillation of Industrial Brine: Effect of Brine\nConcentration and Temperature. Ind Eng Chem Res 63, 1546–1553 (2024).\n6. Khayet, M., Khulbe, K. & Matsuura, T. Characterization of membranes for membrane\ndistillation by atomic force microscopy and estimation of their water vapor transfer coefficients in\nvacuum membrane distillation process. J Memb Sci 238, 199–211 (2004).\n7. Eykens, L., De Sitter, K., Dotremont, C., Pinoy, L. & Van der Bruggen, B. Characterization and\nperformance evaluation of commercially available hydrophobic membranes for direct contact\nmembrane distillation. Desalination 392, 63–73 (2016).\nResponse: We thank the reviewer for this valuable suggestion. In response, we have added\ndetailed information about the two commercial membranes used in our comparisons. These details\nhave been included in the SI document (Section 1.1; Page 2).\nFurthermore, following the reviewer’s recommendation, we have extended our comparative\nanalysis by incorporating performance data from other commercially available membranes\nreported in the literature. This additional data, presented in Table S7, pages 31-32 of the SI\ndocument, provides a more comprehensive context for evaluating the performance of our new\nmembrane relative to a wider range of commercial membrane technologies in similar direct contact\nMD set ups.\nReferences\n1- Eykens, L., De Sitter, K., Dotremont, C., Pinoy, L. & Van der Bruggen, B. Characterization and\nperformance evaluation of commercially available hydrophobic membranes for direct contact\nmembrane distillation. Desalination 392, 63–73 (2016)\nThe manuscript titled \"Scalable thin films for ultrahigh-efficiency brine desalination\" by Obaid et\nal. describes the development of superhydrophobic thin films for desalination using membrane\ndistillation, aiming to achieve high efficiency, low energy consumption, and scalability. The key\ninnovation in the manuscript is the incorporation of fluorinated trichlorosilane monomers with\nlong hydrophobic tails during interfacial polymerization to manufacture thin-film composite\nmembranes suitable for membrane distillation.\nMethodology: The synthesis of thin films using interfacial polymerization with fluorinated\ntrichlorosilanes and diamine-based monomers is well-executed. The authors present a\ncomprehensive material characterization of the thin films, utilizing a wide range of techniques\nsuch as SEM, AFM, Raman spectroscopy, and FTIR to establish their chemical and morphological\nproperties. However, the membrane distillation characterization of the new membranes is limited\n(see major comments below). The scalability of the membranes is well-supported by the successful\nfabrication of large-area films (up to 324 cm²) without compromising mechanical strength.\nNovelty and Impact: Membrane distillation membranes are typically ultrafiltration membranes\nmade using phase inversion. The novelty of the work lies in the development of ultrathin\nmembrane distillation membranes using interfacial polymerization, which is widely used for\nreverse osmosis or nanofiltration membrane manufacture. This approach represents a valuable\ncontribution to the field of membrane science. However, for a broader journal such as Nature\nCommunications, significant changes are required to strengthen the manuscript before it can be\nreconsidered for publication.\nThe changes must address the following Major Comments:\n1. Energy Efficiency Calculations:\nAll discussion of the energy efficiency calculations needs to be revised to:\na) Explicitly clarify when only electrical energy consumption has been included in energy\nefficiency calculations.\nThe manuscript should make it explicitly clear in both the abstract and main text when only\nelectrical energy consumption is being considered. This distinction is crucial because it prevents\nmisunderstanding about the total energy requirements of the process.\nb) Include thermal energy consumption more clearly in the main manuscript and not only in the\nSupplementary Information.\nThermal energy or heat consumption is critical to the driving force for mass transfer in thermal\ndesalination processes like membrane distillation and cannot be omitted from the main manuscript\ntext. The authors currently emphasize low specific energy consumption (SEC) values but focus\nprimarily on electrical energy consumption (parasitic losses). A comprehensive energy analysis\n19\nthat includes both thermal and electrical energy inputs is essential for accurately assessing the\nprocess's overall energy efficiency and comparing it with other desalination technologies.\nResponse: We would like to thank the reviewer for the invaluable comment. To address the\nreviewer’s comment, we have revised our approach to energy efficiency calculations in the\nmanuscript to ensure clarity and thoroughness in distinguishing between electrical and thermal\nenergy consumption.\nOriginally, we presented the specific energy consumption (SEC) as a total energy value, combining\nboth electrical (SEC ) and thermal (SEC ) contributions. However, to address the reviewer’s\nelec the\nconcern and prevent any misunderstanding about the process’s total energy requirements, we have\nrevised the “Energy Savings” section in the main manuscript (Page 16 -17) and Section S7 in the\nSI document (Pages 38 – 42) to clarify this distinction. Throughout the revised manuscript, we\nuse the term “SEC” explicitly to refer to the total specific energy consumption, which now clearly\nencompasses both SEC and SEC . Furthermore, we have changed “specific energy\nelec the\nconsumption” in the abstract to “total specific energy consumption”. Whenever only electrical\nenergy is considered, it is clearly mentioned in the main text as well as the SI document as SEC\nelec\nto distinguish it from the combined total SEC calculation. We believe that these changes ensure a\ncomprehensive energy analysis that accurately reflects both the electrical and thermal energy\ninputs, facilitating fair comparison with other desalination technologies and providing a clear\nassessment of the overall energy efficiency of the process.\n2. Use of Exergy Efficiency:\nGiven that cheap low-temperature heat is key for the calculations presented, the authors should\nuse exergy efficiency rather than just energy efficiency to gauge process performance. Exergy\nefficiency, taking into account the quality of the heat supplied, would provide readers with a more\naccurate description of how efficient the various membrane distillation processes are with these\nnew membranes compared to the existing literature.\nBy employing exergy analysis, the authors can account for the actual useful work potential of the\nthermal energy sources utilized, especially when low-grade heat is considered. This approach\nprovides a more meaningful metric for evaluating the thermodynamic efficiency of the process\nand facilitates a fair comparison with other desalination technologies that might use higher-grade\nenergy sources.\nResponse: To address the reviewer’s insightful comment, we proceeded with a detailed exergy\nanalysis, which can be found in Supplementary Appendix A, with comments provided in the SI\ndocument (Page 42), and in the manuscript (Page 17). Indeed, the analysis provided additional\ninsight into the new membrane effectiveness, which showed a low exergy destruction when\noperating at low temperatures, far outperforming the commercial membrane by two orders of\nmagnitude. We would like to emphasize again that the flow rate in our study was purposely set to\na high value to minimize polarization and capture the membrane intrinsic behavior. Therefore, it\n20\nis expected to lower the exergy destruction in an optimized system design. We would like to thank\nagain the reviewer for the scrutiny and for suggesting the analysis, which we will take to the next\nlevel with the development of a pilot prototype.\n3. Membrane Thermal Conductivity:\nThe authors need to measure the thermal conductivity of the membrane and report this in the\nmanuscript. Thermal conductivity is a critical performance parameter in membrane distillation\nmembranes, as it significantly impacts the actual energy efficiency of the process.\nA membrane that is thin but has high thermal conductivity can lead to large entropy generation\nfrom uncontrolled transmembrane heat transfer, reducing the process's overall thermal efficiency.\nAdditionally, the modeling detailed in Section S6 does not consider heat transfer, despite latent\nheat transfer during evaporation and condensation being critical in membrane distillation.\nIncorporating thermal conductivity measurements and heat transfer modeling will provide a more\naccurate assessment of the membrane's performance and its impact on energy consumption.\nResponse: To address the reviewer’s important comment, we have measured the membrane’s\nthermal conductivity using a Physical Property Measurement System (PPMS) device. The\nmeasured values have been added to both the main manuscript (Page 7) and the SI document\n(Section 2.8, Page 7 and Page 14).\nRegarding the modeling strategy used in the investigation, it is a fully coupled approach that\nconsiders the momentum, heat and species transport in the feed and permeate compartments,\ncoupled to heat and mass transfer across the porous membrane. The heat transfer across the\nmembrane includes conduction through the membrane-air-water vapor composite structure and\nconvection of the latent heat as the hot water vapor molecules are transferred from the feed side to\nthe permeate side of the membrane. Moreover, the heat extracted from the feed and the heat added\nto the permeate as water evaporates and condensates, respectively, is included in the calculations.\nAnalogously, are considered the water loss from the feed across the feed-membrane interface and\nthe water gain to the permeate across the permeate-membrane interface.\nIn order to address the reviewer’s concern, these additional explanations were added to the\nmanuscript in the “Desalination mechanism” section pages 13-14. Moreover, the equations that\ngovern heat transfer across the membrane have been added to section S6 (Mathematical and\nnumerical modeling), pages 33-37 of the SI document. Relevant citation has been included to\nrefer to the detailed momentum, heat and species transport in the bulk1.\nReferences\n1- Soukane S, Ghaffour N. Showerhead feed distribution for optimized performance of large-scale\nmembrane distillation modules. Journal of Membrane Science 2021, 618: 118664.\n21\n4. Membrane Thickness:\nMembrane thickness is a critical parameter in membrane distillation, as the authors’ research\ngroups and others have shown. Previous studies have demonstrated that reducing membrane\nthickness beyond a certain point can have diminishing returns or adverse effects, such as increased\nheat transfer leading to reduced thermal efficiency and reduced exergy efficiency (the authors have\nshown that potential mechanical instability is not a major issue here).\nThe authors should discuss the implications of their membrane's ultrathin structure on both mass\ntransfer and heat transfer. While thinner membranes can reduce mass transfer resistance and\npotentially increase flux, they can also lead to higher heat losses due to increased conductive heat\ntransfer. An analysis of the trade-offs between membrane thickness and thermal efficiency is\nnecessary to understand the membrane's optimal design parameters.\nResponse: We totally agree that membrane thickness is a critical parameter in membrane\ndistillation. Indeed, many studies including those listed herein1,2,3, report the negative and positive\neffects of the membrane thickness, particularly as a function of the solution salinity. However, it\nis important to note the range of thicknesses that were investigated and the theoretical models used\nto seek for optimums. As such, we believe that these studies consolidate the disruptive nature of\nthe new category of thin films we synthesized. First, thicknesses of the order of a tenth of a micron\nwere never considered in the literature and second, the permeabilities used in the theoretical\napproaches are either set to values that fall within the range of known commercial membranes or\nestimated using theoretical models such as the dusty gas model (DGM) or the Knudsen-Molecular-\nPoiseuille transition (KMPT) model.\nWithin the scope of MD, the DGM shortness to explain temperature effects on permeate fluxes led\nto the widely used KMPT4. However, it is very interesting to note that a significant flaw of DGM\nhas been reported earlier, showing the model inability to predict a simple diffusion with a stagnant\ncomponent5. Nevertheless, the model has been used as long as it was able to replicate experimental\ninvestigations for MD membranes within specific ranges of operating conditions. In our\ninvestigation, we are facing an outstandingly fast vapor transfer across films that are two orders of\nmagnitude thinner with pores that are two to three orders of magnitude smaller compared to\ncommercial MD membranes. Nevertheless, in our quest to understand the transfer mechanism, we\ninitially used the KMPT model. Beyond the vapor-pressure enhancement, which we believe is\ncaused by the meniscus curvature, the KMPT model was unable to explain the flux variations with\ntemperature. We came to conclude that the attempt of using these known models out of the range\nof common membrane properties is very likely not valid, which naturally led us to question the\nliterature results related to the membrane thickness as the optimality range may have shifted with\nthe new membrane properties. As such, we used the Beskok-Karniadakis (BK) model because it\nis based on an extension of the Navier-Stokes momentum equation with a second-order slip\nboundary condition further corrected with direct simulation Monte Carlo and experiments to\nextend its validity to very high Knudsen numbers, which was able to reflect the flux dependency\non temperature. In our case, as shown in the experimental results of Fig 3b of the manuscript, the\n22\nobserved flux keeps increasing with thinner films. However, films thinner than 60 nm were\npractically not possible to make due to the limited reaction time and their fragility. The results of\nFig.3b let us assume that the ultra-fast vapor transport has shifted the optimal range of membrane\nthicknesses commonly used up to date to much thinner values.\nReferences\n1. Martínez L, Rodríguez-Maroto JM. Membrane thickness reduction effects on direct contact\nmembrane distillation performance. Journal of Membrane Science 2008, 312(1): 143-156.\n2. Wu HY, Wang R, Field RW. Direct contact membrane distillation: An experimental and\nanalytical investigation of the effect of membrane thickness upon transmembrane flux. Journal of\nMembrane Science 2014, 470: 257-265.\n3. Eykens L, Hitsov I, De Sitter K, Dotremont C, Pinoy L, Nopens I, et al. Influence of\nmembrane thickness and process conditions on direct contact membrane distillation at different\nsalinities. Journal of Membrane Science 2016, 498: 353-364.\n4. Ding Z, Ma R, Fane AG. A new model for mass transfer in direct contact membrane\ndistillation. Desalination 2003, 151(3): 217-227.\n5. Kerkhof PJAM. A modified Maxwell-Stefan model for transport through inert membranes: the\nbinary friction model. The Chemical Engineering Journal and the Biochemical Engineering\nJournal 1996, 64(3): 319-343.\n5. Membrane Porosity:\nUnderstanding membrane porosity is vital in membrane distillation membranes because it gives\ninsight into how membrane permeance and thermal conductivity are impacted by (1) the membrane\nmaterial and (2) the membrane architecture.\nPorosity influences both the mass transfer (permeate flux) and the thermal conduction through the\nmembrane. High porosity generally enhances mass transfer but may also increase thermal\nconduction if not properly managed. The authors should measure and report the membrane's\nporosity, and discuss how it affects the membrane's performance, including any trade-offs between\nflux and energy efficiency.\nResponse: We totally agree with the reviewer that the porosity of the conventional MD membrane\nwith pore size ranging from 100 nm to 1000 nm is an important factor in traditional MD process\nas our group, among others, reported it in earlier investigations1. However, this novel class of\nmembrane with subnanometer pores (0.495 nm), seems to exhibit a different mechanism where\nthe driving force is significantly enhanced by the meniscus curvature reaching values far beyond\nany highly porous membrane can achieve. We believe that these new findings represent a paradigm\nshift in membrane design for membrane distillation where focus should be set on\nsuperhydrophobicity, thinness and reduced pore size. Nevertheless, although not reported in the\n23\noriginal manuscript, we measured the porosity of the thin film by dividing the volume of\nmicropores, which was determined using the Brunauer–Emmett–Teller (BET) method, by the total\nvolume of the thin film. The porosity was estimated at approximately 5%. The details are provided\nin the SI document (section 2.9 and 3.6) page 7 and page 23, and in the revised manuscript,\npage 7.\nReferences\n1. Xu J, Singh YB, Amy GL, Ghaffour N. Effect of operating parameters and membrane\ncharacteristics on air gap membrane distillation performance for the treatment of highly saline\nwater. Journal of Membrane Science 2016, 512: 73-82.\n6. Membrane Pore Diameter:\nMembrane pore diameter is an important parameter for membrane distillation membranes because\nit provides key insight into liquid entry pressures (LEP), which affect operating pressure and\npressure drop limitations. The authors should measure the pore size distribution of their\nmembranes and report the average pore diameter. Understanding the pore size is crucial for\nensuring that the membrane can prevent liquid intrusion while allowing vapor transport. This\ninformation will also help assess the membrane's suitability for various operating conditions and\nits stability during long-term operation.\nResponse: To address the reviewer’s concern, we characterized the pore sizes by employing\ngrazing-incidence wide-angle X-ray scattering (GI-WAXS) to analyze the in-plane structure of the\nTF membranes. The GI-WAXS in-plane profiles for the three variants (TFC0.1-1, TFC0.1-2, and\nTFC0.1-3) demonstrate consistent peaks at 1.27 Å-1, which corresponds to a pore size of 0.495 nm.\nThis consistent peak across several samples suggests a similar pore size throughout these thin\nfilms. For a comprehensive description of the GI-WAXS method, please refer to the SI document,\nSection 2.7, pages 6-7. The results of this analysis are discussed in detail within the main\nmanuscript (Pages 6-7, Figs 2i-j added) and the SI document (Pages 12-14).\n7. Revision of the Title:\nThe authors need to revise the title to more accurately reflect the content and focus of the\nmanuscript. There is no mention of either the chemistry being used or that the desalination is being\nperformed via membrane distillation. Most readers seeing the current title will think of the\nfabrication of thin-film composite reverse osmosis membranes, which are already highly scalable.\nMaking it clear that these are membrane distillation membranes is vital because thin-film\ncomposite reverse osmosis membranes are not new. Furthermore, using terms like “ultrahigh-\nefficiency brine desalination” is potentially misleading because the authors have not fully\naccounted for thermal energy in their calculations. The process itself is not more energy efficient\ncompared to reverse osmosis when considering all energy inputs.\nreviewer on the importance of the title relevance to the content. We would like to convince the\nreviewer to keep “ultrahigh-efficiency” since we provided a detailed revision of the process\nenergetics. However, we clarify the nature of the thin film as hydrophobic in the revised title. As\ndiscussed above in several comments, membrane distillation has been used in relevant sections in\nthe revised manuscript and SI document and added to the keywords.\n8. Discussion of technoeconomics:\nA large part of the value described in the manuscript is based on “free” low-grade heat or low-\ntemperature heat. If this argument is central to the paper the authors need to have a discussion\nabout costs and not just energy efficiency. As the authors note water desalination or brine\nconcentration costs are only partly dependent on energy consumption. Typically, it is possible to\nmake a separations process more energy efficient by reducing driving forces and increasing\nmembrane or adsorbent area. Of course in practical applications, membrane area or separations\nfootprints are limited by cost.\nResponse: We totally agree with the reviewer that desalination or brine concentration costs are\njust partly dependent on energy consumption. However, in our investigation, we emphasized the\nnew membrane sustainability, which encompasses both energy saving and productivity. At the\nlowest temperature difference set with a feed at 25 °C and a distillate at 20 °C, the membrane\nexhibited a SEC of 2.5 kWh.m-3 and 4.4 kWh.m-3 for seawater desalination and brine concentration\nwhile delivering a permeate flux of 40 L.m-2.h-1 and 12 L.m-2.h-1, respectively. This level of\nproductivity is higher than already commercialized spiral wound air gap MD modules, which\nexhibit thermal energy consumption ranging between 150 and kWh.m-3 and 425 kWh.m-3 for a\nproduction of 2 L.m-2.h-1 and 4 L.m-2.h-1, respectively1. Similarly, commercially available flat sheet\ntechnology2 showed a specific thermal energy consumption between 300 kWh.m-3 and 700\nkWh.m-3 for a productivity not exceeding 7 L.m-2.h-1. Regarding our membrane, with flat sheets\nof 70 cm x 30 cm (as depicted in Fig. 1 of this document), a stacking of 10 sheets for instance\n(2.1 m2 of total membrane area) would result roughly in a 75 cm x 35 cm x 15 cm compact module\nable to produce more than 2 m3 a day, which represents twice the need of a household. We would\nlike also to stress the fact that our calculations remain based on a single pass scenario and further\nreduction of energy requirements will be achieved with optimized parallel/series modules\nconfigurations. To concur with the reviewer’s vision, we are in the process of scaling up our\nfindings to a higher technology readiness level. The new prototype will be a key step to optimize\nthe membrane and adjust the equipment to fully capture the performance. We will share our\nfindings, including the economics, with the scientific community as we progress in our\ndevelopments.\n1- A. Ruiz-Aguirre, J.A. Andres-Manas, J.M. Fernandez-Sevilla, G. Zaragoza, Comparative\ncharacterization of three commercial spiral-wound membrane distillation modules, Desalination\nand Water Treatment, 61 (2017), 152-159.\n2- E.Sh. Mohammed, P. Boutikos, E. Mathioulakis, V. Belessiotis, Experimental evaluation of the\nperformance and energy efficiency of a Vacuum Multi-Effect Membrane Distillation system,\nDesalination, 408, 2017, 70-80.\nConclusion:\nThe manuscript represents a valuable contribution to the field of membrane science by introducing\na novel approach to fabricating superhydrophobic thin films for membrane distillation. However,\nsignificant revisions are necessary to enhance its suitability for publication in a broad journal like\nNature Communications.\nBy addressing the major comments outlined above, the authors can improve the clarity, accuracy, and impact of their work.\nIncluding comprehensive energy analyses, detailed membrane characterization (thermal conductivity, thickness, porosity,\npore diameter), and a revised title will strengthen the manuscript and provide a more accurate assessment of the\nmembrane's performance and potential applications.\nRecommendation:\nI recommend that the authors revise the manuscript to address the points raised before it can be\nreconsidered for publication.\nWe would like to thank you for your consideration and for the insightful comments, which\nimproved the overall quality of the manuscript. We addressed all the comments and concerns\nraised by the editor and the reviewers.\nBest regards\n26"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided an-depth and sometimes detailed point-by-point response to the comments provided by this reviewer.\nRegarding comment 1, it is assumed that the authors talk in their manuscript about the membrane distillation process, this was still not mentioned in the title, abstract or introduction.\nIn comment 2, in reference to accurately estimating permeate flux in such a small cell, this reviewer is aware of potential bias effects that could mask intrinsic membrane properties with a larger test cell. However, despite the polarization effect, a high-performance membrane would still demonstrate significantly better performance than commercial membranes when tested under the same conditions. The authors prepared a membrane with an area of 2100 cm2 and continued to include the word “Scalable” in the title; therefore, experimental results with a scaled membrane cell would be mandatory. To grasp the real advantage of these membranes, it is necessary to have clear proof that reported transmembrane fluxes are not due to an artefact generated by the small size of the testing cell. The authors used a relatively large apparatus with feed and permeate volumes of 1.5 L each, equipped with a 2-digit scale but a 2 cm2 membrane, in experiments lasting 2 hours (normally, in my experience, it takes 1 to 2 hours to reach steady state conditions before starting the experiment). This point is even more crucial considering that the transmembrane flux also affects the calculated SEC and the claimed energy savings for the relevant membranes.\nAs an additional comment not included in the initial review report, it is suggested that the Authors discuss the environmental sustainability of the membrane preparation method using IP, particularly in the perspective of large-scale production of such membranes for water desalination. IP is generally considered a “dirty” membrane preparation technique due to the use of organic solvents and/or toxic chemicals.", "reviewer_2": "The authors have done a good job of responding to reviewer comments. Additional notes are below for specific comments\n• The authors provided sufficient analysis on the comparison between the morphology of membranes fabricated by supported and support-free interfacial polymerization, including high-quality images of the membrane surface and cross-section, and the detailed performance analysis. Notably, despite the performance drop relative to the membranes made at a free interface, supported IP membranes still maintained high rejections and fluxes comparable to those of commercial membranes across a low temperature gradient. The authors also acknowledged the difference between the two fabrication methods in terms of cost and performance, and well elaborated on my concern.\n• The authors provided good explanations on the choice of their experimental conditions and made a strong argument on why it is different from some literature. There is also a section comparing the performance of the lab-made and commercial membranes at different transmembrane temperatures, which answers my original question. I would like to ask the authors to put these arguments somewhere in this paper, probably in the experimental section or the discussion where the specific temperature condition was mentioned, to avoid potential confusion for the readers.\n• The new GIWAXS experiments added to the manuscript provide valuable insight into the membrane chemical and physical structures. However, I would suggest the authors to just report the d-spacing and the spectra collected from the GIWAXS experiment, and try not to link it with the free volume or pore size of the membrane, since sometimes they do not match each other. For example, in polyamide network, the calculated ‘pore size’ from GIWAXS may represent the spacing between two aromatic planes rather than the size of channels for transport. I suggest the authors read the following reference [1] on using x-ray scattering to characterize RO material, as it explains well why we cannot use the d-spacing as a direct proxy of membrane pore size, and refine their discussion in this section.\n[1]. Fu Q, Verma N, Hsiao BS, Medellin-Rodriguez F, Beaucage PA, Stafford CM, Ocko BM. X-ray Scattering Studies of Reverse Osmosis Materials. Synchrotron Radiat News. 2020;33(4):10.1080/08940886.2020.1784700. doi: 10.1080/08940886.2020.1784700. PMID: 34121807; PMCID: PMC8194099.\n• The added discussion on the cost of membrane fabrication is valuable to the overall discussion on the economic analysis of the novel membranes presented in this work. I appreciate the quality of this section and the insightful comments the authors made regarding the trade-off between fabrication cost and membrane performance.\n• The authors’ insight into the different mechanisms of water and vapor transport through the same type of membrane, and I agree with the author that although rougher surface (i.e. higher surface area) is usually favorable to water transport, but it might cause resistance to vapor transport.\n• Regarding the GIWAXS data, it is unclear how the d-spacing correlates with membrane thickness. It’s explained well in the manuscript that the different d-spacing values of different membranes can indicate the different degree of molecular packing, but the ‘correlation’ between d-spacing and thickness lacks theoretical and literature support. I suggest the authors add more references to support their claim.\n• The authors have clarified the use of MD in this study, which allows for easier comparisons between studies and highlights the performance of the new membranes. The discussion on the different transport mechanism observed for the membrane described in this work compared to conventional polymeric MD membranes was insightful.", "reviewer_3": "I would like to thank the authors for their extensive revisions and for the expanded discussion on exergy efficiency. The improvements made so far are appreciated. However, there are a few important points that I would request be addressed before the manuscript can be reconsidered for publication:\n1. Reporting of water recovery ratios with SEC: The authors need to give the water recovery ratio with any SEC numbers quoted (along with the initial feed salinity, which they have already included). For example, in the abstract the authors write “Notably, these films achieve the lowest total specific energy consumption reported to date; 1.88 kWh.m−3 for simulated seawater, 3.47 kWh.m−3 for real seawater, and 4.43 kWh.m−3 for real seawater reverse osmosis desalination brine” but in order to know the amount of separation achieved by this energy input we need to know what the water recovery ratio was (i.e., the transmembrane membrane mass or molar flux multiplied by the membrane area and divided by the feed mass or molar flow rate) in order to gauge whether this energy consumption figure is high or low. Quoting energy consumption based on zero- or low-recovery measurements can mislead the reader, especially when compared with the reverse osmosis values for 50% water recovery later in the paper (“This low SEC is close to the reported minimum theoretical energy required for seawater desalination using a large RO system at 50% recovery, estimated at 1.06 kWh.m−3”, referencing Elimelech & Phillip 2011, #78).\n2. Impact of Membrane Density and Porosity on Heat Transfer: The authors report a membrane that is dense, relatively non-porous (~5% porosity), and thin. Previous studies have shown that, in membrane distillation, cumulative heat transfer is one of the major challenges with the membranes developed here. Linking back into Point 1, having a thin, low porosity membrane would mean that cumulatively, over the area of a membrane module, the conductive heat loss from the feed to the permeate side would accumulate, drastically reducing the driving force for further water permeation, limiting the water recovery ratio available. The authors should discuss or model this.\n3. Clarification on Pore Size / Separation Mechanism: the reported pore sizes measured are very small (sub-nanometer), which could suggest that the membrane might operate more like a pervaporation system—where the polymer sieves salt ions—instead of following a conventional membrane distillation process that relies on a water–vapor interface for the water-salt separation? It would be very helpful if the authors could clarify whether the separation mechanism is driven by a phase change or by molecular sieving, and explain how the small pore size influences the transport mechanism.\n4. Keeping / demonstrating the Ultra-High Efficiency Claims: For the ultra-high efficiency claim I think the authors need to show what the energy efficiency of the process is in terms of useful energy out (least work of separation for a given feed salinity and water recovery ratio, assuming 100% salt rejection) divided by the total energy input (including both the electricity and heat). I believe for reverse osmosis with efficient pumps and energy recovery devices the energy efficiency is around 30-40%. If the authors want to keep this claim they should use the flux measurements to estimate the membrane permeance (flux per unit transmembrane chemical potential driving force) and then use this as part of a one-dimensional module-scale model to determine the water recovery. It’s hard to claim ultrahigh efficiency based without knowing the water recovery and how this compares to other processes in the space (e.g., reverse osmosis) even though the figures look promising, this needs to be confirmed.\nRecommendation:\nI am grateful to the authors for the changes that they have made thus far (and I think they strengthen the manuscript quite a bit), and I recommend that the authors revise the manuscript to address the points raised before it can be reconsidered for publication."}, "author_response": "Responses to reviewers’ comments (R2): Manuscript NCOMMS-24-48277A\nDear reviewers,\nThe authors would like to thank you for your thoughtful consideration and time in reviewing the\nmanuscript. Your comprehensive review improved the overall quality of the manuscript. All\ncomments and suggestions have been incorporated in the revised version, and individual responses\nto comments are provided below. The changes made in the manuscript and SI document are\nhighlighted.\nReviewer #1 (Remarks to the Author)\nThe authors have provided an in-depth and sometimes detailed point-by-point response to the\ncomments provided by this reviewer.\nComment 1; It is assumed that the authors talk in their manuscript about the membrane distillation\nprocess, but this was still not mentioned in the title, abstract, or introduction.\nResponse: Following the reviewer’s recommendation, we added “membrane distillation” to the\ntitle, abstract, and introduction (Pages 1-3).\nIn comment 2, in reference to accurately estimating permeate flux in such a small cell, this reviewer\nis aware of potential bias effects that could mask intrinsic membrane properties with a larger test\ncell. However, despite the polarization effect, a high-performance membrane would still\ndemonstrate significantly better performance than commercial membranes when tested under the\nsame conditions. The authors prepared a membrane with an area of 2100 cm2 and continued to\ninclude the word “Scalable” in the title; therefore, experimental results with a scaled membrane\ncell would be mandatory. To grasp the real advantage of these membranes, it is necessary to have\nclear proof that reported transmembrane fluxes are not due to an artefact generated by the small\nsize of the testing cell. The authors used a relatively large apparatus with feed and permeate\nvolumes of 1.5 L each, equipped with a 2-digit scale but a 2 cm2 membrane, in experiments lasting\n2 hours (normally, in my experience, it takes 1 to 2 hours to reach steady state conditions before\nstarting the experiment). This point is even more crucial considering that the transmembrane flux\nalso affects the calculated SEC and the claimed energy savings for the relevant membranes.\nResponse: We sincerely appreciate the reviewer’s detailed feedback. We now fully understand the\nreviewer’s concern, and to avoid any potential misinterpretation, we removed the word “scalable”\nfrom the manuscript title. While we successfully fabricated a 2100 cm² membrane, its desalination\nperformance at a large area requires a larger module and a new setup. Thus, we emphasize in the\nconclusion section that larger-scale testing is an important direction for future research (Page 19).\nNevertheless, to address the reviewer’s concern about any potential artefact, we ran 24 hr long\n1\nexperiments on newly fabricated membranes. We confirm the 40 LMH stable flux and a total\nproduction of 190 mL of distillate (Please see Figure S16d - page 26 in SI).\nFigure. 24-hour DCMD experiment\nRegarding the experiment duration, we fully agree with the reviewer that sufficient time must be\nallowed before data collection to ensure reliable measurements. While the reviewer noted that\nsteady-state conditions typically require 1 to 2 hours, our experimental observations indicate that\nstabilization consistently occurs within 25 to 35 minutes, after which data collection begins at the\n40-minute mark. To clarify this point, we have explicitly stated in the experimental section of the\nrevised manuscript (Page 21) that all reported measurements were recorded only after the system\nhad reached stability.\nWe greatly appreciate the reviewer’s constructive feedback, which has helped us improve the\nclarity of our manuscript. We hope that these clarifications and modifications adequately address\nthe concerns raised.\nComment 3: As an additional comment not included in the initial review report, it is suggested that\nthe Authors discuss the environmental sustainability of the membrane preparation method using\nIP, particularly in the perspective of large-scale production of such membranes for water\ndesalination. IP is generally considered a “dirty” membrane preparation technique due to the use\nof organic solvents and/or toxic chemicals.\nthe IP method. In response, we have added a detailed discussion addressing this important point in\nthe revised manuscript (Page 18).\nWe acknowledge the environmental concerns associated with IP, notably its dependence on\norganic solvents and potentially toxic chemicals. However, these chemicals are widely used in the\nfabrication of commercial reverse osmosis (RO) and nanofiltration (NF) membranes, where IP\nremains the standard industrial technique. While we totally agree that global solutions need to be\nfound for more sustainable desalination in the future, the new membrane takes full advantage of\nall RO existing facilities and industrial-scale manufacturing operations, which are governed by\nstringent environmental and safety regulations, including solvent recovery, waste management,\nand emission control protocols, enforced through national and international policies. We\nacknowledge that achieving sustainable membrane manufacturing will require long-term research\nand development efforts involving research institutions, industry stakeholders, and policymakers.\nAgain, we appreciate the reviewer’s raising this concern, as it highlights an important aspect of\nmembrane technology that requires further attention.\nReviewer #2 (Remarks to the Author)\nThe authors have done a good job of responding to reviewer comments. Additional notes are below\nfor specific comments.\nResponse: We sincerely thank the reviewer for their invaluable feedback and thoughtful\nassessment of our work. Thank you for your valuable insights, which have helped strengthen our manuscript.\n The authors provided sufficient analysis on the comparison between the morphology of\nmembranes fabricated by supported and support-free interfacial polymerization, including\nhigh-quality images of the membrane surface and cross-section, and the detailed performance\nanalysis. Notably, despite the performance drop relative to the membranes made at a free\ninterface, supported IP membranes still maintained high rejections and fluxes comparable to\nthose of commercial membranes across a low temperature gradient. The authors also\nacknowledged the difference between the two fabrication methods in terms of cost and\nperformance, and well elaborated on my concern.\nResponse: We sincerely appreciate the reviewer’s positive feedback. Thank you for your valuable\ninsights, which have helped strengthen our manuscript.\n The authors provided good explanations on the choice of their experimental conditions and\nmade a strong argument on why it is different from some literature. There is also a section\ncomparing the performance of the lab-made and commercial membranes at different\ntransmembrane temperatures, which answers my original question. I would like to ask the\nauthors to put these arguments somewhere in this paper, probably in the experimental section\nor the discussion where the specific temperature condition was mentioned, to avoid potential\nconfusion for the readers.\nResponse: We sincerely appreciate the reviewer’s valuable suggestion. To enhance clarity and\navoid potential confusion for readers, we have incorporated these arguments into the experimental\nsection of the manuscript (Page 21).\n The new GIWAXS experiments added to the manuscript provide valuable insight into the\nmembrane chemical and physical structures. However, I would suggest the authors to just\nreport the d-spacing and the spectra collected from the GIWAXS experiment, and try not to\nlink it with the free volume or pore size of the membrane, since sometimes they do not match\neach other. For example, in polyamide network, the calculated ‘pore size’ from GIWAXS may\nrepresent the spacing between two aromatic planes rather than the size of channels for transport.\nI suggest the authors read the following reference [1] on using x-ray scattering to characterize\nRO material, as it explains well why we cannot use the d-spacing as a direct proxy of membrane\npore size, and refine their discussion in this section.\n[1] Fu Q, Verma N, Hsiao BS, Medellin-Rodriguez F, Beaucage PA, Stafford CM, Ocko BM. X-\nray Scattering Studies of Reverse Osmosis Materials. Synchrotron Radiat News.\n2020;33(4):10.1080/08940886.2020.1784700. doi: 10.1080/08940886.2020.1784700. PMID:\n34121807; PMCID: PMC8194099.\nResponse: We appreciate the reviewer’s insightful comment. As suggested by the reviewer, we\nhave revised the discussion to unlink the observed d-spacing from pore size. These revisions have\nbeen incorporated into the main manuscript (Page 7) and SI (Pages 12-13). The suggested\nreference is cited in the manuscript and SI documents.\n The added discussion on the cost of membrane fabrication is valuable to the overall discussion\non the economic analysis of the novel membranes presented in this work. I appreciate the\nquality of this section and the insightful comments the authors made regarding the trade-off\nbetween fabrication cost and membrane performance.\nResponse: We sincerely appreciate the reviewer’s positive feedback.\n The authors’ insight into the different mechanisms of water and vapor transport through the\nsame type of membrane, and I agree with the author that although rougher surface (i.e. higher\nsurface area) is usually favorable to water transport, but it might cause resistance to vapor\ntransport. Regarding the GIWAXS data, it is unclear how the d-spacing correlates with\nmembrane thickness. It’s explained well in the manuscript that the different d-spacing values\nof different membranes can indicate the different degree of molecular packing, but the\n‘correlation’ between d-spacing and thickness lacks theoretical and literature support. I suggest\nthe authors add more references to support their claim.\nResponse: We totally agree with the reviewer that there is no direct correlation between the d-\nspacing and the film thickness. Our point was to highlight that both the increase in membrane\nthickness and the decrease in d-spacing were influenced by the higher FTCS concentration used\nduring fabrication. To avoid any potential misinterpretation, we have removed the sentence “These\nvariations in the out-of-plane profiles correlate with the increasing film thickness (Table S3).”\nfrom the SI (Pages 12-13) and the revised manuscript (Page 10).\n The authors have clarified the use of MD in this study, which allows for easier comparisons\nbetween studies and highlights the performance of the new membranes. The discussion on the\ndifferent transport mechanism observed for the membrane described in this work compared to\nconventional polymeric MD membranes was insightful.\nResponse: We sincerely appreciate the reviewer’s positive feedback.\nReviewer #3 (Remarks to the Author)\nI would like to thank the authors for their extensive revisions and for the expanded discussion on\nexergy efficiency. The improvements made so far are appreciated. However, there are a few\nimportant points that I would request be addressed before the manuscript can be reconsidered for\npublication.\nResponse: We would like to thank the reviewer for the positive feedback and invaluable\ncomments.\n1. Reporting of water recovery ratios with SEC:\nThe authors need to give the water recovery ratio with any SEC numbers quoted (along with the\ninitial feed salinity, which they have already included). For example, in the abstract the authors\nwrite “Notably, these films achieve the lowest total specific energy consumption reported to date;\n1.88 kWh.m−3 for simulated seawater, 3.47 kWh.m−3 for real seawater, and 4.43 kWh.m−3 for real\nseawater reverse osmosis desalination brine” but in order to know the amount of separation\nachieved by this energy input we need to know what the water recovery ratio was (i.e., the\ntransmembrane membrane mass or molar flux multiplied by the membrane area and divided by\nthe feed mass or molar flow rate) in order to gauge whether this energy consumption figure is high\nor low. Quoting energy consumption based on zero- or low-recovery measurements can mislead\nthe reader, especially when compared with the reverse osmosis values for 50% water recovery\nlater in the paper (“This low SEC is close to the reported minimum theoretical energy required for\nseawater desalination using a large RO system at 50% recovery, estimated at 1.06 kWh.m−3”,\nreferencing Elimelech & Phillip 2011, #78).\nResponse: We fully understand the reviewer’s point and totally agree that the statement may\nmislead readers. As suggested by the reviewer in the sequel, we calculated the water recovery ratio\nof our membranes using a one-dimensional module-scale model for a 1 m2 membrane area. The\nrecovery ratio was found to be 44.4%, 15.5%, and 13.3% for NaCl, real untreated seawater\n(compared to 8% for 2.2 m2 RO membrane for pretreated seawater), and SWRO desalination brine,\nrespectively, which underscores the potential for high recovery in a multistage configuration. To\naddress the reviewer’s comment, we added to the manuscript (Page 16) the recovery values\ncorresponding to a 1 m2 of membrane and a new section on water recovery in the SI document\n(Section S7.3 Page 40), which will allow the readers to assess the significance of the membrane\nperformance.\nMoreover, recovery ratios are inherently scale-dependent and are generally improved by adding\nstages (membrane area) to the process flow. For example, small-scale RO systems employing a\nsingle element typically achieve 8–10% water recovery (e.g., an 8% recovery was reported for a\n2.2 m² RO module2). In contrast, industrial RO systems, which use 6-7 element configurations\n(typically 6-8 elements placed in a pressure vessel) and energy efficient recovery devices, can\nachieve recovery ratios of 40–50%3. Similarly, while lab-scale MD systems may initially show\nrecovery ratios in the range of 7–9%4, advanced configurations and multi-stage operations have\ndemonstrated recovery ratios of 60–80%5. Moreover, in RO processes, increasing the recovery\nratio necessitates a corresponding increase in applied pressure for subsequent stages due to higher\nosmotic pressures caused by higher salinities, which leads to a higher energy consumption further\npenalizing the SEC. In contrast, the new membrane operates at atmospheric pressure and room\ntemperature, which means that recovery ratios can be readily increased with additional stages,\npotentially exceeding 70% owing to the smaller effect of the stream concentration on the vapor\npressure compared to its significant effect on the osmotic pressure in the case of RO. Using\nAntoine’s equation and a correction function of salinity, one can readily verify that when the salt\nconcentration reaches 10% (~3 times seawater), the vapor pressure is reduced by about 6%\n(p /p0 = 0.94); and when the salt concentration reaches 20% (~5 – 6 times seawater)\nwater water\n(p /p0 = 0.85), the vapor pressure is reduced by only 15%6. This shows that for a similar\nwater water\npermeate flux, the new MD membrane can reach as high of a recovery ratio as RO’s. Additionally,\nbench evaluation of seawater desalination by RO shows a permeate flux below 18 LMH7 when the\nosmotic pressure increases beyond 30 bars, while the new membrane delivers 40 LMH under\natmospheric pressure and room temperature.\nReferences:\n1- V.G. Gude, “Energy consumption and recovery in reverse osmosis”, Desalination and Water\nTreatment, 36 (2011) 239-260. (https://doi.org/10.5004/dwt.2011.2534)\n2- D.S. Ayou, H.M. Ega, A. Coronas, “A feasibility study of a small-scale photovoltaic-powered\nreverse osmosis desalination plant for potable water and salt production in Madura island: A\ntechno-economic evaluation”, Thermal Sciences and Engineering Progress, Volume 35, 1\nOctober 2022, 101450. (https://doi.org/10.1016/j.tsep.2022.101450)\n3- M. Elimelech, W.A. Phillip, “The Future of Seawater Desalination: Energy, Technology, and the\nenvironment”, Science, 333, 5 August 2011, 6043, 712-717.\n(https://www.science.org/doi/10.1126/science.1200488)\n4- F. He, J. Gilron, K.K. Sirkar, “High water recovery in direct contact membrane distillation using\na series of cascades”, Desalination, 323, 15 August 2013, 48-54.\n(https://doi.org/10.1016/j.desal.2012.08.006)\n5- R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey Jr, M. AlGhouti, M. Gad-el-Hak, H.V.\nTafreshi, “Energy efficiency of direct contact membrane distillation”, Desalination, 433, May\n2018, 56-67. (https://doi.org/10.1016/j.desal.2018.01.025)\n6- L. Song, B. Li, K.K. Sirkar, J.L. Gilron, “Direct Contact Membrane Distillation-Based\nDesalination: Novel Membranes, Devices, Larger-Scale Studies, and a Model”, Industrial &\nEngineering Chemistry Research, 46, 27 February 2007, 2307-2323.\n(https://doi.org/10.1021/ie0609968)\n7- D.A. Ladner, A. Subramani, M. Kumar, S.S. Adham, M.M. Clark, “Bench-scale evaluation of\nseawater desalination by reverse osmosis”, Desalination, 250 (2), 15 January 2010, 490 - 499.\n(https://doi.org/10.1016/j.desal.2009.06.072)\n2. Impact of Membrane Density and Porosity on Heat Transfer:\nThe authors report a membrane that is dense, relatively non-porous (~5% porosity), and thin.\nPrevious studies have shown that, in membrane distillation, cumulative heat transfer is one of the\nmajor challenges with the membranes developed here. Linking back into Point 1, having a thin,\nlow porosity membrane would mean that cumulatively, over the area of a membrane module,\nthe conductive heat loss from the feed to the permeate side would accumulate, drastically reducing the\ndriving force for further water permeation, limiting the water recovery ratio available. The authors\nshould discuss or model this.\nResponse: We appreciate the reviewer’s insightful comment. While we acknowledge that a thin,\nlow-porosity membrane could lead to increased conductive heat transfer across its matrix, our\nexperimental results show an exceptionally low membrane thermal conductivity (0.07 ± 0.02\nW·m⁻¹·K⁻¹) (Figure S9, SI, Page 14), which helps reducing heat losses considerably and preserve\nthe temperature gradient necessary for an efficient water vapor transport. Moreover, it is important\nto recall that the driving force in our case is higher than that of standard MD membranes owing to\na higher water vapor pressure caused by the nanosized pores, which we expressed using Kelvin’s\nequation (Eq S7, Page 33). This enables the membrane to operate effectively at low feed\ntemperatures (e.g., 25 °C) with a small transmembrane temperature difference (~5 °C), and further\nreduces cumulative conductive heat transfer, as conductive heat losses (Q , Eq. S9, Page 34) are\nc\nproportional to the temperature gradient. We would like to reassure the reviewer that we modeled\na fully coupled approach of CFD and pore-scale calculations. The model considers both convective\nand conductive heat transfer modes from feed to permeate across the membrane as detailed in Eqs.\nS11 to S13 (section S6 of the SI document, Pages 32-35). The analysis confirms that the\nmembrane’s low thermal conductivity and vapor-driven mass transfer mechanism effectively\nprevent significant heat loss accumulation.\nTo address the reviewer’s concern, we added in page 14 of the manuscript a statement highlighting\nthe important contribution of the low thermal conductivity of the thin film in sustaining the driving\nforce for distillation.\n3. Clarification on Pore Size / Separation Mechanism:\nThe reported pore sizes measured are very small (sub-nanometer), which could suggest that the\nmembrane might operate more like a pervaporation system—where the polymer sieves salt ions—\ninstead of following a conventional membrane distillation process that relies on a water–vapor\ninterface for the water-salt separation? It would be very helpful if the authors could clarify whether\nthe separation mechanism is driven by a phase change or by molecular sieving, and explain how\nthe small pore size influences the transport mechanism.\nResponse: We appreciate the reviewer’ comment on this critical point. In our study, we confirm\nthat the separation mechanism is driven by phase change, rather than molecular sieving or\npervaporation. The membrane is superhydrophobic, which prevents liquid water penetration and\nensures that only vapor-phase transport occurs through the subnanometer pores. This is readily\nverified during the experiments by monitoring the conductivity of the distillate stream. Notably,\nthe membrane was tested for seawater desalination at the same temperature on both sides. While\na concentration gradient prevails in these experiments, no water flux was observed further\nconfirming that phase change (evaporation/condensation) is the primary mechanism rather than\ndiffusion or pervaporation (which requires applying vacuum at the permeate side). Regarding\nmolecular sieving, although the reported pore sizes in the subnanometer range may suggest a\npossible molecular sieving mechanism, the zero flux observed in the isothermal desalination\nexcludes such a mechanism. This conclusion is further reinforced by the agreement between the\nvapor flux enhancement due to nanoconfinement as expressed by Kelvin’s equation and the\npermeate flux evolution as a function of feed temperature, as discussed in the manuscript. As such,\nthe modeling results (see Equation S7 and Figure S20 in the SI, Pages 33-34) show that for a\npore size of ~0.495 nm, the vapor pressure enhancement can reach 10x at 20 °C and 6x at 60 °C,\nexplaining the observed ultrafast vapor transport.\n4. Keeping / demonstrating the Ultra-High Efficiency Claims:\nFor the ultra-high efficiency claim I think the authors need to show what the energy efficiency of\nthe process is in terms of useful energy out (least work of separation for a given feed salinity and\nwater recovery ratio, assuming 100% salt rejection) divided by the total energy input (including\nboth the electricity and heat). I believe for reverse osmosis with efficient pumps and energy\nrecovery devices the energy efficiency is around 30-40%. If the authors want to keep this claim\nthey should use the flux measurements to estimate the membrane permeance (flux per unit\ntransmembrane chemical potential driving force) and then use this as part of a one-dimensional\nmodule-scale model to determine the water recovery. It’s hard to claim ultrahigh efficiency based\nwithout knowing the water recovery and how this compares to other processes in the space (e.g.,\nreverse osmosis) even though the figures look promising, this needs to be confirmed.\nResponse: To address the reviewer’s concern, we have added 3 new sections (Sections 7.3, 7.4,\nand 7.5 in the SI, Pages 40-42), discussing water recovery ratio, membrane permeance, and\nenergy efficiency, respectively.\nWe have explicitly calculated the energy efficiency as the ratio of the useful energy output (least\nwork of separation) to the total energy input (SEC, including both electricity and heat). As\nsuggested by the reviewer, we evaluated the membrane permeance using the flux data and the\ncorresponding driving force for all tested solutions. The permeance was found one order of\nmagnitude higher than that of commercial membranes. Using the permeance values in a one-\ndimensional scale model we calculated, for 1 m2 of our membrane, water recovery ratios of 44.4%,\n15.5%, and 13.3% for simulated seawater (30 g/L NaCl), untreated seawater (40.9 g/L) and brine\n(74.4 g/L), which leads to energy efficiencies of 58.1%, 29.5%, and 25.0%, respectively.\nRemarkably, the 15.5% (1 m2) water recovery of the new membrane using real untreated seawater\nis higher than the 8% (2.2 m2) recovery typical of small-scale RO systems (where advanced treated\nseawater is used). As it is the case for RO, recovery values can increase at larger scales, further\nhighlighting the scalability of the new membrane. Indeed, industrial RO systems can achieve\nrecovery ratios of 40–50%1 using 6-7 element (placed in a pressure vessel) configurations and\nefficient energy recovery devices. Similarly, while lab-scale MD systems may initially show\nrecovery ratios in the range of 7–9%2, advanced configurations and multi-stage operations have\ndemonstrated recovery ratios of 60–80%3. Additionally, as discussed in our response to comment\n#1, an increase in salt concentration in MD has a less pronounced impact on energy consumption\nthan in RO, which makes the new membrane particularly advantageous for the treatment of high-\nsalinity solutions. For example, when the salt concentration reaches 10% (~3× seawater\nconcentration), the vapor pressure reduction is ~6%, and at 20% salt concentration, the reduction\nis only ~15%4.\n1- M. Elimelech, W.A. Phillip, “The Future of Seawater Desalination: Energy, Technology, and the\nenvironment”, Science, 333, 5 August 2011, 6043, 712-717.\n(https://www.science.org/doi/10.1126/science.1200488)\n2- F. He, J. Gilron, K.K. Sirkar, “High water recovery in direct contact membrane distillation using\na series of cascades”, Desalination, 323, 15 August 2013, 48-54.\n(https://doi.org/10.1016/j.desal.2012.08.006)\n3- R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey Jr, M. AlGhouti, M. Gad-el-Hak, H.V.\nTafreshi, “Energy efficiency of direct contact membrane distillation”, Desalination, 433, May\n2018, 56-67. (https://doi.org/10.1016/j.desal.2018.01.025)\n4- L. Song, B. Li, K.K. Sirkar, J.L. Gilron, “Direct Contact Membrane Distillation-Based\nDesalination: Novel Membranes, Devices, Larger-Scale Studies, and a Model”, Industrial &\nEngineering Chemistry Research, 46, 27 February 2007, 2307-2323.\n(https://doi.org/10.1021/ie0609968)\n5- Sarah Almahfoodh, Sofiane Soukane, Khalid Alhamdan, Ingo Pinnau, Noreddine Ghaffour “A\nstandardized approach for permeance assessment in direct contact membrane distillation”,\nJournal of Membrane Science Letters, 4, 1, June 2024, 100071.\n(https://doi.org/10.1016/j.memlet.2024.100071)\nRecommendation:\nI am grateful to the authors for the changes that they have made thus far (and I think they strengthen\nthe manuscript quite a bit), and I recommend that the authors revise the manuscript to address the\npoints raised before it can be reconsidered for publication.\nWe would like to thank you for your consideration and for the insightful comments, which\nimproved the overall quality of the manuscript.\nThank you\nBest regards"}, {"round": 3, "reviewer_comments": {"reviewer_1": "After several revisions, the authors have provided convincing answers to nearly all of the reviewers' concerns, and the manuscript has now been satisfactorily reviewed.", "reviewer_3": "I thank the authors for their revisions. The manuscript is stronger and a couple of the issues previously raised have been addressed. However, one key issue tied to two earlier comments is still unclear and needs resolution.\nOn page 16 the authors write:\n“To validate energy savings under scalable conditions, we conducted strongly coupled multiscale CFD-membrane modeling, which revealed negligible temperature polarization at a feed temperature of 25 °C, enabling consistent operation across varying module lengths and flow rates (SI document Fig. 23a). Under these ambient conditions, the TF0.1–1 membrane achieved a total SEC as low as 1.88 kWh.m−3 at a water recovery ratio (WRR) of 44.4% (Fig. 4d and SI document Fig. 24), without requiring external heating and with minimal cooling demand (SI document Figs. 22a).”\nThese numbers are quoted elsewhere in the paper and perhaps I am missing something but I do not understand how a WRR of 44.4 % with SEC of 1.88 kWh/m3 is feasible at or near ambient in a single-pass configuration. This remains unclear from a first-law energy balances perspective. In single-pass MD without external heat input, the latent heat to evaporate water must come from sensible cooling of the feed. Assuming enthalpy of vaporization is approximately ΔH_v ≈ 2440 kJ/kg at 25 °C and the specific heat capacity c_p ≈ 4.18 kJ/kg, evaporating 44.4 % of the feed requires approximately1080 kJ per kg of feed (2440 kJ/kg × 0.444). Without external heating, recovering 44.4% of the water through membrane distillation would lead to a temperature drop of about 1080 / 4.18 ≈ 258 °C on the feed side to close the energy balance, which is of course not possible. This back of the envelope calculation assumes that the membrane material is a perfect thermal insulator and that there is no heat loss by conduction. (If the flow rate on the permeate side was equal this would lead to a temperature increase of 258 °C on the permeate side, which is impossible too).\nWriting the first-law energy balance the other way around, with inlet temperatures of 25 °C (feed) and 20 °C (permeate), the maximum sensible heat energy available is c_p ΔT, so the bound on the single pass recovery is WRR_max ≈ c_p ΔT / ΔH_v. For ΔT = 5 °C, WRR_max ≈ (4.18 kJ kg-1 K-1 × 5 K) / 2440 kJ kg-1 ≈ 0.86 % far below the 44.4 % reported. Even for a larger temperature difference between the feed and the permeate, say 40 °C feed with 20 °C permeate (ΔT = 20 °C), the same bound gives WRR_max ≈ (4.18 × 20) / 2440 ≈ 3.4 % for a single pass with no heat recovery, still far below 44.4 % WRR mentioned. This is not even accounting for the fact that the permeate temperature will increase as it passes through the membrane module so the actual practicable WRR even lower, regardless of how good the membrane is.\nGiven these energy balances, the authors need to clarify what the heat or energy source is to make the 44.4 % WRR possible if the system is single-pass at \"near-ambient conditions\", or else correct the SEC or water recovery rate numbers. For example, explicitly highlighting any external or environmental heat input even if it is considered waste or free heat. As written, the 1.88 kWh/m³ at 44.4 % WRR in single-pass ambient operation appears inconsistent with first-law limits unless additional heat was supplied or if there was membrane wetting. But if there was membrane wetting you no longer have water-salt separation and so those values should not be reported."}, "author_response": "Responses to reviewers’ comments (R3): Manuscript NCOMMS-24-48277B\nWe thank the reviewers for their thorough and constructive feedback on our manuscript. We have\ncarefully addressed each comment and clearly marked all changes in the manuscript and SI\ndocument. Below, we restate each comment (in italics) and provide the detailed response,\nincluding clarifications, revised analyses, references to the literature, and revised figures, as\nappropriate. The changes made in the manuscript and SI document are highlighted.\nReviewer #1 (Remarks to the Author)\nThe main result of this work is that the authors have prepared, in an intrinsically low-\nenvironmental method, membranes capable of achieving direct contact distillation transmembrane\nfluxes of up to 40 LMH with a driving force of only 5 °C with a feed solution of 30 g/L NaCl. In\naddition to the claimed outstanding transport properties, the other main improvement is the very\nlow energy (heat) requirement that makes the process energetically comparable to, or even better\nthan, current best-performing RO processes. But, the “ultrahigh-efficiency”, understood as\noutstanding transport properties, must be combined with an adequate salt rejection capacity of\nthe membrane, which the authors reported at 99.8%, calculated with Eq. S2, otherwise they would\njust observe a filtration that would be ineffective for desalination. However, Eq. S2 does not take\ninto account the overall mass balance, since it considers the conductivity of the whole distillate at\nthe end of the test (only 190 mL of “water passed through the membrane in 24h, migrating into\nthe distillate tank that contains 1.5 L of water). Furthermore, Eq. S16 for the SEC calculation\nincludes in the first term of the numerator the difference between T , \"the feed temperature at the\nf,i\nmodule inlet\", and T , \"the temperature of the preheated feed after passing through the heat\nf,h\nexchanger\". This term, therefore, contains the amount of heat that, according to Figure S15, is\n\"wasted\" in the line section immediately downstream of the heat exchanger and the module inlet,\nrather than the difference between the module inlet and outlet, i.e., the amount of heat used to\nevaporate the water (plus that lost by conduction through the membrane). These calculation\nmethods are misleading, as they overestimate the membrane rejection capacity and underestimate\nthe energy consumption, which, combined with the uncertainty on the flux due to the small\nmembrane area (also relative to the equipment size), makes this work unreliable.\nunderstand the concerns. These comments have significantly improved the clarity and robustness\nof our manuscript. Below, we address each concern raised regarding the salt rejection calculations\n(Eq. S2) and the SEC calculation (Eq. S16) considering the system’s performance at various\nmodule scales through rigorous modeling and simulation.\nSalt rejection calculation (Eq. S2):\nWe would like to reassure the reviewer that our calculation of salt rejection (Eq. S2) explicitly\naccounts for dilution effect using the mass balance 𝐶 𝑉 = 𝐶 𝑉 , where 𝐶 and 𝑉 represent the\n1 1 2 2 1 1\nconcentration and volume of the collected permeate, and 𝐶 and 𝑉 correspond to the concentration\n2 2\nand total volume of the diluted distillate, respectively. Given the direct proportionality between\nconductivity and salt concentration in dilute solutions, the concentration in the mass balance can\nbe replaced by the solution conductivity, allowing a straightforward estimation of solute rejection.\nThis methodology ensures that the reported rejection accurately reflects the actual separation\nperformance, independent of any subsequent dilution occurring in the distillate tank.\nTo enhance clarity, we have revised the explanation of Eq. S2 in the SI document, page 22.\nSpecific energy consumption calculation (Eq. S16):\nWe thank the reviewer for raising this important point regarding the SEC calculation and,\nparticularly, the interpretation of the temperature difference term (∆𝑇 = 𝑇 −𝑇 ) in the\n𝑓,𝑖 𝑓,ℎ\nnumerator of Eq. S16. We appreciate the opportunity to clarify how this term is interpreted in our\nstudy, and we have substantially expanded the relevant sections of the SI document (Section S7.2,\nPages 35–42) to provide additional explanation and justification.\nIt is worth pointing out that our SEC calculation methodology aligns with established approaches\nin the literature1, 2, 3, 4, 5. Our SEC calculation in Eq. S16 (Eqs. S16a and S16b in the revised SI)\nis built on a single-pass DCMD configuration, where the feed is not recirculated, and the concentrate\n(brine) is not recycled (see below Figure S22 (a and b) and Figure R1 (a and b)). This configuration\nis not fortuitous and was selected owing to the ability of the membrane to operate at ambient\nconditions, in which no heating is required, and thus no need to recirculate the feed. In these\nequations (Eq. S16(a and b)), the external heat input (𝑄 = 𝑄 = 𝑚̇ 𝐶 ∆𝑇) represents the\n𝑖𝑛 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 𝑓 𝑝,𝑓\nnet external thermal energy required to raise the feed temperature to the module inlet temperature\n(𝑇 ). Likewise, the cooling energy (𝑄 = 𝑄 = 𝑚̇ 𝐶 ∆𝑇) represents the heat removed\n𝑓,𝑖 𝑜𝑢𝑡 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑐 𝑝,𝑐\n2\nfrom the distillate temperature to maintain the necessary driving force across the membrane (𝑇 ).\n𝑐,𝑖\nTo clarify how ∆𝑇 is applied in our SEC calculation, we distinguish between two operational cases,\nambient-temperature and high-temperature feed, both in a single-pass configuration. Each case\ncorresponds to a specific form of Eq. S16, with ∆𝑇 reflecting the actual net thermal input required\nbased on the system setup, as elaborated in the sequel.\n Single-pass system; low-temperature case (25°C feed, Figure S22a, and Figure R1a):\nFor low-temperature processes (e.g., 25°C feed, Figures S22a and R1a), the feed enters the\nmodule at an ambient temperature (~25°C). Here, heat recovery is neither necessary nor\npractical due to the minimal temperature difference, which aligns with our goal of ultra-low\nenergy operation. In this context, (∆𝑇 = 𝑇\n𝑓,𝑖\n−𝑇\n𝑓,𝑠\n), where 𝑇\n𝑓,𝑠\nis the source temperature. Since\n𝑇 is ambient (~25°C), then ∆𝑇 = 0 and 𝑄 = 0, indicating that no additional heating is needed\n𝑓,𝑠 𝑖𝑛\nbeyond ambient conditions. Thus, for the low-temperature operation case (25°C), no heat is\n\"wasted\", and recovery is unnecessary. This reinforces our central claim of low energy demand\nfor ambient operation, as illustrated in Figures R1a and S22a.\n Single-pass system; high-temperature case (60°C feed, Figure S22b and Figure R1b):\nIt is important to recall that this configuration is presented only for benchmarking against\ncommercial membranes at elevated temperatures. To minimize energy loss, the feed is\npreheated in this case using a heat exchanger that recovers energy from the warmer outlet brine\nto reach 𝑇\n𝑓,ℎ\n. The feed is then heated further from 𝑇\n𝑓,ℎ\nto the required inlet feed temperature 𝑇\n𝑓,𝑖\nusing an external heating source (Fig. R1b). Thus, 𝑄 = 𝑚̇ 𝐶 (𝑇 −𝑇 ) indicates the net\n𝑖𝑛 𝑓 𝑝,𝑓 𝑓,𝑖 𝑓,ℎ\nthermal energy input after heat recovery rather than wasted heat, as shown in Figures R1b and\nS22b.\nIt is worth emphasizing again that the calculations in this investigation do not consider any closed-\nloop configuration. Indeed, as noted by the reviewer, in the case of a closed-loop system (Figure\nR1c) where the feed is recirculated, using inlet minus outlet temperature (∆𝑇 = 𝑇 −𝑇 ) is the\n𝑓,𝑖 𝑓,𝑜\nappropriate calculation, which would represent the actual heat utilized across the membrane\nmodule. This configuration is not employed in our SEC calculations in Eq. S16, since the focus\nwas set on single-pass configuration only (Figure S22(a and b)).\nAdditionally, in the single pass configuration for both low inlet and high inlet feed temperatures,\nthe heat transferred from the feed to the permeate side (loss in the feed) through the membrane\n3\n(via conduction and convection) is included by calculating 𝑄 (the heat removed from the\n𝑐𝑜𝑜𝑙𝑖𝑛𝑔\ndistillate), which is accordingly accounted for in Eq. S16.\nFigure R1. Schematic illustrating the calculation of the energy 𝑄 required to bring the feed\n𝑖𝑛\ntemperature to 𝑇 for different system configurations. (a) single-pass system for low temperature\n𝑓,𝑖\noperation, (b) single-pass system with heat exchanger for high-temperature operation. (c) Closed-\nloop system. (The full schematic, including both heating and cooling for the single-pass\nconfiguration, is presented in Figure S22.)\nFinally, it is crucial to recall that the ultra-high efficiency claimed in this investigation concerns\nthe membrane operation at 25 ℃ only (focus of this work, as explained in the manuscript), which,\nwith the reviewers’ feedback, is now clearly stated for the readers in the manuscript. As such, to\nproperly address the reviewer’s concern, we have mentioned the single-pass configuration in the\nrevised manuscript (Page 16). Additionally, the SI document (Section S7.2, Pages 35–42) has\nbeen revised to include schematic labeling of all relevant temperatures (𝑇 ,𝑇 ,𝑇 ,𝑇 ,𝑇 ) and\n𝑓,𝑠 𝑓,ℎ 𝑓,𝑖 𝑐,𝑜 𝑐,𝑖\nclearly explain the ∆𝑇 definition for each configuration. In the same sections, we have clearly\nexplained our methodology for energy calculations with membrane performances at various\nmodule lengths through rigorous numerical analysis and simulation validated based on\nexperimental data (more details on energy consumption for different scenarios can be found in\nresponses to comments raised by Reviewer #3).\nThe authors have done an exceptional job of responding to this reviewer's and to other reviewer's\ncomments. I have no further comments on this manuscript and think it is of high quality and\npresents a valuable set of data and analysis.\nResponse: We sincerely thank the reviewer for the encouraging and positive feedback. We are\ndelighted that the reviewer finds the revised manuscript improved, of high quality, and valuable to\nthe field.\nReviewer #3 (Remarks to the Author)\nI would like to thank the authors for their revisions, the improvements made do strengthen the\nmanuscript, but they also raise some issues that need to be addressed before the manuscript can\nbe reconsidered for publication:\nResponse: We would like to thank the reviewer for the positive feedback and invaluable\ncomments.\n1. Regarding my previous comment on Reporting of Water Recovery Ratios with SEC:\nThe authors need to give the water recovery ratio with any SEC numbers quoted (along with the\ninitial feed salinity, which they have already included). For example, in the abstract the authors\nwrite “Notably, these films achieve the lowest total specific energy consumption reported to date;\n1.88 kWh.m−3 for simulated seawater, 3.47 kWh.m−3 for real seawater, and 4.43 kWh.m−3 for real\nseawater reverse osmosis desalination brine” but in order to know the amount of separation\nachieved by this energy input we need to know what the water recovery ratio was (i.e., the\ntransmembrane membrane mass or molar flux multiplied by the membrane area and divided by\nthe feed mass or molar flow rate) in order to gauge whether this energy consumption figure is high\nor low. Quoting energy consumption based on zero- or low-recovery measurements can mislead\nthe reader, especially when compared with the reverse osmosis values for 50% water recovery\nlater in the paper (“This low SEC is close to the reported minimum theoretical energy required for\nseawater desalination using a large RO system at 50% recovery, estimated at 1.06 kWh.m−3”,\nreferencing Elimelech & Phillip 2011, #78).\nResponse: We fully understand the reviewer’s point and totally agree that the statement may\nmislead readers. As suggested by the reviewer in the sequel, we calculated the water recovery ratio\nof our membranes using a one-dimensional module-scale model for a 1 m2 membrane area. The\nrecovery ratio was found to be 44.4%, 15.5%, and 13.3% for NaCl, real untreated seawater\n(compared to 8% for 2.2 m2 RO membrane for pretreated seawater), and SWRO desalination brine,\nrespectively, which underscores the potential for high recovery in a multistage configuration. To\naddress the reviewer’s comment, we added to the manuscript (Page 16) the recovery values\ncorresponding to a 1 m2 of membrane and a new section on water recovery in the SI document\n(Section S7.3 Page 40), which will allow the readers to assess the significance of the membrane\nperformance.\nMoreover, recovery ratios are inherently scale-dependent and are generally improved by adding\nstages (membrane area) to the process flow. For example, small-scale RO systems employing a\nsingle element typically achieve 8–10% water recovery (e.g., an 8% recovery was reported for a\n2.2 m² RO module2). In contrast, industrial RO systems, which use 6-7 element configurations\n(typically 6-8 elements placed in a pressure vessel) and energy efficient recovery devices, can\nachieve recovery ratios of 40–50%3. Similarly, while lab-scale MD systems may initially show\nrecovery ratios in the range of 7–9%4, advanced configurations and multi-stage operations have\ndemonstrated recovery ratios of 60–80%5. Moreover, in RO processes, increasing the recovery\nratio necessitates a corresponding increase in applied pressure for subsequent stages due to higher\nosmotic pressures caused by higher salinities, which leads to a higher energy consumption further\npenalizing the SEC. In contrast, the new membrane operates at atmospheric pressure and room\ntemperature, which means that recovery ratios can be readily increased with additional stages,\npotentially exceeding 70% owing to the smaller effect of the stream concentration on the vapor\npressure compared to its significant effect on the osmotic pressure in the case of RO. Using\nAntoine’s equation and a correction function of salinity, one can readily verify that when the salt\nconcentration reaches 10% (~3 times seawater), the vapor pressure is reduced by about 6%\n(p /p0 = 0.94); and when the salt concentration reaches 20% (~5 – 6 times seawater)\nwater water\n(p /p0 = 0.85), the vapor pressure is reduced by only 15%6. This shows that for a similar\nwater water\npermeate flux, the new MD membrane can reach as high of a recovery ratio as RO’s. Additionally,\nbench evaluation of seawater desalination by RO shows a permeate flux below 18 LMH7 when the\nosmotic pressure increases beyond 30 bars, while the new membrane delivers 40 LMH under\natmospheric pressure and room temperature.\nReferences:\n1- V.G. Gude, “Energy consumption and recovery in reverse osmosis”, Desalination and Water\nTreatment, 36 (2011) 239-260. (https://doi.org/10.5004/dwt.2011.2534)\n2- D.S. Ayou, H.M. Ega, A. Coronas, “A feasibility study of a small-scale photovoltaic-powered\nreverse osmosis desalination plant for potable water and salt production in Madura island: A\ntechno-economic evaluation”, Thermal Sciences and Engineering Progress, Volume 35, 1\nOctober 2022, 101450. (https://doi.org/10.1016/j.tsep.2022.101450)\n3- M. Elimelech, W.A. Phillip, “The Future of Seawater Desalination: Energy, Technology, and the\nenvironment”, Science, 333, 5 August 2011, 6043, 712-717.\n(https://www.science.org/doi/10.1126/science.1200488)\n4- F. He, J. Gilron, K.K. Sirkar, “High water recovery in direct contact membrane distillation using\na series of cascades”, Desalination, 323, 15 August 2013, 48-54.\n(https://doi.org/10.1016/j.desal.2012.08.006)\n5- R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey Jr, M. AlGhouti, M. Gad-el-Hak, H.V.\nTafreshi, “Energy efficiency of direct contact membrane distillation”, Desalination, 433, May\n2018, 56-67. (https://doi.org/10.1016/j.desal.2018.01.025)\n6- L. Song, B. Li, K.K. Sirkar, J.L. Gilron, “Direct Contact Membrane Distillation-Based\nDesalination: Novel Membranes, Devices, Larger-Scale Studies, and a Model”, Industrial &\nEngineering Chemistry Research, 46, 27 February 2007, 2307-2323.\n(https://doi.org/10.1021/ie0609968)\n7- D.A. Ladner, A. Subramani, M. Kumar, S.S. Adham, M.M. Clark, “Bench-scale evaluation of\nseawater desalination by reverse osmosis”, Desalination, 250 (2), 15 January 2010, 490 - 499.\n(https://doi.org/10.1016/j.desal.2009.06.072)\n2. Impact of Membrane Density and Porosity on Heat Transfer:\nThe authors report a membrane that is dense, relatively non-porous (~5% porosity), and thin.\nPrevious studies have shown that, in membrane distillation, cumulative heat transfer is one of the\nmajor challenges with the membranes developed here. Linking back into Point 1, having a thin,\nlow porosity membrane would mean that cumulatively, over the area of a membrane module,\nthe conductive heat loss from the feed to the permeate side would accumulate, drastically reducing the\ndriving force for further water permeation, limiting the water recovery ratio available. The authors\nshould discuss or model this.\nResponse: We appreciate the reviewer’s insightful comment. While we acknowledge that a thin,\nlow-porosity membrane could lead to increased conductive heat transfer across its matrix, our\nexperimental results show an exceptionally low membrane thermal conductivity (0.07 ± 0.02\nW·m⁻¹·K⁻¹) (Figure S9, SI, Page 14), which helps reducing heat losses considerably and preserve\nthe temperature gradient necessary for an efficient water vapor transport. Moreover, it is important\nto recall that the driving force in our case is higher than that of standard MD membranes owing to\na higher water vapor pressure caused by the nanosized pores, which we expressed using Kelvin’s\nequation (Eq S7, Page 33). This enables the membrane to operate effectively at low feed\ntemperatures (e.g., 25 °C) with a small transmembrane temperature difference (~5 °C), and further\nreduces cumulative conductive heat transfer, as conductive heat losses (Q , Eq. S9, Page 34) are\nc\nproportional to the temperature gradient. We would like to reassure the reviewer that we modeled\na fully coupled approach of CFD and pore-scale calculations. The model considers both convective\nand conductive heat transfer modes from feed to permeate across the membrane as detailed in Eqs.\nS11 to S13 (section S6 of the SI document, Pages 32-35). The analysis confirms that the\nmembrane’s low thermal conductivity and vapor-driven mass transfer mechanism effectively\nprevent significant heat loss accumulation.\nTo address the reviewer’s concern, we added in page 14 of the manuscript a statement highlighting\nthe important contribution of the low thermal conductivity of the thin film in sustaining the driving\nforce for distillation.\n3. Clarification on Pore Size / Separation Mechanism:\nThe reported pore sizes measured are very small (sub-nanometer), which could suggest that the\nmembrane might operate more like a pervaporation system—where the polymer sieves salt ions—\ninstead of following a conventional membrane distillation process that relies on a water–vapor\ninterface for the water-salt separation? It would be very helpful if the authors could clarify whether\nthe separation mechanism is driven by a phase change or by molecular sieving, and explain how\nthe small pore size influences the transport mechanism.\nResponse: We appreciate the reviewer’ comment on this critical point. In our study, we confirm\nthat the separation mechanism is driven by phase change, rather than molecular sieving or\npervaporation. The membrane is superhydrophobic, which prevents liquid water penetration and\nensures that only vapor-phase transport occurs through the subnanometer pores. This is readily\nverified during the experiments by monitoring the conductivity of the distillate stream. Notably,\nthe membrane was tested for seawater desalination at the same temperature on both sides. While\na concentration gradient prevails in these experiments, no water flux was observed further\nconfirming that phase change (evaporation/condensation) is the primary mechanism rather than\ndiffusion or pervaporation (which requires applying vacuum at the permeate side). Regarding\nmolecular sieving, although the reported pore sizes in the subnanometer range may suggest a\npossible molecular sieving mechanism, the zero flux observed in the isothermal desalination\nexcludes such a mechanism. This conclusion is further reinforced by the agreement between the\nvapor flux enhancement due to nanoconfinement as expressed by Kelvin’s equation and the\npermeate flux evolution as a function of feed temperature, as discussed in the manuscript. As such,\nthe modeling results (see Equation S7 and Figure S20 in the SI, Pages 33-34) show that for a\npore size of ~0.495 nm, the vapor pressure enhancement can reach 10x at 20 °C and 6x at 60 °C,\nexplaining the observed ultrafast vapor transport.\n4. Keeping / demonstrating the Ultra-High Efficiency Claims:\nFor the ultra-high efficiency claim I think the authors need to show what the energy efficiency of\nthe process is in terms of useful energy out (least work of separation for a given feed salinity and\nwater recovery ratio, assuming 100% salt rejection) divided by the total energy input (including\nboth the electricity and heat). I believe for reverse osmosis with efficient pumps and energy\nrecovery devices the energy efficiency is around 30-40%. If the authors want to keep this claim\nthey should use the flux measurements to estimate the membrane permeance (flux per unit\ntransmembrane chemical potential driving force) and then use this as part of a one-dimensional\nmodule-scale model to determine the water recovery. It’s hard to claim ultrahigh efficiency based\nwithout knowing the water recovery and how this compares to other processes in the space (e.g.,\nreverse osmosis) even though the figures look promising, this needs to be confirmed.\nResponse: To address the reviewer’s concern, we have added 3 new sections (Sections 7.3, 7.4,\nand 7.5 in the SI, Pages 40-42), discussing water recovery ratio, membrane permeance, and\nenergy efficiency, respectively.\nWe have explicitly calculated the energy efficiency as the ratio of the useful energy output (least\nwork of separation) to the total energy input (SEC, including both electricity and heat). As\nsuggested by the reviewer, we evaluated the membrane permeance using the flux data and the\ncorresponding driving force for all tested solutions. The permeance was found one order of\nmagnitude higher than that of commercial membranes. Using the permeance values in a one-\ndimensional scale model we calculated, for 1 m2 of our membrane, water recovery ratios of 44.4%,\n15.5%, and 13.3% for simulated seawater (30 g/L NaCl), untreated seawater (40.9 g/L) and brine\n(74.4 g/L), which leads to energy efficiencies of 58.1%, 29.5%, and 25.0%, respectively.\nRemarkably, the 15.5% (1 m2) water recovery of the new membrane using real untreated seawater\nis higher than the 8% (2.2 m2) recovery typical of small-scale RO systems (where advanced treated\nseawater is used). As it is the case for RO, recovery values can increase at larger scales, further\nhighlighting the scalability of the new membrane. Indeed, industrial RO systems can achieve\nrecovery ratios of 40–50%1 using 6-7 element (placed in a pressure vessel) configurations and\nefficient energy recovery devices. Similarly, while lab-scale MD systems may initially show\nrecovery ratios in the range of 7–9%2, advanced configurations and multi-stage operations have\ndemonstrated recovery ratios of 60–80%3. Additionally, as discussed in our response to comment\n#1, an increase in salt concentration in MD has a less pronounced impact on energy consumption\nthan in RO, which makes the new membrane particularly advantageous for the treatment of high-\nsalinity solutions. For example, when the salt concentration reaches 10% (~3× seawater\nconcentration), the vapor pressure reduction is ~6%, and at 20% salt concentration, the reduction\nis only ~15%4.\n1- M. Elimelech, W.A. Phillip, “The Future of Seawater Desalination: Energy, Technology, and the\nenvironment”, Science, 333, 5 August 2011, 6043, 712-717.\n(https://www.science.org/doi/10.1126/science.1200488)\n2- F. He, J. Gilron, K.K. Sirkar, “High water recovery in direct contact membrane distillation using\na series of cascades”, Desalination, 323, 15 August 2013, 48-54.\n(https://doi.org/10.1016/j.desal.2012.08.006)\n3- R. Ullah, M. Khraisheh, R.J. Esteves, J.T. McLeskey Jr, M. AlGhouti, M. Gad-el-Hak, H.V.\nTafreshi, “Energy efficiency of direct contact membrane distillation”, Desalination, 433, May\n2018, 56-67. (https://doi.org/10.1016/j.desal.2018.01.025)\n4- L. Song, B. Li, K.K. Sirkar, J.L. Gilron, “Direct Contact Membrane Distillation-Based\nDesalination: Novel Membranes, Devices, Larger-Scale Studies, and a Model”, Industrial &\nEngineering Chemistry Research, 46, 27 February 2007, 2307-2323.\n(https://doi.org/10.1021/ie0609968)\n5- Sarah Almahfoodh, Sofiane Soukane, Khalid Alhamdan, Ingo Pinnau, Noreddine Ghaffour “A\nstandardized approach for permeance assessment in direct contact membrane distillation”,\nJournal of Membrane Science Letters, 4, 1, June 2024, 100071.\n(https://doi.org/10.1016/j.memlet.2024.100071)\nRecommendation:\nI am grateful to the authors for the changes that they have made thus far (and I think they strengthen\nthe manuscript quite a bit), and I recommend that the authors revise the manuscript to address the\npoints raised before it can be reconsidered for publication.\nWe would like to thank you for your consideration and for the insightful comments, which\nimproved the overall quality of the manuscript.\nThank you\nBest regards"}, {"round": 4, "reviewer_comments": {"reviewer_3": "I thank the authors for their revisions and for adding clarifications to the numerical values presented in the manuscript and supplementary information text. I am happy for the manuscript to be published."}, "author_response": "Responses to reviewers’ comments (R4): Manuscript NCOMMS-24-48277B\nWe thank the reviewers for their thorough and constructive feedback on our manuscript. We have\ncarefully addressed each comment and clearly marked all changes in the manuscript and SI\ndocument. Below, we restate each comment (in italics) and provide the detailed response,\nincluding clarifications, revised analyses, references to the literature, and revised figures, as\nappropriate. The changes made in the manuscript and SI document are highlighted.\nReviewer #1 (Remarks to the Author)\nAfter several revisions, the authors have provided convincing answers to nearly all of the\nreviewers' concerns, and the manuscript has now been satisfactorily reviewed.\nResponse: We sincerely thank the reviewer for the encouraging and positive feedback. We are\ndelighted that the reviewer finds the revised manuscript improved, of high quality, and valuable to\nthe field.\nReviewer #3 (Remarks to the Author)\nI thank the authors for their revisions. The manuscript is stronger and a couple of the issues\npreviously raised have been addressed. However, one key issue tied to two earlier comments is\nstill unclear and needs resolution.\nOn page 16 the authors write: “To validate energy savings under scalable conditions, we\nconducted strongly coupled multiscale CFD-membrane modeling, which revealed negligible\ntemperature polarization at a feed temperature of 25 °C, enabling consistent operation across\nvarying module lengths and flow rates (SI document Fig. 23a). Under these ambient conditions,\nthe TF0.1–1 membrane achieved a total SEC as low as 1.88 kWh.m−3 at a water recovery ratio\n(WRR) of 44.4% (Fig. 4d and SI document Fig. 24), without requiring external heating and with\nminimal cooling demand (SI document Figs. 22a).”\nThese numbers are quoted elsewhere in the paper and perhaps I am missing something but I do\nnot understand how a WRR of 44.4 % with SEC of 1.88 kWh/m3 is feasible at or near ambient in\na single-pass configuration. This remains unclear from a first-law energy balances perspective.\nsensible cooling of the feed. Assuming enthalpy of vaporization is approximately ΔH_v ≈ 2440\nkJ/kg at 25 °C and the specific heat capacity c_p ≈ 4.18 kJ/kg, evaporating 44.4 % of the feed\nrequires approximately1080 kJ per kg of feed (2440 kJ/kg × 0.444). Without external heating,\nrecovering 44.4% of the water through membrane distillation would lead to a temperature drop\nof about 1080 / 4.18 ≈ 258 °C on the feed side to close the energy balance, which is of course not\npossible. This back of the envelope calculation assumes that the membrane material is a perfect\nthermal insulator and that there is no heat loss by conduction. (If the flow rate on the permeate\nside was equal this would lead to a temperature increase of 258 °C on the permeate side, which is\nimpossible too).\nWriting the first-law energy balance the other way around, with inlet temperatures of 25 °C (feed)\nand 20 °C (permeate), the maximum sensible heat energy available is c_p ΔT, so the bound on the\nsingle pass recovery is WRR_max ≈ c_p ΔT / ΔH_v. For ΔT = 5 °C, WRR_max ≈ (4.18 kJ kg-1 K-\n1 × 5 K) / 2440 kJ kg-1 ≈ 0.86 % far below the 44.4 % reported. Even for a larger temperature\ndifference between the feed and the permeate, say 40 °C feed with 20 °C permeate (ΔT = 20 °C),\nthe same bound gives WRR_max ≈ (4.18 × 20) / 2440 ≈ 3.4 % for a single pass with no heat\nrecovery, still far below 44.4 % WRR mentioned. This is not even accounting for the fact that the\npermeate temperature will increase as it passes through the membrane module so the actual\npracticable WRR even lower, regardless of how good the membrane is.\nGiven these energy balances, the authors need to clarify what the heat or energy source is to make\nthe 44.4 % WRR possible if the system is single-pass at \"near-ambient conditions\", or else correct\nthe SEC or water recovery rate numbers. For example, explicitly highlighting any external or\nenvironmental heat input even if it is considered waste or free heat. As written, the 1.88 kWh/m³\nat 44.4 % WRR in single-pass ambient operation appears inconsistent with first-law limits unless\nadditional heat was supplied or if there was membrane wetting. But if there was membrane wetting\nyou no longer have water-salt separation and so those values should not be reported.\nResponse: We sincerely thank the reviewer for the scrutiny and patience throughout the many\nrevisions of the manuscript. Indeed, we fully agree with the reviewer that in a single-pass direct\ncontact MD configuration without recirculation and external heating, the maximum water recovery\nis set by the first law of thermodynamics. While our initial calculations were based on the widely\nadopted methodology used in membrane filtration processes (Eq. S17 of the previous version of\nthe manuscript and SI document), which computes the ratio of the permeate production to the inlet\nfeed, the heat balance needs to be consistent in MD as explained in the simplified calculation\npresented by the reviewer. Fully adopting the reviewer’s approach, and to avoid confusions for\nreaders, we have brought all the required changes in the manuscript and the supplementary\ninformation document using a more accurate calculation of the recovery ratio for a single pass,\nwhich also accounts for the evaporation efficiency, as reported in the reference below (Reference\nadded in the manuscript [74] and SI document [67]).\n[74] O.R. Lokare, S. Tavakkoli, V. Khanna, R.D. Vidic, “Importance of feed recirculation for the\noverall energy consumption in membrane distillation systems”, Desalination, 428, 15\nFebruary 2018, 250-254.\nHere are the modifications brought to the revised manuscript and SI document:\n- Abstract (Page 2): Please see the modifications of the WRRsp values for each SEC.\n- Modification brought to the “Energy saving” Section of the manuscript (Pages 16 - 17).\n- WRRsp values corrected for each SEC throughout the manuscript and SI document. No\nrecirculation is adopted.\n- To avoid confusion, SEC comparison with RO paragraph is deleted (from Page 17).\n- Please see the modification of the WRR calculations in Section 7.2, including new Eq. S17,\nin Pages 37-38 of the SI document, where 2 new references have been added [67, 69,\ndetails below].\n- Energy efficiency (η) values with corresponding WRR are corrected in Page 41 of the SI\ndocument.\n[67] Lokare OR, Tavakkoli S, Khanna V, Vidic RD. Importance of feed recirculation for the overall\nenergy consumption in membrane distillation systems. Desalination 428, 250-254 (2018).\n[69] Yao K, Qin Y, Yuan Y, Liu L, He F, Wu Y. A continuous-effect membrane distillation process\nbased on hollow fiber AGMD module with internal latent-heat recovery. AIChE Journal 59,\n1278-1297 (2013).\nWe are deeply grateful to the reviewer for this insightful observation, which enabled us to clarify\nthis point and enhance the scientific rigor and clarity of our work.\nSincerely,\nThe authors\n3"}]

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