# Advancing Surface-Enhanced Electrospun Nanofiber Membranes: Customizing Properties for Enhanced Performance in Membrane Distillation

Michaela Olisha S. Lobregas, Ratthapol Rangkupan, David Riassetto, Chalida Klaysom

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Michaela Olisha S. Lobregas, Ratthapol Rangkupan, David Riassetto, Chalida Klaysom. Advancing Surface-Enhanced Electrospun Nanofiber Membranes: Customizing Properties for Enhanced Performance in Membrane Distillation. Macromolecular Materials and Engineering, 2024, 309 (7), pp.2300461. 10.1002/mame.202300461. hal-04728902

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# Advancing Surface-Enhanced Electrospun Nanofiber Membranes: Customizing Properties for Enhanced Performance in Membrane Distillation

Michaela Olisha S. Lobregas, Ratthapol Rangkupan,* David Riassetto, and Chalida Klaysom*

Electrospun nanofiber membranes (ENMs) have emerged as a cutting-edge solution for membrane distillation (MD), recognized for their highly porous and interconnected architecture. This distinctive structure enables them to offer minimal mass transfer resistance, making them exceptionally suited for high-efficiency membrane-based separation processes. However, the very porosity that defines their strength also renders them vulnerable to fouling, scaling, and wetting during operation, which in turn compromises their performance. Current research efforts are geared toward overcoming these obstacles by refining the surface design and characteristics of ENMs. This review delves into the latest advancements in surface-enhanced electrospun nanofiber membranes tailored for MD applications. It discusses the existing gaps in research and provides forward-looking insights into the future of ENMs, spotlighting the development of membranes with precisely tunable surface attributes for optimized performance.

## 1. Introduction

Electrospun nanofiber membranes (ENMs) mark a significant advancement in membrane separation technology, showcasing a highly porous and interconnected pore architecture. These ENMs are a fabric of non-woven nanofibers originating from an electrospinning process, a method that propels a charged polymer jet into a high-voltage electric field. Influenced by Coulombic repulsion, the charges within the polymer jet cause it to elongate in a linear and complex trajectory due to electrically-induced bending

instability phenomena. This results in a high draw ratio, culminating in the creation of fibers with notably small diameters over a brief distance.[1] As this charged jet traverses towards a collector of opposite polarity, driven by Coulombic attraction, it forms a non-woven nanofibrous sheet. The orientation of the electrospun nanofibers (ENFs), whether partially or fully aligned, varies based on the collector's design and other processing parameters. Throughout its journey to the collector, the jet undergoes solidification, either through solvent evaporation in the case of solution-based electrospinning or thermal dissipation for melt-electrospinning. The ability to finely control the properties of these electrospun nanofibers (ENFs) by adjusting the electrospinning conditions such as the polymer solution's viscosity, applied voltage, flow rate,

and the gap between the needle tip and the collector is thoroughly documented in recent literature.[2,3] These parameters are instrumental in determining the final characteristics of ENMs, making electrospinning a pivotal technique for customizing membrane properties for various applications.

ENMs have high porosity and interlinked pore structure, that significantly reduce mass transfer resistance and enhance permeate flux, offering a pathway with minimal tortuosity essential for efficient membrane performance. The unique characteristics of ENFs have positioned them as a material of choice for

M. O. S. Lobregas, R. Rangkupan, C. Klaysom
Nanoscience and Technology Interdisciplinary Program
Graduate School
Chulalongkorn University
Bangkok 10330, Thailand
E-mail: ratthapol.r@chula.ac.th; chalida.kl@chula.ac.th

<figure><img src="image_2.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.202300461

© 2024 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/mame.202300461

R. Rangkupan
Metallurgy and Materials Science Research Institute (MMRI)
Chulalongkorn University
Bangkok 10330, Thailand

D. Riassetto
LMGP
Grenoble INP
CNRS
University Grenoble Alpes
Grenoble 38000, France

C. Klaysom
Center of Excellence in Particle and Material Processing Technology
Department of Chemical Engineering
Faculty of Engineering
Chulalongkorn University
Bangkok 10330, Thailand
membrane technologies especially membrane distillation (MD). Specifically, ENMs produced from electrospinning technique offer significant control over film thickness and the ability to customize the architecture of pores through the interconnection of ENFs. This capability allows for the development of multi-layered membranes equipped with desired functional properties, optimizing their performance with specific feed streams and catering to emerging applications in MD processes.[4-6]

ENMs have positioned themselves at the cutting edge of membrane distillation (MD) technology, captivating the interest of the research community. This is evidenced by a substantial uptick in publications over the past decade.[2,7-12] While existing literature thoroughly explores the principles of electrospinning, ENM fabrication techniques, and MD basics,[7,13] this review seeks to offer a novel outlook and a fresh perspective. It delves into the inherent challenges of ENMs, particularly their proneness to wetting and fouling—issues that have impeded their widespread application. Surface modification is identified as a crucial approach to equip ENMs with enhanced resistance to water contaminants, as well as increased stability and longevity, making them more suitable for MD applications.

Acknowledging wetting and fouling as significant hurdles in membrane-based water separation technologies, recent studies have endeavored to engineer membranes with superior resistance to these contaminants. Various methods have been developed to refine, enhance, and instill the desired surface characteristics in ENMs, as reported recently.[7,14] Yet, a structured synthesis and organization of these approaches could deepen our comprehension and stimulate further exploration into membrane surface modification techniques.

This review focuses on ENMs engineered with specialized surface properties designed to enhance their durability and anti-wetting characteristics for MD application. First, it outlines the core principles of MD, underscoring the vital attributes a membrane must possess for an effective and enduring desalination process. It highlights the inherent advantages of ENM that align with the demands of membrane distillation. Subsequently, it examines the latest strategies to augment ENMs for MD purposes, with a particular focus on surface modifications that bolster resistance to liquids and contaminants. The review categorizes these surface enhancement tactics based on their implementation phase in the membrane fabrication process – either during (in-situ) or after (post) electrospinning process. A variety of techniques are systematically detailed, categorized, and differentiated, exhibiting a structured and clear concept of tailoring ENM for practical and durable membrane performance. Lastly, the primary challenges of ENM in MD applications were presented, providing insights into its future research expansion.

# 2. Membrane Distillation (MD)

## 2.1. Operating Principle

Membrane distillation is a thermally driven separation technique wherein the hot feed side and the cold permeate side are divided by a porous hydrophobic membrane. The difference in temperature on both sides generates the difference in vapor pressure. As the feed is heated to a specific temperature just below its boiling point, vapor molecules are formed, passing through the mem-

brane, and eventually condensing back to the liquid phase, as it reaches the cold permeate side. In the MD process, low- or non-volatile solutes and impurities like inorganic salts, mineral oils, grease, and surfactants remain in the solution. Due to high volatility, the water turns into vapor and passes through the membrane pores.[12] MD can thus produce highly pure water and the system has no trade-off problem between flux and selectivity like most membrane processes. Compared to conventional distillation techniques, MD is normally operated at a lower operating temperature. Moreover, compared to reverse osmosis (RO), MD operates at ambient pressure. Operating at such milder conditions ensures that the MD process has a lower carbon footprint and a more cost-effective separation process.[7]

## 2.2. MD Configuration

In Figure 1, it is seen that the MD system can be classified into four basic configurations. All configurations have a similar arrangement at the feed side whereby the membrane is in direct contact with the feed solution. On the permeate side, the designs are seen to have two arrangements: 1) the permeate vapor is immediately condensed inside the membrane module, and 2) the permeate vapor is drawn out first and condensed outside the membrane module. In the first arrangement, the vapor can be condensed by direct contact with cold liquid as in direct contact membrane distillation (DCMD). The vapor can also be condensed on a surface with a small air gap in between as in air gap membrane distillation (AGMD). In the second arrangement, the vapor is drawn out from the membrane module either by the suction of a vacuum pump as in vacuum membrane distillation (VMD) or by an inert sweeping gas as in sweeping gas membrane distillation (SGMD).[12]

As shown in Figure 1a, DCMD is the most straightforward MD configuration and is widely used in laboratory-scale MD research studies. The direct contact of the membrane to both the hot feed side and the cold permeate side offers low mass transfer resistance leading to high flux. However, its direct contact with liquids with opposite temperatures imposes a challenge in heat recovery. A high amount of heat is lost through internal heat conduction in the membrane matrix which could result in low-energy efficiency. Also, the permeate quality is sensitive to membrane wetting as the condensation occurs at the membrane-permeate interface due to direct exposure of the membrane to the cold permeate liquid.

Figure 1b illustrates the AGMD configuration that has a 2-10 mm close air gap between the membrane and the condensation surface. The air gap addresses the high heat loss and potential wetting at the permeate side in the DCMD configuration. It has a low thermal loss, and thus it has the highest energy efficiency.[5] It is also employed in small-scale studies involving the desalination of highly saline water. However, the air gap could give additional vapor resistance leading to low permeate flux when used in lab-scale studies and compared to DCMD. Nevertheless, pilot scale studies show a higher flux and gain-output ratio, making it desirable for commercial applications.[7,15]

In Figure 1c, VMD includes a vacuum of air on the permeate side of the membrane that directs the vapor into a condenser connected outside the MD module. The heat loss through
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The four conventional MD configurations: a) direct contact membrane distillation (DCMD), b) air gap membrane distillation (AGMD), c) vacuum membrane distillation (VMD), and d) sweep gas membrane distillation (SGMD).</figcaption></figure>

conduction is negligible, leading to a very low thermal polarization. It is suitable for recovering aromatic compounds in aqueous and volatile aqueous solutions.[12,16] However, it is more susceptible to pore wetting and fouling as the vacuum might draw in the contaminants from the feed side to move inside the membrane pores. Also, the vacuum could deplete the air at the air-liquid-solid interface on the feed side of the membrane, thus inducing pore-wetting.[17] The additional necessity of a vacuum pump and an external condenser makes it complex for small-scale and basic research studies.

As shown in Figure 1d, SGMD requires a sweeping cold, inert gas as a vapor carrier from the permeate side to an external condenser. Like AGMD, the air barrier diminishes heat loss, and in SGMD, the vapor transfer resistance is reduced due to the air movement, thus enhancing mass transfer. It is also ideal for removing volatile components from aqueous solutions.[9] However, its main disadvantage is that a small volume of permeate diffuses in a large volume of sweep gas and therefore requires a large condenser.[12]

As MD research continuously expands, new configurations are developed based on improving heat recovery for high energy efficiency and reducing mass transfer resistance for high vapor flux. New designs primarily involve hybrids of AGMD wherein there is always a gap between the membrane and the cold stream. The gap can be filled with the permeate liquid as in permeate gap MD (PGMD), or with thermally conducting materials like material gap MD (MGMD). The gap and the cold liquid stream are separated by an impermeable thin condensing film. Such new configurations have lower mass transfer resistance compared to AGMD and lower heat consumption compared to DCMD. A thorough discussion of the recent MD configurations is found elsewhere.[7,12,18,19]

In all MD configurations, one side of the membrane directly contacts the feed liquid, rendering it vulnerable to wetting. Additionally, the membrane is exposed to a variety of contaminants from the feed, including both inorganic and organic substances. These contaminants can lead to fouling and damage the membrane surface, hastening the wetting process and reducing the separation efficiency. Although advancements in module configurations have led to improvements in the energy efficiency

and mass transfer of the membrane distillation system, overcoming the challenge of wetting remains critical. Therefore, it is essential to focus on developing new membrane materials and designs that possess outstanding anti-wetting, anti-fouling, and anti-scaling properties to ensure the efficacy and durability of the MD process.

## 2.3. Membranes for MD

Membranes used in MD act as a barrier between the feed and permeate. Hence, the physical properties of membranes play a crucial role in separation efficiency. MD membranes being highly porous, thin, having small pore tortuosity, and low thermal conductivity offer low resistance to mass transfer, resulting in lower heat loss, high liquid entry pressure (LEP), and blocking the liquid from entering the membrane pores.[10,19] LEP or wetting pressure is the minimum pressure required for the feed solution to penetrate the membrane pores and can be estimated, as expressed in Equation (1):

$$ \text{LEP} = \frac{-2B\gamma_1 \cos \theta_a}{r_{\max}} \qquad (1) $$

$$ \cos \theta_a = \frac{r(\gamma_{SG} - \gamma_{SL})}{\gamma_{LG}} \qquad (2) $$

$$ \cos \theta_a = f_s \left[ \frac{(\gamma_{SG} - \gamma_{SL})}{\gamma_{LG}} + 1 \right] - 1 \qquad (3) $$

where B is the pore geometry coefficient (ranging from 0.4 to 0.85 for non-cylindrical pores,[20] γ₁ is the liquid surface tension, rmax is the maximum membrane pore size. θa is the apparent contact angle for the rough surface, which is a function of surface chemical characteristics and physical morphology, as expressed by the Wenzel model (Equation 2) for completely wet surfaces and Cassie-Baxter model (Equation 3) for unwetted surfaces due to the presence of entrapped air. R is the surface roughness factor and fs is the area of the solid-liquid interface (0 ≤ fs ≤ 1).[14] γSG,
<table><caption>**Table 1.** Desired characteristics of MD membranes.</caption><thead><tr><td>Characteristics</td><td>Recommended values</td><td>Reference</td></tr></thead><tbody><tr><td>Pore size</td><td>0.1-0.6 µm</td><td>[21]</td></tr><tr><td>Porosity</td><td>70-90%</td><td>[22-24]</td></tr><tr><td>Thickness</td><td>30-60 µm</td><td>[21]</td></tr><tr><td>Liquid Entry Pressure</td><td>&lt; 250 kPa</td><td>[19, 23]</td></tr><tr><td>Water contact angle</td><td>&gt; 90°</td><td>[7]</td></tr><tr><td>Thermal conductivity</td><td>Low</td><td>[10]</td></tr><tr><td>Surface energy</td><td>&gt; 1 N/m²</td><td>[7, 10]</td></tr><tr><td>Wetting</td><td>Low</td><td>[7]</td></tr></tbody></table>

$\gamma_{SL}$ and $\gamma_{LG}$, are the interfacial tensions at solid-gas, solid-liquid, and liquid-gas interfaces, respectively.

LEP should be as high as possible to avoid membrane wetting, prolonging the performance stability of the membranes. As can be seen from Equation (1), LEP is highly influenced by the size of pore and hydrophobicity, in which small pore size and high hydrophobicity promote large LEP. In order to maximize the permeate flux and the filtration efficiency these two parameters must be optimized. If the pore size is too small, permeate flux decreases while too large a pore size can cause wetting. Generally, hydrophobic surfaces are used in MD to inhibit the diffusion of water molecules from the feed into the membrane pores. In **Table 1**, the essential characteristics of the MD membrane are summarized.

Besides the essential characteristics listed above, the membrane is required to possess some surface features so that it can maintain its stability when being used with feed containing different contaminants. In **Table 2**, the recommended surface characteristics for controlling contaminants normally found in different feed sources are listed. These surfaces are classified into hydrophobic, oleophobic, and omniphobic surfaces based on their contact angle with water and oils, as graphically presented in **Figure 2**.

In **Figure 2**, it is seen that surfaces with water contact angle (WCA) <90° are hydrophilic, while surfaces with WCA>90° and WCA>150° are hydrophobic and superhydrophobic, respectively. Oleophobic surfaces have contact angles greater than 90° upon interaction with oils and alike. Such surfaces are highly applica-

ble in water treatment regards oil and gas industries where oil contaminates the membrane surface, leading to a decrease in flux. Omniphobic and amphiphobic surfaces have the characteristics of hydrophobic and oleophobic surfaces and are generally desired in industrial applications for MD desalination.

For the MD process, hydrophobic polymers such as poly(vinylidene difluoride) (PVDF), poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP), and polytetrafluoroethylene (PTFE) are widely utilized as materials for membrane fabrication. Polymeric MD membranes can be prepared by four methods: phase inversion, stretching, sintering, and electrospinning.[10] Commercial membranes used in MD are fabricated mainly via phase inversion. This fabrication technique primarily involves demixing a homogeneous polymer solution forming a polymer-rich phase that consequently precipitates into a porous membrane. It is a rapid and easy approach for preparing large-sized membranes, making it common for membrane fabrication.[25,26] Despite the popularity, the tight porous structure of phase inversion membranes gives a drawback in MD. Thus, electrospinning has recently gained interest in MD due to its versatility in producing nonwoven nanofibrous and porous hydrophobic polymer membranes. **Table 3** shows the performance of phase inversion-fabricated membranes and ENMs.

As presented in **Table 4**, the two techniques have different advantages and disadvantages. The membrane in the phase inversion has a more compact and dense structure, providing a more stable pore shape and higher LEP with better anti-wetting characteristics.[14] The membrane has good mechanical stability that is advantageous in resisting pore deformation. However, its dense membrane structure and tiny pore size can increase mass transfer resistance for the permeate vapor, thereby lowering the permeate flux.[8] In contrast, ENM has a looser structure and is more porous, providing less mass transfer resistance and hence higher flux. It is acknowledged that due to its poor interaction among the fibers, fiber slipping during operation arises, causing deformation of the ENM porous structures. The nonwelded fibers of ENM are prone to peeling by the rapid feed and permeate flow that can cause fast membrane wetting and an instant decline of water flux. In the long run, mechanical stability becomes a big challenge for ENMs. As observed in **Table 3**, ENMs demonstrate much higher flux owing to their larger pore size and

<table><caption>**Table 2.** Suggested surface properties for controlling different contaminants.</caption><thead><tr><td>Contaminants</td><td>Feed source/application</td><td>Problem</td><td>Suggested surface property</td></tr></thead><tbody><tr><td>Inorganic salt</td><td>RO brine<br>High-salt wastewater<br>Shale gas produced water</td><td>Scaling can lead to a reduction in permeability and membrane wetting</td><td>Superhydrophobic and small sliding angle (&lt;10°)</td></tr><tr><td>Oils</td><td>Shale gas produced water<br>Industrial wastewater</td><td>Pore blockage causes a decrease in water flux<br>Oils with low surface tension can lower membrane LEP and enhance membrane wetting</td><td>Janus membrane and omniphobic membrane (combined layers of hydrophilic surface and hydrophobic surface)</td></tr><tr><td>Surfactants (amphiphilic contaminants)</td><td>Wastewater from the surfactant using industry</td><td>Reduction of membrane pores leads to flux decline</td><td>Omniphobic membrane</td></tr><tr><td>Natural organic matter</td><td>Surface water</td><td>High affinity of organic compounds to membrane surface causes pore blockage and flux reduction</td><td>Janus membrane and omniphobic membrane</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Different surface types according to water and oil contact angles.</figcaption></figure>

porosity.[14] Incorporating the stable welded structure of phase inversion membrane into the fibrous and porous structure of ENM can address such difficulty in ENM. This strategy combines the advantages of the two membranes, resulting in efficient membrane performance. Compared to the nonwelded ENM, a welded

nanofibrous membrane can have longer sustained performance for up to a week due to its durable structure.[27] Additionally, compared to the commercial phase inversion membrane, it has higher permeate flux and water recovery due to its large pore size and high porosity. Thus, maintaining its highly porous structure

<table><caption>**Table 3.** Characteristics and DCMD performances of MD membranes prepared using phase inversion and electrospinning.</caption><thead><tr><th rowspan="2">Membrane material</th><th colspan="5">Membrane physical characteristics</th><th colspan="2">Membrane performance</th><th rowspan="2">Reference</th></tr><tr><th>WCA [°]</th><th>PS<sub>ave</sub> [µm]</th><th>P [%]</th><th>δ [µm]</th><th>LEP [kPa]</th><th>Feed solution</th><th>ΔT [°C]</th><th>Flux [LMH]</th></tr></thead><tbody><tr><td colspan="9"><strong>Phase inversion</strong></td></tr><tr><td>PVDF<sup>a)</sup></td><td>118</td><td>0.45</td><td>69.33</td><td>100</td><td>117</td><td>Dye wastewater</td><td>≈50</td><td>22.4</td><td>[28]</td></tr><tr><td>PVDF<sup>a)</sup></td><td>117</td><td>0.45</td><td>84.2</td><td>89</td><td>118</td><td>Shale gas wastewater</td><td>40</td><td>19.6</td><td>[29]</td></tr><tr><td>PVDF<sup>a)</sup></td><td>118</td><td>0.22</td><td>75</td><td>125</td><td>204</td><td>Anaerobic digestate</td><td>40</td><td>17.5</td><td>[24]</td></tr><tr><td>PVDF<sup>a)</sup></td><td>131</td><td>0.22</td><td>70.3</td><td>107</td><td>213</td><td>RO brine</td><td>40</td><td>5.24</td><td>[30]</td></tr><tr><td>PTFE/Pp<sup>b)</sup></td><td>137</td><td>0.22</td><td>81.22</td><td>107</td><td>148</td><td>Dye wastewater</td><td>≈50</td><td>32.3</td><td>[28]</td></tr><tr><td>PVDF / graphene</td><td>87.2</td><td>0.11</td><td>84.7</td><td>88.1</td><td>389</td><td>RO brine from CSG</td><td>40<sup>c)</sup></td><td>20.9</td><td>[31]</td></tr><tr><td>PDMS / PVDF / SiO<sub>2</sub> NP</td><td>>160</td><td>0.17</td><td>67.23</td><td>44</td><td>52</td><td>Mixed brine solutions with humic acid</td><td>80 °C feed, negative pressure<sup>d)</sup></td><td>12.2</td><td>[32]</td></tr><tr><td>PVDF / GO-ODA</td><td>146</td><td>0.057</td><td>60.5</td><td>85</td><td>185</td><td>Saline water</td><td>65<sup>c)</sup></td><td>16.7</td><td>[33]</td></tr><tr><td>PVDF / rGO</td><td>59</td><td>0.2</td><td>61</td><td>26</td><td>—</td><td>Saline water</td><td>60<sup>c)</sup></td><td>7</td><td>[34]</td></tr><tr><td>Mullite HFM / FAS</td><td>139</td><td>0.31</td><td>43</td><td>—</td><td>110</td><td>Saline water</td><td>50</td><td>11.8</td><td>[35]</td></tr><tr><td colspan="9"><strong>Electrospinning</strong></td></tr><tr><td>PVDF</td><td>160</td><td>0.81</td><td>86</td><td>150</td><td>187</td><td>RO brine</td><td>40</td><td>15.3</td><td>[30]</td></tr><tr><td>PVDF</td><td>150</td><td>0.43</td><td>86.5</td><td>112</td><td>121</td><td>Heavy metal-contaminated water</td><td>53</td><td>23</td><td>[5]</td></tr><tr><td>PVDF-HFP</td><td>155</td><td>0.62</td><td>85</td><td>232</td><td>160</td><td>Desulfurization wastewater</td><td>40</td><td>15</td><td>[36]</td></tr><tr><td>PVDF-HFP</td><td>139</td><td>0.58</td><td>87.4</td><td>104</td><td>93</td><td>Dye wastewater</td><td>≈50</td><td>37.6</td><td>[28]</td></tr><tr><td>PVDF-HFP</td><td>162</td><td>0.39</td><td>—</td><td>50</td><td>177</td><td>Hypersaline wastewater</td><td>40</td><td>40</td><td>[37]</td></tr><tr><td>PVDF/PPO</td><td>136</td><td>0.6</td><td>83.3</td><td>5</td><td>126</td><td>Shale gas wastewater</td><td>40</td><td>15.7</td><td>[29]</td></tr><tr><td>PVDF/PTFE</td><td>122</td><td>—</td><td>—</td><td>271</td><td>80</td><td>Geothermal water</td><td>45</td><td>27.7</td><td>[38]</td></tr><tr><td>PU/PDMS</td><td>165</td><td>4.24</td><td>—</td><td>95</td><td>12.2</td><td>Saline water</td><td>40</td><td>36.59</td><td>[39]</td></tr></tbody></table>

<footnote>a) Commercial membranes under the trade name “Durapore® Merck Millipore”; b) Commercial membranes under the trade name “TF200 Pall Gelman”; c) AGMD; d) VMD. (PS<sub>ave</sub>) average pore size, (P) porosity, (δ) membrane thickness, (ΔT) temperature difference between feed and permeate.</footnote>
<table><caption>**Table 4.** Comparison of membranes fabricated from phase inversion and electrospinning method.</caption><thead><tr><th></th><th>Phase Inversion</th><th>Electrospinning</th></tr></thead><tbody><tr><td>Morphology</td><td></td><td></td></tr><tr><td>Porosity</td><td>65 – 85%</td><td>80–90%</td></tr><tr><td>Pore size</td><td>0.20 – 0.45 µm</td><td>0.4 to 0.85 µm</td></tr><tr><td>Scaffold ability</td><td>No</td><td>Yes</td></tr><tr><td>Membrane configuration</td><td>Self-stand flat sheet and hollow fiber membrane</td><td>Non-self-stand flat sheet</td></tr><tr><td>Surface modification</td><td>Yes</td><td>Yes</td></tr><tr><td>Inertia modification</td><td>Limited by the composition of the cast solution</td><td>Various options</td></tr><tr><td>Production cost</td><td>Lower</td><td>Higher</td></tr><tr><td>Key challenges</td><td>Very small pore size</td><td>Pore deformation</td></tr></tbody></table>

Inset images were Reproduced with permission.[40] Copyright 2019, Elsevier Science Ltd.

while having a robust interfiber network is the key for ENM to outperform the phase inversion membranes in terms of water quality and quantity.

Despite the distinct benefits of phase inversion and electrospun nanofibrous membranes, they both remain vulnerable to fouling, wetting, and scaling when encountering complex water contaminants. Enhancing the utility of these membranes in MD for processing challenging saline solutions necessitates a high level of resistance to diverse water pollutants. Surface modification techniques, involving innovative surface designs and chemical treatments, have been extensively investigated to achieve the preferred membrane surface characteristics, as illustrated in Figure 2. In this context, ENMs hold an advantage over phase inversion membranes due to the flexibility of the electrospinning process. This process readily incorporates modifiers such as nanomaterials and silane reagents at various stages of membrane production, including both before and after spinning. The subsequent section delves into various strategies employed to instill the desired anti-wetting, anti-fouling, and anti-scaling properties in ENMs.

### 3. Surface Modification of Electrospun Nanofiber Membrane (ENM) for MD

To tailor the properties of ENM for MD application, lowering surface energy and increasing surface roughness is the main direction of the current research. Recent research works employ various techniques such as polymer blending, nanoparticle doping, surface fluorination, and silanization to develop ENMs with tunable properties.[41–46] As illustrated in Figure 3, these techniques can be grouped into two categories: in-situ electrospinning approach and post-electrospinning approach. The in-situ electrospinning approach refers to the adjustment of electrospinning

parameters, such as the composition and concentration of the electrospinning solutions, to obtain the design of the final fibers. In the post-electrospinning approach, ENMs are first prepared and then modified. As such, this approach involves two or more steps for preparing ENMs.

#### 3.1. In Situ Electrospinning Approach

Herein, different additives are incorporated into the polymer solution to form a mixture, which is subsequently electrospun or co-electrospun to form nanofibrous membranes. These additives can be hydrophobic polymers, surfactants, inorganic nanoparticles, and carbon-based nanomaterials, as shown in Table 5. The base polymer and the additives are dissolved in one solution. Hence, both components must have compatible solubility to obtain homogenous jets and fibers. The added component can alter the rheological properties of the polymer solution and can affect both the size and mechanical stability of the fibers, the porosity of the fabric, and the performance of the membrane. Thus, the amount of the additive must be optimized.

##### 3.1.1. Addition of Additives in the Spinning Solution

Blending another polymer into a matrix polymer is one of the easiest and thus most popular options to enhance hydrophobic properties. Due to their higher mechanical strength, lower thermal conductivity, and adequate processability compared to other polymers used in electrospinning, PVDF and PVDF-HFP are commonly used as based polymers.[10] However, the hydrophobicity of these materials is not sufficient for MD processes. Therefore, polymers having higher hydrophobicity, such as PTFE and fluorinated polyurethane (FPU) are added to the electrospinning base polymer solutions. From an economic perspective, the
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Two types of ENM surface enhancements: in-situ electrospinning approach, and post-electrospinning approach.</figcaption></figure>

amount of additive needs to be minimized for maximum benefit but it should also be maximized for a notable and desirable enhancement; optimized concentration is important. Dong et al.,[47] added PTFE micro-powders in PVDF and found that the optimized PTFE concentration that can create a superhydrophobic surface was 12 wt.% owing to the formation of rough particles on the nanofibers, as illustrated in **Figure 4c**. At such a PTFE concentration, it is seen that the hierarchical micro/nanostructure on the surface improved surface roughness (Ra = 472 nm), as shown in Figure 4d.[47] Likewise, a blended FPU and polysulfone (PSF) membrane can provide a superhydrophobic surface and a notable increase in the porosity, thickness, fiber diameter, and mechanical strength compared to the neat PSF membrane.[41] Adding too much FPU (> 6 wt.%) resulted in an enlarged fiber diameter, void volume fraction, and inter-fiber space. Such large inter-fiber space inevitably caused a decrease in LEP, consequently devastating the performance of the membrane due to wetting. In addition, overuse of FPU additives can cause poor fiber alignment, negatively affecting the mechanical properties of the membrane.[41]

Surfactant additives, i.e., tetrabutylammonium chloride (TBAC) and benzyltriethylammonium chloride (BTAC) were used for altering the external morphology of fibers and changing the internal structure of the membrane fabric.[40,50,54] For instance, Li et al. [40] added TBAC, a cationic surfactant, into PVDF to increase the electrical conductivity of the polymer solution and create a repulsion effect on the jet streams.[40] Subsequently, the jets are split, forming a branched structure and a tree-like nanofibrous membrane, as illustrated in **Figure 5**. The spreading

branches decreased the size of the pores and increased LEP, which is beneficial for MD. It was also discovered that when TBAC was added to fluorinated acrylate (FA) copolymer in PVDF, the hydrophobic properties were further enhanced: WCA increased from 119° to 135°. The addition of FA resulted in additional repulsive electric forces. Thus, more hydrophobic branches were generated. The final PVDF/TBAC/FA membrane (60 µm) was also thinner compared to the pure PVDF (100 µm) because the ionic surfactants increased the charge density of the solutions, leading to the extension of jets, forming thinner fibers. The developed superhydrophobic membrane had a remarkably high flux of 54.39 LMH when tested with 3.5 wt% NaCl.[40]

Metal oxide nanoparticles (NPs): ZnO, SiO₂, TiO₂, along with carbon-based materials including graphene oxide (GO), carbon nanotubes (CNT), activated carbon, and carbon sphere nanoparticles, are utilized as fillers to roughen membrane surfaces, thereby enhancing their hydrophobic characteristics.[20,48,49,51,53,55–57] For instance, electrospinning a mixture of ZnO NPs with PVDF in DMAc/acetone solvent leads to the formation of an ENM with randomly distributed ZnO NPs on the surface. Incorporating ZnO NPs into the ENM has roughened its surface, which significantly enhances its resistance to wetting.[20]

In addition to improving roughness and hydrophobicity, carbon-based nanomaterials can enhance the thermal and mechanical strength of the fibers. Research has shown that integrating graphene oxide (GO) and carbon nanotubes (CNT) alters the electrical conductivity of the spinning solution. This change aids in producing finer fibers, more compact membranes, and
<table><caption>**Table 5.** The physical properties and DCMD performances against 3.5 wt% NaCl solution of modified ENM using the in-situ electrospinning approach.</caption><thead><tr><th colspan="2">Membrane composition</th><th colspan="9">Membrane physical properties</th><th colspan="2">Membrane performance</th><th colspan="2">Ref.</th></tr><tr><th>Polymer matrix</th><th>Additive</th><th>WCA [°]</th><th>PS<sub>ave</sub> [µm]</th><th>P [%]</th><th>δ [µm]</th><th>σ [nm]</th><th>LEP [kPa]</th><th>ΔT [°C]</th><th>Flux [LMH]</th><th>t<sub>s</sub> [h]</th><th></th><th></th><th></th></tr></thead><tbody><tr><td>Polymer</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PVDF</td><td>PTFE</td><td>151.2</td><td>0.49</td><td>69</td><td>98</td><td>580</td><td>137</td><td>60 °C feed, 9 kPa<sup>a)</sup></td><td>18.5</td><td>15</td><td>[47]</td><td></td><td></td></tr><tr><td>PSF</td><td>FPU</td><td>155</td><td>—</td><td>95.4</td><td>293</td><td>440</td><td>108</td><td>60</td><td>53.8<sup>b)</sup></td><td>32</td><td>—</td><td>[41]</td><td></td></tr><tr><td>Nanoparticle</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PVDF</td><td>ZnO NP</td><td>145.2</td><td>0.42</td><td>—</td><td>—</td><td>200</td><td>115</td><td>40</td><td>13.3<sup>c)</sup></td><td>10</td><td>[20]</td><td></td><td></td></tr><tr><td>PVDF</td><td>SiO<sub>2</sub> NP</td><td>160.5</td><td>0.26</td><td>77</td><td>99</td><td>340</td><td>195</td><td>60 °C feed, 9 kPa<sup>a)</sup></td><td>31.5</td><td>15</td><td>[48]</td><td></td><td></td></tr><tr><td>PVDF-HFP</td><td>SiO<sub>2</sub> NP</td><td>154</td><td>1.34</td><td>82.3</td><td>393</td><td>175</td><td>87.8</td><td>60</td><td>48.6</td><td>—</td><td>[49]</td><td></td><td></td></tr><tr><td>Surfactant</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PVDF</td><td>TBAC/ FA</td><td>135</td><td>0.27</td><td>71.28</td><td>60</td><td>—</td><td>220</td><td>40</td><td>54.39</td><td>9</td><td>[40]</td><td></td><td></td></tr><tr><td>PVDF-HFP</td><td>BTAC</td><td>≈150</td><td>0.42</td><td>—</td><td>—</td><td>197</td><td>—</td><td>40</td><td>≈15<sup>d)</sup></td><td>8</td><td>[50]</td><td></td><td></td></tr><tr><td colspan="13">Carbon-based nanofillers</td></tr><tr><td>PVDF</td><td>CO</td><td>143.9</td><td>—</td><td>90</td><td>138.6</td><td>143.7</td><td>61</td><td>60</td><td>40<sup>b)</sup></td><td>10</td><td>[51]</td><td></td><td></td></tr><tr><td>PVDF</td><td>MWCNT</td><td>141.5</td><td>—</td><td>92.3</td><td>98.4</td><td>124.3</td><td>35</td><td>—</td><td>54<sup>b)</sup></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td colspan="13">Coaxial electrospinning</td></tr><tr><td>PVDF-HFP</td><td>SiA</td><td>167.7</td><td>0.39</td><td>89.7</td><td>140.5</td><td>86.18</td><td>191</td><td>—</td><td>14.5</td><td>72</td><td>[52]</td><td></td><td></td></tr><tr><td>PVDF-HFP</td><td>PVDF-HFP/TiO<sub>2</sub></td><td>149.9</td><td>0.6</td><td>90.4</td><td>99</td><td>300</td><td>95.1</td><td>—</td><td>40<sup>e)</sup></td><td>168</td><td>[53]</td><td></td><td></td></tr></tbody></table>

<footnote>a) VMD configuration; b) 3.0 wt% NaCl as feed solution; c) 3.5 wt% NaCl + 1 wt% CaSO<sub>4</sub> as feed solution; d) 1 M NaCl + 0.3 mM SDS as feed solution; e) 7.0 wt% NaCl as feed solution; f) fiber diameter.</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a,c) SEM and b,d) AFM surface images of a,b) neat PVDF and c,d) PVDF/PTFE ENM. Reproduced with permission.[47] Copyright 2014, Elsevier Science Ltd.</figcaption></figure>

fewer voids, leading to a compact inter-fiber arrangement.[51]
While these additives generally enhance the mechanical integrity
of ENMs, they also increase the mass transfer resistance, which
in turn, reduces the flow of permeate. Optimal performance, in-
cluding the highest permeate flow and superior salt rejection,
was observed at a nanofiller concentration of 0.1 wt.%. Addition-
ally, leveraging the inherent thermal conductivity of carbon-based
additives has proven effective in establishing a stronger thermal
gradient within the membrane. For instance, embedding carbon
sphere nanoparticles into a PVDF-HFP not only markedly en-
hances surface roughness of the ENM but also improves thermal
differentiation, thereby increasing water vapor transport.[55]

The addition of nanofillers is a relatively facile one-step
method for obtaining enhanced roughness on the membrane
surface. As the fillers are incorporated into the spinning solu-
tion, they interfere with the fiber formation resulting in individ-
ual fibers with nano-sized grooves. This distinct feature of fibers
collectively creates an ENM with remarkable rough surface mor-
phology. Nevertheless, the tendency of nanofillers to accumulate
and agglomerate poses challenges, necessitating their uniform

distribution through ultrasonication or mechanical stirring be-
fore electrospinning. Particle agglomeration can lead to reduced
breaking elongation of the membrane, signifying the brittleness
of the polymer-nanofiller bonds.[48] Excessive nanofiller concen-
tration has an adverse effect that can reduce inter-fiber space and
decrease membrane permeability.[49] Optimized nanofiller con-
centration is vital to maximize the enhancement of the mem-
brane and avoid detrimental effects.

3.1.2. *Co-Axial Electrospinning*

In contrast to the above-mentioned techniques that modify the
compositions of polymer solutions in one pot, coaxial electro-
spinning can simultaneously electrospin the polymer matrix and
the additive in separate nozzles, producing nanofibers with core
and sheath layers, as illustrated in **Figure 6b**. This technique
ensures the appropriate dispersion and suitable interfacial ad-
hesion between the nanomaterial additive and polymer matrix,
which makes it advantageous over the physical blending and

<figure><img src="image_7.png" /><figcaption>**Figure 5.** FESEM images of ENM: a) pure PVDF, b) PVDF/TBAC, c) PVDF/TBAC/1% FA, and d) PVDF/TBAC/3% FA. Reproduced with permission.[40]
Copyright 2019, Elsevier Science Ltd.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** TEM images (a,b) and AFM images (c,d) of neat PVDF ENM (a,c) and co-axially electrospun PVDF/SiA membrane (b,d). Reproduced with permission.[52] Copyright 2021, Elsevier Science Ltd.</figcaption></figure>

in-situ sol-gel preparation.[53] Coaxial electrospinning of silica aerogel (SiA) as sheath and PVDF-HFP as core can remarkably improve the mean surface roughness from 284.65 nm to 451.15 nm, as shown in Figure 6c,d, increasing WCA from 143.8° to 167.7°, thereby resulting in a superhydrophobic surface. This method also decreased the pore size from 0.58 µm to 0.39 µm and increased LEP from 145 kPa to 191 kPa.[52] The ENM with the core/sheath structure showed a very stable performance of 10.45 LMH and 99.99% salt rejection for one month without any notable salt deposits on the surface after a long-time of usage. The excellent performance of the membrane is attributed to its 3D-hierarchically structured surface with a very low sliding angle of 3.2° that inhibits foulant adhesion on the surface. The low thermal conductivity of the membrane lessened the conductive heat losses during the MD process.[52] Similar enhancement was also observed for the coaxial electrospinning of PVDF-HFP/TiO₂ as a sheath and PVDF-HFP as a core. The membranes showed excellent water-repellent properties as they contained nanofibers with protrusions indicating the clusters of TiO₂ on the fibers' surface. These clusters contributed to increased surface roughness, resulting in a superhydrophobic property. Interestingly, the presence of particles did not significantly change the pore size distribution of the membrane, indicating that the modification was successfully done at the surface, while still preserving the

porous internal structure of the membrane. The modified membrane exhibited an outstanding anti-wetting property, which was particularly noticeable when compared to the early deterioration and wetting of the unmodified membrane.[53]

## 3.2. Post-Electrospinning Approach

In the post-electrospinning approach, ENM is first fabricated, and then new functional groups are introduced on the surface either through physical interaction or chemical bonding. Through this follow-up technique, the surface of the pre-constructed ENMs turns out to be rougher with multilevel re-entrant structures similar to a Cassie-Baxter surface model.[58,59] The post-electrospinning approach has the advantage of creating stronger binding affinity and better stability of the grafted layer on the membrane when subjected to rigorous conditions.[60] Moreover, this method can produce diverse surface types such as omniphobic[61–64] and underwater oleophobic surfaces.[60] This approach can be categorized into two types based on the interactions formed between the substrate and the added functional layer: physical and chemical. In Figure 7, an overview of the different post-electrospinning techniques for ENM surface enhancements is shown. Taking advantage of the utilization of pre-fabricated
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Post-electrospinning approaches for ENM surface enhancement.</figcaption></figure>

membranes, this technique can be extended to surface enhancements of other non-electrospun polymeric membranes made from phase inversion, stretching, and sintering.

### 3.2.1. Physical Surface Deposition

Physical surface deposition involves electrostatic, van der Waals forces, and hydrogen bonding interactions between the polymer substrate and the additional functional layer. As listed in **Table 6**, electrospraying, physical vapor deposition, and dip-coating are

examples of physical surface modifications used for MD applications.

Electrospinning another polymer layer on a pre-fabricated ENM can produce superhydrophobic and omniphobic features for membranes. It provides additional surface features and geometries leading to the desired anti-wetting and anti-scaling properties. Du et al. [29] electrospun poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) on top of the PVDF membrane and found that WCA increased from 117° to 140° due to the inherent hydrophobic characteristic of PPO. However, AFM analysis showed a smoother surface at the macroscale for the PPO-PVDF membranes due to the smooth appearance of the individual PPO

<table><caption>**Table 6.** The physical properties and DCMD performances of modified ENM using the physical surface coating.</caption><thead><tr><th colspan="2">Membrane composition</th><th colspan="6">Membrane physical properties</th><th colspan="4">Membrane performance</th><th>Reference</th></tr><tr><th>Polymer matrix</th><th>Coating</th><th>WCA [°]</th><th>PS<sub>ave</sub> [µm]</th><th>P [%]</th><th>δ [µm]</th><th>ø [nm]</th><th>LEP [kPa]</th><th>Feed comp.</th><th>ΔT [°C]</th><th>Flux [LMH]</th><th>t<sub>s</sub> [h]</th><th></th></tr></thead><tbody><tr><td colspan="13"><strong>Electrospinning</strong></td></tr><tr><td>PVDF</td><td>PPO</td><td>≈137</td><td>0.7</td><td>88.9</td><td>—</td><td>1800</td><td>126</td><td>1 wt% NaCl</td><td>40</td><td>17</td><td>100</td><td>[29]</td></tr><tr><td colspan="13"><strong>ElectrospRAY</strong></td></tr><tr><td>PVDF-HFP</td><td>SiNP</td><td>154.5</td><td>0.62</td><td>85</td><td>232.4</td><td>201.1</td><td>160</td><td>8400 ppm Ca<sup>2+</sup>, 8500 ppm Mg<sup>2+</sup> and 18,874 ppm SO<sub>4</sub><sup>2-</sup></td><td>40</td><td>15</td><td>30</td><td>[36]</td></tr><tr><td>F-POSS/ PVDF-HFP</td><td>CB/ F-POSS/ PVDF-HFP</td><td>162</td><td>0.39</td><td>—</td><td>50</td><td>≈362</td><td>176.8</td><td>14 mM CaCl<sub>2</sub>, 14 mM Na<sub>2</sub>SO<sub>4</sub></td><td>40</td><td>40</td><td>—</td><td>[37]</td></tr><tr><td>PVDF</td><td>TiO<sub>2</sub> NP</td><td>157</td><td>0.52</td><td>89.2</td><td>110</td><td>—</td><td>—</td><td>10 mg L<sup>-1</sup> HA, 10 mg L<sup>-1</sup> SA and 5 mg L<sup>-1</sup> BSA in real seawater</td><td>47</td><td>32.26</td><td>24</td><td>[65]</td></tr><tr><td>PVDF-HFP</td><td>PDMS/PVDF</td><td>170</td><td>0.47</td><td>85.8</td><td>91.1</td><td>—</td><td>129.5</td><td>3.5 wt% NaCl</td><td>40</td><td>34.6</td><td>168</td><td>[66]</td></tr><tr><td colspan="13"><strong>Physical vapor deposition</strong></td></tr><tr><td>PVDF-HFP</td><td>FDTS</td><td>154.1<sup>a)</sup></td><td>0.55</td><td>78</td><td>112</td><td>133</td><td>225</td><td>3.5 wt% NaCl + 0.4 mM SDS</td><td>50</td><td>10.5</td><td>8</td><td>[61]</td></tr><tr><td colspan="13"><strong>Dip coating</strong></td></tr><tr><td>PVDF</td><td>Teflon AF2400</td><td>148.41<sup>b)</sup></td><td>—</td><td>80.8</td><td>—</td><td>—</td><td>70</td><td>0.6 M NaCl + 0.4 mM SDS</td><td>50</td><td>27</td><td>90</td><td>[62]</td></tr><tr><td>PVDF</td><td>PDA/PEI</td><td>25.7<sup>c)</sup></td><td>0.017</td><td>—</td><td>—</td><td>—</td><td>—</td><td>3.5 wt% NaCl</td><td>40</td><td>≈13</td><td>24</td><td>[60]</td></tr><tr><td>PVDF-HFP</td><td>FDTES</td><td>138<sup>d)</sup></td><td>0.84</td><td>78</td><td>103</td><td>94</td><td>160</td><td>3.5 wt% NaCl</td><td>40</td><td>19</td><td>5</td><td>[58]</td></tr></tbody></table>

<footnote><sup>a)</sup> 146.0° for MI; <sup>b)</sup> 143.13° for glycerol; 134.16° for EG; 117.02° for SO; 112.67° for tol; 101.75° for etOH; <sup>c)</sup> underwater oleophobic; <sup>d)</sup> 127° for oil.</footnote>
<figure><img src="image_10.png" /><figcaption>**Figure 8.** FESEM-EDX surface images: a) neat PVDF ENM, and b) PVDF/PPO ENM after DCMD experiment with simulated shale gas wastewater. The yellow lines are the profiles of the scaling crystals accumulated on the membrane surface. Reproduced with permission.[29] Copyright 2019, American Chemical Society.</figcaption></figure>

nanofibers. Despite contradicting results of improved hydrophobicity and reduced surface roughness, the PPO-coated PVDF membrane showed excellent ant-scaling properties against a simulated shale gas water containing NaCl, CaCl₂, KCl, NH₄Cl, MgCl₂, FeCl₃, NaHCO₃, Na₂SO₄, and SDS.[29] As illustrated in the FESEM-EDX surface images (Figure 8), the deposition of crystal salts on the modified membranes after MD experiments was much less compared to the unmodified membranes. In addition, the PPO/PVDF membrane maintained its flux of 17 LMH for 100 h against a 1 wt% NaCl solution.

Electrospraying is another electrohydrodynamic technique like electrospinning that is based on liquid atomization by electrical forces to form fine droplets, subsequently depositing on the substrate. Versatile materials ranging from NPs, polymers, and the combination of NPs and polymers can be used as spraying solutions to form micro and nanobeads on the substrate. In the context of solution preparation, the addition of nanomaterials into the solution is similar to that in the in-situ electrospinning approach. However, in this technique, the nanomaterial-containing solution is intended to form droplets or spherical structures, instead of the typical cylindrical fibers in the electrospinning technique. As a result, a membrane with enhanced roughness and hierarchical structure is achieved.[36,37] Wu et al.[36] electrosprayed SiO₂ NP on a PVDF-HFP ENM substrate. The prepared membranes showed a rough surface with

evenly distributed and uniform-sized microbeads connected by thin nanofibers. Such morphology resulted in a lower sliding angle, implicating the “lotus-leaf effect”, which accounted for the notable decrease in the direct contact area between the water surface and the membrane surface. The membrane had a stable permeate flux and good anti-fouling properties against humic acid and excellent anti-scaling properties against inorganic scalants such as 8400 ppm Ca²⁺, 8500 ppm Mg²⁺, and 18 874 ppm SO₄²⁻.[36]

Nanoparticles can also be functionalized with organosilanes and afterward deposited on the ENM surface through electrospraying. In the study of Deka et al.,[6] organosilane-functionalized ZnO NP was electrosprayed on organosilane-functionalized PVDF-HFP ENM, resulting in rough fiber strings with a layer of low-surface energy spherical shape particles. Such a hierarchical structure resulted in an omniphobic ENM with contact angles as high as 160°, 144.5°, 131.6°, and 129.5°, corresponding to water, saline, ethanol, and oil, respectively. The developed membrane had better wetting resistance against water, inorganic scalants, and organic foulants compared to a ZnO NP-containing membrane prepared using the pre-electrospinning approach that only had a superhydrophobic property with WCA of 145.2°.[20] This outcome was due to the presence of organosilane groups that are well known to have low-surface energy properties. The omniphobic membrane demonstrated anti-scaling
properties against highly saline water with contaminants such as CaCl₂ and sodium alginate. It is noted that both WCA and LEP before and after an 80 h operation against a feed containing 1 M NaCl and 0.1 mM SDS slightly changed from 161.6° to 158.3° and 187 kPa to 183 kPa, respectively, denoting the durability of the modified membrane against low surface tension pollutants.[6] Undoubtedly, electrospraying can provide the desired enhancement in membrane surface roughness, but it is important to consider the possibility of pore blockage caused by the sprayed particles depositing on the void spaces of the nanofibrous structure of ENM. Small microbeads on the surface are beneficial to obtain medium pore size, but large agglomerated microbeads would result in smaller pore size.

Vapor deposition and dip-coating are known surface modification strategies to coat a thin layer on different materials aside from polymeric membranes. They involve physical and chemical interactions. However, in this context, in the absence of cross-linking agents and surface activators, vapor deposition and dip-coating are considered to only have physical interactions between the modifier and the substrate. For example, 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) was vapor-deposited on PVDF-HFP, producing extended polysiloxane networks with long fluoroalkyl chains directed towards the surface, creating low surface energy and improved hydrophobicity.[61,62] Likewise, in the dip-coating of PVDF ENM in Teflon AF2400 solution, hydrophobic-hydrophobic interactions between the two layers created an omniphobic surface.[62] Further, a heat-pressed PVDF-HFP ENM was coated with organosilanes through dip-coating in FDTES solution, resulting in a highly rough amphiphobic ENM with superior hydrophobicity and oleophobicity with WCA of 138° and OCA of 127°, respectively. It is seen that the hydrophobic interactions between the fluoride-rich groups of FAS and that of PVDF-HFP bind both the polymer matrix and the coating.[58] Nevertheless, in dip-coating a loose-structured material like ENMs, it is possible that the modification could penetrate too deeply into the membrane's internal structure. Therefore, it is essential for the dip-coating process to precisely coat the individual nanofibers, ensuring the membrane's pores remain unblocked to preserve its overall porosity. This approach guarantees that the membrane's internal porous architecture is maintained, while simultaneously enhancing its surface with anti-wetting and anti-scaling properties.

### 3.2.2. Chemical Surface Deposition

Chemical surface deposition is associated with the formation of covalent bonds, making a strong connection between two layers. A good interfacial interaction between the substrate and the coating layer is a critical factor in sustaining the membrane performance for extended operation. Delamination and degradation of the coating layer during operation could result in pore blockage and induced membrane wetting, leading to the limited performance of the membrane. Examples of chemical surface deposition techniques used in ENM are fluoroalkyl silane treatment and plasma treatment. These techniques increased the surface resistance against various liquids and resulted in a longer MD performance.

**Fluoro Alkyl Silane Treatment:** Organosilane and organofluorine compounds with long alkyl chains and fluorine groups, respectively, are normally used to impart lower surface energy characteristics. The chemical bonds between the modifier and substrate are created via a grafting method using silane coupling agents like alkoxysilanes and fluoroalkyl silanes (FAS). Such coupling agents normally contain organoalkyl and hydrolyzable groups, including alkoxy and chlorine in their structure that can instantly form a chemical bond with the hydroxyl group. As shown in **Scheme 1**, hydroxyl groups on the substrate surface form Si-O bonds with the alkoxy groups, which can then form Si-O-Si with neighboring alkoxy groups creating a stable network. Listed in **Table 7** are the FAS reagents grafted onto ENMs and their effects on the physical properties and performance in MD.

In general, surfaces of the substrates are treated with strong hydroxides such as NaOH or functionalized with oxide nanoparticles (SiO₂ NP, ZnO NPs, and GO nanosheets) to attain as many hydroxyl groups as possible to link with the coupling agents. A widely used approach involves the in-situ development of nanoparticles directly on the fiber surface, followed by the application of the FAS reagent. For instance, SiO₂ NPs were synthesized using the sol-gel method on a cellulose acetate ENM, and then FAS was grafted onto these nanoparticles. This process endowed the ENM with remarkable resistance to surfactants, oil, and ethanol, outperforming both commercial and unmodified PVDF and PTFE membranes.[64] In another study, ZnO nanowires were grown on PVDF nanofiber membrane via hydrothermal method, producing a grass-like morphology on the fiber structure. This unique surface structure resulted in a superhydrophobic membrane surface, minimizing liquid contact and enhancing anti-wetting and anti-scaling properties.[67] The in-situ growth of the nanoparticles ensures their homogeneity on the nanofiber surface prior to the grafting of the FAS compound.

In addition, nanoparticle-nanofiber composite membranes produced from the in-situ electrospinning approach described in Section 3.1.1 can be used for further surface modification using FAS. For example, a PVDF/SiO₂ ENM was post-modified and grafted with FAS via dip-coating, resulting in a notable wetting resistance of the membrane surface. The dip-coating in this context involves chemical bond formation between the hydroxyl groups of SiO₂ NPs and the alkoxy groups of FAS, making this method to have better interfacial interaction between materials compared to the regular dip-coating described in Section 3.2.1. In **Figure 9**, the change in surface structure, as depicted by the hills and valleys in its 3D AFM image, is seen.[45] In this regard, the added nanomaterials provide dual functions: 1) provide enhanced roughness, and 2) as anchor points for the silane compound. The desired surface roughness and low surface energy are attained creating an enhanced resistance to different organic and inorganic compounds. Thus, such a technique commonly produces membranes with omniphobic surfaces. However, uniform distribution of such anchor points on the surface is indeed important to avoid irregularities on the membrane surface.

For the preparation of omniphobic or amphiphobic membranes, FAS is much desired. FAS can repel low-surface tension contaminants like surfactant, oil, and alcohol that are prevalent in hypersaline industrial wastewater. Xu et al.[63] grafted FAS on SiO₂ NP/PVDF ENMs creating an amphiphobic ENM with very high WCA and OCA of 166.9° and 134.5°, respectively. The
<figure><img src="image_11.png" /><figcaption>**Scheme 1.** Silanization reaction on the membrane surface.</figcaption></figure>

presence of nanoparticles and FAS groups primarily generated rough micro/nano surface structures having low surface energy, creating anti-wetting for water and oil. The PVDF/SiO₂ NP/FAS membrane exhibited stable flux and consistent salt rejection for 24 h against a feed containing diesel-in-water emulsion.[63] Similar enhanced resistance against low-surface tension surfactants like SDS and oil-containing feed was observed for a

fluorosilanized PDA-coated PVDF and a fluorosilanized PET membrane.[70,71] Omniphobic fluorinated membranes can repel many liquids (Figure 10a). Such membranes can prevent the precipitation of scalants. As illustrated in Figure 10b,c, the FAS-grafted-SiO₂ NP/PVDF-HFP exhibited anti-scaling properties, demonstrating a stable performance for 72 h against a saline feed containing NaCl, CaCl₂, and NaSO₄. Only the pristine PVDF

<table><caption>**Table 7.** The physical properties and DCMD performances of modified ENMs using fluoroalkyl silane with different alkyl groups.</caption><thead><tr><th colspan="2">Membrane composition</th><th colspan="4">Membrane physical properties</th><th colspan="4">Membrane performance</th><th rowspan="2">Reference</th></tr><tr><th>Polymer matrix</th><th>Activator</th><th>WCA [°]</th><th>PS<sub>ave</sub> [µm]</th><th>P [%]</th><th>δ [µm]</th><th>LEP [kPa]</th><th>Feed comp.</th><th>ΔT [°C]</th><th>Flux [LMH]</th><th>t<sub>s</sub> [h]</th></tr></thead><tbody><tr><td colspan="11"><strong>1H, 1H, 2H, 2H Perfluorodecyltriethoxysilane</strong></td></tr><tr><td>PVDF</td><td>NaOH/ etOH</td><td>162.3</td><td>0.4</td><td>—</td><td>—</td><td>160</td><td>3.5 wt% NaCl + 1 wt% CaSO<sub>4</sub></td><td>40</td><td>11.5</td><td>10</td><td>[20]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-Perfluorooctyltriethoxysilane</strong></td></tr><tr><td>PVDF</td><td>GO nanosheet</td><td>140.5</td><td>0.17</td><td>96.8</td><td>26</td><td>233.9</td><td>3.5 wt% NaCl</td><td>50 °C feed,<br>31.3 kPa<sup>g)</sup></td><td>36.4</td><td>60</td><td>[68]</td></tr><tr><td>PVDF</td><td>ZnO</td><td>150</td><td>0.31</td><td>91</td><td>22</td><td>19</td><td>3.5 wt% NaCl + 1.26 g L<sup>-1</sup> CaCl<sub>2</sub><br>+ 10 mg L<sup>-1</sup> BSA + 30 g L<sup>-1</sup><br>MgSO<sub>4</sub> + 10 mg L<sup>-1</sup> SDBS</td><td>40</td><td>15.7</td><td>60</td><td>[67]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-heptadecafluorodecylsilane</strong></td></tr><tr><td>PTFE/PVA/PAN</td><td>NaOH</td><td>166.9<sup>a)</sup></td><td>—</td><td>64</td><td>64.8</td><td>220</td><td>3.5% wt NaCl + 1000 ppm diesel<br>oil<br>3.5% wt NaCl + SDS<br>25% wt NaCl</td><td>60 °C feed,<br>0.03 MPa<sup>g)</sup></td><td>6.48</td><td>24</td><td>[63]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-Perfluorodecyltrichlorosilane</strong></td></tr><tr><td>CNF</td><td>SiO<sub>2</sub> NP</td><td>> 150<sup>b)</sup></td><td>1</td><td>90.2</td><td>52</td><td>58</td><td>1 M NaCl + 0.4 mM SDS</td><td>40</td><td>50.9</td><td>10</td><td>[64]</td></tr><tr><td>PVDF-HFP</td><td>SiO<sub>2</sub> NP</td><td>≈150<sup>c)</sup></td><td>0.42</td><td>—</td><td>—</td><td>—</td><td>1 M NaCl + 0.3 mM SDS</td><td>40</td><td>≈15</td><td>8</td><td>[50]</td></tr><tr><td>PVDF-HFP</td><td>SiO<sub>2</sub> NP</td><td>151.5<sup>d)</sup></td><td>0.35</td><td>80</td><td>113</td><td>223</td><td>3.5 wt% NaCl + 0.4 mM SDS</td><td>40</td><td>19.11</td><td>—</td><td>[69]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane</strong></td></tr><tr><td>PVDF</td><td>SiO<sub>2</sub> NP</td><td>154.6</td><td>0.27</td><td>77</td><td>98</td><td>143</td><td>3.5% wt NaCl</td><td>70 °C feed, 6<br>kPa<sup>g)</sup></td><td>11.5</td><td>22</td><td>[45]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane</strong></td></tr><tr><td>PVA</td><td>GA</td><td>158</td><td>0.46</td><td>82</td><td>100</td><td>179</td><td>3.5 wt% NaCl</td><td>60 °C feed, 9<br>kPa<sup>g)</sup></td><td>25.2</td><td>16</td><td>[16]</td></tr><tr><td colspan="11"><strong>1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane</strong></td></tr><tr><td>PA6</td><td>None</td><td>149<sup>e)</sup></td><td>0.177</td><td>73</td><td>—</td><td>220 × 10<sup>3</sup></td><td>2% w/v EG + 0.1 mM SDS</td><td>40</td><td>8.7</td><td>196</td><td>[44]</td></tr><tr><td>PVDF</td><td>PDA</td><td>170<sup>f)</sup></td><td>0.358</td><td>89</td><td>190</td><td>216</td><td>3.5 wt% NaCl + 0.5 mM SDS<br>3.5 wt% NaCl + 480 mg L<sup>-1</sup> MO /<br>water emulsion</td><td>40</td><td>17</td><td>6</td><td>[70]</td></tr></tbody></table>

<footnote>a) ≈125° for diesel oil; 130° for MO, 140° for EG, 140° for 0.2 mM SDS; b) 146.0° for MI; c) 130° for 3 mM SDS; 140° for MO; 110° for decane, 115° for etOH; d) 140.64° for MO, 119.59° for 4 mM SDS, 107.5° for etOH; e) 143.13° for glycerol; 134.16° for EG; 117.02° for SO; 112.67° for tol; 101.75° for etOH; f) 150° for 3.5 wt% NaCl + 0.25 mM SDS; 145° for MO; g) using VMD configuration.</footnote>
<figure><img src="image_12.png" /><figcaption>**Figure 9.** 3D AFM surface images: a) neat PVDF ENM, and b) PVDF/SiO₂ NP/FAS ENM. Reproduced with permission.[45] Copyright 2022, Elsevier Science Ltd.</figcaption></figure>

membrane had CaSO₄ on its surface, indicating its scaling, while the modified membrane had no presence of CaSO₄, denoting its superior anti-scaling property.[69]

Aside from the common hydroxyl groups, amide functional groups in polyamides can chemically bond with chlorinated silane coupling agents and form stable Si-N-Si networks. In the works of Koh et al.,[44] FAS was grafted onto a hydrophobic Nylon 6 ENM to create an omniphobic surface exhibiting a repellent surface against water, ethylene glycol, and n-hexadecane. The bulky nanofiber structure and the self-assembled nanoparticles on the membrane coating promoted an extremely rough surface and a barrier for low surface tension liquids. The developed omniphobic membrane showed excellent and stable performance against low surface tension liquids like ethylene glycol (PEG) and SDS for 196 h. Other hydrophilic polymers like poly(vinyl alcohol) (PVA) can be pre-treated with crosslinking agents like glutaradehyde, and subsequently with FAS for enhancing the membrane's mechanical stability against mechanical vibration and its chemical stability against corrosive solutions like HCl and NaOH as well as polar organic solvents like DMF and NMP.[16]

**Plasma Etching Treatment:** Plasma etching is one of the plasma treatment techniques used in surface modification. It involves radical formation on the surface through a bombardment of ions generated by a particular process gas such as CF₄ and O₂.[72] The radicals on the etched surface act as anchor points for grafting new functional groups, creating an excellent adhesion between the surface and the added layer. Plasma etching treatment is reproducible and easily controlled through its main pa-

rameters such as current, pressure, and voltage.[73] It does not involve organic solvents and is much faster and easier to do than conventional chemical grafting. It alters both the physical and chemical surface properties of the substrate while retaining the bulk properties of the material. Hence, plasma etching treatment is very desirable for thin-layer formation and coatings. Likewise, the versatility of this technique can be used to enhance hydrophilicity and hydrophobicity, depending on the nature of the reactive gas used for ion generation. Generally, CF₄ gas is used for hydrophobicity, while O₂ is for hydrophilicity. In using CF₄ gas for plasma treatment, CF₃ and CF₂-CF₂ bonds are formed on the surface, producing a fluorine-rich surface with low surface energy. In the study of Woo et al.,[30] plasma treatment of electrospun PVDF using CF₄ resulted in an omniphobic membrane with remarkable resistance against ethylene glycol, mineral oil, and methanol. Likewise, the loose porous structure of ENM enabled the radicals to slightly penetrate the membrane pores creating enhanced wetting resistance. The optimum penetration depth of the thin fluorine layer was sufficient to obtain strong adhesion to the PVDF substrate while still maintaining the structural integrity of the porous PVDF membrane.

**Table 8** provides a summary of the advantages and limitations of the in situ electrospinning and post-electrospinning methods for surface modification of ENMs. For a thorough comparison of various methods, SiO₂ NP was selected as the sample modifier. In in-situ electrospinning, the nanoparticle is incorporated inside the fiber and near the fiber surface, while in post-electrospinning, the nanoparticle remains on the fiber network, creating a

<figure><img src="image_13.png" /><figcaption>**Figure 10.** a) Photos of different testing liquids on the control PVDF-HFP membrane and the omniphobic PVDF-HFP / SiO₂ NP / FAS membrane, SEM images of membranes after MD test: b) control PVDF-HFP, and c) omniphobic PVDF-HFP / SiO₂ NP / FAS membrane. Reproduced with permission.[69] Copyright 2021, Elsevier Science Ltd.</figcaption></figure>
<table><caption>**Table 8.** Comparison between the two main approaches of membrane surface enhancement.</caption><thead><tr><th></th><th colspan="2">In-situ electrospinning</th><th colspan="2">Post-electrospinning</th></tr><tr><th>Ease of preparation</th><th colspan="2">One-step</th><th colspan="2">Multiple steps</th></tr></thead><tbody><tr><td>Possible interactions between base substrate and coating</td><td colspan="2">Electrostatic interaction, van der Waals forces, H-bonding, covalent bonding</td><td colspan="2">Electrostatic interaction, van der Waals forces, H-bonding, Hydrophobic interaction, covalent bonding</td></tr><tr><td>Possible surface types</td><td colspan="2">Hydrophobic, superhydrophobic, omniphobic</td><td colspan="2">Hydrophobic, superhydrophobic, omniphobic, underwater oleophobic</td></tr><tr><td>Example</td><td>Physical blending</td><td>Co-axial electrospinning</td><td>Electrospray</td><td>Chemical surface deposition</td></tr><tr><td>Sample morphology</td><td><img src="image_0_0.png" alt="Image of physical blending sample"></td><td><img src="image_0_1.png" alt="Image of co-axial electrospinning sample"></td><td><img src="image_0_2.png" alt="Image of electrospray sample"></td><td><img src="image_0_3.png" alt="Image of chemical surface deposition sample"></td></tr><tr><td>Sample membrane material</td><td>PVDF-HFP/SiO<sub>2</sub> NP</td><td>PVDF / SiA</td><td>PVDF-HFP/SiO<sub>2</sub> NP</td><td>PVDF-HFP/SiO<sub>2</sub> NP</td></tr><tr><td>Reference</td><td>[49]</td><td>[52]</td><td>[36]</td><td>[69]</td></tr></tbody></table>

Inset images were Reproduced with permission.[36] Copyright 2022, Elsevier Science Ltd. Reproduced with permission.[49] Copyright 2017, Elsevier Science Ltd. Reproduced with permission.[52] Copyright 2021, Elsevier Science Ltd. Reproduced with permission.[69] Copyright 2021, Elsevier Science Ltd.

complex particle-fiber structure on the membrane surface. Both methods can effectively enhance the surface roughness of an MD membrane. However, the post-spinning method offers an advantage in achieving resistance to organic compounds and surfactants. Low surface energy materials like silane compounds, which can interfere with fiber formation, are typically added after membrane fabrication. As a result, the post-electrospinning technique can produce more unique surface types and designs. Lastly, it is important to note that both techniques may reduce the pore size and increase the pore tortuosity leading to a decrease in permeate flux. As the surface is enhanced by the modifiers, the internal structure of the membrane may become more complex due to modifier-nanofiber network formation. Such a condition is an inevitable trade-off between high flux and excellent anti-wetting of membranes. Therefore, it is critical to balance the two important qualities to achieve the full potential of ENM in MD.

## 4. Limitations and Challenges

ENM fabrication and surface modification is a facile solution to address the common wetting problem of membranes for MD application. Additional functional properties provide improvement while maintaining the integrity of the membrane substrate. However, more realistic desalination operations and longer stable performances are still needed to gain insights into the commercial viability of ENM. Likewise, the intrinsic difficulty of balancing excellent anti-wetting properties and high permeate flux continues to be the hurdle of ENM as MD membranes.

### 4.1. Oil Fouling

ENM with an omniphobic surface is proven to have superior anti-scaling and anti-wetting properties that are highly applicable to MD operations. However, when faced with oil-contaminated saline feed, it becomes susceptible to oil fouling. It may be oleophobic in air, but it becomes oleophilic under water due to the persistent hydrophobic-hydrophobic interactions between the oil and the surface in the presence of water.[74,75] Though the oil droplets do not wick the omniphobic surface due to the rough re-entrant surface structures, the droplets could still accumulate on the surface, leading to limited membrane performance. Thus, underwater oleophobicity becomes vital in developing robust membranes, especially for the MD process, wherein the membranes are always in direct contact with the feed water. Underwater oleophobicity is an inherent property of hydrophilic surfaces. But since the hydrophilic surface is highly susceptible to wetting and scaling, it cannot be used alone for the MD process. Thus, dual-layered membranes with a thin hydrophilic layer on top of nonwetting substrates (i.e., omniphobic and hydrophobic substrates) have been developed. With this membrane design having asymmetric wettability, the hydrophilic layer acts as a protective layer, while the nonwetting layer acts as the active layer. The design of these membranes is different from the conventional dual-layered ENM here the latter primarily involves nonwetting top and bottom layers, as discussed in the previous section. Membranes with opposite wettability, also known as Janus membranes, possess anti-wetting, anti-scaling, and anti-fouling properties.
<figure><img src="image_14.png" /><figcaption>**Figure 11.** In-air WCA and underwater OCA of pristine PTFE membrane, and the dual-layered PTFE/PAN membrane, and b) schema of the anti-oil fouling property of the dual-layered membranes with hydrophobic substrate and hydrophilic coating. Reproduced with permission.[76] Copyright 2019, Elsevier Science Ltd.</figcaption></figure>

For example, in a study by Tang et al.,[76] a dual-layered membrane consisting of a hydrophobic commercial PTFE substrate and a hydrophilic electrospun PAN nanofibers has shown excellent performance against an oily saline feed. The hydrophilicity of the PAN ENM was further enhanced through hydrolysis to provide stronger interaction with water, creating a water film that prohibits the surface from interacting with oil. As shown in **Figure 11**, the PAN ENM provided underwater oleophobicity, enabling the membrane to withstand the fouling action of the oil. The dual-layered membrane with asymmetric wettability had a stable permeate flux of 15.2 LMH and a perfect salt rejection for 25 h against an oily saline feed.[76]

## 4.2. Complex Saline Water

High feed concentration reduces the permeate vapor flux due to reduced water activity and increased concentration polarization. With the advantage of MD operating at low-pressure conditions, MD membranes have a lower fouling tendency and can accommodate higher saline feed concentrations (i.e., >80 g L⁻¹) than RO membranes.[77] Moreover, the tailorable pore size, hydrophobicity, and LEP of ENM benefit the fabrication of MD membranes with remarkable endurance against high saline feed concentrations. Evaluating the performance of ENM against hypersaline water feed can have a valuable industrial impact on ENM and MD. For example, in the study of Su et al.,[78] a superhydrophobic electrospun PTFE/PEO hollow fiber membrane showed a sustained performance despite increasing the saline feed from 35 to 150 g L⁻¹. The excellent performance of the membrane was attributed to the rough multi-level structure and the superhydrophobic surface of the membrane that reduced the concentration polarization at the feed side.

In addition to laboratory-prepared saline feeds, evaluating the performance of ENM against actual water samples, such as shale gas wastewater and industrial flue gas desulfurization wastewater, is helpful for practical MD applications. For example, a CF4 plasma-modified ENM was used in AGMD desalination treatment of actual RO brine coal seam gas-produced water.[30] It demonstrated the realistic potential of ENM in generating fresh water from highly saline- and organic matter-contaminated waters. Likewise, it exhibited prospective operating designs and

strategies in water treatment, such as integrating MD in the RO process to create a zero-liquid discharge system. In such a system, the brine disposal is managed as the RO brine is treated using the MD process, while the MD brine into the brine crystallizer for solid salt recovery.[79]

## 4.3. Long-Term Performance

Wetting is inevitable in membrane technologies due to the thermodynamic-favorable transition from the non-wetting Cassie-Baxter state to the fully wetting Wenzel state of a liquid on a surface. All surface modification strategies only prolong the stability of the Cassie-Baxter state of a liquid on the membrane surface. They delay the occurrence of membrane wetting, but continuous operation will still eventually lead to membrane deterioration. Thus, the long-term performance of the membranes MD process is critical to expand the competency of MD against other desalination technologies. Commercial RO membranes can last up to 12–24 months of continuous usage. In contrast, most MD membranes in literature are limited to a few days up to a week-long operation. The most extended MD performance is a month-long stable operation using a co-axially spun ENM against a 35 g L⁻¹ NaCl feed.[52] This exceptional stability is due to the membrane’s remarkably low thermal conductivity and low sliding angle. The former is a critical factor in sustaining the vapor pressure difference across the membrane, while the latter promotes a lotus effect to resist foulant adhesion on the membrane surface. Despite efforts to develop highly non-wetting membranes with superhydrophobic and omniphobic surfaces, the minimal performance stability is still a real challenge for MD membranes.

## 4.4. Diversity of MD Module Design

ENM is commonly fabricated as a thin flat sheet and used mainly in a plate-and-frame module of MD. In this module, the flat membrane is sandwiched between plates supported with appropriate frames. This setup allows the membrane to be observed, substituted, or cleaned, making it suitable for small-scale research. However, it is limited to a small membrane
area of < 10 cm², which may restrict the total assessment of the fabricated ENM sheet. It may also restrain the efficiency of the membrane and the process in such an operational setup. Thus, fabricating electrospun hollow fiber membranes for hollow fiber MD modules is an excellent expansion for MD membranes. In this setup, a hollow fiber membrane is directly placed inside a tube-like holder. It draws much attention for industrial applications because of its high ratio of specific surface area to module volume. In the study of Su et al.,[⁷⁸] a hollow fiber PTFE membrane fabricated using electrospinning and sintering has proven to have a permeate flux eight times higher than that of a commercial PTFE hollow fiber membrane. Moreover, it is highly efficient against hypersaline feed with concentrations as high as 150 g L⁻¹. With such excellent membrane performance as tested on a bigger sample size, more valuable insights are provided for larger-scale studies than flat sheet membranes.

**5. Conclusion and Future Outlook**

Electrospinning is a new technology for membrane preparation producing nonwoven fibrous and porous structures known as electrospun nanofiber membranes. Due to their large surface area and tunable physical attributes, i.e., porosity and thickness, such membranes have gained much interest in membrane distillation technology. With the primary goal of prolonging and sustaining membrane performance, ENMs were modified to enhance their surface hydrophobicity and water contaminant repellence. Two approaches were used to improve the surface of ENM: in-situ electrospinning and post-electrospinning. The former is a one-step preparation method where nanomaterials are blended and electrospun together with the polymer matrix. The latter is a series of post-functionalization steps conducted on the prepared membrane. Such techniques impart hierarchical micro and nanostructures on the surface of membranes to increase surface roughness and provide low-surface energy materials to repel low-surface tension pollutants. Apart from surface hydrophobicity, physical properties of the membrane, such as porosity, pore size, and LEP, have been improved by surface enhancements, leading to better MD performance. Long-term stability performance and real wastewater application are excellent research areas to denote the commercial application of ENMs persuasively. Developing new membrane designs and creating combinations of different surface types are key points for further exploration of ENMs in MD technologies and to primarily address the wettability, fouling, and scaling of MD membranes.

**Acknowledgements**

The authors acknowledge the financial support from the Second Century Fund (C2F), Chulalongkorn University. This research has received funding support from the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [grant number B16F640164].

**Conflict of Interest**

The authors declare no conflict of interest.

**Keywords**

electrospinning, electrospun nanofibers, membrane distillation, omniphobic, superhydrophobic

Received: January 3, 2024
Revised: March 26, 2024
Published online: May 14, 2024

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<figure><img src="image_15.png" /></figure>

**Michaela Olisha Lobregas** is currently pursuing her Ph.D. in Nanoscience and Technology at Chulalongkorn University in Bangkok, Thailand. She is a recipient of the academic scholarship from the Second Century Fund of Chulalongkorn University. She received her M.Sc. in Chemistry degree from De La Salle University in Manila, Philippines. Her research interests include the development and characterization of polymer nanocomposites for desalination, textile, colorimetric sensing, and energy conversion applications.

<figure><img src="image_16.png" /></figure>

**Ratthapol Rangkupan** is the program director of the Nanoscience and Technology Program and an assistant professor at the Metallurgy and Materials Science Research Institute of Chulalongkorn University in Bangkok, Thailand. He obtained his D.Sc. in polymer science from the University of Akron, USA. His research interests are electrospinning process development and the application of electro-spun nanofibers in various fields such as biomedical, environmental, energy, and health care.

<figure><img src="image_17.png" /></figure>

**David Riassetto** is an associate professor at Grenoble-INP/PHELMA and a member of the Laboratoire des Matériaux et du Génie Physique in Grenoble, France. He obtained his Ph.D. from Grenoble-INP in Grenoble, France. His research area is centered on prospective studies in the field of nanomaterials synthesis using wet chemistry methods: sol-gel chemistry, photochemistry, and hydro-/solvo-thermal methods.

<figure><img src="image_18.png" /></figure>

**Chalida Klaysom** is an assistant professor at the Department of Chemical Engineering of Chulalongkorn University in Bangkok, Thailand. She obtained her Ph.D. in chemical engineering from the University of Queensland in Queensland, Australia. Her research interest involves the synthesis and development of membranes based on polymeric and composite materials. Her works mainly focus on designing and tailoring a membrane to serve specific applications such as water purification, desalination, energy storage, gas separation, and industrial product separation.