# Adsorption Selectivity of Water-Ethanol Mixtures on Organosilica Surfaces: Role of Hydrophilicity

Simon Gravelle, Benoit Coasne

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## To cite this version:

Simon Gravelle, Benoit Coasne. Adsorption Selectivity of Water-Ethanol Mixtures on Organosilica Surfaces: Role of Hydrophilicity. 2025. hal-04917234

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# Adsorption Selectivity of Water-Ethanol Mixtures on Organosilica Surfaces: Role of Hydrophilicity

Simon Gravelle*,† and Benoit Coasne†,‡

†Univ. Grenoble Alpes, CNRS, LIPhy, 38000, Grenoble, France

‡Institut Laue Langevin, F-38042 Grenoble, France

E-mail: simon.gravelle@cnrs.fr

## Abstract

The competition between hydrophilic and hydrophobic interactions in water-ethanol mixtures gives rise to abnormal properties, such as the formation of an ethanol-rich surface layer at non-polar interfaces like graphite. This phenomenon, resulting from the balance between solid-liquid and liquid-liquid interactions, could benefit the development of microporous membranes for separating water-alcohol mixtures. In this study, we used molecular dynamics simulations of water-ethanol mixtures near planar organosilica surfaces, focussing on the role of surface properties in interfacial ethanol enrichment. Organosilica surfaces were chosen for their tunable hydrophobicity, achieved by varying the ratio of methyl to hydroxyl groups. Our findings reveal significant ethanol enrichment at hydrophobic organosilica surfaces decorated with methyl groups, similar to previous observations with graphite. We analyze ethanol enrichment in relation to mixture composition and surface properties, such as roughness and the spatial organization of surface groups. By demonstrating the ability of organosilica materials to preferentially adsorb ethanol over water, this work identifies a cost-effective alternative to carbon-based materials for the fabrication of filtration membranes.

## Introduction

Ethanol is a widely used biofuel that provides a lower-emission alternative to gasoline [1]. Biofuels are produced from the fermentation of crops, a process that yields a mixture of water and ethanol. To obtain high-purity ethanol for use as fuel, the mixture must be separated. The separation of water-ethanol mixtures is typically done using distillation-based methods that require heat and high pressures, resulting in significant energy consumption [2]. In response to this challenge, ongoing efforts are aimed at developing more efficient separation technologies, such as isothermal filtration with microporous membranes [3].

Binary mixtures like water-ethanol have been the focus of intensive research since the pioneering work by Frank and Evans in 1945 [4]. Yet, the properties of such mixtures are still under active investigation due to the complex nature of their interactions [5]. Ethanol is a weakly amphiphilic molecule with a hydrophilic head and a non-polar tail. When mixed with water, it exhibits numerous anomalous behaviors compared to pure solvents. For instance, the hydrophobic segments of ethanol tend to cluster together to avoid interaction with water, while the hydrophilic heads form polar interactions with water [6]. The competition between hydrophilic and hydrophobic interactions gives rise to several anomalies, such as incomplete mixing of water and alcohol, local microheterogeneity, and a negative excess entropy [7].

When a water-ethanol mixture is in contact with an interface, an equilibrium configuration that best satisfies the competition between entropy and enthalpy is established. Next to non-polar solids such as graphene or hexagonal boron nitride, ethanol and other alcohols tend to accumulate over water within the first density layer [8–13]. A similar surface accumulation of ethanol occurs at liquid-vapor interfaces, where the surface ethanol molar fraction can exceed the bulk molar fraction by a factor that depends, among other factors, on the mixture composition [14, 15]. Conversely, polar interfaces like hydrophilic silica surfaces show little to no preference for ethanol adsorption over water [16, 17], while polar alumina surfaces exhibit a preference for water adsorption over ethanol [18, 19].

The molecular enrichment of ethanol over water at non-polar interfaces could form the basis for microporous membrane separation. To achieve effective large-scale separation, membranes must feature well-controlled hierarchical porosity, strong mechanical properties to withstand the high pressures involved in separation processes, and must also be cost-effective and durable [20–23]. Fabricating membranes based on graphene or its derivatives, such as carbon nanotubes, is currently impractical [24], but a promising alternative is nanoporous organosilica [25, 26]. Organosilica materials are composed of nanoporous silica functionalized with organic compounds, such as methyl groups or hydrocarbons [16, 27].
Organosilica membranes combine excellent mechanical properties with tunable hydrophobic/hydrophilic surfaces [28, 29], making them a compelling candidate for fluid separation applications.

In this study, the properties of water-ethanol mixtures near organosilica surfaces with varying functionalizations and structures are examined using molecular dynamics simulations. The organosilica surfaces are functionalized with a fraction $r_{C/O}$ of non-polar (–CH$_3$) groups and a fraction $1 - r_{C/O}$ of polar (–OH) groups. By exploring a wide range of $r_{C/O}$ values (from 0 to 1), we create surfaces with a broad spectrum of hydrophilicities. The composition and structure of the fluid near these surfaces are analyzed and compared with those near a graphite surface and a liquid-vapor interface.

## Method

Molecular dynamics simulations of a mixture of water and ethanol in a silica-based slit pore were performed using GROMACS [30]. The composition of the mixture was varied from $x_{total}^{eth} = 0$ (pure water) to $x_{total}^{eth} = 1$ (pure ethanol), where

$$x_{\text{total}}^{\text{eth}} = \frac{N^{\text{eth}}}{N^{\text{eth}} + N^{\text{wat}}}, \qquad (1)$$

is the total ethanol number fraction, with $N^{eth}$ and $N^{wat}$ representing the numbers of ethanol and water molecules in the simulation. In most of the simulations presented here, the mixture contained 150 ethanol molecules and 2850 water molecules, corresponding to an ethanol number fraction $x_{total}^{eth} = 0.05$ and a mass fraction $r_{total}^{eth} = 0.12$. For each simulation, the ethanol mass fraction in the bulk (i.e., far from the surfaces), $r_{bulk}^{eth}$, was measured after the equilibrium was established. Generally, $r_{bulk}^{eth}$ differs from $r_{total}^{eth}$ due to the preferential surface adsorption of one species over the other, as illustrated, for example, in Fig. 1 A.

Unless stated otherwise, the surfaces of the slit pores were constructed from a Q³ silica material characterized by a surface group density of $5 \text{ nm}^{-2}$ (Fig. 1). Here, the index ‘3’ in Q³ refers to the number of bridging oxygens connected to a silicon atom. The dangling Si atoms of the silica were functionalized with non-polar (–CH$_3$) groups at a fraction $r_{C/O}$ and polar (–OH) groups at a fraction $1 - r_{C/O}$. Slit pores made from Q² silica, mixed Q²/Q³ silica, mixed Q³/Q⁴ silica, Q⁴ silica, amorphous silica, and graphite were also generated (Fig. 1). For comparison, simulations of a liquid-vapor interface were also carried out. For $r_{C/O} = 0$, the Q³ solid surface is hydrophilic, exhibiting a measured water contact angle of approximately 40° (Fig. S1). In contrast, for $r_{C/O} = 1$, the surface is hydrophobic, with a measured water contact angle of approximately 90° [31].

The distance between the two walls of the slit pore, h, was approximately 8 nm, while the system dimensions along the x and y axes were approximately 3 nm (Fig. 1). The system was first equilibrated at ambient

temperature ($T = 300$ K) and pressure ($p = 1$ atm) for a total duration of 50 ns, allowing the density profile to stabilize. During this equilibration step, the temperature was maintained using the Canonical Sampling through Velocity Rescaling (CSVR) thermostat [32] with a time constant of 0.1 ps, and the pressure was maintained using an anisotropic Berendsen barostat [33] with a time constant of 1 ps. Water was modeled with the TIP4P/ε model [34], and ethanol was modeled using the OPLS/AA force field [35]. Cross parameters were calculated using the Lorentz-Berthelot mixing rule. The SURFACE force field was used for silica [36], along with the GROMOS force field, which was used for the –CH$_3$ groups [37]. The simulation scripts are available in a public online repository [38].

## Results

### Density profiles

First, density profiles normal to the solid surface were extracted for both water and ethanol, using the MAICoS toolkit [39] (Fig. 2). Here, the fluid is mostly composed of water, with an ethanol mass fraction $r_{total}^{eth} = 0.12$. For one of the most hydrophobic silica surfaces considered here, where $r_{C/O} = 1$, our results indicate ethanol accumulation within the first density layer next to the solid surface (blue line in Fig 2 A). Our results also show that, when $r_{C/O} = 1$, water is partially depleted from the first density layer (blue line in Fig. 2 B). By contrast, for a more hydrophilic silica surface with $r_{C/O} = 0$, our results show an accumulation of both species within the first density layer (red lines in Fig. 2 A and B). Regardless of the surface hydrophilicity, there is a clear structuring of the fluid next to the solid interface, as indicated by the oscillations in the total fluid density $\rho^{\text{tot}} = \rho^{\text{eth}} + \rho^{\text{wat}}$ (Fig. S2). The maximum density value is approximately 0.75 u/Å³ (about 25 % higher than in bulk) and is largely independent of the value of $r_{C/O}$.

For comparison, density profiles were also extracted for a graphite slit pore and a liquid-vapor interface. Both the graphite slit pore and the liquid-vapor interface show an accumulation of ethanol within the first density layer (dark lines in Fig. 2), in good agreement with previous works [10, 14, 15]. In the case of graphite, the maximum fluid density within the first layer reaches a higher value compared to silica surfaces: $1.0 \text{ u/Å}^3$ (dark dashed line in Fig. S2). In contrast, silica reaches about 0.75 u/Å³. Our results also show that the first layer of fluid next to a graphite surface is thinner than that next to a silica surface (dark dashed lines in Fig. 2). In contrast, the first fluid layer at a liquid-vapor interface is broader than those at any of the solid surfaces considered here (dark dotted lines in Fig. 2), highlighting differences in the fluid structuring that will be analyzed below.
<figure><img src="image_3.png" /><figcaption>Figure 1: (A) Side view of a water-ethanol mixture with $x_{\text{total}}^{\text{eth}} = 0.05$ confined inside an organosilica slit pore with $r_{\text{C/O}} = 0$ (a and b), a graphite pore (c), and two liquid-vapor interfaces (d). Oxygen atoms are represented in red, carbon atoms in gray, silicon in yellow, and hydrogen in white. In panel b, water is represented as a continuous transparent field for clarity, and in panel c, the carbon atoms of graphite are represented as pink bonds. (B) Top view of different silica surfaces considered here: $Q^3$ surface with $r_{\text{C/O}} = 1.0$ (a), $r_{\text{C/O}} = 0.5$ (b), and $r_{\text{C/O}} = 0.0$ (c), $Q^3/Q^4$ surface with $r_{\text{C/O}} = 1.0$ (d), $Q^2$ surface with $r_{\text{C/O}} = 1.0$ (e), $Q^3/Q^4$ surface with $r_{\text{C/O}} = 1.0$ (f), amorphous surface with $r_{\text{C/O}} = 1.0$ (g), and $Q^4$ surface (h).</figcaption></figure>

## Surface enrichment

To quantify the surface enrichment of ethanol over water at the interfaces, we measure the ethanol mass fraction within the first density layer, which we define as

$$r_{1}^{\text{eth}} = \frac{\rho_{1}^{\text{eth}}}{\rho_{1}^{\text{eth}} + \rho_{1}^{\text{wat}}}, \qquad (2)$$

where the index ‘1’ refers to the first fluid layer adjacent to the interface. The molecules of the first density layer were identified using the ITIM algorithm of Pytim [40, 41] (see examples of the first four detected layers in Fig. S3). For comparison, the ethanol mass fraction was also measured in the bulk region, defined as the fluid molecules not belonging to the four detected layers (typically corresponding to molecules located at distances greater than approximately 1.2 nm from any interface):

$$r_{\text{bulk}}^{\text{eth}} = \frac{\rho_{\text{bulk}}^{\text{eth}}}{\rho_{\text{bulk}}^{\text{eth}} + \rho_{\text{bulk}}^{\text{wat}}}. \qquad (3)$$

The ethanol enrichment factor was then defined as

$$E_{1}^{\text{eth}} = \frac{r_{1}^{\text{eth}}}{r_{\text{bulk}}^{\text{eth}}}. \qquad (4)$$

$E_{1}^{\text{eth}} > 1$ indicates that ethanol is enriched in the first density layer relative to the bulk.

For the most hydrophobic organosilica surfaces considered here, and for an ethanol mass fraction $r_{\text{total}}^{\text{eth}} = 0.12$, the ethanol enrichment factor $\gtrsim 10$ (Fig. 3 A). The best-performing silica surface, for which $E_{1}^{\text{eth}} \approx 13$, is the $Q_2$ silica surface with $r_{\text{C/O}} = 1$. The $Q_2$ silica surface has the highest surface group density of all the organosilica structures considered here (Fig. 1 B), a property that will be discussed below. Interestingly, for the

most hydrophilic surfaces considered here with $r_{\text{C/O}} = 0$, the ethanol enrichment factor exceeds one ($E_{1}^{\text{eth}} \approx 3$), indicating preferential ethanol adsorption even on polar surfaces. By comparison, for graphite, $E_{1}^{\text{eth}} \approx 14$, slightly exceeding the best organosilica surfaces (dashed line in Fig. 3 A). At the liquid-vapor interface, $E_{1}^{\text{eth}} \approx 9$, which is lower than that of all organosilica surfaces with $r_{\text{C/O}} = 1$ (dotted line in Fig. 3 A).

Results for the organosilica surfaces with $r_{\text{C/O}} = 1$ and varying reservoir composition $r_{\text{bulk}}^{\text{eth}}$ show a non-homogeneous variation in $E_{1}^{\text{eth}}$, with an optimum near $r_{\text{bulk}}^{\text{eth}} = 0.05$ (Fig. 3 B). In the case of reservoirs of pure ethanol (i.e., $r_{\text{bulk}}^{\text{eth}} \to 1$), $E_{1}^{\text{eth}} \to 1$ as expected. For reservoirs rich in water (i.e., $r_{\text{bulk}}^{\text{eth}} \to 0$), $E_{1}^{\text{eth}}$ is lower than when $r_{\text{bulk}}^{\text{eth}} \approx 0.05$, which reveals an enhancement of surface ethanol enrichment at intermediate concentrations. The origin of this enhancement could be the hydrophobic interactions between ethanol molecules, as will be discussed below. For the hydrophilic silica with $r_{\text{C/O}} = 0$, $E_{1}^{\text{eth}} \gtrsim 2.5$, regardless of the reservoir composition.

The ethanol enrichment factor was also calculated within the second density layer (i.e., $E_{2}^{\text{eth}}$) next to the solid surface (Fig. S4 A). Our results show that $E_{2}^{\text{eth}}$ is nearly independent of $r_{\text{C/O}}$, suggesting that the surface properties mainly affect the fluid composition of the first density layer, with only a minor impact on the second density layer. However, with $E_{2}^{\text{eth}} \approx 1.1$, the second layer is slightly richer in ethanol than the bulk reservoir, implying that the surface influence extends beyond the first density layer, as also seen in the oscillations of the total density (Fig. S2). Our results also show that within the second layer and in the limit $r_{\text{bulk}}^{\text{eth}} \to 0$, $E_{2}^{\text{eth}}$ is lower than 1, indicating a slight depletion in ethanol (Fig. S4 B).

The results in Fig. 3 A indicate that the ratio of polar
<figure><img src="image_4.png" /><figcaption>Figure 2: Density profiles next to the interface for ethanol, $\rho^{eth}$, (A) and water, $\rho^{wat}$, (B), respectively. The position $z = 0$ corresponds to the Gibbs dividing plane, and the bulk ethanol number fraction far from the solid surface is $r_{\text{bulk}}^{eth} \approx 0.05$. The results were obtained with Q³ organosilica surfaces with two different values for the ratio $r_{\text{C/O}}$: $r_{\text{C/O}} = 1$ corresponding to a hydrophobic surface (full blue line) and $r_{\text{C/O}} = 0$ corresponding to a hydrophilic surface (full red line). The dashed dark line was obtained with graphite, and the dotted dark line with a liquid-vapor interface.</figcaption></figure>

to non-polar groups, $r_{\text{C/O}}$, is the primary factor determining ethanol enrichment at an organosilica surface. However, Fig. 3 A also demonstrates that $r_{\text{C/O}}$ is not the sole determinant, as surfaces with $r_{\text{C/O}} = 1$ exhibit a wide range of enrichment values, $E_1^{eth}$, from approximately 9.5 (for the Q³/Q⁴ surface) to approximately 13 (for the Q² surface). This suggests that additional parameters, beyond $r_{\text{C/O}}$, influence ethanol enrichment. One likely candidate is the surface group density, characterized by the number of -CH₃ groups per unit surface area, $n_{\text{CH3}}$. This parameter ranges from 0 (Q⁴ surface) to approximately 9 nm⁻² (Q² surface) (Fig. 4 A). Interestingly, the relationship between $n_{\text{CH3}}$ and $E_1^{eth}$ is non-monotonic, with relatively high ethanol enrichment observed at both small and large values of $n_{\text{CH3}}$. This observation indicates that other factors, alongside $n_{\text{CH3}}$ and $r_{\text{C/O}}$, likely contribute to ethanol enrichment at the interface. These factors are further explored in the next section.

## Surface -CH₃ patches

The results obtained with different types of silica surfaces (Fig. 4 A) indicate that while the number of surface groups, $n_{\text{CH3}}$, is a useful indicator of surface efficiency in separating ethanol and water, it does not fully account for the preferential adsorption of ethanol. Notably, the Q⁴ surface achieves reasonable a value for $E_1^{eth}$ despite having no surface groups. We hypothesize that surface atomic roughness and the specific organization of surface groups play significant roles in the ethanol enrichment

<figure><img src="image_5.png" /><figcaption>Figure 3: The ethanol enrichment factor, $E_1^{eth}$, within the first density layer next to the interface [see Eq. (4)]. A) $E_1^{eth}$ as a function of the carbon-to-oxygen ratio, $r_{\text{C/O}}$, of the organosilica surfaces, from $r_{\text{C/O}} = 0$ (hydrophilic surface) to $r_{\text{C/O}} = 1$ (hydrophobic surface). The ethanol mass fraction is $r_{\text{total}}^{eth} = 0.12$. The dark dashed line is $E_1^{eth}$ in the case of the graphite surface, the blue dashed line corresponds to the Q⁴ surface, and the dark dotted line corresponds to the liquid-vapor interface. Disks are the default Q³ surface, the square is Q², the pentagone is Q²/Q³, the hexagone is the amorphous surface, and the triangle is Q³/Q⁴. B) $E_1^{eth}$ as a function of the bulk ethanol mass fraction, $r_{\text{bulk}}^{eth}$, in the case of the hydrophobic surface with $r_{\text{C/O}} = 1$ (blue), and hydrophilic surface with $r_{\text{C/O}} = 0$ (red).</figcaption></figure>

(Fig. 1 B). To test this hypothesis, -CH₃ patches were defined as the largest possible ellipses connecting neighboring -CH₃ groups within the (x, y) plane (red ellipses in Fig. 4 B, C). We found that the most heterogeneous surfaces considered here, such as amorphous and Q³/Q⁴, exhibit -CH₃ patches with surface areas as large as $A_p \approx 1$ nm², alongside smaller patches with $A_p \approx 0.3$ nm². In contrast, more regular surfaces, like Q², show only smaller patches ranging in size from 0.2 to 0.3 nm² (Fig. 4 B C and Fig. S5).

The ethanol mass fraction, $r_1^{eth}$, was measured within individual patches (Fig. S6). Although no clear relationship between patch size and local ethanol enrichment could be established, certain trends emerged. Large patches with $A_p > 0.4$ nm² consistently exhibit high fraction $r_1^{eth}$, which aligns with the high $E_1^{eth}$ value observed for the Q⁴ surface despite its lack of surface groups. Conversely, all patches with low ethanol fraction (i.e., $r_1^{eth} < 0.8$) fall within the size range of $0.1$ nm² < $A_p < 0.3$ nm². However, many patches within this range, including all patches from the Q² and Q³ surfaces, exhibit high ethanol fraction (see for example patches n°2 and n°3 in Fig. 4 B). These findings suggest that while patch size may play a role in local ethanol enrichment, a more detailed characterization of the atomic structure is required. Such characterization could involve surface topography (e.g. positions of -CH₃ groups along the z axis normal to the surface) and/or the distribution of local surface dipole moments.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

Figure 4: A) Ethanol enrichment factor within the first density layer, $E_1^{eth}$, as a function of the surface density of CH$_3$ groups, $n_{CH_3}$, for the six organosilica surfaces considered: Q$^4$, Q$^3$, Q$^2$, Q$^3/Q^4$, Q$^2/Q^3$, and amorphous. The dashed line represents $E_1^{eth}$ for graphite, and the dotted line corresponds to the liquid-vapor interface. B-C) Density maps of the ethanol mass fraction within the first density layer, $r_1^{eth}$, for the amorphous and Q$^2$ organosilica surfaces with $r_{C/O} = 1$. Yellow indicates a low ethanol fraction ($r_1^{eth} = 0$), while blue indicates a high ethanol fraction ($r_1^{eth} = 1$). White dots mark the average positions of the -CH$_3$ groups. In panel B, red ellipses highlight a large patch (n°1) with surface area $A_p \approx 1$ nm$^2$, and two medium-sized patches (n°2 with $A_p \approx 0.33$ nm$^2$ and n°3 with $A_p \approx 0.28$ nm$^2$). In panel C, all patches are of similar size ($A_p \approx 0.23$ nm$^2$).

## Hydrogen bonding

To further characterize the state of water-ethanol mixtures within the first layers next to the solid surfaces, the number of OH···O hydrogen bonds per molecule, $n_{HB}$, was measured [31]. Our results indicate that for a fixed reservoir composition ($r_{total}^{eth} = 0.12$), the number of hydrogen bonds per ethanol molecule in the first density layer decreases with the enrichment factor $E_1^{eth}$ (Fig. 5 A) [42]. On surfaces with low $E_1^{eth}$ (i.e., hydrophilic surfaces), the number of hydrogen bonds per ethanol molecule is approximately 2, similar to what is measured in bulk. However, on surfaces with high $E_1^{eth}$, this number decreases to 1.75. A qualitatively similar trend is observed for water, for which $n_{HB}$ decreases from approximately 3.1 to 2.6 as $E_1^{eth}$ increases. This reduction can be partially attributed to the absence of hydrogen bonds between molecules and non-polar surface groups (Fig. S7).

Results from Fig. 5 A indicate that for both water

<figure><img src="image_8.png" /></figure>

Figure 5: A) Number of OH···O hydrogen bonds, $N_{HB}$, per ethanol (blue) and water (red) molecule as a function of the enrichment factor $E_1^{eth}$. Filled symbols represent measurements from the first density layer, while open symbols are from the bulk. Symbols correspond to different surfaces: disks for Q$^3$, squares for Q$^2$, pentagons for Q$^2$/Q$^3$, hexagons for amorphous, upward triangles for Q$^3$/Q$^4$, and rightward triangles for Q$^4$. The gray pentagones are results from the vapor interface, and the gray squares are from graphite. The ethanol mass fraction is $r_{total}^{eth} = 0.12$. B) Number of neighboring carbon atoms per carbon atom, $N_{C-C}$, from ethanol molecules in the first density layer (filled symbols) and in the bulk (open symbols) as a function of $E_1^{eth}$. Only carbon atoms located within a distance $r < 4.6$ Å and from different molecules are considered, see Ref. 31.

and ethanol, adsorbing at non-polar surfaces implicate a reduction in the number of hydrogen bonds compared to the bulk. That reduction is however slightly larger for water (about 16%) than for ethanol (about 12%). In addition to the value of $n_{HB}$, there is a well known advantage associated with concentrating the ethanol molecules at the interfaces: it mitigates the disturbance to the hydrogen bond network in the fluid by allowing water molecules to form more hydrogen bonds with themselves [7, 43]. To characterise this effect, we counted the number of neighboring ethanol carbon-carbon atoms, $N_{C-C}$. Our results show that, for low $E_1^{eth}$ (i.e., hydrophilic surfaces), $N_{C-C} \approx 0.75$ for both ethanol molecules from the bulk and the first density layer (Fig. 5 B). This indicates that in the case of the most hydrophilic surfaces, there is no visible clustering of ethanol molecules within the first density layer. As $E_1^{eth}$ increases, $N_{C-C}$ from the first density layer increases up to almost 2, while $N_{C-C}$ from the bulk decreases below 0.5. Interestingly, all data points in Fig. 5 B fall on the same master curve, except for the vapor interface and the amorphous silica surface, for which $N_{C-C}$ is lower, suggesting that the specificity of the interface can favor or hinder the contact between ethanol carbon-carbon atoms, with possible impact on the selective adsorption.

## Discussion

Our results for varying reservoir composition reveal a maximum in the ethanol enrichment at an ethanol mass
fraction of $r_{\text{bulk}}^{\text{eth}} = 0.05$, in the case of organosilica surfaces with $r_{C/O} = 1$ (Fig. 3 B). This suggests that collective effects may enhance surface enrichment. One likely contributor is the hydrophobic interaction between clustering ethanol molecules [5, 44], an effect that is expected to only be significant at intermediate ethanol concentrations and to diminish as $r_{\text{bulk}}^{\text{eth}} \to 0$. Indeed, the typical number of neighboring ethanol carbon-carbon atoms, $N_{C-C}$, shows an inflection point near $r_{\text{bulk}}^{\text{eth}} \approx 0.1$ (Figs. S8-S9). This indicates that, below $r_{\text{bulk}}^{\text{eth}}$, there is a substantial decrease in ethanol-ethanol hydrophobic interactions within the first layer. Such a decrease aligns with the observed reduction in ethanol enrichment as $r_{\text{bulk}}^{\text{eth}} \to 0$. These findings suggest that membrane separation based on the preferential adsorption of ethanol over water could lose efficiency in the limit of vanishing ethanol concentrations.

The surfaces considered in this study exhibit varying degree of atomic roughness, ranging from the atomically smooth graphite to the disordered amorphous silica surface (Fig. 1). These differences in surface roughness lead to distinct structuring of molecules within the first density layer, as revealed by 1D (Fig. 2) and 2D (Fig. 4) density profiles, the number of neighboring ethanol carbon-carbon atoms $N_{C-C}$ (Fig. 5 B), as well as orientation distribution of molecules within the first layer (Fig. S10). However, despite these differences, certain generic trends emerge across all surfaces. For example, the number of hydrogen bonds per molecule, $n_{\text{HB}}$, from all systems follows the same master curves (Fig. 5 A). Additionally, the thickness of the first density layer, $\sigma_1$, measured as the variance of the density profile $\rho_1(z)$ [see Eq. (S2)], decreases with increasing ethanol enrichment factor, $E_1^{\text{eth}}$ (Fig. S11). In other words, surfaces with thinner first density layer tend to exhibit better ethanol-water separation performance.

Although our results show that silica surfaces decorated with $-\text{CH}_3$ are not as effective as pristine graphite for selectively adsorbing ethanol over water, they also reveal that even the least effective surfaces considered here (i.e., the $Q^2/Q^3$ and amorphous surfaces) show an ethanol enrichment factor of about 10 for $r_{\text{total}}^{\text{eth}} = 0.12$. This indicates that the enrichment of ethanol at hydrophobic silica surfaces is quite robust and should be measurable with any nanoporous silica-based material primarily decorated with non-polar groups, regardless of its structure.

## Conclusion

In conclusion, we used molecular simulations to study the preferential adsorption of ethanol over water at organosilica surfaces. Our results reveal that the primary predictors of an organosilica surface ability to separate ethanol from water are the type and density of surface groups. The organosilica surface exhibiting the highest ethanol enrichment is a $Q^2$ surface fully decorated with non-polar groups ($r_{C/O} = 1$). This surface features a high density

of $-\text{CH}_3$ groups ($n_{\text{CH}_3} \approx 9 \text{ nm}^{-2}$), and performs nearly as well as pristine graphite in preferentially adsorbing ethanol over water. Our study also highlights the non-trivial influence of surface atomic structure on ethanol enrichment. Surfaces with disordered groups, such as amorphous and $Q^2/Q^3$ surfaces, exhibit the poorest performance. This reduced enrichment could arise from geometric effects, as smaller water molecules may more readily adapt to rough landscapes (thereby decreasing ethanol enrichment), or from an increased local polarity associated with disordered surfaces.

## Acknowledgments

This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement N° 101065060. Most of the computations presented in this paper were performed using the GRICAD infrastructure (https://gricad.univ-grenoble-alpes.fr), which is supported by Grenoble research communities. In addition, this work was granted access to the HPC resources of IDRIS under the allocation 2023-AD012A14560 made by GENCI.

## Availability of Data

All simulation input files and post-processing scripts are available on GitHub, under the https://github.com/simongravelle/publication-data repository [38].

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