Alcohol-Assisted Water Condensation and Stabilization into

Article pubs.acs.org/JPCC

Alcohol-Assisted Water Condensation and Stabilization into Hydrophobic Mesoporosity Mickael Boudot,†,‡ Davide R. Ceratti,† Marco Faustini,† Cédric Boissière,† and David Grosso*,† †

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de le Matière Condensée de Paris, F-75005, Paris, France ‡ Polyrise SAS, F-33607, Pessac, France ABSTRACT: In this work, we report the condensation and stabilization of water vapors into water-repellent, methyl-functionalized, mesoporous, silica-based films at room temperature and atmospheric pressure using a soft alcohol-assisted method. The capillary pore fillings due to water adsorption were ensured by a fine control of partial vapor pressures of alcohol and water. A synergic coadsorption of alcohol and water was observed due to a reversible surface energy switching from hydrophobic to hydrophilic induced by the preferential interaction of the alkyl side of the surfactant alcohol molecules with the hydrophobic walls. The influences of the film mesoprosity as well as the type of the alcoholic coadsorbate (methanol, ethanol, and 1propanol) were investigated, revealing various Kelvin’s law-like adsorption behaviors, depending on the alcohol carbon chain length. Interestingly, the stabilization of condensed domains of quasi-pure water phases was obtained in confined hydrophobic spaces by a presumed selective alcohol desorption. It was demonstrated that a small quantity of remaining Si−OH groups at the pore surface was sufficient to stabilize water nanodomains in hydrophobic pores. These results were based on in situ ellipsometric investigations, that further allowed measuring the wetting angle of water droplets trapped within the porous network, from which we could evaluate the quantity of polar −OH groups remaining in the methyl-functionalized, mesoporous, silica-based films. This first insight in alcohol/water coadsorption may open the door toward new generation of alcohol sensors.

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INTRODUCTION The weak interactions between water molecules and nonpolar surfaces, opposed to the strong hydrogen bonds, are responsible of the hydrophobic property characterized by the high contact angles observed when a sessile droplet of water lies on a hydrophobic surface. Hydrophobic surfaces associated with the presence of nonpolar functions are still intensively investigated today, because of their potential antisticking, anticontamination, and self-cleaning properties.1 Wetting a hydrophobic surface by water seems an antinomy at first glance, nevertheless many groups have already demonstrated that the static contact angle of water droplets can be reversibly switched in the range of 0° to 150° by surfaces energy modulation induced by stimuli such as temperature, UV-light, electrical potentials, or pH.2−5 In superhydrophobic surfaces, mimicking lotus leaves, for instance, a hydrophobic interface is combined to a submicron-scale texture. However, the latter differs from the real porosity into which water can simply condense at relative humidity below saturation due to capillary forces. The relative humidity threshold at which capillary condensation occurs is described by the Kevin’s law6 and is generally experimentally observed for mesopores having hydrophilic interfaces and dimension between 2 and 50 nm. When the interface is composed of nonpolar chemical groups, static contact angles with water are usually ≥90°, preventing water capillary condensation. Nevertheless, computer simulations of water properties when confined in hydrophobic mesocavities © 2014 American Chemical Society

predicted surprising behaviors such as a lower density of water at the vicinity of the hydrophobic surface compared to bulk water, or the spontaneous water evaporation when pore size becomes small enough, to name a few.7−11 Demonstrating experimentally these predictions requires to find the way to trap water into hydrophobic mesopores, since relying on stimulus induced switching wetting properties, as mentioned above, remains difficult to apply in confinement. On the other hand, some groups have shown that infiltration of hydrophobic nanoporous materials by liquid water was possible using high hydrostatic pressures (tens of atmospheres were required to reversibly wet 50 nm large pores)12 or using electric fields to create a reversible gate system for transport of aqueous solutions into partially hydrophobic nanopores.13 However, further understanding of water confinement in hydrophobic nanopores is relevant for many domains of materials science in general, such as biomedical, fuels cells, analytical science, or selective adsorption for depollution. Especially, the present work is the first stone of a project aiming at designing alcohol sensors that relies on optical detection after adsorption, but that takes into account and exploits the relative humidity of water in the atmosphere. Received: August 19, 2014 Revised: September 19, 2014 Published: September 19, 2014 23907

dx.doi.org/10.1021/jp508372d | J. Phys. Chem. C 2014, 118, 23907−23917

The Journal of Physical Chemistry C

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Figure 1. (a) Illustration of the environmental ellipsometry setup. (b) Total, ethanol and water vapor pressures (Ptot = PH2O + PEtOH) vs the ethanol content in the bubbling aqueous solution (X wt %) at 20 °C and 1 atm, adapted from ref 18.

M), F127 Pluronic (EO106PO70EO106), CTAB (cetyltrimethylammonium bromide), and DTAB (decyltrimethylammonium bromide) were purchased from Aldrich. All products were used as received. Film Processing. Mesoporous methyl-functionalized silica thin films with F127 template, sample labeled L for large pores, were prepared from solutions composed of TEOS/MTEOS/ F127/HCl/H 2 O/EtOH with respective molar ratio of 0.5:0.5:0.002:0.005:5:41. Mesoporous methyl-functionalized silica thin films with either CTAB (labeled M for medium pores) or DTAB (labeled S for small pores) template were prepared from solutions composed of TEOS/MTEOS/CTAB or DTAB/HCl/H2O/EtOH with respective molar ratio of 0.5:0.5:0.14:0.15:5:23. TEOS and MTEOS were first dissolved in EtOH, HCl (2 M), and H2O before addition of the template. Solutions were stirred at least for 24 h at room temperature before use. Films were prepared on silicon wafer by dip-coating at room temperature, at relative humidity of 30 ± 5% with a withdrawal rate of 3 mm·s−1. After coating, hybrid films were immediately calcinated at 450 °C for 10 min. Methyl remains in the pores and provides hydrophobicity.16,17 In order to make sure that a minimal concentration of silanol groups remains on the surface, a post methylation was performed by aging 72 h the film in 20 vol % of hexamethyldisilazane dissolved in anhydrous toluene, before drying and stabilization at 450 °C for 10 min. Only some S films underwent such a treatment; they are labeled S*. Characterization. Environmental ellipsometric (EP) analyses were performed on a UV−visible variable angle spectroscopic ellipsometer (M2000 Woolam) equipped with a controlled atmosphere cell, in which relative vapor pressures of H2O and alcohol were adjusted using mass flow controllers at room temperature (20 ± 2 °C). Alcohol and H2O mixed vapors were obtained by bubbling a dry and filtrated air (flux: 2L·min−1) in 200 g of solution composed of alcohol and water with the respective mass ratio in alcohol of X (weight %) = 0, 5, 10, 20, 50, 80, or 100%. After each EEP measurement, films were washed with acetone and dried at 350 °C for 30 s. Each point was taken after 20 s, which was verified to be sufficient to reach equilibrium. Film thicknesses and refractive indices were extracted from conventional Ψ and Δ dispersions using a Cauchy model (Wase 32 software). Water contact angles (CA) were measured on a Kruss DSA 30 system at ambient temperature with 4 μL droplets of different alcohol and water mixtures (methanol−water; ethanol−water, and 1-propanol− water) with various alcohol contents of X (weight %) = 0, 5, 10,

In this present work and contrary to previously mentioned works, we demonstrate that water can be adsorbed and capillary condensed within hydrophobic nanopores, at atmospheric pressure conditions and without external stimulus, using alcohol coadsorption from the vapor phase. The investigated systems consisted in mesoporous methyl-functionalized silica films, bearing interconnected pores of mean diameters adjusted between 2.3 and 5.9 nm through the evaporation-inducedmicellar-template-self-assembly (EISA)14 method, standing in atmospheres where relative vapor pressure in water and alcohol were accurately adjusted between 0 and saturation. Alcohols were selected between methanol and n-propanol so as to investigate the influence of the alkyl chain size on the hydroalcoholic liquid phase capillary condensation. We demonstrate that the adsorption of alcohol from the atmosphere, even if present at low vapor pressures, is sufficient to increase the surface energy of the pore interface to allow water capillary condensation. Alcohol plays here the role of surfactant and allows significant modification of the molecular exchange dynamics between the vapor phase and the confined condensed fluid phase. A critical parameter here is the density in remaining hydroxyl groups at the pore SiO2 surface that permits the stabilization of the condensed aqueous phase by localized anchoring. Finally, we propose a method to assess the “effective” contact angle in the pores, based on the Kelvin model for capillary condensation associated with the Cassie model15 for wetting. Practically, these observations reveal that any volatile compound, such as VOC, may modify the capillary condensation equilibrium in nanopores in real atmospheric conditions. The results also indicate that this kind of porosity could be exploited to break azeotropic mixtures for purification, or could be adapted to realize sensors of alcohol when under film form. Such potential applications will be explored in further works. Structural characteristics of the water repellent mesoporous films were deduced from field-emission-gun scanning electron microscopy (SEM-FEG) and environmental ellipsometric porosimetry (EP) investigations. Alcohol and water vapor adsorption and capillary condensation were followed by in situ environmental ellipsometry, by mean of refractive index variation.

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EXPERIMENTAL DETAILS Chemical Solutions. Methanol 99.9% (MeOH), absolute ethanol 100% (EtOH), and 1-propanol 100% (PrOH) were purchased from Normapur. TEOS (tetraethylorthosilicate), MTEOS (methyltriethoxysilane), hydrochloride acid (HCl 2 23908



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silane.17 It has been shown that water condensation does not occur at room temperature and normal pressure for hydrophobic thin films with pores in the range of 1 to 50 nm. In these cases, single organic solvents such as pure ethanol, isopropanol or toluene are used as absorbate to perform EEP analysis using configuration 1 with X = 100% (αEtOH = 1) to generate the atmosphere.19,21 Here, ethanol adsorption− desorption isotherms recorded from ellipsometry porosimetry are shown in Figure 2 for L, M, and S films. Isotherms are typical of mesoporous structures with interconnected porous networks. Only the isotherm of the large pore film exhibits a hysteresis due to the ethanol desorption that takes place above the critical desorption pressure of ethanol (P/P0EtOH ≈ 0.55). Pore volume of 33, 41, and 38% were determined for S, M, and L films, respectively, using the Bruggeman effective medium approximation.22 The slight difference of pore volume and thickness of the three films will not be taken into account in the further discussion since they do not affect the capillary condensation equilibrium in the case of thin films.23 Capillary condensations are observed at ethanol partial vapor pressures of 0.32, 0.48, and 0.72 for S, M, and L films, respectively. The pore size distributions were determined using the Kelvin equation:

20, 50, 80, and 100%. The average CA values were obtained by measuring at three different positions on the films. Microscope images showing the film structure were collected using a field− emission gun scanning electron microscopy (FEG-SEM Hitachi SU7000 instrument). Generation of the hydro-alcoholic vapor phase introduced in the ellipsometry cell (Figure 1) was obtained in two different configurations, depending on the experiment. The first configuration consisted in mixing in controlled proportion dry air with air enriched in ethanol and water vapors. Air enriched in ethanol and water vapors was produced by bubbling dry air in hydro-alcoholic solutions of various compositions (weight ratios X = 0, 5, 10, 20, 50, 80, and 100%) at 20 °C and 1 atm. The real composition of the atmospheres, in terms of relative vapor pressures (P/P0EtOH, P/P0H2O), were obtained by assuming that the air extracted from the bubbler (see Figure 1a) has the composition of the air in equilibrium with the solution, as defined by O’Hare et al. (Figure 1b), corrected by the relative flux of dry/enriched airs.18 For instance, mixing 1 L· min−1 of dry air with 1 L·min−1 of enriched in 50%/50% EtOH/H2O solution (X = 50%) means having a final injected air loaded with ((P EtOH /P 0 EtOH ) X=50 )/2 and ((P H 2 O / P0H2O)X=50)/2, with values of P0 and P directly extracted from the curves for X = 50% (Figure 1b). Because of the restrictions imposed by configuration 1, not all (P/P0EtOH, P/P0H2O) combinations could be generated. A second configuration (configuration 2) had to be used in certain experiments where dry air in configuration 1 was exchanged by a second bubbler filled by pure water. Whatever the configuration, data are always related to relative vapor pressure of either water or alcohol, entering the measurement cell. The ratios of αEtOH = PEtOH/Ptot = PEtOH/(PH2O + PEtOH) in the atmosphere, which indicate roughly the mole ratios of ethanol over the total mole of water and ethanol in the atmosphere, in equilibrium with solutions composed of X (weight) = 0, 5, 10, 20, 50, 80, and 100% are extracted from Figure 1b and reported in Table 1.

ln

where P/P0 is the relative pressure and rp, γ, Vl, θ, R, and T are the Kelvin pore radius, the surface tension, the molar volume of liquid, the contact angle, the gas constant, and the temperature, respectively. Pore sizes distribution taken from adsorption curves (Figure 2, inset), reveal fairly narrow distributions in the range of 1.5−3, 2.5−4, and 4−11 nm for S, M, and L films, respectively, with corresponding mean pore diameters of 2.30 ± 0.15, 3.50 ± 0.20, and 5.90 ± 0.40 nm. Water−Ethanol Coadsorption Investigations. The effect of ethanol adsorbed onto the hybrid surface of the pores has been first observed by following in situ by ellipsometry the variation of the refractive index of the postmethylated S* film, induced by the direct variation of the atmosphere composition in the chamber. Results are gathered in Figure 3. The sequence of successive steps is described in Figure 3a together with the corresponding liquid phase content in the film. The first drying step aims at ensuring full desorption of the porosity. One observed that the following abrupt increase of humidity at saturation in step II is accompanied by only less than 3 vol % of water uptake after 3 min, confirming the good hydrophobicity of the film (the uptake seems to be linear with time with a slope of 0.2% min−1, suggesting that the films will carry on slowly pumping water if maintained in this environment, but this was not investigated further here). However, it also suggests that the surface is not homogeneously coated with methyl groups at nanometer scale, and that few hydroxyl groups remain into the mesopores, despite the post methylation, −OH groups can be seen as “defects”, breaking the continuity of the hydrophobic surface within the pores, that prevent full water repellence. When the atmosphere is modified to P/P0EtOH = 0.07 and P/P0H2O = 0.97 in step III, capillary condensation occurs as testified by the sudden increase in refractive index, and the full porosity filled up with a liquid phase. This effect occurs only if a little amount of ethanol is introduced into the humid atmosphere, revealing that the latter organic moiety plays a critical role in the water

Table 1. Correspondence between X (Weight%) of the Solution in Bubbler and αEtOH in the Atmosphere X (weight%) αEtOH = PEtOH/(PH2O + PEtOH)

0 0

5 0.16

10 0.27

20 0.44

50 0.62

80 0.74

2γV cos θ P =− 1 P0 rpRT

100 1

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RESULTS AND DISCUSSION Film Structure. Hydrophobic methyl-functionalized thin films with average thicknesses of 200 nm and different mesopore sizes were synthesized by the evaporation induced self-assembly (EISA) method14 using TEOS and MTEOS as precursors and various conventional micellar templates (DTAB, CTAB, and F127). The final thermal curing for 10 min at 450 °C induces the creation of the porosity and confers the hydrophobicity.19 Films were labeled S (small pore), M (medium pore), and L (large pore) for DTAB, CTAB, and F127 templates, respectively. Figure 2 shows the SEM images of the three types of film, revealing a similar worm-like organization of the porosity. GI-SAXS analysis of each film showed a large scatter signal that cannot be exploited, but are typical of a poorly ordered Worm-like structure,20 often encountered with hybrid mesoporous silica films prepared by co-condensation with a high proportion (>20 mol %) of 23909



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Figure 2. SEM-FEG cross-section images and corresponding ethanol adsorption/desorption isotherms (pore size distributions inset) for L, M, and S films.

Figure 3. (a) Time-resolved porosity filling for S* films, deduced from the refractive index variation, upon successive adjustment of the atmosphere composition from step I (P/P0EtOH = 0; P/P0H2O = 0), followed by step II (P/P0EtOH = 0; P/P0H2O = 1), followed by step III (P/P0EtOH = 0.07; P/ P0H2O = 0.97), to finished with step IV (0 < P/P0EtOH < 0.024; P/P0H2O = 1). (b) Indication of the volume of liquid phase remaining in the pores at step IV, depending on the composition of the atmosphere at step III (lozenges: P0EtOH = 0.07; P/P0H2O = 0.97; and squares: P/P0EtOH = 0.14; P/P0H2O = 0.95) and of the composition of the atmosphere at step IV (0 < abscise P/P0EtOH < 0.065 with P/P0H2O = 1).

adsorption and its capillary condensation. It is clear that this effect can only be due to the modification of the surface energy of the pore surface induced by the alcohol adsorption. Very interestingly, if the atmosphere composition changes to P/ P0EtOH = 0 and P/P0H2O = 1, as in step IV, the porosity remains partly filled with liquid (up to 23%), which is much higher than what was first obtained in step II with the same vapor phase composition, suggesting the condensed liquid phase into the porosity can be out of equilibrium with the atmosphere. Because the volume of liquid phase that is lost is relatively high, it is highly probable that modifying the equilibrium state between steps III and IV provokes first a preferential evaporation of ethanol, which then destabilizes liquid/ mesopore interfaces, leading to water spontaneous desorption, even at 100% relative humidity. Ethanol seems thus to play a

role of mediator between the hydrophobic surface of the pores and water. The significant amount of water (23% of the porous volume) remaining trapped within the porosity at P/P0EtOH = 0 and 100% relative humidity, can evidently not be homogeneously distributed into the porosity but are more likely distributed into localized dispersed liquid nanodomains stabilized in regions of higher surface −OH groups density (water molecules have indeed not choice but to interact with themselves and with silanol groups to maximize the density of more enthalpically favored hydrogen bonds). Water-rich nanodomains are then stabilized by surface anchoring, as illustrated in Figure 4. The existence of such aqueous nanodomains into hydrophobic nanopores with diameters of 3−4 nm were already described by Molinero et al. using molecular dynamics simulations.24 There is no reason that 23910



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Figure 4. Time resolved porosity filling of S films, deduced from the refractive index variation, upon successive adjustment of the atmosphere composition from step I (P/P0EtOH = 0; P/P0H2O = 0), followed by step II (P/P0EtOH = 0; P/P0H2O = 1), followed by step III (P/P0EtOH = 0.07; P/ P0H2O = 0.97), to finish with step IV (P/P0EtOH = 0; P/P0H2O = 1). Illustration of ethanol surfactant role during step III and water-nano domain anchoring effect attributed to the presence of surface silanol groups.

postmethylation after thermal curing at 450 °C) were subjected to similar successive atmosphere variations (see Figure 4). First, one observes a water uptake of 8% at 3 min after the beginning of step II in saturated humidity, compared to 3% at 3 min for the postmethylated S* film in Figure 3. The up-taking slope in step III is also 6 time faster than for the S* film. This indicates that a higher density of −OH must be available at the surface as expected. Surprisingly and very interestingly, passing from step III to step IV (P/P0EtOH = 0; P/P0H2O = 1) induces only a slight decrease of the condensed liquid volume of only 7%. Contrary to S*, the same decrease was observed when step III conditions were fixed at P/P0H2O = 0.94 and P/P0EtOH = 0.14. It means that a higher content of ethanol introduced into the porosity during step III does not noticeably influence the stability of the liquid domains during step IV. This is likely due to the higher density of surface silanol groups, and the 7% of freed porous volume must correspond to porous regions where the density of silanol groups is inferior to the critical value required to stabilize liquid nanodomains. In step IV, one notices, however, that water slowly condenses back into the mesoporosity after the 7% loss, with a slope of 0.5%·min−1. Such a water up-take is expected as a result of the saturated humidity. The equilibration mechanism would probably involve the sudden early departure of EtOH, which is then slowly replaced by water. The slow condensation process is likely to be related to the difficulty of the water molecule to adsorb and diffuse through the hydrophobic porosity to the condensed liquid domains. The condensation seems however to slow down and stabilize at 95% after 10 min of stabilization (data not shown). Here again, it is sensible to propose that the liquid domains confined in the hydrophobic porosity, and stabilized by the presence of OH groups, are mainly composed of water. However, the real composition of the liquid phase is not extractable from the available data. By mastering the balance between surface density of silanol groups (Si−OH) and −CH3 we demonstrate that it is possible to simply trap water into hydrophobic pores by adopting an adsorption and desorption sequence involving ethanol coadsorbate. It is clear in Figure 4 that the same atmospheric composition of P/P0EtOH = 0; P/P0H2O = 1 applied in steps II and IV are not associated with the same state concerning the volume of condensed liquid phase. This furthermore reveals the important role of volatile organic contaminants in the dynamic

permits to affirm here that the liquid phase, and therefore the porosity, does not contain any more ethanol molecules participating to the stabilization of the liquid domains. The composition of the liquid phase in steps III and IV can therefore not be determined, but it is likely that it is mainly water. It is important to clarify that the same trend was observed if step II was skipped (no shown), suggesting that the 3% of the porosity already fill with water are not responsible for the capillary condensation observed in step III. Finally, three distinct stabilized states have been obtained in step IV (Figure 3a), that correspond to three different applied final atmospheric compositions. First, and in view of the exact overlapping of the water up-taking curved in step III for the three reported attempts, we conclude that the effect is highly reproducible and is not affected by cycling that may induces porosity structural and surface modification. Here, the equilibrium is established only with the ethanol pressure, since only the latter value varies while the atmosphere remains saturated in water in step IV. We observe that the higher is the ethanol pressure in step IV, the higher is the volume of liquid phase stabilized in the pores. Such a dependency is more clearly evidenced in Figure 3b, showing linear variations of the remaining amount of condensed liquid phase with respect to the atmospheric composition in ethanol finally applied in step IV. Here two slopes were plotted that correspond each to a specific applied ethanol pressure in step III. Both show a linear tendency but with very different slope and origin, that point out the crucial influence of the atmospheric composition during step III on the final equilibrium state in step IV. A higher ethanol pressure during step III is associated with a much lower volume of remaining liquid phase in step IV for the same conditions, and a lower slope. It is likely that the incorporation of a higher amount of ethanol in the liquid phase in step III leads to the desorption of a higher volume of liquid in step IV, when ethanol vapors are removed. Therefore, the equilibrium attained in step IV depends not only on the relative densities of Me, silanol and alcoholic −OH groups at the pore surface and on the composition of the atmosphere in step IV, but also on the composition of the atmosphere in step III. In order to confirm the role of the density of silanol groups, S films containing less methyl but more silanol groups (i.e., no 23911



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Figure 5. Scheme of the dynamic capillary condensation occurring in step III (top) and of the equilibrium achieved during step IV (down) for hydrophobic and partly hydrophobic S* and S films.

Figure 6. Adsorption/desorption isotherms for L, M, and S films plotted vs partial vapor pressures of ethanol (black line) and water (red dotted). Fraction of vapor pressures αEtOH = PEtOH/(PH2O + PEtOH) are indicated on the right side.

of capillary condensation in nanopores. The roles of ethanol as coadsorbate are critical in adsorbing and condensing water in the present porosity, since it plays the role of surfactant by

adapting a conformation with its alkyl tail interacting with the surface methyl groups and its OH groups turning toward the pore centers (see Figure 4,inset). In step IV, the presence of 23912



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hydrophilic Si−OH sites are necessary to stabilize the condensed phase after elimination of ethanol from atmosphere, that is likely accompanied by the partial evaporation of ethanol from the confined liquid domains. The wetted surface associated with the anchoring effect is represented in Figure 4,inset. At another scale, Figure 5 shows a schematic representation of the porous network of S and S* films, during steps III and IV. The equilibrium, achieved in step IV where the alcohol-free atmosphere is applied, is due to the presence of OH- groups at the surface that stabilize the liquid/vapor interfaces of the liquid nanodomains. In such conditions, the wetted surface methyl groups of these domains do not play their water repellent role anymore since water molecules have no choice but to stay in the confinement, stabilized by the localized surface silanol groups. The higher is the density of hydroxyl groups on the surface, the higher is the fraction of the liquid phase that can be stabilized, due to the greater statistics to create polar enough zones in the porous network to pin the liquid domains. It is clear now that such a final equilibrium state can only be achieved if the capillary condensation occurs in the presence of EtOH. Influence of Pore Size on the Capillary Condensation from Hydro-ethanolic Atmosphere. In the next experiment, the atmosphere is generated by controlled-volume mixing of dry air and air saturated from ethanol/water solutions at fixed composition (mass ratio in alcohol of XEtOH = 0, 5, 10, 20, 50, 80, or 100%). In this configuration, the vapor pressure ratio of ethanol (αEtOH) in the generated atmosphere remains the same for given XEtOH values (see Table 1), but each relative vapor pressure is simultaneously and proportionally modulated by the combination with dry air. Figure 6 displays adsorption/ desorption isotherms of the volume fraction of condensed phase present in the film with respect to relative vapor pressure for S (small), M (medium), and L (large) films, with averaged pore diameters of 2.3 ± 0.1(5), 3.5 ± 0.2, and 5.9 ± 0.4 nm, respectively, and for the different XEtOH values mentioned above, extracted from ellipsometry. Each isotherm is plotted twice on the same graphic, in black with respect to P/P0EtOH and in dashed red with respect to P/ P0H2O. Isotherms obtained for XEtOH = 0% (P/P0EtOH = 0; 0 < P/P0H2O < 1) show the expected low water uptake for the three S, M, and L films, confirming the hydrophobicity of the pores, as already observed for films S in Figure 4. Isotherms obtained for XEtOH = 100% (P/P0H2O = 0; 0 < P/P0EtOH < 1) are those reported in Figure 2 and have thus already been exploited to extract the pore size distribution. One recalls that the full porosity can be filled-up with ethanol by capillary condensation. When αEtOH ranged from 0.16 to 0.74, one observes that capillary condensation on the adsorption curves occurs at different relative vapor pressures whatever the size of pores. The relative vapor pressures in ethanol and water at which condensation occurs in all investigated conditions are reported in Table 2. First, it is clear that for a fixed αEtOH, and whatever its values, the capillary condensation occurs always at higher pressures for films with larger pores, as expected from the Kelvin model. For all individual S, M, and L films, P/ P0EtOH Cap.Cond increases and P/P0H2O Cap.Cond decreases when αEtOH increases. Such displacements of the capillary condensation with αEtOH, despite the constant pore sizes, highlight a synergetic mechanism of adsorption/condensation between alcohol and water that corroborates the conclusions of the previous experiments. This synergy is also the reason why it is difficult to dissociate P/P0EtOH and P/P0H2O to discuss these

Table 2. Partial Vapor Pressures of Ethanol and Water at Capillary Condensation versus αEtOH = PEtOH/(PH2O + PEtOH) in the Atmosphere for S, M, and L Films L αEtOH 0 0.16 0.27 0.44 0.64 0.74 1

P/P0

EtOH

0.28 0.43 0.56 0.72

M P/P0H2O

P/P0EtOH

0.89 0.68 0.50

0.14 0.22 0.31 0.39 0.48

S P/P0

H2O

0.95 0.68 0.48 0.35

P/P0

EtOH

0.07 0.12 0.17 0.25 0.29 0.32

P/P0H2O 0.95 0.79 0.59 0.41 0.26

isotherms. A distinct couple of minimal partial vapor pressures of ethanol and maximal partial vapor pressures of water per type of porosity can be distinguished to observe the capillary filings into the pores. In S films, a 0.07 relative vapor pressure in ethanol is sufficient to condense a liquid phase at 0.95 relative vapor pressure of water. In M films, P/P0EtOH = 0.14 is necessary to condense a liquid phase at P/P0H2O = 0.95. In L films, P/P0EtOH = 0.28 is necessary to condense a liquid phase at P/P0H2O = 0.89. In other words, full capillary condensation occurs only if both relative vapor pressures are above these critical values. Only adsorption, or partial pore filling with liquid phase, is observed if this condition is not verified. It is important to notice here that for any value of αEtOH, and for all S, M, and L of films, capillary condensation occurs at ethanol relative vapor pressures that are inferior to the ethanol vapor pressure at which capillary condensation occurs for αEtOH = 1 (XEtOH = 100%). This is to say that, in all cases for which αEtOH ≤ 0.74 (XEtOH = 80%), the capillary condensation must only be attributed to water molecules, ethanol molecules serving only to reverse surface energy. However, it is not clear if in these conditions a co-condensation occurs, and if after condensation the composition of the confined liquid remains only water combined to the adsorbed EtOH molecules or progressively evolves toward that imposed by the dynamic liquid/vapor equilibrium. As already deduced, water condensation into hydrophobic mesopores occurs only thanks to the coadsorption with ethanol vapor due to the surfactant behaviors of the alcohol molecules. If a high density of ethanol is present in the atmosphere, a higher quantity of ethanol can be physically adsorbed in the porosity, making it less hydrophobic. Then, as soon as surface energy of pores is high enough, the first liquid droplets can condense and initiate locally the capillary condensation. In addition, ethanol adsorption tends to slightly reduce the average diameter of the pores, contributing to the lowering of the critical relative vapor pressure for condensation. It is complicated to rationally interpret the condensation here because, if we consider the adsorbate molar volume Vl to be constant in a first approximation (only water condenses), P/P0 at capillary condensation is function of γ that varies with the adsorption of ethanol, and certainly locally, as already illustrated in Figure 5. Such an inhomogeneity in γ, associated with the statistical presence of localized regions with higher density of hydroxyl groups coming from the silanol or from ethanol, is the reason for which isotherms present a hysteresis at low αEtOH values that disappears at higher αEtOH values. Indeed, regions of low density in −OH groups could play the role of constrictions, thus favoring the pore blocking effect responsible for the presence of a hysteresis loop. This applies mainly for S and M 23913



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films, since L films already exhibits an isotherm with a hysteresis for αEtOH = 1. Film Thickness Behavior under Hydro-ethanolic Atmosphere. In this part we report the thickness evolution measured by ellispometry of an S film during an adsorption desorption isotherm whose vapor is created from water and EtOH mixture XEtOH = 5 wt % and composition is αEtOH = 0.16, see Figure 7. For this vapor composition similar to the one used

are proportional to the density of curved liquid/vapor interfaces and also to the curvature radius, as described by the Laplace law. This effect will be studied in the future. Influence of the Alcohol Molecular Weight. In addition to ethanol as coadsorbate, methanol and 1-propanol vapors were used to evaluate the influence of the ROH carbon chain length on the ability of water vapors to condense into hydrophobic pores of S films. Partial vapor pressures for MeOH and PrOH could not be determined, as for EtOH, because diagrams of vapor pressures in equilibrium with alcohol/water solutions of different concentrations are not available in the literature. We will directly thus use XROH to discuss the experimental data. As seen previously, a minimal XEtOH value of 5 wt % in water is required to allow capillary condensation in mesoporosity of S films. In order to observe the same full capillary condensation on the same S films, XMeOH = 10% and XPrOH = 2% are required. The isotherm of adsorption desorption of the S films for the three different alcohols at the latter critical X values are plotted versus global P/P0, deduced from the mixing using configuration 1 to generate the atmosphere, in Figure 8. All three isotherms

Figure 7. Relative thickness evolution of an S film during an adsorption/desorption cycle performed with vapor composed of water and ethanol, αEtOH = 0.16.

in step III of the time-resolved porosity filling of S films, we can identified the capillary contraction and relaxation of the film (blue arrows) induced by the capillary stresses created by adsorption and desorption of the hydro-ethanolic vapors into the porosity. Such breathing phenomenon of mesoporous film during adsorption/desorption is often observed but has never been fully described and interpreted yet. The transversal film contraction results from the apparition of liquid−vapor interface meniscuses during capillary condensation into pores. Negative curvatures of meniscuses (concavities) generate a negative pressure inside the porous volume of film causing its contraction. When the liquid/vapor interface curvatures disappear the porous network relaxes and the thickness regains its initial value. Right before the capillary condensation and after the capillary relaxation, the film structure underwent unusual swellings (of about 1% of the film thickness). These behaviors are also observed for αEtOH ≤ 0.27 and αEtOH ≤ 0.44 for S films and M, L films. It could be interpreted as an over pressure effect into porous volume due to the presence of positive curvatures (convexities) of the meniscuses. It is likely that for low vapor compositions in ethanol at a given P/P0 of the mixture, P/P0H2O is high enough to generate water patches into porosity (see in previous part: the effect of ethanol adsorbed onto the hybrid surface),24 whereas P/P0EtOH is too low to create a uniform hydrophilic ethanol adsorbed layer. These aqueous nanodomains do not wet the hydrophobic surface of pores and therefore possess a positive liquid−vapor interface. They are expected to grow from polar-group-rich regions until they reach the film surface or coalesce together, as illustrated in Figure 5 (state IV). Eventually, water patches are destabilized by the formation of an ethanol monolayer when P/P0EtOH reaches the critical value leading to the good wetting of the water domains and creation of negative curvature (capillary adsorption) or by the desorption of water. The swelling and contraction degree

Figure 8. Adsorption/desorption isotherms for S films versus partial vapor pressures created with water and EtOH, or MeOH, or PrOH, and obtained for critical XROH given on the right side.

present the same aspect and positions of the capillary condensation, with a hysteresis loop due to the heterogeneity of alcohol adsorption already described above. It clearly appears that the longer is the carbon chain length of alcohol, the lower is the XROH value necessary to observe capillary condensation of water. One expects the same mechanism as those described previously for EtOH to govern the capillary condensation with other alcohols. Difference of partial vapor pressure between methanol, ethanol, and 1-propanol is related to alcohol affinities for the surface and their vapor pressures. Methanol, ethanol, and 1-propanol have similar chemical structures and polarities (1.70, 1.69, and 1.55 D for methanol, ethanol, and 1-propanol, respectively),25 however, they have relatively different saturated vapor pressures, 13.0, 5.95, and 2.03 kPa at 20 °C for methanol, ethanol, and 1-propanol, respectively. The lower volatility of PrOH associated with a lower critical αPrOH, compared to the other two lighter alcohols, means that an even much lower proportion of PrOH is necessary in the atmosphere to allow water condensation, since the latter takes place at roughly the same P/P0 ≈ 0.9. This could only be attributed to a better affinity/stability of the PrOH with the hydrophobic walls, combined with the slight reduction of the average pore dimension upon adsorption. 23914



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Figure 9. (a) Molecular volume of water and alcohol solution depending on the mass ratio of alcohol. (b) Surface tension of water and alcohol solutions depending on mass ratio of alcohol.

Figure 10. (a) Calculated effective static contact angles into the pores depending on the alcohol ratio (XROH) for the three hydro-alcoholic solutions. (b) Measured macroscopic static contact angles for S films depending on the alcohol ratio (XROH) for the three hydro-alcoholic solutions. (c) Comparison of effective static contact angles (nano) and macroscopic contact angles (macro) for the three hydro-alcoholic solutions.

Evaluating Effective Contact Angles of Water Phase into Confinement. In this part, we considered only S films. The objective is to compare the macroscopic contact angle, governing the macroscopic wetting of the film surface, with the effective contact angle of the liquid phase into the nano porosity, deduced from Kelvin law based on the values of relative vapor pressures at which capillary condensation occurred. From this effective contact angle, the Cassie law of wetting was used to deduce the proportion of the inner pore surface composed of adsorbed ROH, for the three investigated alcohols (MeOH, EtOH, and PrOH). Macroscopic contact angles of 0° are assumed in the following study since they were measured to be below 5° with the pure alcohol. We also assumed that alcohol/water solutions in confined environments have the same physical-chemical properties than the bulk ones (i.e., polarizability, molecular volume (Vl)26,27 and the surface tension (γ)28) at 20 °C. The evolutions of these properties are extracted of the latter references and are provided in Figure 9. In addition, the variation of the pore dimension (rp) due to alcohol adsorption is taken to be negligible.

The effective static contact angles of condensed liquid into the pores can be estimated by the following rewritten Kelvin’s equation, using relative vapor pressure measured at the capillary condensation from the adsorption curves. P

θ = cos

−1

−ln P rpRT 0

2γVl

For simplicity, the composition of the condensed liquid into the pores was considered to be similar to the composition of the solution used to generate the atmosphere (equilibrium state), so that Vl and γ can be introduced into the equation, together with rp, and P/P0 at the capillary condensation. The effective static contact angles as well as the measured macroscopic angles are plotted versus the ratio of alcohol XROH from 5 to 100% for S films in Figure 10. Contact angles for XROH = 0% are not considered since they are above 90°. The evolutions of the calculated effective static contact angles in Figure 10a show that the combination of water with long carbon chain alcohols, such as PrOH, allows to wet the hydrophobic surface of the pores more efficiently than with 23915



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surfaces. More precisely, XROH conditions of 10, 5, and 2% were selected for methanol, ethanol, and 1-propanol, respectively, because they correspond to the minimal alcohol concentrations to observe the capillary condensation into the pores. With these conditions, the effective contact angle of water into the pores can be related to the fraction of ROH adsorbed onto the pore interface using the Cassie relation.15

smaller alcohol. Complete wetting (θ = 0°) is observed for ratio of alcohol (XROH), starting from 10, 50, and 80% for 1propanol, ethanol, and methanol, respectively. Macroscopic contact angles shown in Figure 10b exhibit similar tendencies as those reported for the effective contact angle, with also a better wetting efficiency with PrOH. Plots in Figure 10c reveal that there are significant differences between calculated effective contact angles in nanopores (Nano) and macroscopically measured ones (Macro), with Δθ found between 40 and 0°, depending on the conditions. For methanol and ethanol aqueous mixtures, the Nano contact angles are greater for low XROH values, with differences that tend to diminish with increasing XROH values, to eventually match. The mismatch could be due to two effects: (i) Calculated effective contact angles in nanopores are in fact dynamic contact angles measured at the capillary condensation, which are classically higher than the static contact angles. Liquid intrusion calorimetry of water29 reported by Gomez et al. is a technic that allows the measurements of advancing contact angles into hydrophobic pores and could be a means to complete our results. Moreover, Spagnolo et al.30 reports with the same technic the use of a water−alcohol mixture to probe contact angles of hydrophobic pores; it could be also a way to complete our interpretation concerning the role of alcohol in the pore filling mechanism. (ii) The other possibility lies in the fact that external contact angle is a macroscopic measurement at the triple line of the liquid droplet. Intrinsically, its large size is the average of the Si−OH and Si−CH3 compositions of the matrix, thus, allowing measurement of the mean contact angle. On the other hand, the silanol heterogeneities of the porous network allows nanodroplet nucleation growth on more hydrophilic patches until they reach more hydrophobic patches, which reverses the curvature of the liquid meniscus and stops their growth. As a consequence, the measured nanowetting surface angle could be more representative of the more hydrophobic domains of the network, explaining that its value is higher than the macroscopic average value. This tendency is also observed for PrOH, but only at very low XROH values, since an inversion of the nano and macro curve very rapidly occurs at around XROH = 10%. This effect could be attributed to the decrease of the effective pore dimension upon PrOH adsorption that has not been considered. At low XROH values, we considered that ROH molecules present in the condensed liquid phases are mainly located at the pore interface in interaction with the methyl groups, and that core of the liquid domains is mainly composed of water. The decrease of the nano contact angle with increasing XROH values is then attributed to the increase of the polarity of the interfaces, associated with the increase of the density of adsorbed ROH but also to the diminution of the surface tension of the liquid phase due to its ROH enrichment. This tendency is more marked with PrOH than with the other alcohols because of its ability to considerably reduce the surface tension of mixed aqueous solution, as shown in Figure 9b. The deduced effective contact angles in nanopores at low XROH values, where we considered that alcohol is mainly located at the interface and liquid phase is mainly composed of water, are now used to evaluate the fraction of ROH covering the pore

cos θeff = ΥOH cos θOH + ΥCH3 cos θCH3

where θeff, θOH, θCH3, ΥOH, ΥCH3 are the calculated effective contact angle, the macroscopic water contact angle on hydrophilic surfaces (fixed at 10°), the macroscopic water contact angle on methyl-functionalized hydrophobic surfaces (fixed at 90°), the hydrophilic surface fraction, and the hydrophobic surface fraction, respectively, with ΥOH + ΥCH3 = 1. ΥOH would thus correspond to the fraction of the inner surface occupied by an alcohol molecule. These fractions have been reported in Figure 11.

Figure 11. (a) Calculated ratio of hydrophobic surface covered by adsorbed alcohol for S films with XROH = 2%, 5%, and 10% of methanol, ethanol, or 1-propanol, respectively. (b) Illustration of the alcohol adsorption onto the pore surface for methanol, ethanol, and 1propanol.

We observed that a lower surface of S films need to be cover by adsorbed 1-propanol compared to ethanol and methanol to turn up the surface energy of porosity and condensed water, suggesting that alcohols with longer carbon chains are more efficient to reverse the hydrophobicity of pores. This tendency can be attributed to a higher interaction of the propyl chain with the surface methyl groups, and the localization of the polar hydroxyl groups of the alcohol relatively far from the surface, where interaction with water is more favorable.

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CONCLUSIONS It was shown that the presence of a slight amount of alcohol into the atmosphere preferentially adsorbed into hydrophobic methyl-functionalized SiO2 nanopores and plays the role of surfactant (surface energy switching from hydrophobic to hydrophilic) agent that allows capillary condensation of water. We also pointed out the critical role of remaining silanol groups to stabilize water phase nanodomains by localized anchoring. Investigations using methanol, ethanol, and 1-propanol as coadsorbate revealed that this effect is more efficient with 1propanol as a result of its longer carbon chain. The conditions at which capillary co-condensation occurs were then used to deduced effective contact angles into the nanocavities. Finally, swellings and contraction of such porous films were observed and were then attributed to the presence of curved liquid/vapor interfaces into the pores during water adsorption/desorption. Further investigation concerning quantification of −OH, H2O, EtOH, and −CH3, using indirect methods such as ellipsometric detection of specific adsorption and microcalorimetry, needs to 23916



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Substrates: A Detailed Investigation of Ammonia Vapor Treatment (AVT). Chem. Mater. 2014, 26, 1822−1833. (17) Cagnol, F.; Grosso, D.; Sanchez, C. A General One-Pot Process Leading to Highly Functionalised Ordered Mesoporous Silica Films. Chem. Commun. (Cambridge, U. K.) 2004, 1742−1743. (18) O’Hare, K. D.; Spedding, P. L. Evaporation of a Binary Liquid Mixture. Chem. Eng. J. 1992, 48, 1−9. (19) Faustini, M.; Nicole, L.; Boissière, C.; Innocenzi, P.; Sanchez, C.; Grosso, D. Hydrophobic, Antireflective, Self-Cleaning, and Antifogging Sol−Gel Coatings: An Example of Multifunctional Nanostructured Materials for Photovoltaic Cells. Chem. Mater. 2010, 22, 4406−4413. (20) Tate, M. P.; Urade, V. N.; Kowalski, J. D.; Wei, T.; Hamilton, B. D.; Eggiman, B. W.; Hillhouse, H. W. Simulation and Interpretation of 2D Diffraction Patterns from Self-Assembled Nanostructured Films at Arbitrary Angles of Incidence: From Grazing Incidence (above the Critical Angle) to Transmission Perpendicular to the Substrate. J. Phys. Chem. B 2006, 110, 9882−9892. (21) Vayer, M.; Nguyen, T. H.; Grosso, D.; Boissiere, C.; Hillmyer, M. A.; Sinturel, C. Characterization of Nanoporous Polystyrene Thin Films by Environmental Ellipsometric Porosimetry. Macromolecules 2011, 44, 8892−8897. (22) Niklasson, G. a; Granqvist, C. G.; Hunderi, O. Effective Medium Models for the Optical Properties of Inhomogeneous Materials. Appl. Opt. 1981, 20, 26−30. (23) Boissiere, C.; Grosso, D.; Lepoutre, S.; Nicole, L.; Bruneau, A. B.; Sanchez, C. Porosity and Mechanical Properties of Mesoporous Thin Films Assessed by Environmental Ellipsometric Porosimetry. Langmuir 2005, 21, 12362−12371. (24) De la Llave, E.; Molinero, V.; Scherlis, D. a. Role of Confinement and Surface Affinity on Filling Mechanisms and Sorption Hysteresis of Water in Nanopores. J. Phys. Chem. C 2012, 116, 1833− 1840. (25) CRC Handbook of Chemistry and Physics 85th ed.; CRC Press: New York, 2004. (26) Natividade, N. A. T.; Ferreira, A. G. M.; Fonseca, I. M. A. Densities and Excess Molar Volumes of Water + Propyl Acetate + Propan-1-ol and Its Constituent Binaries at 303.15 K. J. Chem. Eng. Data 1997, 42, 1232−1234. (27) Arce, A.; Blanco, A.; Soto, A.; Vidal, I. Densities, Refractive Indices, and Excess Molar Volumes of the Ternary Systems Water + Methanol + 1-Octanol and Water + Ethanol + 1-Octanol and Their Binary Mixtures at 298.15 K. J. Chem. Eng. Data 1993, 38, 336−340. (28) Vazquez, G.; Alvarez, E.; Navaza, J. M. Surface Tension of Alcohol Water + Water from 20 to 50 °C. J. Chem. Eng. Data 1995, 40, 611−614. (29) Gomez, F.; Denoyel, R.; Rouquerol, J. Determining the Contact Angle of a Nonwetting Liquid in Pores by Liquid Intrusion Calorimetry. Langmuir 2000, 16, 4374−4379. (30) Spagnolo, D. a.; Maham, Y.; Chuang, K. T. Calculation of Contact Angle for Hydrophobic Powders Using Heat of Immersion Data. J. Phys. Chem. 1996, 100, 6626−6630.

be tackled in the near future to complete this work after design and construction of appropriate measurement cells. This very fundamental work is the first stone on which we would like now to base further works dedicated to sensing alcohol vapors in humid environments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for funding provided by CIFRE Fund granted by ANRT and the AVATAR Project, which is supported by Polyrise SAS and DGA. The authors thank D. Montero for Semfeg microscopy conducted on a Hitachi Su-70 + Oxford XMax facilited by the IMPC (FR2482) financially supported by the C’Nano projects of the Region Ile-de-France and M. Selmane for GI-SAXS experiments.

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REFERENCES

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Alcohol-Assisted Water Condensation and Stabilization into

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