Confinement of Water in Hydrophobic Nanopores: Effect of the

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Confinement of Water in Hydrophobic Nanopores: Effect of the Geometry on the Energy of Intrusion Thomas Karbowiak,† Guy Weber,‡ and Jean-Pierre Bellat*,‡ †

UMR PAM, Agrosup Dijon, Université de Bourgogne, 1 Esplanade Erasme, F-21078 Dijon, France Laboratoire Interdisciplinaire Carnot de Bourgogne, ICB UMR 6303 CNRS, Université de Bourgogne, 9 Avenue Alain Savary, F-21078 Dijon, France

‡

ABSTRACT: Water confinement in the hydrophobic nanopores of highly siliceous zeolite having MFI and CHA topology is investigated by high pressure manometry coupled to differential calorimetry. Surprisingly, the intrusion of water is endothermic for MFI but exothermic for CHA. This phase transition depends on the geometry of the environment in which water is confined: channels (MFI) or cavities (CHA). The energy of intrusion is mainly governed by the change in the coordination of water molecules when they are forced to enter the nanopores and to adopt a weaker, hydrogen-bonded structure. At such a nanoscale, the properties of the molecules are governed strongly by geometrical restraints. This implies that the use of classical macroscopic equations such as Laplace−Washburn will have limitations at the molecular level.

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INTRODUCTION

Water confinement in hydrophobic environments has gained considerable interest in recent years. Some of this work has dealt with the modification in physical properties of water during nanoscale confinement, as occurs, for example, in carbon nanotubes or zeolites.1,2 This concept is also applicable in the field of biology, when considering protein folding in restricted spaces in living cells.3−6 How relevant is the hydrophobicity of a material? Hydrophobicity generally refers to wetting by water and especially to the value of the contact angle when a water drop is deposited onto a surface. Another, more rigorous, way to define hydrophobicity takes into consideration the interaction of water with a surface. Obviously, this interaction depends on the physical and chemical properties of the surface, namely its geometry (flat vs curved, smooth vs rough), and its surface chemistry (presence of hydrophilic chemical functions). Taking this into consideration, and assuming the entropic contribution is negligible, the knowledge of the enthalpy of interaction between water and a surface compared to the enthalpy interaction of water molecules between themselves (enthalpy of liquefaction) can be used to estimate hydrophobicity. When the enthalpy of the water−surface interaction is lower than the enthalpy of water liquefaction, the surface is hydrophobic. When it is equivalent or higher, the surface is hydrophilic. In a thermodynamic system composed of a porous material interacting with water at constant temperature, pore filling depends on pressure. This is classically given as adsorption or intrusion isotherms dictated by the hydrophobic character of the material, as illustrated in Figure 1. For hydrophilic materials, © XXXX American Chemical Society

Figure 1. Effect of hydrophobicity on pore filling pressure.

pore filling occurs by adsorption of water vapor at a pressure lower than the saturated vapor pressure (ps). As the hydrophobic character increases, the pore filling pressure increases. For very hydrophobic materials, the pore filling pressure is higher than the saturated vapor pressure. Under these conditions, the water is liquid, and pore filling occurs as an intrusion process. This situation, where water is forced to fill the porosity under high pressure, is particularly relevant for applications in energy Received: November 7, 2013 Revised: December 16, 2013

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storage.7−9 Early work in this field started in the mid-90s10 and has expanded with the synthesis of new hydrophobic nanomaterials like hydrophobic zeolites, so-called zeosils, or grafted mesoporous silicas.11−13 However, these studies essentially focused on the determination of the mechanical work by measuring the volume of water entering the porosity as a function of the applied pressure (intrusion pressure). This mechanical work is given by

Wint =

∫

that the intrusion is endothermic on MFI while it is exothermic on FAU. Up to now, this has not been validated by experimental data, partly due to the difficulty in achieving complete hydrophobicity in all types of zeolitic structures. Though the synthesis of MFI zeosil is well-known, zeosil with FAU topology has not yet been elaborated. The objective of this work is to bring new insights in the intrusion process of water in zeosils having different pore sizes and geometries. Two zeolitic structures, the synthesis of which are possible in pure siliceous form, are studied: MFI, with cylindrical channels, and CHA, with a cage-like structure. Mechanical and thermal energies of intrusion of water in these zeosils are determined by using high pressure calorimetry. The exothermicity of the intrusion process in relation with the structure of the confined water is discussed, as well as the validity of the Laplace−Washburn equation. Lastly, we compare the stability of these two materials by analyzing the mechanical and thermal energies of intrusion as well as the chemical modifications occurring along successive cycles of intrusion and extrusion.

p dV

According to the reversibility of the intrusion and extrusion process, some systems have been described as molecular springs, dampers, or shock-absorbers when they are able to restore, absorb or dissipate mechanical energy, respectively.7 Thus, an efficient system for mechanical energy storage can be achieved for an accessible porous volume of great size and high intrusion pressure. However, as dictated by the Laplace−Washburn equation, the pore opening must be as small as possible to have a high intrusion pressure: pc = −

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2γ cos θ r

EXPERIMENTAL SECTION

Porous Materials. The two hydrophobic porous materials used for this work were purely siliceous zeolites, namely silicalite-1 (MFI type) and chabazite (CHA type), synthesized in the group Matériaux à Porosité Contrôlée of the IS2M UPR7228 CNRS in Mulhouse, France. They were prepared in fluoride media according to the procedure described by Guth et al.19 for silicalite-1 and by DiazCabanas et al.20 for chabazite, using tetrapropylammonium cations and trimethyladamantammonium as structure-directing agents, respectively. After synthesis, the products were filtered, washed with demineralized water, and dried at 353 K overnight. To completely liberate the porosity, the solids were then calcined at 823 K under air for 15 h to eliminate the templating agent. The porosity of these two materials was characterized by nitrogen adsorption at 77 K (Table 2). High Pressure Calorimetry Experiments. The volume of intruded water and the intrusion heat were measured using a differential calorimeter (Tian-Calvet Setaram C80, Setaram, Lyon, France) coupled to a high pressure manometric device equipped with a high pressure pump. This experimental setup has been described in detail in a previous publication.21 Basically, the sample calorimetric vessel containing the zeolite (around 0.5 g previously outgassed in situ under primary vacuum) is first filled with distillated and degassed liquid water. Then, the liquid water is compressed step by step, with successive small pressure increments Δp, in the range 0.1 to 120 ± 0.1 MPa, at 298 K. For each pressure step, the change in volume ΔV is measured from displacement of the piston of the high pressure pump used for compressing liquid water. The accuracy on the intruded volume is around ±0.01 cm3 g−1. The reference vessel, also containing water, is maintained at atmospheric pressure. The heat flow is recorded over time until equilibrium is reached for each pressure step. This method uses integration to determine calorimetric differential heat per gram of zeolite δQcalo/mdp with an accuracy of around ±0.05 J MPa−1 g−1. Blank runs were performed under the same conditions, but without a sample, to give the thermal effect of water compression around the zeolite. These values can then be subtracted from the calorimetric differential heat to obtain the differential heat of intrusion δQint/mdp. Finally, the heat of intrusion Qint is obtained by integrating this value against pressure. FTIR Spectroscopic Characterization. FTIR spectra were recorded on a BRUKER Equinox 55 spectrometer equipped with a specific ultrahigh vacuum chamber.22 Spectra were measured in situ, by transmission, after outgassing under secondary vacuum at 298 K to remove all physisorbed water molecules. 200 scans were averaged with a resolution of 2 cm−1. Spectra collected under the same conditions without a sample were used for background correction. IR measurements were performed on self-supported samples (compacted under

pc being the capillary pressure (Pa), γ the surface tension of the fluid (N m−1), r the pore radius (m), and θ the contact angle. For mesopores, the value taken for γ is usually that one of the liquid bulk (γ = γbulk = 72.14 mN m−1) Tzanis et al.11 attempted to establish the relation between the intrusion and the pore diameter for water-zeosil systems having different pore geometries (channels and cages). They found that the pressure of intrusion is inversely proportional to the diameter of pore opening for the same pore geometry. However, their experimental data do not satisfactorily match the Laplace−Washburn equation. This raises the question of whether the Laplace−Washburn equation, though very robust for macroscopic systems, remains valid at lower scales. In particular, some deviations may appear at the nanoscale. First, the surface tension and the contact angle depend on the pore diameter.14 Second, when the pore diameter is reduced to a value close to the hydrodynamic diameter of the molecules entering the porosity, the concept of two phases separated by a hemispheric meniscus, on which the Laplace−Washburn equation is based, becomes questionable. The knowledge of the mechanical energy is not enough to understand the intrusion process of water in porous materials. The thermal energy (Qint) involved must also be determined because it can be a non-negligible part of the total energy exchanged. We have shown in the case of silicalite-1 that thermal energy represents almost half of the mechanical energy.15 Moreover, the enthalpy of intrusion can be calculated with the following relationships: ΔUint = Wint + Q int ΔHint = ΔUint + p(Vm,int − Vm,l) ≈ ΔUint

assuming that the intruded phase has the same molecular volume (Vm,int) as the liquid phase (Vm,l). Few experimental works have dealt with the determination of the enthalpy of intrusion for water in silicalite-1 or in functionalized mesoporous silica gel,16,17 though some values have been obtained by molecular modeling for water in MFI and FAU zeosils.18 Surprisingly, molecular simulation shows B

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an uniaxial pressure of 0.1 GPa) for MFI, and on a small volume of powder deposited on a KBr wafer for CHA, as self-supported samples cannot be created for this material.

These volumes are 53% lower than the microporous volumes determined by nitrogen adsorption (Table 2). Water molecules

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Table 2. Characteristics of the Porosity of MFI and CHA Zeosils As Determined by Nitrogen Adsorption at 77 K

RESULTS AND DISCUSSION Mechanical Energy of Water Intrusion. Intrusion isotherms of water in MFI and CHA zeosils are given in Figure 2. They are characterized in both materials by a one-step

zeosil MFI CHA

process that occurs when the capillary pressure is reached, at around 92 and 36 MPa for MFI and CHA, respectively. It may be noted that, before intrusion, the volume of intruded water for MFI is slightly negative. This is due to imperfect correction calculated through subtraction of blanks and may be caused by powder packing under the application of hydraulic pressure. In addition, thermodynamic values relative to the intrusion and extrusion of water in MFI and CHA zeosils are summarized in Table 1. Only small differences in pressure and volume are

a

(MPa) (cm3 g‑1) (J g‑1) (J g‑1) (J g‑1) (MPa) (cm3 g‑1) (J g‑1) (J g‑1) (J g‑1) (J g‑1)

cycle >1

MFI

CHA

MFI

CHA

92 0.096 8.83 7.8 16.63 88 0.096 −8.45 −1.5 −9.95 6.68

36 0.16 5.76 −7.85 −2.09 30 0.16 −4.8 7.43 2.63 0.54

90 0.096 8.64 3.5 12.14 88 0.096 −8.45 −1.6 −10.05 2.09

31 0.15 4.65 −7.6 −2.95 30 0.15 −4.5 6.74 2.24 −0.71

0.51 × 0.55 0.53 × 0.56 0.38 × 0.38

0.181

10-ring aperture channel

0.300

8-ring aperture cage

pore geometry

δ being the Gibbs adsorption at the intruded liquid/gas interface (m), 0.162 nm for water, and r the pore radius (m). For the latter, no satisfactory experimental values are given in the literature. This is due to the fact that it is extremely difficult to determine a finite contact angle for water on hydrophobic powdered materials. Most available values, in fact, are deduced from the pressure of intrusion using the Laplace−Washburn equation! For siliceous materials, the contact angle with water lies in the range 100° to 130°.25 Assuming a mean value of 115° and a mean pore radius of 0.28 and 0.19 nm for MFI and CHA, respectively, we have calculated the corresponding theoretical pressure of intrusion using the Laplace−Washburn equation with the above cited correction on the surface tension. This leads to a pressure of intrusion of 100 MPa for MFI and 119 MPa for CHA. The value for MFI is in agreement with the experimental value of 92 MPa (Table 1). However, for CHA the Laplace−Washburn equation greatly overestimates the pressure of intrusion (experimental being 36 MPa). Conversely, starting from the experimental pressure of intrusion for CHA, the value of the contact angle calculated with the Laplace− Washburn equation is 83°, an unrealistic value for such a hydrophobic siliceous material. This means that the use of the Laplace−Washburn equation is very sensitive to pore geometry; subsequently, its use appears questionable at the nanometric scale, because the notion of hemispheric meniscus at the liquid/gas interface vanishes when the pore radius

Table 1. Experimental Thermodynamic Values of Intrusion (int) and Extrusion (ext) of Water in MFI and CHA Zeosils at 298 Ka

pint Vint Wint Qint ΔUint pext Vext Wext Qext ΔUext ΔUcycle

microporous volume (cm3 g−1)

do not completely probe the inner space offered by such hydrophobic materials. In other words, the apparent density of the intruded water, given by the ratio of the intruded mass of water over the microporous volume of the material, is lower than the density of bulk liquid water. In both hydrophobic materials, the density of the intruded phase is around 0.55 g cm−3, almost two times lower than for the bulk liquid water at those intrusion pressures. This result is consistent with a water density of around 0.6 g cm−3 previously reported for MFI zeolite topology from GCMC simulations.23,24 This indicates that the intruded phase is different than the bulk liquid water. Surprisingly, the pressure of intrusion is lower in CHA than in MFI, while the pore opening diameter of MFI is higher (Table 2). It is interesting to compare these experimental values with the calculated ones using the Laplace−Washburn equation. The main difficulties in using this equation lie in finding the right values for the surface tension and the contact angle. For the former, Defay and Prigogine14 have shown that the surface tension depends on the pore radius and proposed the following relation: γ γ = bulk2δ 1+ r

Figure 2. Volume of water intruded in MFI and CHA zeosils during compression in the range 0.1−120 MPa at 298 K (data on chabazite have been obtained from ref 35).

cycle 1

pore opening diameter (nm)

Values are expressed per gram of zeolite.

observed between intrusion and extrusion, indicating a quasireversibility in the intrusion−extrusion phenomenon. These two water−zeosil systems can therefore be considered “molecular springs”. The volume of intruded water is equal to 0.096 and 0.16 cm3 per gram of anhydrous sample for MFI and CHA, respectively. C

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Figure 3. Differential heat of water intrusion and extrusion in MFI and CHA during the first cycle of compression and pressure release at 298 K.

approaches the same order of magnitude as the size of the intruding molecules. Thermal Energy of Water Intrusion. The differential heats of intrusion δQint/mdp measured during the first cycle of compression and pressure release are given in Figure 3 for MFI and CHA. The corresponding integral heat of intrusion Qint and the internal energy of intrusion are displayed in Table 1. The integral heat of intrusion (absolute value) is in the same order of magnitude as the mechanical work for both materials. When considering the total energy stored in such systems, the thermal energy represents a non-negligible part. It is also worth noting that the intrusion of water in MFI is endothermic, while it is exothermic in CHA. This is in line with the results found by Caillez et al.18 by GCMC simulation on MFI and FAU zeosils. This result seems very surprising at first glance because these two materials have a similar surface chemistry; subsequently, the interactions of water with both MFI and CHA siliceous surfaces are expected to be equivalent. Therefore, the opposite thermal effects observed in MFI and CHA can only originate from the difference in their pore size and geometry. From a very simple geometric calculation, taking into account the density of intruded water (0.55 g cm−3) and the volume offered by the pores (either a cylindrical channel, a zigzag channel or an intersection for MFI, or a cage for CHA), the number of water molecules inside each pore type can be estimated (Figure 4). In MFI, a straight channel, a zigzag channel, and an intersection can accommodate 1.9, 1.1, and 2.4

water molecules, respectively, corresponding roughly to 2 water molecules per channel. As the kinetic diameter of water (0.265 nm) is close to the channel radius (0.28 nm) and as the water is rather far from the surface because the interaction between the water and the hydrophobic surface is not favorable, we can admit that two water molecules cannot be located beside each other in the plane perpendicular to the channel axis. So, we could reasonably consider that water molecules confined in this porous system are in single file. In CHA, cages offer more free space, which leads to a pore filling of approximately 11 water molecules. In CHA the distance between the water molecules i.e. the strength of hydrogen bonds are probably different than those in the liquid bulk but the number of hydrogen bonds per water molecule is similar. The ratio of surface water molecules is obviously high (9/11). However as the inner silica surface is hydrophobic the interaction between the water molecule and this surface is weak. The water-surface distance is then longer than the water−water distance. So we can assume that the water molecules intruded in the CHA cage form a “bulk-like” water cluster. Clearly the coordination of water molecules in MFI and CHA is not the same. Considering the geometrical restraint for MFI, it should be around two, most certainly lower than the tetrahedral coordination of liquid bulk water. For CHA, the coordination of intruded water molecules is probably not too different than that of the bulk. In order to explain why the phase transition from the liquid bulk to the intruded water can be either endothermic or exothermic, as suggested by Caillez et al.,18 the internal energy of intrusion can be seen as the sum of the energy of interaction of water with the zeolite surface (ΔUw/z) and the energy due to the change in the number of hydrogen bonds between water molecules (ΔUHB): ΔUint = ΔUw/z + ΔUHB

The former is always negative (exothermic effect), while the latter can be positive or close to zero (endothermic effect). In MFI, the intrusion of water takes place with a loss of hydrogen bonds because the coordination of water molecules decrease from 4 for the bulk to roughly 2 for the intruded phase (Figure 4). This endothermic phenomenon counterbalances the exothermicity of the interaction of molecules with the surface, and leads to a positive internal energy of intrusion. For MFI, taking −30 kJ per mol of water for ΔUw/z26,27 and 2.28 kJ per mole of water for ΔUint (considering 12.14 J g−1 in Table 1, a value which becomes constant for cycle >1, as

Figure 4. Schematic representation of how water molecules huddle in the MFI straight channels or in CHA cages. This very straightforward view of a probably more complex molecular arrangement gives a good idea of some limitations imposed by the restricted geometry of these nanoporous systems. D

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Figure 5. FTIR absorbance spectra in the wavenumber range 4000−3000 cm−1 for MFI and CHA before (blue) and after (red) water intrusion at 298 K. These spectra were collected after complete outgassing of the sample.

Figure 6. Differential heat of water intrusion and extrusion in MFI and CHA at 298 K during the second and third cycles of compression and pressure release.

detailed in the next section), the value of ΔUHB is estimated to be 32.28 kJ mol−1. For bulk liquid water, the hydrogen bond energy is equal to 23.3 kJ per mole of hydrogen bonds.28 Assuming that the hydrogen bonds in the intruded phase have the same energy, ΔUHB corresponds to a loss of 1.4 in the average number of hydrogen bonds. This corresponds nicely with the hypothesis of single file water molecules confined in the channels of MFI. Running the same calculation for CHA, there is not sufficient change in the coordination of water from the bulk to the intruded phase to counterbalance the energy of interaction of water with the zeolite surface. Therefore, the internal energy of intrusion is exothermic. Nevertheless, with a ΔUHB estimated to 29.65 kJ mol−1, there is a significant change in the hydrogen bond network between the liquid bulk and the intruded phase. This can be due either to a decrease in the number of hydrogen bonds with the nearest neighboring molecules, as for MFI, or to a decrease in the energy of hydrogen bond interactions. In addition, the internal energy of intrusion can be expressed as kJ per mole of intruded water. This gives a value of around 2.3 kJ mol−1 for MFI and −0.4 kJ mol−1 for CHA. These values are greatly higher than the internal energy of solidification of water (−6 kJ mol−1 at 273 K). Thus, the intruded water also has less hydrogen bonding than the ice.

It may also be pointed out that, after intrusion, the differential heat of water intrusion does not return to zero (Figure 3). Nevertheless, these results have been corrected assuming the compressibility of the intruded phase is the same as that of the liquid bulk. This, therefore, reinforces the hypothesis that the intruded water in hydrophobic nanopores is not in the same physical state as the liquid bulk water. Moreover, considering the first intrusion-extrusion cycle of water, a slight difference is observed in pressure between intrusion and extrusion for both MFI and CHA (Figure 3 and Table 1). However, the intruded and extruded volumes remain the same. This hysteresis phenomenon probably results from the metastability of the intruded fluid29 and the nucleation of vapor bubbles during extrusion, as is often encountered in mesoporous silica materials.30,31 Stability of Materials under High Hydrostatic Pressure. In the case of MFI, it is noteworthy that the change in internal energy ΔUcycle is not equal to zero (Table 1). This inconsistency with the first principle of thermodynamics comes from the fact that, in absolute value, the heat of intrusion is greatly higher than the heat of extrusion, while mechanical work is similar. This difference between intrusion and extrusion heats is attributed to a modification in the structure of the material.15,17 During the first intrusion, some siloxane bonds are broken (endothermic effect), leading to the creation of E

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maintain the same coordination as that of bulk water. This intruded phase would be a weaker hydrogen-bonded water structure lacking tetrahedral coordination. Nanoscale can be defined here as the scale at which there is a change in the properties of water molecules imposed by geometrical restraints. It therefore implies that classical and robust macroscopic equations such as Laplace−Washburn are limited in their top-down use at this scale. In particular, the notion of liquid/gas interface with a hemispheric meniscus does not exist at this molecular level. A very few silanols are created from siloxane bond breakage when water is forced to enter the hydrophobic nanopores, primarily during the first intrusion. Thus, the thermal energy involved during intrusion includes the phase transition energy from the bulk phase to the intruded phase, but also the energy involved in the formation of silanol defects and the energy of interaction of water molecules with these defects. However, these last two contributions are minor, and the endothermicity or exothermicity of intrusion is governed mainly by the change in the coordination of water molecules as a function of the internal space, namely, channels or cavities in the present study. From a more practical point of view, the two systems studied behave as molecular springs (after a very slight damper behavior during the first intrusion) and are fairly stable along cycles. Complete thermodynamic study using high pressure calorimetry shows they are not, however, equivalent. Due to the higher intrusion pressure in MFI, it is able to store two times more mechanical energy than CHA, although CHA offers higher porosity. The absolute value for thermal energy, which is two times lower for MFI than for CHA, is not negligible compared to the mechanical work. As a consequence, any further development of energy storage based on these systems would require efficient dissipation of thermal energy.

hydrophilic silanol groups on which water molecules are then able to adsorb (exothermic effect). Figure 5 displays the infrared spectra before and after water intrusion in MFI, after in situ outgassing at room temperature under secondary vacuum. The sharp and intense band at 3731 cm−1 is assigned to the OH symmetrical stretching vibration of isolated silanol groups. The second broad band at lower frequency (3650−3700 cm−1) is attributed to the stretching vibration of terminal silanols, whose oxygen atoms are involved in hydrogen bonding with nearer hydroxyls. The third very large band in the 3200−3650 cm−1 region is due to the stretching vibration of vicinal hydrogenbonded silanols, whose hydrogen atoms are involved in weak hydrogen-bonding with nearer hydroxyls.32−34 It is not possible to know where these silanol defects are exactly located. However we showed in a previous paper that they are formed inside the micropores and not on the external surface.15 In the case of CHA, ΔUcycle is lower than that of MFI, but it is still not equal to zero (Table 1). This indicates that a few silanol defects are also created during intrusion in CHA. The infrared spectrum for CHA displays the same narrow vibrational band for isolated silanols as for MFI, but a poorly resolved band for silanols interacting by hydrogen-bonding (Figure 5). This suggests that silanol defects are in less extent in CHA than in MFI. After water intrusion the surface chemistry is then slightly modified, the formation of silanol groups making it less hydrophobic. It may be noticed that the microporosity is also slightly altered by the presence of silanol defects. Though ex situ XRD analysis does not evidence any structural modification (because not enough sensitive), we observed by nitrogen and nhexane adsorption that the microporous volume is slightly decreased after intrusion. The pore size distribution is however maintained. The decrease in the adsorption capacity is probably due to the obstruction of some micropores by silanols groups. Intrusion-extrusion cycles of water performed in the same pressure range evinced that intrusion pressure is still slightly higher than the extrusion pressure (Figure 6 and Table 1). This result, observed for each cycle, is due to the metastability of the confined fluid, as previously explained. The pressure for the first intrusion is also observed to be higher than for those following. As hydrophilic sites appear during the first intrusion (Figure 5), the intrusion pressure for the next cycles is necessarily reduced. This suggests that the silanol defects are essentially created during the first intrusion of water into pores. Nevertheless, for cycles over the first intrusion the change in internal energy ΔUcycle still remains greater than zero and is greatly reduced for MFI. The breaking of siloxane bonds still occurs, but to a lesser extent than for the first intrusion. These results are, moreover, in line with the characterization of silanols performed by NMR spectroscopy after successive intrusion−extrusion cycles.15,35,36 For both MFI and CHA, there is no change in the X-ray diffractograms, suggesting that only a very few silanol defects are created when water enters the nanopores.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the French Agence Nationale de la Recherche through the ANR program “Heter-eau”, under Contract No. BLAN 06-3_144027.

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REFERENCES

(1) Deschamps, J.; Audonnet, F.; Brodie-Linder, N.; Schoeffel, M.; Alba-Simionesco, C. A thermodynamic limit of the melting/freezing processes of water under strongly hydrophobic nanoscopic confinement. Phys. Chem. Chem. Phys. 2010, 12 (7), 1440−1443. (2) Smirnov, S.; Vlassiouk, I.; Takmakov, P.; Rios, F. Water Confinement in Hydrophobic Nanopores. Pressure-Induced Wetting and Drying. Acs Nano 2010, 4 (9), 5069−5075. (3) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108 (1), 74−108. (4) Chandler, D. Hydrophobicity: Two faces of water. Nature 2002, 417 (6888), 491. (5) Mallamace, F.; Corsaro, C.; Baglioni, P.; Fratini, E.; Chen, S. H. The dynamical crossover phenomenon in bulk water, confined water and protein hydration water. J. Phys.-Condens. Mater. 2012, 24, 6.

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CONCLUSIONS This thermodynamics study of water confinement in nanoscale hydrophobic environments clearly evidences that such a transition is an endo- or exothermic phenomenon, depending on the geometry of the environment in which water is confined. This result is of fundamental importance. It points out that there is clearly a modification in the structure of confined water, probably dictated by the relative difficulty of water molecules to F

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