Hydration of MCM-41 Studied by Sorption Calorimetry - The Journal of

12906

J. Phys. Chem. C 2007, 111, 12906-12913

Hydration of MCM-41 Studied by Sorption Calorimetry Vitaly Kocherbitov*,† and Viveka Alfredsson‡ Biomedical Laboratory Science, Faculty of Health and Society, Malmo¨ UniVersity, SE-205 06 Malmo¨, Sweden, and Physical Chemistry 1, Center for Chemistry and Chemical Engineering, P. O. Box 124, Lund UniVersity, SE-221 00 Lund, Sweden ReceiVed: March 29, 2007; In Final Form: June 18, 2007

Hydration of mesoporous silica MCM-41 was studied using the method of sorption calorimetry. By combining water sorption and nitrogen sorption experiments, we calculated the density of silanol groups on the MCM41 surface as 1.6 nm-2. Comparison of capillary condensation regimes of water and nitrogen showed that the apparent density of water confined in MCM-41 pores is ca. 0.88 g/cm3. The pore diameter calculated using a combination of X-ray and sorption data is 39 Å. Calculations based on application of the Kelvin-Cohan equation on the water sorption data are in reasonable agreement with this value. The sorption calorimetric results show that the capillary condensation of water in the pores is driven by enthalpy; the entropic effect is negative. A mechanism of hydration that involves formation of small unfilled cavities adjacent to pore walls can be used to explain the observed enthalpy end entropy effects. Comparison of sorption and desorption data indicates the presence of air trapped in pores when hydration is performed by mixing MCM-41 with liquid water. The heat effect of pre-capillary condensation adsorption of water on hydroxylated MCM-41 is much more exothermic compared to the original calcined material.

Introduction Sorption of gases, especially nitrogen, is a standard method of characterization of porous materials. Using sorption of gases, one can determine the surface area of a porous material, the volume of the pores, and estimate the size of the pores.1 The advantage of the use of inert gases such as nitrogen for sorption studies is their low specificity to studied surfaces: one can, for example, expect only a small difference between nitrogen adsorption properties on hydrophilic and hydrophobic surfaces. This low specificity allows application of universal algorithms for the assessment of properties of porous materials from gas sorption experiments. However, sorption of gases provides only geometrical information about studied systems and is not able to characterize the intermolecular interactions of the material with the molecules typical for the environment at the conditions at which the materials are normally used. Water is a substance that is present almost everywhere unless special precautions are taken. Water vapor can interact with porous materials, causing adsorption on the surface of the pores and capillary condensation in the pores. This makes studies of interactions of water vapor with porous materials an important part of their physical characterization. Water sorption experiments may provide information not only on geometry but also on the physics and chemistry of the surface of a material. Moreover, the physical state of water confined in the nanopores is of fundamental importance. One of the porous materials often used as a model material for studies of sorption of gases on solids is MCM-41.2 This material has a highly ordered two-dimensional hexagonal structure and a narrow pore-size distribution. Several studies * Corresponding author. Address: Faculty of Health and Society, Malmo¨ University, SE-205 06 Malmo¨, Sweden. Ph: +4640 6657946. Fax: +4640 6658100. E-mail: [email protected]. † Malmo ¨ University. ‡ Lund University.

of nitrogen sorption on MCM-41 showed the absence of the sorption-desorption hysteresis.1,3-6 The absence of the hysteresis in these cases is a result of a small pore size (typically in the range of 2-4 nm). However, in MCM-41 samples with larger pore sizes hysteresis loops can be observed.7,8 The presence of hysteresis in gas sorption experiments depends not only on the pore size but also on the temperature of the sorption experiment. The sorption-desorption hysteresis is observed only below hysteresis critical temperature, Tch. The larger the pore size, the higher the hysteresis critical temperature.9 Nitrogen sorption data can be used to calculate the pore diameters of mesoporous materials. The simplest way to assess the pore diameter is to calculate it form the volume of the pore to the surface area ratio. The accuracy of this method is limited by the fact the surface area calculated from the gas adsorption data can be overestimated because of the surface roughness. Another approach is to use the Barrett-Joyner-Halenda (BJH) method10 based on the Kelvin equation. This method, however, underestimates the pore size because it neglects the effect of the curvature of the pore walls on the thickness of adsorbed gas layer.11 The Broekhoff-de Boer method12 and some later works11,13 take this effect into account. An alternative way for accurate calculation of the pore diameter was introduced by Kruk, Jaroniec, and Sayari14 who used an empirical correction to Kelvin equation. Unlike the methods based on the Kelvin equation, the methods based on density functional theory15-17 (DFT) are derived from microscopic considerations. According to Ravikovitch et at,17 the method of determination of the pore size based on nonlocal DFT calculations appears to be the mostaccurate method. The methods mentioned above are normally used to treat the sorption isotherms of inert gases. Studies of water sorption on mesoporous silicas are focused mostly on the hydrothermal stability of the materials and on the presence of hysteresis but not on characterization of the geometry of the pores. Several

10.1021/jp072474r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/16/2007

Hydration of MCM-41 Studied by Sorption Calorimetry studies of sorption of water by different types of MCM-41 materials have been reported. Branton et al.18,19 have studied the sorption of water and other substances by MCM-41. They found a pronounced capillary condensation step in the sorption isotherm of water that showed a sorption-desorption hysteresis. The precondensation regime of the sorption isotherm was reversible, and the authors concluded that exposure to water leads to very little, if any, rehydroxylation of the silica surface. Alternatively, several other authors presented results that indicate hydroxylation of the surface of MCM-41. Data on MCM-41 and FSM-16 presented by Takahara et al.20 and Inagaki et al.,21 respectively, show significant differences between the first and second runs in the water sorption experiments. Matsumoto et al.22-24 found a large difference between the first and the second sorption runs both before and after the capillary condensation regime of hydration of MCM-41 and FSM-16. Llewellyn et al.25 reported a water sorption isotherm of aluminosilicate MCM-41 with a pore size of 2.5 nm. They noted a relatively hydrophobic character of the material and characterized the sorption isotherm as type-V according to IUPAC classification. From the temperature dependence of the sorption isotherm, they calculated the heat of adsorption of water. Smirnov et al.26 presented a sorption isotherm of MCM-41 that contained a hysteresis loop. They also reported that when the humidity is increased up to 100% the sample is seriously damaged. Rozwadowski et al.4 studied water sorption on AlMCM-41 with different Si/Al contents and discussed the dependence of the sorption capacity of the material on the Si/ Al ratio. Floquet et al.27 presented a water sorption isotherm of MCM-41 and noted that the adsorption film of water does not grow; only the capillary condensation phenomenon is observed. Gru¨nberg et al.28 studied hydrogen bonding of water confined in MCM-41 and SBA-15 using NMR. They found that no water molecules strongly bound or confined in inaccessible places are present in dried MCM-41. At water content of 23 wt %, two NMR lines were visible, which was attributed to the coexistence of filled pores and pores with water adsorbed on the surface. Here we present a sorption calorimetric study of the hydration of MCM-41. The method of sorption calorimetry29,30 allows simultaneous measurements of the water activity and the enthalpy of hydration of solid materials. Earlier, a very high resolution of the method allowed an accurate thermodynamic description of small low-energetic phase transitions in surfactant and lipid systems.31-34 The method was also used to study proteins35,36 and DNA.37 In this paper, we focus on a thermodynamic analysis of processes of hydration of mesoporous silica MCM-41 and demonstrate that a combination of water and nitrogen sorption experiments provides additional physicochemical information for the characterization of mesoporous materials. Materials and Methods MCM-41 was synthesized according to protocols published previously.3,38 Cetyl trimethyl ammonium bromide (0.60 g) was dissolved in 30 mL of Milli-Q water. NH3 (25%, 2.45 mL) was added, and the solution was tempered to 30 °C. Thereafter, 2.5 g of tetraethylorthosilicate was added to the solution. The reagent solution was left stirring at 30 °C for 24 h after which it was hydrothermally treated at 90 °C for another 24 h. The resulting powder was filtered and washed with water. Calcination was performed according to the following: from RT to 550 °C, 1 °C/ min, held at 550 °C at 300 min, and from 550 to RT, 5 °C/min. SAXS experiments were performed on the as-synthesized and calcined materials using a Kratky small-angle system equipped

J. Phys. Chem. C, Vol. 111, No. 35, 2007 12907 with a position-sensitive detector. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID 3000 X-ray generator. Nitrogen sorption isotherms were recorded at 77 K using a Micrometrics ASAP 2400 instrument. Electron micrographs were recorded on a Philips CM120 Biotwin operated at 120 kV using a CCD camera (Gatan). Sorption calorimetric experiments were conducted at 25 and 40 °C in a two-chamber sorption calorimetric cell inserted in a double-twin microcalorimeter.29,39 The studied samples were placed in the upper chamber, while pure water was injected in the lower chamber. In a sorption experiment, water evaporates from the lower chamber, diffuses through the tube that connects the two chambers, and is adsorbed by the sample in the upper chamber. The thermal powers released in the two chambers are monitored simultaneously. The activity of water, aw, in the sorption experiments was calculated from the thermal power of vaporization of water in the lower chamber as described in ref 40. The partial molar enthalpy of mixing of water was calculated using the following equation vap vap Hmix w ) Hw + Hw

Psorp Pvap

(1)

where Pvap and Psorp are thermal powers registered in the vaporization and sorption chambers, respectively, and Hvap w is the molar enthalpy of evaporation of pure water. For accurate calculation of the partial molar enthalpy of mixing of water, the sorption calorimeter was calibrated using magnesium nitrate as a standard substance.41 The partial molar entropy of mixing of water was calculated as follows:

Sm w )

Hm w - R ln aw T

(2)

In desorption calorimetric experiments, the hydrated sample was placed in the upper chamber, while in the lower chamber a solution of magnesium nitrate was injected. The salt solution was used as a vapor sink.30 The desorption experiments were done at 25 °C. Results and Discussion The water and the nitrogen sorption isotherms of MCM-41 are shown in Figure 1. The sorption isotherms are presented as functions of water (or nitrogen) content, not as functions of relative pressure. This is done in order to be able to compare this data with the enthalpy and entropy data measured by sorption calorimetry. The sorption isotherms as functions of relative pressure (as well as the complete data on all sorption calorimetric experiments) are presented in the Supporting Information. No significant hysteresis is seen in the nitrogen sorption isotherm, which is due to the small size of the pores in the studied material. The water sorption isotherm presented in Figure 1 does not show any desorption data; the water desorption experiments will be discussed later. In both nitrogen and water sorption isotherms, one can see three regimes: the surface adsorption of the adsorbate, the capillary condensation plateau, and the final regime of high pressures. The most pronounced difference between the two sorption isotherms is in the first regime: it is very short in the case of adsorption of water and very long in the case of nitrogen (more than a half of the sorption isotherm). The reason for this is not only in the low hydrophilicity of the material of the walls but also in the large difference of the surface tensions of the two liquids. Adsorption of a liquid

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Figure 1. Water (upper curve) and nitrogen (lower curve) sorption isotherms of MCM-41. In the nitrogen sorption isotherm, triangles denote sorption and circles denote desorption; the curve is only for the guidance of the eye.

in a cylindrical pore leads to the formation of a long cylindrical meniscus (an interface between the liquid and vapor). Because of the high surface tension of water, this is strongly energetically unfavorable and hence the capillary condensation of water starts at low water contents. Nitrogen and Water BET Areas. Nitrogen adsorption is a standard method for the determination of surface area of porous materials. Most often the results of nitrogen adsorption experiments are treated using the BET model42 that assumes formation of multilayers of the adsorbed substance. From the amount of the adsorbed substance that is required to build a monolayer (and knowing the area per molecule, which is 0.162 nm2 for nitrogen1), one can calculate the surface area of the sample. The surface area, A, of the present sample of MCM-41 calculated using nitrogen adsorption data is 994 m2/g. BET calculation from the data on sorption of water gives the value of the amount of water needed for the monolayer coverage hw ) 0.048 g of water per g of silica. Assuming the surface area of a water molecule to be 0.105 nm2 (ref 25), this amount would correspond to the surface area of 170 m2/g. Such a big difference between the areas obtained by nitrogen and water adsorption data indicates that the “monolayer” of water assumed in BET calculations does not cover the whole silica surface but is formed by adsorption of water molecules on separated individual hydroxyl groups. If this “monolayer” of water molecules corresponds to one water molecule per hydroxyl group, then one can calculate the number of hydroxyl groups per square nanometer:

nOH )

hwNa MwA × 1018

(3)

The number calculated according to eq 3 is 1.6 nm-2, which is about three times less than the amount of OH groups (4.6 nm-2) corresponding to the fully hydroxylated surface.43 The calculated silanol number of 1.6 nm-2 is in good agreement with the data on the OH group surface density as function of pretreatment temperature.43 The value presented here is lower than the value determined by Zhao et al.44 in their NMR study (2.5 to 3.0 nm-2) but higher than the value estimated by Llewellyn et al.25 for Al-MCM41 (1.2 nm-2) from the weight loss upon heating. Widenmeyer

and Anwander45 presented silanol numbers of 1.4, 1.75, and 1.9 nm-2 for different types of mesoporous silica MCM-48 determined using silylation. Comparing the silanol numbers presented by different authors, one should keep in mind that the results can be dependent on both techniques used and on procedures of synthesis of mesoporous materials. Capillary Condensation of Water and Nitrogen. The capillary condensation regime in the sorption isotherm of water is much longer than that in the sorption isotherm of nitrogen, but its end corresponds to a lower volume of the sorbate (calculated per bulk liquid properties). The amount of water in Figure 1 is expressed as mass of water per mass of silica (g/g); the amount of nitrogen is expressed as the volume of liquid nitrogen per mass of silica (cm3/g). Assuming that the density of liquid water is close to 1 cm3/g, one should expect that the end of the water capillary condensation regime would show the total volume of the pores in the material. For this reason, the ends of the capillary condensation regimes of the two sorption isotherms should coincide. Nonetheless, the capillary condensation regime of nitrogen continues to higher values of volume of nitrogen, VN2, compared to the values of masses of water, mw. One possible explanation for this fact would be a decrease of the density of water in the pores compared with that of the bulk. If this is the case, then the apparent density of water in the pores can be calculated using the volume corresponding to the end of the capillary condensation regime of nitrogen (0.780 cm3/g) and the mass of water that corresponds to the end of the same regime (0.684 g/g). The apparent density of the confined water calculated in this way is 0.877 g/cm3. One has to note that the data on nitrogen and water sorption were collected at different temperatures: 77 K in the case of nitrogen and 298 K in the case of water. One should therefore take into account the possibility of thermal expansion/contraction of silica. The data on thermal expansion of silica show, however, that this effect is very small: the thermal expansion coefficient of amorphous silica is very small (on the order of 10-7 K-1). Moreover, it is negative at temperatures below 150 K and positive at higher temperatures.46 Another possible source of errors is adsorption and capillary condensation of water or nitrogen on the surface of MCM-41 particles and in the space between them. Considering that the size of the particles is in the range of 100-1000 nm (Figure 2a), we suggest that the surface of the particles is several orders of magnitude lower than the inner surface of the pores and the adsorption on the outer surface of the particles would have a minor effect on the results. The amount of water condensed between the particles is also insignificant because the end of the capillary condensation plateau corresponds to the filling of the pores that are equal or less than 4 nm in diameter (see the discussion below). Some uncertainty may arise from the change of density of liquid nitrogen upon adsorption in the pores. Still, the change of structure of water (that is determined by specific hydrogen bonding) upon capillary condensation is expected to be much larger than the corresponding change of structure of liquid nitrogen. Therefore, the discrepancy between water and nitrogen sorption data indicates the change in the density of water rather than the density of nitrogen. The formal density of water in the cylindrical pores of MCM-41 calculated here (0.877 g/cm3) is in good agreement with the density of water in the 4-nm pores of Vycor glass used in ref 47 (0.888 g/cm3) calculated from small-angle neutron scattering data.48 Although the surface properties of MCM-41 and Vycor glass may be different, the data obtained by the two different methods indicate similar decrease of the density of water in the pores of similar

Hydration of MCM-41 Studied by Sorption Calorimetry

J. Phys. Chem. C, Vol. 111, No. 35, 2007 12909 be calculated from the area of the unit cell, Auc, and the volume fraction of the pores, φp

r)

x

Aucφp )d π

x

2φp

πx3

) 0.6063‚dxφp

(4)

where d is the (100) repeat distance measured in an X-ray scattering experiment (40.3 Å for this particular calcined MCM41 sample). The volume fraction of the pores can be calculated from the water to silica mass ratio, hw, that corresponds to the end of the capillary condensation regime

φp )

1 1 + dw/(hwds)

(5)

where dw and ds are the densities of water and silica, respectively. In a similar way, the volume fraction of the pores can be calculated from the nitrogen sorption data.49,50 The radius of the pores calculated according to eqs 4 and 5 is 19.4 Å. We assumed that the density of amorphous silica is 2.2 g/cm3 and the density of water in the pores is 0.877 g/cm3. The pore size calculated in this way is not strongly dependent on the used value of the density of water: assuming the bulk density of water we obtain is 18.9 Å. Thus, the combination of X-ray and water-vapor sorption is a good tool for the determination of the pore size of ordered mesoporous materials. It is more accurate than the calculations based on the volume to surface ratios obtained in nitrogen sorption experiments because the surface area calculated using the BET method can be overestimated because the surface is not perfectly smooth. However, calculations based on combination of X-ray and nitrogensorption data can be slightly more accurate than those based on a combination of X-ray and water-sorption data because the change of the density of liquid nitrogen upon capillary condensation is expected to be smaller than the respective change of density of water. Calculation of Pore Size using the Kelvin-Cohan Equation. The activity of water in the capillary condensation regime is remarkably constant (especially in the beginning of this regime) and was reproducible in two separate sorption experiments. One can therefore use the Kelvin-Cohan equation51 to calculate the radius (r) of the pores:

r-t)Figure 2. Transmission electron micrographs of MCM-41. (a) Lowmagnification TEM showing particles (dark gray) dispersed on the supporting holey carbon film (light gray). The particles are generally elongated with the pores aligned along the longest axis of the particles. (b) Higher magnification image of a particle along the [001] direction. The characteristic honeycomb pattern of pores aligned along the direction of the electron beam is clearly visible.

diameter. It should be kept in mind, however, that the decrease of the apparent density of water does not necessarily reflect only the change of the structure of water. Surface roughness and formation of small unhydrated cavities at the silica-water interface may contribute to the apparent decrease of water density (see the discussion below). Calculation of the Pore Size from X-ray Data and the Sorption Isotherm. Combination of the X-ray and sorption data allows the calculation of the pore size with good accuracy. From a simple geometrical consideration, the radius of the pores can

2γ cos θ‚Vm RT ln aw

(6)

Here γ is the surface tension of water (72.0 mJ/m2 at 25 °C), aw ) 0.607 is the water activity measured in the sorption experiment at 25 °C, t is the thickness of the preadsorbed water layer, and Vm is the molar volume of liquid water (18.07 × 10-6 m3/mol in the bulk or 20.5 × 10-6 m3/mol in the pores). When eq 6 is used for the calculation of the pore size from the nitrogen sorption data, it is assumed that the contact angle (θ) is zero. This assumption is based on the fact that nitrogen condenses on a preadsorbed layer of nitrogen molecules. In the case of water, the amount of preadsorbed water molecules is very small (not the whole surface is covered); therefore, the contact angle is not necessarily zero. The exact value of the contact angle for the present sample of MCM-41 is not known, but we tried to estimate it from literature data on the contact angle of water on quartz. Lamb and Furlong52 proposed a formula that connects the fraction of silanol groups on the surface a(SiOH) with the contact angle of water on quartz:

cos θ ) 0.257‚a(SiOH) + 0.743

(7)

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Assuming a(SiOH) to be 1.6 nm-2 /4.6 nm-2, where 4.6 nm-2 is the maximum possible number of OH groups per square nanometer,43 we obtain a contact angle value of about 34°. Equation 6 calculates the radius of the capillary minus the thickness of the preadsorbed layer of water. Because the volume of condensed liquid is proportional to (r - t)2 and the total volume of the pores is proportional to r 2, one can write

Vtot r 2 ) Vtot - Vads r-t

( )

(8)

The correction for the preadsorbed water will modify eq 6 in the following way:

r)-

x

2γ cos θ‚Vm Vtot ‚ Vtot - Vads RT ln aw

(9)

The ratio between the preadsorbed and the total volumes can be estimated from the water-to-silica mass ratios that correspond to the beginning and end of the capillary condensation regime. In the present case, Vads/Vtot ) 0.068/0.684. The radius calculated using eq 9 is 20.9 Å in the case of the pore density of water and 18.4 Å using the bulk density of water. We suggest that it is more correct to use the molar volume of water in pores in eq 9 rather than the bulk molar volume because in the free-energy balance between the condensation of water and the elimination of the solid-vapor interface, the molar volume defines the amount of condensed water. In the cases when the actual density of water in the pores is not known, the bulk density can be a good approximation. The higher value of the radius of pores calculated using eq 9 compared to that calculated using eq 4 can be explained by two factors. First, the contact angle is not accurately known; higher values of the contact angle would give lower pore radii in eq 9. Second, the thermodynamic properties of water in the capillaries may be different from the bulk ones, assumed in the derivation of the Kelvin-Cohan equation (eq 6). If the Gibbs energy of water in pores is different from the Gibbs energy of bulk water, then

Gpore gGbulk

(10)

because otherwise the bulk water would spontaneously turn into the pore water in the absence of pores. Thus, the activity of water in the pores apores should not be less than 1. The activity w of water, aw, used in eqs 6 and 9 can be thought of as the activity of water outside pores at which water fills the pores, whereas is the actual activity of water inside the pores. Equation 6 apores w can than be rewritten as

r-t)-

2γ cos θ‚Vm RT ln(aw/apores w )

(11)

Equation 9 can be modified in an analogous way. The value of apores cannot be determined independently, but one can roughly w estimate its value comparing the results of the calculations of pore radius from eqs 4 and 9. Correction of eq 9 with an apores w value of 1.04 produces the radius close to that calculated from ) 1.04 is calculated based on several eq 4. The value of apores w approximations and therefore cannot be considered as particularly accurate. Nonetheless, it shows an approximate magnitude of deviation of the activity of water in pores from that in the bulk.

Figure 3. Water sorption isotherms of initial MCM-41 (solid curve), MCM-41 hydroxylated at mild conditions (dashed curve), and MCM41 hydroxylated at elevated temperature in the presence of ammonia (dotted curve).

Hydroxylated MCM-41. One of the sources of uncertainties in the presented calculations is the value of the contact angle. The contact angle of the fully hydroxylated silica surface would be close to zero, which would make the calculations more accurate. We attempted to hydroxylate the calcined MCM-41 using two different procedures: mild hydroxylation and intensive hydroxylation at more severe conditions. For mild hydroxylation, the material was equilibrated with water vapor provided by a saturated salt solution of KNO3 (about 93 % RH). For intensive hydroxylation, the sample was dispersed in water, treated at 80 °C in the presence of a small amount of ammonia, and than washed with water. After these procedures, the samples were dried in vacuum and sorption experiments were performed. The sorption isotherms of the hydroxylated materials in comparison with that of the original MCM-41 are shown in Figure 3. Both hydroxylated materials adsorb more water than the initial material does prior to the capillary condensation. This shows that both mild and intensive procedures lead to hydroxylation of the surface of MCM-41. The sorption isotherm of MCM-41 changes from type V to type IV after mild hydroxylation. The sorption isotherm of the material hydroxylated at mild conditions has a pronounced capillary condensation regime with the water activity lower than in the original material. The lowering of the water activity at the capillary condensation regime is due to two factors. First, the value of the contact angle decreased after hydroxylation and, second, the amount of water adsorbed on the silica walls prior to capillary condensation is higher, which decreases the diameter of the cylinder that needs to be filled during the capillary condensation. At high relative humidities, the amount of water taken up by the material hydroxylated at mild conditions is lower than the amount taken up by the original material. This might be interpreted as an indication of degradation of the silica skeleton of MCM-41. In a similar case of hydration of FSM-16, Matsumoto et al.24 interpreted a decrease in the amount of water taken up by the material in the second run as a result of a collapse of the porous structure. For our data on the mildly hydroxylated MCM-41, we propose another interpretation. Every water molecule that hydroxylates a surface siloxane group turns it into two silanol groups. This has two effects related to capillary condensation of water: first, the mass of the silica skeleton increases exactly by the mass of one water molecule per reaction; second, the

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J. Phys. Chem. C, Vol. 111, No. 35, 2007 12911

volume of the pore decreases approximately by the volume of one water molecule per reaction. This means that the amount of liquid water that condenses in the hydroxylated material (per mass of the material) is lower than the amount of water that condenses in the original material. The sorption data presented in Figure 3 show that the difference between water-to-silica mass ratios of the initial and hydroxylated material after the capillary condensation plateau (taken at 80% RH) is 0.05. Interestingly, this value is close the difference between mass ratios corresponding to water monolayer coverage in the hydroxylated (hw ) 0.095) and initial MCM-41 (hw ) 0.048) calculated using the BET model. Even if one water molecule chemically reacted with the surface during hydroxylation should not exactly correspond to one water molecule adsorbed on the surface of hydroxylated material, this comparison shows that the assumption that the silica skeleton is preserved after mild hydroxylation is reasonable. Another possible contribution to the decrease of the amount of water taken by MCM-41 after hydroxylation may come from changes in the geometry of the material. Our X-ray data indicate a small decrease in the repeat distance of the material after hydroxylation. This decrease is probably due to the increase of the density of the amorphous silica during hydroxylation. Unlike the material hydroxylated at mild conditions, MCM41 hydroxylated at higher temperature in the presence of ammonia lost its original structure. This is clear from the absence of a pronounced capillary condensation plateau on the sorption isotherm of this material (Figure 3). The total amount of water taken up by the intensively hydroxylated material (measured at high water activities) was much lower than that of the original MCM-41 and of MCM-41 hydroxylated at mild conditions. Enthalpy and Entropy of Sorption. The sorption calorimetric technique allows direct measurements of partial molar enthalpy of mixing of water Hm w (without evaluation of the temperature dependence of water activity). Hm w is the derivative of the enthalpy of the mixing of liquid water with a material with respect to the number of moles of water in the sample. It is related to the enthalpy of sorption as sorp cond Hm w ) Hw - Hw

(12)

where Hcond is the enthalpy of condensation of one mole of w pure water (-44 kJ/mol at 25 °C). The partial molar enthalpy of mixing and the water activity can be used to calculate the partial molar entropy of mixing of water (see eq 2). The partial molar enthalpy and entropy of mixing of water with initial calcined MCM-41 measured at 25 °C are shown in Figure 4a. In the beginning of sorption (in the adsorption regime), the enthalpy effect is negative but relatively small and the entropy changes from positive to negative values. The reason for this type of entropy change is the following. When water content is very low and most of the OH groups are not occupied, the addition of water molecules that adsorb on OH groups increases the disorder in the system (the number of ways in which one can distribute a given number of water molecules on adsorption sites increases). As a result, Sm w is positive at very low water contents. At higher water contents (close to the water content needed to occupy all sorption sites), the addition of water molecules decreases disorder in the system because it decreases the number of sorption sites available for adsorption. As a result, Sm w is negative at higher water contents of the adsorption regime. In the capillary condensation regime, both enthalpy and entropy are negative. The enthalpy effect, Hm w , is more nega-

Figure 4. Partial molar enthalpy of mixing of water, Hm w , (lower curve) and the partial molar entropy of mixing of water, Sm w , (upper curve) as functions of water to silica mass ratio, hw. (a) Initial calcined MCM-41. (b) MCM-41 hydroxylated at mild conditions.

tive than the entropy effect, TSm w , and this makes the Gibbs energy of hydration negative. Thus, the driving force of capillary condensation in this system is enthalpy. The negative entropy effect implies an increase of order in the system compared to pure water. To find the reason for negative entropy and enthalpy effects, let us consider the processes that occur when the water fills the capillaries that contain preadsorbed water. Water molecules adsorbed on silanol groups have unsaturated (“dangling”) hydrogen bonds. When the bulk water comes into the capillary, it saturates those hydrogen bonds, giving rise to an exothermic heat effect. Another process that should take place during capillary condensation is the creation of the interface between the liquid water and the walls of silica. Assuming that the area of one OH group is about 0.1 nm2 and using the silanol number of 1.6 nm-2, one can estimate that ca. 84% of the silica surface is hydrophobic. Creation of an interface between water and a hydrophobic surface leads to disruption of hydrogen bonds, which gives an endothermic heat effect. Nonetheless, despite the fact that most of the MCM-41 surface is hydrophobic, the observed heat effect of the capillary condensation is exothermic. This indicates that most of the hydrogen bonds in water are preserved at the conditions of capillary condensation. To explain this phenomenon, one should recall the fact that hydrogen bonds on the surface of water are disrupted if the surface is flat or it is a surface of a large cavity; hydrogen bonds between water

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Figure 5. Proposed structure of water in pores of MCM-41 (a cross section of a cylindrical pore). The bulk water fills the pore and hydrates the silanol groups, leaving hydrophobic patches on silica surface unhydrated. The small cavities adjacent to the walls may contain molecules of air.

TABLE 1: Apparent Density of Water in Porous Materials material

pore diameter, nm

vycor glass MCM-41

∼4 3.3

MCM-41

3.9

method neutron scattering H2O and N2 sorption H2O and N2 sorption

temperature, K

density, g/cm3

303

0.89 0.89

48 18,19

298

0.88

this work

ref

molecules on the surface of a small cavity are preserved.53 If liquid water does not fill the whole volume close to the silica surface but leaves some cavities close to the hydrophobic walls (Figure 5), then those cavities can be hydrated without disruption of hydrogen bonds. This scheme of hydration of the pores may explain the apparent decrease of density of water in MCM-41. The density of water is then close to one, but the presence of small cavities where the density of water may be close to zero gives the average value shown in Table 1. The presence of cavities is favorable from the point of view of thermodynamics of condensation of water. In capillary condensation, there are two driving forces that act in opposite directions: saturation of hydrogen bonds of preadsorbed water molecules (this process drives water molecules into the pores) and condensation of water at relative humidities that are too low to condense water in the absence of other driving forces. The balance between the two driving forces is the balance between the mass of condensed water and the surface covered in the process of capillary condensation. The lower the required mass of condensed water, the more favorable the process of condensation is. The condensation with the formation of small cavities decreases the mass of condensed water, still saturating the “dangling” hydrogen bonds. In the presence of air (our experiments were carried out at atmospheric pressure), it is possible that the small cavities contain molecules of air (nitrogen and oxygen). At higher water contents (after the capillary condensation regime), water uptake is relatively small and can be associated with filling of the small cavities that are left unfilled during capillary condensation. During this process, the enthalpy effect increases dramatically and at the end of the process it is positive (endothermic). The probable reason for that is the disruption of hydrogen bonds upon creation of an interface between water and hydrophobic patches of silica surface. The negative partial molar entropy of the mixing of water in the capillary condensation regime can be explained by the fact that the formation of small cavities is accompanied by the decrease of entropy. Water molecules that hydrate the small cavities adopt orientations that allow hydrogen-bonding patterns to go around the cavities, which implies ordering in the system.53 After the capillary condensation regime, this ordering is removed

Figure 6. Water sorption and desorption isotherms of MCM-41. The thick dashed curve shows the sorption experiment with the initial calcined sample. The thin dashed curve shows the sorption experiment with the hydroxylated sample. Two thin curves show two desorption experiments. The right thin curve shows the results of the experiment where vacuum was applied to remove air bubbles prior to the water desorption experiment. The left curve shows the water desorption results obtained without degassing. The arrows indicate the direction of the sorption/desorption process.

gradually upon further addition of water, which produces a positive entropy effect (Figure 4a). The partial molar enthalpy of mixing of water with MCM41 hydroxylated at mild conditions is strongly exothermic at low water contents (Figure 4b), whereas in the case of initial MCM-41 it is weakly exothermic. This indicates that the mechanism of adsorption of water on these two materials is different. In the hydroxylated material, the surface density silanol groups is much higher than that in the unhydroxylated MCM41; therefore, adsorbed water molecules can form hydrogen bonds with two silanol groups, while in the unhydroxylated MCM-41 the assumption of one water per silanol group is reasonable. The entropy curve for hydroxylated MCM-41 has more complicated shape than that of the initial MCM-41. We suggest that the main reason for this is the availability of different types of sorption sites (one water per one OH group and one water per two OH groups) in the hydroxylated material. Desorption of Water. To check the presence of sorptiondesorption hysteresis, we performed desorption calorimetric experiments with MCM-41 samples. In the first desorption experiment, we mixed dried MCM-41 samples with a known amount of liquid water and then conducted desorption experiments according to the usual procedure.30 The results of this experiment are shown in Figure 6 (the left desorption curve). The obtained desorption isotherm differs significantly from the sorption isotherm (shown in the same figure as a thick dashed curve), the amount of water needed to fill the pores is lower than that in the case of sorption. This result was reproducible in two desorption experiments. A possible reason for the observed sorption-desorption difference is the presence of trapped air bubbles inside the pores. To verify it, we have performed a third desorption experiment with a slightly changed procedure. In this experiment, we mixed a dried MCM-41 sample with some amount of water, degassed the mixture in vacuum for several minutes in order to remove possible air, and measured the mass of the mixture to obtain the amount of water still left after vacuuming. The obtained mixture was then used for the desorption calorimetric experiment. The desorption isotherm obtained in this experiment is

Hydration of MCM-41 Studied by Sorption Calorimetry in good agreement with the post-capillary condensation branch of the sorption isotherm (Figure 6). This result confirms the presence of trapped air bubbles in MCM-41 pores when this material is mixed with liquid water (that has RH ) 100%). This also indicates that the mixing of MCM-41 with water vapor (RH < 100%) does not produce trapped air bubbles. We suggest that these air bubbles are larger than the small cavities adjacent to the silica walls (shown in Figure 5), and the former are present only in the case of mixing of liquid water with the silica material, whereas the latter is more typical in the case of mixing with gas-phase water. The structure of water proposed in Figure 5 provides a straightforward explanation for the observed difference between uptake of liquid water and of water vapor by MCM-41. When water vapor at a relative humidity of ca. 60% condenses in the pores, the emerging liquid water leaves unfilled cavities along the walls of silica (Figure 5). These cavities serve as escape paths for air that was in the pores prior to condensation. On the contrary, when liquid water (RH ) 100%) comes into the pores, it does not leave unfilled cavities and therefore the escape paths for the air molecules are blocked. Conclusions We have studied hydration of MCM-41 silica using sorption calorimetry. The calorimetric results were compared with the results on sorption of nitrogen on the same material. The main conclusions arethe following: • The silanol number calculated from comparison of water and nitrogen adsorption isotherms of the studied material is 1.6 nm-2. • The apparent density of water in the pores at RH of capillary condensation is 0.88 g/cm3. • The data on water sorption can be used for accurate calculations of the pore sizes of mesoporous materials. • The capillary condensation of water in the pores of MCM41 is driven by enthalpy; the entropy effect is negative. • The observed heat effect and the apparent density of pore water are in agreement with the mechanism of capillary condensation that involves the formation of small unfilled cavities adjacent to the silica walls. • The uptake of liquid water by MCM-41 leads to entrapment of air in the pores, whereas the uptake of water vapor does not. Acknowledgment. We thank Håkan Wennerstro¨m and Thomas Arnebrant for fruitful discussions. Supporting Information Available: The water sorption isotherms of MCM-41 measured at 25 and 40 °C, the water sorption isotherm of hydrothermally treated MCM-41, and the plots of partial molar enthalpy of mixing of water and the nitrogen sorption isotherm. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Grun, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Microporous. Mesoporous. Mater. 1999, 27, 207. (4) Rozwadowski, M.; Lezanska, M.; Wloch, J.; Erdmann, K.; Golembiewski, R.; Kornatowski, J. Langmuir 2001, 17, 2112. (5) Sklari, S.; Rahiala, H.; Stathopoulos, V.; Rosenholm, J.; Pomonis, P. Microporous. Mesoporous. Mater. 2001, 49, 1.

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