Study of Hydrated Structures on the Surface of Mesoporous Silicas

Mar 5, 1997 - Sophie Le Caër , Patricia Rotureau , Francine Brunet , Thibault Charpentier , Guillaume Blain , Jean Philippe Renault , Jean-Claude Mia...
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Langmuir 1997, 13, 1237-1244

1237

Study of Hydrated Structures on the Surface of Mesoporous Silicas and Carbosils by 1H NMR Spectroscopy of Adsorbed Water† V. V. Turov,‡ R. Leboda,*,§ V. I. Bogillo,‡ and J. Skubiszewska-Zie¸ ba§ Institute of Surface Chemistry of NAS, 252022 Kiev, Ukraine, and Department of Chemical Physics, Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland Received December 10, 1995. In Final Form: October 18, 1996X 1H NMR spectroscopy of adsorbed water was used to study mesoporous silica gel and carbosils synthesized with carbon in the range from 4 to 14.5% w/w in the surface layer. It has been revealed that both silica gel and carbosil are characterized by a rise in temperature of water freezing in an aqueous medium in mesopores. This phenomenon may be explained by contact of water adsorbed in pores with a continuous aqueous medium. The changes in free energy of water in pores due to adsorption have been calculated. It has been shown that carbonization of the mesopore silica gel surface leads to formation of carbon layers which are denser than those on a nonporous silica and the main types of water adsorption sites on the surface are basic graphite planes of carbon and oxygen-containing groups on the carbon part of the carbosil surface.

Introduction The structure of adsorption sites of the silica surface was studied in detail by various NMR techniques,1-7 infrared spectroscopies,8-10 and calorimetric measurements.11-13 It has been ascertained according to the data of 29Si cross polarized magic angle spinning (CP MAS) NMR spectroscopy that the three signals caused by Si atoms are in the spectra of silica gel. They correspond to Si atoms of siloxane bridges, silanol, and silane diol groups. The concentration of silane diol groups is comparatively small and according to calculations constitutes 10% of the total concentration of hydroxyl groups. The relationship between Si atoms of silanol groups and those of siloxane bridges may vary in a wide range depending on the type of silica.7 The primary centers of water adsorption on the silica gel (SG) surface are the single and geminal silanol groups.1,2,7 As the hydration degree of the surface increases, the clusters of adsorbed water are being formed. The coalesce together forming a continuous aqueous film and filling the silica pores. Adsorbed water is characterized by a nonuniform energy interaction with the surface. While the SG surface is being covered by adsorbed water molecules, a drop in adsorption heat is observed. The data of calorimetric measurements show that the interaction energy of the initial portions of the water with the † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. ‡ Institute of Surface Chemistry of NAS. § Maria Curie-Sklodowska University. X Abstract published in Advance ACS Abstracts, February 15, 1997.

(1) Morrow, B. A.; Gay, I. D. J. Phys. Chem. 1988, 92, 5569. (2) Kinney, D. R.; Chuang, I. S.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 6786. (3) Maciel, G. E.; Sindirf, D. W. J. Am. Chem. Soc. 1980, 102, 7606. (4) Sindorf, D. M.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 7606. (5) Albert, K.; Bayer, E. J. Chromatogr. 1991, 544, 345. (6) Haukka, S.; Root, A. J. Phys. Chem. 1994, 98, 1695. (7) Chuang, I. S.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 401. (8) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (9) Hoffmann, P.; Knozinger, E. Surf. Sci. 1987, 188, 181. (10) Morrow, B. A.; McFarlane, R. A. J. Phys. Chem. 1986, 90, 3192. (11) Bolis, V.; Fubini, B.; Marchese, L.; Marta, G.; Costa, D. J. Chem. Soc., Faraday. Trans. 1991, 87, 497. (12) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793. (13) Etzler, F. M. Langmuir 1988, 4, 878.

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SG dehydrated surface exceeds 150 kJ mol-1, which is in agreement with the dissociation mechanism of adsorption on the strained siloxane bonds of the surface.11 The water molecules, which participate in the formation of two hydrogen bonds with the silanol groups simultaneously, are weakly bonded (50-90 kJ mol-1); the weaker bond with the surface is observed for the water molecules in a multilayer film of physically adsorbed water (45 kJ mole-1). When the silica surface is subjected to carbonization, areas covered with a carbon layer appeared on the silica surface. The dominant adsorption sites for water adsorption on the surface of carbon-containing silicas are the basic graphite planes and oxygen-containing (C-OH, COOH, CdO, and C(H)dO) groups.14-18 Accordingly, the strongest associates are observed in the process of forming the hydrogen-bonded complexes with the carbonyl and carboxylic groups of the oxidized carbon surface. The properties of water at the adsorbent interface differ to a great extent from its bulk characteristics. The hydration forces are characterized by a sign-changeable character when moving away from the surface.19,20 The surface strongly influences only the first molecular layers of adsorbed liquid.21 Changes in density and dissolving capacity are observed in such layers.22 The above phenomena in the pores, whose dimensions are less than 10 water molecular diameters, are observed in the form of an oscillating dependence of molecular mobility,23 heat capacity24 of the adsorbed molecules upon the dimensions of pores. One may expect that the influence of the surface in the mesoporous adsorbents will affect most of the substance located in pores, and therefore, the properties of the water layers at the interface may differ significantly (14) Tabony, J. Prog. NMR Spectrosc. 1980, 14, 1. (15) Tabony, J.; White, J. W.; Delachanme, J. C.; Coulon, M. Surf. Sci. 1980, 95, L282. (16) Gross, R.; Boddenberg, B. Z. Phys. Chem. 1987, 152, 259. (17) Turov, V. V.; Pogorely, K. V.; Burushkina, T. N. React. Kinet. Catal. Lett. 1993, 50, 279. (18) Karpenko, G.; Turov, V.; Chuiko, A. In Extended Abstracts of the International Carbon Conference, 16-20 July, 1990, Paris, 1990; p 679. (19) Delville, A. J. Phys. Chem. 1993, 97, 9703. (20) Attard, P.; Parker, J. L. J. Phys. Chem. 1992, 96, 5086. (21) Gruz, M.; Van Caugh, L.; Fripiat, I. J. Bull. Sci. Acad. Belg. 1972, 58, 439. (22) Deriygin, B. V.; Churaev, N. V. In Water in Dispersed Systems; Deriygin, B. V., Ed.; Nauka: Moscow, 1989. (23) Vanderlick, T. K.; Davis, H. N. J. Chem. Phys. 1987, 87, 1791. (24) Etsler, F. M. Langmuir 1988, 4, 878.

© 1997 American Chemical Society

1238 Langmuir, Vol. 13, No. 5, 1997

Turov et al.

Figure 2. Distribution curves of parent silica gel (1) and carbosils CS1 (2), CS2 (3), and CS3 (4) mesopores on their radius. Table 1. Structural Characteristics of Silica Gel (SG) and Carbosils, Synthesized on Its Basis (CS) specific content av radius of adsorption area total volume of carbon mesopores of mesopores of mesopores sample (% w/w) (nm) (m2 g-1) (cm3 g-1)

Figure 1. Adsorption isotherms of nitrogen on the surface of silica gel (1) and synthesized on its basis carbosils CS1 (2), CS2 (3), and CS3 (4).

from the corresponding characteristics on the surface of nonporous adsorbents. We have studied earlier the structure of adsorption complexes and hydrated layers on the surface of nonporous pyrogenic silica25,26 and synthesized composite carbon-containing silicasscarbosils.27,28 The aim of the present work is the examination of changes in hydration properties of porous silica due to its carbonization. As a main method of investigation the 1H NMR spectroscopy of adsorbed molecules has been chosen.17,26 Contrary to the CP-MAS 1H NMR spectroscopy and CRAMPS (combined rotation and multiple-pulse spectroscopy),2 this method is characterized by a lesser resolution. However, in combination with the bulk freezing29,30 procedure, the above method permits the signal of molecules of a substance that has undergone perturbation originating from the adsorbent surface to be resolved from the total proton signal of solid and liquid phases. The advantage of the above method lies in its high accuracy in determining the relatively weak intensities of the signals. Experimental Section Carbosils were synthesized by a low-temperature carbonization of acetylacetone on a parent silica gel.31,32 The carbon varied in a range of 4-14% (w/w). Adsorption characteristics of adsorbents were determined on a Carlo Erba Sorptomat (Italy) device by nitrogen adsorption at 77 K. The adsorption isotherms are shown in Figure 1, and distribution curves of mesopores according to (25) Turov, V. React. Kinet. Catal. Lett. 1993, 50, 243. (26) Turov, V. V.; Zarko, V. I.; Chuiko, A. A. Ukr. Khim. Zh. 1990, 56, 1262. (27) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zie¸ ba, J. Ads. Sci. Technol., in press. (28) Bogillo, V. I.; Turov, V. V.; Leboda, R. Extended Abstracts of the International Conference on Carbon 94, 3-8 July, 1994, Granada, Spain, 1994. (29) Gorbunov, B. Z.; Lazareva, L. S.; Gogolev, A. Z.; Hugilev, E. L. Kolloidn. Zh. 1989, 51, 1062. (30) Turov, V. V.; Kolychev, V. I.; Bakaj, E. A. Teor. Exp. Khim. 1990, 26, 111. (31) Gierak, A.; Leboda, R. Mater. Chem. Phys. 1989, 19, 503. (32) Gierak, A.; Leboda, R.; Tracz, E. J. Anal. Appl. Pyrol. 1988, 13, 89.

SG CS1 CS2 CS3

0 4.0 9.1 14.5

4.05 4.24 3.81 3.20

394 359 303 281

0.744 0.718 0.599 0.560

their radii, which have been calculated on the basis of the desorption branch of the above isotherms, are presented in Figure 2. The structural characteristics of studied materials are given in Table 1. The 1H NMR spectra were recorded in the temperature range of 160 < T < 280 K on a high-resolution spectrometer WP 100 SY (Bruker, Germany). The temperature in the detector was maintained constant with an accuracy up to (1 K using a heating appliance B-VT 1000. Deviation in measurements of integral intensities did not exceed 15%. To prevent a formation of water metastable states at low temperatures, the 1H NMR spectra were recorded under conditions of heating samples, previously cooled up to 160 K. The relaxation period between the probe pulses in the process of accumulation was 5 s (pulse duration 4 µs). The increase in the probe pulse duration did not result in the change of signal intensity, which made it possible to conclude that the signal was unsaturated in the whole temperature range. The dosage of water adsorbed on the porous adsorbents was carried out by adding predetermined portions of water to the samples using a microdropper. The samples were preheated in air up to 473 K for 2 h. The changes in the water content were determined on the basis of the signal intensities in the 1H NMR spectra. All adsorbents contained some amount of water adsorbed from air. In order to determine the water content in powders, the previously weighted samples of adsorbents (m ) 100 - 200 mg) were thoroughly mixed with a certain portion of water, and after the establishment of equilibrium (24 h, 293 K), the dependencies of the water signal intensity (I) on the content of water added to a sample were measured. As a result, graphs of the dependence I ) f(m) for every adsorbent were constructed. The graph yields straight lines, and a point of intersection with the ordinate gives the initial degree of hydration of samples. In view of the fact that times of the transverse relaxation of protons for a substance in the solid state are by several orders of magnitude smaller than for an adsorbed phase, such a procedure allows one to separate a signal of molecules at the adsorbent interface from the total signal of protons. In order to determine unfrozen water concentration in aqueous suspensions, we drew a comparison between the intensities of water signal for a starting sample of hydrated adsorbent and for an aqueous suspension at such a temperature (T < 273 K) when both signals have a comparable intensity. All measurements of the chemical shifts were taken using standard ampules of glass inserts 4 mm in diameter which were

Study of Hydrated Structures

Figure 3. Temperature dependencies of 1H NMR spectra of water adsorbed on the mesoporous silica gel surface at different content of water in the sample: (a) CH2O ) 6% (w/w); (b) CH2) ) 7% (w/w); (c) CH2O ) 24% (w/w); (d) frozen suspension. placed in measuring ampules 5 mm in diameter. The chemical shifts were determined with reference to tetramethylsilane used in the mixture, with CDCl3 (in 1:20 ratio) in the gap between ampules 4 and 5 mm in diameter.

Results and Discussion i. Adsorption of Water on the Mesoporous Silica Gel Surface. It has been established that the 1H NMR spectroscopy technique can be used to study water adsorbed on the surface of nonporous silicas and that in a wide region of the hydrated surface its spectrum is observed in the form of a single signal, whose width monotonously increases while the temperature and adsorbed water concentration decrease.25,26 The lower the concentration of adsorbed water, the larger the signal width and the weaker its dependence upon the temperature. Different regularities are observed in the mesoporous materials. Contrary to the nonporous silica sample, the character of the temperature changes in the 1H NMR spectra of adsorbed water depends on the degree of hydration of mesoporous silica (Figure 3). Spectra of water adsorbed from the gaseous phase (Figure 3a-c) and in the frozen state are presented. In the latter case all the pores and the space between the adsorbent granules are filled by water (Figure 3d). As seen from this figure, at CH2O ) 6% (w/w) (8.46 µmol m-2) the spectrum of adsorbed water is in the form of a single signal whose width is approximately 1 kHz (Figure 3a) and weakly depends on the temperature. As the concentration of hydroxyl groups on the SG surface is about 2.5 µmole m-2,33 we may conclude that at these values of CH2O each surface hydroxyl group attracts approximately three water molecules. The wide width of the signal gives evidence in favor of a low mobility of the adsorbed water molecules. Apparently, there is no condensation of water in pores under the above mentioned conditions and it is distributed uniformly on the adsorbent surface interacting predominantly with the surface hydroxyl groups. The rise in CH2O (Figure 3b,c) results in appearance of a bulk component of water due to filling of the adsorbent (33) Tertykh, V. A.; Belyakova, L. A. Chemical Reactions with Participation of Silica Surface; Naukova Dumka: Kiev, 1991.

Langmuir, Vol. 13, No. 5, 1997 1239

pores by liquid water. As the silica gel studied is characterized by a wide distribution of pores in their radii (Figure 2), water is filling the narrower pores first. Actually two signals are observed in that concentration range. The broad line is connected with water present in large pores, where water localized on the surface is characterized by low mobility, and the sharp line is due to water in narrower pores, filled with condensed water. In the process of rising CH2O when the pores of the adsorbent are being filled with water, the intensity of the narrow signal is increasing, while that of the wide one is decreasing. The water condensed in the pores does not freeze at low temperature due to drop in the freezing temperature of water in the pores because of the process of its dispersing.34 For example in the pores having the radii of about 4 nm, the water freezing temperature is lowered by 60 K, which is in agreement with the observed decrease in the intensity of water signal at T < 200 K for the samples having the water content of 24.0 and 33% (w/w). From the silica/water/air system to the silica/water/ice frozen aqueous suspension system, significant widening of the adsorbed water signal is observed (Figure 3d). This may be regarded as evidence of decrease in mobility of the adsorbed water molecules in the presence of ice. The most probable reason of the decrease in mobility of water molecules located in the gap between the solid phases of silica and ice is the formation of hydrogen bonds between the water molecules and hydroxyl groups of the ice surface. Therefore, the degree of water ordering increases, which leads to decrease in mobility of its molecules. Besides, as we may conclude from comparison of the data given in Figure 3a and Figure 3d, the intensity of the adsorbed water signal in the frozen suspensions decreases as a function of drop in temperature considerably stronger than in the case of contact with air. Thus, at T < 215 K the signal intensity in the ice medium becomes smaller than that at CH2O ) 6% (w/w). Therefore, the presence of the ice phase gives rise to crystallization of water in the adsorbent pores. This phenomenon is observed clearly, as we compare the dependencies of the unfrozen water signal intensity on the temperature for the samples having a different degree of the surface hydration and in the frozen suspensions in the temperature range from 170 to 273 K (Figure 4). As we can see in the case when adsorbed water interacts with air, water in mesopores freezes at T < 200 K, while in the frozen suspensions this process comes to an end at 240 K. The dependence I ) f(T), where I is a signal intensity of water in the 1H NMR spectrum, shows that the process of bulk water freezing in the adsorbent pores is observed in the form of a segment, which is characterized by a smaller decline of the curve toward the intensity axis. If the metastable states are not observed, water present at the interface freezes in the case, when the free energy of the adsorbed molecules becomes equal to that of the water molecules in ice.35,36 The higher the adsorption energy, the lower the temperature needed for adsorbed water freezing. As the thermodynamic functions of water are tabulated in a wide temperature range,37 each deviation of the freezing temperature from 273 K may be correlated with the corresponding change in the water free energy. Then the dependence of the unfrozen water (34) Atkins, P. W. Physical Chemistry, 4th ed.; Oxford University Press: Oxford, 1993. (35) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zie¸ ba, J. Langmuir 1995, 11, 931. (36) Turov, V. V.; Zarko, V. I.; Chuiko, A. A. Zh. Fiz. Khim. 1995, 69, 677. (37) Handbook of Thermodynamic Properties of Individual Substances; Glushko, V. P., Ed.; Nauka: Moscow, 1978; Vol. 1 (in Russian).

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Turov et al. Table 2. Effect of Surface Hydration on the Water Free Energy at Mesoporous Silica Gel/Water Interface CH2O (µmol m-2) mol-1)

∆Gav (kJ ∆Gmax (kJ mol-1) ∆Gtot (mJ m-2)

Figure 4. Effect of temperature on the unfrozen water concentration in mesoporous silica gel at different content of water in the sample: (1) CH2O ) 6% (w/w); (2) CH2O ) 10% (w/w); (3) CH2O ) 24% (w/w); (4) CH2O ) 33% (w/w); (5) frozen suspension.

8.5

10.0

33.8

47.0

84.0

5.3 8.7 38

4.8 6.8 66

4.0 4.7 132

3.9 4.2 181

2.0 3.2 151

For the systems, where the adsorption of water has been carried out from the gaseous phase, the ∆G ) f(CH2O) dependencies have two segments, one of which is parallel to the ∆G axis. It is in agreement with the constancy of the unfrozen water concentration under the conditions of the temperature decrease. The second segment is linear in shape. It corresponds to partial freezing of water in the adsorption layer. By extrapolating the dependencies obtained for the last segment to the zero value of CH2O, the maximum change of the water free energy induced by the presence of interface (∆Gmax) may be calculated. As the average values of the water free energy in the adsorption layer, the ∆G value in the center of the linear segment may be chosen. For the frozen aqueous suspensions the ∆G ) f(CH2O) dependence is linear at T < 235 K (Figure 4 and Figure 5, curves 5). The thickness of the water layer perturbed by the surface may be determined by extrapolating the ∆G ) f(CH2O) dependencies to the ∆G ) 0. The average ∆G values calculated from the data of Figure 3 are presented in Table 2. The three main reasons responsible for the drop of the water free energy at the adsorbent/water interface during sample freezing may be revealed. They are as follows: a change of the water free energy induced by the adsorption on the SG surface (∆Gads) as well as the change in the free energy at interaction of liquid water with ice surface, which is defined by the free energy of the water/ice interface (∆Gice) and free energy of the water/air interface (∆Gwa). Then the expression for the total change in the water free energy on the adsorbent surface may be written as follows

∆Gtot ) ∆Gads + ∆Gice + ∆Gwa

(1)

The change in the water free energy in the adsorbent pores is defined by the following equation

∆Gwa ) 2γwaVmr-1

Figure 5. Dependencies of free energy of unfrozen water on the silica gel surface versus water concentration: (1) CH2O ) 6% (w/w); (2) CH2O ) 10% (w/w); (3) CH2O ) 24% (w/w); (4) CH2O ) 33% (w/w); (5) frozen suspension.

content, related to the weight of the adsorbent on the temperature may be transformed into dependence of the water free energy at the interface on the content of the unfrozen water. Thus, we calculate the concentration of unfrozen water at different temperatures T < 273 K on the basis of a signal’s intensity and plot the corresponding values on the abscissa. The difference between the free energy of ice at T ) 273 K and the temperature of measurement (∆G) is plotted on the ordinate. ∆G is in agreement with a decrease in a free energy of water at the adsorbent/ice interface. Figure 5 presents these dependencies of the Gibbs free energy on the temperature for water adsorbed in the silica gel pores at its different concentrations. Numeration of the curves coincides with that given in Figure 4.

(2)

where γwa is a coefficient of the surface tension of water at the water/air interface, Vm is the water molar volume, and r is the curvature radius of the surface. The ∆Gtot value determines the change of the free energy of the water molecules localized at the adsorbent interface irrespective of their distances to the surface. Therefore, on the basis of the data of Figure 5, the ∆Gtot value may be determined as an area restricted by the ∆G ) f(CH2O) dependence, where CH2O is expressed in terms of the moles of unfrozen water related to the adsorbent surface. The ∆Gtot calculated in accordance with the above method are presented in Table 2. The data from Figure 5 and Table 2 show that if the CH2O value does not exceed the monolayer thickness, the ∆Gmax decreases as a function of rise in the degree of surface hydration. That reflects the decrease in the average values of the water free adsorption energy as a function of rise in the thickness of the adsorption layer. Accordingly, the monotonous increase of the ∆Gtot as a function of the CH2O rise is observed, and at CH2O > 33 µmol m-2, the rise becomes higher than the ∆Gtot value for the frozen suspension. The scheme of the system mesoporous silica gel/adsorbed water vapors under conditions of the partial freezing of

Study of Hydrated Structures

Langmuir, Vol. 13, No. 5, 1997 1241

Figure 7. 1H NMR spectra of water adsorbed in the carbosil CS1 pores at different temperature (a) and water concentration (b).

Figure 6. Scheme of liquid/porous silica interface structure in the freezing process at water adsorption from gas phase (a) and in frozen suspensions (b).

medium the adsorbent pores are connected with water in the volume of the sample. In this case the condition of water location in pores as an individual dispersed particle is broken (Figure 6b). Therefore, the registered changes in the ∆Gtot values in the process of the freezing of the main portion of water in pores are attributed to adsorbed water in the silica gel/water/ice system. As the water/air interface is not observed in the case of the frozen suspensions, eq 1 may be written as follows

water in the adsorption layer is shown in Figure 6a. The three types of interfaces are clearly seen in the above system: ice/silica, water/ice, and water/air. Each of them is characterized by its own surface area of the interface (Sls, Sli, and Slv, respectively). As filling of the adsorbent pores with water occurs, when the average thickness of the adsorbed water layer exceeds that of one monolayer (Figure 3), we may make a conclusion that the silica surface is covered with the adsorbed water layer completely. Then S1s ) Sads; i.e., it is equal to the specific surface area of the adsorbent (Sads). However, the remaining areas of the interface boundaries have not been determined as well as the dimensions of water microdrops in the pores under the conditions of their partial filling with water. The phenomenon of the increase in the temperature of water freezing in the silica gel pores in the water suspensions (Figure 4) turned out to be unexpected. A series of reasons leading to the enhancement in the water freezing temperature in the adsorbent pores may be regarded as responsible. Thus, the silica gel surface in the presence of the ice phase may exert a perturbation effect on the water structure in the adsorption layer. Particularly, it has been ascertained by the use of calorimetry that the rise in the temperature of the ice melting occurs in the silica pores.38 Being involved in the interaction with the hydroxyl groups of the ice surface, water in the gap formed between the ice and adsorbent surface acquires a higher degree of ordering and that may exert an influence on the freezing temperature. However, when hydrated silica is brought into contact with the air medium in the cooling process, ice microcrystals are formed in the pores, but the quick freezing of water is not observed, and the region of the phase transition is stretched out by more than 20 K (Figure 4). Thus, the condition for the higher temperature of water freezing in pores is considered to be the existence of the continuous aqueous medium in this system. One of the parameters influencing the temperature of water freezing is pressure. Because of water expansion while freezing, the regions of elevated pressure may be observed in the suspensions at the interface. However, one may expect a drop in the temperature of water freezing.39 The most probable cause responsible for the above phenomenon we consider to be the partial loss of dispersity of water in the adsorbent pores. In the aqueous

The free surface energy of ice in accordance with ref 40 is 100 mJ m-2, which gives chance to determine the free surface energy of water on the SG surface. However, the value of ∆Gice must depend on the curvature of the ice surface as well. Besides, it is not clear what are the limits of the intensity of the signal of the water adsorbed on the ice surface involved into the average intensity of unfrozen water signal. The definition of ∆Gtot on the basis of the experimental data seems to be more correct. ∆Gtot for water on the SG surface given in Table 2 is close to that for the free energy of water adsorbed on the surface of the nonporous silica.35 ii. Adsorption of Water on the Surface of Mesoporous Carbosils. Carbonization of the silica surface exerts a significant effect on its adsorption properties. The increase in the carbon concentration in the silica matrix (Ccarb, in % w/w) results in a decresae of the specific adsorption surface area of the material as well as the average radius of the mesopores (Table 1). That makes it possible to conclude that a layer of the carbon has been formed in the silica mesopores in the process of carbosil synthesis. Electron microscopy data show that the rise in Ccarb on the silica surface results in appearance of carbon globules,32 which fuse to form a quite loose carbon coating. Conclusions concerning the possible structure of the adsorption complexes on the carbon surface can be made while comparing the 1H NMR spectra of the adsorbed molecules on the basis of the procedure described in refs 17, 27, and 28, where relationships between the differences of the chemical shifts of molecules in the adsorbed and condensed states and the structure of the adsorption complexes have been established. The 1H NMR spectra of water adsorbed on the carbosil (CS) sample (Ccarb ) 4% (w/w)) from the gaseous phase under different temperature conditions and concentrations of water are presented in Figure 7. Water adsorbed under normal conditions (T ) 293 K; P ) 1 atm; 20% relative humidity) is observed as two signals having the chemical shifts δ ) 2.8 and 0.6 ppm. A drop in the temperature and a rise in the concentration

(38) Drost-Hausen, W.; Etsler, F. M. Langmuir 1989, 5, 1439. (39) Kanno, H.; Speedy, R. J.; Angell, C. A. Science 1975, 189, 880.

(40) Turov, V. V.; Kolychev, V. I.; Burishkina, T. N. Teor. Exp. Khim. 1992, 28, 80.

∆Gtot ) ∆Gads + ∆Gice

(3)

1242 Langmuir, Vol. 13, No. 5, 1997

Figure 8. 1H NMR spectra of water adsorbed in the carbosil CS2 (a) and CS3 (b) pores at different temperatures and water concentrations.

of the adsorbed water result in increase of the signal intensity in the weak magnetic field. The higher degree of carbonization of the parent silica gel gives rise to the widening of the signals, and water adsorbed on the carbosils having Ccarb > 4% (w/w) is observed as the averaged signal having the chemical shift at δ ) 2.8 ppm (Figure 8). As in the case of the parent silica gel, at transition to freezing suspensions the width of the adsorbed water signal rises significantly and its intensity decreases due to freezing of water in the adsorbent mesopores. The carbonized silica samples follow the same regularities concerning the change of the signal width as takes place in the case of the parent silica gel sample. The transition to the frozen suspensions is accompanied by a sharp widening of the signal. Therefore, the spectra of water given in Figures 7 and 8 may be attributed to bulk water in the adsorbent mesopores. The values of the obtained chemical shifts correspond to displacement of the water signals toward the strong magnetic fields in comparison with that of liquid water. According to ref 17, the above values of displacement are attributed by water adsorbed on basic graphite planes of the carbon surface. The temperature of carbonization for the carbosil samples used in our study was lower than 1273 K. The afore-mentioned temperature leads to appearance of relatively small carbon areas formed by a system of condensed benzene nuclei. As a rule the above mentioned areas do not exceed 2 nm2.41 The carbon component of carbosils may contain a considerable amount of oxidized carbon atoms. On the basis of the fact that the water molecules form the strongest associates with the oxygen-containing groups of the carbonaceous materials,14 we can conclude that the carbon layer on the silica gel surface is nonuniform and some surface parts comprise the oxidized carbon groups (COOH, CdO, C(H)dO) and water molecules form hydrogen bonds with them. While these complexes are formed with electron donor sites, which are stronger than that of oxygen in the water molecule, the protons of the water molecules are subjected to a deshielding effect.42 Thus, e.g., the chemical shift of water in oxidized graphite is about 7 ppm.18 The hydroxyl groups of the silica part of the carbosil surface may also participate in the formation of the water adsorption complexes. This is due to the fact that the average thickness of a carbon layer does not exceed a monolayer for the carbosil sample having 4 wt % carbon on its surface. The chemical shift of water on the silica surface depends on a hydration degree of the surface. In the case, when water forms the thick adsorption layers, in which each molecule participates in formation of several hydrogen bonds simultaneously, its chemical shift is close to that of liquid water. The chemical shift of adsorbed (41) Lucas, P.; Marchand, A. Carbon 1990, 28, 207. (42) Bilibrov, V. M. Hydrogen Bond; Naukova Dumka: Kiev, 1993.

Turov et al.

water is approaching δ ) 2 ppm as a function of a decrease in a layer thickness. It corresponds to the individual water molecules having hydrogen bonds with surface hydroxyls.22 Probably, the signals of water adsorbed on the surface are averaged on some types of adsorption complexes. Thus, the water molecules adsorbed on the carbon areas formed by a system of condensed benzene nuclei and individual water molecules having hydrogen bonds with the surface hydroxyls give the main contribution to the chemical shift value. Then the signal of water having a chemical shift of δ ) 0.6 ppm may be attributed to the surface areas having a higher carbon concentration. The temperature changes in intensities of the signals of adsorbed water which are observed in Figure 7 may be caused by an increase in a width of a signal having a chemical shift of δ ) 0.6 ppm at a low temperature, as well as a higher stability of hydration structures responsible for the signal having a chemical shift of δ ) 2.8 ppm, and differences in a temperature dependence of a spinlattice relaxation rate for the above mentioned signals. The widening of the water signal while the degree of the silica gel surface carbonization is growing may occur because of decrease in the average radius of the pores (see Table 1). Actually, the decrease in the radius of pores results in increasing the portion of the water molecules localized near the surface. As the translation mobility in the layer having thickness of 1 mm is about as half of order as much as in the pore volume,43 the sharp decresae in the averaged time of the lateral relaxation of the adsorbed molecules is observed as a function of the decrease in the radius of pores. This phenomenon may be regarded as responsible for the signal width. The structure of the adsorption sites on the surface of carbosils synthesized by carbonization of acetylacetone in silica gel pores differs significantly from the surface structure of the carbonized nonporous (pyrogenic) silica.27,35 Thus, the surface of carbosils on the basis of pyrogenic silica has a great number of oxygen-containing sites leading to displacement of the main signal contributed by adsorbed water toward the weak magnetic fields (δ ) 7 ppm). Besides, up to 10% micropores formed by the closely located graphite planes have been determined on the carbon part of this carbosil surface. Having been involved with these pores, the adsorbed molecules are influenced by the screening effect from both graphite planes. Accordingly, the signal of the water protons is displaced toward the strong magnetic fields by 15 ppm, taking the signal of liquid water as a reference.35,44 The signal arising from the water molecules bounded with the silica part of the carbosil surface (δ ) 4.5 ppm) is not observed in the pores of carbonized silica gel. It may be concluded that the carbon clusters are uniformly distributed on the surface of silica gel, when it is subjected to carbonization in such a manner that predominantly individual water molecules are involved in a process of binding with the silica component of the surface at Ccarb ) 4% w/w. These carbosils do not contain micropores in the carbon component. Similarly as shown for the parent silica gel sample, the thickness of the hydrated layer and the change in the free energy of water localized between the adsorbent and ice phases may be calculated on the basis of the temperature dependencies of the signal intensities of unfrozen water (Figure 9). The freezing curves represent the complex dependencies where the segment of the sharp decrease in the unfrozen water concentration may be revealed. This (43) Holle, B.; Piculell, L. J. Chem. Soc., Faraday Trans. 1 1986, 82, 415. (44) Fletcher, N. H. The Physics of Rainclouds; Cambridge University Press: Cambridge, 1962.

Study of Hydrated Structures

Langmuir, Vol. 13, No. 5, 1997 1243

Figure 9. Temperature effect on the unfrozen water concentration in carbosils CS1 (1), CS2 (2), and CS3 (3).

Figure 10. Dependencies of free energy of unfrozen water on the carbosils CS1 (1), CS2 (2), and CS3 (3) surface versus water concentration.

segment is caused by the freezing of water in the largest pores as well as in the gap between the adsorbent granules. Then the transition region is observed (255 < T < 265 K). It corresponds to freezing of the main portion of water in the adsorbent mesopores. Similarly as it takes place in the pores of parent silica gel, the temperature of freezing is displaced toward the region of high temperatures. At T < 255 K the almost linear segment is observed. In accordance with Figure 6b, water freezing occurs at the adsorbent/ice interface in the above mentioned temperature region. Therefore, this segment of the temperature dependence may be used to determine the change of free energy of water located between adsorbent and ice phases. Then the CH2O ) f(T) dependence is characterized by gentle sloping toward the T axis. The main portion of water in the adsorbent mesopores freezes in the above temperature region. At 255 K these dependencies are close to linear. In accordance with Figure 6b water freezing at the carbosil/ water interface occurs. Therefore, these segments of the dependencies may be used for determining the change of free energy of water in the gap between the adsorbent and ice phases. Figure 10 gives the ∆G ) f(CH2O) dependencies for carbosils having a different degree of the surface carbonization. As in the case of silica gel, in the region of the low CH2O values these dependencies are linear, which gives a chance to determine the maximum thickness of the unfrozen water layer and the maximum changes in the water free energy caused by the adsorption (Table 3). By comparing data in Tables 2 and 3, we can make a conclusion that the ∆Gmax values for the CS1 and CS3 samples practically coincide with the corresponding ∆Gmax values for water adsorbed on the parent silica gel, while in the case of the CS2 sample a drop in the water free energy in the adsorption layer is considerably lower. The decrease of the ∆Gmax value is accompanied by the rise in thickness of the unfrozen water layer. Thus, in the case of CS1 and CS3 only 15 and 30% water content, respectively, are bonded with the carbosil surface, but in the case of the CS2 sample the surface influences more than half of the water located in the pores. The rise in thickness of the bounded water layer at Ccarb ) 9.1% (w/w) makes it possible to conclude that perturbation of bulk water structure nearly the carbon part of carbosil surface occurs.

Table 3. Characteristics of Hydrated Layers on the Surface of Mesoporous Carbosils CH2O (µmol m-2) ∆Gmax (kJ mol-1) ∆Gtot (mJ m-2)

CS1

CS2

CS3

14 3.7 26

70 2.6 91

36 3.4 61

The same phenomenon has been ascertained earlier for nonporous carbosils,35 but the reasons responsible for appearance of the above anomalies at the adsorbent/water interface are to be studied in detail. For all carbosils studied the ∆Gtot value is lower than that for the parent silica gel sample. Moreover, this is due to the fact that the carbonized silica gel surface has less hydrophilic character than silica surface. Probably, the silica surface carbonization leads to considerable increase of its hydrophobic properties. This phenomenon is clearly observed at a low degree of the surface carbonization (Figure 10, curve 1). Conclusion 1

The H NMR spectroscopy of adsorbed molecules in combination with the bulk phase freezing may be successfully used to study the interaction of water with the surface of mesoporous adsorbents. The value of chemical shift of the adsorbed molecules characterizes the type of surface adsorption sites and temperature dependence of the unfrozen water layer may be used to determine the perturbation effect of the surface on the bulk structure of water at the adsorbent/liquid interface. The main types of adsorption sites on the carbonized surface of mesoporous silica gel are the basic graphite planes of the carbon part of the material. In contrast to nonporous carbosils, the adsorption sites of the carbon part as gaps formed between the closely located graphite planes have not been revealed on the surface of carbonized silica gel. It has been revealed that both parent silica gel and carbosils synthesized on its base are characterized by a rise in the water freezing temperature in the adsorbent pores. This may be explained by a comparatively wide mouth of the adsorbent mesopores. Accordingly, the area

1244 Langmuir, Vol. 13, No. 5, 1997

of contact of water in the pores with bulk water is sufficiently large and a water microdrop in the pore is not a particle separated from the outer medium. Therefore, in the temperature region corresponding to the frozen bulk water in pores, the temperature dependencies of the unfrozen water content related to the adsorbent weight may be used to determine a decrease in the water free energy caused by the adsorption. On passing from silica gel to carbosils synthesized on its base, a considerable decrease in the free energy of adsorbed water is observed. Probably, this may be connected with the higher hydrophobic properties of the carbon part of the carbosil surface.

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A drop in the water free energy on the silica gel surface at adsorption of water vapors is more pronounced that it occurs during freezing of water suspensions. This may be explained by the fact that during the contact with air besides the adsorbent/water and water/ice interfaces the water/air interface has appeared. As in the experiments, where the bulk phase freezing is used, all the unfrozen water interfaces have an influence on the chemical potential; the decrease in the free energy of the water molecules in the air medium has a maximum value. LA951565P