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Influence of the Base Size and Strength on the Acidic Properties of Silica Gel and Monodispersed Silica Beads: Interest of Impedance Measurements for the in Situ Monitoring of the Ionization Process C. Despas, A. Walcarius,* and J. Bessie`re Laboratoire de Chimie Physique pour l’Environnement, Unite´ Mixte de Recherche UMR 7564, CNRS - Universite´ H. Poincare´ Nancy I, 405, rue de Vandoeuvre, F-54600 Villers-les-Nancy, France Received September 9, 1998. In Final Form: February 4, 1999 The reactivity of silica gel and Sto¨ber silica toward molecular and anionic bases of various size and strength was examined in aqueous medium. The extent of the deprotonated silanol groups was discussed with respect to the size, strength, and concentration of the reactants, as well as to the nature of the silica. Measurements of the complex impedance performed on the decanted samples were found to be a rapid, sensitive, and nondestructive way for the in situ characterization of the surface silica ionization process in aqueous alkaline solutions. The apparent equilibrium constant for the deprotonation reaction was found to be strongly affected by the degree of dissociation of both silica samples, so that the complete neutralization was never observed. The molecular-sieving properties of the Sto¨ber silica was demonstrated experimentally by using bases of various dimensions: while the ionization of silica gel was nearly unaffected by the base size, that of the monodispersed silica beads was strongly influenced by the ability or inability of the base to enter the bulk of the beads. The use of reactants larger than 0.3 nm totally prevents accessibility to the internal silanols. Reactivity of bases of critical dimensions ranging from 0.2 to 0.3 nm, though they were able to diffuse freely within the porous structure, was found to be hindered by volume exclusion effects.

Introduction Silica gels are amorphous inorganic solids made of spatially arranged SiO4 units in the bulk and hydroxyl moieties referred to as silanol groups (Si-OH) in the outward region. They have been widely applied in industrial processes (many of them are commercially available) such as catalyst supports, stationary phases in chromatography, polymer fillers, or optical and electronic materials, for example.1-5 In addition, silica was used as a model system in various fields of surface science.6,7 The surface chemistry of silica, strongly related to the presence of silanol groups distributed on the surface, has been extensively studied using various classical methods of analysis.8-11 However, concerning the investigation of the mass transfer reactions across the silica/solution interface, many of these methods prevent an in situ analysis under nondestructive conditions, most often because they require the separation of the solid and liquid phases before their * Fax: (+33) 3 83 27 54 44. E-mail: [email protected]. (1) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (2) Unger, K. K. Porous Silica; Elsevier: Amsterdam, 1979. (3) Ribbe, P. H., Ed. Silica: Physical Behavior, Geochemistry, and Materials Applications, Rev. Mineral., 1994; Vol. 29. (4) Levy, D.; Esquivias, L. Adv. Mater. 1995, 7, 120. (5) Van der Voort, P.; Vansant, E. F. J. Liq. Chromatogr. 1996, 19, 2723. (6) Brinker, C. J.; Scherer, G. W. J. Non-Cryst. Solids 1985, 70, 301. (7) Badley, R. D.; Ford, W. T. J. Org. Chem. 1989, 54, 5437. (8) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J. P.; Hemidy, J. F.; Lavalley, J. C.; Barre`s, O.; Burneau, A.; Grillet, Y. Adv. Colloid Interface Sci. 1990, 33, 91. (9) Vansant, E. F.; Van der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: The Netherlands, 1995. (10) Robertson, A. P.; Leckie, J. O. J. Colloid Interface Sci. 1997, 188, 444. (11) Legrand, A. P., Ed. The Surface Properties of Silicas; Wiley: 1998.

respective analysis. It appears therefore attractive to promote the development of nondestructive methods for the in situ characterization of the surface reactivity involving solid materials in direct contact with a solution of a chemically similar neighbor in constant evolution. The surface reactivity of silica powders depends on the nature, distribution, and accessibility of the surface sites.9,11,12 The silanol groups, either single or geminal, are of particular interest and considered to be strong adsorption sites,13,14 whereas the siloxane sites are usually regarded as hydrophobic.2 The reaction of bases with silica has been largely investigated in the past, and the adsorption of ammonia and organic amine vapors on silica surfaces is well-documented.15-26 The dissociative chemisorption of ammonia on the siloxane sites on poorly hydroxylated silicas was observed (i.e., when silica was subjected to a vacuum degassing at temperature above 400 °C),15-19 whereas acid-base type interactions of varying strength with the silanol groups were found to occur with silica surfaces in highly hydroxylated state.20-25 Understanding of this last system has been significantly (12) Nawrocki, J. Chromatographia 1991, 31, 177. (13) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 4269. (14) Hair, M. L. ACS Symp. Ser. 1980, 137, 1. (15) Bartell, F. E.; Dobay, D. G. J. Am. Chem. Soc. 1950, 72, 4388. (16) Peri, J. B. J. Phys. Chem. 1966, 70, 2937. (17) Blomfield, G. A.; Little, L. H. Can. J. Chem. 1973, 51, 1771. (18) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1976, 80, 1998. (19) Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Surf. Sci. 1989, 222, 95. (20) Felden, M. Compt. Rend. 1959, 249, 682. (21) Boyle, T. W.; Gaw, W. J.; Ross, R. A. J. Chem. Soc. 1965, 240. (22) Ross, R. A.; Taylor, A. H. Proc. Brit. Ceram. Soc. 1965, 5, 167. (23) Fripiat, J. J.; Van der Meersche, C.; Touillaux, R.; Jelli, A. J. Phys. Chem. 1970, 74, 382. (24) Zhdanov, S. P.; Kosheleva, L. S.; Titova, T. I. Langmuir 1987, 3, 960. (25) Titova, T. I.; Kosheleva, L. S. Colloids Surf. 1992, 63, 97. (26) Tripp, C. P.; Kazmaier, P.; Hair, M. L. Langmuir 1996, 12, 6407.

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advanced by workers who explicitly demonstrate the proton transfer between the surface silanol groups and the base to form the corresponding silanolate groups interacting with the protonated base (eq 1).

(tSiOH) + B h (tSiO-BH+)

(1)

Much work has been devoted in the past20-23 and more recently24-26 to the study of the interaction between gaseous basic reactants and silica surfaces, because such systems are of interest in adsorption and catalysis.27 Despite the fact that the fixation of metal species on silica surfaces, for example, has long been known to be promoted by the presence of aqueous ammonia,28 studies dealing with the deprotonation of silanol groups by a solutionphase basic reactant are not widespread, most probably because of the relatively low stability of silica at high pH.1,29,30 Anyway, the acid-base properties of the silanol/ silanolate couple have been exploited for the determination of the specific surface area of silica gel, most often by acid-base titration.31-36 The procedure, however, did not involve the deprotonation of all of the silanol groups36 and did not take into account differences between the type of silanols although it is demonstrated that the reactivity of these groups strongly depends on their form.12,37 We have recently shown how the deprotonation of silica surfaces by ammonia is influenced by the base concentration in the solution.38 In agreement with Sonnefeld’s model which predicts a decrease in the acid constant of silanol by an increase in the extent of dissociation,39 the degree of completion for the reaction between silanol groups and ammonia was found to be limited by the deprotonation ratio, so that no more than 70% of the silanol groups can be deprotonated by 1 M NH3.38 In this paper, we have extended this study to the reaction of silanol groups with other bases (alkylamines and hydroxides) to investigate the effects of their strength and size on the deprotonation process. This knowledge is important in connection to the acid-base and ion-binding behaviors of silica surfaces,10,40 especially for studies on metal ion adsorption for which nitrogen-containing compounds act as either a base or a ligand.41,42 Beside the commercially available colloidal silicas, Sto¨ber et al.43 have reported the preparation of spherical silica particles displaying a narrow size distribution, according to an ammonia-catalyzed hydrolysis and condensation of tetraethyl orthosilicate in an aqueous alcohol solution. After this pioneering work, studies were directed to the optimization of the experimental conditions leading (27) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (28) Smith, G. W.; Reyerson, L. H. J. Am. Chem. Soc. 1930, 52, 2584. (29) Kondo, S.; Igarashi, M.; Nakai, K. Colloids Surf. 1992, 63, 33. (30) Sjo¨berg, S. J. Non-Cryst. Solids 1996, 196, 51. (31) Sears, G. W., Jr. Anal. Chem. 1956, 28, 1981. (32) Meffert, A.; Langenfeld, A. Z. Anal. Chem. 1970, 249, 231. (33) Unger, K. K.; Kittelmann, U. R.; Kreis, W. K. J. Chem. Technol. Biotechnol. 1981, 31, 435. (34) Cheng, W.; McCown, M. J. Chromatogr. 1985, 318, 173. (35) Khurama, A. L.; Ho, C.-T. J. Liq. Chromatogr. 1988, 11, 3205. (36) Takeuchi, T.; Miwa, T. Anal. Chim. Acta 1993, 282, 565. (37) Nawrocki, J.; Buszewski, B. J. Chromatogr. 1988, 449, 1. (38) Despas, C.; Walcarius, A.; Bessie`re, J. Talanta 1997, 45, 357. (39) Sonnefeld, J. J. Colloid. Interface. Sci. 1993, 155, 191. (40) Davis, J. A.; James, R. O.; Leckie, J. O. J. Colloid Interface Sci. 1978, 63, 480. (41) Park, Y. J.; Jung, K.-H.; Park, K. K. J. Colloid Interface Sci. 1995, 171, 205. (42) Park, Y. J.; Jung, K.-H.; Park, K. K. J. Colloid Interface Sci. 1995, 172, 447. (43) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

to the synthesis of these monodispersed silica beads.44-48 Efforts have been mainly devoted to their characterization by various physicochemical techniques, the understanding of particle nucleation and growth mechanisms, and the analysis of particle microstructure.48-61 Some studies also involved surface modification reactions using silane coupling agents and polymers.62 Several authors have pointed out that Sto¨ber silica beads display an anomalously high hydroxyl concentration relative to their apparently low specific surface area, as measured by BET sorption experiments using Ar or Kr as the inert gas, indicating that these solids present a microporous structure.46,48,57,60,63 Moreover, Giesche46 has suggested a pore cutoff size of about 0.3 nm by using various gases of different size (He, Ar, Kr, Xe) when performing the sorption studies. Despite the growing interest in Sto¨ber silica, very few studies have been directed toward demonstrating the chemical reactivity of the internal silanol groups.38,60,63 Burneau and co-workers60,63 have reported the deuteration of these internal silanols although they are not accessible to argon, and our group38 has demonstrated that ammonia is able to deprotonate them. Preliminary results have also indicated molecular-sieving properties for these monodispersed silica beads,64 and the present work aims to investigate thoroughly the acid-base reactivity of the internal silanol groups by using bases of various size. The accessibility to the interior of the bead will be discussed and further compared to results obtained from water adsorption results. In this work, we have thus examined how steric factors, as well as the strength of the reactants, influence the extent of deprotonation of the silanol groups on two silicas displaying very different structures: a mesoporous amorphous silica gel and microporous monodispersed Sto¨ber silica beads. Reactions have been monitored by in situ high-frequency impedance measurement as well as quantitative chemical analyses. The results of in situ Raman spectroscopy are also briefly presented. Attention has been given to performing experiments under conditions which (44) van Helden, A. K.; Jansen, J. W.; Vrij, A. J. Colloid Interface Sci. 1981, 81, 354. (45) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F., IV J. Non-Cryst. Solids 1988, 104, 95. (46) Giesche, H. J. Eur. Ceram. Soc. 1994, 14, 189. (47) Giesche, H. J. Eur. Ceram. Soc. 1994, 14, 205. (48) Labrosse, A.; Burneau, A. J. Non-Cryst. Solids 1997, 221, 107. (49) Lecloux, A. J.; Bronckart, J.; Noville, F.; Dodet, C.; Marchot, P.; Pirard, J. P. Colloids Surf. 1986, 19, 359. (50) Philipse, A. P. Colloid Polym. Sci. 1988, 266, 1174. (51) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1988, 124, 252. (52) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1989, 132, 13. (53) Look, J.; Bogush, G. H.; Zukoski, C. F., IV Faraday Discuss. Chem. Soc. 1990, 90, 345. (54) Bogush, G. H.; Zukoski, C. F., IV J. Colloid Interface Sci. 1991, 142, 1. (55) Bogush, G. H.; Zukoski, C. F., IV J. Colloid Interface Sci. 1991, 142, 19. (56) Jelinek, L.; Dong, P.; Rojas-Pazos, C.; Taı¨bi, H.; Kovats, E. Langmuir 1992, 8, 2152. (57) van Blaaderen, A.; Kentgens, A. P. M. J. Non-Cryst. Solids 1992, 149, 161. (58) van Blaaderen, A.; van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481. (59) van Blaaderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1. (60) Burneau, A.; Humbert, B. Colloids Surf. A 1993, 75, 111. (61) Chen, S.-L.; Dong, P.; Yang, G.-H.; Yang, J.-J. J. Colloid Interface Sci. 1996, 180, 237. (62) Ketelson, H. A.; Pelton, R.; Brook, M. A. Langmuir 1996, 12, 1134 and references therein. (63) Gallas, J. P.; Lavalley, J.-C.; Burneau, A.; Barres, O. Langmuir 1991, 7, 1235. (64) Walcarius, A.; Despas, C.; Bessie`re, J. Microporous Mesoporous Mater. 1998, 23, 309.

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allow us to neglect the dissolution of silica ( 7 for these geminal silanol groups1,12,24 and the fact that about 30% of the silanol population are geminal76 explain why deprotonation ratios higher than 70% were never observed, even in highly concentrated base solutions. 3.2.3. Model of the Interface. Figure 4 depicts the impedance spectra (Nyquist plots) recorded from silica contacting either EtNH2 solutions of increasing concentration or various base solutions of different nature but at the same concentration. Because of the action of the base on the surface silanol groups, the decanted solid made of ionized silica appears as a dispersed system of particles with fixed charges. For such a system, two kinds of dielectric relaxations are expected to take place.83 The relaxations observed in a lower frequency region are thought to be caused by surface conductivity originating from ions in diffuse double layer. The relaxations in a higher frequency range are considered to be due to interfacial polarization mechanism. The recorded impedance diagrams of Figure 4 show semicircles slightly shifted with respect to the real axis and with a center lying under this axis, because of the superimposition of several relaxation phenomena with narrow frequency, as explained by the rather heterogeneous distribution of the decanted silica particles. It is important not only to note here that the dielectric response was not affected by electrode polarization but also that the effect of interfacial polarization can be observed at high frequency, as depicted by the data moving aside from the main semicircle (Figure 4). In the present study, only the low-frequency relaxations (80) Stolen, R. H.; Walrafen, G. E. J. Phys. Chem. 1976, 64, 2623. (81) Humbert, B.; Burneau, A.; Gallas, J. P.; Lavalley, J. C. J. NonCryst. Solids 1992, 143, 75. (82) Nawrocki, J. J. Chromatogr. 1997, 779, 29. (83) Ishikawa, A.; Hanai, T.; Koizumi, N. Bull. Inst. Chem. Res., Kyoto Univ. 1984, 62, No. 4, 251.

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(i.e., the main semicircle) reporting on the ionization process will be discussed. Among the different theoretical transfer functions Z(ω) which have been applied to the quantitative analysis of the experimental impedance data, we have chosen the simplest approach as a satisfactory approximation. This requires Z(ω) to be represented by a parallel combination of a resistance Rs and a capacitance Cs, both in series with another resistance Rc (Figure 5). The model of the interface is then made of an ohmic resistance, Rc, associated to two surface parameters, the double-layer capacity, Cs, and the charge-transfer resistance, Rs, which is a direct measurement of the interface resistance.84 By applying this simple model to the results of Figure 4, it

appears that Rc did not vary significantly with either the base concentration or the nature of the base used to ionize silica. However, the interfacial resistance was found to strongly evolve depending on the experimental conditions. As illustrated in Figure 4A, in the case of the deprotonation of silanol groups by EtNH2, an increase in the solution-phase EtNH2 concentration resulted in a decrease in the interfacial resistance. This is explained by the gradual increase of the SiO-/SiOH ratio leading to the growth of the surface charge density on the silica particles. This evolution is parallel to that of the corresponding dielectric losses and is directly proportional to the amount of ionized silanol (i.e., the silanolate groups), as shown in Table 2. Similar evolution as a function of the deprotonation ratio is more or less observed with all of the bases studied here, except that the nature of the base significantly affects the absolute values of the interfacial resistance. As an example, for a selected base concentration of 0.1 M, corresponding to approximately the same ionization degree, the use of NH3, MeNH2, EtNH2 or Et3N as the deprotonation agent resulted in Rs equal to 59, 81, 86, and 94 Ω, respectively (Figure 4B). These values are in good agreement with those of the corresponding equivalent conductivity of the protonated bases, so that the interfacial conductivity is related not only to the deprotonation ratio but also to the mobility of the chargecompensating cations (Table 2). This demonstrates the ability of complex-impedance measurements to get information about the evolution of a solid/liquid interface, under nondestructive conditions, as was previously shown in corrosion studies.85 Although dielectric data indicate that the interactions between silanolate groups and cations are mainly electrostatic in nature, it should be pointed out however that they do not allow for the unambiguous establishment of whether cations are accumulated in the fixed or diffuse part of the double layer. 3.3. Ionization of Monodispersed Silica Beads. We have demonstrated in a previous paper38 that ammonia was able to react not only with silanol groups located on the outer surface of the Sto¨ber beads but also with the inner groups, because of the microporosity of Sto¨ber beads.46,48,49,57,60 The number of formed silanolate groups was much higher than the theoretical number of silanol groups which one could calculate on the basis of the specific surface area measured by Kr adsorption (about 7 × 10-2 mmol of outer silanols/g of beads). In the present work, the ionization of the monodispersed silica beads will be studied in detail by using bases of different size, strength, and concentration, as a follow-up to our preliminary note.64 3.3.1. Reaction with Molecular Bases. Figure 6 illustrates the difference between silica gel and Sto¨ber silica beads as concerns their reaction with amines of various sizes. Although the ionization of silica gel was not influenced significantly by the base size (in the limit of the base size considered here), that of the monodispersed silica beads were strongly affected by the reactant size. For comparison purposes, the y-axes scale has been chosen so that each maximum corresponds to the maximal theoretical capacity of each silica sample. As mentioned above, the amount of fixed base on silica gel was unaffected by the base size; the experimental values under the average loading are those of ammonia, trimethylamine, and ethanolamine, which are known to be weaker bases than the others, leading to a lower degree of completion for the ionization reaction, in agreement with the Sonnefeld’s model.39 With the Sto¨ber silica, small bases were

(84) Coster, H. G. L.; Chilcott, T. C.; Coster, A. C. F. Bioelectrochem. Bioenerg. 1996, 40, 79.

(85) Popova, A.; Raicheva, S.; Sokolova, E.; Christov, M. Langmuir 1996, 12, 2083.

Figure 4. Impedance spectra recorded for 1.0 g of silica gel in equilibrium with (A) ethylamine at different concentrations and (B) various bases at the same concentration (0.1 M).

Figure 5. Model for the silica/water interface.

Acidic Properties of Silica Gel and Beads

Figure 6. Evolution of the amount of deprotonated silanol groups of (a) silica gel and (b) Sto¨ber silica beads as a function of the size of the reactant: 1.0 g of silica gel or 0.5 g of Sto¨ber silica in 10 mL 0.5 M base.

found to react to a large extent (proportionally larger than with silica gel). By increasing the base size, however, the amount of fixed species was found to decrease down rapidly. Bases as large as (and larger than) i-BuNH2 were excluded from the internal structure and did not allow the ionization of inner silanol groups. This confers on the Sto¨ber beads a molecular-sieving property agreeing well with the pore cutoff size of 3 Å suggested by Giesche46 for similar materials. On the other hand, the partial reaction of bases of critical dimensions ranging from 2 to 3 Å with respect to their initial concentration in solution was explained by volume-exclusion effects. These reactants are small enough to enter the pore structure of silica beads, but their size may be such that before that particular base can react with an inner silanol, no room is left within the pores for further reactant. In fact, the sum of the volume of all species required for the complete neutralization of silanol groups (within the limit of the base concentration in solution) is greater than the available space within the bead. When this occurs, partial reaction is observed, not because of a sieving effect but because of a volume-exclusion effect. As shown in Table 2, a similar effect was observed for silica gel with the very large NBu4OH species. Finally, it should be emphasized that the equilibrium state of the deprotonation reaction was also reached in less than 15 min with the microporous Sto¨ber silica (similar to the mesoporous silica gel); this could be rationalized by considering the small size of Sto¨ber beads (0.62 µm) as compared to that of silica gel (125 µm). Figure 7 shows that ionization of silica beads not only depends on the size of the base but also on its initial concentration in solution. As expected, for each base able to fit inside the porous structure, the degree of ionization was found to increase when increasing the initial base concentration in solution. This means that the Sto¨ber beads behave with nonsize-excluded reactants similar to silica gel. The evolution of the apparent dissociation constant of inner silanol groups strictly obeys the theory derived for silica gel.38 Moreover, the deprotonation rate of the Sto¨ber silica did not depend significantly on the strength of the base. As an example, the dissociation constant of MeNH2 (pKa ) 10.6) is more important than that of NH3 (pKa ) 9.2), but the number of silanolate groups was slightly lower for MeNH2 than for NH3 at a same concentration. 3.3.2. Reaction with Hydroxylated Bases Associated to Various Countercations. The molecular-sieving effect pointed out above for molecular bases was also observed

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Figure 7. Evolution of the amount of deprotonated silanol groups of Sto¨ber silica beads as a function of the base concentration: 0.5 g of silica in 10 mL of (a) ethylamine, (b) methylamine, and (c) ammonia.

Figure 8. Evolution of the amount of deprotonated silanol groups of Sto¨ber silica beads in 0.05 M NBu4OH as a function of the concentration of added cations: (a) Li+, (b) Na+, (c) K+, (d) Rb+, and (e) Cs+.

with OH- associated to various countercations. Reaction of silanol with OH- requires the presence of an additional cation (M+) for the neutralization of the silanolate group (eq 2). Of course, OH- can diffuse freely inside the porous

(tSiOH) + M+ + OH- h (tSiO-M+) + H2O (2) structure, so that the limiting factor is now the entrance of the countercation within the bead. The use of 5 × 10-2 M NaOH resulted in the ionization of 1.8 mmol silanol/g of Sto¨ber silica, whereas the same concentration of NBu4OH did not give any reaction in the time scale of the experiment (overly long times would result in the dissolution of silica; see below). Whatever the size of the tetraalkylammonium hydroxide, no cation was fixed by Sto¨ber silica. Figure 8 demonstrates that the sieving effect is controlled by the size of the charge-compensating cation. Increasing concentrations of nonsize-excluded cations were added to a 5 × 10-2 M NBu4OH solution containing a weighed amount of silica beads. The extent of reaction was determined by monitoring both the disappearance of the nonsize-excluded cations and the decrease in the OHconcentration. The presence of alkali metal cations in the medium induces the ionization process (eq 2). As long as

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Table 4. Titrimetric Determination of the Amount of Silanolate Groups (tSiO-) Formed during the Deprotonation of Sto1 ber Silica Beads by OH- (0.1 M) as a Function of the Countercation equiv amount of cation size (cm3 mol-1) conductivitya cation on silica cation (mmol g-1) (Ω cm2) nonhydratedb hydratedc Li+ Na+ K+ Rb+ Cs+ Me4N+ Et4N+ Pr4N+ Bu4N+

38.7 50.1 73.5 77.8 77.3 44.9 32.6 23.4 19.5

0.78 2.29 5.96 8.01 11.7 83.2 142.7 208.0 269.3

125.9 109.0 94.4 93.0 93.0 115.4 143.4 206.9 270.1

2.5 2.3 1.7 1.8 2.1 Cs+ Results of Figure 8 show that this was not exactly the case, most probably because cations should be considered hydrated, resulting in faster diffusion within the micropores of cations displaying the smallest hydrated size. Such diffusional limitation has been reported for zeolites, especially in the indirect amperometric determination of alkali metal cations using zeolite-modified electrodes.86 The observed selectivity series is probably due to the competition between these two opposite effects (Table 4):

Li+ > Na+ > Cs+ > Rb+ > K+ Finally, the addition of increasing concentrations of tetraalkylammonium cations of various alkyl chain length (n ) 1-4) in the 5 × 10-2 M NBu4OH solution did not give any incorporation of these species within the bead, conferring to the solid material a size selectivity at least as restrictive as that displayed by the crystalline zeolites. 3.3.3. Dielectric Monitoring. Dielectric permittivity, ′, and dielectric losses, ′′, are, respectively, the charge and current densities induced in response to an applied electric field of unit amplitude.87 For nonsize-excluded bases such as NH3, both of these parameters can indicate the ionization of the silica beads, similarly to that of silica gel, except that dielectric values were much lower for the beads, most probably because of a higher resistance to the movement of charges in the tiny internal volume of this material (Table 5). For example, ′ values for Sto¨ber silica rose from 52 to 85 when adding 0.05 M NH3 in the medium (86) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electroanalysis 1995, 7, 120. (87) Foster, K. R.; Schwan, H. P. Crit. Rev. Biomed. Eng. 1989, 17, 25.

(from 73 to 174 for the gel), whereas ′′ values were multiplied by more than 1 order of magnitude after ionization (Table 5). Moreover, in the absence of any interfering ion in the medium, ′′ values were found to be proportional to the amount of silanolate groups formed within the bead, as ascertained by varying the base concentration in the medium. However, data of Table 5 were however given for a selected base concentration (0.05 M) in order to compare the behavior of both silica samples in the same experimental conditions. Also, ′ and ′′ values varied with the applied frequency, but at any frequency experienced here (from 0.1 to 100 MHz), the same trends were observed when passing from one suspension composition to another, and the results of Table 5 are those obtained at 1 MHz. For bases larger than 3 Å, such as i-BuNH2, neither ′ nor ′′ were found to change as compared to those characteristics of Sto¨ber silica beads in pure water (′ ) ∼50 and ′′ ) ∼150; Table 5), indicating the absence of a significant ionization process. In comparison, values obtained for silica gel were increased (from 73 to 93 for ′ and from 99 to 2600 for ′′) because of the deprotonation reaction. The high-frequency impedance technique thus appears to be a good tool for the in situ monitoring of the ionization of monodispersed silica beads in basic media and for pointing out their sieving properties. In the presence of salt (i.e., NaCl), the situation is somewhat more complicated, and the rationalization of the results requires one to distinguish between small (such as NH3) and large bases (such as i-BuNH2). Let’s consider the evolution of ′′ values which are characteristics of the conductivity changes of the dispersion. Two processes are able to induce an increase in the ′′ values: the deprotonation of silanol groups, as largely discussed above, and the increase in the ionic strength of the medium. When using NH3, ionization occurs independently of the nature of the silica sample, and ′′ is a function of both the deprotonation ratio and the ionic strength of the medium. Indeed, values observed for silica samples in the presence of 0.05 M NH3 + 0.05 M NaCl are much higher than those measured in 0.05 M NaCl in the absence of ammonia (Table 5), confirming that the surface ionization of silica still can be monitored by high-frequency impedance experiments in the presence of salt.38 On the other hand, we have demonstrated that the addition of i-BuNH2 in a suspension containing Sto¨ber silica did not result in a deprotonation reaction and dielectric values stayed low (Table 5). By adding a nonsize-excluded cation (Na+) in the medium, however, ′′ values increased (from 144 to 9100) because of both the formation of internal silanolate groups and the increase in the ionic strength. The control experiment performed using Sto¨ber silica suspended in 0.05 M NaCl in the absence of base (′′ ) 5700) confirms the intervention of both of these effects on the dielectric response. Of course, even in the presence of nonsize-excluded cations (such as Na+), i-BuNH2 cannot enter the internal structure (eq 3),

(tSiOH) + i-BuNH2 w no reaction

(3)

but it maintains a high pH in the medium so that the overall reaction is due to the action of OH- (eq 4), with

i-BuNH2 + H2O h i-BuNH3+ + OH-

(4)

Na+ acting as a countercation for allowing charge balance (eq 5). Also, the Sto¨ber silica can be characterized by the

(tSiOH) + Na+ + OH- h (tSiO-Na+) + H2O (5)

Acidic Properties of Silica Gel and Beads

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Table 5. Comparison of Silica Gel and Sto1 ber Silica with Respect to Their Reaction with NH3 and i-BuNH2 in the Presence or Absence of NaCl in the Medium, Along with Their Ionization Ratio, Dielectric Permittivity, and Dielectric Lossesa silica gel

Sto¨ber silica

composition of the slurry

pHb

% reactionc

′

′′

pHb

% reactionc

′

′′

SiO2 in pure water SiO2 in 0.05 M NH3 SiO2 in 0.05 M NH3 + 0.05 M NaCl SiO2 in 0.05 M i-BuNH2 SiO2 in 0.05 M i-BuNH2 + 0.05 M NaCl SiO2 in 0.05 M NaCl

6.9 10.5 10.2 11.2 10.9 5.6

0 23 29 25 30 0

73 174 127 93 92 67

99 6.3 × 103 9.1 × 103 2.6 × 103 4.5 × 103 5.3 × 103

7.2 10.5 10.3 11.4 11.0 5.8

0 30 35 0 36 0

52 85 70 53 196 48

144 1.8 × 103 9.9 × 103 144 9.1 × 103 5.7 × 103

a

Field frequency ) 1 MHz. b Slurry at the equilibrium. c SiO-/SiOH ratio expressed in percent.

Figure 10. Scheme illustrating the ion-sieving properties of Sto¨ber silica beads. The spatial representation of the bases has been achieved using the Hyperchem Software.

Figure 9. Adsorption (O) -desorption (b) isotherm of water at 25 °C on Sto¨ber silica.

same model as that proposed for the silica gel. A 4-fold increase in the interface resistance was found by passing from silica gel to silica beads in the presence of i-BuNH2, because only the surface silanol groups of the beads can react, whereas all those of the gel can theoretically be reached by the alkaline reactant. 3.3.4. Microporosity and Molecular Sieving. The characterization of the porosity of Sto¨ber silica beads has generated many research efforts and is still a current area of investigations.46,48,57,60,63 If it is well-established that this property should be directly related to the particle growth mechanism, the exact nature of this mechanism is still a matter of debate.48 Without entering this discussion, we want to give a new result which could help in the understanding of the microporous behavior of this peculiar solid. Considering that the water molecule is able to enter the microstructure of the beads, the water adsorption isotherm was constructed and then compared to both the results discussed above and those previously obtained by using other probes such as Kr or N2. The water adsorption isotherm characteristic of the Sto¨ber silica sample is clearly of type IV, displaying an hysteresis on reverse scan (Figure 9). BET analysis gives a specific surface area of 310 m2 g-1 (cumulated surface area obtained after t-plot) and an estimated pore size of 18 Å (model of cylindrical pores). This result is quite surprising when considering the adsorption data obtained with other probes that revealed nonporous materials (type II isotherms) and the pore cutoff size of 3 Å suggested by Giesche46 and further demonstrated in this work. On the other hand, it is noteworthy that type I isotherms were recently obtained for such a material for Kr adsorption

when very long equilibrium times were allowed, indicating the microporous behavior with large specific surface areas (100-200 m2 g-1) and a pore size close to 9 Å.48 All of these results are consistent with a structure of the Sto¨ber silica beads displaying a bulk porosity ranging from 10 to 20 Å and covered with a small layer of microporous silica on the surface, which gives ion-sieving properties to the material, as illustrated in Figure 10. The entrance to the porous structure is mainly governed by the pore cutoff size of 3 Å, whereas the large surface area measured by water adsorption (as well as Kr adsorption at low rate) is explained by the larger pores in the bulk of the beads. Conclusions The ionization of surface silanol groups of silica by neutral (amines) or anionic (hydroxides) bases can be characterized in situ by high-frequency impedance measurements. The calculation of dielectric permittivity (′) and losses (′′) allows one to discriminate between the surface silanol and silanolate groups. ′ values were found to be directly related to the counterion associated with the silanolate groups, and the recording of ′′ values allowed the quantitative analysis of the ionization processes. Deprotonation of silica gel was almost unaffected by either the base strength (in the range from ammonia to strong bases) or the size of the reactant. On the other hand, the deprotonation ratio was found to increase with the base concentration. The interfacial resistance, estimated from a dielectric model, was directly related to the mobility of the charge-compensating cation of the silanolate groups. The behavior of the Sto¨ber silica was totally different. Because of its microporosity, only bases smaller than the pore aperture were able to deprotonate the internal silanol groups, conferring on this material a molecular-sieving property. Moreover, it was found that the pore size in the bulk of the beads was larger than the pore cutoff aperture of 3 Å. Although both silica gel and Sto¨ber silica produced an amount of silanolate groups proportional to the base concentration, the decrease in the apparent equilibrium

3196 Langmuir, Vol. 15, No. 9, 1999

constant, which rose as fast as the degree of dissociation of silica, made impossible the completion of the neutralization reaction. Acknowledgment. We gratefully acknowledge Prof. A. Burneau and A. Labrosse for providing us with the

Despas et al.

sample of silica beads. We thank Dr. B. Humbert for recording the Raman spectra and for helpful discussions. We are also grateful to Dr. F. Villieras and M. Franc¸ ois for water adsorption measurements. LA981217Y