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Influence of Ion Size on Short-Range Repulsive Forces between Silica Surfaces Miroslav Colic,*,† Matthew L. Fisher,‡ and George V. Franks‡ R&D Division, ZPM Inc., 5770 Thornwood Drive, Suite C, Goleta, California 93117, and Materials Department, University of California at Santa Barbara, Santa Barbara, California 93106 Received July 13, 1998. In Final Form: July 27, 1998 The influence of counterion size on short-range repulsive forces at high salt concentrations was investigated with silica slurries at various pHs. Ions from the lyotropic series of monovalent electrolytes (LiCl, NaCl, KCl, and CsCl) were used to coagulate dispersed slurries. Measurements of viscosity and sedimentation rate were performed. The results at high salt concentrations and volume fractions of silica clearly show that the extent of the short-range repulsive forces correlates with the size of the unhydrated ion. This trend is opposite to the ion-adsorption sequence on silica at low volume fractions of solids or lower ionic strength. Our results are also contrary to the widely accepted hydration force model but concur with the recently developed reference-hypernetted chain statistical mechanics models describing the interaction of ions with solvated surfaces.
Introduction The forces acting between colloidal particles and surfaces govern their behavior in solution. The classical Derjaguin-Landau-Verwey-Overbeek theory (DLVO)1,2 describes the total interparticle interaction energy as the sum of electrostatic forces and van der Waals attractive forces. DLVO theory is a continuum approach, considering ions as point charges and neglecting ion-solvent interactions, surface-solvent interactions, and ion-ion correlations.3 The development of direct surface force measurement techniques with the surface force apparatus (SFA) and the atomic force microscope (AFM) in the last 2 decades has confirmed the accuracy of the DLVO predictions for low electrolyte concentrations and large separation distances. These direct force measurement techniques have also led to the discovery of several new forces such as hydration forces4 and short-range repulsive steric forces5 that are important at small separation distances. The hydration (or solvation) force, as its name implies, is correlated to the structuring of water (solvent) at surfaces. Pashley and Israelachvili6-8 investigated the effect of ion size on short-range repulsive hydration forces between mica plates at low-to-moderate ionic strength. The results showed that smaller, more hydrated ions such as lithium produced short range repulsive forces to a greater extent. The repulsive hydration force was attributed to the dehydration of counterions when surfaces are pushed together. Thus, more strongly hydrated ions such as Li+ would be expected to produce short-range repulsions of greater magnitude and extent, as compared to poorly * To whom correspondence should be addressed. † ZPM Inc. ‡ University of California at Santa Barbara. (1) Derjaguin, B.; Landau, L. Acta Physiochim. URSS 1941, 14, 633. (2) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, The Netherlands, 1948. (3) Greathouse, J. A.; McQuarrie, D. A. J. Colloid Interface Sci. 1996, 181, 319. (4) Israelachvili, J. N. Adv. Colloid Interface Sci. 1982, 16, 31. (5) Horn, R. G. J. Am. Ceram. Soc. 1990, 73, 1117. (6) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (7) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (8) Pashley, R. M.; Israelachvili, J. N. J. Colloid Interface Sci. 1984, 97, 446.
hydrated ions such as Cs+. Such behavior was postulated for other surfaces. Velamakanni et al.9 indirectly studied surface forces by measuring the rheology and particle packing density of alumina slurries and consolidated bodies. These researchers demonstrated that coagulation by salt produces a weaker network than that produced by flocculation at the isoelectric point (iep). This observation suggested that the high concentrations of salt produced a short-range repulsive force, which was theorized to be due to hydration forces. Using the surface force apparatus, Ducker et al.10 confirmed the existence of a short-range repulsive force between sapphire plates at high ionic strengths. On the basis of rheological and particle packing measurements, Colic and colleagues11 recently found that the extent of the short-range repulsive forces between alumina particles dispersed and coagulated with 0.5 M lithium, sodium, potassium, and cesium chlorides correlated to the size of the unhydrated counterion. Similar results were obtained with viscosity measurements on other materials such as zirconia, silicon nitride, and mullite.12 This is opposite to the trend predicted by the hydration force model described above. Using the SFA with silica, Chapel13 found that the strength and range of the hydration force decreased with a decreasing bare counterion size. All of the materials studied by Colic and Chapel have hydroxyl groups on the surface that can be either neutral or positively or negatively charged (depending on the pH) and at least a primary hydration layer. On the other hand, the mica studied by Pashley and Israelachvili is a constant charge system with ion exchange as the main mechanism of charging. Silica has been extensively studied for decades,14 yet its behavior in aqueous solutions is unusual and still poorly (9) Velamakanni, B. V.; Chang, J. C.; Lange, F. F.; Pearson, D. S. Langmuir 1990, 6, 1323. (10) Ducker, W. A.; Xu, Z.; Israelachvili, J. N.; Clarke, D. R. J. Am. Ceram. Soc. 1994, 77, 437. (11) Colic, M.; Franks, G. V.; Fisher, M. L.; Lange, F. F. Langmuir 1997, 13, 3129. (12) Colic, M.; Lange, F. F., unpublished data. (13) Chapel, J. P. Langmuir 1994, 10, 4237. (14) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979.
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understood.15 When silica is coagulated with the addition of salt, the maximum stability is observed at the isoelectric point where oxides bare no net surface charge, while it coagulates with the lowest amount of salt at high pHs where silica surfaces are highly negatively charged.16,17 One possible explanation for such behavior is the presence of a thick hydration layer on the silica surface and its destruction at high pH by ion exchange for surface protons,16,17 which prevents hydrogen bonding of water to the surface. Another proposed explanation is the presence of a surface-protruding layer of silicic acid which stabilizes the particles sterically.18 Israelachvili and Wennerstrom recently proposed that there is only one structured water layer near the surfaces of particles and monotonic shortrange repulsive hydration forces actually might not exist.19 Presented in this letter are the results of rheological measurements on silica slurries at high salt concentrations, from which short-range forces are inferred. Our results clearly show that the short-range repulsive force with the longest extent is observed with the largest (in the unhydrated state) ion studied (Cs+). This is contradictory to the widely accepted hydration force theory. The results cannot be attributed to any kind of specific ion adsorption since cesium adsorbs stronger at silica than lithium.16 If ion adsorption was governing the interparticle forces, slurries with higher viscosities would be produced with cesium ions. As will be suggested in the Discussion, our results also strongly contradict the recent hypothesis of Israelachvili and Wennerstrom (IW) that there is no long-range ordering of water near silica surfaces. Experimental Section High-purity silica slurries (0.3 ( 0.02 µm particle size) were obtained from Nissan Chemicals and used as received. Particles were prepared through hydrolysis and were never dried or heattreated. The isoelectric point of the silica particles was found to be at pH 1.8. Salts and hydroxides were purchased from Sigma Chemicals, St. Louis, Missouri. Viscosity measurements of 0.15 volume fraction slurries at various pHs and 4 or 1 M salts were made with a dynamic stress rheometer (Rheometrics DSR) using a couette-type measuring cell (29.5-mm diameter, 44.0-mm long).20 Slurries were subjected to a high shear rate which was decreased until the measured torque was below the sensitivity of the instrument (0.1 g‚cm). Sedimentation rate measurements were performed as described in Hunter and Ekdawi.21 Electrophoretic light scattering (Malvern Zeta Sizer 3) and acoustophoresis (Acoustosizer) were used to determine zeta potentials at high ionic strengths (0.5 M of salts).
Results The viscosities of silica slurries at pH 1.8, 6, 9, and 11 and 4 M salts are presented in Figure 1a-d. For the suspensions containing 4 M salts, it is evident that the slurries formulated with lithium chloride always have the highest viscosity and slurries formulated with cesium the lowest. It is especially intriguing that at the isoelectric point of the slurry (pH 1.8), where the surface is neutral and ions are not expected to adsorb, the effect of ion size still exists (Figure 1a). For other systems, the viscosity at the iep is the highest and strongly shear-rate-dependent. Such anomalous behavior is characteristic for silica. It (15) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375. (16) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1969, 31, 287. (17) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1970, 33 420. (18) Frens, G.; Overbeek, J. Th. G. J. Colloid Interface Sci. 1972, 38, 376. (19) Israelachvili, J. N.; Wennerstrom, H. Nature 1996, 379, 219. (20) Yanez, J. A.; Shikata, T.; Pearson, D. S.; Lange, F. F. J. Am. Ceram. Soc. 1996, 79, 2917. (21) Hunter, R. J.; Ekwadi, N. Colloids Surf. 1986, 18, 325.
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is quite amazing that at its iep even in the presence of 4 M salt silica still shows no shear rate dependence of viscosity (Newtonian behavior) which is characteristic of well-dispersed colloidal systems with repulsive interparticle networks. Figure 2 shows the effect of ion size on the sedimentation rate of pH 11 silica slurries at different volume fractions coagulated with 4 M salts. The results presented show that the transition from a low to high sedimentation rate occurs at similar volume fractions for all ions studied, namely between 1.5 and 2 vol %. This indicates a similar particle network structure for all of the systems studied. As in the viscosity studies, slurries formulated with lithium showed the fastest rate of sedimentation and hence the deepest attractive potential well. The electrophoretic light scattering and electroacoustophoretic measurements at high ionic strengths showed that cesium always neutralizes the silica surface stronger than lithium, which is consistent with the critical coagulation concentration measurements performed by Allen and Matijevic.16 For instance, with pH 4 and 0.5 M salts the zeta potential of silica slurries containing cesium was 0 V and lithium slurries -7 mV. Similar results were found at pH 6 (zeta potential 0 for 0.5 M cesium slurry and -4 mV for 0.5 M lithium slurry). Zeta potential measurements at higher ionic strengths could not be measured due to overheating. The effect of ion size on interparticle forces between silica particles was also studied at a lower concentration of ions where both ions and surfaces are more completely hydrated. Unfortunately, at such concentrations the critical coagulation concentrations needed to produce coagulated slurries were not reached. Such behavior was also shown in the work of Allen and Matijevic.16 Nonetheless, the obtained results were interesting and are presented in Figure 3a-d. Figure 3a shows the viscosity versus the shear rate behavior for 0.15 volume fraction silica slurries formulated at pH 1.8 with 1 M of different salts. At pH 1.8, the isoelectric point of silica, no effect of added ions was expected. However, as with the slurries prepared with 4 M salts, a clear effect of ion size on the viscosity was observed; in fact, the increase in viscosity with the addition of lithium was greater at 1 M than at 4 M. Lithium slurries once again had the highest viscosity and cesium the lowest. Figure 3b presents silimilar results for silica slurries formulated at pH 6. As mentioned before, silica is very stable at low pHs and slurries prepared at pH 6 and with 1 M salts are still somewhat charged and not fully coagulated. No effect of ion size at this pH was observed. Figure 3c shows similar results obtained for systems prepared at pH 9. As shown by Allen and Matijevic,16 the ccc of silica sols at pH 9 is much lower than at low pH’s. Consequently, at pH 9 silica particles are already charged to a lesser extent and closer to the coagulated state. And indeed the effect of ion adsorption was observed at this pH with cesium coagulating the silica stronger than lithium. Both ion adsorption and competition for hydration water play a significant role at this pH. As in the case of silica slurries with a low volume fraction used in coagulation experiments, slurries formulated with cesium were more coagulated, probably due to stronger adsorption.16 At pH 11 the inversion of the ion size adsorption sequence occurred. This was also observed in experiments at low volume fractions performed by Allen and Matijevic.16 Matijevic suggested that the inversion in the lyotropic sequence effects at pH 11 occurred due to the ion exchange of hydrogen ions from surface silanol with the counterion cations.
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Figure 1. Viscosity versus strain rate plots for silica slurries (0.15 volume fraction solids) formulated at (a) pH 1.8 (the iep), (b) pH 6, (c) pH 9, and (d) pH 11 without salt and with the addition of 4 M of various chlorides, as labeled.
Figure 2. Sedimentation rate versus volume fraction for silica slurries formulated at pH 11 with 4 M of various chlorides, as labeled.
Discussion The viscosity of slurries and strength of particle networks are well-correlated to the interparticle forces.22-24 The current results clearly show that the strengths of the attractive particle networks follow from strongest to (22) Bergstrom, L. Surface and Colloid Chemistry in Advanced Ceramic Processing; Surfactant Science Series; Marcel Dekker Inc.: New York, 1994; Vol. 51. (23) Goodwin, J. W.; Hughes, R. W.; Partridge, S. J.; Zukoski, C. F. J. Chem. Phys. 1986, 85, 559. (24) Leong, Y. K.; Scales, P.; Healy, T. W.; Boger, D.; Buscall, R. J. J. Chem. Soc., Faraday Trans. 1993, 89, 2473.
weakest as follows: Li+, Na+, K+, and Cs+. This shows that lithium counterions produce the deepest potential well and strongest attractive forces. Deeper potential wells are produced by short-range repulsive interparticle forces of a lesser extent. Consequently, the size of the bare (rather than hydrated) ion correlates to the extent of short-range repulsive forces. When the salt is added to a slurry at pHs different from the iep, the mechanism for the short-range repulsion is the collapse of the counterion cloud to its minimum extent (surface charge is completely neutralized in the first layer of adsorbed counterions) as described in the previous report on alumina.11 The current results are opposite to the trend predicted by the hypotheses of Pashley and Israelachvili. According to their model, the dehydration of adsorbed cations leads to the repulsive short-range hydration force. Thus, repulsions with the longest extent are expected with the strongly hydrated lithium ion which has the largest hydrated size of the ions studied here. At low ionic concentrations and volume fractions, cesium adsorbs stronger at silica than lithium. This produces a lower critical coagulation concentration (ccc) for cesium than for lithium and consequently stronger networks would be expected with slurries formulated with cesium as compared to lithium. At high salt concentrations where the electrical double layer is completely neutralized in the first layer of the adsorbed counterions, some other mechanism must be responsible for the observed behavior. Chapel12 observed the same trend with direct surface force measurements on silica. In Chapel’s experiments, care was taken to remove silicic acid adsorbed on the surface.
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Figure 3. Viscosity versus strain rate plots for silica slurries (0.15 volume fraction solids) prepared at (a) pH 1.8 (the iep), (b) pH 6, (c) pH 9, and (d) pH 11 without salt and with the addition of 1 M of various chlorides, as labeled.
Ion-surface-solvent interactions must be taken into account. It is important to note that the silica in Chapel’s work was prepared by flame treatment. The influence of flame and plasma treatment on the surface properties of silica will be addressed below. It is not surprising that lithium produces short-range repulsive interparticle forces with the longest extent on mica. Mica is poorly hydrated, and exchange of fully hydrated ions with the surface potassium ions accounts for the observed adsorption and ion size effect in such systems. Secondary hydration of the adsorbed ions rather than a primary surface hydration layer controls the behavior of mica. Figure 4 schematically shows the proposed mechanism of counterion adsorption for both silica at pH > pH (iep), where the counterions are cations, and mica, respectively. The remaining question concerns the origin of the observed behavior on silica and other oxides.11 Statistical mechanics models used to further improve the DLVO theory might yield an answer. According to the referencehypernetted chain model (RHNC) of Torrie et al.25 the layer of structured water on charged surfaces is more favorable for the accommodation of smaller ions with a high affinity for water. Such ions have a tendency to strongly organize water and prefer to crowd near the charged surface where the water is already highly polarized and more structured. Larger ions such as cesium prefer to reside outside the hydration layer and thus give rise to short-range repulsions of a longer extent. It was (25) Torrie, G. M.; Kusalik, P. G.; Patey, G. N. J. Chem. Phys. 1989, 91, 6367.
Figure 4. Schematic representation of the structure of the oxide/water interface (pH > pH (iep) and the mica/water interface with various counterions adsorbed to the surface. Small counterions with a greater affinity for water penetrate deeper into the surface hydration layer of oxides than large ions. At the mica surface, ion exchange of fully hydrated counterions for the lattice potassium ions occurs.
recently shown that it is more difficult to remove smaller ions when surfaces are pushed together.26 Moreover, NMR relaxation time studies indicated strong interactions between the surface and sodium ions.27 Most amazingly, the effect of ion size is observed even at the isoelectric point (iep) where no ion binding to the neutral surface is expected. The presence of excess ions increases the viscosity compared to that of silica slurries (26) Franks, G. V.; Colic, M.; Fisher, M. L.; Lange, F. F. J. Colloid Interface Sci. 1997, 193, 96. (27) Colic, M., unpublished data.
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with no added salts, and thus the effect of ion binding can be excluded since it would be expected to provide shortrange repulsive forces and further stabilize the slurry. We believe that at the iep and at high ionic strength the electrolyte ions are abstracting water from the silica surface, decreasing the thickness of the hydration layer and the extent of the short-range repulsion. Lithium, having a stronger affinity for water, removes more hydration water from the silica surface and produces slurries with higher viscosity than cesium. At very high ionic concentrations, there is a competition for water between the silica surface and ions in solution. The removal of structured water molecules from the silica surface produces a less structured hydration layer and subsequently short-range repulsive forces with a shorter extent. Lithium has a stronger affinity for water than cesium and thus steals more water from the surface. In this way lithium reduces the surface hydration layer of silica more effectively than cesium and allows the surfaces to come closer together before the hydration repulsion is felt by the particles. Why are the viscosities for slurries at pH 11 and 4 M salts closer than at other pHs investigated in this work (see Figure 1d)? The surface charge of silica at pH 11 is very high. It is likely that the ions adsorb very close to the interface with little or no water left in between. Since lithium is larger than cesium, this would produce a more stable system. However, lithium penetrates deeper inside the hydration layer for which it has more affinity. The two effects could neutralize each other. Also, the high solubility of silica at pH 11 might contribute to the observed behavior. Spectroscopic experiments should be performed to understand this phenomenon in more detail. The results described in Figure 3 also need additional explanation. As explained before, at the iep silica is exceptionally stable, which is anomalous behavior. We believe that, at high ionic strengths, competition for the water of hydration between cations and silica is severe. Potassium and cesium are the “structure breakers” and are not competing for water as successfully as lithium or sodium. Therefore, more water is scavenged in the presence of lithium or sodium. This results in a thinner hydration layer and short-range repulsion with a shorter extent, which produces an interparticle potential with a secondary minimum. Consequently, the viscosity is higher and since the system no longer possesses a repulsive interparticle network, the viscosity is non-Newtonian. It seems that the thickness of the hydration layer in the absence of lithium or sodium is so extensive that, despite the lack of charge, the interparticle forces are repulsive. An alternative explanation could be that the cations promote silica gelation to a different extent, lithium being more successful than cesium. Particle networks which are more gelled would stick together more strongly and yield higher viscosity. It is also interesting to note that with 1 M salts (see Figure 3) no complete inversion in the lyotropic sequences occurs. Similar incomplete inversions in lyotropic sequences at high pH were also previously observed with silica by Allen and Matijevic.16,17 With 4 M salts, however, a complete inversion is observed, suggesting that extremely high ionic strengths are necessary to perturb the surface hydration layer. The viscosity of the salt solutions could also influence the lyotropic sequence at the isoelectric point. The addition of a 4 M salt affects the viscosity of the solutions differently for each of the ions investigated. Viscosities of salt solutions in the absence of silica were measured to determine if the increase was due to a change in the interparticle forces or simply an increase in solution
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viscosity. The increase in viscosity with the addition of 4 M Li was the highest, but not nearly as great as was the case for the silica slurries. Also, at the iep, the viscosity of 4 M salt slurries were lower than those at 1 M. DePasse recently pointed out that at high ionic strength in silica systems such behavior could be observed due to attenuation of van der Waals attractions. Ninham and coworkers also proposed such behavior in their recent papers. If the viscosity of the salt solutions controlled the total viscosity, the opposite would have been observed. The behavior of silica at pH 6 is another anomaly: at this pH silica is very unstable. It takes some amount of acid and base to adjust the pH of the slurry to where it is desired; this adds some amount of salt to the system. It does not matter for the systems with the high salt concentration but can significantly influence the viscosity of the systems with no added salt. This likely happened in the viscosity measurements of the slurries prepared with no salt additions whose viscosities are presented in Figures 1b and 3b. Israelachvili and Wennerstrom proposed that adsorbed surface silicic acid is promoting the unusual colloidal stability of silica. It is difficult to explain how a surface layer of silicic acid could yield such strong stability at the isoelectric point at which silicic acid, like silica, bears no charge. It is well-known that, in order to achieve complete electrosteric stabilization with a layer of adsorbed polymers, the molecules have to be charged. Uncharged polymer molecules collapse to the surface and do not provide any significant steric stability. Even if the silicic acid was present on the surface and was collapsing to a different extent with different counterions, this would likely be caused by different abilities of ions to perturb the hydration layers around silicic acid. Other evidence suggests the existence and importance of a thick hydration layer at the silica surface. Shen and co-workers29 performed a series of experiments with sumfrequency generation (SFG) spectroscopy, which is sensitive only to noncentrosymmetric molecules such as adsorbed oriented water at the silica surface. They confirmed the existence of icelike water layers near the silica surface as well as the presence of up to seven layers of hydration water at higher pHs where the surface is strongly charged. Similar results were observed by Miller and co-workers30 with the attenuated total reflectance FTIR spectroscopy at the silica/water interface. Heterocoagulation experiments of Matijevic and colleagues also suggested the existence of a large hydration cage near the silica surface which collapses when protons are ionexchanged with the metal cations.31 SFA measurements by Grabbe32 on silica surfaces coated with aminosilane showed no evidence of the short-range repulsive force present for well-hydrated, uncoated surfaces. So what are the reasons for the differences in observed behavior between surface force apparatus and AFM measurements with pyrogenic silicas and colloidal stability studies on precipitated silica such as that used in the current work? Ninham and co-workers33 recently found that silica surfaces which were produced by flame treatment or cleaned by plasma eventually gelled, given enough time. Such surfaces always carry some intrinsic silicic (28) Depasse, J. J. Colloid Interface Sci. 1997, 188, 229. (29) Du, Q.; Freysz, E.; Shen, Y. R. Phys. Rev. Lett. 1994, 72, 238. (30) Yalamanchili, M. R.; Atia, A.; Miller, J. D. Langmuir 1996, 12, 4176. (31) Kihira, H.; Matijevic, E. Langmuir 1992, 8, 2855. (32) Grabbe, A. Langmuir 1993, 9, 797. (33) Yaminsky, V.; Ninham B. W.; Pashley, R. Langmuir 1998, 14, 3223.
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acid which indeed controls their behavior. We have also realized that by adding silicic acid to our silica samples the viscosity can be lowered. Yet all such samples gelled in 1-4 days. The silica sols obtained from Nissan Chemicals and used in our work have not gelled in more than 3 years. NMR analysis of our silica samples did not identify any silicic acid on the surface. Colic and co-workers34 recently realized that silica samples treated with the plasma or electromagnetic radiation always dissolved at pHs where silica is not commonly soluble. It was also observed that plasma, flame, or electromagnetic radiation produce free radicals inside silica. Atomic hydrogen can be trapped inside silicas for years.35 When such silica samples are submerged under water, atomic hydrogen produces protons and free electrons which are ultimate structure breakers.36 Bartels and co-workers recently proposed a model which shows the reasons for strong structure-breaking properties of free electrons.36 Structure-breaking properties of electrons produce smaller, more active water clusters which might be responsible for some silica dissolution even at pHs as low as 3. Consequently, water penetrates inside the silica structure and gels the surface. When two gelled surfaces come together, they will ultimately adhere strongly, if given enough time. Electron spin resonance measurements shall be performed to identify the presence of atomic hydrogen, free electrons, or other free radicals in flameor plasma-treated silicas. Until spectroscopic results are available, our hypothesis remains speculative. The above-described results, we believe, will help explain a long dispute about what is causing the unusal stability of silica samples in water solutions. Both the adsorbed layers of silicic acid and extensive hydration of the surface can contribute to that stability, depending on the sample, experimental conditions, and time given for the equilibration. Finally, other possible explanations for the observed phenomenon such as ion-correlation effects or an inadequacy of the linear summation of electrostatic double-layer forces and van der Waals forces should be considered. Such possibilities were recently scrutinized by Ninham and co-workers.37 Some researchers suggest that chemical bonds can be formed when two surfaces stick together. This was intially proposed by
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Depasse and Watillon as a possible reason for the unusual silica behavior.38 Kekicheff and co-workers proposed the same model based on surface force studies.39 Depasse also recently performed a detailed analysis of colloidal stability of silica sols in the presence of various cations.40 On the basis of previously published results, it was proposed that the formation of chemical bonds rather than dehydration was responsible for the unusual colloid stability behavior of silica. It was also proposed that such a model can explain the observed lyotropic sequences. Such models shall be scrutinized with spectroscopic studies. Conclusions The current rheological experiments on silica slurries support the previous surface force apparatus12,15,32 and spectroscopic measurements28-30 in suggesting that the short-range repulsive forces which stabilize silica are due to the existence of layers of organized water near the surface. The ion size effect observed in the present work and in previous surface force12 and rheological studies,11 namely that smaller, more strongly hydrated ions such as lithium produce short-range repulsions to a lesser extent, is contrary to that of the hydration force model. However, the results correlate well with the predictions of the reference-hypernetted chain statistical mechanics models which take into account ion-solvent-surface interactions. The differences in behavior observed for metal oxides and for mica indicate that ion adsorption as well as ion interactions with surface hydration layers must be taken into account when considering short-range forces. The behavior of silica is strongly influenced by its history of treatment. Silica surfaces prepared with plasma or flame can be stabilized by adsorbed layers of silicic acid. The results obtained in various laboratories can be compared only if the samples are treated in a similar fashion. Acknowledgment. The authors wish to acknowledge the Nissan Chemical Co. for supplying the silica used in the present study. We would also like to acknowledge the Office of Naval Research for supporting this work under Contract No. N00014-92-J-1808, and thank Prof. Fred Lange for allowing us the use of the ceramic processing facilities at UCSB. LA980489Y
(34) Colic M.; Morse, D. E. J. Colloid Interface Sci. 1998, 200, 265. (35) Sasamori, R.; Okaue, Y.; Isobe, T.; Matsuda, Y. Science 1994, 265, 1691. (36) Han, P.; Bartels, D. M. J. Phys. Chem. 1991, 95, 5367. (37) Ninham, B. W.; Yaminsky, V. Langmuir 1997, 13, 2097.
(38) Depasse, J.; Watillon, A. J. Colloid Interface Sci. 1970, 33, 430. (39) Atkins, D.; Kekicheff, P.; Spalla, O. J. Colloid Interface Sci. 1997, 188, 234. (40) Depasse, J. J. Colloid Interface Sci. 1997, 194, 260.
