J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1261-1263 Comparison of the Electrokinetic Properties of the Silica Surface Dave E. Dunstan Department of Chemical Engineering The University of Melbourne Parkville, Victoria 3052, Austra lia The electrokinetic potentials and surface charge densities of the silica surface are interpreted from both electro- phoretic mobility and electro-osmosis measurements. Comparison of the data shows smaller magnitudes for both the potentials and surface charge interpreted from the electrophoretic mobility over the range of KCI concentration studied (10-5-0.1 mot dm-3). The interpreted surface charge densities increase indicating that effective adsorption of the negative chloride ions to the negative surface occurs. The silica surface is therefore postulated to be non-classical in nature.It is suggested that the retardation effect and therefore the electro-phoretic potentials are underestimated in the electrophoresis problem. The electrokinetic equations describing colloidal phenomena are undergoing close experimental examination at present. There are several reasons for this. The continuum model pro- posed in the theory is of fundamental physical interest and the practical implications of the measurements and their interpretation are wide.' It is generally accepted that a test of the theory is that it yields the same electrokinetic potential from different measurements on the same surface. To date this has not been satisfactorily achieved. Theoretical modifi- cations have been made in an attempt to obtain consistency between the interpreted potential^.^,^ However, no com-prehensive study of a single system which enables vindication of the modified or fundamental theory has yet been made. This work presents and compares data obtained from two direct current (dc) electrokinetic measurements of the silica surface over a range of electrolyte concentrations.The elec- trophoretic mobilities of silica particles and the electro-osmotic mobilities of silica plates were measured over a range of KCl concentrations and the interpreted potentials com-pared.4 Further insight into the nature of the silica surface and the electrokinetic equations is obtained. Experimental KCl obtained from Fluka (99.5%) was recrystallized and baked at 600°C for 4 h.Deionized water was further purified by double distillation from a KOH-KMnO, first stage and stored in a Pyrex vessel until used. The water used was of surface tension 72.6 mN m- ',conductivity 0.9 mS cm- ' and pH 5.8 (1.5 x mol dmP3 H,CO,). All measurements were conducted at pH 5.8. Two sizes of silica particles were prepared using the method of Stober et a1.' to yield particle sizes of 50.4 and 165.0 nm radius. These particles are reported to be of lower density (2.0 g cm-3) than amorphous silica (2.2 g cm-3).6*7 Particle sizing was done using electron micros- copy on more than 300 particles. The silica particles were extensively cleaned by centrifugation-decantation ca. 100 times over a period of one year in Teflon centrifuge tubes.This ensured that all ammonia and other possible impurities were removed. Polystyrene lattices, used as tracers, were obtained from Interfacial Dynamics Corporation (sulfate variety, 300 nm in diameter). These were extensively cleaned by centrifugation-decantation 50 times to form an opalescent suspension in H20 at a volume fraction of 0.05 which showed no surface-active contamination to be present. All the reported electrokinetic measurements were made using a Coulter Delsa 440 Electrophoresis apparatus. The mobilities of the silica particles were measured as a series by addition of high concentrations of KC1 to a dilute stock solu- tion. Using this method the volume fraction of the suspension was kept approximately constant. The final concentration was determined by mass.The silica particle mobilities were measured at both stationary layers to ensure that no con- tamination artefacts were present during the measurement. Measurement of the electro-osmotic mobility required the use of a tracer particle, polystyrene latex, and measurement of the apparent mobility profiles. Latex particles were used as tracers in preference to silica particles as the latices give nar- rower, less noisy mobility peaks. To evaluate the electro- osmotic mobility of the silica cell wall from the apparent mobility profile in the Delsa cell (aspect ratio 3.25) the Komogata equation was used.' The Komogata equation describes the relationship between the apparent particle mobility, U,, , the electrophoretic mobility, U, , and the electro-osmotic mobility, U,, ,as a function of position in the cell.U,, = U, -O.86Ue0+ 1.86Ue0~/h)2 where y/h is the fractional distance from the centre of the cell to the wall. The symmetry of the parabolic profiles indicated that the cell was free of contamination. The electro-osmotic potentials were interpreted from the electro-osmotic mobilities of the latex tracer particles using the Smoluchowski equation.' This analysis was consistent with that of O'Brien and White4 for infinite particle radius. Results and Discussion The electrokinetic potentials and surface charge densities interpreted from the measurements are shown in Fig. 1 and 2. Both the electro-osmosis and electrophoresis measurements are interpreted using the theory of O'Brien and White.4 In the limit of high KU the theory turns to the well-known Smol- uchowski result.Here, K is the reciprocal Debye length and a the particle radius. Several trends are immediately obvious. The potentials and charge densities interpreted from the electro-osmosis mea-surements are greater in magnitude than those interpreted from the electrophoretic mobilities of the particles over the range of electrolyte concentration employed. The surface charge densities increase with increasing electrolyte concen- tration for the electro-osmotic measurements. The surface charge densities interpreted from the electrophoresis mea- surements on the particles show a slight maximum at ca.0.01 mol dm-3 KCl. Both data sets indicate that negative ions are being adsorbed to the negative silica surface. [KCl]/mol dm-3 Fig. 1 Interpreted electrokinetic potentials of the silica surface us. KCl concentration. The curves are best fits to the data and are shown to help distinguish the different data sets. The derived electro- phoretic potentials for the two different particle sizes are: (+) 504 nm particles and (0)1650 nm particles. (W) Electro-osmotic and (0) streaming potential" data. The possibility that experimental artefacts are present must be discussed before a physical interpretation of the data is made. The particles are deemed to be This would imply that the surface area of the particles is larger than expected.Therefore, the driving force on the particles should be larger and the interpreted potentials and surface charge densities should therefore be higher than those interpreted for the flat plates. The derived potentials and surface charge den- sities for the particles of two different sizes would also not be the same, as their surface areas are different. Essentially the surface-to-bulk ratio will be different for the two particles and thus any porosity should not give the same contribution to the mobility for the different particles. The possibility that the particle surfaces are porous or that contamination is present does not account for the increasing electrokinetic N E Q)P r 0 -2 Q) f 10-6 lo4 lo-' loo [KCl]/mol dm-3 Fig.2 Interpreted electrokinetic surface charge densities of the silica surface us. KCl concentration. (+) 504 and (D) 1650 nm particles. (0)Values interpreted from the electro-osmosis measurements. The curves are drawn to guide the eye. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 surface charge density with electrolyte concentration. Both the particles and plates are made of silica and were cleaned stringently, as discussed in the Experimental. As such, both should behave as silica exposed to electrolyte. To ensure that the electro-osmotic measurements were interpreted correctly using the Komogata equation to describe the hydrodynamics of the Delsa cell, data obtained by other workers for silica plates in streaming potential measurements are also shown in Fig.1 (0)''The data for the two sets of independent mea- surements agree within experimental error, indicating that the cell walls are free of contamination and that the Komo- gata equation is correct for the aspect ratio of the cell. The streaming potential measurements are for an effectively infin- ite aspect ratio. Given that the experimental data represent a true compari- son of the silica surface measured by several techniques, let us return to a physical discussion of the data. If the theory was a correct physical description of the measured phenomena, the interpreted potentials would be the same. The physical reason for the occurrence of both phenomena, electrophoresis and electro-osmosis, is the finite spatial separation of charge which occurs at the electrolyte/solid interface.The plane of shear in both systems defines the elec- trokinetic potential. The spatial distribution of ions and the fluid flow in the electrolyte, outside the shear plane, are described by the continuum equations, the Poisson-Boltzmann and Navier-Stokes equations, re~pectively.~ The usual assumptions of a continuous dielectric, constant vis- cosity solvent, with point charges for the ions, apply. In the electro-osmotic measurements, electro-osmotic mobility is induced in the solvent under the influence of the applied field by the flow of the asymmetric charge that resides in the inter- facial region. The flow of charges carries liquid with it.This is a contradiction of the continuum model. The point charges should have zero interaction with the solvent and therefore induce no flow. Point charges would also have infinite con- ductivity. This is accounted for in the theory by using the measured values of the specific ionic conductivities. The electrophoresis problem is of a similar nature at high electrolyte concentration. In the Smoluchowski limit, the relative motion of the particle and electrolyte is induced by the electro-osmotic motion around the particle. At lower elec- trolyte concentrations the particle moves due to the force exerted on it by the electric field and the electrokinetic surface charge. The counterion charge in the diffuse double layer is distorted i.n the electric field and gives rise to the retardation effect.The dominant retardation effect on the particle mobility is due to the fact that the electric field seen by the particle is less than the applied field combined with an extra viscous drag due to the diffuse double layer being carried with the particle. This polarization of the diffuse double layer around the particle is responsible for the very large dielectric response observed for colloidal particles.' From the above discussion it is apparent that one reason for the differences in the interpreted potentials and surface charge densities is that the retardation effect is incorrectly calculated by the theory. The spatial distribution of ions in the double layer is necessary to calculate the retardation effect in the electrophoresis problem, while in the electro- osmosis problem the exact spatial distribution of the charge need not be known. It is therefore readily concluded that the Poisson-Boltzmann equation is not an accurate description of the diffuse layer in this case.This interpretation is consis- tent with dielectric response measurements on colloidal silica which show the theory to underestimate the dielectric response significantly. l2 Essentially, the experimentally mea- sured dipole moments are much larger than could be expected reasonably from the theory. Therefore, the interpre- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tation of the electrophoresis measurement underestimates the electrokinetic potential. Consistent with the above discussion is the observed increase in magnitude of the electrokinetic surface charge with electrolyte concentration.The electrokinetic surface charge should decrease with electrolyte concentration as positive ions are adsorbed to the negative surface as the bulk electrolyte concentration increases. The data indicates that negative ions are adsorbed to the negative surface as the con- centration increases. This is non-classical behaviour and further evidence that the Poisson-Boltzmann description of the interfacial charge distribution is not complete. A classical mechanism for the adsorption of negative ions to a negative surface has yet to be proposed. Recent measurements of the electrokinetic properties of hydrocarbon particles in an elec- trolyte have shown the presence of finite mobilities, which are interpreted as resulting from changes in the chemical poten- tial of the solvent due to the presence of an interface.l39l4 While the silica surface is intrinsically charged oia the disso- ciation of hydroxy groups at the measurement pH, the surface may still cause a perturbation in the interfacial water.As such, the interfacial region may be described by a com- bination of both Poisson-Boltzmann electrostatics and a spa- tially varying solvent chemical potential. Note that recent measurements on the interaction forces between silica surfaces have shown the continuum description to be remarkably good down to small separation distances of the order of several nanometres.” This may be reconciled with the data presented here by assuming that the classical electrostatic interaction is observed at small separation dis- tances.It is hoped that future work on different electrolytes will further elucidate these interesting phenomena. References 1 R. J. Hunter, The Zeta Potential of Colloid Science, Acadmic Press, New York, 1987. 2 E. H. B. de Lacey, Ph.D. Thesis, Australian National University, 1982. 3 C. S. Mangelsdorf and L. R. White, J. Chem. SOC., Faraday Trans., 1990,86,2859. 4 R. W. O’Brien and L.R. White, J. Chem. SOC.,Faraday Trans. 2, 1978,74, 1607. 5 W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62. 6 R. K. Iler, The Chemistry of Silica, Wiley, New York, 1979. 7 G. H. Bogush, M. A. Tracy and C. F. Zukoski, J. Non-Crystalline Soids, 1988, 104, 95. 8 S. Komogata, Res. Electrotech. Lab. Tokyo, Comm. No. 1933, 348. 9 M. Von Schmoluchowski, Z. Phys. Chem., 1918,92, 129. 10 P. J. Scales, F. Grieser and T. W. Healy, Langmuir, 1990,6, 582. 11 E. H. B. de Lacey and L. R. White, J. Chem. SOC., Faraday Trans., 1981, 77, 2001. 12 L. A. Rosen and D. A. Saville, Langmuir, 1991,7,32. 13 D. E. Dunstan and D. A. Saville, J. Chem. SOC.,Faraday Trans., 1992,88,2031. 14 D. E. Dunstan and D. A. Saville, J. Chem. SOC., Faraday Trans., 1993,89, 527. 15 A. Grabbe and R. G. Horn, J. Colloid Inter$ace Sci., 1993, 157, 375. Paper 3/07106E; Received 1st December, 1993