Probing Surface Charge Potentials of Clay Basal Planes and Edges by Direct Force Measurements

Abstract

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.

Pressure solution – The importance of the electrochemical surface potentials

“…Effective surface potentials of muscovite, silica glass, magnetite and zircon as function of the pH. Muscovite mica, silica glass and zircon were measured using AFM (for details see electronic annex) and compare well with values reported in the literature (Zhao et al, 2008;Grabbe and Horn, 1993;Mao et al, 1994;Kirby and Hasselbrink, 2004) using various different techniques. The indicated range of values for magnetite is taken from zeta potential measurements (Sun et al, 1998).…”

Direct molecular-level characterization of different heterogeneous freezing modes on mica – Part 1

“…In addition, this change does not mean that water becomes more alkaline at lower temperatures because, in the case of pure water and according to Le Châtelier's principle, there is always the same concentration of hydrogen and hydroxide ions and, hence, the water is still neutral (pH = pOH) even if its pH changes. The pH 7.47 at 0 • C is simply the new way of referencing neutral water pH at 0 • C. In addition, assuming that the surface potential has an influence on the background signal, this will not change even if the pH changes with temperature because the surface potential values of the muscovite basal plane (the surface under study) is pH independent in the range of pH 5.6 to 10 (Zhao et al, 2008).…”

Absolute acidity of clay edge sites from ab-initio simulations

“…Nevertheless, there has been to date no direct way to probe the charge density on the edge and to measure the deprotonation free energies of the different sites. Another indirect strategy to determine the overall protonation state of clay edges has been proposed by Zhao et al who found using Atomic Force Microscopy that the force between a silica tip and a surface consisting of muscovite edges turned from repulsive at pH $10 to attractive at pH $6, from which they concluded that these edge surfaces had a point of zero charge of pH 7-8 (Zhao et al, 2008).…”