ACID-BASE EQUILIBRIA IN CLAY SUSPENSION

The thermodynamics governing the equilibria of weakly basic compounds in dilute clay suspensions is developed in considerable detail. Quantities suitable for characterizing the equilibrium are shown to be: Keff, an apparent equilibrium constant for the acid-base reaction in suspension; Δ G ̄BH, and Δ G ̄B, which are the changes in the partial molar Gibb's free energy for the protonated and neutral forms of the base B, respectively, which occur upon addition of clay to the system at constant T, P, and composition. The above thermodynamic treatment is then used to examine the frequently observed phenomenon that basic compounds in clay suspensions are often protonated to a much greater extent than that predicted on the basis of the pH of the bulk solution phase and the pK of the compound. It is shown that the enhanced protonation is due to the ability of the clay-water interface to significantly lower the potential of the protonated form of the molecule relative to that of the neutral form. This relative difference in stabilization of the two forms displaces the equilibrium and is shown by experiment to depend on the apparent magnitude of negative charge on the clay surface as encountered by the basic compound. © 1979.