THE MODIFIED GOUY-CHAPMAN THEORY - COMPARISONS BETWEEN ELECTRICAL DOUBLE-LAYER MODELS OF CLAY SWELLING

The modified Gouy-Chapman (MGC) theory of the electrical double layer is used to calculate the swelling pressure (PI) between planar double layers using three possible boundary conditions: constant surface char ge density (sigma(o)), constant surface potential (psi(o)), and consta nt potential at the plane of closest approach of the ions (psi(s)). It is shown that, at charge densities typical of montmorillonite clays, the swelling pressures of the three models are different only at plate separations of a few ionic diameters. Beyond those separations, the t hree pressures are identical. The trend of increasing PI with increasi ng ionic size is shown for the two constant potential models, in agree ment with previous work on the constant surface charge density model. Finally, the MGC theory is compared with experimental data from clay s welling experiments. The comparisons show that the MGC theory agrees q ualitatively with experiment, while the absence of a plane of closest approach results in poor agreement.