One-phase and two-phase regions of colloid stability

Abstract

The Coulombic attraction theory of colloid stability, first proposed by Sogami in 1983, is shown to be well adapted to explain the existence and extent of both the one-phase (suspension) and two-phase (gel) regions of colloid stability which are observed in clay swelling. The central prediction of the Sogami theory is that there is a weak attractive tail in the thermodynamic electrostatic interaction potential between colloidal particles in electrolyte solutions. The position of the minimum in the pair potential between parallel clay plates is given as a function of the plate thickness and the electrolyte concentration (c) and is used to estimate how much solvent the clay will absorb as a function of its initial volume fraction (r). This yields a prediction for the position of the r–c phase boundary, whose curvature is in excellent agreement with recent studies of vermiculite swelling. The standard theory of colloid stability, the DLVO theory, is a limiting case of the Coulombic attraction theory, in the one-phase region. In the two-phase region, Sogami theory combined with the Dirichlet boundary condition (constant surface potential) predicts that the ratio (s) of the salt concentration in the supernatant fluid to the average slat concentration in the gel phase will be constant. For a surface potential of 70 mV, s is equal to 2.8, in excellent agreement with the experimental results on n-butylammonium vermiculite gels.