A REASSESSMENT OF THE APPLICABILITY OF THE DLVO THEORY AS AN EXPLANATION FOR THE SCHULZE-HARDY RULE FOR COLLOID AGGREGATION

Shortcomings in the application of the DLVO theory in explaining the observations of the Schulze-Hardy rule occur because of inappropriate simplifications. Previous authors (14, 15) have commented that an approximation valid for high surface potentials is difficult to reconcile with the aggregation of low potential particles. In addition to this, we suggest that a further simplification, assuming only weak electrostatic interaction between particles, is also inappropriate in explaining the Schulze-Hardy rule. We derive a new expression that determines the critical ionic strength for the coagulation of low potential colloids in terms of the interfacial charge density. The new equation is more general than previous expressions since the ionic strength may be used to describe nonideal unsymmetrical electrolytes, and the charge density is a more fundamental parameter than surface potential. The concept of restabilization in coalescing emulsions is described and discussed in terms of the new expression for critical aggregation ionic strength. © 1991.