(N,V,T) Monte Carlo simulations are performed by using hyperspherical geometry in order to investigate the mechanical stability and the ionic affinity of clay particles neutralized by a mixture of counterions of different valency. In this study, clay particles are described as uniformly charged surfaces and the interactions between ions and these charged surfaces are described within the context of the ‘primitive model'. The purpose of this study is to derive the maximum amount of monovalent counterions necessary to remove the stability of clay materials neutralized by divalent or trivalent counterions. From the equation of state of these charged materials we quantify the influence of ionic exchange on the interfacial free energy.