The Esin and Markov coefficient for double layers with colloid chemical importance

Abstract

A thermodynamic derivation is given of the Esin and Markov coefficient β for double layers on oxides, metals and silver iodide in the presence of an electrolyte of arbitrary type and at arbitrary surface charge. The coefficient is defined as the partial derivative of the negative logarithm of the activity of potential-determining ions with respect to the logarithm of the salt activity at constant surface charge. β is the same for substances for which the p.d. ions are constituent ions and for substances for which this is not the case. In the absence of specific adsorption the coefficient reverses its sign at the point of zero charge and attains limiting values depending only on the valency of the counterion. Application to a number of systems (AgI, SiO2, SnO2, Fe2O3) shows that the theoretical predictions are essentially corroborated. For very porous double layers, β=0. The theory can also be used as a means to determine ionic components of charge.