Development of a rationale for decoupling osmotic coefficient of electrolytes into electrostatic and nonelectrostatic contributions

Abstract

In this work, a method has been developed to decouple the electrostatic and the nonelectrostatic contributions to the osmotic coefficient. The observation, that the osmotic coefficient-molality plot exhibits a linear region over a significant range of concentration, allows estimation of the primary hydration number of electrolytes and also to hypothesize that the primary hydration number does not depend on electrolyte concentration. The estimated value of the primary hydration number agrees well with that obtained using NMR spectroscopy and exhibits correct dependence on temperature. It is also shown that in a mixture of electrolytes, each electrolyte retains its own primary hydration number irrespective of the presence of the other electrolytes. The estimate of the primary hydration number allows us to determine the nonelectrostatic contribution to the osmotic coefficient for the single and the mixed electrolytes over the entire range of electrolyte concentration. Subtraction of this contribution from the osmotic coefficient yields the electrostatic contribution. The secondary hydration number, which is responsible for modulating electrostatic interaction, attains a constant value beyond the electrostatic screening limit and at a fixed temperature, this value is found to be independent of the type of electrolyte. The sum of the primary hydration number and the limiting secondary hydration number agrees well with the hydration number estimated using the extended X-ray absorption fine structure spectroscopy. The most important contribution of this work is the isolation of the electrostatic contribution to the osmotic coefficient of solutions of single and mixed electrolytes. The electrostatic interaction is shown to much stronger than that predicted by the extended Debye-Hückel theory, which points to a need for revision of the existing theories for electrostatics of concentrated electrolytes. The electrostatic contribution estimated using the present method, for both single as well as mixed electrolytes, would provide the basis for validation of the revised theories.