Prediction of the Water Activity in Multicomponent Solutions by a Simple and Accurate Equation and Derivation of Zdanovskii’s Rule

Abstract

A simple equation is derived for the prediction of the water activity as a function of the molal concentrations of a multicomponent mixture, a1(m2 , m3 ,.., mn+1). It consists basically of a series expansion whose coefficients are determined from the limiting behavior imposed by the thermodynamics of solutions. The validity of this equation was verified on 96 binary, ternary and quaternary systems involving electrolytic and molecular solutes as well as solutions containing both electrolytes and nonelectrolytes. The error was always lower than 1% for solutions with a 1 ≥ 0.90. For electrolytic solutions its applicability range is moderately concentrated and dilute solutions (maximum molality m max ≤ 1 mol·kg−1), while for the case of molecular solutes it extends to significantly more concentrated mixtures (3 ≤ m max /mol·kg−1 ≤ 6). Moreover, it also allowed the derivation of a generalized form of Zdanovskii's rule for multicomponent systems involving both electrolytic and molecular solutes as well as the verification of the Frolov?Rard hypothesis. Finally, the equation was used for the estimation of the vapor pressure of ternary solutions of non-volatile solutes, showing better descriptive capabilities than those calculated by the application of the perturbed-chain statistical associated fluid theory (PC-SAFT).