Thermodynamics of Electrolyte Solutions.

Thermodynamics of electrolyte solutions has been studied and a new equation of state for mixtures containing electrolytes has been derived using perturbation theory. In this equation, short-range interactions between molecules are calculated using the Perturbed-Anisotropic-Chain theory (PACT) of Vimalchand and Donohue. A perturbation expansion based on Henderson's restricted primitive model is used for charge-charge interactions among ions. Solvation effects caused by charge-molecule interactions very near ions are taken into account through an effective dielectric constant. Additional new expressions, a third-order perturbation expansion for charge-dipole interactions and a first-order perturbation expansion for charge-induced dipole interactions, were derived for interactions of ions with molecules in the bulk of solution. This equation of state contains four parameters: s, number of segments per particle, q, normalized surface area per particle, epsilon, a characteristic energy per unit external surface area of a particle, and c, one-third number of external degree of freedom. For neutral molecules, these parameters have been determined using PACT by fitting simultaneously experimental vapor -pressure and liquid-density data. For ions, the parameters are calculated using literature values for polarizability and ionic radius which are adjusted by an ionic size parameter, C_{s}. In this work, preliminary calculations involving mean ionic activity coefficients for fifty strong electrolytes in water, specific volumes of several strong aqueous solutions, K factors for argon and methane in aqueous solutions of NaCl and aqueous solutions of KOH and vapor-liquid equilibra for aqueous solutions containing volatile weak electrolytes are carried out. The results show the usefulness of this new equation of state. In these caulations, the ionic size parameter, C_{s}, is the only adjustable parameter used for strong electrolytes over a range of molarity form infinite dilution to 6 molal. The calculations involving weak electrolytes here are carried out without using any adjustable mixture parameter. Average absolute errors are less than 5% for activity coefficients of most strong electrolytes in water, 1.5% for specific volumes for the two electrolyte systems tested, 5% for partial pressures of weak electrolytes in water, and 5% for K factors for the ternary systems containing strong electrolytes.