Models Having the Thermodynamic Properties of Aqueous Solutions of Tetraalkylammonium Halides

The hypernetted-chain integral-equation-approximation method is used to calculate the ion-ion pair correlation functions and the thermodynamic properties of models of the kind previously studied,((1)) which are based on an ion-ion pair potential having four terms: the usual Coulomb term, a core repulsion term, a term to represent a well-known dielectric repulsion effect, and a "Gurney" term to represent the effect of the overlap of the structure-modified regions, solvation shells or "cospheres," when the ions come close together. The coefficient A(ij) of the last term. for each pair i,j of ionic species is the only parameter that is adjusted to fit the solution data. It is determined by fitting excess-free-energy (osmotic-coefficient) data. It is scaled to represent the molar free-energy change of water displaced from the cospheres when they overlap. The corresponding entropy change S(ij) and volume change V(ij) are determined by fitting, respectively, excess-enthalpy and excess-volume data. Problems of finding a uniquely "best" set of parameters are discussed together with many examples of variations of the model, most of which require further investigation.. 4 consistent set of these parameters, which represents much of what is known about the thermodynamic excess functions of these solutions at concentrations up to about 0.5 M, is interpreted as far as possible in terms of the data for thermodynamic solvation functions for the same systems.