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A theoretical study of ammonia-salt mixtures in bulk solutions and interfacial regions Perkyns, John Stephen


Five models, ranging from a full molecular polar/polarizable model with C₃v symmetry, to the primitive model of ions in a dielectric continuum, have been used to study the properties of ammonia both as a pure liquid and as a solvent. Ammonia is modelled as a multipolar polaxizable hard sphere and ions as charged hard spheres. Using these models, in conduction with the Reference Hypernetted-chain integral equation theory, ammonia has been studied as a pure liquid, as a solvent near charged and uncharged surfaces, and in electrolyte solutions of finite concentration. The formalism of Kirkwood and Buff was used to obtain thermodynamic quantities of ionic solutions from calculated distribution functions. Structural, thermodynamic and. dielectric properties were calculated for pure ammonia and were compared with experiments where possible. Values for the dielectric constant and the configurational energy were found to compare favorably with experiment. Ammonia formed a relatively dense, highly structured layer within two solvent diameters of an uncharged surface. This structure was analyzed in terms of angular probability distributions of the molecular dipole vector and the NH-bond direction, and was found to be similar to that of frozen ammonia. The extreme asymmetry of solvation of unlike charges in ammonia was also investigated. Small cations were found to be more favorably solvated than small anions, but as the ion size was increased, the situation reversed. Estimates of the number of ion pairs in liquid ammonia and their effects on the behavior of mean molar activity coefficients were examined. Large differences between experimental activity coefficients and the Debye-Hückel hmiting law could not be explained by the usual ideas of ion pairing. It was found that the integral equation theories appear to have no solution between ionic concentrations of about 2 x 10⁻⁴M and 2 x 10⁻²M for either molecular or continuum models. Using rigorous stability criteria, this behavior was shown to be consistent with the onset of a phase change. It is proposed that such phase separation phenomena might explain the unusual behavior of the experimental activity coefficients.

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