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A theoretical study of dilute aqueous electrolyte solutions Kusalik, Peter Gerard


In the past, studies of electrolyte solutions have generally treated the solvent only as a dielectric continuum. This was the approach taken in the theory of Debye and Huckel and is still widely used today. A significant improvement to this approach would be to include the solvent as a true molecular species. This theoretical investigation of aqueous electrolyte solutions considers a polarizable hard-sphere fluid with embedded point dipoles and tetrahedral quadrupoles with water-like parameters as its solvent. The model solutions are systems of hard spherical ions immersed in this water-like solvent at 25°C. The linearized hypernetted-chain theory is applied to these solutions in the infinite dilution limit. The properties of solution are studied as functions of ion size and charge. Both dynamical and equilibrium contributions to the apparent dielectric constant of solution are examined and compared with experimental measurements at low concentrations. In the present theory, the ion-solvent correlation functions for this model electrolyte solution are found to scale exactly with charge. The ion-ion potentials of mean force demonstrate strong dependence on ion size and for small ions scale to a fair approximation with ion charge. For ions in the water-like solvent the potentials of mean force are observed to be less structured and approach the continuum limit more rapidly than for ions in a simple dipolar solvent. The equilibrium contribution to the dielectric decrement for alkali metal and halide ions is found to be negative but not strongly dependent upon ion size. The values for the kinetic dielectric decrement are also negative and are in fair agreement with previous theoretical results. The total dielectric decrement is dominated by the equilibrium term and is relatively insensitive to ion size for aqueous alkali halides. The limiting slopes for 1:1 and 2:1 electrolytes at 25°C obtained from experimental data at low concentrations are found to be in fair agreement with those predicted by the present theory.

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