Fluidity and conductance in aqueous electrolyte solutions. An approach from the glassy state and high-concentration limit. I. Ca(NO₃)₂ solutions

This paper is concerned with a treatment of aqueous electrolyte solution transport properties which differs from conventional approaches by the absence of any initial assignment of separate solute and solvent roles to the constituent particles of the solution. In this approach the embarrassments met by conventional theory on passing to compositions at which there is no longer any recognizable bulk solvent may be avoided. To provide the experimental basis for the treatment, a study of electrical conductance and fluidity of Ca(NO₃)₂-water solutions in the composition range 0-26 mol % (0-20 m) and temperature range +80 to -60° has been performed. Results are analyzed utilizing concepts and expressions for relaxation processes in glass-forming liquids, of which aqueous solutions provide many excellent examples. The data provide the basis for quasiempirical expressions in which the composition dependence of the transport properties can be expressed in simple form incorporating the parameters which describe the transport temperature dependence. A simple general form which describes the isothermal composition dependence of viscosity in these solutions as it varies over four orders of magnitude between infinite dilution and extreme concentration is η^-1 = A exp [-B/(x₀-x)], where x is the solution composition in mole per cent salt and x₀, B, and A are constants. This form is shown to be almost the equivalent of the Vand equation which has previously been invoked to describe the η composition dependence of highly viscous solutions. In the viscous region, solution equivalent conductances are described by the same form of equation with almost the same value of x₀. From our analysis we conclude that the behavior of this type of electrolyte system may be described in terms of the following three composition regions: (i) a relatively simple high-concentration region in which it is suggested there is no "bulk" water present and all modes of motion are presumed to be strongly coupled (the cohesive energy, indicated by the glass temperature, T_g, and "ideal" glass temperature, T₀, decreases in simple linear fashion with composition as the ionic charges are diluted by intervening oriented water molecules) ; (ii) a complex intermediate region in which the solution structure is probably inhomogeneous on a microscopic scale ("worm-hole" structure) due to cooperative overlap of oppositely charged (ion-solvent cosphere) complexes, and is subject to increasingly large water-rich and hydrated salt-rich composition fluctuations with decreasing temperature; and (iii) a dilute region in which, as normally conceived, the ions plus their solvent cospheres are dispersed in a bulk water continuum.