Transport in ionic liquids under pressure. II. Concentrated calcium nitrate-water and magnesium chloride-water solutions

The interpretations of transport properties of concentrated aqueous solutions by the "physics of glass-forming liquids" approach recently applied to normal pressure measurements is extended by analysis of the pressure dependence of the electrical conductance of Ca(NO₃)₂ and MgCl₂ solutions. The conductance of these solutions has been determined to 3 kbar over the temperature range 20-100°C for compositions 9-20 mole% Ca(NO₃)₂ (5.6-13.9 m) and 80-200°C for MgCl₂ solutions of composition 12.4 and 14.2% MgCl₂. Using high-pressure-data determination of the glass-transition temperature to eliminate one adjustable parameter, it is found that the pressure dependence of the conductance of these solutions can be adequately described by ascribing all the pressure dependences to a single parameter, the "ideal glass" temperature T_o(P) of the Vogel-Tammann-Fulcher (VTF) equation known to describe temperature dependences of transport in viscous liquids. The results are discussed in terms of the Adam-Gibbs entropy theory and the Angell-Rao bond lattice model. The latter relates the pressure dependence of conductance to the volume change involved in configurational excitations of the liquid quasi-lattice, for which a mean volume increment of 1.6 cm³/mole is obtained.