Transport in molten salts under pressure. I. Glass-forming nitrate melts

The electrical conductances of calcium nitrate-potassium nitrate solutions have been measured as a function of temperature to 200°C and pressure to 3000 atm, by imposing oil pressure directly on samples held within an externally heated bomb. Log (conductance) varies linearly with pressure at high temperatures, but curvilinearly at low temperatures, the "activation volume" appearing to increase with increasing pressure. "Activation volumes" increase rapidly with decreasing temperature. The observations may be accounted for if the "ideal glass transition temperature" T ₀ is taken to be the only pressure-dependent variable in a simple equation previously found to describe temperature and composition-dependent variations in conductance in this, and other, systems. Within uncertainty limits, T₀ varies linearly with pressure and mole fraction, thus pressure equivalents of given composition changes can be simply stated. Using a thermodynamic relation applicable at the glass transition, the observed dependence of T₀ on pressure is used to predict the change of heat capacity at the glass transition. The value is confirmed within error by direct measurements. Application of the Adam-Gibbs theory of relaxation processes leads to the derivation of approximate "constant (configurational) entropy" parameters, E (S_c), which exhibit qualitatively the properties of "constant volume" parameters, E(r>, observed in studies of nonionic liquids.