Pressure dependence of the glass transition temperature in ionic liquids and solutions. Evidence against free volume theories

A simple DTA method of determining the glass transition temperature, T_g, of a supercooled liquid using small samples under high pressure has been developed and applied to the study of several anhydrous ionic liquid binary systems, and salt + water systems. Values of the pressure dependence of T_g have been compared with those predicted (a) from the pressure dependence of electrical conductance and (b) from the changes of heat capacity and expansion coefficient observed at T_g by use of the quasi-thermodynamic relation dT_g/dP = T_gV_gΔα/ΔC_p. In the case of alkali acetate + water systems, T_g is almost independent of pressure, and, in the case of the composition LiOAc·10 H₂O, dT_g/dP is actually negative. This result, which implies a decrease in expansion coefficient on increase of temperature through the glass transition, is conceptually incompatible with a free volume interpretation of liquid relaxational properties.