Partial molar heat capacities of selected electrolytes and benzene in methanol and dimethylsulfoxide at 25, 40, and 80°C

A high temperature-high pressure flow heat capacity calorimeter, designed to operate to 350°C and 20 Mpa, has been constructed and tested with aqueous sodium chloride solutions to 80°C. The calorimeter has been used to measure the specific heats for solutions of NaBr, NaClO₄, ₄PBR, NaB₄, and benzene in methanol (MeOH) and dimethylsulfoxide (DMSO) at 40 and 80°C. A commercial calorimeter was used to measure the same systems at 25°C. Apparent molar heat capacities C>_p, have been evaluated and extrapolated to infinite dilution to obtain standard partial molar heat capacities. For electrolytes are positive and insensitive to temperature to 80°C in DMSO, but in MeOH, C_{p, 2}⁰ for simple electrolytes are negative and become increasingly negative with temperature. The behavior in MeOH is attributed to strong electrostriction by ionic charge and solvation of anions by MeOH molecules which increases with temperature. This is similar to observed behavior of electrolytes in water above 100°C. For benzene is positive in MeOH and DMSO, and increases with temperature.