HEAT-CAPACITIES AND ISENTROPIC COMPRESSIBILITIES OF ELECTROLYTES IN PROPYLENE CARBONATE AT 25-DEGREES-C

The heat capacities per unit volume and sound velocities of many solutions of 1:l electrolytes in propylene carbonate (PC) were measured at 25°C. Combining these data with previously published densities, apparent molar heat capacities and isentropic compressibilities were calculated. By extrapolation to infinite dilution, the corresponding standard partial molar quantities were obtained. The heat capacities and compressibilities of water in PC were also calculated. Through ionic additivity, it was possible to calculate these functions for all alkali and tetraalkylammonium halides. The magnitude and trends with ionic size of the heat capacities of electrolytes in PC are similar to those in other aprotic solvents such as acetonitrile (ACN) and dimethylformamide (DMF) and significantly different from those in water. On the other hand, with compressibilities, the magnitudes are the same in PC and in water for alkali halides but significantly different for tetraalkylammonium halides. The trends with ionic size are similar to those in ACN. These effects can be interpreted through ion-solvent interactions. The separation of these thermodynamic functions into their ionic contributions is also discussed.