Partial molal heat capacities of sodium perchlorate in N,N-dimethylformamide from 10 to 75°C

Abstract

Enthalpies of solution of NaClO₄ have been measured calorimetrically in the aprotic solvent N,N-dimethylformamide at several temperatures ranging from 5 to 80°C. The data have been extrapolated to infinite dilution to obtain standard enthalpies of solution. The integral enthalpy of solution method was used to evaluate the standard partial molal heat capacity c ^°_p2 of NaClO₄ in N,N-dimethylformamide as a function of temperature. This function is almost temperature invariant in N,N-dimethylformamide, in contrast to its complex behavior in aqueous and methanolic solutions. This suggests that ionic heat capacities are extremely sensitive to the structure of solutions and that this function can be used as a probe for studying the structure of electrolytic solutions. The complex temperature dependence of c ^°_p2 in water and methanol can be explained in terms of the decreased hydrogen bonding and dielectric constant of the solvents at the higher temperatures. The data show that one must be cautious in interpreting single-temperature heat capacities of transfer between solvents.