Thermodynamics of 1:1 Electrolyte Solutions in Water + N,N-Dimethylformamide Mixed Solvent System at 25°C: Molar Excess Enthalpy of Mixing

The molar excess enthalpies of mixing for the six possible binary combinationsof solutions of NaCl, KCl, NaBr, and KBr as a function of ionic strength fractionhave been measured at 25°C. The experiments were performed at constant ionicstrengths of 1.000 and 2.000 mol-kg−1 with an LKB flow microcalorimeter inthe mixed solvent water + dimethylformamide. The equations of Friedman'sModel were fitted to the results. Our parameters differ appreciably from thecorresponding results in water and Young's cross square rule does not apply.While Pitzer's ion-interaction model was able to represent the enthalpy data forcommon ion mixings, it was unable to model the data of the noncommon ionmixtures. The data suggests that the problem may arise from variations in solvationof the ions and ion clusters including preferential solvation of certain species.