THERMODYNAMICS OF AQUEOUS MIXTURES OF ELECTROLYTES AND NONELECTROLYTES .8. TRANSFER OF SODIUM CHLORIDE FROM WATER TO AQUEOUS UREA AT 25 DEGREES

Enthalpies of transfer of sodium chloride over the mixed-solvent range from pure water to urea mole fraction (X3) = 0.18 (42 wt %) were determined calorimetrically at 25°. These were combined with free energies of transfer at constant molality (per 1000 g of mixed solvent) calculated from isopiestic activity coefficients to yield entropies of transfer. The positive free energy is a linear function of X3. Since the coulombic part of the free energy is qualitatively predicted to decrease with X3, the structural contributions to the free energy are positive. Both the enthalpy and entropy of transfer decrease with X3 and are considerably greater in magnitude than the free energy. The negative values are in qualitative agreement with those predicted by the Born equation and also indicate that urea may act as a net water structure breaker across the entire measured composition range. The relationships between this transfer and those of amino acids are briefly discussed. © Copyright, 1969, by the American Chemical Society.