Enthalpies and heat capacities of dissolution of some sodium carboxylates in water and hydrophobic hydration

Integral heats of solution ΔH_s of sodium carboxylates, C_nH_2n+1COONa (n=0, 1, 2, 3, 4, 5, and 7) and C₆H₅(CH₂)_nCOONa (n=0, 1, 2, and 3), in water at 25 and 35°C have been determined at very low concentrations. The heat capacities of dissolution at infinite dilution, ?C _p ^o , of sodium carboxylates have been derived by the integral heat method. The-CH₂-increment of ?C _p ^o in aliphatic carboxylates has been found to be 14 cal-deg^?1-mole^?1, which is close to the value derived from other series of compounds, indicating that the interaction of nonpolar moieties with water is independent of the hydrophilic group attached to it. On the other hand, the-CH₂-increment for the aromatic sodium carboxylates is much less (about 6 cal-deg^?1-mole^?1) than for the aliphatic sodium carboxylates, indicating that the hydrophobic interaction is affected by the aromatic end group.