Enthalpies of Alkyl Sulfonates in Water, Heavy Water, and Water-Alcohol Mixtures and the Interaction of Water with Methylene Groups

The heats of solution of the sodium n-alkyl sulfonates from methyl to octyl have been measured at 25 degrees C in water, D2O, and three alcohol-rich mixtures with water, all at solute concentrations low enough so that solute-solute interaction is negligible. Methylene-group contributions to the enthalpies of transfer are readily identified. Collation with the results of earlier studies of enthalpies of transfer leads to the conclusion that methylene-group enthalpies of transfer can be identified as follows: 0.87 kcal-mole(-1) for transfers to the gas phase from water and -0.035 kcal-mole(-1) for transfers to D2O from H2O, independent of the solute type provided only that methylene-group contributions can be identified in the data. Compared to other solvents, water is about as lipophilic as dimethylsulfoxide or a mixture of 27 moles of water with 73 moles of methanol. However, the range of the interactions between different groups on a given solute molecule seems to be much greater in water than in any of the other solvents studied, making it more difficult to identify group contributions to solvation in water. Another example of the complexity of the solvation of alkyl groups in water is encountered when one compares the solvation enthalpy of hexane in various solvents with some of the above results.