Freezing points and enthalpies of dilution of aqueous formic, acetic, propionic, and butyric acids. Free energies and enthalpies of solute-Solute interactions

Freezing-point depressions and enthalpies of dilution for aqueous solutions of the straight chain, aliphatic carboxylic acids, C₁ through C₄, have been measured. These data, together with the corresponding apparent molal heat capacities, have been used to calculate the pairwise free energy and enthalpy of interaction of undissociated acid molecules at 298.15⁰K. As expected, the effect of dimer and triplet interaction increases with chain length, reflecting the hydrophobic nature of the hydrocarbon portion of the molecules. The group-interaction additivity principle of Savage and Wood applied to these results gives an excellent correlation and yields values of the group interaction parameters, G_i,j and H_i,j for the CH₂ and COOH groups. These parameters confirm previous results for the interaction of polar and hydrophobic groups in water. The primary data have been fitted to the activity expansion equations of Wood, Lilley, and Thompson to yield values of dimer and trimer "sociation" constants and their temperature derivatives. This procedure allows for the simultaneous treatment of the dissociation effects of these acids as weak electrolytes together with their tendency to form associated clusters.