Dilution Enthalpies of Alkanols in Concentrated Aqueous Solutions of Urea at 25° C

Enthalpies of dilution of some aliphatic alcohols were determined at 25°C in aqueous 7M urea solutions by flow microcalorimetry. The excess enthalpies were expressed as power expansion series in molalities referred to 1 kg of constant composition urea-water mixture. This urea-water mixture was utilized throughout as a mixed solvent. The values of the second enthalpic virial coefficients were all found to be positive and generally lower than the corresponding values in water. Large differences were encountered, as in water, by comparing normal and branched isomeric propanols and butanols. For one system it was possible to measure the third coefficients, which were also positive. The second enthalpic coefficients were found to increase with the molecular weight of the alkanols. These facts suggest that in the presence of a large concentration of urea, the excess enthalpies are mainly determined by apolar interactions. This is surprising and potentially rich in consequences for a better understanding of the interactions among amino acid residues distantly situated in the primary sequences but topologically near in the loops of globular proteins. An analysis, carried out using the Savage-Wood additivity group method, shows that the enthalpic contributions (that appear to play a crucial role in water in making the polar interaction to be favorable) become essentially unfavorable in urea-water solvent. The hypothesis that the peptide-peptide interactions are prevented by the preferential solvation of urea is also discussed