Thermochemical investigation of possible interactions between urea and some sparingly soluble solutes in aqueous solution

The interaction of alkyl-p-hydroxybenzoates and of beta-phenylalanine with urea in dilute aqueous solution which has been proposed by some workers, has been investigated by microcalorimetry. The derived enthalpies are all less than -50 J mol(-1) and hence are too small to be considered as describing specific interactions between urea and the study solutes. Furthermore a linear relationship is shown to exist for a wide variety of compounds, including the study solutes, between solute size (surface area and molecular or van der Waals' volume) and Delta(trs) G, the Gibbs function for solute transfer from water to aqueous urea solution. Both these findings support the view that the effect of urea upon these solutes' aqueous solubility is an entropically controlled bulk solvent process rather than specific complex formation between urea and solute. The results are discussed in the light of recent models of the hydrophobic effect which propose displacement by urea of water solvation molecules in the solute's hydration shell.