Solubility of the cosolutes 4-hydroxybenzoic acid and its alkyl esters (‘alkylparabens’) in aqueous urea: evidence for 1 : 1 cosolute–urea association in solution and evaluation of the methylene group contribution to the free energy of association

The effect of urea (U) on the aqueous solubility of the cosolutes (S) 4-hydroxybenzoic acid and its C1–C4 alkyl esters (‘alkylparabens’) has been studied. In each case the cosolute solubility increases linearly with concentration of U, which has been interpreted as due to association of the type SxU, where x is assumed to be either 1 or 2. The results are discussed in terms of the interaction forces (dipole–induced dipole forces, hydrogen bonds, hydrophobic interactions) expected to occur in the association, and on the basis of some amphiphilic character for the urea resulting from the expected ‘peripheral’ pattern of hydration around its molecule. The ‘water structure’ interpretation of interactions in these systems is also discussed.