NMR study of the influence of urea on the self-association of propan-1-ol in water and comparison with Kirkwood-Buff integral results

The self-association of hydrophobic propan-1-ol has been studied in binary aqueous mixtures and in mixtures containing urea as a third component. We applied an NMR technique to obtain the so-called association parameter A(22), which is based on the measurement of intermolecular H-1-H-1 dipole-dipole relaxation rates and on self-diffusion coefficients of the component of interest. The composition dependence of A(22) reflects a weak tendency for the self-association of propan-1-ol in the binary mixture. Addition of urea leads to a strongly increased self-association of propan-1-ol. The same qualitative result has been obtained from recent studies of the Kirkwood-Buff integral G(22) by one of the present authors. A(22) is governed by short-range contributions but G(22) by long-range contributions and the common study of the two quantities has delivered interesting complementary information about liquid mixtures. Propanol-propanol correlations in binary systems were never found to extend longer than one to two propanol diameters, however when urea is added the correlation length can extend up to four diameters. Measurements of B-D parameters, showing the influence of urea on the translational motion of propan-1-ol, revealed only a weak effect. Thus no rigid self-associated species could be detected and the self-association found from A(22) and G(22) can be interpreted as 'preferential self-solvation' by relatively weak intermolecular interactions.