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 bi nary aqueous mixtures and in mixtures containing urea as a third compo nent. 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 coefficien ts of the component of interest. The composition dependence of A(22) r eflects a weak tendency for the self-association of propan-1-ol in the binary mixture. Addition of urea leads to a strongly increased self-a ssociation of propan-1-ol. The same qualitative result has been obtain ed from recent studies of the Kirkwood-Buff integral G(22) by one of t he present authors. A(22) is governed by short-range contributions but G(22) by long-range contributions and the common study of the two qua ntities has delivered interesting complementary information about liqu id mixtures. Propanol-propanol correlations in binary systems were nev er found to extend longer than one to two propanol diameters, however when urea is added the correlation length can extend up to four diamet ers. 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-a ssociation found from A(22) and G(22) can be interpreted as 'preferent ial self-solvation' by relatively weak intermolecular interactions.