NMR-STUDIES ON HYDROPHOBIC INTERACTIONS IN SOLUTION .2. TEMPERATURE AND UREA EFFECT ON THE SELF-ASSOCIATION OF ETHANOL IN WATER

The self-association of ethanol has been investigated in binary aqueou s mixtures and in mixtures containing urea as a third component by the application of NMR. We obtain the so-called association parameter A(2 2) by means of the measurement of intermolecular H-1-H-1 dipole-dipole relaxation rates and of self-diffusion coefficients of the ethanol mo lecules. The composition dependence of A(22) shows a surprisingly high self-association of ethanol. This result is in agreement with that fo und recently from other authors using different experimental methods. Addition of urea further increases the self-association of ethanol. Fo r the first time we measure the A(22) parameter as a function of tempe rature, ranging from 10 to 40 degrees C. We observe that the associati on of ethanol increases with increasing temperature in agreement with theoretical predictions. The structure breaking effect of urea obvious ly acts on the hydrophobic association in the same way as an increase of temperature.