NMR studies on hydrophobic interactions in solution Part 2.—Temperature and urea effect on the self-association of ethanol in water

Abstract

The self-association of ethanol has been investigated in binary aqueous mixtures and in mixtures containing urea as a third component by the application of NMR. We obtain the so-called association parameter A ₂₂ by means of the measurement of intermolecular ¹ H– ¹ H dipole–dipole relaxation rates and of self-diffusion coefficients of the ethanol molecules. The composition dependence of A ₂₂ shows a surprisingly high self-association of ethanol. This result is in agreement with that found recently from other authors using different experimental methods. Addition of urea further increases the self-association of ethanol. For the first time we measure the A ₂₂ parameter as a function of temperature, ranging from 10 to 40°C. We observe that the association of ethanol increases with increasing temperature in agreement with theoretical predictions. The structure breaking effect of urea obviously acts on the hydrophobic association in the same way as an increase of temperature.