NMR Studies on hydrophobic interactions in solution Part 3 Salt effects on the self-association of ethanol in water at two different temperatures

Abstract

The salt effect on the self-association of ethanol has been investigated in water-rich aqueous mixtures at 25 and 40 °C by the NMR A-parameter method. In order to investigate the anion dependence we used salts with the common sodium cation, namely Na₂SO₄, CF₃COONa, NaBr, NaI, NaClO₄ and NaSCN at fixed concentration. The association parameter A₂₂ was obtained by means of the measurement of intermolecular ¹H–¹H dipole–dipole relaxation rates and self-diffusion coefficients of the ethanol molecules in D₂O. The influence of the anions on the alcohol self-association at 25 °C follows the sequence: SO₄²⁻ > CF₃COO⁻ > Br⁻ > ClO₄⁻ ≈ I⁻ > SCN⁻, which corresponds exactly to the well known Hofmeister or lyotropic series determining e.g. the stabilisation and destabilisation of proteins in aqueous solutions. The two salts Na₂SO₄ and CF₃COONa promote the alcohol self-association, whereas the other salts reduce the association tendency. Thus, small organic molecules can be used for basic investigations with respect to salt effects on biologically interesting molecules. At 40 °C the relative promotingeffect of the two salts is weakened but the opposite effect of the other salts is strengthened. At this temperature the sequence ofthe anion influence is slightly different in the sense that Br⁻ and ClO₄⁻ exchange position. Finally, we propose consideration of a special anion–apolar group interaction in water in the explanation of the anion sequence in the Hofmeister series.