NMR studies on hydrophobic interactions in solution Part 4. Temperature and concentration dependence of the hydrophobic self-association of tert-butanol in water

Abstract

The hydrophobic self-association of tert-butanol was investigated in binary aqueous mixtures at 10, 25 and 40°C by the application of NMR techniques. The association tendency of the organic compound was monitored using the so-called (self)-association parameter A₂₂ obtained by means of the measurement of intermolecular ¹H–¹H dipole–dipole relaxation rates of the methyl protons and of self-diffusion coefficients of tert-butanol molecules. The composition dependence of A₂₂ shows that the self-association of tert-butanol increases with dilution and reaches a maximum at a concentration of about 2–3 mol% at 25°C. At lower concentrations a decrease is observed. The concentration corresponding to the maximum of A₂₂ can be viewed as a transition point separating the tert-butanol concentration range (within the water-rich domain) in two regions, the low concentration region being dominated by single hydrophobically hydrated tert-butanol molecules whereas the high concentration region is characterised by microstructures where tert-butanol molecules are hydrophobically associated. With increasing temperature the association tendency is enhanced and the A₂₂ maximum is shifted to lower tert-butanol concentrations, diminishing the extent of the hydrophobic hydration region. The occurrence of two structurally different regions is discussed in the light of literature results from other methods.