Nuclear magnetic resonance study of self-association of small hydrophobic solutes in water: salt effects and the lyotropic series

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Abstract
The self-association behaviour of the relatively small hydrophobic organic components in mixtures of dimethyl sulfoxide (DMSO)–water, of tert-butyl alcohol (ButOH)–water and of tetramethylurea(TMU)–water has been studied by the application of an NMR technique. The so-called association parameter A22, which essentially is obtained from experimental intermolecular dipole–dipole relaxation data of 1H nuclei and from NMR measurements of the self-diffusion coefficients of the organic components, has been determined. The composition dependence of A22 in all the systems under investigation shows that self-association of the organic component occurs. The association parameters Aij and the Kirkwood–Buff integrals Gij are related quantities, which might yield complementary information. Thus a qualitative comparison between A22 and G22 is given for aqueous mixtures of DMSO and of ButOH. The main aim of the present work is the study of the influence of salts on the association behaviour. The A22 parameters obtained for salt-containing mixtures show that the salts (Lil, Csl and CsAc in DMSO–water; LiBr and CsBr in ButOH–water; LiBr, LiClO4 and LiAc in TMU–water) change the attractive interaction between the organic species. For ButOH and TMU the influence of salt essentially follows the lyotropic or Hofmeister series; however, for DMSO the reverse sequence of effectiveness of the ions is found. Our results indicate hydrophobic association for ButOH and TMU, while for DMSO obviously a short-range interaction of another kind is acting. Finally, a new attempt is made to explain the well known reverse order in the Hofmeister series for cations and for anions with respect to their structure-breaking ability.

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