DIRECT DETECTION OF AN ATTRACTIVE INTERACTION BETWEEN ANIONS AND HYDROPHOBICALLY HYDRATED UNIPOLAR GROUPS

There exist some indirect hints that in multicomponent aqueous solutions anions are attracted to hydrophobically hydrated alkyl groups and it seems that this phenomenon is of general nature. In order to detect for the first time directly such an attractive interaction we have determined intermolecular dipole-dipole relaxation rates (1/T1)inter of protons residing on the hydrophobically hydrated species dimethylformamide (DMF) and dimethylacetamide (DMA), respectively, in aqueous solutions containing as probe ions either tetraphenylborate (Ph4B-) or tetraphenylarsenate (Ph4As+) ions. We have also measured the translational diffusion coefficients D of DMF, DMA and the ions in the solutions DMF/D2O + phenyl salt (NaPh4B or Ph4AsCl) and DMA/D2O + phenyl salt in function of the salt concentration. This allowed the determination of the so-called A(ij)-parameter with respect to the amide-ion interaction. This A(ij)-parameter is connected with an integral over the pair correlation function and the results clearly show that there is a stronger attractive interaction between the hydrophobically hydrated CH3-groups and anions than with cations. In a simple model we explain this peculiar attraction by preferentially oriented water molecules at the hydrocarbon-water interface.