Tuesday, November 13, 2007

Sticking hydrophobes with salts and smallness

Do Hofmeister effects after all depend on altering ‘water structure’? Frankly, I doubt it. But a suggestive case is made in a paper by Andrew Thomas and Adrian Elcock (JACS doi:10.1021/ja073097z). Their MD simulations of various salt solutions show that changes in water-water hydrogen bonding appear to be correlated with experimental solubility data for hydrophobic solutes. Strongly salting-out salts, for instance, cause significant decreases in the water-water hydrogen-bonding fraction. Lithium ions, previously considered anomalous in their salting-out behaviour, form linear ionic chains, with correspondingly unusual hydration structures. But is all this behaviour seen in neutron-scattering studies of salt solutions? I don’t recall that it is. In any event, Thomas and Elcock also find that for simulations that include hydrophobes, an increase in hydrophobic association for certain hydrophobes and salts also correlates with solubility data. There’s a way to go yet before we understand all this.

Relevant to this paper is an experimental study by Jared Smith, Rich Saykally and Phillip Geissler (JACS 129, 13847; 2007) on the effects of dissolved halide ions on hydrogen bonding in water. In contrast to the old ideas about structure-making/breaking, they find that the effects on Raman and IR vibrational spectra can be explained by the action of the ions’ electric fields on adjacent water molecules, and that H-bond strengths are altered very little beyond the first hydration shell. In other words, the H-bond network seems rather robust to such perturbations.

Hydrophobic association in pure water is studied by K. G. Ayappa and colleagues at the Indian Institute of Science in Bangalore (Langmuir doi:10.1021/la7022902). They consider the effects of nanoconfinement on the interaction, looking at 2.82-nm diameter water droplets in reverse micelles. They find that the attraction is enhanced by the confinement, which they explain by the lack of sufficient water to solvate and stabilize the solvent-separated solutes. Plausible? I guess so – after all, hydration of lone hydrophobes is thermodynamically favourable. Dave Thirumalai has considered this issue recently (JACS 128, 13490; 2006) – I must remind myself of what he found…

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