Return of the iceberg model?

A paper by Huib Bakker and Yves Rezus in Phys. Rev. Lett. (vol. 99, 148301; 5 Oct.) seems bound to stir up some debate. The work seems nice: an ultrafast IR spectroscopic study of water motions in the hydration spheres of some small organic molecules, which apparently indicates that the (four or so) waters hydrating the methyl groups are rotationally retarded by a factor of at least 4-5 relative to the bulk, while the other waters in the hydration sphere are barely affected. Bakker has used this technique extensively, and one would imagine the results are reliable. Indeed, they seem very much in line with what has been reported previously, for example from NMR studies.

They interpret the slowing as being due to steric hindrance of the breaking of hydrogen bonds via a five-coordinate species – somewhat akin, if I remember rightly, to the kind of slowing down of SN2 substitution reactions in organic chemistry when they are similarly sterically blocked.

What is curious is that the discussion is framed in the context of the Frank & Evans hypothesis from 1945 of an ‘ice-like’ hydration sphere for hydrophobic groups (H. S. Frank & M. W. Evans, J. Chem. Phys. 13, 507; 1945). The paper itself seems to imply that the findings validate this picture – and as a consequence, support the 1959 idea of Walter Kauzmann of an entropic basis for the hydrophobic interaction.

The problem is that that idea seems inconsistent with just about all previous experimental evidence (see, for example, Blokzijl and Engberts, Angew. Chem. Int. Ed. 32, 1545; 1993). And I can’t for the life of me see why a factor of several-fold slowing of rotation should be equated with ‘immobilization’ of the water. Yet this is how the work seems to be getting sold by the APS. (See http://focus.aps.org/story/v20/st11; note in particular, “Biophysicist Kim Sharp of the University of Pennsylvania considers this the first direct observation of the iceberg model, thus completing a long history of trying to confirm this theory” – and the statement in the paper itself that “Our results provide a molecular picture of these icebergs”. Gulp.) Given how entrenched the Kauzmann model has become, without good reason, it seems unfortunate that it as apparently going to receive further support from this work, without any real justification that I can see.