Tuesday, October 9, 2007

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.


Jan B.F.N.Engberts said...

I fully support your comments, Phil. The new work is useful and provides further significant knowledge about an important issue, and it is consistent with previous NMR results. It does not support iceberg formation for small hydrophobic solutes. The existing literature provides, I think, overwhelming evidence that no "" icebergs"" are present in the hydrophobic hydration spheres.

王小黑 said...
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王小黑 said...

I'm happy to see that we share the same feeling, Mr. Ball. This article (Huang, C.; Wikfeldt, K. T.; Tokushima, T.; Nordlund, D.; Harada, Y.; Bergmann, U.; Niebuhr, M.; Weiss, T. M.; Horikawa, Y.; Leetmaa, M.; Ljungberg, M. P.; Takahashi, O.; Lenz, A.; Ojamae, L.; Lyubartsev, A. P.; Shin, S.; Pettersson, L. G. M.; Nilsson, A. Proceedings of the National Academy of Sciences of the United States of America 2009, 106, 15214.) is the one that gives me a strong hunch about the return of the "Iceberg" model.

Tom said...

Philip, I quite enjoyed stumbling upon this blog (again). The iceberg model is much maligned but really shouldn't be. I have a graphic on it at the beginning of just about every presentation I give about water since it so nicely relates the competition between the energetically favorable formation of hydrogen bonds and the entropic cost of doing so. The traditional notion of hydrophobic collapse being entropy driven is certainly something that is being revamped as we increase our understanding of how water solvates more complex surfaces.

Tom Young