It is now well-appreciated that the hydrophobic interaction (HI) is truly 'multiscale,' with a distinct scaling with size and temperature for small (molecular) versus large (macroscopic) solutes, crossing over in the nanoscale regime. We show that, rather remarkably, the HI does not depend on detailed hydrogen bonding considerations, but is quite simply reproduced by a �two-length scale� molecular model.  In this picture, waters interact isotropically with two energy minima at different separation distances, favoring a competition between open, tetrahedrally-coordinated structures with dense, close-packed ones.  We show that such a model naturally emerges upon coarse-graining realistic all-atom force-fields.  We then use Monte Carlo simulations to compute the HI (free energy vs. distance) of associating solutes, for molecular to macroscopic solute sizes.  Despite its simplicity, this coarse picture of water quantitatively reproduces the peculiar scaling behaviors and surface tension of real water and is in excellent agreement with experiments.

Surprisingly simple, two length-scale picture of hydrophobic interactions

243rd National Meeting (2012)

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