Using an atomistic model we are studying the structure and fluctuations of water at the interface of different hydrophobic solutes. In real systems, even non-polar solutes attract water molecules through small dispersive interactions. One of our goals is to develop a better understanding of the effect these small attractions have on the properties of the solute-solvent interface. In one approach we have performed a set of simulations containing both a small length-scale spherical solute and a large length-scale planar solute. Both solute species interact with the surrounding water through a two-body interaction potential with a tunable parameter which sets the magnitude of the water-solute attraction.

This study illuminates the fundamental difference between the interface between small length-scale and a large length-scale hydrophobic solvation. A small length-scale solute can be thought of as existing within the hydrogen bonding network of the liquid. Adjacent to the solute, molecular orientations are constrained as they conform their hydrogen bonds to contain the solute. Consequently, for a small length-scale solute, the solute-solvent interface is rather insensitive to dispersive like solute-solvent attractions.

Larger hydrophobic solutes (those whose surface extends, with low curvature, over about 1nm2) significantly disrupt the hydrogen bonding network of the liquid. At the solute interface, where hydrogen bonds must be broken, water molecules rearrange to form a structure which is very much like the interface between the coexisting liquid and vapor phases. Fluctuations in this interface are entropic and relatively susceptible to even small solute-solvent attractions. Consequently , the layer of depleted density seen between water and an ideal (purely repulsive) hydrophobic solute can be removed with the application of very moderate solute-solvent dispersive attractions.

For further reading refer to Reference 1.

1. A.P. Willard, Faraday Discuss., 141, 309, (2009).

(left) A snapshot of the model system where water molecules are colored red/white and the hydrophobic solutes are colored green. The front quarter of the system has been omitted for clarity. (below) The tunable solute-solvent potential.

solute-solvent distance (Angstroms)