Pair hydrophobicity and potential of mean force between two hydrophobic solutes

5 Pair hydrophobicity and potential of mean force between two hydrophobic solutes 

While insertion of a non-polar solute into water is entropically unfavorable (at room temperature) and can now be understood nearly quantitatively, the effective interaction between a pair of non-polar solute molecules is more difficult to understand. This effective interaction is termed the potential of mean force, often just called 

Understanding PMF is of great importance in biology and chemistry, in such phenomena as aggregation, cluster formation, protein folding, and protein-DNA interaction, to name a few. When large hydrophobic solutes are present in water, then they introduce a distortion in the HB network around them. Therefore, two hydrophobic solutes can interact with each other even when they are far apart, at the scale of solvent molecular diameter. Such large-scale distortion of the water structure is not present for small solute molecules, such as methane, which can be easily accommodated within the water structure. 

In the same figure we also show the distance dependence of the effective interaction energy between the two spheres. The results have been obtained by eomputer simulations where the spheres are methane molecules. Note the pronounced minimum between the two spheres at contact and then the maximum at intermediate distances. The interaction energy falls off to zero as the two spheres move away. In some cases, one finds a second minimum at a distance beyond the maximum [10]. Such a minimum at a larger separation is referred to as a solvent separated pair and arises due to the structuring around the hydrophobic spheres. 

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