Molecular-level understanding of hydrophobic interaction

The difficulty with the lattice models of hydrophobicity is that they do not do full justice to the HB network of water. However, a molecular-level theory is also rather difficult because the hydrophobic effect is a complex collective phenomenon involving many water molecules. The hydrophobic solute perturbs the HB network of water, resulting in a change of both entropy and enthalpy of the solute-solvent 

As we discussed earlier, hydrophobicity is considered at two levels. First is the hydration of a single non-polar solute and the second is pair hydrophobicity, where a water-mediated interaction between two non-polar solutes is articulated. The former is often referred to as hydrophobic hydration. 

Understanding hydrophobic hydration requires an estimate of the chemical potential of the non-polar solute in water. Actually, one measures the change in chemical potential as the non-polar solute is transferred from its own liquid to water. This quantity is related to the hydropathy scale discussed earlier in the context of protein folding. 

While there is no general expression for the arbitrary value of the cavity radius X, one can use macroscopic considerations to obtain the energy function W(X) for X much greater than the diameter of the solvent molecules. In this case the solvent molecules near the solute see a hard wall. In the case of water, Stillinger showed that a vapor-like very-low-density state of the solvent will be present near the surface and the density will increase to bulk density as we move away fi-om the wall. The density profile may look like that of a gas-liquid interface. Indeed the energy function W(X) involves the vapor-liquid surface tension term, and is given by [16] 

Here p is the pressure, a is solvent diameter, y is the surface tension in the planar interface limit, and 0(1) provides a correction for the curvature dependence of the surface [16]. 0(1) is also known as Tolman s length. 

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