Computer simulation of the effect of salt on the hydrophobic effect

Molecular dynamics simulations of aqueous solutions containing methane and sodium chloride reveal the existence of a structured hydration shell around the non-polar solutes, although this hydration shell is less well defined than in the case where salt is absent. It was also observed that hydrogen bonds made from the hydration shell across to the bulk are weaker than those in the bulk, with or without salt. Although the geometry of the hydrogen bonds is preserved, there is a larger fraction of broken hydrogen bonds in the hydration shell of the non-polar solutes than in the bulk of the solution, the difference increasing with increasing temperature. The number of broken H bonds is significantly larger in the presence of salt, and should contribute to an increase in the free energy of dissolution and hence to a lowering of the solubility and an increase in the hydrophobic interaction.