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 b onds 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 b onds is preserved, there is a larger fraction of broken hydrogen bonds in t he hydration shell of the non-polar solutes than in the bulk of the solutio n, the difference increasing with increasing temperature. The number of bro ken H bonds is significantly larger in the presence of salt, and should con tribute to an increase in the free energy of dissolution and hence to a low ering of the solubility and an increase in the hydrophobic interaction.