Salting-in and salting-out of hydrophobic solutes in aqueous salt solutions

We present results on the thermodynamic and structural aspects of the hydra tion of hydrophobic solutes in three tetramethylammonium [N(CH3)(4)(+)] Sal t solutions at various concentrations obtained from molecular dynamics simu lations. Monovalent counterions of different sizes-F-, Cl-, and a relativel y large model ion BI--are chosen in order to cover a range of kosmotropic t o chaotropic behaviors. Chemical potentials of hard-sphere solutes obtained using test particle insertions display both salting-in and salting-out eff ects depending on the type of salt. Water and salt-ion densities in the vic inity of hard-sphere solutes are calculated. Small and strongly hydrated F- ions (kosmotropes) are excluded from the vicinity of hydrophobic solutes, leading to an increase in local water densities near hydrophobic solutes (i .e., preferential hydration). This increases the excess chemical potential of hydrophobic solutes in solution which leads to salting-out. Opposite beh avior is observed for large, less favorably hydrated BI- ions (chaotropes) which associate strongly with hydrophobic solutes. Compressive forces due t o neighboring water molecules, cations, and anions on the surface of the ha rd sphere solute are calculated. We find that water molecules make the most significant contribution toward the total compressive force. This explains the observed linear correlation between the extent of preferential hydrati on or dehydration of the solute surface and salting-out or salting-in effec ts. The trends in the thermodynamics of hydration of hydrophobic solutes up on addition of salts are explained in terms of the structural hydration of individual salt ions.