ATOMIC-SCALE ANALYSIS OF THE SOLVATION THERMODYNAMICS OF HYDROPHOBIC HYDRATION

Molecular dynamics simulations are used to model the transfer thermody namics of krypton from the gas phase into water, Extra long, nanosecon d simulations are required to reduce the statistical uncertainty of th e calculated ''solvation'' enthalpy to an acceptable level. Thermodyna mic integration is used to calculate the ''solvation'' free energy, wh ich together with the enthalpy is used to calculate the ''solvation'' entropy, A comparison series of simulations are conducted using a sing le Lennard-Jones sphere model of water to identify the contribution of hydrogen bonding to the thermodynamic quantities. In contrast to the classical ''iceberg'' model of hydrophobic hydration, the favorable en thalpy change for the transfer process at room temperature is found to be due primarily to the strong van der Waals interaction between the solute and solvent, Although some stabilization of hydrogen bonding do es occur in the solvation shell, this is overshadowed by a destabiliza tion due to packing constraints. Similarly, whereas some of the unfavo rable change in entropy is attributed to the reduced rotational motion of the solvation shell waters, the major component is due to a decrea se in the number of positional arrangements associated with the transl ational motions.