Monte Carlo computer simulation study of the hydrophobic effect. Potential of mean force for [(CH4)2]aq at 25 and 50 °C

Abstract

A series of liquid-state computer simulations has been performed on the system [(CH₄)₂]_aq at 25 °C as a function of the separation R between the dissolved CH₄ molecules. The potential functions for H₂O–H₂O, CH₄–H₂O and CH₄–CH₄ are based on quantum-mechanical calculations of the corresponding intermolecular interactions. The simulation was carried out by the Monte Carlo method augmented with convergence acceleration techniques involving force-bias and the method of preferential sampling. The potential of mean force w_ss(R) for the interaction of the apolar species in water was determined using umbrella sampling procedures on four windows on the intersolute coordinate. Convergence acceleration was found to be necessary for obtaining satisfactory results. Contact and solvent-separated contributions to the statistical state of the system are discussed in terms of the radial distribution functions, quasi-component distribution functions for coordination number, binding energy and pair interaction energies. Stereographic views of significant structures are presented, and contributions of clathrate structures in the solvent are discussed. The results are discussed in the context of the experimentally observed concentration dependence of hydrophobic effects and related theoretical work on w_ss(R) by Marcelja, Mitchell, Ninham and Scully, Pratt and Chandler, and Pangali, Rao and Berne. Prospects for quantitative study of the temperature dependence of the hydrophobic interaction and other quantitative extensions of the work are discussed.