Monte Carlo studies on the hydrophobic hydration in dilute aqueous solutions of nonpolar molecules

Monte Carlo simulations have been carried out for 64 water molecules and two aqueous solutions each containing 63 water molecules and one nonpolar molecule in the canonical NVT ensemble at 298.15 K and 18.07 cm3 mol−1 using the Metropolis method. The ST-2 and Lennard-Jones potentials are used for water–water and water–solute interactions. Two nonpolar molecules roughly simulate methane and isobutane, respectively. The energy stabilization due to the introduction of nonpolar solute is clearly recognized. This energy stabilization is dependent on the size of solute and is pronounced in the case of methane. Internal energy, radial distribution function and related properties have been evaluated separately for vicinal and distant water molecules to the solute. Water molecules in the first hydration shell of methane are influenced by the presence of the solute and lead to a kind of structuration. Thermodynamic and structural properties of vicinal water to methane differ from those of pure water. This is also observed, though less pronounced, for isobutane solution. The Helmholtz free energy difference between pure water and aqueous methane solution has also been evaluated by the umbrella sampling technique.