Monte Carlo simulation of <i>n</i>-butane in water. Conformational evidence for the hydrophobic effect

Monte Carlo statistical mechanics simulations have been carried out for a dilute solution of n-butane in water at 25 °C and 1 atm. The intermolecular interactions were described by Coulomb and Lennard-Jones terms in the TIPS format including the TIPS2 parameters for water. The internal rotation about the central CC bond in n-butane was included using a rotational potential based on molecular mechanics (MM2) calculations. The precision of the simulation results was enhanced by preferential sampling and by umbrella sampling for the internal rotation over chopped barriers. Conformational results are also reported from a long Monte Carlo run for pure liquid n-butane using umbrella sampling. Although no condensed phase effect is found on the conformational equilibrium in pure liquid n-butane, there is a pronounced increase in the gauche population of n-butane upon transfer from the gas phase to aqueous solution. The latter finding is in near quantitative accord with the shift predicted by Pratt and Chandler from their theoretical model. It is also consistent with the basic tenet of the hydrophobic effect regarding the folding of natural and synthetic polymers in water. In addition, detailed structural results for the system are reported. Notably, the water molecules in the first shell around n-butane have normal bonding energies and hydrogen bonding profiles for bulk water. However, since their coordination numbers are low, this situation can only be achieved by greater ordering which is entropically costly. The validity of the observations is supported by the computed heat and volume of solution which are in accord with experimental data.