The thermodynamics of solvophobic effects: A molecular-dynamics study of n-butane in carbon tetrachloride and water

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Abstract
We have carried out molecular‐dynamics simulations with holonomic dihedral angle constraints on two models of n‐butane in CCl4 and in water to study the effects of apolar and polar solvents on the gauche‐trans equilibrium. We calculated distributions of conformers from the torsional free‐energy surfaces for each model of butane in both solvents. For a four‐atom model of butane, the gauche‐trans equilibrium constant in either solvent is unchanged relative to its gas‐phase value. For an all‐atom model of butane, the equilibrium population of gauche conformers is increased relative to its gas‐phase value by 14% and 31% in CCl4 and water, respectively. The all‐atom results are consistent with the idea of solvophobic stabilization of the gauche conformation. We also computed finite‐difference temperature derivatives of the free energy to determine its energetic and entropic components. The gauche conformer of the four‐atom model is stabilized by entropy and destabilized by energy in CCl4. We find the opposite thermodynamic driving forces for the all‐atom model in CCl4. The gauche conformer is favored entropically and opposed energetically for both models in water. This result supports the idea that the butane hydrophobic effect is a manifestation of the hydrophobic interaction. Average interaction energies show that changes in solute–solvent interactions contribute significantly to the trans‐gauche internal energy differences in CCl4, while the internal energy differences in water are dominated by changes in the solvent–solvent interactions. Solvent–solute radial distribution functions show that CCl4 packing around the butane molecular is similar to that around other CCl4 molecules, and it is not sensitive to the butane conformation. The water distributions around the butane molecule are very flat, but they show that the water is more disordered around the gauche conformer than the trans.