(367b) Higher-Order Hydrophobic Effects: Capturing the Distinct Heat Capacity and Compressibility Signatures of Non-Polar Gas Hydration with Molecular Simulations

The poor solubility of non-polar gases in water at room temperature is marked by a favorable hydration enthalpy that is opposed by a more unfavorable hydration entropy. The enthalpy and entropy of hydration, however, are only first derivative properties of the chemical potential. While it well appreciated that hydrophobic hydration is also accompanied with significant temperature dependencies resulting from a large positive heat capacity of hydration, a second derivative quantity, the impact of pressure on non-polar species hydration is less well understood. We present here a large scale molecular simulation study of the dissolution of simple gases in water and decane over several hundred state points, over temperatures from 0 C to 100 C and pressures from -500 bar to 1500 bar. The results from these simulations are fitted to free energy expressions that accurately describe gas solvation free energies in these solvents with a mean square accuracy of ~0.01 RT over the full simulation range. These free energy expressions subsequently permit us to explore connections of the solvation heat capacity to the distinctive equation-of-state properties of water as well as examine anomalous pressure effects in water, like the negative partial compressibilities of gases in water at low temperature. These heat capacity and compressibility effects provide insights into the origins of lower critical solution phenomena in aqueous solutions and maxima in the speed of sound in mixtures of water with polar organic solvents.