Insights on the hydrophobic effect in a simple model of water

The present work aims to understand the molecular basis of the hydrophobic effect, particularly how hydrophobic surfaces perturb water and how this results in the anomalous thermodynamics observed---an unfavorable free energy with a large, positive heat capacity. A simplified statistical mechanical model of water, the Mercedes Benz model, is used in NPT Monte Carlo simulations to examine the aqueous solvation of a nonpolar solutes, as a function of solute radius. We find a very different mechanism for the aqueous solvation of large nonpolar solutes (much larger than a water) than for smaller solutes. Small solute transfer involves a large hydrophobic heat capacity; its disaffinity for cold water (room temperature or below) is due to the ordering of the neighboring waters (entropic), while its disaffinity for hot water is due to the breaking of hydrogen bonds among the neighboring waters (enthalpic). In contrast, transferring large nonpolar solutes into water involves no such large changes in heat capacity or entropy. Perturbations to hydrogen bonding strongly correlates with the anomalous thermodynamics of the hydrophobic effect, including enthalpies, entropies, and heat capacities of solvation. In agreement with previous work, we find that the formation of small cavities in water resembles that of hard disk fluids, and can be modeled using the excluded volume work of forming a cavity. However large cavity formation in water is different. It is less costly to open large cavities in MB water than excluded volume models would predict. Hydrophobic interactions between hydrophobic solutes are driven by this same physics. For molecular-sized solutes, variations in solvent density around one solute affect the transfer of a second solute into solution by affecting the excluded volume work of transfer. For larger cavities, the net burial of hydrophobic surface between solutes ultimately drives the association of solutes. Finally, a simple model of ions in water is developed in the context of the MB model and used to examine how salt affects the hydrophobic effect---the Hofmeister series. Hofmeister effects are mitigated through changes in water density around the ions, thereby perturbing hydrophobic solute transfer into those regions of solution.