Quantitative Assessment of Thermodynamic Theory in Elucidating the Behavior of Water under Hydrophobic Confinement

A macroscopic thermodynamics-based theory that can quantitatively describe the behavior of water confined between hydrophobic solutes has so far remained elusive. In this work, we progress toward this goal by comparing the predictions of macroscopic theory with the results from computer simulations. We have determined free energy profiles of water confined between two nanometer-sized surfaces of varying hydrophobicity using molecular simulations and have estimated thermodynamic properties such as contact angle, line tension, and size of the critical vapor tube from independent simulations. We show that the scaling of free energy barrier to evaporation is fairly well captured by the factor (D/2 + λ/ϒLV)2, where D is the confinement gap and λ/ϒLV is the ratio of line-tension and liquid–vapor surface tension. The radius of the critical vapor tube necessary for nucleating evaporation scales by the factor (D/2 + λ/ϒLV). Exclusion of the line-tension term from thermodynamic theory leads to a qualitative disagreement between theoretical predictions and results from molecular simulations. We also demonstrate that macroscopic theory that includes the line-tension term is able to quantitatively match the entire free energy profile associated with the formation of a vapor-tube inside the confined region for conditions when the vapor state is the most stable state. The match is however only qualitatively correct for the conditions when the liquid state is more stable. Overall, the conclusion is that the inclusion of line-tension in macroscopic theory is necessary to describe the behavior of water under nanoscale confinement between two hydrophobic solutes