Quantitative Modeling of Behavior of Water in Hydrophobic Confinement: Role of Line Tension

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 have calculated 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 inclusion of line tension is important for a quantitative match between thermodynamic theory and experimental results. The free energy barrier to evaporation scales as a quadratic function of the confinement gap, and the radius of the critical vapor tube scales linearly with the confinement gap. 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. 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.