(715a) The Effect of Material Flexibility on the Drying Transition of Water Between Hydrophobic Objects: Thermodynamics and Kinetics

Liquid water confined between hydrophobic objects of sufficient size becomes metastable with respect to its vapor at separations smaller than a critical drying distance. Macroscopic thermodynamic arguments predicting this distance and nearly all molecular simulations of this phenomenon have been restricted to the limit of perfectly rigid confining materials. However, no material is perfectly rigid, and it is of interest to account for this fact in both theoretical and in silico analyses. First, we present a theory that combines the current macroscopic theory with the thermodynamics of elasticity to derive an expression for the critical drying distance for liquids confined between flexible materials [1]. The thermodynamic arguments show that flexibility increases the critical drying distance (i.e. it promotes evaporation). Second, we present calculations of the rate of evaporation of water as a function of both object separation and flexibility. Rate calculations, obtained via forward flux sampling [2] in conjunction with molecular dynamics simulations, show that the rate increases substantially as the material is made more flexible, suggesting that more flexible materials provide lower kinetic barriers to evaporation. Third, we present calculations of the free energy barriers to evaporation and show how they evolve as one changes the flexibility at fixed object separation. We also discuss how flexibility affects the kinetic mechanism of evaporation as well as condensation of water in hydrophobic confinement.

[1] Y.E. Altabet, P.G. Debenedetti â??The role of material flexibility on the drying transition of water between hydrophobic objects: a thermodynamics analysisâ? Journal of Chemical Physics, 2014, 141: 18C531.

[2] R.J. Allen, C. Valeriani, P.R. ten Wolde â??Forward flux sampling for rare event simulationsâ? Journal of Physics: Condensed Matter, 2009, 21: 463102.