Wetting-drying transitions for water in hydrophobic confinement.

PHYS 156

Limei Xu and Valeria Molinero, valeria.molinero@utah.edu. Department of Chemistry, University of Utah, 315 South 1400 East, Rm 2020, Salt Lake City, UT 84112
We characterize the process of wetting/drying equilibrium for water confined between hydrophobic plates of nanoscopic dimensions using MD simulations with a monatomic model of water. The speed-up in computing time provided by the coarse-grained model allows us to study rare events over hundreds of nanoseconds.

In agreement with theoretical models and atomistic simulations, we observe the existence of wet and dry states for water confined between hydrophobic interfaces. We use long unconstrained simulations to characterize the thermodynamics and dynamics of the wetting/drying transition as a function of temperature, pressure and distance between the surfaces. We find that the coexistence of (or rather oscillation between) the dry and wet states occur at each temperature and pressure over a narrow surface-separation range, of about one angstrom. Within this range of coexistence we fully characterize the free energy of the system as a function of two reaction coordinates: the density of water between the plates and the magnitude of the interface between the confined liquid and vapor phases. We investigate whether this first order-like transition ends in a critical point. We find that the free energy barrier between the two states decreases with temperature, and levels off at a value comparable to the thermal energy of the system, leading to extremely fast oscillations, without ever erasing the distinction between dry and wet states. We study the nature of the transition states between wet and dry, and discuss the relevance of the two selected reaction coordinates –density and line of surface- to the kinetics of the phase transformation. For a soft graphite-water interaction potential we observe that the “barrierless” state is reached at ambient pressure and temperature for interfaces with diameters ~1 nm at separations of ~0.8 nm. Hydrophobic patches with these characteristic dimensions should be common in membranes and biomaterials.