Long-Range Effects of Confinement on Water Structure

Molecular dynamics simulations were performed for water confined between two extended hydrophobic surfaces, which were represented by either planar surfaces or, more realistically, linear alkane monolayers, separated up to hundreds of angstroms. These simulations show that the confinement significantly affects the structure and density distribution of water, as well as the hydration of the confining Surfaces. The results are largely independent of model Surfaces used. At large separation distances (up to 800 angstrom) between the confining hydrophobic Surfaces, it was found that the water distribution is inhomogeneous: the water density is high near the surfaces and low in the middle of the water slab. Using a simple free energy density functional approach, these features seen from Simulations were reproduced when a nonlocal free energy density term was introduced. The confinement was also found to have a noticeable effect on the hydrogen bond network of water. For water confined between two hydrophobic plates separated at distances in the range of 100-800 angstrom, the tetrahedrality parameter q was found to be more liquid-like near the Surface (distribution shifts toward random arrangements) and more ice-like in the middle (distribution shifts toward more perfectly tetrahedral bonding). These rather long-range effects are expected to vanish at even longer distances, which shall be Simulated in the future with the state of the art computational resources. On the other hand, confinement was found to have little effects oil water orientational relaxation dynamics.