Viscosity of Ultrathin Water Films Confined between Aluminol Surfaces of Kaolinite: Ab Initio Simulations

Ab initio molecular dynamics simulations of water confined between kaolinite walls were conducted in an effort to make contact with experiments implying dramatic viscosity enhancement for water in nanometer-scale, hydrophilic channels. An earlier ground-state structural optimization of a single water layer on a flat kaolinite(001) surface had yielded a molecular arrangement, which, by appearing hydrophobic to subsequent layers, suggested the possibility of very low flow resistance. Well above the freezing point, however, and under shear, the surface became hydrophilic, as a percentage of first-layer water molecules flipped to expose dangling hydroxyls. This led to simulated steady-state velocity fields consistent with a "no-slip" boundary condition and viscous flow. The magnitude of the viscosity derived from the simulations, only a few times that of bulk water, does not lend theoretical weight to the notion of dramatic enhancement under nanoconfinement.