PNNL

Abstract

Thin fluid layers confined between anoparticulate solids play an important role in a number of natural and industrial systems, including the saltcakes in the waste tanks at U.S. Department of Energy’s Hanford and Savannah River sites. The properties of these materials are, however, significantly different from the bulk, and strongly constrain the rheology of the material. An array of neutron and computational approaches has been to examine the properties of 1, 2, and 4 monolayers of water (H2O or D2O) on nanoparticulate, hydrous or deuterated boehmite (g-AlOOH, g-AlOOD). Molecular dynamics simulations showed that single water molecules on the (010) surface bond to the structure via four bonds: two from hydrogens on the water to surrounding oxygens, and two from the water’s oxygen to surrounding OD/OH. Exposure of deuterated boehmite to H2O at 90°C yielded rapid H/D isotopic exchange likely driven by a Grotthus-like proton-hopping mechanism or phonon assisted. The in-plane and out-of-plane vibrations of the structural hydroxyls involved in this exchange were observed for both the hydrated and deuterated boehmite. These identifications were confirmed by molecular dynamics simulations. Bulk waterlike properties began to appear once 4 monolayers of water had been added (that is, 7 - 8 between two particles), but steric crowding limited water diffusion rates once two monolayers had been added. Super-Arrhenius temperature-dependent behavior was observed at four monolayers indicating glass-like behavior in a well-formed hydrogen-bonding network. Such a network is not sufficiently developed, however, when the surface water coverage is low.