The structural environments, dynamical behavior, and energetics of water molecules and ions in 2-dimensional nano-confinement or on mineral surfaces can be effectively studied using a combination of computational molecular dynamics (MD) modeling and experimental NMR spectroscopy. New MD and 2H variable temperature NMR results show that the cation exchanged into the 2-dimensional interlayer galleries of the expandable smectite mineral hectorite greatly influences the structural environments and dynamical behavior of the H2O molecules. The MD results show that H2O molecules occur on sites in nearest neighbor coordination to the cations and on sites coordinated to only the basal surface. The rate of exchange between these two types of sites decreases dramatically with increasing hydration energy of the cation. These effects are directly observed in the 2H2O NMR spectra. At 233K and 253K signals for these two types of sites are readily resolved with Mg2+ present but not with other alkali or alkaline earth cations with lower hydration energies present. In addition, the temperature at which well-resolved site hopping dynamics at frequencies of the order of 25 – 50 kHz is observed increases with increasing hydration energy. The variable temperature 2H2O data for Pb-exchanged hectorite are very similar to those for Mg-hectorite, despite the lower hydration energy of Pb2+. This observation suggests that cation polarizability also plays an important role in controlling H2O dynamics. For paste samples with inter-particle pore fluid, the 2H2O spectra near room temperature resolve separate resonances for H2O in the interlayers and external to the clay particles.

Water dynamics in 2-dimensional nano-confinement: 2H NMR and molecular dynamics modeling of smectite interlayers

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.