Historically, geochemical water exchange rates have been measured using Nuclear Magnetic Resonance spectroscopy.  This technique has a practical ceiling of ~10^9 1/s for the rates of reactions it can access, which leaves the water exchange rates of many elements of the periodic table and mineral surfaces unaccessible.  Coupled to this is that measurements of water exchange rates of mineral surfaces are difficult to interpret due to contributions from multiple surface sites.  Quasi-elastic neutron scattering can access diffusional motions within the picosecond to nanosecond timescale, faster than NMR and slower than Raman spectroscopy.  In this work, we have used QENS to measure the diffusional motions of few monolayers of water adsorbed to barite (barium sulfate) nanoparticles.  Fits to the QENS measurement are compared to molecular dynamics simulations of the same system, which allows us to directly link specific water exchange reactions to the net rate measured by QENS.

Fast water exchange on a mineral surface measured by quasi-elastic neutron scattering and classical molecular dynamics

245th National Meeting (2013)

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