Layer charge effects on anisotropy of interlayer water and structural OH dynamics in clay minerals probed by high-resolution neutron spectroscopy

Abstract The mobility of confined water at clay mineral surfaces underpins many of their functional properties as industrial minerals. Oriented thin films of the sodium (Na+) and calcium (Ca2+) forms of beidellite (Bd) and charge-reduced montmorillonite (Mt), equilibrated at 55% relative humidity using D2O, were studied using high-resolution neutron spectroscopy to determine the overall mobility of the OH groups and interlayer water, as well as the effect of layer charge location and total layer charge on anisotropy of collective motions at clay mineral surfaces. An elastic fixed window approach was applied to follow temperature (T) and scattering vector (Q) dependent changes in the elastic scattering signal at two different film orientations with respect to the instrument configuration. The mean square displacement associated with interlayer D2O was greater when the film (and layer) plane was aligned perpendicular with the instrument's average scattering Q vector. A minor displacement was observed near T = 100 K and assigned to localised isotropic rotational contributions of structural OH. While the anisotropic effect at higher T was minor, Q-dependence was present in all samples and dominated by incoherent scattering from interlayer D2O. The Ca2+-form of reduced charge Mt. had larger correlation lengths in the direction perpendicular to the lattice plane than in the parallel direction. While no obvious direct effect of anisotropy due to total layer charge was observed, higher layer charge resulted in greater overall mobility of both structural OH and interlayer D2O. Despite the challenges of studying smectite thin films by neutron spectroscopy, this study confirmed that hydrated interlayer cations mobilised water above 190 K.