Abstract
Silica microfibers with highly oriented cylindrical mesopores embedded in alumina membrane were used as model system to characterize the water dynamics anisotropy at a picosecond timescale. To reach this goal, we first studied the optimal preparation conditions to obtain longitudinal silica mesopores orientation at macroscopic scale. Accordingly, we characterized the impact of ethanol content and aspiration pressure on the orientation and filling of the mesopores using scanning electron microscopy, transmission electron microscopy, small angle X-ray scattering, and dynamic vapor sorption. Based on the obtained results we selected the most appropriate sample as a model system. The model system was filled with water and used to characterize the dynamics of water molecules in parallel and perpendicular directions using quasi-elastic neutron scattering measurements collected in two orientations at two timescales (7 ps and 30 ps). The results revealed the anisotropy of water dynamics in the silica mesopores and highlighted a larger perpendicular diffusion coefficient compared to the parallel one at both timescales. This suggests that it may be more difficult for water molecules to move from one hydroxyl surface site to another compared to forming a hydrogen bond with a water molecule not adsorbed at the surface.