Dynamics of water in LiCl and CaCl2 aqueous solutions confined in silica matrices: A backscattering neutron spectroscopy study

Backscattering neutron spectroscopy was used to probe the dynamics of water molecules in LiCl and CaCl2 aqueous solutions confined in 2.7, 1.9, and 1.4 nm diameter pores of various silica matrices. The pore size of 2.7 nm was found to be sufficiently large for the confined liquids to exhibit characteristic traits of bulk behavior, such as a freezing-melting transition and a phase separation. On the other hand, none of the fluids in the 1.4 nm pores exhibited a clear freezing-melting transition; instead, their dynamics at low temperatures gradually became too slow for the nanosecond resolution of the experiment. The greatest suppression of water mobility was observed in the CaCl2 solutions, which suggests that cation charge and perhaps the cation hydration environment have a profound influence on the dynamics of the water molecules. Quasielastic neutron scattering measurements of pure H2O and 1 m LiCl-H2O solution confined in 1.9 nm pores revealed a dynamic transition in both liquids at practically the same temperature of 225-226 K, even though the dynamics of the solution at room temperature appeared to slow down by more than an order of magnitude compared to the pure water. The observation of the dynamic transition in the solution suggests that this transition may be a universal feature of water governed by processes acting on the local scale, such as a change in the hydrogen bonding.