Behavior of water and aqueous LiCl solutions confined in cylindrical silica pores: A wide temperature range molecular dynamics simulation study

Abstract: We report here a molecular dynamics simulation study on water and aqueous LiCl solutions confined in 1.6 nm cylindrical pores of silica to investigate a dynamical cross-over, observed earlier experimentally, wherein LiCl slows down confined water at high temperatures but makes it faster at lower temperatures. The cross-over observed in the experiments is reproduced in the simulations, albeit at lower temperature. Moreover, the cross-over encompasses all aspects of dynamics including translation as well as rotation. Both addition of LiCl and confinement result in a breaking of hydrogen bond network in confined water, eliminating the need for long jumps via exchange of hydrogen bonded partner molecules. This lowers the activation energy for diffusion in the electrolyte solution compared to pure confined water and leads to the dynamical cross-over seen at lower temperatures. Our results thus provide an explanation to the experimentally observed phenomena and provide important insights on the interplay of confinement, temperature and presence of electrolytes on the dynamical behavior of nano-confined water.