Local structure and dynamic properties of molecules in anisotropic liquids extracted from molecular dynamic simulations. The case of acetonitrile in polyacrylamide hydrogels

Abstract

The diversity of the local mobility of probe molecules in hydrogels, caused by local structural inhomogeneity has been investigated by molecular simulation. Molecular dynamics (MD) is used, to examine both qualitative concepts to get insight into molecular processes in local structures and quantitative concepts to clarify and calculate the local MD in anisotropic liquids. We extend the global correlation concept, to calculate the reorientational correlation on pieces of trajectories of individual probe molecules in equivalent local environments. It shows that, as experimental results indicate, local water structures exist around acetonitrile molecules in a water–acetonitrile system and are conserved in water-rich domains of a polyacrylamide (PAA)-hydrogel–acetonitrile system. Over and above that, local structures exist in water-poor regions of the PAA-hydrogel, where the dynamics of acetonitrile probe molecules slow down. An understanding at the atomic level of the variation in dynamics of acetonitrile in PAA-hydrogel is given. The averaged reorientational time over all time origins and all acetonitrile molecules in the simulation system reveals very good agreement with Raman spectroscopical data.