Ultraslow Dynamics of Water in Organic Molecular Solids

Abstract

The relaxation dynamics of water in hygroscopic molecular solids is probed by broadband dielectric spectroscopy in the temperature range from 200 to 450 K. Evidence is found for three types of dynamic processes. The intermediate process is common to all probed materials and is associated with the reorientation of bound water molecules that are attached directly onto organic molecules and counterions. A faster process is observed in rhodamine chloride and fullerol, which is the dynamic signature of water in higher hydration layers, either at grain boundaries (rhodamine) or in interstitial clusters (fullerol). All these processes are observed near room temperature and exhibit nonmonotonic temperature dependence and decreasing spectral strength upon heating. In fullerol a third, ultraslow relaxation is observed at high temperature, which may be due to the reorientation of water–fullerol complexes or to the diffusion of water vapor through intermolecular voids.