Dielectric Analysis of Water in Microcrystalline Cellulose

Abstract

Water content and the states of water are important factors with respect to the physical, chemical and microbiological-growth characteristics of pharmaceutical materials. In this study, the interaction of water with microcrystalline cellulose (as a function of water content between 9–37.5% w/w added water) was investigated by high frequency (1–20 GHz) dielectric relaxation spectroscopy. A dielectric dispersion (centred on frequencies between 4 and 8 GHz) was observed and attributed to the dipolar relaxation of water. The results suggest that up to 35% w/w, water is adsorbed onto the surface of microcrystalline cellulose and at the surface of the water-soluble components, and there is no free water; up to 20% w/w, water is strongly associated as the first hydration layer; between 20–35% w/w, additional hydration layers form at the surface; and up to 37.5% w/w, the water in the system is affected in some way by the surface of the solid. It is proposed that this technique could be used for determining the water content, and states of water, in pharmaceutical materials and could therefore be of great value in gaining an understanding of water-excipient interactions. A number of potential applications for this technique are given.