The interaction of various types of microcrystalline cellulose and starch with water

Sorption isotherms of water onto/into potato, wheat and maize starch and Avicels PH-101, RC-581 and CL-611 have been prepared. In previous publications, a vacuum microbalance and a sorption microcalorimeter have been used to study interactions between water vapour and crystalline solids. In an attempt to extend the application of these techniques, the thermodynamic functions for sorption of water have been calculated for solids where water sorbs in the amorphous regions (i.e. maize and potato starch and Avicel PH-101 and CL-611). Microcalorimetric methods were also used to study the kinetics of the sorption process. As was expected the potato starch sorbed more water than the other two starches, and the Avicel grades sorbed water in proportion to their sodium carboxymethylcellulose content. The numerical values for the thermodynamic functions were lower than those obtained previously from similar studies on crystalline pharmaceutical powders. For starches and celluloses, it is proposed that water binds to anhydroglucose units within the amorphous regions of the solid. It has been suggested that over the sorption process water initially binds as one molecule between two anhydroglucose units, but that these bonds are then broken to form a 1 : 1 stoichiometry. This process of forming and breaking bonds could explain the low values for the net change in thermodynamic functions. The kinetic interpretation demonstrated that the sorption process was split into three discrete regions. The results from vapour sorption microcalorimetry, in combination with a vacuum microbalance, are valuable in probing interactions between water and crystalline or semi-amorphous solids.