Dynamic moisture sorption and desorption of standard and silicified microcrystalline cellulose.

Moisture sorption and desorption isotherms of standard and silicified microcrystalline cellulose (MCC and SMCC) were determined using an automatic multi-sample gravimetric analyzer, and compared by fitting different kinetic models, including the excess surface work model (ESW), the BET and GAB model, Young and Nelson model and recently developed parallel exponential kinetics (PEK) model. It was found that silicification affects the moisture sorption and desorption properties of SMCC mainly at high relative humidity (above 50% and 70%, respectively). In general, the differences in the moisture sorption and desorption properties of MCC and SMCC can be elucidated by the different kinetic models. Particularly the PEK model shows that hysteresis is related primarily to a fast sorption process, which corresponds to bound water, and secondarily to a slow process, which corresponds to sorption of free water and that SMCC acquires more water than MCC at RH higher than 50% by the slow (secondary) sorption process. A possible mechanism for this process is presumably the hydrolysis of SiO2 particles and formation of silanol groups that act as a water reservoir, preventing the accumulation of more water in the polymer matrix and thus may be protecting the structure of SMCC from undergoing irreversible structural changes that would impair its performance as an excipient.