Hygroscopic study of glucose, citric acid, and sorbitol using an electrodynamic balance: Comparison with UNIFAC predictions

The equilibrium water sorption and desorption characteristics of three phar maceutical excipients, namely, glucose, citric acid, and sorbitol, were inv estigated at 25 degreesC using an electrodynamic balance (EDB). The mass fr action of solute (mfs), defined as the ratio of the dry solute mass to the solution mass, of the additives was measured as a function of relative humi dity (RH) at equilibrium. Bulk measurements and literature data suggest tha t these three additives all crystallize at their respective saturation cond itions. However, crystallization or deliquescence was not observed for the suspended particles of the three additives. They behave as nondeliquescent materials and sorb and desorb water reversibly. At RH = 5%, citric acid par ticles still contain similar to8 wt% residual water, which corresponds to a stoichiometry of citric acid monohydrate. Glucose and sorbitol particles a re essentially water-free. Universal Functional Activity Coefficient (UNIFA C) model predicts the water activities of glucose, citric acid, and sorbito l (expressed as mfs of the particles) to within +/-16%, 24%, and 6% of thos e determined by the EDB, respectively.