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 pharmaceutical excipients, namely, glucose, citric acid, and sorbitol, were investigated at 25°C using an electrodynamic balance (EDB). The mass fraction 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 humidity (RH) at equilibrium. Bulk measurements and literature data suggest that these three additives all crystallize at their respective saturation conditions. 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 particles still contain ∼8 wt% residual water, which corresponds to a stoichiometry of citric acid monohydrate. Glucose and sorbitol particles are essentially water-free. Universal Functional Activity Coefficient (UNIFAC) model predicts the water activities of glucose, citric acid, and sorbitol (expressed as mfs of the particles) to within ±16%, 24%, and 6% of those determined by the EDB, respectively.