Moisture sorption kinetics for water-soluble substances. IV: Studies with mixtures of solids.

This paper extends earlier work from this laboratory concerning the sorption kinetics of water vapor on deliquescent water-soluble substances to mixtures of these solids. A theoretical model, based on heat transport control, excellently predicted a priori the rate of water uptake by a variety of binary mixtures of alkali halides and sugars. The rates for mixtures containing highly water-soluble quaternary ammonium salts, as either one or both of the components, were less successfully predicted as the combined water solubilities of the two components increased. It is concluded that water-soluble deliquescent substances, normally encountered in pharmaceutical dosage forms, rapidly form saturated aqueous solutions in the aqueous film formed as water vapor uptake proceeds, and that the water uptake rate can be predicted a priori from known and experimentally determinable parameters using the heat transport model.