The Change of the Rate‐Controlling Processes with Temperature in the Hydration of Anhydrous Nedocromil Sodium by Water Vapor

Abstract

A six‐stage model is proposed to describe the overall process of sorption of water vapor on and into anhydrous nedocromil sodium. The way in which temperature, pressure, and time affect the rate of reaction for each of the stages has been analyzed. Experimental data for the measured rates, where temperature, pressure, and time are variables, are compared with the predictions obtained from each of the six stages. The most useful comparator is a graphical representation of reduced time versus hydration rate. The theoretical equations presented as a shape analysis of the experimental curves show the process to have different controlling mechanisms in three temperature regions: up to 27 °C, hydration is controlled by a nucleation and growth mechanism; between 27 and 31 °C, the process is dominated by diffusion of water molecules into the crystal; and >31 °C, neither nucleation nor diffusion are controlling but some, as yet, undetermined physical processes. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89:1187–1195, 2000