The change of the rate-controlling processes with temperature in the hydration of anhydrous nedocromil sodium by water vapor

A six-stage model is proposed to describe the overall process of sorption o f water vapor on and into anhydrous nedocromil sodium. The way in which tem perature, pressure, and time affect the rate of reaction for each of the st ages has been analyzed. Experimental data for the measured rates, where tem perature, pressure, and time are variables, are compared with the predictio ns obtained from each of the six stages. The most useful comparator is a gr aphical representation of reduced time versus hydration rate. The theoretic al equations presented as a shape analysis of the experimental curves show the process to have different controlling mechanisms in three temperature r egions: up to 27 degrees C, hydration is controlled by a nucleation and gro wth mechanism; between 27 and 31 degrees C, the process is dominated by dif fusion of water molecules into the crystal; and >31 degrees C, neither nucl eation nor diffusion are controlling but some, as yet, undetermined physica l processes. (C) 2000 Wiley-Liss, Inc. and the American Pharmaceutical Asso ciation J Pharm Sci 89:1187-1195, 2000.