Adsorption of water by anhydrous nedocromil sodium from 20 to 40 degrees C

Three different powder preparations of the drug disodium 9-ethyl-4,6-dioxo- 10-propyl-4H,6H pyrano[3,2-g]quinoline-2,8-dicarboxylic trihydrate, Nedocro mil sodium (trade name Tilade), have been fully dehydrated in a vacuum and their water vapor adsorption characteristics quantitatively assessed at dif ferent water vapor pressures over a temperature range 20 to 40 degrees C. A t saturated vapor pressures; 100% RH, rates of adsorption are around 0.1 s( -1/2). Graphs of square root of time against reduced mass during uptake of water vapor at vapor pressures in the range 20 to 47 mm of Hg, all equivale nt to 100% RH, indicate control by a diffusion mechanism with activation en ergies in the range 8 to 24 kJ mol(-1), dependent on the powder preparation method. In two of the powders nonlinear Arrhenius-type plots are interpret ed as showing that control of the process is dependent on the surface's abi lity to hold water molecules at the experimental temperature. The variation in activation energies and the calculated values for diffusivities, around 1 x 10(-13) m(2) s(-1), are used to explore structural involvement in the overall water adsorption process. The measured values of water vapor diffus ivity into the structure have been used to predict the water solubility of nedocromil sodium trihydrate, and the results show good agreement to report ed solubilities. This approach to solubility prediction is an alternative t o the Noyes and Whitney method where ions leaving the surface are monitored .