STUDY OF HYGROSCOPIC PROPERTIES OF AQUEOUS MIXTURES OF DISODIUM FLUORESCEIN AND SODIUM-CHLORIDE USING AN ELECTRODYNAMIC BALANCE

Purpose, The purposes of this study are: a) to demonstrate the use of an Electrodynamic Balance (EDB) to investigate the hygroscopic propert ies of pharmaceutical aerosols; and b) to evaluate the applicability o f the Zdanovskii-Stokes-Robinson model (ZSR) in the associated data an alysis with multicomponent pharmaceutical aerosols. Methods. The compo sitional dependence of the water activity of two model materials commo nly employed in the study of pharmaceutical aerosols, namely, NaCl and Disodium Fluorescein(DF), was investigated using an EDB. The water co ntents of single levitated droplets of NaCl and DF and their mixtures at mass ratios of 1:3, 1:1, 3:1, and 6:1 from dilute concentration to high supersaturation were determined as a function of relative humidit y (RH). Results. At decreasing ambient RH, supersaturated aqueous NaCl droplets lose water and crystallize to form dry solid particles at an RH of similar to 50%. Aqueous DF droplet continues to lose water unti l it reaches a final state containing about 20% by mass of residual wa ter. Mixed solutions of DF and NaCl crystallize at an RH of similar to 50% and then continue to lose water at lower RHs. The resulting ''dri ed'' particle still contains water whose amount depends on the mass ra tios of DF and NaCl in the mixture. Good prediction of water activity of the DF-NaCl mixture can be achieved with the ZSR model. Collection of a full set of water activity-composition data at each mass ratio of DF-NaCl requires only a few hours. Conclusions. The EDB, together wit h the application of the ZSR model in data treatment, appears to be a valuable tool for studying the hygroscopic properties of pharmaceutica l aerosols.