A diffusing wave spectroscopy study of pharmaceutical emulsions for physical stability assessment.

Emulsions are broadly used in pharmaceutics either as intermediate products or as final dosage forms. Such disperse systems are only kinetically stabilized and therefore early detection of physical instability is highly desirable. This work employed diffusing wave spectroscopy (DWS) to study a series of model emulsions that were categorized, based on their composition, as either "simple" or "complex". DWS data were compared with results of droplet size imaging, apparent viscosity obtained by microfluidics, and near-infrared (NIR) analytical centrifugation. A mathematical model of the droplet mean square displacement (MSD) was modified by us regarding improved fitting of experimental data. Although the emulsions showed different types of instability like creaming and sedimentation, a good rank correlation was found between the DWS parameters and results from the comparative stability methods. Our findings indicate that DWS provides a highly attractive method for stability analysis of pharmaceutical emulsions because it requires only low sample volumes, is rapid and non-invasive. The proposed data modeling provides the means for a better understanding of emulsion microstructure that in turn will help designing quality into pharmaceutical dispersions.