Strategies for Formulation and Evaluation of Emulsions and Suspensions: Some thermodynamic Considerations

Abstract

The difficulties in the development of reasonably stable emulsions and suspensions and the problems associated with the evaluation of such products are, for the most part, related to their non-equilibrium state. A conceptual understanding of the thermodynamics of these dispersions, particularly their surface and/or interfacial free energy, will facilitate successful formulation of such disperse systems. A strategy that fails to utilize surface behavior to stabilize thermodynamically unstable formulations is unlikely to be productive whereas a strategy based on an appreciation of the surface properties of the system is much more likely to succeed. Stability testing of liquid disperse systems is one of the most difficult problems faced by pharmaceutical and cosmetic chemists. The scientist is often asked to predict the shelf-life of a product or choose between experimental formulations based on estimates of how well they will hold up with time. While there are no standardized tests available to determine stability, a conceptual understanding of the thermodynamics of these systems, particularly their interfacial properties, can provide some insight into the expected types of instability, and equally important into the most sensible tests for predicting shelf-life.