APPLICATION OF A NEW METHOD BASED ON CONDUCTIVITY MEASUREMENTS TO DETERMINE THE CREAMING STABILITY OF O/W EMULSIONS

A kinetic model, based on equations used in biological population dynamics, is proposed to describe the creaming profile of oil-in-water emulsions at the top of a storage cell. The creaming profile itself is determined by conductivity measurements through the emulsion at different heights, and conversion of conductivities into volume fraction values of the dispersed phase in terms of dispersion dielectric theory. The reliability of the proposed model was successfully verified. This methodology (conductivity method and kinetic model) was applied to model emulsions, to analyze the most reliable test scheme for stability assessment (room temperature storage, temperature stress test, and low centrifugational stress test), The conductivity method is sensitive to small changes in the volume fraction of dispersed phase observed at room temperature. The creaming profile can then be analyzed with the kinetic model allowing extrapolations for long-term predictions and shortening the time needed for stability assessments. Additionally, an Arrhenius-type equation relating the creaming rate constants with the corresponding storage temperatures can principally be employed. With regard to centrifugation stress testing, if under centrifugation no creaming can be observed, it is normally correct to assume that the emulsion will be stable at normal gravitational condition. However, the reverse argument does not always apply.