A novel fiber-optic photometer for in situ stability assessment of concentrated oil-in-water emulsions.

The purpose of this research was to evaluate a novel fiber-optic photometer for its ability to monitor physical instabilities occurring in concentrated emulsions during storage. For this, the fiber-optic photometer was used to measure transmission of oil-in-water emulsions stabilized with hypromellose (HPMC) as a function of oil volume fraction and droplet size distribution (DSD). To detect physical instabilities like creaming and coalescence, the transmissivity of the samples was studied at 2 different height levels over a certain period of time. The corresponding droplet size distributions were determined by laser diffraction with PIDS. Transmissivity was found to depend on the number of dispersed droplets and thus is sensitive to both the variation of phase volume fraction as well as the emulsions droplet size distribution. At constant DSD, light transmission decreased linearly with increasing oil content within a large interval of phase volume fractions from 0.01 to 0.3. At constant phase volume fraction, an increase in droplet size increased light transmission. Investigation of creaming on emulsions with different droplet size distributions showed changes in the initial delay times and creaming velocities. In contrast to creaming phenomenon coalescence can be identified by height independent changes of the transmissivity. In conclusion, transmissivity of oil-in-water emulsions observed by the novel fiber-optic photometer is sensitive to phase volume fraction, droplet size distribution, and thus can be used as a tool for stability studies on concentrated emulsions.