The structure and behaviour of self-emulsifying drug delivery systems (SEDDS) containing Labrafil M2125 CS and Tween 80 have been examined and the effects of changing the formulation via the addition of a non-polar model drug (L-365,260) investigated. Low frequency dielectric spectroscopy (LFDS) was used to examine the individual components in order to investigate the effects of drug inclusion. The presence of the drug resulted in a decrease in the dielectric response of the Labrafil M2125 CS, Tween 80 and the oil-surfactant vehicles. The surface tension of the emulsions decreased on addition of the drug, while particle size analysis showed that the emulsions containing no drug and 2% w/v drug had a bimodal distribution and the emulsions containing 6% w/v drug were unimodal. It was found that the bimodal distribution changed over a period of 14 h, with a decrease in modal value of the larger distribution peak and, for samples containing no drug, an increase in the proportion of droplets in the lower size distribution. The results therefore indicate that the drug interacts with one or more components of the self-emulsifying system, leading to a change in droplet size distribution which varies as a function of drug concentration.
