Light-scattering studies of testosterone enanthate containing soybean oil/C18:1E10/water oil-in-water microemulsions

Total-intensity light scattering (TILS) and photon correlation spectroscopy (PCS) techniques have been used to determine the droplet size of concentrated, oil-in-water microemulsions formed from soybean oil, polyoxyethylene-10-oleyl ether (C18:1E10), and water, both in the presence and absence of the lipophilic drug, testosterone enanthate. The TILS data were analyzed using the hard-sphere model of Percus-Yevick to account for interparticulate interactions experienced in the concentrated systems studied and the volume fraction of the hard-sphere droplet obtained from these analyses used to correct the PCS data. Correction of the light-scattering data in this manner yielded a satisfactory agreement between the size of the microemulsion droplets calculated using both techniques. Both the TILS and PCS data showed that, for a constant surfactant concentration, the size of the microemulsion droplets increased with increasing oil content. For example, droplets of radius 52.5 and 65.1 Angstrom (as determined by TILS) were obtained at 10.0% w/w C18:1E10 and 0.5 and 2.0% w/w soybean oil, respectively. In contrast, for a constant oil concentration, microemulsion droplet size decreased with increasing surfactant concentration. For example, droplet sizes of 65.1, 59.3, 56.6, 54.5, and 53.3 Angstrom were seen with 2.0% w/w soybean oil and 10, 14.0, 18, 22, or 26% w/w C18:1E10, respectively. Furthermore, in the presence of 1.0% w/w of the lipophilic drug, testosterone enanthate, the size of the microemulsion droplet increased by about 6-10 Angstrom depending on the concentration of the surfactant; the higher the concentration of the surfactant, the smaller the increase in size. The slight increase in size of the drug-containing microemulsion droplets suggests that some of the drug has penetrated into the core of the droplet. (C) 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association.