Microemulsions as potential ocular drug delivery systems: phase diagrams and physical properties depending on ingredients.

The microemulsion systems, both o/w and w/o, composed of isopropyl myristate, soybean lecithin (Epikuron 200), Polysorbate 80, Cremophor EL, n-butanol and triacetine, taken at various amounts and in various combinations, were tested in order to assess their physical-chemical properties. Some ingredients of the microemulsions were physiologically acceptable, except one formulation containing n-butanol used as a model reference system. Phase diagrams were made for all microemulsions studied in order to test the influence of components on the boundaries of the stable microemulsion domains. The greatest area of stable microemulsion system was obtained for the formulation in which n-butanol was used, while the smallest area was obtained when soybean lecithin (Epikuron 200) was used as a surfactant. The following physical parameters were analyzed: osmotic tension, density, viscosity, refractive index. pH and particle size. Microemulsions stored at 25 degrees C for the period up to 12 months, showed physical changes depending on ingredients. The study made it possible to select the most stable microemulsion system meeting the requirements of eye drops.