Phase distribution studies on an oil-water emulsion based on a eutectic mixture of lidocaine and prilocaine as the dispersed phase.

The distribution conditions in oil-water emulsions prepared by emulsifying a 1:1 eutectic mixture of lidocaine and prilocaine with a nonionic surfactant in water were studied by membrane and gel filtration methods. In this system, the local anesthetics are considered to be freely dissolved, surfactant solubilized, and emulsified in three separate phases. The dispersity of the oil phase was investigated by light microscopy and light-scatter spectroscopy. The majority of drops in the lidocaine-prilocaine emulsions were less than 1 micron in size. The concentration of freely dissolved drug in the aqueous phase of the emulsions was equal to the aqueous solubility of lidocaine-prilocaine in a 1:1 ratio. At constant lidocaine/prilocaine/surfactant ratio, increasing the total drug concentration in the emulsion resulted in an increase of the emulsified fraction of lidocaine-prilocaine, whereas the surfactant-solubilized fraction remained constant.