Phase behavior of water-oil systems containing mixed nonionic surfactant or commercial surfactants, such as nonylphenylpoly (oxyethylene) ether, Tween 20 has been studied. In these systems, a three-phase region consisted of a water, a surfactant and an oil phases moves toward higher temperature with the decrease in a surfactant content. Accordingly, O/W-type emulsion can be inverted by not only the rise in temperature but increasing the surfactant content.
The effect of added ionic surfactant on the phase behavior of a nonionic surfactant/water/oil system has been also studied. The three-phase region moves toward higher temperature and diminishes by adding a small amount of ionic surfactant to a nonionic surfactant/water/oil system. Moreover, the solubilization of water and oil in a surfactant phase is abruptly increased.