A finely monodispersed oil-in-water (O/W) emulsion was obtained under high temperature and pressure conditions (240 o C and 10 MPa). A coarse emulsion stabilized by Tween 20 was prepared in advance from the decane / water system to be treated under the subcritical conditions. The treated coarse emulsion was thereafter mixed with the aqueous solution including surfactants to improve the monodispersity of the oil droplets. The subcritical condition could supply sufficient energy to produce finely dispersed oil droplets (~233 nm in diameter).

The formation of finely monodispersed oil-in-water (O/W) emulsions under high temperature and pressure conditions (240 o C and 10 MPa) depended on the solubility of the oil in water. Fatty acids with large solubilities in water favored the formation of oil droplets in emulsions of less than 100 nm in diameter. The water dissolved in the oil droplets participated throughout the cooling process, followed by the break-down of the oil droplets. In contrast, organic solvents with lower solubilities in water formed oil droplets in emulsions that had at most a diameter of 300 nm.