Ultrafine Emulsions and Application to Cosmetics

A new method for preparation of ultrafine emulsions was developed. The ultrafine emulsions can be obtained easily by rapid cooling of the high temperature microemulsions, i.e., the swollen micellar solutions in the oil-water-nonionic surfactant systems, to room temperature. They are the thermodynamically unstable dispersions of which the mean droplet diameters are bellow 100nm, and look like so-called microemulsions, i.e., transparent or translucent. The droplet diameter was related to the ratio of oil to surfactant by a linear function. The droplet size of the ultrafine emulsions can thus be controlled by changing this ratio.The stability of the ultrafine emulsions depends on the chemical nature of the oil and surfactant. The stability rises dramatically with increase in the carbon number of the oil, and decreases with increase in the polarity of the oil. The droplets of the liquid paraffin (Lp) and squalane (Sq) ultrafine emulsions do not practically change in size for over a year at 25°C. The addition of small quantities of a long-chain alkane to the disperse phase can have a profound effect on ultrafine emulshion stability. The unstable ultrafine emulsions of short-chain alkanes such as dodecane can be stabilized by added Lp or Sq. The results suggest that the main cause of instability in the ultrafine emulsions may be molecular diffusion (Ostwald ripening) rather than droplet coalesence. The stability of the ultrafine emulsions increases with the size of the surfactant molecule.The ultrafine emulsions were prepared by using the commercial materials for the purpose of applying to the products of cosmetics. The transparent or translucent lotions containing oil components were obtained by using small quantities of the surfactants, and were very stable. Since this method can keep the droplet size under precise controll, the turbidity of the products can be controlled easily.