192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Officers of the Ohio Valley Chapter of the Society of Cosmetic Chemists for 1970 LeJt to right: Mr. Herbert L. Bradley, Secretary Dr. Fred H. Snyder, Chairman-elect Dr. Robert B. Hutchison, Treasurer Mrs. Rose•narie •Vallisch, Chairman and Mr. Kenneth Hartley, Treasurer of the Society of Cosmetic Chemists

.]. Soc. Cosmetic Chemists, 21, 193-204 (Mar. 4, 1970) Microemulsions LEON M. PRINCE, A.B., M.A. • Presented September 8-9, 1969, Seminar, St. Louis, Mo. Synopsis--Recent advances concerned with the theory of MICROEMULSIONS are revicwed and a theorctical interpretation of the H¾•)ROemum-LiPom•mm BALANCE (HLB) scheme for rating EMULSIFIERS is offered. It is submitted that HLB measures the partitioning of emulsifier be- tween the oil phase and interphase rather than bctween the oil phasc and the water phase of an emulsion. Under thcse circumstances, the interrelationship betwcen the flat film pressure, oil/water interfacial tension and HLB determines the character of the emulsion to be formed. INTRODUCTION Coarse (macro) emulsions of water and certain oils stabilized by soap may be titrated with alcohols to transparent, optically isotropic, fluid, water-in-oil (W/O) or oil-in-water (O/W) systems. Schulman coined the term "microemulsion" to identify such dispersions of spheri- cal droplets as emulsions and to distinguish them from systems of swollen micelles (1). The definition was subsequently expanded to include emulsions whose droplets exhibit faint light scattering (2). Such emul- sions, the droplet diameters of which are less than 1400 •., are thermo- dynamically stable, differentiating them from macroe•nulsions which achieve equilibrium by separating into their original two mutually in- soluble liquid phases. It is the purpose of this paper to point out some of the commercially important microemulsions and to describe the conditions which favor their formation. In the process, macroemulsions will be viewed from a Lever Brothers Co., Research Center, Edgewater, N..l. 07020. 193