SEGREGATION OF SOLIDS BY VIBRATION 233 REFERENCES (1) J. L. Olsen and E.G. Rippie, Segregation kinetics of particulate solid systems. I: Influence of particle size and particle size distribution. J. Pharm. Sci., 53, 147-150 (1964). (2) E.G. Rippie, D. C. Kriesel, and H. Rettig, Electronic monitoring of events within dynamic par- ticulate beds: Conductance and capacitance measurements, J. Pharm. Sci., 67, 1121-1127 (1978). (3) L. van Beek, "Dielectric Constants of Heterogeneous Systems," in Progress in Dielectrics, Vol. 7 (Academic Press, New York, 1965). (4) M. Hayashi, A. Suzuki, and T. Tanaka, Behavior of a particle bed in the field of vibration. V. Evaluating flow rate of particles in vibratory conveying system, Powder Technol., 6, 353-362 (1972).

j. Soco Cosmet. Chem., 42, 235-247 (July/August 1991) Characterization of swollen micelies containing linoleic acid in a microemulsion system SHARON K. SWAFFORD, WOLFGANG R. BERGMANN, KENNETH G. MIGLIORESE, J. LEON LICHTIN, and ADEL SAKR, Helene Curtis, Inc., Chicago, IL 60639-4769 (SoK. S., W.R.B., K.G.Mo), and College of Pharmacy, University of Cincinnati, Cincinnati, OH 45267 (J.L.L., A.S.). Received July 23, 1991. Synopsis This paper describes the use of experimental design techniques to examine and optimize swollen micelles containing linoleic acid in an oil-in-water microemulsion system. An experimental design was utilized to understand the complex relationships between the components and their measured properties and to assess how various levels of the components (surfactant, cosurfactant, water, and oil) affect the particle size, conductivity, clarity, and cloud point. Optimal formulas were generated by maximizing the oil content and minimizing the surfactant concentration while holding the linoleic acid at a cosmetically acceptable level. Results were interpreted using statistical and graphical tools. INTRODUCTION Microemulsions or swollen micellar mixtures have many unique properties and numer- ous potential applications. They are very appealing because of their transparency, spon- taneous formation, and extended shelf-life (1). Additionally, they offer the promise of improved delivery systems for active ingredients (2). During the past decade, these systems have become valuable agents in many cosmetic and pharmaceutical applications including bath oils, skin moisturizers, drug delivery systems, and hair fixatives (2-5). Alternate terminology such as swollen miceliar solution, micellar emulsion, middle phase, and spontaneous transparent emulsions have also been used to describe colloidal solutions of this kind (6). Swollen miceliar solutions appear as transparent or slightly translucent systems. Their clarity results from very small particle sizes of the dispersed phase (7). Although there is no defined dividing line between true emulsion droplet size and microemulsion droplet size, the upper microemulsion droplet size limit is generally 500 nm. Others set the upper limit of microemulsion particle sizes as being less than 1400 ]k or 140 nm. This is due to the fact that at one quarter the wavelength of visible light, particles will no longer scatter light, thus appearing transparent (8). The lower particle size limit for microemulsions remains undefined. 235