900 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This adaptation in the '50's gave birth to the now well-recognized field of "Physical Pharmacy," which means physical chemistry applied to pharmacy. It is believed that the time has now come for the cosmetic industry to start adapting these same physical-chemical principles in its own development work, in such a manner that it meets the needs of the cosmetic industry. This paper, therefore, represents an application of some of these physical-chemical principles in cosmetic product develop- ment. Two main topics will be covered in this presentation namely, the application of electrokinetic phenomena and the critical micelle phe- nomena in cosmetic developments. ELECTROKINETIC PHENOMENA The electrokinetic phenomena refer to the motion of colloidal particles in an electric field. The term "zeta potential" is the measure of this : PARTICLE ':ELECTRIC SURROI/N I) • __ PL.•NE ':OP {mm}•N oF Figure 1. Concept of the zeta potential. Zeta potential is the difference between the charge of the diffused moveable negatively charged layer and that of the bulk of the suspending liquid. (Reprinted by permission of the American Chemical Society)

PHYSICAL CHEMISTRY AND PRODUCT DEVELOPMENT 901 electrokinetic effect on the surface of the particle (Figs. 1 and 2). Elec- trokinetic effect, or zeta potential, is most noticeable in colloidal sus- pensions, emulsions, or in general at all solid-solid interfaces and liquid- liquid interfaces. This presentation is limited to a discussion of aqueous systems in which a large portion of the molecular size particles composing the medium are ionized. To illustrate, consider a spherical insoluble solid particle suspended in an aqueous medium. The particle may be iron oxide, talc, pigment, guanine, bismuth oxychloride, etc. A differ- ence in electrical potential will exist between the surface of the particle and the bulk of the solution at a distance away from the surface. This electrical potential or charge can originate in either of two ways: 1. From the presence of ionic sites on the surface, that is, chemical radi- cals capable of ionizing when brought into contact with a suitable liquid medium or, 2. From adsorption of ions out of the solution onto the particle surface. The fact that a charge exists on the colloidal particle surface will lead to the rise of a concentration of oppositely charged ions near the surface in the solution. The ions charged oppositely to the surface charge are referred to as "counter-ions" or "gegen-ions." The combination of charge on the surface and the diffused array of counter-ions near the surface is called the "electrical double layer" (EDL). The EDL, •, may ..... :4: •.:-- ,: • '.,., ,:' ,..•,. :'.'. " -,._• Figure 2. Concept of zeta potential present• in model form