J. Soc. Cosmetic Chemists, 17, 313-327(1966) Product Stability: Prognostication, Placement, Parameters Part II* LLOYD KENNON, Ph.D. Synopsis--Chemical kinetics provides the basic principles which can help study of the deterioration of pharmaceutical and cosmetic products. Principles of kinetics, which could be used to predict long-term stability of finished formulations, are reviewed, and techniques are described which can be used in programming stability studies. Finxlly, those properties of emulsions, suspensions, and solids are discussed which are amenable to measurement and can be used as paramcters for establishing and predicting detcrioration of finished con- sumer products. PARAMETERS The purpose of this section will be to present a discussion of parameters, that is, physical properties of a formulation which can be measured and to which numerical values can be given. Furthermore, we will wish to examine parameters which change with the stress of time alone or after other challenging stresses (either with or without elapsed time) are applied to the formulations under study. Therefore, we will not be interested in all of the general stresses which are used to generate product development laboratory reports but rather in those which will cause changes in parameters which can be plotted kinetically so that we may try to predict stability by noting how the parameters are changed by the stresses. These are not always easy to find as some stresses, tempera- ture and time included, will destroy the products, and we will be up against a limitation not unlike the Heisenberg Uncertainty Principle. Also, we want stresses and parameters which can be used on products per * Part I of this paper was published previously: .L Soc. Cosmetic Chemists, 17, 135 (1966). ,• Research and Development Laboratories, Bristol-Myers Products, Hillside, N.J. 07207. 313

314 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS se and not just on components, because the latter usually act differently when separated from their system. This section will then be a partial and eclectic guide to the literature it will be designed to suggest that what has been done in certain areas of the all-encompassing fields of pharmacy and chemistry may also be applicable to cosmetic product development work. It appears that not a great deal of work has been done on physical stability testing tech- niques this observation was also noted by Lachman (17) in an article in which he discussed various aspects of physical and chemical stability testing. Some of the work cited here will illustrate the measurement of parameters the usefulness of which may be of a low order. However, it is hoped that these items will be of sufficient general interest so that the reader will consider them as pertinent as the author does because they touch upon this discussion. The references will be divided into cate- gories based on general formulation type. Besides this categorization, no particular order will be followed some references will describe theory, some suggest stresses or techniques, and others will illustrate kinetic plots. Emulsions Becher (18) pointed out in a discussion of spreading, HLB, and emul- sion stability that a correlation exists between these factors. To effect correlation he used the concept of spreading coefficient, i.e., a parameter obtained by measurement of the surface (cohesion) and interfacial (adhe- sion) tensions involved. These are related to the mutual spreading properties of the two phases. When the work of adhesion is greater than the work of cohesion, spreading takes place (positive spreading coeffi- cient) this is generally bad for stability. However, the author points out that too negative a spreading coefficient is not necessarily ideal because a balance between this coefficient and a low interfacial tension is needed. An early discussion of the theoretical considerations concerning the electric double layer and its influence on emulsion stability is that of Cheesman and King (19). They especially noted the influence of elec- trolyte content on stability. Experimentally they followed stability by observing the separation represented by the rate of movement of the separation meniscus in a tube. Higuchi and Misra (20) raised some very interesting points concern- ing emulsion stability. Usually we think of physical emulsion degrada- tion as taking place only through coalescence. These investigators