j. Soc. Cosmet. Chem., 30, 167-180 (May/June 1979) Low-surfactant emulsification T.J. LIN, 628 Enchanted IVay, Pacific Palisades, CA 90272. Received January 15, 1979. Presented at loth I.F.S.C.C. Congress, Sydney, Australia, October 26, 1978. Synopsis Although surfactants are indispensable in formulating modern cosmetic emulsions, their presence has caused many problems such as skin irritation, preservative inactivation, reduction of waterproofing property, increase of drying time or stickiness, air bubble entrainment during processing and promotion of skin penetration by undesirable impurities. From the consideration of increasing consumer demand on product safety in recent years as well as from pragmatic view points of product performance and manufacturing, it is often desirable to keep the total surfactant content in a cosmetic formulation as low as possible. One way to compensate for the increase in emulsion droplet size or decrease in stability resulting from a reduction in surfactant concentration is to increase the rate of shear in processing the emulsion. However, the effectiveness of such a method is often limited. A better way to formulate a LOW-SURFACTANT emulsion proposed here is by carefully choosing the surfactant-oil combination which is most effective for the given system and process it under an optimum condition for EMULSIFICATION. By using a newly developed technique utilizing solubilization, one can find the most effective surfactant-oil combination quickly and reliably. The solubilization technique can be also applied to facilitate processing of fine emulsions with minimum mechanical energy. The technique is particularly useful for O/W emulsions and basically involves a careful control of the mode and rate of phase combination such that the amount of solubilization is maximized during the critical period. INTRODUCTION Surfactants are, unquestionably, the most versatile and most widely used class of cosmetic raw materials. Many anionic and amphoteric surfactants are used for cleansing, foaming and detergency. Cationic surfactants are popularly used in hair conditioning and for germicidal purposes. A wide variety of nonionics are available for emulsification, wetting, solubilization, dispersion or antifoaming. It is nearly impossi- ble today to find a cosmetic preparation which does not contain some kind of surfactant or related materials. Whereas the availability of an increasing number of surfactants has enabled cosmetic chemists to formulate unique and elegant personal-care products, it has also caused many problems for which satisfactory solutions are still unavailable. For example, many modern, nonionic surfactants are very efficient emulsifiers and they are now widely used in many cosmetic emulsions in place of the conventional soap emulsifiers. However, a major disadvantage of nonionic surfactants is their tendency to interact 167

168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS with preservatives such as the esters of p-hydroxybenzoic acid and reduce their antimicrobial activity (1,2). Skin irritation is another problem often associated with the use of surfactants. Although modern synthetic surfactants are purer and safer than their predecessors, many adverse skin reactions experienced by consumers from the use of cosmetics are directly related to the presence of the surfactants. Moreover, even though many surfactants, by themselves, are not irritating to the human skin, they can facilitate penetration of other irritants or toxic impurities in the formulation, since they can, indirectly cause adverse reactions. In addition, the presence of excess surfactant in make-up and skin-care preparations can prolong drying time, increase stickiness, reduce waterproofing or encourage entrainment of undesirable air bubbles during the processing which may ultimately affect the product appearance or stability. Therefore, from the practical aspects of manufacturing, product performance as well as stability, it is frequently desirable to reduce the amount of surfactants used in cosmetic formulations. Moreover, in view of the increasing consumer demand and legislative pressure on the safety of cosmetics, it would be prudent for formulation chemists today to limit the use of surfactants to the minimum quantity required to achieve the necessary function. Although it is desirable that the amount of surfactant used in cosmetic preparations should be limited to the minimum, there is no easy way to determine what is the required minimum and how best to go about reducing its content in commercial emulsions. The main purpose of this paper is first to reexamine the use of surfactant in emulsification and then to suggest ways to reduce the total surfactant content without adversely affecting the emulsion quality. Since the great majority of cosmetic emulsions belong to oil-in-water type, the discussions will mainly be concerned with the preparation of O/W emulsions. THEORETICAL CONSIDERATIONS It is believed that the amount of surfactant used in most commercial cosmetic emulsions is far in excess of the amount actually required to stabilize the emulsions. However, reduction of the surfactant in a typical cosmetic emulsion can result in the formation of emulsion with larger average droplets and consequently a reduction in the product shelf life. Although a homogenizer or colloid mill can be used to reduce the droplet size to compensate for the reduction in surfactant content, it requires a considerable consumption of mechanical energy and, furthermore, the effectiveness of such a technique involving the use of brute force is very much limited. In a recent investigation of the effects of surfactant location in emulsions, this author and his co-workers found an interesting correlation between solubilization and emulsification efficiency (3). It was suggested in an earlier publication that the amount of water solubilized in an oil phase containing surfactants could be used to facilitate the finding of the optimum surfactant mixture (4). In another paper dealing with the subject of conservation of energy used in emulsion processing, the author suggested that by combining the technique of low-energy emulsification and solubilization it was