.I. Soc. Cosmet. Chem., 24, 797-814 (December 9, 1973) Effect of $urfactant Migration on the Stability of Emulsions T. J. LIN, Ph.D.,* H. KURIHARA, B.S.,* and H. OHTA, B.S.? Presented September I1-15, 1972, Seventh IFSCC Congress, Hamburg, Germany Synopsis-A new technique, which involves analyzing solutions from successively centri- fuged emulsions, was developed and applied to determine the SURFACTANT CONCEN- TRATIONS in the aqueous phase of O/W emulsions. By following the surfactant MIGRA- TION from one phase of the freshly prepared emulsion to the other phase, the effects of the migration on the DROPLET SIZE DISTRIBUTION and EMULSION STABILITY were investigated. It was found that, at a low mixing speed, the surfactant location had a significant in- fluence on the stability as well as the droplet size distribution of the newly formed O/W emulsions. In general, the emulsion prepared [½y initially placing the surfactant in the oil phase produced a more stable emulsion than a corresponding emulsion, with an identical composit/on, prepared by placing the surfactant in the aqueous phase. Microphotographic examinations suggested that the initial formation of a double emulsion was probably responsible for the difference. At a high mixing speed, this effect was not very pronounced. The data indicated that under intensive mixing, the time required from the moment of emulsification to the time when the average emulsion droplet size reached the minimum was approximately equal to, or slightly less than, the time required to reach surfactant equilibrium. Once the surfactant equilibrium was attained, further mixing of the emulsion did not improve the emulsion stability. INTRODUCTION Cosmetic chemists are often puzzled as to why the first batch of the emul- sion manufactured in the factory differs so much in physical characteristics or stability from those of the emulsion originally developed in the laboratory. If there is no weighing error or raw material variation, it is most likely due to the difference in manufacturing conditions. A plant's equipment is rarely identical to its laboratory equipment and the difference can easily influence one of the many physical factors which can affect the formation of the emulsion (1). *Shen Hsiang Tang Chemical Co., P.O. Box 150, Taichung, Taixvan. ? Takasago Perfumery Co., Ltd., Tokyo Central P.O. Box 1033, Tokyo, Japan. 797

798 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS However, among many important factors such as mixing and cooling rates which are known to affect emulsions, one factor which has not been thorough- ly studied is the location of the surfactant in the emulsion (2). In manufacturing emulsions, it is generally regarded best to form th• emul- sifier in situ (3). However, except when fatty acid soaps are used as the emul- sifters, it is usually impractical to prepare emulsions by such a method. More frequently, the surfactant is either dispersed in the oil or aqueous phase prior to emulsification. Since all surfactants have some solubility in both oil and wa- ter, if the surfactant is first dispersed in the oil phase containing all oil-soluble components and then added to the aqueous phase to form an emulsion, some of the surfactant originally in the oil phase will migrate to the aqueous phase until an equilibrium is established. Conversely, migration would take place from the aqueous phase to the oil phase if the surfactant were first placed in the aqueous phase. The main purpose of this work was to determine if the initial surfactant lo- cation and the migration of the surfactant immediately after emulsification had any significant effect on the stability and droplet size distribution of the emulsion prepared. Theoretically, since the surfact.ant plays a major role in stabilizing the emul- sion, any movement of the surfactant during the emulsification process can affect the adsorption of the surfactant at the interface and thus influence the quality of the emulsion formed. If the equipment or procedure used for emul- sification affects surfactant migration, the formation of the emulsion may also be indirectly affected. Similarly, the migration of the surfactant after the for- mation of an emulsion can also alter the emulsion properties and emulsion stability. Conceivably, this may be one of the factors controlling the often troublesome changes in the rheological properties of freshly prepared cosmet- ic emulsions upon aging. However, in order to quantitatively study the effect of surfactant migration on emulsion stability, one must accurately know the distribution of the surfac- rant in both phases of the emulsion at a given time. There is definitely a paucity of such data as the measurement of surfactant distribution in an emul- sion is no simple task. This is because, first of all, the surfactant is not only dis- solved in the bulk phases of the emulsion but is also present as micelles in both phases and a substantial portion of it is adsorbed at the oil/water interface of the emulsion droplets. Secondly, no method is available which allows a direct measurement of the surfactant concentration in either the continuous or dis- persed phase of a stable emulsion without first breaking or creaming the emul- sion. If an emulsion is first cracked by coalescing the dispersed droplets using chemical or physical means, the surfactant present in one of the phases or in both phases can then be analyzed without too much difficulty. However, this would only provide the information on the surfactant distribution in a cracked emulsion. In order to know the distribution in the original stable emulsion,