J. $oc. Cosmet. them., 25, 171-188 (March 3, 1972) Diffe ' Ult 'fug 1 rences racentrl a Stability of Various Oil-in-Water Emulsions* ROBERT D. VOLD, Ph.D., and KASHMIRI L. MITTAL, Ph.D.t Presented in part May 26, 1970, Society of Cosmetic Chemists, New York City, and in part March 29, 1971, American Chemical Society, Los Angeles, Calif. Synopsis--Studies of Nujo1-water and olive OIL-WATER EMULSIONS stabilized with sodium dodecyl sulfate, cetyl pyridinium chloride, Tween 20, or Triton X-100 show that generally the rate of separation of oil in an ultracentrifuge decreases with time of CEN- TRIFUGATION, app•,oaching zero at sufficiently long times. The marked differences in be- havior of these systems suggest that the chemical nature of the oil and of the SURFACTANT, and the molecular geometrical compatibility, may be more important than such general characteristics as VISCOSITY and INTERFACIAL TENSION. INTRODUCTION A large and discordant literature exists with respect to the sta- bility of emulsions but only in relatively recent times, with the applica- tion of the ultracentrifuge to these systems, has it become possible to make quantitative measurements in reasonable time. Much of this work has recently been summarized by Garrett (1, 2). The present work was undertaken (a) to investigate the ultracentrifugal behavior of emulsions with a greater variety of oils and emulsifying agents than had been * The authors gratefully acknowledge support in the form of a Fellowship (for K.L.M.) from the Foods Division of the Anderson Clayton Co., Dallas, Tex. t Department of Chemistry, University of Southern California, Los Angeles, Calif. Dr. Mittal's present address is Department of Chemistry, University of Pennsylvania, Philadel- phia, Pa. 171

172 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS studied previously, (b) to establish and compare different criteria for quantitative comparison of the stability of the different emulsions, (c) to assess the relative importance of physical and chemical factors in deter- mining the stability of emulsions prepared with different oils and differ- ent stabilizing agents, and (d) to obtain further information leading to identification of the rate-determining step or steps in'the demulsification process and the locus of coalescence in ultracentrifugal demulsification. There is much disagreement in the literature concerning the effect of the nature of the oil and the nature of the surfactant on the stability of the emulsions formed. Garrett (1) reports that in many experiments the nature of the oil has no effect except for oils of high polarity, as was also found by King and Mukherjee (3), although King (4) later recognizes its probable importance. Similarly, although the importance of proper choice of surfactant for attainment of desired emulsion properties is clearly recognized (5), there are also reports of quantitative experiments (6) showing no difference in stability between, for example, sodium oleate and various nonionic surfactants, as determined by size distribu- tion measurements on emulsions subjected to ultracentrifugal stress. Possibly, much of the confusion concerning the stability of emulsions is due to the multiplicity of meanings given to the term (7), particularly to the failure to distinguish clearly between creaming and coalescence, to- gether with insufficient sensitivity of some methods (8)--such as surface area--to detect subtle changes occurring in the emulsions. In addi- tion, different methods of characterization may well be measuring entirely different properties of the emulsion, or the emulsions may be in a quite different physical state in the various experiments. Particularly, in ul- tracentrifuge experiments, the emulsion is present as a flocculated system resembling a foam (9) and not as a system of free drops, so that presum- ably only factors involving the rate of coalescence are determined, with little or no effect due to those governing the rate of fiocculation. Hence, great caution is necessary in extrapolating the conclusions from such experiments to free-standing emulsions. In the present work, Nujol and olive oil were used as examples of polar and nonpolar oils. Sodium dodecyl sulfate, cetyl pyridinium chlo- ride, Triton X-100©, • and Tween 20 ©t served as emulsifiers, representing positively and negatively charged ionic surfactants and two types of non- ionics. It was hoped to determine whether observable differences in sta- * Rohm 8c Haas Co., Philadelphia, Pa. + Atlas Chemical Industries, Wilmington, Del.