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.

ULTRACENTRIFUGAL STABILITY OF EMULSIONS 173 bility could be explained in terms of physical characteristics (e.g., viscos- ity, interfacial tension, drop size distribution, etc.) or whether less quan- titative concepts (e.g., the chemical nature and the geometric fit between the oil and the surfactant molecules) were of greater importance, the lat- ter having been emphasized by Schulman et al. (10). Since not only the quantity and rate of separation of oil from the emulsion but also the qualitative nature of the time dependence of the rate of separation of oil were found to change markedly with both the nature of the oil and the nature of the surfactant, it seems apparent that chemical and geometrical factors must be considered in addition to the nonspecific physical factors. EXPERIMENTAL Materials The sodium dodecyl sulfate (SDS) was a pure sample used in another investigation (11) and estimated to contain not more than 0.05% lauryl alcohol as an impurity. Cetyl pyridinium chloride (CPC) • was used as received and was found to be 98% active by titration (8) against pure SDS. Triton X-100 (Lot 2788) was used directly as furnished and is re- ported to be 100% polyoxyethylene p-t octyl phenyl ether with 9 to 10 ethylene oxide groups. Tween 20 was also used directly as furnished and is reported to be polyoxyethylene (20) sorbitan monolaurate, HLB number 16.7. Preparation of Emulsions Emulsions were generally prepared as in previous work (8) by first stirring 150 ml of oil and 120 ml of 0.2% aqueous solution of the surfac- tant for 5 min at 5000 rpm in a Brookfield counter-rotating mixer, fol- lowed by eight passes through a motorized Cenco hand homogenizer. After standing overnight, 45-ml aliquots were taken and gently blended with 5-ml solutions of surfactant of appropriate concentration to give 50 ml of emulsion containing a 50-50 volume ratio of oil and water and any desired concentration of surfactant. By this method, it was possible to prepare a set of emulsions of the same drop size distribution but of vary- ing initial concentration of surfactant in the aqueous phase. In the case of emulsions stabilized with Tween 20 and Triton X-100, 0.15% solu- tions of the surfactant were used in the initial preparation, and 0.1% solutions in the case of CPC, since 0.2% solutions of the latter gave such stable emulsions that oil did not separate at a sufficiently rapid rate in the * Matheson, Coleman 8c Bell, East Rutherford, N. J.