TRANSPARENT EMULSIONS 281 the surface film is too highly condensed to produce the high curvature necessary for the formation of completely transparent systems. The added 6.12 penetrates the interfacial film and increases its fluidity, per- mitting a higher curvature and the formation of smaller droplets. It may be noted that the emulsion breaks if too much 6.12 is added. Table II shows how the appearance of these emulsions changes with temperature based on the first appearance or disappearance of haze. Re- sults appear to be reproducible to within iøC. Emulsion 2A became crystal clear at 80øC and remained clear to 90øC, the highest temperature of the experiment. Clarification of this emulsion on heating was to be expected. Tween 60 dehydrates and becomes less water soluble with an increase in temperature. The reduced solubility tends to the formation of a condensed film. At the same time, the elevated temperature reduces the effect of van der Waals interaction between the hydrocarbon chains of the Tween 60 molecules. The result is a liquid condensed film and microemulsion formation. TABLE II.--EFFECT Or FILM MODIrlEP. S ON TP. ANSPAP. ENC¾ RANGE -Parts By Weight , 271 2B 2C 2D 2E Tween 60 20 20 20 20 20 Isopropyl palmirate 20 20 20 20 20 Water 25 25 25 25 25 Span 80 6.5 13 13 13 2-Ethylhexanediol-l,3 2 4 Transl•arent Range, øC 80-90 74-90 62-90 33-90 20-77 The addition of Span 80 to emulsion 2A reduces the temperature re- quired for the clarification of the emulsion. Thus, with 10% of Span 80 added to 2A the emulsion became transparent at 74øC. With 20% of Span 80 added, clarification occurred at 62øC. Since the presence of Span 80 results in a condensed interface, it is only necessary to heat the emulsion sufficiently to reduce molecular interaction to give a liquid con- densed interphase. Since it is not known which Tween-Span ratio gives the more condensed film, the ratio that will produce clearing at the lower temperature cannot be predicted. Small additions of 6.12 to emulsion 2C, containing both Tween and Span, result in transparency at decreasing temperatures, until transparency occurs at room temperature and below. However, these clear emulsions cloud when heated. The 6.12 expands the condensed film to give condi- tions suitable for the formation of a microemulsion. Upon heating, the interfacial film expands to the point where it is no longer condensed, and the emulsion becomes unstable. The addition of too much 6.12 has exactly the same effect.

282 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE Ill.--EFFECT OF FILM MODIFIERS 01•' TRANSPARENCY Ra•oz . Parts By Weight 3d 3B 3C Tween 60 20 20 20 Mineral oil 20 20 20 Water 25 25 25 Span 80 6.5 6.5 2-Ethylhexanediol-l,3 4 Appearance opaque gel opaque gel transparent fluid Transparency Range, øC none 74-90 20-90 One requirement specified by Schulman for the formation of a micro- emulsion is that the chain length of the nonpolar oil should not be greater than the hydrocarbon chain length of the emulsifiers. Isopropyl palmi, rate presumably meets this requirement since the fatty acid group has only 16 carbon atoms Carnation ©* mineral oil probably does not. Table III is similar to Table II, with Carnation mineral oil substituted for isopropyl palmirate. Emulsion 3A, without Span 80 or 6.12, does not clarify even when heated to 90øC, suggesting that it is more difficult to form a microemulsion with the longer chain oil. However, the effects of addition of Span 80 and 6.12 are quite similar to those obtained with isopropyl palmkate. It should also be noted that the addition of 6.12 causes a substantial reduction in the viscosity of the emulsion. This is consistent with general observations to the effect that a highly condensed in terphase gives a stiff emulsion. A number of experiments were conducted by first forming a transparent emulsion, adding water or mineral oil until the emulsion remained turbid upon heating and cooling, and then adding increments of emulsifier. After each addition of emulsifier, the emulsion was heated, to effect more rapid enqulsification, and then cooled. If the emulsion did not remain trans- parent, additional emulsifier was added. The results of these experiments are summarized in Table IV. The ingredients in each clear composition are expressed as a ratio to the amount of Tween 60 present. Referring to the O/W emulsions at the top of the table, it can be seen that the ratio of Span 80 to Tween 60 varies from 0.32 to 0.88. The only significant correlation between this ratio and that of the other components present is the 6.12 to Tween 60 ratio: The larger the ratio of 6.12 to Tween 60, the higher the ratio of Span 80 required to effect clarification. This find- ing is consistent, since the Span 80 addition condenses the interphase, and the 6.12 addition expands it. In the case of W/O emulsions, the Span 80 is the predominant emulsifier at the interface, and the Tween 60 is the condensing agent. Increasing the ratio of Tween 60 to Span 80 requires a corresponding increase in 6.12 con ten t. * Registered trade name of Sonneborn Chemical and Refining Co., Div. of Witco Chemical Co., New York, N.Y.