SPONTANEOUS EMULSIFICATION By J. T. DAVIES, M.A. (CANTAB.), D.Sc. SYSTEMS which emulsify without any stirring whatever are of considerable fundamental interest. In such spontaneous emulsification the entire energy required for the emulsification comes from the redistri- bution of materials within the system. We shall see that spontaneous emulsification may occur by either condensation or dispersion mechanisms, the visible effect being the same--if the oil is placed quietly on the water, the interfacial region gradually becomes cloudy due to spontaneously formed emulsion. If the oil and water (with certain additives) are brought quietly into contact, spontaneous emulsification may occur on one or both sides of the interface. Usually the requisite energy comes from the free energy release as the additive is redistributed to its equilibrium state in the two phases, but occasionally electrical energy rather than chemical energy can be responsible for spontaneous emulsification, as explained below. In prac- tice, particular care is necessary to distinguish emulsification occurring truly spontaneously from emulsification occurring easily: this easy emul- sification, made possible by a very low interfacial tension, is often referred to commercially as spontaneous emulsification. However, in true spon- taneous emulsification no external mechanical work whatever is required. Three mechanisms have been suggested to account for spontaneous emulsification: (i) INTERFACIAL TURBULENCE This mechanism was originally proposed by Quincke in 1888: it had previously been noticed that, when solutions ooe laurie acid in oil are placed very gently on aqueous sodium hydroxide, an emulsion is formed in the water phase (1). Quincke suggested that the spontaneous emulsification is caused by localized interfacial tension lowerings, due to the unequal formation of soap at different points in the interface. The ensuing violent spreading may tear droplets of oil away from the interface, which are then stabilized by the soap produced. (ii) DIFFUSION aND STRANDING Spontaneous emulsification can occur by diffusion alone when, as in the example mentioned above, a solution of ethyl alcohol and toluene is placed 339

340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS gently in contact with water (2-4). The alcohol, as it diffuses from the oil into the water, carries with it some oil (forming a three-component phase in the immediate vicinity of the interface). As the alcohol diffuses further into the water, the associated oil is thrown out of solution it becomes "stranded" in the water in the form. of fine emulsion drops. An emulsion of water in oil may also be formed on the oil side of the interface, since the alcohol in the oil may initially permit some water to dissolve. As the alcohol passes into the aqueous phase, this water becomes "stranded" in the oil. (iii) NECATIVE INTERFACIAL TENSION If the interfacial tension 7 is locally negative, the area of the interface tends to increase spontaneouly. This occurs when an adsorbed film is present under conditions such that the surface pressure •r (where •r = 3'0 - 3') of m. aterial adsorbed in the interface exceeds the tension 3'0 of the clean interface, i.e. •r 3'0 a zero interfacial tension will also be un- stable, since chance vibrations and thermal fluctuations (i.e. entropy effects) will tend to break up the interface. As the area of the interface increases, the interfacial tension will become less negative, the surface pressure of the adsorbed film decreasing till finally •r is a little less than 3'o, i.e. the over-all interfacial tension is low and positive. This appears to be the mechanism operative in forming spontaneous emulsion when oleic acid dissolved in oil is placed on aqueous alkali (5). To choose between these three possible explanations is not easy, and sometimes more than one of the processes may be occurring. However, it is possible to devise tests (6, 7) to apply to any given experimental system to determine which mechanism is operative. EXAMPLES OF MECHANISM (i) No example of this is definitely established, though it is perhaps the most widely accepted mechanism (2). The obvious system to test for this mechanism {s that of methyl or ethyl alcohol in toluene in contact with water (8). This shows strong spontaneous emulsification (Fig. 1), and it also shows marked interfacial turbulence (9). Surprisingly, however, the emulsification in these systems is accounted for by the second mechanism, since when the interfacial turbulence is completely suppressed by adding a little detergent to the water, by dissolving salt in the water, or by spread- ing a protein film at the interface (6), the spontaneous emulsion {s still produced (Fig. 2). The only effect of the elimination of the interfacial turbulence is that the emulsion is no longer thrown violently away from the interface, but instead streams off quietly. The turbulence is thus no,' responsible (as many had previously thought) fo,' the ernt•lqification in this system.