THE SEVENTH SPECIAL AWARD 343 EXAMPLES Or MECHANISM (iii) The most straightforward illustration of the effect of a negative inter- facial tension is the spontaneous emulsification of mercury in water. Ilkovi• (12) showed that if a negative potential is applied to a mercury drop in an aqueous solution of a quaternary ammonium salt, the interfacial tension can be greatly decreased. The quaternary ammonium ion is so resistant to decomposition at the surface of the mercury that a highly compressed monolayer of these cations is held there, both by adsorption of the hydrocarbon groups and by electrical attraction. At a potential of about -2 volts, the over-all interfacial tension becomes negative, i.e. •r %. The negative interfacial tension at the large applied negative potentials results in the surface of the mercury drop disintegrating into a brown cloud of colloidal mercury in water at -8 volts the spontaneous emulsification is very striking. The energy required to form the emulsion is supplied electrically. It must be noted that zero or negative interfacial tension will occur when- ever an adsorbed film at the interface is so compressed that •r %: in two-component systems, however, zero implies interfacial tension mis- cibility, corresponding to no interfacial tension. The above experiment brings out this point the highly compressed film of quaternary ammonium ions at the mercury-water interface has no effect on the negligibly small miscibility of mercury and water. Another example of a zero interfacial tension was found by Langmuir (13). He compressed rapidly a film of protein at an oil-water interface, tventually reaching the point where •r = 3•0. This is possible because a protein monolayer is unfolded irreversibly by adsorption and becomes insoluble. Eventually, however, the protein monolayer may crumple and the surface pressure will again decrease but this is a rather slow process. In this example, moreover, the very high interfacial viscosity of the monolayer prevents the interface from increasing in area by the spontaneous formation of an emulsion. A negative interfacial tension mechansim also explains the finding (5) that oil containing 5 per cent to 20 per cent of a fatty acid such as oleic, placed gently on aqueous alkali, leads to spontaneous emulsification. By extrapolation the interfacial tension appears to be negative in the pH range of 9 to 12, in which range the emulsification also occurs. Another system, which emulsifies because of a negative interfacial tension, is that of solutions of long-chain salts in contact with solutions of cetyl alcohol or cholesterol in oil (6, 7, 14, 15). With sodium decyl sulfate in water against cetyl alcohol in toluene, the emulsion can form spontaneously at the interface, with concentration limits which are quite sharp for both the long-chain ions and the alcohol (6). Further, these concentration limits agree with those at which the over-all interfacial tension is expected to become negative (according to a short linear extrap-

344 JOURNAL OF THE SOCIETY OF COSMETIC CHFMISTS Figure 3.--Xylene does not form a drop in an aqueous solution of dodecylamine hydro- chloride, as the interfadal tension is too low. Picture taken at 80 frames per second with a cin•-camera operating through a microscope (17). Steel tip as in Fig. 4 diameter of cylinder of oil •0.0! cm. Figure 4.--A drop of xylene is now being formed against the density gradient. Single frame from sequence at 80 frames per second, showing how the emulsion streams away from the freshly formed interface. The movements on the surface of the drop are quite in-- sufficient to explain the formation of the spontaneous emulsion (17). External diameter of end of tip is about 0.05 cm.