THE SEVENTH SPECIAL AWARD 345 olation). It appears that the spontaneous emulsion forms slightly when the tension of the interface is a few tenths of a dyne cm. -• below zero, and becomes pronounced when .• = -1 dyne cm. -•, i.e. when the pressure of the intel facial monolayer, adsorbed form both the oil and the water phases, exceeds by 1 dyne cm -t the tension of the clean interface. In general, ionic and nonionic surface-active agents adsorb almost in- dependently of each other, thus pointing to a general method of obtaining very small or negative interfacial tensions. Local lowerings of the interfacial tension to values below zero are also responsible for the spontaneous emulsification of an oil (e.g., xylene) in aqueous dodecylamine hydrochloride solutions of concentration in excess of M/10. This phenomenon involves only the passage of the oil into the aqueous phase, where it is ultimately solubilized: (16) there is no diffusional reason why excess of oil should appear on the aqueous side of the interface. The explanation (7, 17) is that the dodecylamine ions are strongly adsorbed [possibly with other surface-active impurities (18)] and momentarily reduce the interfacial tension to a negative value: the interface then in- creases in area by spontaneous emulsification (Figs. 3 and 4). Next, the passage of oil into the aqueous phase (as emulsion drops and as solubilized oil) reduces the concentration of free dodecylamine ions near the interface to a level from which adsorption is no longer sufficient to make the inter- facial tension negative. However, stray convection currents or density differences occasionally sweep fresh, undepleted solution of the dodecyl- amine into the interface through the enveloping emulsion, thus momentarily lowering the interfacial tension locally to below zero (17). This fluctuation of the interfacial tension causes both the kicking and the spontaneous emulsification. SUMMARY If the interfacial tension is negative, this is a sufficient explanation, provided the monolayer is not too highly viscous, and provided also that there is not complete miscibility at the interface. Emulsification by th's mechanism occurs at sharp concentration limits. If, however, the interfacial tension is appreciably positive and there is no interfacial turbulence, then the "diffusion and stranding" mechanisms must be operative. If the interfacial tension is positive and there is interfacial turbulence, t•rther investigation (e.g., by inhibiting the turbulence with a monolayer of adsorbed protein) is necessary. REFERENCES (1) Gad, J.,/itch./inat. u. Physiol., Leipzig, 181 (1878). Briicke,/inz./ikad. Wiss. lfien., 79, 267 (1879). Quincke, lfiedman's/Inn., 35, 593 (!888).

346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (2) McBain, J. W., "Colloid Science," Boston, D.C. Heath and Co. (1950). Kruyt, H. R., "Colloid Science," Vol. I, Amsterdam, Elsevier Press (1952), p. 340. (3) Gurwitsch, L., "Wissenschaftliche Grundlagen der Erdolbearbeitung," London, Transla- tion by Moore, Chapman and Hall (1932), p. 430. (4) Raschevsky, N., Z. Physik, 46, 568 (1928). (5) Stackelberg, M. v., Klockner, E., and Mohrhauer, P., Kolloidzschr., 115, 53 (1949). (6) Davies, J. T., and Haydon, D. A., Proc. 2rid Intern. Congress Surface •tctivity, Butter- worths, London 1, 417 476 (1957). (7) Davies, J. T. and Rideal, E. K., "Interfacial Phenomena," New York, Academic Press, Inc. (1961). (8) McBain, J. W., and Woo, T. M., Proc. Roy. Soc. (London), 163A, 182 (1937). Kaminski, A., and McBain, J. W., Ibid., 198A, 447 (1949). (9) Haydon, D. A., Nature, 176, 839 (1955). Lewis, J. B., and Pratt, H. R. C., Ibid., 171, 1155 (1953). (10) Pospelova, K. A., and Rehbinder, P. A., •tcta physicochima. U.R.S.S., 16, 71 (1942). (11) van der Waarden, M., •7. ColloidSci.,7, 140 (1952). (12) Ilkovid, I., Collection tray. chim. tch•coslov., 4, 480 (1932). (13) Langmuir, I., Cold Spring Harbor Symposium, 6, 193 (1938). (14) Cockbain, E.G., and Schulman, J. H., Trans. Faraday Soc., 36, 651 (1940). (15) Matalon, R., Ibid., 46, 674 (1950). (16) Kaminski, A., and McBain, J. W., Proc. Roy. Soc. (London), 198A, 447 (1949). (17) Davies, J. T., Bell, G., and Law, P. J. S., Research Project in Dept. of Chemical Engineering, Cambridge (1960). (18) Hartung, H. A., and Rice, O.K., 5•. Colloid Sci., 10, 436 (1953). A SURVEY OF DR. J. T. DAVIES' CONTRIBUTIONS TO EMULSION STABILITY, FOAM STABILITY AND OLFACTORY THRESHOLDS OF ODORANTS T•E s'rut)•Es of J. T. Davies have contributed very significantly to the understanding of several phenomena highly important to cosmetic chemists. His recent work has led to the development of equations for predicting emulsion stability, foam stability and olfactory thresholds of odorants. Earlier workers had shown that an electrical charge an a monolayer might bring about radical changes in its free energy and surface tension. As early as 1951 (1), as a result of his extensive studies of interfacial po- tentials and reactions, Davies had shown that surface potentials (2) and the amounts and rates of adsorption (4) are also affected by the electrical charge, and had developed an equation of state for oil-water films. In 1956 (8) he developed a similar expression to cover charged monolayers at the air-water interface. In 1957 (10) Davies reported data, obtained by using a new viscous- traction surface viscometer of his own design, which showed a correlation between foam stability and surface viscosity. In the same year he evolved a quantitative kinetic theory of emulsion type, which provided a firm basis for the HLB system of classifying emulsifiers (11). The HLB num- ber was shown to depend upon the ratio of the coalescence rate of the oil-in-