TOWARD EMULSION CONTROL 191 trical conductivity may be used in suitable systems to determine concentra- tion. The effects of emulsifiers on conductivity should always first be ascertained. It is also necessary to warn experimenters that the formation of "multiple emulsions" invalidates the application of the theory (11). DE•EE ov DISPERSION: SIZE F•EQUENC¾ ANAL¾S•S, LIGHT SCATTERING AND DIELECTRIC CONSTANT A direct indication of the work done in creating an emulsion is the area of interface that has been produced. As the emulsion ages the area of interface decreases. The most fundamental way of expressing emulsion stability is by means of the variation of interfacial area with time. The interfacial area is usually expressed in sq. cm. per cc. of emulsified liquid. For an average size of droplet of 2u, the interfacial area would be 30,000 sq. cm./cc., and would decrease to 20,000 sq. cm./cc. when the average droplet size had increased to 3u. A direct way to get both the average particle size and the interfacial area is by means of the microscope. Size distribution curves may be obtained by measuring about 400 droplets, but better accuracy is obtained if the number measured is even larger. Levius and Drommond (12) found a Bausch and Lomb camera lucida more convenient to use than photomi- crography. They report that they traced the outlines of from 400 to 800 drops per determination: "after some practice this operation could be performed in less than 45 minutes." The results reported by this optical method vary from 28,000 to 19,000 sq. cm./cc. The emulsions were pre- pared by premixing in a Waring blender for 10 minutes, and then passing through a hand-operated lever-type homogenizer. Some degree of stand- ardization is produced by this method, but the emulsions are relatively coarse. Emulsions prepared by the method of phase inversion, for example, produce much smaller particles, but they are then not visible under the microscope and some other method must be found. The advantages of the use of the microscope are that it is direct, inex- pensive, and yields a size frequency distribution as well as a value for the interfacial area. Other possible methods give only an average particle size and the interfacial area is based on that value. Where it is applicable, the use of the microscope is to be preferred before any other method. We are forced, however, to turn to other methods for very small emulsion drops. Van der Waarden (13) has reported the preparation of emulsions of medicinal oil in water, stabilized with from 5 to 35 per cent of alkylben- zene sulfonates in the oil, prepared by the method of phase inversion. The particle sizes were below the resolving power of a microscope, but could be measured by/i. ght-scattering methods. As the content of emulsifying agent was increased, average particle sizes decreased in the range of from 900 to

192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) 300 •k.U. The interface contained all the emulsifying agent, and simply increased in area as more agent was present in the system. Emulsions of this type can be centrifuged for half an hour at an acceleration of 25,000 times gravity without any migration of emulsified oil. Some results obtained in the colloid laboratory at Rensselaer Polytechnic Institute showed that the measured dielectric constant of emulsions of ethyl- ene glycol in mineral oil decreased regularly as the coalescence of the drop- lets took place. We were not aware at the time of the prior research of Piekara (Gdansk) on this very subject. A recently published book on emulsions contains an excellent survey of this work revised by Piekara (14). On the basis of our own results there certainly appeared to be here, in the measurement of dielectric constant, a direct and simple de- termination of the area of the interface, and hence a possible way of know- ing both the initial state and the changes due to aging of an emulsion. This conclusion is not contradicted by Piekara's findings, as he reported also a marked increase of dielectric constant with degree of dispersion in emulsions of water in transformer oil, ethyl alcohol and water in paraffin oil, and mercury in paraffin oil. But an extension of his investigations to o/w emulsions showed that the effect, while still present, was almost com- pletely counteracted by another effect, an orientation of adjacent dipoles of the internal phase, which unfortunately affects the measured dielectric constant in the opposite direction. At present the use of dielectric constant measurements for emulsion interface determination appears therefore to be practicable only for wfo emulsions. Nevertheless the same effect, though much less marked, is still present in o/w emulsions, and it remains to be seen if, with modern electronic apparatus, the small differences that make up the effect can be magnified for the important, practical use here suggested. BIBLIOGRAPHY (1 t Griffin, W. C., y. Soc. Cosmetic Chem., 1, 311 (1949). (2 McBain, J. W., "Colloid Science," Boston, D.C. Heath and Company (1950), pp. 19-21. (3) McBain, J. W.,Ibid.,p. 15. (4) Kremnev, L. Ya., and Ravdel, A. A., Kolloid. Zhur., 16, 17-28 (1954). Abstracted in Chem. •lbs., 48, 6780 (1954). Adam, N. K., and Stevenson, D. G., Endearour, 12, 25 (1953). Hutchinson, E., in H. Sobotka (Ed.), "Monomolecular I.ayers," A.A.A.S., Washington, D.C. (1954), pp. 161-174. Manegold, E., "Emulsionen," Heidelberg, Chemie und Technik Verlags (1952), p. 23. Richardson, E.G., Emulsions in Herroans, J. J. (Ed.), "Flow Properties of Disperse Systems," New York, Interscience Publishers (1953), pp. 39-60. Maxwell, J. C., "A Treatise on Electricity and Magnetism," 3rd Ed., Vol. I, Oxford, Clarendon Press (1892), p. 440. Eucken, Arnold, Forsch. Gebiete Ingenieurw., B. Forschungsheft, 353 (1932), also re- ported by Manegold, E., loc. cit., pp. 27-31. Dixon, H. H., and Bennet-Clark, T. A., Proc. Roy. Dublin Soc., 19, 421 (1930) 20, 211 (1932). See Chem. Abs., 24, 3691 and 26, 3713. Levius, H. P., and Drommond, F. G., •. Pharm. andPharmacol., 5, 743 (1953). Van der Waarden, M., •. Colloid Sci., 7, 140 (1952) 9, 215 (1954). Clayton, W., "The Theory of Emulsions and Their Technical Treatment," 5th Ed., by Sumner, C. G., New York, The Blakiston Company (1954), pp. 241-244.