STABILITY ASSESSMENT OF EMULSIONS 357 assessed, and it is my hope that this review will stimulate further inquiry into this topic. SUMMARY AND CONCLUSIONS 1. Most references to emulsion stability deal with relatively simple liquid emulsions which bear little relationship to typical cosmetic lotions and semisolids. 2. Since most cosmetic emulsions are aggregated gel structures, any test must only minimally disturb this system relative to the environmental situation to be encountered during shelf-life and use. 3. Most stresses used to accelerate instability cause it by unrelated mechanisms. Thermal stress is useful only if related to the temperature range to be encountered. There is the danger of over-killing acceptable emulsions. 4. Low shear theological studies which measure viscoelasticity, coupled with continu- ous shear rheometry, are the best methods for monitoring physical stability of emulsions. REFERENCES (1) S. Friberg, Food Emulsions (Marcel Dekker, Inc., New York, 1976). (2) J. A. Kitchener and P. R. Mussellwhite, "The Theory of Stability of Emulsions," Ch. 2 in Emulsion Science, ed. P. Sherman (Academic Press, London, 1%8). (3) K.J. Lissant, Emulsions and Emulsion Technology Parts I and II, (Marcel Dekker, Inc., 1974). (4) G. Zografi, "Emulsions," Ch. 16 in The Theory and Practice of Industrial Pharmacy, 1st edition, ed. L. Lachman, H. A. Lieberman, and J. L. Kanig, (Lea and Febiger, Philadelphia, 1970). (5) P. Sherman, "Accelerated Testing of Emulsion Stability," Soap, Perfumery and Cosmetics, 44, 693-699 (1971). (6) E. R. Garrett, "Prediction and Evaluation of Emulsion Stability with Ultracentrifugal Stress,"J. Soc. Cosm. Chem., 21,393-415 (1970). (7) R. C. Vold and R. C. Grott, "Parameters of Emulsion Stability,"J. Soc. Cosm. Chem., 14, 233 (1%3). (8) T. G. Rochow and C. W. Mason, "Breaking Emulsions By Freezing," Ind. Eng. Chem., 28, 12%-1300 (1936). (9) J.P. Cordiez, G. Grange, and B. Mutaftschiev, "Droplet Freezing Experiments in Stearic Acid-Water Emulsions, Role of Droplet-Medium Interface,"J. Coll. Interf Sci., 85, 451-441 (1982). (10) H. R. Kruyt, Colloid Science, Vols. I and II (Elsevier Publ. Co., New York, 1949). (11) J. D. Ferry, Viscoelastic Propetites of Polymers, 3rd edition (John Wiley and Sons, New York, 1980). (12) H. Schott, "Rheology," Ch. 22 in Remingtons Pharmaceutical Sciences, ed. A. Osol (Mack PuN. Co., Easton, PA, 1980), 323-339. (13) P. Sherman, "Rheology of Emulsions," Ch. 4 in Emulsion Science, ed. P. Sherman (Academic Press, London, 1968). (14) E. N. Hiestand, "Physical Properties of Coarse Suspensions,"J. Pharm. Sci., 61,268-272 (1972). (15) B. W. Barry and E. Shotton, "Structure and Rheology and Sodium Dodecyl Sulfate-Cetyl Alcohol- Water: Continuous Shear Experiments,"J. Pharm. Pharmac., 19, 1105-1205 (1%7). (16) B. W. Barry and E. Shotton, "Rheology of Sodium Dodecyl Sulfate-Cetyl Alcohol-Water: Small Strain Experiments,"J. Pharm. Pharmac., 19, 1215-1295 (1%7). (17) B. W. Barry and B. Warburton, "Some Rheological Aspects of Cosmetics,"J. Soc. Cosm. Chem., I9, 725-744 (1968). (18) B. W. Barry, "Evaluation of Rheological States of Semisolids," J. Soc. Costa. Chem., 22, 487-503 (1971). (19) B. W. Barry, "The Self Bodying Action of the Mixed Emulsifier Sodium Dodecyl Sulfate/Cetyl Alcohol,"J. Coll. and lnterf Sci., 28, 82-91 (1%8).

358 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (20) P. Sherman, "Rheological Changes in Emulsions on Aging,"J. Coll. and Interf Sci., 24, 107-114 (1967). (21) D. E. McVean and A.M. Mattocks, "Simple Rheometer for Definition of Viscoelastic Properties of Structured Liquids,"J. Pharm. Sci., 50, 785-788 (1961). (22) J. H. Wood and G. Catacalos, "Prediction of The Rheological Aging of Cosmetic Lotions,"J. Soc. Cosm. Chem., 14, 147-156 (1963).