682 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS fect of mesophases is not as intense as that of bismuth oxychloride or guanine (com- mon pearlescent additives in nail polishes, lipsticks and mascara), they are usually more stable. CONCLUSIONS The micellar properties of colloidal solutions of nafoxidine hydrochloride represent a very unique and complex system of drug in aqueous media. A detailed knowledge of the theory explaining some of the phenomena observed in formulating the drug has proven to be of invaluable assistance in developing an improved product and/or in understanding and predicting the physicochemical properties involved. To assume that the information gained is entirely or partly applicable to the successful development of a drug product recognizes only a part of the effort. Such a system, in which the active principle of the dosage form is a highly surface-active material possessing a cmc range in the concentration range of interest, represents a selected pharmaceutical product which is not too common. On the average, it has been assumed that about 1 in every 200 compounds is liquid crystalline. Although this is not a high occurrence, the phenomena can occur over a wide range of molecular types and can pose several op- portunities as well as problems. ACKNOWLEDGEMENT The author expresses appreciation to The Upjohn Company for permission to release information regarding nafoxidine hydrochloride. The investigation of this drug was performed at Upjohn in the Product Research and Development Department, Solid Dosage Form Section. Special acknowledgement is given to Engelbert L. Rowe of Upjohn, who assisted in the surface tension studies and contributed much helpful dis- cussion. REFERENCES (1) V. Luzzati, H. Mustacchi and A. Skoulios, The structure of the liquid-crystal phases of some soap and water systems, D/scussions Faraday Sot., 25, 42- 50 ( 1958) (2) P. A. Winsor, Binary and multicomponent solutions of amphiphilic compounds. Solubilization and the formation, structure, and theoretical significance of liquid crystalline solutions, Chem. Rev., 68, No. 1, 1-40 (1968). (3) F. B. Roseyear, The microscopy of the liquid crystalline neat and middle phases of soaps and synthetic detergents, J. Amer. O//Chem. Sot., 31, No. 12,628-639 (1954). (4) P. A. Winsor, The structure ofviscoelastic solutions of colloidal electrolytes, J. Co//oidSci., 10, 88-100 (1955). (5) J. S. Clunie, J. M. Corkill and J. F. Goodman, The structure of lyotropic mesomorphic phases, Proc. Roy. Soc. (London), Ser. A. 285,520-533 (1965). (6) C. A. Gilchrist, J. Rogers, G. Steel, E.G. Vaal and P. A. Winsor, The constitution of aqueous liquid crystalline solutions of amphiphiles, J. Colloid Inter/}tc. SoL, 25,409-420 (1967). (7) F. B. Roseyear, Liquid crystals: the mesomorphic phases of surfactant compositions, J. Soc. Cosmet. Chem. 19, 581-594 (1968). (8) D.C. Poland and H. A. Scheraga, Hydrophobic bonding and micelie stability, J. Phys. Chem., 69, 2341-2442 (1965).

LYOTROPIC MESOPHASE (LIQUID CRYSTAL) 683 (9) D. G. Hall and B. A. Pethica, Thermodynamics of micelle formation, in M. J. Schick, "Nonionic Surfactants," Marcel Dekker, Inc., New York, 1967, pp 516-557. (10) A. A. Abramzon and Y. B. Moiseeva, Temperature dependence of surface tension, Kolloid. Zh. 30, 795-801 (1968). (11) I. Langmuir, The role of attractive and repulsive forces in the formation of tactoids, thixotrol•ic gels, protein crystals and coacervates, J. Chem. Phys., 6, 873-896 (1938). (12) I. Cohen and P. Economou, Temperature studies of coacervating cationic soap solutions, J. Amer. Oil C hem. Soc., 41,461-464 (1964). (13) D. M. Small and M. Bourges, Lyotropic paracrystalh'ne phases obtained with ternary and quarternary systems of amphiphilic substances in water: studies on aqueous systems of lecithin, bile salt and cholesterol, in G. H. Brown, G. J. Dienes and M. M. Labes, "Liquid Crystals," Gordon and Breach, Science Publishers, New York, 1966, pp 221-241. (14) V. A. Usol'tseva and I. G. Chistyakov, Chemical .characteristics, structure, and properties of liquid crystal, Russian Chem. Rev., 32,495-509 (1963). (15) K. Shinoda, An outline of the solvent properties of surfactant solutions and L. Sjoblom, Pharmaceutical applications and physiological aspects of solubilization, in K. Shinoda, "Solvent Properties of Surfactant Solutions," Marcel Dekker, Inc., New York, 1967, pp 1-26 and 189-262.