670 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 10. Micrograph of lyotropic mesophase of aqueous nafoxidine hydrochloride (approximately 10- 30%) showing birefringent liquid layer at 43X objective under crossed Nicol prisms observed at 25øC. perses in solution as micelies. This transition could not be observed at higher concentrations where the liquid was more viscous and turbid. The Krafft point can also be explained as the temperature at which the solubility of the hydrated crystalline drug reaches the cmc (critical micelie concentration) for micelie formation. These transition phases were indistinguishable by polarizing microscopy at 12øC. The viscous isotropic phase could be separated from its conjugate solutions by centrif- ugation, but it was not physically stable alone at room temperature and usually developed neat phase as evaporation occurred. The polarizing microscope can only show textures which suggest a distribution of the molecular orientation, as contrasted with the distribution of molecules. The high degree of orientation observed in the lyotropic mesophases described above has been

LYOTROPIC MESOPHASE (LIQUID CRYSTAL) 671 Figure 11. Micrograph of lyotropic mesophase of aqueous nafoxidine hydrochloride (approximately 70- 90%) showing focal conic and spherulite texture with birefringent liquid layer at 43X objective under crossed Nicol prisms observed at 25øC. structurally analyzed by X-ray and NMR investigations and has been described by many workers (see ref 5 and 6 for reviews). Rosevear (7) has also reviewed his micro- scopic texture classification in a Society of Cosmetic Chemists seminar paper. RESULTS AND DISCUSSION Nafoxidine hydrochloride exhibits both thermotropic and lyotropic mesomorphism. The drug is known to be polymorphic from X-ray diffraction and infrared evidence. Thermal analysis of the/• polymorphic form shows that the compound undergoes a transition to a liquid crystal phase identified as smectic by optical characteristics. This solid form can be investigated in aqueous systems for various phase changes either by