668 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 10o 1.0 l 0.1 0.001 o o o SINGLE SPECIES 25'C 90* 700 m/.z ø••1:• 1.55 mg/rnl I 50mg/ml /--% COACERVATE ' •' ø o MI I I I I 0.01 O, I I, 0 I0.0 I00.0 CONCENTRATION NAFOXIDINE HCI (mg/ml) Figure 8. Log-log plot of turbidimetric data for aqueous solutions of nafoxidine hydrochloride at 25øC showing observed cmc values at 0.7 mg/ml. An extrapolated value of 1.35 mg/ml for a second critical concentration represents the onset of turbidity for a phase change (coacervate) which represents the ap- pearance of the middle phase. The log turbidimetric behavior shows a departure from linearity above 30 rag/mi. the field of view as successive, well defined, opaque regions. The solid-mesomorph, mesomorph-mesomorph, and mesomorph-isotropic solution changes were observed across the preparation as distinct fronts. All mesomorphic phases were found to be birefringent (doubly refractive) as well as showing uniaxial and positive optical signs. These are characteristic of both neat (smectic) and middle (nematic) liquid crystal tex- tures. Figure 9 shows a micrograph taken under a 10X objective, displaying a focal conic texture with fanlike terrace fronts. Occasionally oily streaks can be observed in this turbid, viscous state. This phase was identified as a smectic mesophase. Lyotropic mesophase transitions were observed under controlled temperature condi- tions at 25 and 37øC. For other temperatures, rapid heating rates of 10øC/rain were used to minimize evaporation and may have led to slightly higher temperatures in re- cording a phase change. Three lyotropic phases of nafoxidine HC1 observed at 25øC are shown in Figures 10, 11 and 12. These same phases were also observed at 37øC al- though evaporation represented a problem. By drying the micellar solutions at high temperature (25øC) and 37øC, each system was seen to pass through a sequence of mesomorphic phases. These observations were used to construct a phase diagram, shown in Figure 13. This phase diagram was constructed for phases which were ob- served by various methods listed in Table III. At higher temperatures above 37øC, only a few birefringent textures could be detected (too weak to be photographed). Addi- tionally, the viscous isotropic region usually contained a conjugate solution which separated into a clear isotropic phase combined with either the middle or neat phase. The appearance of a homogeneous isotropic solutions was used to judge the transition points for this phase.

LYOTROPIC MESOPHASE (LIQUID CRYSTAL) 669 i Figure 9. Thermotropic liquid crystal micrograph showing smectic mesophase with birefringent liquid layer at 10X objective under crossed Nicol prisms observed at approximately 183øC. The microscopic texture of the middle phase was easiest to detect (its presence is shown on the phase diagram of Figure 13 by using X as data points). The middle phase was characterized by its threaded texture. It appeared as a turbid liquid with regions of birefringence occurring in various sequences such that the surface-active material it- self, rather than water, was the continuous medium. The thread-like texture, also called fibrous, has a very mobile state. The typical birefringence with characteristic striations is shown in the Figure 10 micrograph. At depressed temperatures, a heterogeneous separation of the miceliar solution was observed in which a soap curd formed. The temperature at which the solution became opaque was observed to be approximately 12øC for several of the dilute systems. This temperature was regarded as the Krafft point. This point can be regarded as the melt- ing point of the hydrated nafoxidine hydrochloride, above which the surfactant dis-