666 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tion (nephelos standard). The third method measured the ratio of intensity of the light transmitted through the solution to that of the incident light. The first two methods (tyndallmeter and nephelometer) are more sensitive to extreme dilutions while the tur- bidimeter is good at middle and higher concentrations. a. Tyndallmetric measurements were made on a Coleman Model 14 absolute angular light scattering photometer using green light at ?, = 546 n• radiation. The photometer was calibrated against the opal glass diffusor supplied with the instrument. The temperature of the cell chamber and solutions were equilibrated and monitored by use of a surface temperature probe to maintain 25 -+ 0.1øC. The results are plotted in Figure 6. b. Nephelometric measurements were performed on the Coleman Model 7 Photo Nephelometer utilizing a concentrated-filament bulb yielding radiation at ?, -- 700 rn/x (Model 7-500 exciter lamp suitably filtered). Coleman Certified Nephelos Standards were used in this operation so that haze readings were recorded in arbitrary Coleman Nephelos Units spanning the region from absolute clarity to the zones of visible turbidity. The temperature was controlled as in the tyndallmeter readings. The nephelos scale extends from 0-! 30 units and it is within that range that is found the most precise linearity in the relationship between Tyndall-light intensity and colloid concentration. The use of the nephelos standards permitted a proportional reduction of the response of the instrument to the standard of known value which gave results in arbitrary units as fractions of the true Standard Nephelos value. This made possible the COLLOID 15-- 50-- z o • 45- z 75-- 90 - S,NGLE S.EC,ES / II I I 0.1 1.0 10.0 I• CONCENTRATION NAFOXIDINE HCI (mg/ml) Figure 6. Absolute angular photometric light-scattering curve for n•oxidine hydrochloride at 25øC show- ing tyndallmetric cmc value extrapolated at 1.2 mg/ml.

LYOTROPIC MESOPHASE (LIQUID CRYSTAL) 667 IOOO • I / - 3000 o // 900r © NEPHELOS UNITS A (O-IOO0) / - 2700 800 I'-- o NEPHELOS UNITS B //o ( 0 - 3000) 2400 -- 700 -- i /o 2100 • 600 -1800 u3 500 - 1500 o a_ 400 LU -- 1200 ,,, z z - 900 200 P' -• •== - 600 I00 - 300 01 -- I00 I BA I 0.01 0.1 1.0 I0.0 CONCENTRATION NAFOXIDINE HCI (mg/ml) Figure 7. Nephelometric determination of aqueous solution of nafoxidine hydrochloride at 25øC plotting nephelos units vs. log concentration. The formation of haze is observed as an increase in light scattered. The extrapolated value of 0% transmission (light scattered) at 0 nephelos units for A and B units is 0.68 mg/ml and 0.75 mg/ml, corresponding to observed cmc values. measurement of more turbid systems. Two series of nephelos units, A and B, were em- ployed to extend the reference range from 0-1000 units and 0-3000 units, respectively. The readings of nafoxidine hydrochloride were extremely temperature dependent and care was needed to avoid thermal body contact or prolonged reading in the instrument. Data are shown in Figure 7. c. Turbidimetric measurements were performed on an Aminco-Bowman Model Spec- trophotometer at k = 700 m/x measuring at an angle of 90 ø. Cuvettes with a 1-cm path were thermally controlled by jacketting the sample chamber and circulating water con- trolled at 25 +- 0. IøC. Because of the wide concentration range followed, a log-log plot was used to record the data shown in Figure 8. Deviations may be expected at high concentrations because of two reasons: part of the scattered light is scattered again, some of it re-entering the transmitted beam, or, as the viscosity of the turbid colloid increases, air bubbles become entrapped within the cuvette, creating a distorted signal. 5. PHASE EXAMINATIONS WITH THE POLARIZING MICROSCOPE were made on a Leitz Ortholux instrument equipped with a Bertrand lens, a 10X eyepiece and a hot stage. Microscopic textures were defined and identified utilizing phase classifica- tions described by Rosevear (3) and Winsor (2,4). Thermotropic phase changes were observed on the crystalline nafoxidine HC1 at heating rates of lø/min in the transition regions. A small sample of the drug was heated on a microscope slide until the crystals passed to the isotropic liquid. The mesophase or mesophases were seen to pass across