MICROEMULSIONS 343 IO 8 9 SYSTEM: 0 I 2 BRIJ $5'.•. gm ARLACEE' 186: 4gm OIL: IOml ISOPROPANOL- VARIABLE WATER - VARIABLE ß n-DECANE OIL ß n-HEXADECANE OIL 0 n-TETRADECANE OIL ,• DECANE / HEXADECANE •,\\ ,•TETRADECANE 3 4 5 6 AMOUNT OF ISOPROPANOL (ml) I I 7 8 9 Figure 5. Maximum solubilization of water at various isopropanol concentrations in microemulsions with different oils. birefringence could be observed. We are not able to offer any logical explanation for this phenomenon at this juncture. Measurements of the viscosity of the microemulsions indicated that with an increase in water-to-oil ratio, the kinematic viscosity increased steadily suggesting that the microemulsion is the water-in-oil type. This observation is in accordance with the known behavior of dispersions where the viscosity increases with increasing volume of the discontinuous phase (31-32). Representative data with tetradecane as oil at various water-to-oil ratios are plotted in Figure 6. Electrical conductance of the pharmaceutical microemulsion with various oils increased with an increase in water-to-oil ratio. Data obtained with pentadecane oil is given Figure 7. The electrical conductivity was found to increase steeply over five orders of magnitude when the concentration of the droplets was increased. The sharp increase in conductance with an increase in the

344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 4O • 35 o • 30 25 o 20 ([ 15 z ,• I0 SYSTEM: Bri•3,•= 2 gm Arlocel'186 = 4 grn n-Tetrodecone = I0 rnl Isoproponol = 3 rnl Woter vorioble. I I I I I O.I 0.2 0.3 0.4 0.5 WATER TO OIL RATIO i I Figure 6. The effect of water-to-oil ratio on the kinematic viscosity of microemulsions. water-to-oil ratio could be attributed to a percolation transition which gives rise to a continuous path through aggregated droplets beyond a critical volume concentration of droplets (33-34). 3. SOLUBILITY OF HYDROCORTISONE IN PHARMACEUTICAL MICROEMULSIONS Solubility of hydrocortisone in isopropanol was determined spectrophotometrically. The maximum solubility obtained by this method (8.5 mg/ml) agreed with the value obtained gravimetrically (8.55 mg/ml). It was observed that the solubility of hydrocorti- sone was greater in isopropanol/water mixtures than in pure isopropanol. The solubility exhibited a maximum when 20% water was present in the alcohol and then