148 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS LIJ #• .06) #9 O. 19) #27 ß 0% Petro[(]tum ZX t0% Petro[(]tum ß 2:0% Petrot(]tum O 30% Petrot(]tum [] 40% PetroR]tum ß 50% Petrol.(]tum ,t 60% Petrooe(]tum I • 70% Petro•(]tum i • ( } Oil/W(]ter Ph(]se-Volume #2 (0.t9) (0.35) #4 Rotio (0.57) # Formule Number (o'.35) #•l (o 8a•,,• #6 ' - • (t34) #4: 0.88) • ,8) (0.88) i- ".z•44)- #43 % (•,34) • (1.34) •7 N (2.10} (2.10) • (3.59) •5 •8 (3.59) (3.59} •34 • 5Z L2' I•,,,•, • •30 #26 (3.59) (3.59) #36 (3.59) #33 (3.59) •35 (3.59} I tO O 20 30 40 50 60 70 % Minerel 0i[ Figure 2. Water evaporation rate (WE) and the percentage of mineral oil in the test o/w emulsions. demonstrates the volatility of the selected twelve emulsions in four hours at ambient conditions. The results in Figure 3 agree well with those of Figure 2. The formulas with the greatest amount of water have the greatest weight loss. From a practical viewpoint, the evaporation of water from the skin or emulsions into the atmosphere is a continuous process. The vehicle effect due to water evaporation will

EMULSION VEHICLES AND VASOCONSTRICTOR ACTIVITY 149 lO0 9O 8O • 7O Formul. o # • ß 36 5.59 ß t3 t.34 0 t9 t.34 [] 24 1.34 X 28 t.34 ß 3t 1.34 © 6 t.34 /x 23 0.88 ß t7 0.57 [] lO 0.35 • 2 O. J9 o 1 0.06 (,•' -- Oi[/Water Phase- VoLume Ratio) 200 i • 1/2 t 2 • 4 Time, hours Figure 3. The percentage of initial weights of the 12 selected hydrocortisone 17-valerate 0.2% o/w emul- sions in four hrs. The bar indicates the standard error of the means (n = 3). always occur if the water vapor from the o/w vehicle is taken away more quickly than water can diffuse upward from the deeper layer of the skin into the stratum corneum. The situation applied to the highly volatile emulsions with low phase-volume ratio, i.e., formulas #1, #2, #10, #17, and #23. In theory, it is speculated that after losing most of their own water, the emulsions will develop a draining effect, which can lead to drying of the underlying tissue (2). However, the evaporation could also cause a corre- sponding rise in concentration of active drug in these highly volatile formulas, which might then increase the rate of diffusion of the active into the skin. Ideally, an optimal formula might be achieveable by adjusting its phase-volume ratio to the optimal point in which the skin permeation of the active is enhanced to a maximum level. The fol- lowing study using the selected twelve formulas is an attempt to achieve this goal.