804 JOUBNAL OF THE SOCIETY OF COSMETIC CHEMISTS It can be seen that different oils required different lengths of time to reach surfactant equilibrium even though the mixer speed was set constant at 11,- 900 rpm in all cases. For example, isopropyl myristate required less than an hour whereas oleic acid required nearly oe hours before the apparent equilib- rium was attained. The particle size distribution of each emulsion was measured from micro- photographic prints by counting and the result was expressed in terms of mean volume diameter, din, defined as follows: dm- • n where n is the number of droplets and d is the droplet diameter. The results of d• as a function of emulsification time, Te, are presented in Figs. 4-7. By comparing the d• curve against the corresponding Co curve, it is interest- ing to note that the time required for the emulsion droplets to reach a mini- mum was •approximately equal to, or slightly less than, the time required to reach surfactant migration equilibrium. Apparently, under a high mixing speed, very little migration took place once the droplets were reduced to the minimum size and the adsorption at the interface was completed. 4'01 ' , -0- 3.0 I I I I I I I I I ] TWEEN 8O IN 01L TWEE N 80 I N WATER E 'o 1.0 I I I I I I I I I I 20 40 60 80 100 120 EMULSIFICATION TIME (MIN) Figure 4. Change of mean volume diameter, d .... of emulsion droplets with mixing time during emulsification (oleic acid system)

EMULSION STABILITY 805 3.0 z O n,,, 2.0 TWEEN 80 I N OIL TWEEN 80IN WATER 1.0 0 i I I I I t I I I I I 0 20 40 60 8o 100 120 EMUSIFICATION TIME (MIN) Figure 5. Change of mean volume diameter, d .... of emulsion droplets with mixing time during emulsification (mineral oil system) --- 3.0 I,/3 Z O n," •: 2.0 1.0 TWEEN 80 IN OIL '11,NEEN 80 IN WATER 0 i I I I I I I I I I I 0 20 40 60 60 100 120 EMULSIFICATION TIME (MIN) Figure 6. Change of mean volume diameter, d,•, of emulsion droplets with mixing time during emulsification (isopropyl myristate)