688 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TWEEN 80 - ARLACEL 80 ( õ% TOTAL) 30 % OIL HLB 12 - LB 14. 0 I I I I I 0 20 40 60 80 I00 % ARLACEL 80 IN OIL PHASE Figure 3. Effect of initial lipophilic sur- factant location on the immediate viscosity of emulsions w/o ,/ o/w ( 5% TOTAL) TWEEN 80- ARLACEL80 HLB 6 7'0% OIL o 2o ,,o ,o eo ,oo % TWEEN 80 IN AQUEOUS PHASE Figure 4. Effect of initial hydrophilic surfactant location on viscosity and type of emulsions tively slow, conceivably, the location of initial surfactant in the aqueous phase can affect many of the above mentioned factors. In this work, no attempt was made to isolate each factor and determine the manner by which the emulsion viscosity is influenced. However, as it will be shown later, the initial surfactant location can have a significant effect on the particle size distribution of the emulsion formed and, consequently, on the emulsion viscosity. Effect on the Type of Emulsion In the systems where the HLB of the surfactants and the volume of the internal phase are such that it can form either an O/W or W/O type emulsion, the initial surfactant location appears to play an important role in determining the type of the emulsion. In the systems investi- gated, placing of the entire surfactants in the oil phase appeared to pro- mote formation of a W/O emulsion or a multiple emulsion. On the other hand, placing of the surfactants in their respective phases (i.e., the hydro- philic surfactant in water and lipophilic surfactant in oil) seemed to en- courage formulation of an O/W emulsion. For example, in a 70% mineral oil system stabilized with Tween 80- Arlacel 80 combination at HLB 0, the viscosity increased sharply when the initial concentration of Tween 80 in the aqueous phase was increased from 0 to 10% (Fig. 4). Conductivity measurements indicated that the

EFFECT OF SURFACTANT LOCATION ON EMULSIONS 689 •--W/O •[• O/W = ( 5 % TOTAL) • • TWEEN 80 - ARLACEL 80 / HLB I0 / • 70ø/001L ./ -- 20 40 60 80 •00 % TWEEN 80 IN AQUEOUS PHASE Figure 5. Effect of initial hydrophilic sur- factant location on viscosity and type of emulsions 8 o/w w/o •= SOLULAN 98 - AMERCHOL L-IOI ( 5% TOTAL) ß HLB I0 60% OIL 0 20 40 60 BO I00 % SOLULAN 98 IN AQUEOUS PHASE Figure 6. Effect of initial hydrophilie sur- factant location on viscosity and type of emulsions sharp increase in the emulsion viscosity was due to phase inversion from W/O type to O/W type. Figure 5 shows the viscosity change for a similar system at HLB 10. In this system, the inversion took place when the amount of Tween 80 initially in the aqueous phase corresponded to approximately 70% of the total Tween 80 used. Figure 6 shows a similar curve for a Solulan 98- Amerchol L-101 system. In these three systems the viscosity readings were taken with the No. 3 spindle at 30 rpm. It has been known that the type of surfactants as well as their con- centration have marked effect on the type of emulsion formed (8, 9). The present work indicates that not only the surfactant type and concen- tration are important, but the initial distribution of the hydrophilic sur- factant prior to emulsification can also have a significant effect on the type of the emulsion formed. Davies reasoned that the type of emulsion formed as the result of shaking of •a mixture of oil and water with an emulsifying agent is deter- mined by the relative coalescence rates (10). He suggested that: Rate 2 O/W emulsion preferentially stable if Rate• 1 and, Rate 2 W/O emulsion preferentially stable if --- 1 Rate 1