240 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Surfactant Surfactol Q4 5 to 15 wt% Oil Linoleic acid 1.17 to 4 wt% Cosurfactant PPG-3-buteth-5 2.42 to 5 wt% Effects Graph of Main Effects on Particle Size (with 95% Confidence Intervals) i I I I I I - 200 - 100 0 100 200 300 400 Increase in Particle Size (nm) Figure 1. Effects graph on response particle size (with 95% confidence intervals). The effects of the weight % of each component on particle size is shown. Surfactol Q4 and linoleic acid have the greatest influence on particle size as they are farthest from zero. Since the PPG-3-buteth-5 crosses zero, there is a 95% chance that this component is not having an effect on particle size. acid at the interface. As additional linoleic acid was added to the system, the fatty acid began to partition and collect in the core of the micelle. This effect is postulated to cause micellar growth and to be responsible for the larger particle sizes that exist at these higher fatty acid concentrations (47). At these higher concentrations of linoleic acid, the "empty surfactant micelies" and "swollen micelies" are in equilibrium, as depicted in Figure 4. This equilibrium is responsible for the bimodal distribution in the particle size analysis. Initial addition of the fatty acid interpenetrates and associates with the interfacial film, forming "micelies." Further addition of linoleic acid will begin to collect in the core, forming

SWOLLEN MICELLES 241 Effects Graph of Main Effects on Clarity (with 95% Confidence Intervale) Surfactant Surfactol Q4 5 to 15 wt% Oil Linoleic acid 1.5 to 4 wt% Cosurfactant PPG-3-buteth-5 2.38 to 5 wt% ! I I I - 1.5 - 1.0 0.5 0 1.0 1.5 2.0 Increase in Absorbance Figure 2. Effects graph on the clarity of the system by absorbance. The main effects on the clarity are contributed by linoleic acid and Surfactol Q4, the primary surfactant. The PPG-3-buteth-5 has a 95% chance of not having a significant effect since it crosses zero. the "swollen micelies." This process is depicted in Figure 4. Previous studies conducted by Mukerjee focusing on the solubilization of benzoic acid derivatives by a series of polyoxyethylene surfactants also suggested the existence of an equilibrium distribution between the polyoxyethylene shell and the hydrocarbon core (47). The effect of the addition of PPG-3-buteth-5 while holding the linoleic acid level at 2.25 wt % is shown in Figure 5. Starting at the baseline (0.0 wt % PPG-3-buteth-5), the micelle size increased with the addition of the cosurfactant until a certain concen- tration was reached, at which point the particle size began to decrease. At higher concentrations of cosurfactant, the PPG-3-buteth-5 appeared to be acting more as a cosolvent.