SURFACE ACTIVITY OF BIOSURFACTANT–SURFACTANT MIXTURES 133 Figure 7 compares the surface elasticity of the ternary CAPB/RL/SL system with the bi- nary CAPB/RL system. Here, it is seen that the surface elasticity of the two samples was approximately the same, which indicates that the RL is the dominant species at the air– water interface. The RL at the air–water interface provides a strong surface layer, which potentially enhances foam stability. FOAMING PROPERTY To directly evaluate the performance aspects of the surface tension and the surface elastic- ity optimization with RL, a foaming test was performed. Figure 7. Comparison of the surface elasticity of CAPB/RL/SL and CAPB/RL. Figure 8. Result of the foaming test. Bottles from left to right: pure CAPB at pH 5.5, pure CAPB at pH 7.0, CAPB/RL at pH 5.5, and CAPB/RL/SL at pH 5.5. Times of photo taken were as following: (A) after shaking (B) after 5 min (C) after 15 min (D) after 30 min (e) after 45 min.

JOURNAL OF COSMETIC SCIENCE 134 The result of the foaming test is shown in Figure 8. After shaking for 10 s, the sample with binary 8:8 CAPB/RL and ternary CAPB/RL/SL formed a denser and an even bubble size foam than the pure CAPB sample by themselves. Five minutes later, the bubbles in pure CAPB sample were signifi cantly coarsening and draining, whereas the bubble size and quality of the samples with RL were still intact. After 45 min, the foam formed by the two pure CAPB samples almost disappeared, whereas the foam in the other two samples was coarsening, but there was no large gap which appeared in between. This strongly complements the results from the surface tension and surface elasticity measurement. With the RL in the surfactant system, the surfactant solution formed a more elastic layer at the air–water interface, which resulted in denser and more stable foam. Com- pared the performance of CAPB/RL and CAPB/RL/SL sample in this foaming test, the differences were subtle. This result supports the surface tension and surface elasticity ex- perimental results, which also have minimal differences between these two samples. CONCLUSION This study has shown the strong impact of biosurfactants such as RL and SL on the surface properties in binary and ternary mixtures with a commonly used zwitterionic surfactant such as CAPB. Signifi cant surface tension reduction and high surface elasticity was ob- served in all formulations, both binary and ternary when RL was present. As shown in Figure 9, this indicates the high surface activity of RL. The RL potentially dominates at the interface for both CAPB/RL and for CAPB/RL/SL mixtures, forming tightly packed elastic layers at the air–water interface as shown by the high values of surface elasticity. This results in denser and more stable foam formation. The SL behavior is signifi cantly different. In the binary SL/CAPB mixtures, it seems to exhibit synergistic interactions and forms a mixed layer. This is further corroborated through the surface elasticity mea- surements. These new insights on binary and ternary mixtures of two biosurfactants, RL and SL, together with CAPB should provide new formulation guidance for personal care products. The study also highlight the importance of surface tension and surface elasticity as two highly complementary techniques to better understand surface structuring in sur- factant and biosurfactant mixtures. Figure 9. Schematics of surfactant molecule orientation at air–water interface in different surfactant systems.