SURFACE ACTIVITY OF BIOSURFACTANT–SURFACTANT MIXTURES 131 RL system. This indicates that the RL is more competitive at the air–water interface compared with the CAPB, which leads to the domination of RL at this interface. The phenomenon observed from the surface tension measurement was further corrobo- rated with the surface elasticity measurements. Figure 4 shows the surface elasticity ex- hibited within the binary system. This result is in agreement with the surface tension measurements. The pure CAPB shows the lowest surface elasticity compared with the sample with pure RL. The surface elasticity of the 8:8 CAPB:RL mixture sample has a similar value to that of the pure RL sample. This indicates that RL forms a strong elastic structure at the air–water interface and is the dominant species at the air–water interface. EFFECT OF SL BINARY AND TERNARY SYSTEM MIXTURE RATIO ON SURFACE BEHAVIOR In general, using surfactant mixtures in the solution may allow a further reduction in surface tension or enhancement of surface elasticity through potential synergistic interac- tions between surfactants or through formation of a mixed surfactant layer exhibiting high surface elasticity (31). The synergistic effect was not achieved with the RL and CAPB system as shown in Figures 3 and 4 in this binary system, RL dominates surface Figure 3. Effect of increasing RL concentration on surface tension. Figure 4. Difference of surface elasticity of pure CAPB, pure RL, and CAPB and RL mixture sample.

JOURNAL OF COSMETIC SCIENCE 132 behavior at air–water interface. SL was explored as a potential addition to the system to understand whether it brings about any additional new synergistic effect. Figure 5 highlights the behavior of SL in both binary and ternary surfactant systems in the binary system with CAPB, CAPB is incrementally substituted with SL. It is observed that a surface tension value below that of either pure SL or pure CAPB is achieved at equal ratios. This clearly shows an synergistic interaction between CAPB and SL. In the ternary system with the addition of SL to CAPB and RL mixture, there does not seem to be sig- nifi cant impact on the surface tension reduction, with lowest surface tension in the sys- tem being 24.9 mN/m, close to pure RL. This indicated that RL seems to dominate at the air–water interface. Figure 6 highlights the surface elasticity behavior of SL in both binary and ternary sys- tems. The surface elasticity of the ternary system with CAPB/RL/SL is higher than both binary system of CAPB/SL and the pure SL system. This further corroborates that RL is competitive at the interface, being substantiated with both surface tension and surface elasticity measurements. This can result in an elastic layer and tighter packing for high foam stability, ideally desired in a personal cleansing application. Figure 6. Surface elasticity comparison of the ternary CAPB/RL/SL system with binary CAPB/SL system and pure SL system. Figure 5. Effect of SL addition in the binary system with CAPB and ternary system with CAPB and RL.