SURFACE ACTIVITY OF BIOSURFACTANT–SURFACTANT MIXTURES 129 used in this study as it has better surface tension reduction effect compared with acidic form (3) for optimal cleansing. CAPB (Lubrizol, Wickliffe, OH) is a zwitterionic surfactant and is commonly used in cosmetic formulations. It is comparatively mild and less irritating as compared with other traditional surfactants (28), and it is used in combination with biosurfactants to aid in surface tension reduction. Besides the three surfactants, citric acid (Sigma-Aldrich, St. Louis, MS) and sodium hydroxide (Fisher Scientifi c, Hampton, NH) were used to adjust the sample to the de- sired pH. Deionized water was added as a solvent. Sodium chloride (Fisher Scientifi c) concentration of all the concentrated samples has been kept fi xed at 2 wt % in all samples. Figure 1. Structure of the two commonly seen RLs R1 and R2. Figure 2. Structure of the two forms of SL. Left: acidic form right: lactonic form.

JOURNAL OF COSMETIC SCIENCE 130 METHODS The RL mixture samples were prepared such that total surfactant concentration is kept constant at 16 wt %, and the ratio of the different surfactants was varied. The SL mixture samples were prepared at a total surfactant concentration of 10 wt %, with the variation of surfactant ratio in the binary to ternary surfactant system. Known masses of each surfactant were added to glass vials, and then known masses of deionized water were added to dissolve the surfactant. The vials were then gently shaken until the surfactant was fully dissolved. Sodium chloride was added to the mixture, and then the mixture was carefully shaken until the salt fully dissolved. After letting the sample sit for 24 h, the concentrated surfactant mixture is then diluted to 10 times of the original concentration to test the surface tension and surface elasticity. After the dilution, the samples were adjusted to the desired pH value using citric acid or sodium hydroxide. All samples were allowed to equilibrate for 24 h before starting the experiment. The surface tensions of the diluted samples were measured at 20°C using du Noüy ring technique with an Attension Sigma 701 Tensiometer (Nanoscience Instruments, Phoenix, AZ). The surface elasticity response of the sample is evaluated with the rheometer Discovery HR-3 (TA Instruments, New Castle, DE) using the Double Wall Ring accessory at 20°C. After adding the sample into the cuvette and letting it rest for 30 min, an amplitude oscillation measurement was performed. All the measurements were repeated fi ve times. The foaming of the samples was tested as follows: 3 ml of the surfactant solutions were added into the same-size glass vials. The samples were allowed to sit until all the bubbles disap- peared. Then the samples were taped into a bundle and shaken by hand for equal length of 10 s. The quality of the foam was observed at 0-, 5-, 15-min, 30-, and 45-min intervals. RESULTS AND DISCUSSION INFLUENCE OF RL ON SURFACE BEHAVIOR IN THE CAPB AND RL BINARY SURFACTANT SYSTEM CAPB is one of the most commonly used zwitterionic surfactants in the personal care industry, and it is usually used in conjunction with anionic surfactants, such as SLES (29). The combination of zwitterionic surfactant and anionic surfactants give rise to not only an optimized rheological property such as proper viscosity in concentrated solutions but also good surface properties allowing good cleansing and foaming performance when diluted. All of the binary surfactant mixtures are adjusted to pH 5.5, which is close to the skin pH. The isoelectric point of the micellar CAPB is 6.5, which means in the pH 5.5 solu- tion, the zwitterionic surfactant CAPB takes on more of cationic nature (30). Figure 3 shows the variation of the surface tension as the RL concentration is increased in this bi- nary mixture. The surface tension is seen to reduce dramatically from 32.37 mN/m to 27.61 mN/m on the addition of a small amount of RL. As the concentration of the RL is increased to 8 g/L, the surface tension decreases even further to 24.67 mN/m, close to the surface tension of pure RL, which is 25.87 mN/m. When the composition of the RL is increased, the surface tension of the sample remains close to the surface tension of pure