JOURNAL OF COSMETIC SCIENCE 128 Rhamnolipids (RL) and sophorolipids (SL) are two such promising microbial glycolipid surfactants. RL biosurfactants consist of a carboxylate head group, which is anionic in nature and alkyl tail groups (15,17), whereas the SLs can exist in two forms, lactone, as a result of esterifi cation of carboxylic acid on the disaccharide ring, and acidic form, be- cause of two head groups of acetylated dimeric sugar sophorose and carboxylic acid with only one long fatty acid tail to each form (15,18–19). These biosurfactants have individu- ally been shown to enhance cleansing performance (5). In a surfactant water solution, the surfactant molecules are attracted to the air/water in- terface because of their amphiphilic nature. These surfactant monolayers at this interface can form a highly elastic interfacial layer and the surfactant monolayer dilational visco- elastic properties can be quantifi ed using elastic modulus (20). In personal care applications, good foaming performance of the product is highly desired by most consumers. In addition to surface tension, the foaming property of a surfactant solution can be also related to its surface elasticity (21). During foam coarsening, the key characteristic of foam stability and the mean bubble size continuously increase. This is caused by the transferring of gas from bubbles (22). Although the actual mechanism of this transportation is still not clear, there are several factors which may infl uence this process such as gas permeability (23) and fi lm thickness (24). In the study by Golemanov et al., high surface modulus of surfactant mixtures has signifi cant effect on foam proper- ties, for instance, the rate and mode of foam fi lm thinning and the rate of bubble Ostwald ripening (25). There are limited studies on the impact of these biosurfactants individually on surface tension, surface elasticity, and surface rheology (5,7,15). However, the impact of RLs and SLs as binary or ternary mixtures on these surface properties has not been investigated (15,18–19). These surface properties of the biosurfactants are related to their cleansing effi cacy and foam stability (3,15,25). Also, their specifi c surface elasticity is related to foam stability, which provides a more pleasant cleansing experience to the consumer (26). High elasticity results in durable foam, which is much more resistant to instability (26,27). In this study, we systematically evaluated how the surface tension and surface elasticity of these biosurfactants are impacted with respect to their ratio, concentration, and as a mixture with a traditional zwitterionic surfactant, cocamidopropyl betaine (CAPB). This study provides new insights into formulation design, which can lead to enhanced perfor- mance in terms of cleansing, foaming, and emulsifi cation. MATERIALS AND METHODS MATERIALS As a fermentation product, RL have various structures. The two commonly seen struc- tures are mono-RLs (R1) and di-RLs (R2). The R2 has an extra rhamnose group com- pared with R1, as shown in Figure 1 (28). The RLs used for the experiment were provided by Natsurfact Laboratories (Fairfax, VA) and have an R1 to R2 ratio of 2:3 w/w. SLs are seen in two forms, the lactonic form (R1 = R2 = COCH3) and the acidic form (R1 = R2 = H), as shown in Figure 2 (11,15). Lactonic form SL (Holiferm, Manchester, UK) is

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.