J. Cosmet. Sci., 70, 127–136 (May/June 2019) 127 Optimization of the Surface Activity of Biosurfactant–Surfactant Mixtures YAO ZHOU, SWARA HARNE, and SAMIUL AMIN, Manhattan College, Riverdale, NY 10471 (Y.Z., S.H., S.A.) Accepted for publication April 25, 2019. Synopsis The impact that rhamnolipid (RL) and sophorolipid (SL) biosurfactants has on solution surface activity when used in conjunction with the commercially important zwitterionic surfactant cocamidopropyl betaine (CAPB) is highlighted for the fi rst time through surface tension and surface rheology measurements on binary and ternary mixtures of these surfactants. It was observed that in both the binary (CAPB/RL) and the ternary (CAPB/RL/SL) mixtures, RL tends to dominate at the air-water interface and primarily control both surface tension and surface elasticity behavior. Signifi cant reduction of surface tension and enhancement of surface elasticity is observed as a result of the competitive adsorbtion/dominance of the RL at the air–water interface and this leads to performance enhancements in terms of foam stability. INTRODUCTION With personal care industry moving toward higher sustainability, the need for greener alternatives for conventionally derived ingredients is increasing signifi cantly. As a result, the demand for novel biosurfactants in the market is anticipated to increase substantially. This demand is due to the expectation of higher sustainability, such as better biodegrad- ability, and more environmental friendly sourcing (1–5). Although biosurfactants have high potential as synthetic surfactant alternatives, their commercial uptake has been lim- ited this is primarily due to the higher costs, limited scale-up, and limited understand- ing of formulation design rules for optimizing performance criteria, such as foaming and cleansing. The number of studies on the surface properties of biosurfactant or biosurfac- tant mixtures is relatively limited (5–14). Biosurfactants are surface-active agents primarily derived from micro-organisms, and they comprise a hydrophilic and hydrophobic group. These microbial surface-active agents have superlative emulsifying, dispersing, foaming, wetting, and coating capabili- ties (5). They can function well at acute temperatures and pH and could be derived from waste products, which can reduce their cost (9–11,14). Address all correspondence to Samiul Amin at samin01@manhattan.edu. Yao Zhou and Swara Harne contributed equally to this work.
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
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