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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.