A versatile family of sophorolipid esters: Engineering surfactant structure for stabilization of lemon oil-water interfaces

• Sophorolipids (SLs) are safe, scalable biosurfactants with promising surface activity.
• Modified SLs outperform natural SLs with low CACs and low IFT with lemon oil.
• By tuning the surfactant structure, adsorption and emulsification can be optimized.
• Modified SLs cause depletion flocculation in lemon oil emulsion systems.
• Emulsion and interfacial data can be used to form a predictive model.

Sophorolipids (SLs) are biosurfactants produced by Candida bombicola from renewable feedstocks in yields > 400 g/L. Molecular editing of natural SLs gave a series of n-alkyl SL-esters that, along with natural SLs, were interrogated to determine how structural changes alter SL interfacial tension (IFT), diffusion, adsorption, and emulsification at the lemon oil/water interface. SL-ethyl ester (SL-EE) reduced the IFT by 95.1% and has a critical aggregation concentration (CAC) of 0.026 mg/mL while SL-hexyl ester (SL-HE) had a lower CAC, 0.02 mg/mL, but a lower IFT reduction (87.5%). Adsorption behavior further highlighted differences between the SL-esters as SL-decyl ester (SL-DE) had the lowest surface excess concentration while SL-HE had the highest adsorption coefficient. The competing effects of these interfacial parameters were manifested in the relative performance of SL-esters in forming lemon oil-in-water emulsions. SL-EE had the largest average emulsion droplet size but showed no oil separation up to 200:1 wt/wt oil/surfactant. Microscopy provided information on macroscopic emulsion morphologies. For example, flocculation was observed for all 1 wt% SL 20 wt% lemon oil emulsions. These results were corroborated by shear thinning rheological behavior. Studies were conducted with a natural SL mixture consisting of 1:1 wt/wt lactonic and acidic SLs (LSL + ASL). LSL + ASL has similar emulsion properties to SL-EE at low SL concentrations while emulsions collapsed at higher SL concentrations.

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