Molecular editing of sophorolipids by esterification of lipid moieties: Effects on interfacial properties at paraffin and synthetic crude oil-water interfaces

Abstract

Sophorolipids (SLs) are naturally produced glycolipid biosurfactants that offer safe alternatives to chemical surfactants that have been identified as ecologically-hazardous. Molecular editing of SLs by esterification of the lipid tail was used to prepare a family of SL-esters whose hydrophobic moiety is extended from 18 carbons (17-HOC18:1Δ9) to 20, 24, or 28 carbons, respectively. The interfacial properties of natural and SL-esters were evaluated with paraffin and a synthetic crude oil. SL-esters reduced paraffin oil/water interfacial tension (IFT) by 89–97% and had critical aggregation concentrations (CACs) between 0.02–0.008 mg/mL. SL-esters were also found to emulsify paraffin and synthetic crude oil with oil/surfactant ratios up to 200:1 wt/wt for 1 week with emulsion droplet sizes below 5 μm. SL-hexyl ester had the lowest CAC, 0.008 mg/mL, and generally, the lowest droplet sizes, particularly at 10 wt% paraffin oil. While, SL-decyl ester had the highest (97.2%) maximum%-IFT reduction and gave paraffin oil emulsions that were most stable over time. SL-ester emulsions of synthetic crude oil resulted in emulsion sizes that are similar to those with paraffin oil. However, the absence of SL-ester precipitation over the range of SL-ester/synthetic crude oil emulsions tested suggest that the affinity of SL-esters to the oil phase is enhanced by the presence of o-xylene and 1,2-dimethylcyclohexane in the crude oil formulation. A natural SL mixture consisting of 1:1 lactonic (LSL) and acidic (ASL) forms has a CAC that is about an order of magnitude larger than SL-esters. Furthermore, the natural SL mixture was unable to emulsify paraffin and synthetic oil when the ratio of oil-to-surfactant is greater than 1:1. The predictive connection between CAC and emulsion sizes is discussed. IFT and emulsion data show that SL-esters are promising surfactants for crude oil related oil compositions.