Surface and interfacial characterization of modified sophorolipid derivatives

Abstract

Finally, SL-esters were evaluated for crude oil applications such as drilling, transportation, and reclamation. Demand for petroleum and natural gas remains steady while the means of extraction become more complex. Significant worry over the release of processing chemicals into the environment due to enhanced oil recovery or the effect of dispersants on the ecology contaminated with crude oil has driven the demand for safer surfactants. SL-esters were found to have interfacial and spreading performance similar to if not better than commercial comparisons. Emulsification of crude oil led to the first water-in-oil (and water-in-oil-in-water) emulsions from SL-esters. It was determined that different components of crude oil had significantly different interactions with the SL-esters, which determined the final performance. This again allows for the opportunity to tune the interfacial performance of the SL-ester to a particular application given the chemical make-up of the oil.

The use of amphiphilic surface active agents, or surfactants, is ubiquitous throughout many industries including pharmaceuticals, petroleum, personal care, and food. There has been great interest in gaining a deeper understanding of the connection between structure and surface/interfacial properties, and also in moving towards greener, safer, and more sustainable surfactants.

Sophorolipids (SLs) are glycolipid biosurfactants produced from the fermentation of Candida bombicola through a commercially scaleable process using sustainable feedstocks. Natural SLs, however, have limitations due to low water solubility or poor surface activity. The Gross lab has developed a suite of simple, scaleable, and green modifications of the SL to enhance antimicrobial activity, solubility, and surface activity.

The work presented here will focus on understanding the structure-property relationships of a simple homologous series of SL-esters at a variety of interfaces. Modified sophorolipids have critical micelle concentrations (CMC) as low as 2μM, one of the lowest values in literature, but this value is very dependent on structure. It was found that sophorolipids with hydrophobic tails extended to 19-22 carbons show a linearly decreasing CMC as is common in literature, but that beyond 22 carbons the CMC no longer decreased. Further surface measurements such as ΔHmic, ΔSmic, and minimum surface area (Amin¬) showed stark transitions at this length as well, suggesting a significant difference in self-assembly around this certain critical length. This was investigated and was found to be due to a previously unidentified pre-micellar behavior.

The work of understanding SL-esters at the interface was extended to almond oil, paraffin oil, and lemon oil all of which have significantly different structures and are expected to interact with the SL-ester differently. All three hydrophobic phases exhibited different interfacial and emulsification behavior and also had a different "optimal" SL-ester. Almond and paraffin oil emulsification performance were dominated by relative degree of surface saturation of the SL-esters while lemon oil emulsification performance was dominated by adsorption, minimum interfacial tension, and flocculation. The ability to tune interfacial properties through the simple extension of the SL-ester tail is a powerful method of optimizing for many applications and should allow for a predictive performance model based on hydrophobic phase structure.