Aqueous food-grade and cosmetic-grade surfactant systems for the continuous countercurrent cloud point extraction

Abstract

Nonionic surfactant aqueous two-phase systems represent an alternative extracting media to conventional organic solvents by ensuring a mild process temperature in an aqueous environment. However, their large-scale implementation is limited due to the challenging stripping of the target substances from the surfactant. In this study, we demonstrate the application of aqueous food-grade and cosmetic-grade surfactant systems as extraction media for solutes of natural origin. We suggest that no separation of surfactant and target product (or only rough separation is sufficient) is needed in this case due to the application of both substances in the final food or cosmetic formulation. To this purpose, sixteen potential commercial surfactants applied in food or in the personal care industry were investigated for their cloud point temperature and their phase separation kinetics. Among those, the aqueous surfactant systems containing Silwet L-7230 (poly[dimethylsiloxane-co-methyl(3-hydroxypropyl)siloxane]-graft-poly(ethylene/propylene glycol) and ROKAnol NL5 (C9–11, branched and linear, ethoxylated alcohol) separated in two phases rapidly at temperatures below 50 °C. Therefore, the liquid-liquid equilibria of the mixtures Silwet L-7230/water and ROKAnol NL5/water were determined, exhibiting lower critical solution temperatures (LCSTs) of 37.3 °C and 33.5 °C, respectively. Additionally, the mixtures were investigated for their capacity to extract the model solute cinnamic acid ((2E)-3-phenylprop-2-enoic acid, CA). Batch separations with 10logPCA=0.9 for ROKAnol NL5 and 10logPCA=1.6 for Silwet L-7230 were achieved, proving the potential of these surfactants for the extraction of hydrophobic compounds. To further design a stable liquid-liquid extraction process, the density and viscosity of the surfactant-water mixtures were measured. The density differences between the micellar and aqueous phases was found to be sufficient for continuous extraction, whereby the system with ROKAnol NL5 consisted of an upper surfactant-rich phase and a lower surfactant-lean phase, and vice versa for the Silwet L-7230 system. The formation of a liquid crystalline structure with high viscosity was observed in the Silwet L-7230 micellar phase. Based on these investigations, the corresponding processes were designed and finally, for the first time, the food-grade Silwet L-7230 and the cosmetic-grade ROKAnol NL5 surfactant systems could be successfully implemented in the batch (yield = 77% and 80%) and in the continuous (yield = 96% and 100%) cloud point extraction of cinnamic acid.