Novel non-ionic surfactants synthesised through the reaction of CO2 with long alkyl chain epoxides

Abstract Two long alkyl chain cyclic carbonates, 1,2-hexadecene carbonate (HDC) and 1,2-dodecene carbonate (DDC), were synthesised via a green catalytic approach from CO2 and the corresponding epoxide, achieving nearly complete conversion with full selectivity. After purification, the two cyclic carbonate products were studied for application as non-ionic surfactants through different tests, including interfacial tension, emulsion stability and droplet size measurements. HDC demonstrated to be suitable for application as a non-ionic surfactant, as it was able to reduce the interfacial tension between water and hexane in an inverse emulsion (i.e. water-in-oil, w/o) at different HDC concentrations (0.5–2.5 wt.%). HDC allowed reaching a similar surface activity compared to a benchmark surfactant as sodium dodecyl sulphate (SDS), although the latter for an oil-in-water (o/w) emulsion, in the concentration range from 1.5 to 2.5 wt.%. On the other hand, DDC was not an efficient emulsion stabiliser, yielding only a slight decrease in the interfacial tension when compared to the results obtained with HDC under the same conditions. This underlined the role of the hydrophobicity of the longer alkyl chain in HDC on the performance of these carbonates as surfactants. For HDC, the optimum ratio of water to hexane for preparing an inverse emulsion was 50/50 vol %, which showed the highest colloidal stability at 2.0 % (w/v) concentration of the surfactant. Increasing the HDC concentration from 1.0 to 2.0 % (w/v) resulted in a significant decrease in average droplet size from 170 to 72 nm, in addition to a decrease in the droplet polydispersity of the dispersion. A further decrease in droplet size and a narrowing in the size distribution, leading to nearly monodisperse nanoemulsions, was achieved upon the addition of CaCl2. The combination of the results obtained in this study reveals a new, promising class of sustainable surfactants consisting of long alkyl chain cyclic carbonates synthesised from CO2.