Structural Characterization of Nonionic Surfactant Micelles with CO2/Ethylene Oxide Head Groups and Their Temperature Dependence

Using CO2 as a resource in the production of materials is a viable alternative to conventional, petroleum-based raw materials and therefore offers great potential for more sustainable chemistry. This study presents a detailed structural characterization of aggregates of nonionic dodecyl surfactants with different amounts of CO2 substituting ethylene oxide (EO) in the head group. The micellar structure was characterized as a function of concentration and temperature by dynamic and static light scattering and, in further detail, by small-angle neutron scattering (SANS). The influence of the CO2 unit in the hydrophilic EO group is systematically compared to the incorporation of propylene oxide (PO) and propiolactone (PL). The surfactants with carbonate groups in their head groups form ellipsoidal micelles in an aqueous solution similar to conventional nonionic surfactants, becoming bigger with increasing CO2 content. In contrast, the incorporation of PO units hardly alters the behavior, while the incorporation of a PL unit has an effect comparable to the CO2 unit. The analysis of the SANS data shows decreasing hydration with increasing CO2 and PL content. By increasing the temperature, a typical sphere-rod transition is observed, where CO2 surfactants show a much higher elongation with increasing temperature, which is correlated with the reduced cloud point and a lower extent of head group hydration. Our findings demonstrate that CO2-containing surface-active compounds are an interesting, potentially "greener" alternative to conventional nonionic surfactants.