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.

Thermodynamics of micellization of nonionic surfactants - The effect of incorporating CO2 moieties into the head group.

HYPOTHESIS: The micellization behavior of nonionic surfactants is significantly influenced by substituting ethylene oxide (EO) units with CO2 in the head group of nonionic C12EOj surfactants. Incorporating hydrophobic units has a major effect on the driving forces of the micellization process by a reduced hydration affinity. Hence, the incorporation of CO2 moieties is favoring micellization and thereby adding a further tuning parameter. EXPERIMENTS: The micellization of the surfactants was characterized in terms of thermodynamics and the assembly properties on the water/air interface by isothermal titration calorimetry (ITC) and surface tension measurements. The incorporation of CO2 moieties was compared to the effect of propylene oxide (PO) and propiolactone (PL) over the temperature range of 25-50 °C. From ITC measurements and the van't Hoff relation, we determined the thermodynamic parameters of micellization: enthalpy (ΔHmic), entropy (ΔSmic), and Gibbs free energy (ΔGmic). FINDINGS: The incorporation of CO2 moieties reduces the critical micellization concentration (cmc) and a transfer energy of -0.36 kT/CO2 unit quantifies favored micellization for CO2 surfactants. The presence of PO or CO2 in the head group has a similar effect on the cmc, but for CO2 ΔHmic is substantially decreased, resulting in a largely reduced temperature sensitivity of the micellization process and indicating a reduced hydration affinity. This thermodynamic analysis reveals that CO2 and PO behave very differently concerning their effect on the micellization process.

Changes in Phase Behavior from the Substitution of Ethylene Oxide with Carbon Dioxide in the Head Group of Nonionic Surfactants.

Nonionic ethylene oxide (EO)-based surfactants are widely employed in commercial applications and normally form gel-like liquid crystalline phases at higher concentrations, rendering their handling under such conditions difficult. By incorporating CO2 units in their hydrophilic head groups, the consumption of the petrochemical EO was reduced, and the tendency to form liquid crystals was suppressed completely. This surprising behavior was characterized by rheology and studied with respect to its structural origin by means of small-angle neutron scattering (SANS). These experiments showed a strongly reduced repulsive interaction between the micellar aggregates, attributed to a reduced hydration and enhanced interpenetration of the head groups owing to the presence of the CO2 units. In addition, with increasing CO2 content the surfactants became more efficient and effective with respect to their surface activity. These findings are important because the renewable resource CO2 is used, and the CO2 -containing surfactants allow handling at very high concentrations, an aspect of enormous practical importance.