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.