Tertiary Amine Esters for Carbon Dioxide Based Emulsions

Abstract

Tertiary amine esters, a new class of surfactants for CO2-based dispersions, stabilize carbon dioxide in water macroemulsions for several hours even at a CO2 density as low as 0.74 g/mL (70 bar) at 298 K. The combination of a weakly hydrophilic tertiary amine, which is protonated by carbonic acid, and branched ester tails provides proper values of the hydrophilic−CO2-philic balance (HCB) for emulsion stabilization. The surfactant nitrilotripropane-1,2-diyl tripivalate (tBu-TIA) lowered the CO2−water interfacial tension to 2.6 mN/m as a result of the stubby architecture (low aspect ratio) of the surfactant tail, which helps block contact between water and carbon dioxide. The high level of methylation produces a smaller interfacial tension and greater emulsion stability relative to nitrilotripropane-1,2-diyl triacetate (TIA). Relative to the high-pressure CO2−water system with a pH 3.3, an increase in pH with the addition of NaOH decreases interfacial activity and reduces emulsion stability, as the surfactant is deprotonated. The adsorption isotherm shows a high interfacial area per surfactant molecule (400 Å2) as a result of the stubby structure of the surfactant. The extremely low aspect ratio of this surfactant compared to other hydrocarbon surfactants shields water from CO2 at the interface, resulting in a lower interfacial tension, and minimizes interactions between surfactant tail groups. These factors make these low-molecular-weight amine esters desirable for tunable CO2-in-water emulsions, as a replacement for more widely used fluorinated surfactants. The facile synthesis of a variety of tertiary amine esters makes this class of surfactants attractive for developing structure−property relationships.