Interfacial Properties of Fluorocarbon and Hydrocarbon Phosphate Surfactants at the Water-CO₂ Interface

With high-pressure pendant-drop tensiometry, the interfacial tension and surface excess for a family of ionic surfactants with identical phosphate headgroups and varying fluorocarbon and hydrocarbon tail structures were examined at the water-CO₂ interface. To compensate for the unusually weak CO2-surfactant tail interactions, we designed hydrocarbon tails with weak tail-tail interactions to achieve a more favorable hydrophilic-CO₂-philic balance. Branching of hydrocarbon surfactant tails is shown to lead to more favorable adsorption at the interface, closer to that of fluorocarbon surfactants. Interfacial tension for a double-tail hydrocarbon phosphate surfactant with a relatively high degree of tail branching was lowered from the water-CO₂ binary interface value of about 20 mN/m at 25 °C and 340 bar to 3.7 mN/m. This reduction in interfacial tension is attributed to both a decrease in the free volume between tails at the interface and reduced tail-tail interactions. In addition to tail structure, the effects of surfactant counterion, salt concentration, temperature, and CO₂ density on interfacial tension and surface excess were investigated. The hydrophilic-CO₂-philic balances of these surfactants are mapped by investigating changes in interfacial tension with these formulation variables. Low-molecular-weight branched hydrocarbon ionic surfactants are shown to stabilize concentrated CO₂-in-water emulsions for greater than 1 h.