Interfacial properties of fluorocarbon and hydrocarbon phosphate surfactants at the water-CO2 interface

high-pressure pendant-drop tensiometry, the interfacial tension (gamma) and surface excess (Gamma(infinity)) 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. gamma 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 degreesC and 340 bar to 3.7 mN/m. This reduction in gamma 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 gamma and Gamma(infinity) 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.