Effect of surfactants on the interfacial tension and emulsion formation between water and carbon dioxide

The lowering of the interfacial tension (gamma) between water and carbon dioxide by various classes of surfactants is reported and used to interpret complementary measurements of the capacity, stability, and average drop size of water-in-CO2 emulsions. gamma is lowered from similar to 20 to similar to 2 mN/m for the best poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide) (PPO-b-PEO-b-PPO) and PEO-b-PPO-b-PEO Pluronic triblock copolymers, 1.4 mN/m for a poly(butylene oxide)-b-PEO copolymer, 0.8 mN/m for a perfluoropolyether (PFPE) ammonium carboxylate and 0.2 mN/m for PDMS24-g-EO22. The hydrophilic-CO2-philic balance (HCB) of the triblock Pluronic and PDMS-g-PEO-PPO surfactants is characterized by the CO2-to-water distribution coefficient and "V-shaped" plots of log gamma vs wt % EO. A minimum in gamma is observed for the optimum HCB. As the CO2-philicity of the surfactant tail is increased, the molecular weight of the hydrophilic segment increases for an optimum HCB. The stronger interactions on both sides of the interface lead to a lower gamma. Consequently, more water was emulsified for the PDMS-based copolymers than either the PPO-or PBO-based copolymers.