The lowering of the interfacial tension (γ) 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. γ is lowered from ∼20 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 γ vs wt % EO. A minimum in γ 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 γ. Consequently, more water was emulsified for the PDMS-based copolymers than either the PPO- or PBO-based copolymers.

Measurements of the interfacial tension γ at the water-CO2 interface with the surfactant perfluoropolyetherCOO-NH4 + (M w = 2500) are utilized to determine the surfactant interfacial area, surface pressure vs area, critical microemulsion concentration, and the thermodynamic properties of microemulsion formation. The measurements were made at equilibrium vs surfactant concentration with a tandem variable-volume pendant drop tensiometer from 25 to 65 °C at a constant CO2 density of 0.842 g·mL-1. The experimental results along with a simple molecular surface equation of state indicate that the area per surfactant at the critical microemulsion concentration is much larger at the water-CO2 vs water−oil interface for two primary reasons. The first reason is that γο (without surfactant) is much smaller for the water−CO2 interface; thus, less surfactant is required to lower γ to a typical value for microemulsions, 1 mN/m. The second reason is the larger entropic contribution to the free energy of the monolayer, due to greater penetration of the small CO2 molecules in the tail region relative to larger oils. The critical microemulsion concentration varies from 0.26 to 1.5 mM for a temperature range of 25−65 °C. Microemulsion formation is driven by the favorable enthalpy of −37.6 kJ/mol.

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Fundamental molecular understanding of surfactants at the CO2/water interface is lacking, especially in the context of the poor performance of hydrocarbon-based surfactants relative to fluorocarbons. We present computer simulations of a dichain fluorinated phosphate surfactant known to promote microemulsion formation and its hydrocarbon analogue which does not. Analysis of the computer simulation results shows that CO2 solvates both tails well. In fact, at equal area per surfactant, CO2 penetrates the hydrocarbon tails somewhat more than the fluorocarbon tails. Water is also found to penetrate the hydrocarbon surfactants to a greater extent than the fluorocarbon ones. This difference in penetration causes an unanticipated orientation of the headgroup in the fluorocarbons that promotes hydration and is absent in the hydrocarbon surfactant case. These results, combined with the structural analysis, lead us to infer that the poor performance of hydrocarbon surfactants is caused by their inability to effectively separate the water and CO2 phases from each other. A geometrically based penetration parameter for surfactants is defined and calculated. This parameter describes the ability of the surfactants to physically separate the bulk phases. The parameter is shown to correlate with interfacial tension. On the basis of this mechanism for surfactant performance, “stubby” hydrocarbon surfactants, which cover more surface area per surfactant, show promise for new surfactant design.

A novel paradigm for the design of surfactants for water/CO 2 (W/C) microemulsions is presented. The paradigm focuses on the fractional free volume (FFV) available to CO 2 at the interface. The FFV is an unambiguous geometric parameter that is calculated directly from surfactant tail geometry and surface coverage. We present an analysis of recent experimental studies indicating that low FFV is a necessary, although not sufficient, condition for W/C microemulsion formation and that both microemulsion and macroemulsion stability correlate qualitatively with FFV. This correlation is understood by noting that a decrease in FFV tends to favor the factors that stabilize W/C microemulsions, namely, decreased interfacial tension, reduced overlap between tails (weakening attractive interdroplet interactions), and increased interfacial curvature. These factors are more challenging to achieve in CO 2 than in alkane solvents, implying that low FFV is especially important for W/C microemulsions.

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The stability of water-in-CO2 (W/C) emulsions stabilized with poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) and PDMS-b-poly(acrylic acid) (PDMS-b-PAA) ionomer surfactants is reported as a function of surfactant architecture, pH, temperature, pressure, and droplet flocculation. For a given PDMS block length, the stability of the emulsion is correlated with the distance from the balanced state where the surfactant prefers the water and CO2 phases equally. When the pH starting at 3 is raised up to 5−6, the hydrophilic/CO2-philic balance of the surfactant increases, because of ionization of COOH, and the emulsion becomes more stable. At the pH of maximum stability, the emulsion becomes more stable with a decrease in the PDMS length, for a given ratio of block lengths, because of gelation of the flocculated 2−5 μm primary droplets. W/C emulsions are stable with respect to sedimentation for >24 h and are resistant to coalescence for more than 7 days. Because of gelation, the W/C emulsions are more stable than water/hexane emulsions (at ambient pressure) formed at the same conditions. The addition of 20% hexane to CO2 as a cosolvent reduced flocculation in some cases to zero.

An important goal is the design of economically viable amphiphiles capable of forming and stabilizing water domains in bulk CO 2 . To enhance the understanding of this class of systems, we present in this work the results from atomistic molecular dynamics computer simulations of a perfluoropolyether ammonium carboxylate surfactant monolayer at the high-pressure CO 2 |water interface. The system is modeled including all internal degrees of freedom for the surfactant anion, and considering all atoms explicitly. At 318 K and 23 MPa, and 84 Å 2 /molecule surface coverage, the calculated decrement in interfacial tension due to the presence of the monolayer agrees with the experimental value previously obtained in our laboratories. We show that the surfactant monolayer retains most of the structural features commonly observed in conventional hydrocarbon amphiphiles at the oil|water interface. However, CO 2 penetrates the surfactant monolayer to a larger extent than conventional hydrocarbon solvents. Correspondingly, CO 2 is also capable of solvating the fluorinated tail-group throughout. The result is a thick, fairly structured monolayer, comparable to analogous hydrocarbon surfactants at the oil|water interface, a surprising observation given the much lower surfactant surface coverage. On the aqueous side, in contrast, the carboxylic carbon is well solvated, but very little contact between water and the tail-group is observed past the CF2 adjacent to the head-group. It is also observed that most of the ammonium counterions are associated with the head-groups.