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.
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.
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.
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