No abstract

Over the past decade, steric stabilization has been achieved for a variety of inorganic and organic colloids in supercritical fluid carbon dioxide (scCO2). Herein we demonstrate that colloids may also be stabilized in CO2 by electrostatic forces, despite the ultralow dielectric constant of 1.5. Zeta potentials of micrometer-sized water droplets, measured in a microelectrophoresis cell, reached -70 mV corresponding to a few elementary charges per square micrometer of droplet surface. This degree of charge was sufficient to stabilize water/CO2 emulsions for an hour, even with water volume fractions of 5%. Hydrogen ions partition preferentially, relative to bicarbonate ions, from the emulsion droplets to the cores of surfactant micelles in the diffuse double layer surrounding the droplets. The micelles, formed with a low molecular weight branched hydrocarbon surfactant, prevent ion pairing of the hydrogen counterions to the negatively charged emulsion droplets. Dielectrophoresis of the water droplets at a frequency of 60 Hz leads to chains containing a dozen droplets with lengths of 50 mum. The ability to form electrostatically stabilized colloids in carbon dioxide is particularly useful in practical applications, because steric stabilization in CO2 is often limited by the poor solvation of the stabilizers.

With high-pressure pendant-drop tensiometry, the interfacial tension (γ) and surface excess (Γ∞) 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. γ 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 γ 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 γ and Γ∞ 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.

Tertiary amine esters, a new class of surfactants for CO2-based dispersions, stabilize carbon dioxide in water macroemulsions for several hours even at a CO2 density as low as 0.74 g/mL (70 bar) at 298 K. The combination of a weakly hydrophilic tertiary amine, which is protonated by carbonic acid, and branched ester tails provides proper values of the hydrophilic−CO2-philic balance (HCB) for emulsion stabilization. The surfactant nitrilotripropane-1,2-diyl tripivalate (tBu-TIA) lowered the CO2−water interfacial tension to 2.6 mN/m as a result of the stubby architecture (low aspect ratio) of the surfactant tail, which helps block contact between water and carbon dioxide. The high level of methylation produces a smaller interfacial tension and greater emulsion stability relative to nitrilotripropane-1,2-diyl triacetate (TIA). Relative to the high-pressure CO2−water system with a pH 3.3, an increase in pH with the addition of NaOH decreases interfacial activity and reduces emulsion stability, as the surfactant is deprotonated. The adsorption isotherm shows a high interfacial area per surfactant molecule (400 Å2) as a result of the stubby structure of the surfactant. The extremely low aspect ratio of this surfactant compared to other hydrocarbon surfactants shields water from CO2 at the interface, resulting in a lower interfacial tension, and minimizes interactions between surfactant tail groups. These factors make these low-molecular-weight amine esters desirable for tunable CO2-in-water emulsions, as a replacement for more widely used fluorinated surfactants. The facile synthesis of a variety of tertiary amine esters makes this class of surfactants attractive for developing structure−property relationships.