Interfacial Properties of Fluorocarbon and Hydrocarbon Phosphate Surfactants at the Water−CO₂ Interface

Abstract

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.

Molecular origins of fluorocarbon hydrophobicity

“…It is also clear from the solvation calculations that a solute may have polar moieties and still be hydrophobic if its partial molar volume is large enough. Correspondingly, one strategy used for surfactants for water-in-CO 2 emulsions, which are important for a number of applications (32), can have the hydrophilic-CO 2 philic balance in their tails adjusted by increasing polar moieties that convey CO 2 -philicity while increasing solvent excluded volume for hydrophobicity (33).…”

Economical and Efficient Hybrid Surfactant with Low Fluorine Content for the Stabilisation of Water-in-CO2 Microemulsions

“…hence, most studies focused on the effects of counterions and headgroup polarity [31][32][33][34]. The most relevant work [15,23,35] In terms of hydrocarbon surfactants, only a few studies have been reported.…”

Synthesis and Evaluation of CO2 Thickeners Designed with Molecular Modeling

“…Research has shown that branched, methylated, and stubby surfactants can be used to form micelles in supercritical CO 2 , because of higher tail solvation and smaller tail-tail interactions [44,45,61,127]. Although some success has been reported in forming fluorine-free microemulsions using hydrocarbon surfactants [46,128] and in making macroemulsions using silica nanoparticles [129], ionic hydrocarbon surfactants [130,131], or trisiloxane surfactants [127] in CO 2 , no reports to date have demonstrated the synthesis or dispersion of nanoparticles within these non-fluorinated surfactant systems in CO 2 . To make full use of the advantages of supercritical CO 2 , a stable dispersion of fluorine-free nanoparticles in a single CO 2 phase would be ideal.…”