Molecular Differences between Hydrocarbon and Fluorocarbon Surfactants at the CO₂/Water Interface

Abstract

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.

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

“…For decades, extensive studies were devoted to define the effect of the surfactant tail architecture [13,15,24,50,51]. However, only few works have alluded to the effects of hydrophilic-headgroup modifications on surfactant performance in CO2 [36,38,39].…”

“…Much effort has been devoted to determine how changes in surfactant molecular structure can affect physicochemical properties of aqueous surfactant solutions and w/c phase stability [13,[18][19][20][21]. A traditional concept of molecular packing parameter (p) [22] has been widely invoked in colloid studies to explain the correlation between the molecular structure and the surfactant self-assembly.…”

Properties of surfactant films in water-in-CO2 microemulsions obtained by small-angle neutron scattering

“…29 On the other hand, for normal hydrocarbon AOT-stabilized microemulsions in a range of alkane solvents 30 , a mean value of 72 Å 2 has been reported for ℎ . The smaller ℎ value of hydrocarbon (H-carbon) surfactants at w/o interface suggests a higher packing density compared with the fluorocarbon (F-carbon) surfactants at w/c interface, which can be rationalized as an enhanced packing efficiency of F-carbon chains at the w/c interface 40,41 .…”

Modeling Solvation in Supercritical CO2

“…[154] The structuralf eatures of surfactants at the water-CO 2 interface were investigated by Rossky's group, who showedt hat CO 2 can solvate the fluorinated tails as wellasthe hydrocarbon tails, leading to an ew orientationalo rdering that allows increasedw ater accessibility of the hydrophobic part. [155,156] Aj oint experimental and computational study by Berkovitz et al characterized am icroemulsion in as ystem containing a fluorinated surfactant, water and scCO 2 . [157] More insights on micelles were provided by the same group in the following years.…”