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.