A liquid CO2-compatible hydrocarbon surfactant: experiment and modelling

Surfactants soluble in liquid CO2 are rare and knowledge on interfacial and self-assembly behaviour is fragmented. We found that polyoxyethylene (5) isooctylphenyl ether is interfacially active at the water–liquid CO2 interface. Water–liquid CO2 interfacial tension was measured at various surfactant concentrations at 50 bar and 283 K using the pendant drop method, and a CMC like cusp was observed at a surfactant concentration of ∼50 mM in the bulk liquid CO2. This system was modelled applying the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We use a free-volume approach, wherein the chemical potential of the vacancies was linked to the pressure and the molecules were described using a freely-jointed chain model on a united atom level. The model indicates that typically the water–vapour interface is wet by CO2. Interestingly, a window of partial wetting was identified at the water–vapour interface as a function of the chemical potential of the surfactant. The second-order nature of both wetting transitions is attributed to the close proximity to the critical point of the CO2–vapour system. Furthermore, the SF-SCF theory was used to study the self-assembly of the surfactant in bulk CO2 or water, focusing on the three-phase coexistence, that is at P/Psat = 1. Above ∼40 mM in the CO2-rich phase, the theory indicates stable water swollen reverse micelles with an aggregation number of ∼100. The analysis further shows the stability of compressible CO2-swollen surfactant bilayers in the bulk water phase at elevated surfactant concentrations. Finally it was found that the critical reverse micellar concentration (in liquid CO2) increases and the aggregation number decreases with increasing pressure.