A liquid CO2-compatible hydrocarbon surfactant: experiment and modelling

Abstract

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/P_sat = 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.