Microemulsions of water in supercritical carbon dioxide: an in-situ NMR investigation of micelle formation and structure

Abstract

High-pressure NMR spectroscopy was used for the first time to investigate microemulsions of water in supercritical carbon dioxide. The emulsions were formed using a family of anionic perfluoropolyether ammonium carboxylate surfactants. This system holds promise as a reaction medium for conducting homogeneous catalytic reactions within the aqueous micellular cores while, at the same time, exploiting the facile mass transfer properties of the supercritical fluid. Ammonium hexafluorophosphate was used as a water-soluble ionic guest to investigate micelle formation and structure. Under micelle-forming conditions, the PF 6 − guest, surfactant, and water were uniformly dispersed throughout the CO 2 phase, as demonstrated by in situ NMR imaging. In addition, the micelles were observed to form even in the absence of mechanical stirring. This spontaneous formation of micelles demonstrates that the NMR spectral properties were obtained under conditions that result in the production of thermodynamically stable microemulsions. The nuclear overhauser effect (NOE) was used to probe the micellular structure through dipole–dipole interactions between the PF 6 − anion and the fluorinated backbone of the surfactant. A strong negative homonuclear NoE was observed between the PF 6 − guest and the fluorine moiety that is located directly adjacent to the surfactant's carboxylate head group. This highly specific negative NOE indicates an ordered arrangement, where the PF 6 − anion and carboxylate ion are located in close proximity to one another. This close association of two negatively charged ionic groups in an aqueous environment is unusual and suggests that the PF 6 − guest is concentrated within the electric double layer that forms at the micellular interface.