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

Hypothesis

The formation, stability and structural properties of normal liquid phase microemulsions, stabilized by hydrocarbon surfactants, comprising water and hydrocarbon oils can be interpreted in terms of the film bending rigidity (energy) model. Here, this model is tested for unusual water-in-CO₂ (w/c) microemulsions, formed at high pressure with supercritical CO₂ (sc-CO₂) as a solvent and fluorinated surfactants as stabilizers. Hence, it is possible to explore the generality of this model for other types of microemulsions.

Experiments

High Pressure Small-Angle Neutron Scattering (HP-SANS) has been used to study w/c microemulsions, using contrast variation to highlight scattering from the stabilizing fluorinated surfactant films: these data show clear evidence for spherical core–shell structures for the microemulsion droplets.

Findings

The results extend understanding of w/c microemulsions since previous SANS studies are based only on scattering from water core droplets. Here, detailed structural parameters for the surfactant films, such as thickness and film bending energy, have been extracted from the core–shell SANS profiles revealed by controlled contrast variation. Furthermore, at reduced CO₂ densities (∼0.7 g cm⁻³), elongated cylindrical droplet structures have been observed, which are uncommon for CO₂ microemulsions/emulsions. The implications of the presence of cylindrical micelles and droplets for applications of CO₂, and viscosity enhancements are discussed.