CO₂ induced phase transition on a self-standing droplet studied by X-ray scattering and magnetic resonance

Acoustic levitation is a suitable approach for studying processes occurring at the gas-liquid interfaces, as it allows its investigation in a contact-free manner while providing control over the gas phase. Here, we hypothesize that phase transitions induced by a CO₂ rich atmosphere can be examined, at different length scales, in a contact-free manner.

A system consisting of 12-hydroxysteric acid (HSA) soaps mixed with different ratios of monoethanolamine (MEA) and choline hydroxide, was prepared. Microliter droplets of the samples were acoustically levitated and monitored with a camera, while exposed to CO₂ to modify the pH through diffusion at the air-liquid interface and inside the droplet. The phase transition and water mobility in the levitated droplets were evaluated through X-ray scattering (SAXS/WAXS) and magnetic resonance studies, in real-time. Finally, the droplets were collected and examined under the microscope.

The introduction of CO₂ gas induced a phase transition from micelles to multi-lamellar tubes, resulting in a gel-like behavior both in the bulk and at the interface. The high stability of the acoustic levitator allowed the investigation of this dynamic phenomenon, in real-time, in a contact-free environment. This study showcases the suitability of acoustic levitation as a tool to investigate complex chemical processes at interfaces.