Chemical exchange in unstable emulsions

Abstract

Nuclear magnetic resonance (NMR) is a routine method to study chemical exchange in reactions and molecular rearrangements in solution. However, when it comes to exchange of molecular species in liquid-liquid, two phase systems like in phase-transfer catalysis, the rate becomes a function of the surface area between two phases, which means that only persistent emulsions could be studied with standard equipment. Unstable emulsions, which rapidly demix, require a continuous application of shear forces by stirring. Here, a setup is described with which unstable emulsions can be produced and studied in-situ by solution NMR spectroscopy. The setup provides sufficient torque and spinning frequency for generating an unstable two-phase water/oil mixture by rapid stirring. The pneumatically driven stirrer in the probe head was designed using ideas borrowed from magic angle sample spinning and a prototype was produced by 3D printing. As proof of concept, the dynamics in an aniline water emulsion over the phase boundary are studied by regular exchange spectroscopy NMR experiments.