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Interactions between CO2 responsive switchable emulsion droplets determined by using optical tweezers.

CO2 responsive switchable emulsions have been of great interest on some industrial process where the stability of emulsion is only required temporarily, such as oil transport, drug delivery, and fossil fuel production. The well understanding of the stability and instability mechanism is vital to the switchable behaviour between emulsification and demulsification. Herein, a novel approach was developed to determine the interactions between two switchable emulsion droplets directly by dual-laser optical tweezers instrument. The repulsive force between a couple of tetradecane droplets occurs to increase progressively with the increasing concentration of switchable surfactant in solutions. However, the repulsive force appears to decrease progressively in turn when the switchable surfactant concentration is far higher than critical micelle concentration (CMC). Moreover, the depletion effect starts to emerge in the higher surfactant concentration which is attributed to the switchable surfactant micelles generated in solutions. In addition, according to the measurements of interaction forces, a mechanism of the switchable behavior is well proposed, which is established by the principle of self-assembly / detachment of the switchable surfactant, resulting in the weakening and re-enhancing of the electrostatic double-layer (EDL) repulsive forces between tetradecane droplets, upon selective introduction and removal of CO2. Based on this work, a novel perspective was provided to study the switchable emulsion, which can contribute instructive messages for the understanding of stability and instability mechanisms of switchable emulsion.