Dual CO2- and thermo-responsive block copolymers as switchable emulsifiers

Emulsions, in which one liquid phase is dispersed as droplets within another immiscible liquid phase, are widely employed in the pharmaceutical, food, cosmetic, and separation industries. A central challenge is to achieve an optimal balance between long-term stability and on-demand destabilization. Such control requires a certain degree of tunability, for which CO2 and thermoresponsive polymers represent a promising strategy. In this work, two families of amphiphilic block copolymers were synthesized with the aim of emulsion stabilization. Both contain a poly((N,N-diethylamino)ethyl acrylamide) (PD) block bearing tertiary amine groups. In its unprotonated form, PD is thermoresponsive, whereas CO2 bubbling induces protonation of the amine groups and suppresses its thermoresponsive behavior. Subsequent bubbling with an inert gas partially restores thermoresponsiveness, depending on the residual degree of protonation.The CO2-responsiveness of these polymers was first investigated in aqueous solution. The reversible protonation of tertiary amine groups enables modulation of their thermal behavior and aggregation properties. Changes in the protonation state also modify the surface tension at both the air–water and oil–water interfaces. Finally, emulsions stabilized by these polymers were prepared, and their formation and long-term stability were evaluated. While the PD homopolymer exhibits limited emulsifying ability, the amphiphilic block copolymers enable the formation of emulsions with enhanced long-term stability, which can be tuned in conjunction with temperature.