pH-Responsive Shape-Changing Molecular Bottlebrush Emulsifier

By utilizing the large size and the tunable amphiphilicity of shape-changing molecular bottlebrushes (MBBs), we show that MBBs with heterografted poly(ethylene oxide) (PEO) and poly(2-(N,N-diethylamino)ethyl methacrylate) (PDEAEMA) side chains (MBB-OE) are an efficient and pH-responsive emulsifier, producing highly stable emulsions that can be disrupted by external stimuli. MBB-OE was wormlike in acidic solution and collapsed into a globular shape with increasing pH due to the insolubility of PDEAEMA in basic water. Upon vortexing, water-in-oil (w/o) emulsions were formed from the mixtures of toluene and a pH 9.5 buffer with mass ratios ≥0.8:1.0, even using only 0.0068% MBB-OE. Vortexing caused the globular brushes to unfold and adsorb at the interface, reconfiguring into a Janus wormlike structure with PEO and PDEAEMA extending into the aqueous and toluene phase, respectively. This is supported by the observations of the partitioning of PEO and PDEAEMA between the two phases and the interfacial wrinkling of emulsion droplets during solvent evaporation. The emulsions were readily disrupted by changing the pH to 4.0 and reformed by vortexing after increasing the pH to 9.5; the emulsion formation and breaking can be repeated multiple times. Interestingly, oil-in-water (o/w) emulsions were formed at the toluene-to-water mass ratio ≤0.6:1.0 and remained stable after the pH was changed to 4.0 by HCl, likely because H+ was unable to cross the interface. However, bubbling with CO2 disrupted the emulsions. This work demonstrates the potential of shape-changing MBBs as stimuli-responsive emulsifiers, opening up opportunities for designing advanced emulsifiers by leveraging the large size, unique characteristics, and responsiveness of MBBs.