Graft-modified cellulose nanocrystals as CO2-switchable Pickering emulsifiers†
We report the use of a bio-sourced Pickering emulsifier based on cellulose nanocrystals (CNCs) grafted with less than 25 wt% of synthetic, CO2-responsive polymer for the reversible emulsification/demulsification of oil and water. CO2-responsive CNCs were previously prepared by grafting poly(N-3-(dimethylamino) propyl methacrylamide) (PDMAPMAm) and poly(N,N-(diethylamino)ethyl methacrylate) (PDEAEMA) to the CNC surface using nitroxide-mediated polymerisation (NMP) in water as a benign solvent. The surface and interfacial properties of the graft-modified CNC were characterised by surface and interfacial tensiometer measurements, and the resulting droplets obtained from emulsions were visualised using optical microscopy. The emulsification/demulsification process was found to be responsive to N2 and CO2, reversible and easily repeatable. The Pickering emulsifiers could be recovered after application, which would make this technology particularly interesting for oil–water separation and enhanced oil recovery. The pKaH of the grafted polymer and the polarity were correlated to the CO2 sparging time required to break the emulsion and to the emulsion stability (lifetime). The effects of the chain length of the grafted polymer, graft densities, total amount of CO2-switchable groups on the CNC surface, and concentration of Pickering stabiliser on the properties of emulsions were investigated.