Graft-modified cellulose nanocrystals as CO2-switchable Pickering emulsifiers

Abstract

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 pK_aH 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.