Surfactant induced gelation of TEMPO-oxidised cellulose nanofibril dispersions probed using small angle neutron scattering

In this work, we studied TEMPO-oxidised cellulose nanofibrils (OCNF) suspensions in presence of diverse surfactants. Using a combination of small angle neutron scattering (SANS) and rheology, we compared the physical properties of the suspensions with their structural behaviour. Four surfactants were studied, all with the same hydrophobic tail length but different headgroups: hexaethylene glycol mono-n-dodecyl ether (C12EO6, non-ionic), sodium dodecyl sulphate (SDS, anionic), cocamidopropyl betaine (CapB, zwitterionic) and dodecyltrimethylammonium bromide (DTAB, cationic). Contrast variation SANS studies using deuterated version of C12EO6 or SDS, or by varying the D2O/H2O ratio of the suspensions (with CapB), allowed focusing only on the structural properties of OCNF or surfactants micelles. We showed that, in the concentration range studied, C12EO6, although it concentrates the nanofibrils thanks to an excluded volume effect observed in SANS, does not affect the rheological properties of the suspensions. Addition of SDS or CapB induces gelation for surfactant concentrations superior to the critical micellar concentration (CMC). SANS results show that attractive interactions between OCNF arise in the presence of these anionic or zwitterionic surfactants, hinting at depletion-flocculation as the main mechanism of gelation. Finally, addition of small amounts of DTAB (below the CMC) allows formation of a tough gel by adsorbing onto the OCNF surface.