Surfactant Induced Gelation of TEMPO-Oxidized Cellulose Nanofibril Dispersions Probed Using Small Angle Neutron Scattering

biodegradable

Cellulose, as the most abundant natural polymer, is extensively studied in the formulation of new biodegradable materials, as it is biocompatible and environmentally friendly [1].It can be isolated from various sources (wood pulp, bacteria, tunicates…) yielding variously shaped particles.Chemical modification allows dispersion of individualised fibrils.Specifically, oxidised cellulose nanofibrils (OCNF), obtained by TEMPO-mediated oxidation of plant or bacterial cellulose [2], present a high negative surface charge making them easy to disperse in water [3].
We have studied the formulation of hydrogels using OCNF, pinpointing the effect of concentration or additives (e.g.salt, surfactants) to the suspension, by comparing the structural properties monitored using small angle scattering and the rheological properties of the samples [4].Among these additives, surfactants are extensively used in personal and home care formulations.Understanding their influence on the OCNF hydrogel rheological and structural properties is hence crucial to tailor the system to the formulation of personal-care products.For example, charged surfactants are expected to strongly interact with the negatively-charged nanofibrils via electrostatic interactions.We hence studied the addition of 4 types of surfactants, all bearing the same hydrophobic tail but different headgroups: hexaethylene glycol mono-n-dodecyl ether (C 12 EO 6 , non-ionic), sodium dodecyl sulfate (SDS, anionic), cocamidopropyl betaine (CapB, zwitterionic), and dodecyltrimethylammonium bromide (DTAB, cationic).
The rheological properties of the mixtures of OCNF 1 wt% and surfactant at different molar concentration were probed.It was observed that the addition of C 12 EO 6 up to 50 mM had no effect of the rheological properties of the suspension, which behaved as a shear-thinning fluid.Alternatively, SDS and CapB addition (≥20 mM, so above the critical micelle concentration (CMC) for both surfactants) resulted in the gelation of the system.Both the viscosity and the storage modulus of the gel increased with the concentration of surfactant up to the highest concentration probed (80 mM).Finally, for CTAB, a strong gelation of the OCNF suspension is observed at 5 mM already (below the CMC of CTAB).For concentrations above the CMC, the sample is unstable and a phase-separation is seen.
To probe the gelation mechanisms, we carried out small angle neutron scattering (SANS) measurements at Sans2D at the ISIS Neutron and Muon Source.Using deuterated version of C 12 EO 6 or SDS, or by varying the D 2 O/H 2 O ratio of the suspensions (with CapB), we did contrast variation studies on the mixtures, allowing us to focus on the structural properties of OCNF or surfactant © 2023 The Author(s).Published with license by Taylor & Francis Group, LLC.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.
Science Snapshot micelles only.For example, Figure 1 shows the SANS patterns of SDS suspensions alone (labelled pure SDS) and OCNF/SDS mixtures at different surfactant concentrations and different contrast (using hydrogenated or deuterated version of SDS, labelled h-SDS and d-SDS respectively): d-SDS/D 2 O was used to focus only on OCNF in the mixture, d-SDS/50%D 2 O to focus on the micelles and h-SDS/D 2 O and d-SDS/H 2 O to probe both micelles and OCNF.Data were fitted using a model of interacting spheres, via a Yukawa potential, for SDS micelles and the PRISM model for interacting rods for OCNF.For mixtures, the sum of the two contributions was used, adjusting the structure factor for both micelles and nanofibrils, but neglecting any interaction between both objects.Results indicated that OCNF experience strong attraction when SDS micelles are present, while the overall repulsion between micelles is decreased.
These results suggest that micelles act as a depletant for OCNF, triggering fibril-fibril attraction and the overall gelation of the suspension.
A similar behaviour was observed with CapB, which also forms spherical micelles in solution.On the other hand, for C 12 EO 6 in the concentration range studied, SANS results suggest that if non-ionic micelles induce crowding of OCNF in the water phase due to an excluded volume effect, they also act as a steric barrier between nanofibrils, preventing gelation.Contrary to SDS and CapB, C 12 EO 6 micelles change shape with concentration, going from quasi-spherical to elongated micelles, which might prevent their use as depletants.Finally, addition of small amounts of DTAB (below the CMC) drastically influences the SANS pattern and the fibril-fibril aggregation.This suggests that CTAB allows the formation of a tough gel by adsorbing onto the OCNF surface, reducing their overall

Science Snapshot
negative charge and increasing their hydrophobicity causing them to aggregate and thus gel.
This study was published in the journal of Chemical Physics (doi: 10.1063/5.0129276)[5].

Figure 1 .
Figure 1.SANS patterns of pure SDS micelles in D 2 O compared with mixtures of SDS at different contrasts with OCNF (1 wt%).Measurements were made at SDS concentrations of (a) 20 mM, (b) 40 mM, (c) 60 mM and (d) 80 mM.The fits (in black) were made using the model of charged spherical micelles for pure SDS and sum of attractive nanorods and charged spherical micelles for mixtures.Reproduced from J. Schmitt, V. Calabrese, M. A. da Silva, K. M. Z. Hossain, P. Li, N. Mahmoudi, R. M. Dalgliesh, A. L. Washington, J. L. Scott and K. J. Edler, J. Chem.Phys., 158, 034901 (2022); licensed under a Creative Commons Attribution (CC BY) license.