Somia Haouache1,2,#, Yu Chen3,#, Clara Jimenez1, Fabrice Cousin4, Pan Chen3, Yoshiharu Nishiyama5, François Jerome2, Isabelle Capron1
INRAE, UR BIA, F-44316, Nantes, France
ICMMP, Université de Poitiers-CNRS, Poitiers, France
Beijing Engineering Research Centre of Cellulose and Its Derivatives, School of Materials Science and Engineering, Beijing Institute of Technology, 100081, Beijing, P.R. China
Laboratoire Léon Brillouin, Université Paris-Saclay, CEA-CNRS, CEA-Saclay, Gif-sur-Yvette, France
CERMAV, University Grenoble Alpes, CNRS, F-38000 Grenoble, France.
# contributed equally
Cellulose is an abundant natural renewable resource that accounts for more than half of the earth’s biomass. It is being developed for a wide variety of applications due to the unique combination of sustainability, scalability, low density, and chemical and mechanical stabilities. One of elementary brick that is largely used to design cellulose-based materials with targeted properties are cellulose nanocrystals (CNCs), obtained from hydrolysis of cellulose fibers. CNCs are rod-like parallelepiped nanoparticles made of elementary crystallites of cellulose (figure 1). Despite being considered hydrophilic, these CNCs adsorb efficiently at oil−water interfaces and enable to produce highly stable emulsions armored by a layer of solid particles, which are called Pickering emulsions (Figure 2). These CNC-stabilized emulsions have raised a strong interest in these last 10 years for further applications but the exact mechanism of their formation remains to be elucidated.
In this paper, we have compared the organization of two crystalline kinds of CNCs once adsorbed on the surface of hexadecane droplets dispersed in water at different CNC concentrations [1]. CNC-I are native CNC, and CNC-II are so-called mercerized CNC, named after the famous mercerization process invented by John Mercer in 19th century. CNC-II have been obtained from cellulose modification in a very alkaline medium and have a different crystallographic structure and size than CNC-I.
Figure 1. Principle of synthesis of CNC I and CNC II from cotton cellulose fibers. The dimensions of both types of CNC have been obtained on CNC aqueous solutions by SANS [2].
In order to depict the mechanisms of adsorption, the thickness of the CNC layer was obtained by Small Angle Neutron Scattering (SANS) experiments thanks to a contrast variation trick that enables to determine the thickness of the adsorbed layer in a very robust way without any fitting. The scattering length of oily and aqueous phases have been tuned by deuterated/hydrogenated mixtures, so that they are exactly similar and only the CNC-adsorbed layer contributes to the scattering. Since the radius of emulsion droplets are micrometric, such layer appears as a disc of radius R and thickness h within the q-range probed by SANS, and the thickness can be extracted from the slope of the scattering curve in a ln(I(q)q2) = f(q2) representation, where I the scattered intensity and q the scattering vector.
Figure 2. CNC-stabilized Pickering emulsions. (a) CNC I. Top panel: SEM picture of the emulsions at two different magnifications. Lower panel: schematic structure of emulsion. CNC are in green, oil in orange, and water in blue. The inset shows a snapshot from MD simulations. (b) CNC II. Top panel. SEM picture of the emulsion at two different magnifications. Lower panel: schematic structure of emulsion. CNC are in green, oil in orange, and water in blue. The inset shows a snapshot from MD simulations.
In case of CNC-I, the layer thickness formed is independent of the cellulose concentration introduced in solution and equals 7 nm, which corresponds almost to the width of a CNC-I crystallite. This shows that CNC-I adsorbs flat on the surface with a “Face-On” process and that further addition of cellulose only induces densification of the layer (Figure 2.a). On the contrary, CNC-II forms a thicker layer that increases from 9 to 14 nm thick with increasing concentration, although the width of a CNC-II crystallite is 3.5 nm only. CNC-II adsorb thus with “Edge-on” process and may partially stand up upon further addition of cellulose (figure 2.b).
In order to get a better insight on the origin of these two different behaviors, molecular dynamics simulations (MD) were driven. It showed that both CNCs, starting dispersed in water, adsorb at the oil/water interface with preferred interacting planes (100 for CNC-I and 010 for CNC-II) that are considered hydrophobic since they expose their CH groups. In both cases, such planes are maintained adsorbed all along the simulation.
In summary, this study suggests that whatever the allomorph, the migration of CNCs to the oil−water interface is spontaneous and irreversible, and is driven by both enthalpic and entropic processes leading to highly stable emulsions.
[1] S. Haouache, Y. Chen, C. Jimenez-Saelices, F. Cousin, P. Chen, Y. Nishiyama, F. Jerome, I. Capron, Edge-on (cellulose II) and face-on (cellulose I) adsorption of cellulose nanocrystals at the oil/water interface; a combined entropic and enthalpic process, Biomacromolecules, 2022, 23(9), 3517–3524.
[2] S. Haouache, C. Jimenez-Saelices, F. Cousin, X. Falourd, B. Pontoire, K. Cahier, F. Jérome, I. Capron, Cellulose Nanocrystals from native and mercerized cotton, Cellulose, 2022, 29, 1567–1581.
