Thermodynamic Study of Ion-Driven Aggregation of Cellulose Nanocrystals

Abstract

The thermodynamics of interactions between cations of the second group of the periodic table and differently negatively charged cellulose nanocrystals was investigated using isothermal titration calorimetry (ITC). The interaction of cations with the negatively charged CNCs was found to be endothermic and driven by an increase in entropy upon adsorption of the ions, due to an increase in degrees of freedom gained by the surface bound water upon ion adsorption. The effect was pH-dependent, showing an increase in enthalpy for cellulose suspensions at near-neutral pH (6.5) when compared to acidic pH (2). Sulfated cellulose nanoparticles were found to readily interact with divalent ions at both pH levels. The adsorption on carboxylate nanocrystals was found to be pH dependent, showing that the carboxylic group needs to be in the deprotonated form to interact with divalent ions. For the combined system (sulfate and carboxylate present at the same time), at neutral pH, the adsorption enthalpy was higher than the value obtained from cellulose nanocrystals containing a single functional group, while the association constant was higher due to an increased favorable entropic contribution. The higher entropic contribution indicates a more restricted surface-bound water layer when multiple functionalities are present. The stoichiometric number n was nearly constant for all systems, showing that the adsorption depends almost completely on the ion valency and on the amount of ionic groups on the CNC surface, independent of the type of functional group on the CNC surface as long as it is deprotonated. In addition, we showed that the reduction in Gibbs free energy drives the ionotropic gelation of nanocellulose suspensions, and we show that ITC is able to detect gel formation at the same time as determining the critical association concentration.

A novel supra coarse-grained model for cellulose

“…The interactions and self-assembly of CNC or CNF have been subject of numerous experimental studies (Kontturi et al 2018;Lagerwall et al 2014;Ding et al 2014;Orts et al 1998;Benselfelt et al 2019;Lombardo et al 2019;Honorato-Rios et al 2018). Depending on the experimental conditions, surface modifications, additives, etc.…”

“…Depending on the experimental conditions, surface modifications, additives, etc. CNC and CNF can assemble into variety of different configurations (Kontturi et al 2018;Lagerwall et al 2014;Ding et al 2014;Orts et al 1998;Benselfelt et al 2019;Lombardo et al 2019;Honorato-Rios et al 2018). CNC can also self-assemble into chiral nematic phases which possess interesting photonic properties and may have applications as optical material (Schütz et al 2015;Surov et al 2017;Liu et al 2019Liu et al , 2018Pospisil et al 2018;Bertsch et al 2019;Parker et al 2016;Yao et al 2017).…”

Engineered Multilayer Microcapsules Based on Polysaccharides Nanomaterials

“…Other groups have prepared alginate-CNC systems for removal of organic and inorganic pollutants from water by encapsulation, which take the advantage of the easy separation method of the solid capsules [ 126 , 127 , 128 ]. The incorporation of carboxylated cellulose nanocrystals can be particularly interesting for such application, as interactions with cationic species are pH dependent and can be easily regenerated by lowering the pH [ 129 ]. Taken together, the microcapsules prepared combining alginate with polysaccharide nanomaterials showed a similar structure, with a large diameter of several hundred µm, and a porous morphology, as shown in Figure 2 .…”

Host–Guest Chemistry Within Cellulose Nanocrystal Gel Receptors

“…These colloids can be disturbed by the addition of inorganic or organic salts, which modifies the ionic strength of the medium and causes gel formation (Figure a). This process is spontaneous and entropically driven, and has been used by several groups to assemble cellulose‐based hydrogels . In fact, there is considerable interest in CNC‐based gels as they are attractive materials for separation/purification purposes, drug‐delivery, sensing, thermal insulation, tissue engineering, and 3D printing …”