Thermodynamics of interactions between cellulose nanocrystals and monovalent counterions.

Alkali and quaternary ammonium cations interact with negatively charged cellulose nanocrystals (CNCs) bearing sulfated or carboxylated functional groups. As these are some of the most commonly occurring cations CNC encounter in applications, the thermodynamic parameters of these CNC-counterion interactions were evaluated with isothermal titration calorimetry (ITC). Whereas the adsorption of monovalent counterions onto CNCs was thermodynamically favourable at all evaluated conditions as indicated by a negative Gibbs free energy, the enthalpic and entropic contributions to the CNC-ion interactions were found to be strongly dependent on the hydration characteristics of the counterion and could be correlated with the potential barrier to water exchange of the respective ions. The adsorption of chaotropic cations onto the surface was exothermic, while the interactions with kosmotropic cations were endothermic and completely entropy-driven. The interactions of CNCs with more bulky quaternary ammonium counterions were more complex, and the mechanism of interaction shifted from electrostatic interactions with surface charged groups of CNCs towards adsorption of alkyl chains onto the CNC hydrophobic planes when the alkyl chain length increased.

Dimensions of Cellulose Nanocrystals from Cotton and Bacterial Cellulose: Comparison of Microscopy and Scattering Techniques.

Different microscopy and scattering methods used in the literature to determine the dimensions of cellulose nanocrystals derived from cotton and bacterial cellulose were compared to investigate potential bias and discrepancies. Atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), depolarized dynamic light scattering (DDLS), and static light scattering (SLS) were compared. The lengths, widths, and heights of the particles and their respective distributions were determined by AFM. In agreement with previous work, the CNCs were found to have a ribbon-like shape, regardless of the source of cellulose or the surface functional groups. Tip broadening and agglomeration of the particles during deposition cause AFM-derived lateral dimensions to be systematically larger those obtained from SAXS measurements. The radius of gyration determined by SLS showed a good correlation with the dimensions obtained by AFM. The hydrodynamic lateral dimensions determined by DDLS were found to have the same magnitude as either the width or height obtained from the other techniques; however, the precision of DDLS was limited due to the mismatch between the cylindrical model and the actual shape of the CNCs, and to constraints in the fitting procedure. Therefore, the combination of AFM and SAXS, or microscopy and small-angle scattering, is recommended for the most accurate determination of CNC dimensions.