Cellulose Nanocrystals Orientation Order Control via Tuning Nanocrystals Coulombic Interactions

Here we present how the ionic strength and effective charge density affect the final structural organization of cellulose nanocrystals (CNCs) after drying suspensions with different ionic strengths in terms of quantitative characteristics of the orientation order. We demonstrated that electrolyte’s introduction into CNCs dispersion results in dramatic changes in electrical double layer thickness and subsequently, the effective charge density, measured experimentally, as well as the interparticle interaction energy calculated from the DLVO theory. The question of whether electrostatic interactions by themselves can control the nanocrystals' chiral organization is frequently debatable. We showed with high-resolution AFM that the reduction of the Debye charge screening length below a critical value of 3 nm results in the loss of the long-range helicoidal order and the transition to a disordered morphology with random packing of nanocrystals within thin films’ layers. Subsequently, very high local orientation ordering with the 2D orientation factor, <i>S</i>, obtained by applying image analysis software that was within 0.8–0.9, gradually decreased to a random order with the final value <i>S</i> = 0.1–0.2. The effect of Coulombic interaction on CNCs chiral assembly is clearly supported by the measured shrinking of the helical pitch length and shifting the photonic band gap from 400 to 250 nm, and increase in helical twisting power from 4 to 6 μm^–1 and doubling of the twisting angle between neighboring monolayers from 5.5 to 9°. Thus, we conducted quantitative characterization of the CNCs molecular organization down to the level of individual nanocrystals and their orientational organization at nanoscale and microscale levels. Simultaneously with these comprehensive molecular characteristics, we measured optical appearance, chiral activity, and evaluated their charged state, offering unique comprehensive and quantitative characterization of the molecular orientational order of the CNCs.