Calcium Ion-Induced Structural Changes in Carboxymethylcellulose Solutions and Their Effects on Adsorption on Cellulose Surfaces

Abstract

The adsorption of carboxymethylcellulose (CMC) on cellulose surfaces is one of the most studied examples of the adsorption of an anionic polyelectrolyte on a like-charged surface. It has been suggested that divalent ions can act as a bridge between CMC chains and the surface of cellulose and enhance the CMC adsorption: they can, however, also alter the structure of CMCs in the solution. In previous investigations, the influence of cations on solution properties has been largely overlooked. This study investigates the effect of Ca 2+ ions on the properties of CMC solutions as well as the influence on cellulose nanofibers (CNFs), which was studied by dynamic light scattering and correlated with the adsorption of CMC on a cellulose surface probed using QCM-D. The presence of Ca 2+ facilitated the multichain association of CMC chains and increased the hydrodynamic diameter. This suggests that the adsorption of CMCs at high concentrations of CaCl 2 is governed mainly by changes in solution properties rather than by changes in the cellulose surface. Furthermore, an entropy-driven mechanism has been suggested for the adsorption of CMC on cellulose. By comparing the adsorption of CMC from H 2 O and D 2 O, it was found that the release of water from the cellulose surface is driving the adsorption of CMC.

Biobased Nanomaterials─The Role of Interfacial Interactions for Advanced Materials

“…An interesting case is the effect of divalent cations on the adsorption of CMC on cellulose substrates. Ca 2+ ions have been observed to affect CMC structure in solution and favor the CMC adsorption on cellulose more than Mg 2+ ions (Figure c,d), which was ascribed to the different polarity of those ions. , …”

“…(d) Schematic of CMC adsorption in the presence of CaCl 2 . (c,d) Reproduced with permission from ref under Creative Commons CC-BY license. Copyright 2017 American Chemical Society.…”

“…Ca 2+ ions have been observed to affect CMC structure in solution and favor the CMC adsorption on cellulose more than Mg 2+ ions (Figure 2c,d), which was ascribed to the different polarity of those ions. 90,134 Although also affected by the pH and the ionic strength, the adsorption of cationic polymers on cellulose surfaces is driven by attractive electrostatic interactions between oppositely charged groups and, especially, the remarkable gain in entropy associated with the release of bound counterions. 135 The a d s o r p t i o n o f c a t i o n i c p o l y m e r s l i k e p o l y -(diallyldimethylammonium chloride) (PDADMAC) and chitosan on CNF has been monitored in real-time by QCM-D and SPR.…”

Anion specific adsorption of carboxymethyl cellulose on cellulose

“…Recently, investigations suggest that the adsorption of polymers on cellulose surfaces is driven by entropy gain due to the release of water that is unfavorably arranged on the surface. 9,13,[41][42][43] The distribution of ions at a charged interface plays a crucial role in determining the effect of charge on the water organization. 36 The vicinity of chaotropic nitrate ion at the interface could be causing a more unfavorable organization of water molecules at the interface, which results in larger entropy gain once released from the surface and thus higher adsorption of CMC.…”

“…12 Divalent cations are more effective in increasing adsorption as they can efficiently screen surface charge and induce favorable structural changes in the polyelectrolyte solution for adsorption. 13,14 However, influence of ions on different processes are governed not just by their charge. Factors such as ionic radii and ion hydration become more decisive, especially in interfacial processes such as adsorption.…”