Carboxylated cellulose nanofibrils (CNFs) have emerged as effective green dispersants for monolayer montmorillonite (MMT) dispersions. However, their dispersion capability is sensitive to the metal ion concentration in aqueous solutions. Hence, this study investigated the effects of Na⁺ and Ca²⁺ concentrations on the dispersive ability of carboxylated CNFs for monolayer MMTs in water. Quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) were utilized to explore the interfacial interactions between the carboxylated CNFs and monolayer MMTs under different Na⁺ and Ca²⁺ concentrations. When the concentration of Na⁺ reached 0.1 mmol/L, the adhesion mass of carboxylated CNFs on MMT-coated wafer peaked at 24.47 mg/m², higher than control sample (carboxylated CNF-dispersed monolayer MMT dispersion without metal ions, 16.03 mg/m²). Moreover, the electrostatic shielding effect promoted a better dispersion of monolayer MMTs by carboxylated CNF dispersant. With a further increase in the Na⁺ concentration, the surface charge of CNFs and MMTs would be reversed resulting from the improved electrostatic shielding effect, which weaken the dispersive ability of carboxylated CNFs. The addition of Ca²⁺ reduced the dispersive ability of carboxylated CNFs for monolayer MMTs, because Ca²⁺ required a lower concentration for the onset of charge reversal compared to Na⁺. This study provides interfacial scale insights into the influence of metal ion concentration on carboxylated CNF-dispersed monolayer MMT dispersions. It also provides a strategy to enhance the dispersive ability of carboxylated CNFs for monolayer MMTs.

Carboxymethylated cellulose nanofibril (CMCNF) is an effective green dispersant to prepare well-dispersed monolayer montmorillonites (MMTs) in water, thereby facilitating the preparation of a high-performance MMT/polymer nanocomposite film. However, not enough attention has been paid to correlating the degree of substitution (DS) of CMCNFs with the mechanical and optical properties of the final nanocomposite films. In this study, a series of homogeneous monolayer MMT nanoplatelet dispersions was prepared initially using CMCNFs with different DS as a dispersant, and the as-prepared CMCNF-dispersed MMT dispersions were then mixed with sodium carboxymethyl cellulose (CMC-Na) to fabricate nacre-like nanocomposite films with different contents of MMTs through self-assembly. The layered nanostructure and optical and mechanical properties of the as-prepared CMCNF-dispersed MMT/CMC-Na nanocomposite films were investigated, which demonstrated that CMCNFs with lower DS have a positive effect on their optical and mechanical properties. This study sheds light on the preparation of MMT-based nanocomposite films with superior optical and mechanical properties.