Molecular Dynamics Simulation of the Interaction within the Carboxymethyl Cellulose-Modified Montmorillonite Lamellae at Aggressive CuCl2 Concentrations

To elucidate the degradation mechanism of the CMC-modified MMT composite at aggressive Cu²⁺ concentrations, large scale molecular dynamics simulation was conducted for CuCl₂ concentrations ranging from 0 to 800 mM. Both CMC and MMT followed the Langmuir isotherm for Cu²⁺ adsorption, and the adsorption capacity of CMC (8.75 mmol/g) was much higher than that of MMT (0.83 mmol/g). Despite the CMC mass ratio being only 4.1%, it adsorbed up to 34.3% of the total adsorbed Cu²⁺. The Cu²⁺ attraction ability hierarchy of oxygen-containing functional groups in the CMC is as follows: carboxylic oxygens > alcoholic oxygens > carbinolic oxygens > bridging oxygens > glucose oxygens. Carboxyls were the most effective in chelating and complexing with Cu²⁺, and they could be intentionally added in artificially synthesized polymer-MMT composites for Cu²⁺ containment. Formation of the Cu²⁺ cation bridge between CMC and MMT at aggressive CuCl₂ concentrations contributed to the transition of CMC density distribution from unimodality to bimodality and enhanced resistance of polymer elution. As the CuCl₂ concentration increased, the stoichiometric ratio between the chelated Cu²⁺ and carboxylic oxygens increased from 1:2 to 1:1, suggesting the evolution of the Cu²⁺ chelation mechanism.