Understanding the Modification Process of Montmorillonite by Cetyltrimethylammonium via Molecular Dynamics...

Cetyltrimethylammonium bromide (CTAB)-modified montmorillonite (MMT) exhibits enhanced adsorption capacity for both anionic and nonpolar contaminants. However, the structure evolution and underlying thermodynamic mechanism during the modification process remain poorly understood. This paper employs molecular dynamics to simulate the adsorption behavior and drying process during CTAB-MMT modification at varying dosages. The simulations reveal that the adsorption process of cetyltrimethylammonium (CTA⁺) ions is an initial enthalpy-driven ion exchange between CTA⁺ ions and compensating Na⁺ ions, followed by entropy-driven hydrophobic interactions between CTA⁺ alkyl chains and the nonsubstituted MMT surface, which facilitates CTA⁺ ions to lie on the surface. At a higher CTAB dosage, the further adsorption of CTA⁺ ions occurs through hydrophobic interactions with preadsorbed CTA⁺ ions, leading to the formation of CTA⁺ stacks and clusters on the MMT surface. The structure evolution of interlayer CTA⁺ ions during the drying process is characterized for the first time. The CTA⁺ alkyl chains progressively move toward the MMT's hydrophobic regions as MMT lamellas collapse during drying. Meanwhile, spontaneous water expulsion is facilitated by hydrophobic aggregation of CTA⁺ alkyl chains, while trimethylammonium groups near isomorphic substitution sites retain water molecules. Nonequilibrium molecular dynamics simulations successfully capture the dosage-dependent zeta potential reversal of CTAB-MMT, which results from charge shielding and overcompensation effects. These findings provide molecular-level insights into the modification mechanism and interfacial properties of organoclays.