Interlayer structure and dynamic properties of CTMAB-montmorillonite: experiment and molecular dynamics

The intercalation of cetyltrimethylammonium bromide (CTMAB) into montmorillonite will cause interlayer expansion and surface charge reversal. In this study, CTMAB-Mt is prepared by adding CTMAB with different multiples of montmorillonite cation exchange capacity (CEC), and the intercalated CTMAB structural arrangement, as well as the dynamics behavior, are investigated by combining molecular dynamics (MD) simulation with experimental characterization. According to RDF analysis of MD simulations, the interaction between CTMA⁺ and the surface of montmorillonite is mostly electrostatic interaction and hydrogen bond production. At low loading (≤1.00CEC), the XRD profile exhibits a peak value corresponding to one type of intercalation structure and interlayer spacing, but at high loading (>1.00CEC), two peaks are visible, each of which has a fixed value but a varied strength, corresponding to the existence of two types of expanded structures. The d-spacing (d ₀₀₁) values obtained from MD simulations are quite close to XRD values when CTMAB loading is lower than 1.00CEC. Density distribution profiles obtained from MD analysis reveal that as loading increases, CTMA⁺ is arranged in the interlayer from a monolayer to a bilayer and then to a pseudo-trilayer. At high loadings (>1.00CEC), due to the fact that the excess loading leads to inhomogenous intercalation, XRD demonstrates the existence of two different arrangements: bilayer and pseudo-trilayer. The self-diffusion coefficients of MD simulations show that the dynamic behavior of CTMA⁺ is influenced by both the interlayer space and the electrostatic interaction of the montmorillonite clay. The abrupt rise in interlayer spacing increases mobility, whereas the increased interaction between alkyl chains decreases mobility.