Understanding the thermal motion of the luminescent dyes in the dye-surfactant cointercalated ZnAl-layered double hydroxides: a molecular dynamics study

Previous work has demonstrated that cointercalation of luminescent dyes and surfactants into layered double hydroxides (LDHs) is an efficient approach to inhibit the aggregation of dye and therefore enhance its photoluminescence behavior. In this work, molecular dynamics simulations are performed on different ZnAl-LDHs cointercalated with dye (fluorescein or 1-anilinonaphthalene-8-sulfonate) and alkylsulfonate with different alkyl chain length (CnH2n+1SO3, n = 5, 6, 7, 10 and 12, respectively), together with dye-alkylsulfonate solutions for comparison. The structure, binding energy and the thermal motion characterized by the diffusion coefficient of each dye are analyzed. In the dye-alkylsulfonate/LDHs, the diffusion coefficient and the binding energy of the dye show a minimum when the dye is cointercalated with heptanesulfonate (HPS, n = 7), whose size is the closest to that of the dye. While in the case of dye-alkylsulfonate solutions, the diffusion coefficient and the binding energy vary monotonously with the increasing alkylsulfonate size. Furthermore, it is found that the increase of Al3+ content in LDH matrix in dye-HPS/LDHs is favorable for the restriction of the dye motion. These results indicate that the dye-alkylsulfonate/LDH system is more effective in restraining both the thermal motion and the aggregation of the dye than that of dye-alkylsulfonate solutions due to the confined microenvironment provided by the LDH matrix. Therefore, it is possible to inhibit the aggregation of the dye in dye/LDHs by two aspects: choosing a surfactant with a size close to that of the dye as the cointercalant and increasing the content of trivalent cations in the LDH matrix.