Inhomogeneity of Organically Modified Montmorillonite Revealed by Molecular Dynamics Simulation

The modification of an interlayer of layered materials by intercalation with an organoammonium ion has been a promising method to control the polarity of the two-dimensional nanospace. Montmorillonite is one of the best-known examples, and the modification with octadecyltrimethylammonium ion (Mont-C-18) results in adsorption of anthracene and pyrene together with specific excimer emission, while the nanostructure is yet to be uncovered at the molecular level because the gallery height is only ca. 27 angstrom. We, herein, investigated the nanostructure of this nanocomposite by molecular dynamics (MD) simulation, combined with analysis of molecular orientations against the Mont layer. The gallery height of Mont-C-18 was well consistent with the experimental value, which was linearly increased along with the intercalation of anthracene. Anthracene was segregated on the Mont layer with its short and long molecular axes vertical in the early and late stages, respectively. In contrast, C-18 was initially rather horizontal, forming the so-called pseudotrimolecular layer. Pushed out by anthracene, distribution and orientation of C-18 were gradually changed: the third molecular layer was distinctly observed in the center of the interlayer in the early stage, and the orientation was changed to vertical in the late stage. Thus, the continuous increase in the gallery height is ascribed to soft response of C-18 to the intercalation. Summarizing the abovementioned results, it was concluded that angstrom-order inhomogeneity is introduced in the interlayer by the intercalation of anthracene, which is significant in ideal design of the two-dimensional nanospace.