Packing behaviors of structurally different polyhedral oligomeric silsesquioxane-imidazolium surfactants in clay

This paper reports the packing structures of two types of polyhedral oligomeric silsesquioxane (POSS)-imidazolium surfactants with different molecular rigidity in the intergalleries of montmorillonite clay. Wide-angle X-ray scattering (WAXS) and molecular modeling studies suggest that the POSS-imidazolium cations have a bilayer packing structure in clay with the long axes of the molecules largely tilted with respect to the basal plane. Direct evidence for the bilayer structure is provided by transmission electron microscopy (TEM). WAXS and differential scanning calorimetry (DSC) results indicate that the relatively flexible POSS-imidazolium cation is able to form a two-dimensionally ordered structure in the clay intergalleries, while the relatively rigid one exhibits a disordered structure in clay. Furthermore, with reducing surfactant loading, the clay modified with the rigid surfactant exhibits increased interlayer d-spacings probably because the surfactant is able to take more extended conformation at lower loadings. With its low organic content and disordered surfactant packing structure, the clay modified with the rigid surfactant also exhibits excellent thermal and thermo-oxidative stabilities.