Dimensionality of Diffusion in Flow-Aligned Surfactant-Templated Mesoporous Silica: A Single Molecule Tracking Study of Pore Wall Permeability

Flow-aligned mesoporous silica monoliths incorporating nanometer-sized cylindrical pores serve as models for materials with applications in chemical separations and catalysis. In many such systems, it is assumed that incorporated analytes or reagents are confined to and travel along the one-dimensional (1D) pores. Anisotropic motion is often inferred from ensemble measurements and knowledge of the material structure. In this report, single molecule tracking (SMT) is used to directly visualize the diffusive motions of uncharged, cationic, and anionic perylene diimide (PDI) dyes within the monoliths. The monolith pores are filled with Pluronic F127 surfactant, water, and alcohol. SMT data depict both isotropic and anisotropic diffusion for all three dyes. The charged dyes exhibit predominantly isotropic motions, suggesting these molecules readily pass through defects in the silica pore walls. In contrast, the motions of the uncharged PDI evolve to become more anisotropic with monolith aging. Results obtaine...