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 obtained from flow-aligned F127 gels in the absence of silica suggest that partitioning plays an important role in limiting passage of the PDIs between pores. Fluorescence correlation spectroscopy (FCS) is used to measure the dye diffusion coefficients and their evolution in time. The FCS data confirm the presence of two populations of diffusing molecules attributable to anisotropic and isotropic motions. All three dyes exhibit mean diffusion coefficients that decrease with monolith age. Taken together, the SMT and FCS data are consistent with aging-time-dependent chemical changes to the materials. These lead to partitioning of the charged PDI dyes into more polar regions that facilitate their passage through the pore walls. In contrast, the uncharged PDI becomes better confined to the hydrophobic micelle cores with monolith aging, leading to enhanced 1D diffusion. This study provides an improved molecular-level understanding of how partitioning contributes to mass transport selectivity in nanoporous membranes