(36c) Small Molecule Probes of the Pore Structure of Mesoporous Silica Thin Films Investigated Using Quartz Crystal Microbalance

Mesoporous silica thin films form diverse structures with varying pore sizes and high surface area, which make them ideal for applications including catalysis, adsorption, and sensing. Silica thin films are synthesized by evaporating the solvent from a homogenous solution of soluble silica and surfactant prepared in ethanol/water, otherwise called evaporation induced self-assembly (EISA).  The ratio of precursor/ template / water/ solvent in the sol determines the hexagonal, cubic or lamellar pore structure, depending on the region it lies in the aqueous phase diagram.  Pore size, orientation, and degree of order can be further controlled by changing the process environment conditions and modifying the substrate.  Adsorption capability of the film depends on the pore accessibility, pore size and surface area. Quartz crystal microbalance with dissipation monitoring (QCM-D) can detect small changes in mass (about 0.5 ng/cm²) along with the viscoelastic properties of the films. Synthesizing thin films directly on QCM-D sensors allows direct probing of the as-synthesized pore structure through the measurement of solute adsorption on the thin film surface and pores. In the present study we analyze and interpret the pore characteristics of the thin films using small analytes whose adsorption is dependent on pore accessibility, pore size and surface area.   Poly (ethylene oxide)-polypropylene oxide (PEO-PPO) triblock surfactant (P123) templated silica thin films were synthesized of varying pore structure, orientation and size were synthesized by varying sol-gel composition , substrate modification and hydrothermal treatment, respectively. Deuterium oxide as a measure of pore volume, and tetramethyl ammonium chloride (TMA), which strongly interacts with negatively charged silica, are used in the adsorption studies to demonstrate the sensitivity of small analytes as probes of accessible pore volume, size, structure, surface area, orientation, and order. Complementary X-ray diffraction studies are used to identify the pore structure.  The ability to probe the pore structure of mesoporous ceramic films using QCM will enhance the design of thin films for selective adsorption and reactivity.