In this project supported by the Chemical Structure, Dynamics and Mechanisms-A Program of the Division of Chemistry, Professor Scott Shaw and his students at the University of Iowa are employing a variety of optical tools (including laser-based techniques) to characterize the structure and dynamics of molecules adsorbed onto or near the surfaces of solid materials. A key tool in this research is a "dynamic wetting" (DW) apparatus, which can create films of liquids of controlled thicknesses from single nanometers to several microns (25 millionths of an inch to a few thousandths of an inch). By measuring the absorption of light (especially infrared light) as the film thickness is systematically varied, chemical and structural information about the molecules nearest the surface can be deduced. The information and insights gained from this research have impacts in other fields of chemistry research, as well as applied technologies such as catalysis, corrosion, lubrication, and semiconductor processing. This research project is also serving as a vehicle for the training of graduate and undergraduate students in physical chemistry, laser technology, and computational science. Outreach activities include Professor Shaw's Rural Scholars Program, which engages aspiring undergraduate students from rural areas in genuine laboratory research experiences.
The experimental strategy employed in the Shaw laboratory is developed around the dynamic wetting (DW) technique, which creates molecular fluids films with controlled thicknesses from about one monolayer to nearly 10 microns thickness. This approach circumvents difficulties in direct observation of the interface by selectively excluding opaque, bulk phases, and allows detailed descriptions of interface formation and behaviors. The net molecular orientation and the absolute thickness of the fluid films are extracted by sum-frequency generation (SFG), infrared reflection-absorption spectroscopy (IRRAS) with polarization modulation (PM-IRRAS), Raman spectroscopy, and variable angle spectroscopic ellipsometry. The experimental spectra are used to benchmark and validate computational and fluid dynamic models of interfacial molecular structure. Ultimately, this research may lead to an understanding of the roles of intermolecular forces and geometric confinement in creating chemical interfaces at defined distances from surfaces, advancing fundamental research in lubrication, tribology, and heterogeneous catalysis.
