Understanding Vibrational Spectroscopic Probes of the Structure and Dynamics of Liquids Confined in Mesoporous Materials

University Of Kansas Center For Research Inc, Lawrence KS

Investigators

Abstract

Professor Ward Thompson of the University of Kansas is receiving an award from the Macromolecular, Supramolecular and Nanochemistry Program. Theoretical and computational approaches are used in the awarded project for inquiring what vibrational spectroscopic techniques can reveal about the properties of liquids and solutions confined in nanoscale silica pores of varying size (~2.4 to 4.5 nm in diameter) and surface functionality (hydroxyl- and alkyl-terminated). Dramatic changes in the molecular-level liquid structure and dynamics occur upon such nanoscale confinement, yet these effects are not readily observable in the linear infrared (IR) spectra. By analyzing vibrational spectroscopic probes, this project sheds light on the complex structure and dynamics of nanoconfined liquids. The project focuses on confined acetonitrile and related systems. The application choice is based on the presence of a dramatic blue shift in the CN stretching frequency upon hydrogen bonding and the relatively slow vibration relaxation, which permits examination of longer timescale dynamics. A combination of molecular dynamics, grand canonical Monte Carlo simulations, electronic structure calculations, and mixed quantum-classical MD are the chosen methodologies. The proposed properties to be studied include: i) determination of the relative intensities of hydrogen bonded and non-hydrogen bonded peaks in the linear infrared spectra of nitriles and isonitriles, ii) prediction of the IR pump-probe spectroscopy of CH3CN confined in silica pores of varying surface chemistry, iii) prediction of the IR photon echo spectroscopy of CH3CN confined in silica pores of varying surface chemistry, and iv) simulation of the spectroscopy of solutes in nanoconfined CH3CN and of a CH3CN solute in other nanoconfined liquids. Porous silica materials are of interest in catalysis, separations, and sensing and are part of a wider class of porous oxide materials similarly important in a variety of applications. Moreover, nanoconfined liquids are present in a number of other systems including supramolecular assemblies, templated materials, reverse micelles, biological systems, hydrogels, membranes, fuel cell electrodes, and nonlinear optical materials. The broader impact aim of this project is to gain deeper mechanistic understanding of how liquids move and interact within nanoconfined structures and how these mechanisms can be probed via spectroscopy to assist in the design and characterization of applications that exploit the unique physical properties of liquids in confined environments. Graduate and undergraduate students are involved in this research, providing them with training in theoretical and computational techniques and a broad background in physical chemistry. The participation of underrepresented groups continues to be encouraged within this research group.