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Surfaces play a central role in many of the chemical and physical processes in our everyday lives. Reactions in the atmosphere are often heterogeneous, occurring at the surfaces of aerosols, ice particulates and oceans. Respiration in the lungs of living organisms occurs across lipid surfactant monolayers. Chemical reactions including polymerization can often be facilitated by the presence of a surface. Many environmentally important chemical separation processes are based upon the partitioning of solute molecules across the interface between two immiscible liquids.
Whereas much is known about reactivity in bulk media, far less is known about factors that control adsorption and reactivity at surfaces. This is particularly true for "wet interfaces", those involving a liquid in contact with another media such as air, solids or other immiscible liquids. In our laboratory we are using laser spectroscopic techniques and theoretical computational methods to probe the molecular structure of molecules at these wet interfaces. We have a number of ongoing projects in this laboratory which include:
- Understanding how water molecules hydrogen bond at a water surface and how this hydrogen bonding, which is so strong at a water surface, is perturbed by the presence of surfactants, organic adsorbates or electrolytes in the bulk solution.
- Probing how water behaves near hydrophobic surfaces. These studies examine water in contact with a variety of liquid, solid and monolayer surfaces as we seek to understand some of the most fundamental interactions that underpin many environmental, chemical and biological processes on our planet.
- Examining important environmental processes at liquid surfaces. A wide range of projects are ongoing in this area including monitoring the adsorption of contaminants on water surfaces, studying how atmospheric gases adsorb and react on water surfaces, exploring how acids and ions in aqueous solutions alter the surface adsorptive and reactive properties at the air/water and oil/water interfaces.
- Characterizing the electron-hole dynamics at the surface of photoactive materials as we seek to develop high efficiency solar conversion materials for solar cell applications.
- Studying the structure and dynamics of thin film growth at mineral/water interfaces. The information that we seek from these studies is important for a number of environmental, material science and energy related challenges today.
- Exploring how biomolecules adsorb and assemble at liquid surfaces and the role that water plays in the surface behavior of these molecules.
These examples of many of the ongoing studies in our group which ask very fundamental questions about these surfaces while having direct relevance to understanding a wide range of biological, environmental and chemical processes.
Because of the molecular complexity of the interfaces that we study, it is essential that we approach the problems with as many tools as possible. For this reason our studies involve a close coupling between our experimental efforts and computation. Our studies of photoinduced processes at solar-active materials involve ultra-fast photoluminescence studies of carrier dynamics. The majority of our other experiments involve surface sum frequency spectroscopy (VSFS) as this is a very powerful tool for measuring the vibrational spectroscopy of interfacial molecules. From these experimental studies we can learn about the molecular structure and orientation of molecules residing in the thin interfacial region. Complementing these experimental studies are our computational studies where we use molecular dynamics (MD) simulations to derive further information about our interfaces. With these MD simulations we calculate the VSF spectra and compare it with our experimental measurements. When we have a good correspondence between the spectral features obtained experimentally and through MD simulations, we can pull information from the MD simulations that we cannot get through the experimental data – information such as population densities, orientational distributions, and bonding interactions. This combination of experiment and theory in a single program as well as collaborations with other theorists provides a unique opportunity to make advances in understanding molecular processes at these complex interfaces while testing the validity of different molecular models describing interfacial interactions.
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