Faculty Profile
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Jeanne PembertonRegents ProfessorEmail: pembertn@email.arizona.edu Building: CSB 302 Phone: 520-621-8245 | Honors
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Education and Appointments
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Research Interests
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Research Summary | |
| Chemistry at Interfaces Important in Molecular Electronic and Photonic Devices, Biosurfactants and Organized Assemblies, and Environmental Systems The interfacial regions between phases are sites of critical importance in many relevant processes and technologies. The catalysis of chemical reactions by metals, the corrosion of metals, the pollution of groundwater by toxic chemicals released from soil surfaces, the organization of surfactants at liquid-liquid interfaces important in phase-transfer catalysis, and the conversion of chlorofluorocarbons to reactive chlorine species which destroy ozone in the upper atmosphere are all examples of important chemical processes which occur at surfaces or within interfaces. Despite decades of intense study, our understanding of the chemistry of these and similar interfacial and surface processes at the molecular level is still poorly developed. Thus, the development of adequate tools with which to study surface and interfacial chemistry and elucidation of the molecular details of such complex chemistry represent two of the most exciting frontiers of modern measurement science. Our research seeks to develop an understanding of such chemistry in several technologically important areas including surface wetting and lubrication, organized assemblies including self-assembled layers and surfactant systems, and environmental systems. Methodologies employed for these efforts include surface vibrational spectroscopies, electrochemistry, surface electron spectroscopies, work function measurements, ellipsometry, fluorescence microscopy,electron microscopy, the scanning probe microscopies (AFM and STM), Langmuir trough methods. These methods are supplemented by more conventional chemical measurement tools (e.g. mass spectrometry, NMR spectroscopy, FTIR spectroscopy, fluorescence spectroscopy) as needed for complete characterization of relevant solution and interfacial systems. Specific systems of current interest include: The Chemistry of Metal/Organic Interfaces in Molecular Electronic & Photonic Devices Understanding the chemistry that occurs upon deposition of a reactive metal on an organic surface is increasingly important in developing efficient molecular photonic and electronic devices. Charge transfer in these devices is largely limited by the interactions and poorly defined-chemistry and electronic structure at these interfaces. Examples of these types of devices are organic light emitting diodes (OLEDs), organic photovoltaics (OPVs) and organic field effect transistors (OFETs). These organic-based photonic and electronic technologies possess several advantages over traditional inorganic-based systems, including inexpensive fabrication costs and the potential for flexible formats. To achieve widespread commercialization, these devices need improved efficiencies and increased device. Understanding and controlling the critical interfaces in these devices is central to overcoming these challenges. One critical interface in these devices typically consists of the junction of a low work function metal such as Al, Ag, Ca or Mg, and an organic electron transport layer, usually a conductive polymer or other highly-conjugated small molecule system. The nature of the products formed upon deposition of the vapor phase metal onto the organic surface are not well understood and are being studied using surface vibrational and electron spectroscopies in ultrahigh vacuum using the system shown in the figure below. Numerous analytical techniques are available in this UHV chamber with which to interrogate these systems including Auger electron spectroscopy, Kelvin probe work function measurements, temperature-programmed desorption-mass spectrometry and Raman spectroscopy. This suite of techniques allows us to develop a greater understanding of metal-organic conjugates formed at these interfaces that have not been studied heretofore.  Interfacial Studies of Microbially-Produced Surfactants This effort combines the scientific expertise of chemists and microbiologists to explore the fundamental surface and interfacial chemistry of four classes of microbially-produced biosurfactants: the rhamnolipids, the siderolipids, the surfactins and the sophorolipids. Biosurfactants are known to aggregate in solution at low concentrations and exhibit powerful surfactant activity at both liquid and solid surfaces. The structural intricacy of these materials is magnified further upon realization that they are produced as complex mixtures in which component congeners can have remarkably different surfactant properties. This work seeks to elucidate the fundamental surface and interfacial chemistries of naturally occurring, individual congeners of these surfactants as well as carefully-chosen (e.g. aided by modeling and dynamics studies) synthetic analogues with specific molecular attributes. Interfacial and surface studies are being undertaken at air-liquid interfaces and at oxide, metal, and polymer surfaces and are supplemented by solution aggregation studies. This work is being pursued under the auspices of Collaborative Research in Chemistry: Microbially-Produced Surfactants (CRC-MiPS) project funded by the National Science Foundation in collaboration with Professor Raina M. Maier of the Department of Soil, Water and Environmental Science at the University of Arizona. Molecular Architecture and Hydrodynamic Properties within Solid-Fluid Interfaces Solid-fluid interfaces play critical roles in many important processes including lubrication, painting, coating, printing, mineral flotation, and oil recovery. Such interfaces are also important in biologically-related systems in areas such as the use and control of biological membranes, development of biocompatible materials and bio-inspired microelectromechanical systems, and in environmental systems including the understanding and control of fluid flow in confined spaces such as soil. Many modern measurement and analysis technologies rely on the dynamic properties of solid-fluid interfaces, including microfluidics, the surface force apparatus, and scanning probe microscopies, yet little is known from experimental measures about molecular structure-hydrodynamic relationships of such interfaces. Despite longstanding and widespread interest in solid-fluid interfaces, it is increasingly understood that control and improvement of these interfaces will come only as molecular structure-function relationships that govern the static and dynamic properties of these interfaces are elucidated more fully. The interest in such questions by researchers from a broad range of scientific and engineering disciplines, including chemistry, physics, biology, environmental science, materials science, chemical engineering, and mechanical engineering, speaks to the central importance of solid-liquid interfaces in emergent scientific technologies and issues. Although some recent success has been realized in characterizing hydrodynamic properties within solid-fluid interfaces, and recent computational and theoretical efforts predict unusual hydrodynamic behavior that arises from unique molecular structure within such interfaces, experimental efforts to define interfacial molecular structure and to correlate that structure with interfacial dynamic and hydrodynamic properties are in their infancy. This work has as its primary goal the determination of interfacial molecular structure in solid-fluid interfaces formed with simple and complex fluids and elucidation of the dependence of interfacial fluid hydrodynamic properties on that molecular structure. These efforts build on past efforts in this laboratory to define molecular structural attributes of solid-fluid interfaces and specifically focuses on improving understanding of the relationship between molecular architecture and hydrodynamic attributes for solid substrates and simple and complex fluids. Novel measurement strategies for spectroscopic characterization of solid-fluid interfaces based on isolation of ultrathin fluid films on solid substrates by forced dewetting are being exploited for providing unique insight into interfacial molecular structure, and new approaches to understanding interfacial hydrodynamic behavior are being developed. This work has the potential to contribute significant new information about interfacial hydrodynamics and their relationship to molecular structure, thereby guiding future chemical design of such interfaces for specific function. Forced dewetting can be thought of as a two-stage process: residual film creation by forced dewetting followed by evolution of film fate prior to and during optical sampling. Films created by forced dewetting can be classified into one of two hydrodynamic regimes depending on the nature of the solvent (surface tension, γ, and viscosity, η), the dewetting velocity, and the macroscopic static (equilibrium) contact angle of the fluid with the solid, θequil that defines the surface energetics between the solid and fluid. For forced dewetting at velocities above a critical value, inertial and viscous forces are balanced by gravity and surface tension forces, leading to a dynamic contact angle (θD) of 0° at a point well above the edge of the macroscopic meniscus. This situation was originally described by Landau and Levich in 1942 and typically results in fluid films on the order of hundreds of nm to μm in thickness. This is the hydrodynamic regime of film formation by dip coating that is at the heart of many industrial processes, and results from the behavior of macroscopic hydrodynamic properties of the fluid. In contrast, however, for dewetting below this critical velocity, inertial forces are small and the residual film formed is the result of intermolecular and molecular-surface forces that occur on much shorter length scales. In this case, films are on the order of nm and reflect unique intermolecular and molecular-surface interactions within the interfacial region that alter the effective hydrodynamic properties of the fluid from their macroscopic values. In other words, the fluid films in this thickness regime exhibit non-Newtonian hydrodynamic characteristics. Effective interfacial viscosities up to several orders of magnitude larger than those of the bulk fluid have been reported for certain fluids in this thickness regime. As a result of this increased viscosity, fluid molecules within the dewet film do not slip under shear stress (or slip to a much smaller extent) and are therefore retained on the surface during the forced dewetting process. Thus, the unique interfacial molecular structural features that are sought as the goal of this research are those characteristics that are responsible for creation of the residual film during forced dewetting. | |
Selected Publications | |
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