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Jeanne E. PembertonRegents Professor of Chemistry John & Helen Schaefer Professor of Chemistry pembertn@u.arizona.edu Chemical Sciences Building 302 Phone: (520) 621-8245 Fax: (520) 621-8248 |
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, Chromatography, 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, chromatography and electrophoresis systems, 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 electronic and organic photonic devices. Charge transfer in these devices is largely limited by the interactions at these interfaces. An example of these types of devices is an organic light emitting diode (OLED), an example of which is shown in Figure 1.  The components of an OLED are shown in Fig. 2. OLEDs possess several advantages over traditional inorganic-based LEDs, including inexpensive fabrication costs, tunable emission wavelengths and potential for flexible displays. To achieve widespread commercialization, these devices need improved electroluminescence efficiencies, increased device lifetimes, and lower power requirements. Understanding and controlling interfacial properties is critical to overcoming these challenges. The metal/organic interface in these electronic devices consists of the junction of the cathode, typically a low work function metal such as Al, Au, Ca or Mg, and the electron transport layer, usually a conductive polymer or other highly conjugated system such as shown schematically in Figure 3. 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 Figure 3. 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.  Vibrational Spectroscopy of Chromatographic Interfaces Chromatographic separations play a central role in fundamental research related to energy, biological systems and the environment. This area of research seeks to develop a molecular-level understanding of the chemical processes that underlie separations for both large-scale and analytical-scale purposes through characterization of the interfacial details of separations processes at the molecular level. Specific projects being pursued include: 1) studies on interfacial stationary phase and mobile phase structure within residual layers created by forced dewetting at stationary phases relevant in RPLC and capillary electrochromatography (CEC), especially horizontally polymerized self-assembled monolayer systems, 2) molecular structure, conductivity and hydrodynamics within silica/aqueous mobile phase interfaces relevant in capillary electrophoresis (CE) and micro- and nanofluidic devices, with a special emphasis on structure and dynamics within the stagnant layer, and 3) investigation of chemical accessibility, reactivity, and molecular structure within silica pores of systematically varying dimensions down to the nanoscale using colloidal crystalline arrays (CCAs). Studies of stationary and mobile phase structure seek a unified molecular picture of stationary phase-stationary phase, solute-stationary phase, solute-mobile phase solvent, stationary phase-mobile phase solvent, and mobile phase solvent-solvent interactions that dictate retention in RPLC and CEC. Current efforts are attempting to extend past efforts on conventional stationary phases of single alkylsilanes through study of mixed trifunctional silane self-assembled monolayer stationary phases prepared by horizontal polymerization. These systems provide a route to systematic control and variation of alkyl conformational order (see Figure 1), and hence stationary phase free volume, within a continuum of accessible states not achievable with single-alkylsilane materials. A new stationary phase synthesis technique has been developed that is based on the use of displaceable surface templates to form mixed trifunctional silane systems, especially those for CEC in which the alkylsilane/surface charge ratio is varied.  Vibrational spectroscopy (FTIR and Raman) studies of residual layers formed by forced dewetting at silica/aqueous solution interfaces relevant in electrokinetically-driven separations (capillary electrophoresis and CEC) are of interest under conditions of electrokinetic flow. Results from these studies will provide insight into interfacial structural details critically important for optimization of microfluidic and nanofluidic devices based on electrokinetic flow strategies. Ellipsometric measurements of residual layer thicknesses created through interfacial hydrodynamic shear will be correlated with streaming potential measures of interfacial zeta potential to understand shear plane position in such systems. We are investigating colloidal crystalline arrays (CCAs) of silica nanoparticles to produce samples compatible with vibrational spectroscopy to allow investigation of the molecular details of pore chemistry important in RPLC and CEC. Arrays produced from silica nanoparticles <100 nm in diameter are being used to produce pore sizes ranging from ~1 to 40 nm. Specific issues in RPLC of interest include alkylsilane binding and conformational order, mobile phase solvent structure, and solute-stationary phase/-mobile phase interactions as a function of pore size. Specific issues targeted for electrokinetically-driven separations on silica include the effect of electric field on mobile phase solvent structure as a function of pore size down to the regime of electrical double layer overlap, and on modified silica, the effect of electric field on alkylsilane conformational order and mobile phase solvent structure as a function of pore size.  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. 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. Emersion 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 emersion 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 emersion 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 emersed 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 emersed film during forced dewetting. |
