The objective of this Early-Concept Grant for Exploratory Research (EAGER) project is to investigate molecular-level transport of (i) reactant species in soups that generate functionalized nanotubes, (ii) probe molecules in designed nanotubes, (iii) fluids in engineered micro-contacters, (iv) macromolecules with multicyclic topologies, and (v) anions in anion exchange fuel cells. These studies will be conducted by diffusion nuclear magnetic resonance (NMR) using specialized equipment acquired with support from this grant (high-gradient-strength diffusion probe and gradient amplifier) on an existing NMR spectrometer. The resulting new experimental capabilities are urgently needed to enable exploratory research in fluid dynamics, separations, nano- and microfluidics, nanoengineering, energy conversion, and fuel cell development.
The intellectual merit of the proposed activity is embodied in the research thrusts for which strong-gradient NMR diffusion measurements are needed:
(i) Mechanistic studies of nanotube growth and engineering -The requested NMR accessories will enable measurements of size and structure of nanoparticles, thus yielding significant insight into growth mechanisms of single-walled inorganic nanotubes prepared in aqueous solution. This knowledge is crucial for the development of nanotubes, which find applications in various transformational technologies such as electronics, separations and energy storage/generation/ management.
(ii) Transport in nanotubes - Nanofluidic transport in tubular materials is poorly understood, in spite of its scientific and technological relevance. Inorganic nanotubes (based on metal oxides) and organic nanotubes (based on crosslinked cyclodextrins) are good model systems for nanotube membranes and biological nanochannels. NMR diffusion measurements of probe molecules within these nanotubes will provide important insights that cannot be achieved with conventional techniques.
(iii) Hierarchically engineered fabrics for large-array micro-contactors Engineered fabrics provide a novel and highly scalable approach to liquid-liquid contacting of immiscible fluids for (bio)chemical separations. Development of this technology requires transport measurements of multiphase flow and characterization of pore structure in 3D amphiphilic microchannels.
(iv) Dynamics in topologically complex fluids - Diffusion of cyclic and multicyclic macromolecules is poorly understood relative to linear polymers due to the existence of unconventional entanglements. Investigation of the diffusion behavior of blends of these topologically complex macromolecules with linear polymers will be key in unraveling the effect of molecular topology on fundamental aspects of polymer dynamics.
(v) Transport in anion exchange membranes - Fuel cell designs that employ anion exchange membranes are being developed to overcome shortcomings of existing proton exchange membranes. Diffusion of hydroxide ions and water confined in cationic polymer membranes with heterogeneous morphologies must be studied in order to optimize these membranes.
Broader Impacts: The NMR diffusion accessories will enable research that is critical for development and understanding of materials, processes and technologies with transformative potential in areas like chemical processing and energy management. Important breakthroughs with inorganic and organic nanotubes, inexpensive fabric-based micro-contactors, and anion exchange membranes could have broad implications for economic development. The proposed equipment is a modest investment that is critical for enabling exploratory research within all of these ?high-risk high-payoff? areas and provides unique interdisciplinary leverage. The requested equipment will significantly enhance the general research capabilities of a large group of researchers across a variety of fields in the greater Atlanta area.
The funds of this project were entirely used for the purchase of a high gradient strength NMR Diffusion Probe and accessories. This probe works in combination with a 400 MHz NMR spectrometer, which was already present at the Georgia Tech NMR Center. The Diffusion Probe measures the motion of molecules through the combined application of radio-frequency pulses and magnetic fields. This motion occurs in all fluids and it increases with the sample temperature. It is known as Brownian motion and commonly termed "Diffusion" or "Self-Diffusion". Our new diffusion probe is able to characterize molecular diffusion in great details. Especially it is possible to: (i) Characterize the diffusion of individual molecules within a mixture of molecules, which will provide information about the nature of the mixture. (ii) Characterize the diffusion of molecules in porous structures, which will provide information about the structure of those materials. (iii) Characterize the diffusion of larger particles, which allows the measurement of particle sizes. (iv) Characterize the interaction of a fluid with a substrate through changes in the diffusion behavior of the fluid. The diffusion probe was installed in May of 2012. Due to technical difficulties and some malfunctions the probe had to be sent back several times to the manufacturer (Bruker). It became available to research only in late 2012. Since then the probe has been used by several researchers for the following innovative projects. (i) Binding of solvent Molecules to Metal Organic Frameworks (co-P.I. Nair) Metal Organic Frameworks are an important new group of crystalline materials with a porous structure. Due to this porous structure the materials MOFs may be used for the storage of gases, for chemical separations and for the delivery of drugs in medications. We have been studying the diffusion of alcohols and other small molecules in a group of MOFs termed ZIFs (Zeolitic Imidazole networks). The results obtained so far show that ZIFs are good candidates when it comes to the separation of alcohols in the production process of biofuels. (ii) Molecular Diffusion of Nanogels in aqueous solutions (co-P.I. Lyon). We investigate the effect of nanogels, which are made of a material termed pNIPAm. Those are particles, which can be suspended in water. Upon heating the particles will change their size and properties. They may therefore be used for the delivery of drugs. Using our new equipment it was possible to measure the diffusion of these particles, which turned out to be quite a technical challenge. The diffusion data allow the calculation of the particle size. We find some discrepancies between particle sizes measured with our method and measured with other methods (dynamic light scattering). However, results from both methods provide a complimentary picture. (iii) Molecular Diffusion in ionic liquids in the presence of nanoparticles (co-P.I. Cola) Ionic liquids are in essence salts, with such low melting temperatures that they are liquid at room temperature. Ionic liquids are very promising as solvents for chemical reactions; one example is the decomposition of biomatter, which is an important step for the generation of biofuels. Carbon nanotubes, i.e. large, tubular carbon structures are hypothesized to bind components of the ionic liquid such that it enhances the performance of the liquid when used as solvent for complex chemical reaction. We are investigating this effect using Diffusion NMR. (i) Molecular diffusion of water molecules in membranes used for fuel cells (co-P.I. Beckham and Kohl) Fuel cells are batteries, which - instead of being recharged with electrical currents – may be charged through the chemical conversion of a fuel. The membranes separating individual components in these fuel cells play a vital role for the performance of these cells. Important for the functioning of these membranes is their capability to transport charges, which is related to the binding of water. We have been using Diffusion NMR to characterize the mobility of water in these membranes. Hence, Diffusion NMR may become an important tool for the development of new and improved fuel cells. The probe has been incorporated into the equipment of the Georgia Tech NMR center. As such it is available to all researchers at Georgia Tech including minorities and underrepresented groups. The capability of the probe has been also advertised during various teaching activities. For instance the P.I. of this proposal has been teaching a short-course "solid-state NMR" in August of 2013. The principles and application of diffusion NMR were covered during this course.
