Per President Fitts’s announcement on March 11, 2020 that all large Tulane gatherings will be cancelled immediately as part of the University’s precautionary response to the global COVID-19 situation, the CBE Department is postponing all of our remaining seminars for this semester, including visits from the following speakers:
|Guest name:||Originally scheduled for:|
|Lars Grabow||March 13|
|Steve Rick||March 20|
|Sangwoo Lee||April 3|
|Christina Tang||April 17|
All seminars will be held in the Lindy Boggs Center room 243 from 2:00 pm – 2:50 pm unless noted otherwise. For more information please call (504) 314-2914 or email email@example.com.
Sponsored through generous gifts from members of The Chemical and Biomolecular Engineering Board of Advisors.
The security of our nation relies on a deep fundamental understanding of materials from structural ceramics and metals to low viscosity silicone oils and everything in between including a range of soft matter. This presentation will focus on two applications on the harder side of the soft matter domain. Glass-ceramics, which are used to manufacture hermetic electrical pass-throughs, contain an embedded crystalline phase surrounded by a glassy matrix. The crystals form a space filling “gel-like” structure which supports a stress above the melting temperature of the glassy matrix. The temperature dependent viscoelastic rheology of a glass ceramic will be compared to a traditional sealing glass. Evidence of microstructural changes is observed and highlights the challenges of developing predictive models of stress development in these materials.
Lithium ion batteries are everywhere around us but are still challenged by issues of capacity and power fade. A unique mesoscale approach will be presented to understand the performance of battery electrodes, especially the role of the polymer composite binder used to hold electrodes together. The properties of those binders will be explored under conditions mimicking battery operation. To better explore the dynamics, battery electrode microstructures are reconstructed into realistic computational geometries in order to perform simulations of electrochemical charging and predict the stresses generated during battery operation. Understanding capacity fade and reliability of rechargeable batteries will significantly impact on our nation’s ability to field advanced chemistries for electronics and energy storage for renewable sources.
With the growing population in the world, global construction output is expected to increase by 85% by 2030 compared to 2015 levels. Given that concrete production generates a large fraction of total CO2 during its production, finding alternatives to concrete as structural materials is becoming critically important. In this talk, I will describe our efforts to explore the possibility of using polymer nanocomposites as structural materials. Using molecular dynamics simulations of model polymer glasses, we show first that molecular dynamics simulations can be used to study the creep dynamics of polymer glasses in a way that is comparable to experiments, despite the simulations probing much shorter time scales. In our studies of polymer nanocomposites, we find that the dynamics near nanoparticle are drastically different from the bulk dynamics, and the changes in the dynamics depend strongly on the polymer-particle interactions. I will show that the changes in the dynamics have surprising features that cannot be rationalized as a simple change in the density near the particle surface, and these findings lead us to probe fundamental questions about the nature of gradients in mobility in supercooled liquids and amorphous solids.
MXenes are large family of two-dimensional (2D) transition metal carbides and nitrides of Mn+1XnTz composition; where M is an early transition metal (e.g. Ti, V, Mo, Nb) and X is either carbon or nitrogen, “Tz” stands for a mixture of surface terminations (e.g. O, OH, F, Cl), and n can be 1, 2, or 3. So far, about two dozens of MXenes have been produced experimentally (e.g. Ti3C2, V2C, Nb2C, Mo2C, (V0.5,Cr0.5)3C2, Ti3CN, Ta4C3, and Nb4C3). In addition, ab initio calculations predicted many others to be stable. Combining the metallic conductivity of transition metal carbide/nitrides with the hydrophilic nature of their terminated surfaces place MXenes in a unique position among all other 2D materials.
MXenes can be intercalated by a wide range of intercalants from mono- and multi-valent ions to organic and inorganic molecules. Since their discovery, intercalation has been of a critical importance for MXenes processing and applications including electrochemical energy storage, water purification and sensing. However, very little has been known for the nature of intercalant and the bonding between MXenes surface and the intercalant. Using various neutron scattering techniques, we studied MXenes intercalation for two systems: Ti3C2/ion/water and Ti3C2/urea/water. In this presentation, the recent fundamental findings and understanding for the complexity of intercalations in MXenes, will be discussed. In addition, the performance of MXenes as electrode materials hosting ions for batteries and supercapacitors and their performance as electrocatalyst will be presented.
“What happens when you add water?” is possibly the most frequently asked question after presentations in heterogeneous catalysis. This question is indeed paramount, and I will report on our group’s recent studies of the promotional and inhibiting role of water for CO and H2 oxidation at Au/TiO2 interfaces, as well as for the reduction of phenolic alcohols over Ru/TiO2.
Preferential oxidation (PrOx) of CO is a promising energy efficient alternative to CO methanation for purifying H2 streams from steam reforming processes. With high-purity H2 being the desired product, the obvious challenge is to find a catalyst that readily oxidizes CO, but does not burn H2. Through integrated experimental and computational studies we have produced evidence suggesting that O2 and H2 activation over Au/TiO2 catalysts occurs at the metal-support interface (MSI). The activation of O2 on Au is assisted by support protons originating from hydroxyl groups or weakly adsorbed water molecules. Meanwhile, H2 dissociation across the MSI occurs heterolytically resulting in a Au-hydride and a proton on the oxide support. Notably, H2 activation is inhibited by reduced charge transfer from Au to the proton acceptor site located on a basic support hydroxyl.
The concept of water-modulated acid/base strength of sites at the MSI is rather general. For example, we show through first principles kinetic Monte Carlo simulations that the selectivity for direct deoxygenation over hydrogenation of phenol and m-cresol during reductive treatment with H2 can be tuned by adjusting the water partial pressure. In this reaction, Brønsted acidic protons co-catalyze C–OH bond cleavage and are recreated by heterolytic activation of H2 across the Ru/TiO2 interface. Thus, rather than invoking the traditional redox terminology such as hydrogen spillover, support reduction or vacancy defects, we provide a new interpretation of the support effects in terms of acid/base chemistry.
Overall, these examples demonstrate the sensitivity of oxide chemistry to the presence of various amounts of moisture, which in turn opens up interesting opportunities to improve catalytic activity and selectivity without the need for time-consuming catalyst design.
Stimuli-responsive polymers are materials which show large reversible structural changes in response to small changes in the environment. Recent work in our lab has examined the various roles that hydrophobicity, electrostatic interactions, and hydrogen bonding play in the response. Results for pH and thermal responsive polymers will be discussed.
All crystalline solids are believed to have polymorphism which is the capability of forming different crystal structures. Polymorphism is important in a myriad of practical applications of solid compounds such as pharmaceutical solids, electronic materials, and catalysts. However, the origin of the polymorphic behavior of solid compounds is still not well understood. We investigated the fluid-to-solid and solid-to-solid phase transitions of close-packed structures of strongly-segregated polymeric block copolymer micelles, model colloidal spheres. In the fluid-to-solid transitions, we observed if the crystal domains of close-packed micelles are formed small by deep temperature quenching, the block copolymer micelles form metastable hexagonal-close packed (HCP) and random stacking of two-dimensional close-packed layers (RHCP) structures instead of the face-centered cubic (FCC) that is believed thermodynamically stable. Interestingly, as the crystal domains of the non-cubic structures grow, these metastable close-packed structures transform to the stable FCC. In the solid-to-solid transition case, we observed that the martensitic shear transformation is employed for the transition from the initial kinetically-arrested FCC structures to stable HCP, but the martensitic transition could be initiated only when the crystal domains become sufficiently small. Furthermore, the martensitic transformation appears to utilize a specific transformation pathway due to the morphology of the crystal domains. These observations show that the polymorphic behavior of solids is strongly regulated by the size and morphology of crystal domains and reveals a physical origin of the Ostwald rule of stages in the phase transitions in a quantitative and thermodynamically well-defined way.
Hybrid materials that synergistically combine properties of multiple components provide new opportunities to improve functional (mechanical, electrical, optical, etc.) properties and provide innovative solutions in drug delivery, protective clothing, etc. Leveraging material interactions, we are developing new methods in self-assembly to produce functional polymer nanoparticles and nanofibers. Flash NanoPrecipitation (FNP) is a rapid and scalable method for polymer directed self-assembly of nanoparticles typically driven by hydrophobic interactions. Since self-assembled structures such as micelles facilitate organic phase reactions in a bulk aqueous environment, we use FNP to incorporate catalytic gold nanoparticles into polymer nanoreactors. Formulation of nanoreactors with tunable functional properties (e.g. catalyst loading, nanoreactor size) will be discussed. The solubility parameters of the core material are of particular interest. Biomedical applications of self-assembled nanoparticles via in situ complexation will also be discussed. Bovine serum albumin, a model protein, was encapsulated via rapid complexation with tannic acid and stabilization with a cationic polyelectrolyte. Encapsulation efficiency was ~80%. We aim to achieve thermochromic fibers by incorporating cholesteryl ester liquid crystals, a class unique soft material with thermochromic properties arising from their molecular structure. Methods to fabricate thermochromic fibers with hierarchical core-shell structure via in situ self-assembly during fiber processing will be discussed.
Sponsored through generous gifts from members of The Chemical and Biomolecular Engineering Board of Advisors.
A better understanding of how molecules interact in aqueous solutions has ramifications across the biosphere, lithosphere, atmosphere, and hydrosphere. For example, aqueous solutions of dissolved organic molecules and salts are central to all of biology and biochemistry. Unsurprisingly, documented studies of how organic solutes, dissolved salts, and water interact with each other arguably go back to at least the late 18th Century with Franz Hoffmeister’s seminal work on protein solubility. However, to date no comprehensive atomistic model of the interactions between this trinity of solute, salt, and water has been forthcoming.
One major facet of our research is building up an atomistic viewpoint of aqueous supramolecular chemistry, and in doing so building systems that engender unusual phenomena; for example, water-based, yoctoliter reaction vessels. This presentation will focus on our recent studies examining the aqueous supramolecular interactions involving deep-cavity cavitands and small ions, and how these interactions control their bulk properties. Relatedly, the presentation will also summarize how understanding aqueous supramolecular chemistry can lead to novel supramolecular containers that function as yoctoliter reaction vessels, and tools for bringing about novel separation protocols.
Polymeric materials, due to their adaptability and array of functionality, touch almost every aspect of our daily lives. This seminar will focus on two areas in which new behavior was engineered into “old” materials. In the first part, I will discuss my lab’s efforts to design new polymers for water treatment membranes. Membrane-based water purification techniques are the current state of the art, but face limitations including thermodynamically limited transport, high material and operation costs, the perm-selectivity tradeoff, and fouling-prone or chlorine-sensitive membrane materials. Through the addition of charged sites, specifically zwitterions, to poly(arylene ether sulfone)s, the hydrophilicity, water permeability, and fouling resistance were all improved while maintaining constant salt rejection.
In the second part, I will discuss a new paradigm for studying molecular-level behavior in nanocomposites. These multifunctional materials enable combinations of tunable optical properties, smart sensing, conductivity, and excellent thermomechanical properties. Currently, mechanoresponsive polymers use a limited subset of active backbone chemistries to yield changes in optical properties. My lab has developed a strategy to yield mechanoresponsive fluorescence by adding quantum dots and fluorescently labeled carbon nanotubes. Pronounced changes in fluorescence emerge following plastic deformation, indicating a transduction of mechanical force into fluorescence. Thus, the force activation of fluorescence for quenching pairs can serve as a general strategy to develop new nanocomposite matrices that impart desirable functionalities, including damage sensing and robust mechanical strength.
Polyimides are at the forefront of advanced membrane materials for CO2 capture and gas purification processes. Recently, “ionic polyimides” (i-PIs) have been reported as a new class of condensation polymers which combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this work, the CO2 separation characteristics of ionic polyimides are modeled using molecular dynamics simulations in combination with grand canonical Monte Carlo calculations. The performance of neat i-PI systems is evaluated, as well as composite structures containing both i-PIs and various ionic liquids (ILs). The i-PI+IL composites are based on combinations of 1-n-butyl-3-methylimidazolium ([C4mim+]) cations with three common molecular anions: (bis(trifluoromethylsulfonyl)imide ([Tf2N-]), tetrafluoroborate ([BF4-]), and hexafluorophosphate ([PF6-]). It is found that 50 mol% IL inclusion can increase CO2/CH4 selectivity by 16% in [BF4-]-based materials and by 36% in [PF6-]-based materials from mixtures of 5% CO2 / 95% CH4. While the [BF4-]-based system shows higher CO2/CH4 selectivity, the [Tf2N-]-based system shows higher CO2/N2 gas selectivity. A comprehensive structural analysis (fractional free volume (FFV), pore size distribution, surface area, etc.) is used to highlight the underlying differences among the different i-PI+IL systems that lead to the different adsorption properties.
13C metabolic flux analysis (MFA) is the gold standard approach for quantifying rates of biochemical reactions in living cells. It has been widely applied to debottleneck the metabolism of industrial host organisms, but it is now being increasingly used to investigate metabolic disease mechanisms both in cellular and in vivo models. Over the past decade, my lab has focused on establishing novel 13C MFA tools and approaches that enable us to probe entirely new aspects of metabolism previously inaccessible to measurement. In particular, we have developed a publicly available software package called INCA that automates the computational workflow of MFA. I will discuss several ongoing studies where my lab has leveraged INCA to (i) identify targets for metabolic engineering of host cell factories and (ii) investigate metabolic disease mechanisms using 2H/13C MFA to simultaneously assess gluconeogenesis, citric acid cycle, and anaplerotic fluxes in conscious, unstressed mice. These studies have established 13C flux analysis and the INCA software package as a comprehensive platform to map carbon fluxes in microbial and mammalian cell cultures, as well as whole animals.
We use models in science and engineering extensively. We use them to make predictions about the behavior of systems, to optimize designs, and to understand why systems behave the way they do. Most of our models are built from physical principles, and the parameters in the models are usually determined from measured data. That data is often expensive to gather, but the model is then cheap to evaluate. The accuracy of these models depends both on the depth of understanding we have, and the quality of the data, and when we hit the limit of our understanding it is difficult to make better models. Machine learning offers a path forward to build models that are not necessarily based on physics, but which more accurately predict outputs.
We are interested in building models that allow us to perform molecular simulations that require many (hundreds of thousands) of calculations. These are not practical with quantum chemical calculations, which are too expensive to run at this scale. Existing molecular force fields are efficient enough for this, however, they lack the accuracy required to obtain meaningful results. I will present how we are using machine learning in conjunction with quantum chemical calculations to develop efficient models that can be used to simulate effects such as segregation, diffusion, etc., which can only be probed using simulation methods such as Monte Carlo and molecular dynamics.
Machine learning has more to offer science and engineering than just model development. I will also discuss some aspects of how machine learning works, particularly the role that automatic differentiation has in machine learning. This has implications for many types of scientific programming, and may enable new ways to think about science and engineering problem solving.
Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermo-sensitive polymer that exhibits a low critical solution temperature (LCST) at around 305 K in aqueous solutions. The coil-to-globule transition of PNIPAM can be induced by a small temperature variation (1~2 K) accompanied by abrupt conformational changes. The LCST behavior of PNIPAM has been attracting research interests for several decades because of its implication in a number of living phenomena, especially on protein folding and DNA packing. However, the coil-to-globule transition of PNIPAM has been greatly affected by some additives, such as surfactants and inorganic salts. Therefore, the researches on the effects of the additives on the coil-to-globule transition of PNIPAM have gained great realistic significance. In this work, with the applications of laser light scattering (LLS), viscometry as well as high-resolution nuclear magnetic resonance (NMR), the effects of the anion surfactant sodium n-dodecyl sulfate (SDS) on the coil-to-globule transition of PNIPAM under various temperatures and surfactant concentrations have been systematically studied. Besides, the effects of eight inorganic salts on the coil-to-globule transition of this polymer have been also explored. Several interesting results have been drawn.
The multiscale self-assembly of atoms, molecules, and particles is the origin of all physical mesoscopic matter. The spatial organization, symmetry, and physical properties of the assembled structures are determined by thermodynamic characteristics of their building blocks. Colloidal particles are emerging as models for understanding governing principles of directed-assembly and non-equilibrium response of advanced materials. Here, I will present the concept of using external field driven interactions to direct the assembly and spatial migration of colloids. First, I will present the principle of using magnetostatic interactions to direct surface patterning using sessile drop drying. In droplets of magnetite nanoparticles, magnetic establish a microconvection from droplet edge to center. This magnetostatic convection is used to assemble secondary nonmagnetic particles in droplets, allowing for the assembly of four distinct kinetically stable states, and enabling a new route for surface patterning. Second, I will introduce the concept of directing spatial motion and non-equilibrium behavior of metal-dielectric patchy colloids using external electric field. The electric field drives a local force imbalance around the particle, resulting into its direction motion. I will demonstrate that the particle’s velocity, chirality, and its 3D trajectory can be programed by engineering the patchy particle/cluster size and shape. I will show that the coupling of translation and rotational component of the energy enables programming helical motion in spherical colloids, and provides an alternative mode of navigating through complex cross-linked matrices. This approach introduces a new method of engineering the assembly and self-propulsion of microparticles, which could lead to the development of advanced micro-motors and miniature robots capable of navigating through complex biological environments.
Sponsored through generous gifts from members of The Chemical and Biomolecular Engineering Board of Advisors.
Rafael Verduzco | Rice University
Nanette Boyle | Colorado School of Mines
Christopher G. Arges | Louisiana State University
Kristala Jones Prather | Massachusetts Institute of Technology