Search

Redox hopping is the dominant charge transport mechanism in many catalyst-modified metal-organic frameworks (MOFs). Previous studies have shown that ion diffusion is the rate-determining step of redox hopping, but the realization regarding to the fundamental mechanism of redox hopping in MOF is still infantile. In this dissertation, we will discuss the redox hopping process in MOFs from multiple perspectives, including how to use the Scholz model to analyze the coefficients in redox hopping, the influence of the type of carrier, the influence of electrolyte concentration, and the influence of temperature on redox hopping, so as to try to reveal the mechanism of the redox hopping process and make some constructive suggestions for the future design and application of MOF based on this topic.

An implantable closed-loop insulin delivery device (INsmart device) containing a glucose responsive gel has been developed within the INsmart research group, over a period of 10 years, to mimic pancreas. In this thesis, the reliability and performance capability of the INsmart device was studied for future clinical use. Investigations into the device material compatibility with insulin solution, assessed by monitoring insulin loss and degradant formation over a period of 31 days using RP-HPLC have shown that stainless steel and titanium are the most compatible materials. Polycarbonate contributes to insulin loss after 11 days, resin might not be the best material and polyurethane is the least compatible for future device designs. To study insulin delivery mechanism and kinetics from the device, fluorescently labelled human insulin (FITC-insulin) was synthesised and characterised using RP-HPLC and MS, to produce a product with predominantly di-labelled conjugate (>75%) with no unreacted FITC or native insulin. Clinically used insulin analogues were also fluorescently labelled to produce predominantly di-labelled FITC-insulin conjugate with potential future biological and in vitro applications. The drug release mechanism from the glucose sensitive gel held in the INsmart device, studied using fluorescein sodium was determined as a Fickian diffusion controlled release mechanism. The diffusion coefficient (D) for FITC-insulin in the non-polymerised dex2M-conA gel (NP gel) determined using mathematical models, QSS and TL slope methods was 1.05 ± 0.02 x 10-11 m2/s and in the cross-linked dex500MA-conAMA gel (CL gel) was 0.75 ± 0.06 x 10-11 m2/s. In response to physiologically relevant glucose triggers in the NP gel, the diffusivity of FITC-insulin increases with increasing glucose concentrations, showing a second order polynomial fit, device thus showing glucose sensitivity and graded response, mimicking pancreas. Rheological measurements further confirmed the gel glucose responsiveness demonstrated by a third order polynomial fit between FITC-insulin D and the NP complex viscosity in response to increasing glucose concentration. The knowledge of FITC-insulin diffusion kinetics in the gel has aided in making some theoretical predictions for the capability and performance of the INsmart device. Alternate device geometry and design optimisation is also explored.

This thesis primarily focuses on the electrochemical properties of the H2/H+ redox couple, at various metallic electrodes in room temperature ionic liquids. Initially, a comprehensive overview of room temperature ionic liquids, RTILs, compared to conventional organic solvents is presented which identifies their favourable properties and applications, followed by a second chapter describing the basic theory of electrochemistry. A third chapter presents the general experimental reagents, instruments and measurements used in this thesis. The results presented in this thesis are summarized in six further chapters and shown as follows. (1) Hydrogenolysis, hydrogen loaded palladium electrodes by electrolysis of H[NTf2] in a RTIL [C2mim][NTf2]. (2) Palladium nanoparticle-modified carbon nanotubes for electrochemical hydrogenolysis in RTILs. (3) Electrochemistry of hydrogen in the RTIL [C2mim][NTf2]: dissolved hydrogen lubricates diffusional transport. (4) The hydrogen evolution reaction in a room temperature ionic liquid: mechanism and electrocatalyst trends. (5) The formal potentials and electrode kinetics of the proton_hydrogen couple in various room temperature ionic liquids. (6) The electroreduction of benzoic acid: voltammetric observation of adsorbed hydrogen at a Platinum microelectrode in room temperature ionic liquids. The first two studies show electrochemically formed adsorbed H atoms at a metallic Pt or Pd surface can be used for clean, efficient, safe electrochemical hydrogenolysis of organic compounds in RTIL media. The next study shows the physicochemical changes of RTIL properties, arising from dissolved hydrogen gas. The last three studies looked at the electrochemical properties of H2/H+ redox couple at various metallic electrodes over a range of RTILs vs a stable Ag/Ag+ reference couple, using H[NTf2] and benzoic acid as proton sources. The kinetic and thermodynamic mechanisms of some reactions or processes are the same in RTILs as in conventional organic or aqueous solvents, but other remarkably different behaviours are presented. Most importantly significant constants are seen for platinum, gold and molybdenum electrodes in term of the mechanism of proton reduction to form hydrogen.

Electrostatics plays a crucial role in the membrane biology. Negatively charged lipids (such as PS, PA and PIP2) are subject to redistribution under the action of electrostatic forces during various signalling events. Membrane recruitment of multiple signalling proteins (such as MARCKS or Src kinase) is often maintained by positively charged polybasic domains (PD). Even though adsorption of these proteins to the cellular membrane has been extensively investigated, very little is known about how electrostatic interactions contribute to their membrane lateral dynamics. This thesis presents an investigation of the contribution of electrostatic interactions to the membrane lateral dynamics by means of novel computational tools. First, I developed a dynamic Monte-Carlo automaton that faithfully simulates lateral diffusion of the adsorbed positively charged PD of a peripheral membrane protein, as well as the dynamics of mono- (PS, PA) and polyvalent (PIP2) anionic lipids within the bilayer. This model allowed to investigate the major characteristics of protein-membrane diffusion on the uniform membrane. In agreement with earlier results, the simulations revealed the following microscopic phenomena: 1) Electrostatic lipid demixing in the vicinity of the PD; 2) PD interacts with PIP2 stronger than with monovalent lipids. On the spatially heterogeneous membrane the automaton predicted a directional drift of the PD, which was validated by a simple mean-field analytical model. The predicted phenomenon could potentially play a major role in membrane domain formation. To test this hypothesis and to investigate the membrane dynamics on larger scales I developed a continuous model, which was based on the results of the automaton simulations. The results of the continuous model and the Monte-Carlo simulations were shown to be in quantitative agreement. The continuous model allows one to simulate the electrostatic membrane dynamics on micrometer scales and can be used to describe various biologically important processes, such as endocytic cup initiation.

Release of small molecules from polymeric materials has wide applications in the delivery systems of active molecules such as drug, fragrance, and semiochemical. Rubber materials are good candidates for the excipients of those systems due to properties such as good flexibility, permeability, and biocompatibility. Factors like material properties, mechanical loading, and molecular interactions may affect the release of small molecules in those systems. Therefore, understanding how those factors affect the release is key to the formulation, design, and evaluation of those systems. To study the effects of materials properties, vulcanized natural rubber sheets with different crosslink densities, loaded with small molecules with different boiling points, molecular weights, and chemical moieties were prepared. Release experiments were performed to determine the mass transfer and diffusion coefficients of those molecules and corresponding numerical models were built. Good agreements between experiments and numerical simulation were observed. It was found that the mass transfer coefficients of those small molecules decreased with increasing boiling points but remained practically constant among rubber sheets with different crosslink densities. The diffusion coefficients did not show evident correlation with molecular weights but their relationships with crosslink densities indicated that there might be an optimal crosslink density where a small molecule reached its maximum diffusion coefficient. To study the effects of uniaxial tensile strain, silicone rubber sheets loaded with triacetin were stretched and held at different lengths up to 125% engineering strain. The mass transfer and diffusion coefficients of triacetin were determined experimentally. It was found that there was no significant change of diffusion coefficient as the applied strain increased, which might result from two microstructure changes that had conflicting effects on diffusion: chain orientation and free volume deformation. To study the effects of molecular interactions, the mass transfer and diffusion coefficients of three model molecules, octanol, octyl acetate, and octyl butyrate were determined from silicone rubber sheets loaded with only one or two of the three molecules. Differential scanning calorimetry and Fourier transform infrared spectroscopy were used to characterize molecular interactions. It was found that the release of octanol conformed to the Fickian diffusion pattern at low initial concentration but deviated as the concentration increased due to hydrogen bonding between octanol and the silanol group of the silica filler. When two small molecules were released simultaneously, different effects of one model molecule on the diffusion coefficient of another were observed and explained by the competing effects of plasticizing and hydrogen bonding/dipole-dipole interaction. In summary, the outcome of this work promoted the understanding on important influencing factors of the release of small molecules from rubber and other polymeric materials and can serve as support and reference for the formulation, design, and evaluation of such delivery systems.

Karst landscapes can serve as carbon sinks when carbon dioxide (CO2) reacts with water to form carbonic acid, which then weathers carbonate rocks. However, CO2 can also move through the subsurface via gas diffusion, a process that is not well-understood in karst systems. This study focused on quantifying CO2 diffusion within a karst sinkhole. The objectives of this study were to: 1) identify the depth of the zero-flux plane (i.e., depths of local maximum CO2 concentrations), analyze the distributions of concentration gradients, and investigate the validity of a uniform concentration gradient throughout the profile; and 2) assess the influences of vertical position and seasonality on CO2 fluxes within this sinkhole. The study site contained three locations within the sinkhole, including shoulder, backslope, and toeslope locations. Each location had three soil CO2 and three soil water content/temperature sensors placed at 20, 40, and 60 cm depths. Zero-flux planes were seldom detectable during the warm season (April-September) but were frequently found near the surface (20 or 40 cm) during the cool season (October-March). The common assumption of a uniform concentration gradient was often invalid based on relative concentrations between sensor pairs. As for the second objective, CO2 fluxes generally followed a trend of upward fluxes in warmer months that was partially offset by downward fluxes during the cooler months. These study results provide new insight into CO2 dynamics in a karst system, and suggest that subsurface processes such as chemical weathering and cave ventilation affect the direction and magnitude of CO2 fluxes.

Abstract: In the first part of this study, we use an analytical approach and the interpolation-supplemented lattice Boltzmann method (ISLBM) to quantify convective and diffusive transport during CO2 dissolution in the oil bulk phase. In the first step, we use a turbulence analogy and the ISLBM to determine the relationship between the Rayleigh number (Ra) and the ratio of the pseudo-diffusion coefficient to the molecular diffusion coefficient (D^*/D ). We then use experimental data from two oil samples, condensate and crude oils, to validate the obtained relationship between D^*/D and Ra. We also use the Sherwood number (Sh), total mixing and diffusive transport curves to analyze different periods during CO2 dissolution for condensate and crude oils. We focus in particular on how Ra affects the characteristics of density-driven fingers and the convection field. Our results show that there is a logarithmic trend between D^*/D and Ra. Analysis of the total mixing and diffusive curves indicates that the CO2 dissolution process can be divided into three distinct periods, namely diffusive transport, early convection, and late convection. We find that more than 50% of the ultimate CO2 dissolution occurs in the early convection period. We also show that the analytical results obtained for the critical time and critical depth at the onset of convection is in good agreement with those of ISLBM. After the onset of convection, the formation of initial fingers leads to enhanced convective transport, with marked implications for the concentration variance and mixing rate. In the second part of this study, we propose a novel analytical solution to predict the diffusion coefficient and depth of gas (C1 and a mixture of C1/C2 with the molar ratio of 70/30) penetration during the soaking period of the cyclic gas injection process. Our analytical solution is derived from the modeling of gas-phase pressure declines by use of mass-balance and continuity equations. We model mass transport during the soaking period as a counter-diffusion process, and found that diffusion coefficient and velocity are controlled by the pressure gradient at the early soaking times and the concentration gradient when the soaking progresses. The estimated diffusion coefficients through our solution for a mixture of gas/oil under tight porous media conditions are in agreement with published literature. We calculate the depth of gas penetration in the plug, and show that the gas front reaches the other end of the plug at the end of the soaking period in the gas-mixture case. Also, the model is capable of predicting swelled oil volume by gas dissolution. A thermodynamic consistency check was performed by comparing the amount of leaked-off gas in the experiment and that of the model. The results show that these values are in the same range.

Abstract: Viscosity reduction is critically important for heavy oil extraction and experimental studies have shown this can be achieved by injection of a light solvent into a reservoir and dilution of heavy oil in place. Solvent-based methods have challenges like slow diffusion process and oil production rate as well as high cost of solvent. There is presently a lack of understanding regarding the pore-scale mechanisms involved in dilution of bitumen. Therefore, microfluidic as the science of fluid flow in micro-scale, has a great potential to play a significant role in the investigation and development of solvent-based heavy oil extraction methods. However, microfluidic is limited in applicability due to materials and operating condition.In the first section of this work, possibility of using poly(dimethylsiloxane) (PDMS) microchannels in solvent-based bitumen extraction methods is studied. Swelling ratio of PDMS samples in different organic solvents and diluted bitumen were evaluated. We were particularly interested in the kinetic of swelling, deformation, and discoloration of PDMS. The hypothesis was that a PDMS microchip can still be used if the experiment time is much shorter than the time it takes for PDMS discoloration and deformation. The effect of surface coating on bitumen resistivity and compatibility of PDMS slabs was also studied.Choosing the best material for micromodel fabrication, this thesis focuses on the experimental observation and characterization of solvent-based bitumen extraction. In the second section, a microchannel was used to measure hexane diffusion in bitumen with tremendously tiny amount of solvent and bitumen compared with prevalent methods. Semitransparent nature of bitumen facilitates the application of light transmission imaging during the time and relating intensities to solvent concentration. Diffusion mass transfer was studied with consideration of Fickian and Single-File mass transfer mechanisms and corresponding equation was applied to calculate diffusion coefficient. The accuracy of Fickian mass transfer mechanism was confirmed with calculating values of fitting parameter (i.e. nw) in addition to superimposition of data for both techniques. Comparison between constant and concentration dependent coefficients revealed that application of constant value causes up to 100 times larger coefficients specially at high (>0.85) and low (

Cuprous and cupric bromide electrochemistry and speciation was examined in electrolytes and concentrations of interest to an all-copper flow battery (CuFB) in a bromide ion supported electrolyte. The all-CuFB was also examined and it was shown that an all-CuFB can operate at geometric current densities of up to 300 mA cm-2 at a temperature of at least 50°C. In addition, the battery was cycled for 50 cycles from 0 to 60% SOC with plating capacities of 125 mAh cm-2 with a voltaic efficiency of 64% at a temperature of 50°C. However, capacity fade (and thus fading coulombic efficiency) was observed due to non-adherent plating on the carbon electrode. Morphology and nucleation of cuprous bromide was examined via current time transients and qualitative plating experiments. Cuprous bromide electrodeposits adhered to gold, silver, and platinum but did not adhere to titanium or glassy carbon. This may due to a lattice mismatch between the copper crystalline structure and the substrate structure. It was determined that electronucleation of cuprous bromide at 30°C between overpotentials of 10 and 150mV on glassy carbon substrates most closely resembled progressive nucleation. Utilizing in situ methods to study the electrochemical nucleation and morphology of cuprous bromide led to the examination of high aspect ratio small gap height cells with side by side electrodes which are typically used for in situ ec-S/TEM. A Wagner number was derived which allows one to determine whether uniform current distribution is expected in the current ec-S/TEM cell design and chemistry of interest.

Many applications in reactor physics still use the diffusion equation on a full reactor core with homogenized parameters for each assembly. Many homogenized diffusion coefficient definitions have been proposed over the years, and some others are still proprietary. Inspired by the promising results reported for the Cumulative Migration Method (CMM) by Z. Liu, B. Forget and K. Smith at MIT, this work formulates a mathematical framework that fits that method. In this framework and the original CMM papers, the definition of diffusion the coefficients is based on choosing the diffusion coefficients to preserve a physical property of the system. The proposed framework provides insights into the workings of CMM, and allows for an equivalent implementation with deterministic transport codes. Moreover, this framework provides two competing definitions for the diffusion coefficients that share the same motivation, and shows that CMM can be seen as an implementation of one of those. The framework is developed both for infinite homogeneous problems, where the solutions can be derived by hand, and for infinite lattice problems of a single unit cell, where solutions must be solved numerically.

Thin films of Nafion, p-phenylenediamine (PPD) and crosslinked choline oxidase (ChOx) were coated onto a Pt rotating disk electrode (RDE). A MSR electrode rotator with a three-electrode system was used to evaluate the mass transport properties of hydrogen peroxide (H2O2) within these thin films at room temperature. Diffusion coefficients of H2O2 within Nafion, PPD and crosslinked ChOx were obtained from Koutecky-Levich analysis. The obtained values are (2.4 � 0.2) � 10-7 cm2/s for Nafion, (1.7 � 0.3) � 10-8 cm2/s for PPD and (2.9 � 0.3) � 10-6 cm2/s for crosslinked ChOx using a simple diffusion model. An extensive comparison of the results to many reported in the literature suggests good agreement. These diffusion coefficient estimates provide good parameter values for mathematical models of thin-film coated biosensors that require Nafion and PPD coatings for permselectivity.

The goal of this research was to study the electrochemical behavior of tri-nuclear clusters of molybdenum and tungsten. In addition, the feasibility of using these clusters as catalysts for the purpose of oxidizing ethanol was investigated. Four tri-nuclear cluster compounds were studied: hexa-µ2-acetatotriaquadi-µ3-oxotrimolybdenum (IV, IV, IV) trifluoromethanesulfonate [Mo3O2(O2CCH3)6(H2O)3](CF3SO3)2, hexa-µ2-acetatotriaquadi-µ3-oxodimolybdenum (IV, IV) tungsten (IV) trifluoromethanesulfonate [Mo2W2O2(O2CCH3)6(H2O)3](CF3SO3)2, hexa-µ2-acetatotriaquadi-µ3-oxomolybdenum (IV) ditungsten (IV, IV) trifluoromethanesulfonate [MoW2O2(O2CCH3)6(H2O)3](CF3SO3)2, and hexa-µ2-acetatotriaquadi-µ3-oxotritungsten (IV, IV, IV) trifluoromethanesulfonate [W3O2(O2CCH3)6(H2O)3](CF3SO3)2.Data was gathered from experimental results with cyclic voltammetry for the four tri-nuclear clusters. Initially, an ionic liquid, EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), was used as the solvent. Subsequent solvents for use with these clusters were investigated, including ACN (acetonitrile) and NMF (N-methylformamide). The secondary solvent system settled on was the DMSO-TBAHFP solvent system. Each tri-nuclear cluster displayed a reversible redox reaction and one or more irreversible reduction reactions. The redox peak potentials were found to be Ep,a: -0.44V and Ep,c: -0.42V for Mo3, Ep,a: -0.32V and Ep,c: -0.43V for Mo2W, Ep,a: -0.31 V and Ep,c: -0.44 V for MoW2, and Ep,a: -0.42 and Ep,c: -0.46 for the W3 tri-nuclear cluster. The irreversible reduction reactions for each tri-nuclear cluster were observed at Ep,c(2): -0.74 for Mo3, Ep,c(2): -1.15 for Mo2W, Ep,c(2): -1.14 for MoW2, and Ep,c(2): -0.84 for the W3 tri-nuclear cluster. The diffusion coefficients in DMSO were determined to be DMo3 = 9.105E-06 cm2s-1, DMo2W = 1.743E-05 cm2s-1, DMoW2 = 1.764E-05 cm2s-1, and DW3 = 1.991E-05 cm2s-1.Exploring the electrocatalytic capability of these compounds was another effort made, by attempting to electroplate the compounds on platinum electrodes. Although some types of deposition events did appear to occur, it is unlikely they were of the intact tri-nuclear clusters. Thus far, the ethanol molecule has been partially oxidized, but breaking the carbon-carbon bond in the molecule proved to be a challenge that was not achieved.

Pyroprocessing is an electrochemical method that is capable of separating uranium (U) and minor actinides from LiCl-KCl eutectic salt where used nuclear fuel (UNF) is dissolved. During the process, fission products including rare earth metals (RE) continually accumulate in the salt and eventually affecting uranium recovery efficiency. To reduce the salt waste after uranium and minor actinides recovery, electrolysis is performed to drawdown rare earth materials from molten salt to restore salt initial state. Present research focus on the development of RE fundamental physical properties in LiCl-KCl eutectic salt. These properties includes apparent potential, activity coefficient, diffusion coefficient and exchange current density. Additional properties including charge transfer coefficient and reaction rate constant are calculated during the analysis. La, Nd and Gd are three RE that we are particularly interested in due to the high ratio of these elements in UNF (La, Nd), the well-studied properties in dilute solution to provide a base for comparison, and the highest standard potential among all RE (Gd). Fundamental properties of La, Nd, Gd in LiCl-KCl eutectic salt are studied at a temperature ranging from 723 K to 823 K and RE concentration ranging from 1 wt% to 9 wt%. These properties are studied by electroanalytical methods including Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), Tafel method, Chronocoulummetry (CC) and Chronopoentiometry (CP). BET model that considers the RE adsorption on the electrode is developed for diffusion coefficient analysis. Electrode kinetic model is developed to account for mass transfer effect during the analysis of exchange current density. Correlations of diffusion coefficient, apparent potential, exchange current density with temperature and concentration are developed. These fundamental data are integrated with a electrolysis model to predict the electrolysis process for RE drawdown from LiCl-KCl salt. The model considers both the diffusion in electrolyte and Faraday process on the electrode surface and a surface layer is introduced to account for the fact that diffusion current is not necessarily equal to the current due to the Butler-Volmer equation. The model is validated by chronoamperometry and chronopotentiometry.

There are diverse technological contexts where fluids and suspensions are perturbed by applied fields like interfaces or intrinsically governed by complex interparticle potentials. When these interactions act over lengthscales comparable to the fluid particle size and become strong enough to frustrate particle packing or rearrangements, they drive systems to exhibit microscopically inhomogeneous (i.e., position-dependent) structural and relaxation responses. We use computer simulations and statistical-mechanical tools to find connections between such frustrating interactions and inhomogeneous fluid responses, which can profoundly impact macroscopic material properties and processing requirements. We first consider how to measure and predict the position-dependent and average diffusion coefficients of particles along inhomogeneous free-energy landscapes (i.e., potentials of mean force). Characterizing diffusion in such inhomogeneous fluids is crucial for modeling, e.g., the transit of colloids across microfluidic devices and of solutes through biological membranes. We validate a practical technique based on the Fokker-Planck diffusion formalism that measures diffusivities based solely on particle trajectory data. We focus on hard-sphere fluids confined to thin channels or subjected to external fields that impose density fluctuations at various wavelengths. We find, for example, that hydrodynamic predictions of tracer diffusion in confinement are surprisingly robust given non-continuum solvents. We also demonstrate that correlations between fluid static structure and diffusivity can qualitatively depend on the lengthscale of density fluctuations or the onset of supercooling. We next examine fluids governed by competing short-range attractions and long-range repulsions that drive formation of equilibrium cluster phases, which comprise monodisperse aggregates of monomers. The formation of such morphologies greatly impacts, e.g., the manufacturing of therapeutic protein solutions. We first address a major challenge in probing the real-space structure of such suspensions: detecting and characterizing cluster phases based on the static structure factor accessible via scattering experiments. Using computer simulations and liquid-state theory, we validate rules for interpreting low-wavenumber features in the structure factor in terms of cluster emergence, size, spatial distribution, etc. We then validate a thermodynamic model that predicts cluster size based on the strengths of monomer interactions, adapting classical nucleation theory to incorporate new empirical scalings for the surface energies of small stable droplets.

Solute uptake and release govern the efficacy of hydrogels in controlled drug delivery, tissue engineering, and chromatographic separations. In soft contact lenses, uptake and release of wetting, packaging, and care-solution agents is extensively employed to improve on-eye lens performance. Key physical parameters are the equilibrium solute partition coefficient and the solute diffusion coefficient in the gel that dictate the amounts and rates of uptake/release, respectively. To investigate the mechanisms of solute uptake and release in hydrogels, this work experimentally and theoretically determines equilibrium partition and diffusion coefficients of prototypical macromolecules and drugs in hydrogels over a wide range of water contents. A hydrogel is a crosslinked polymer network with water-filled voids arranged in an unstructured three-dimensional mesh. Solutes (e.g., drugs, sugars, proteins, polymers) typically partition into and diffuse through the water-filled mesh but are excluded from mesh voids smaller than solute size. Consequently, solute size and the distribution of mesh sizes in the hydrogel-polymer network are vital to understand solute uptake and release. Solutes may also exhibit specific interactions with and, accordingly, adsorb to hydrogel polymer chains by hydrogen bonding or counterion binding. Specific solute adsorption to hydrogel-polymer strands results in larger partition coefficients and diminished effective diffusion rates. To elucidate size effects on aqueous-solute transport rates, diffusion coefficients of large macromolecules in hydrogels with relatively small mesh sizes are investigated experimentally and theoretically. Two photon-confocal microscopy measures transient uptake and release concentration profiles of fluorescently labeled dextrans of varying molecular weight, and fluorescently labeled cationic avidin protein. Dextrans are highly water-soluble polysaccharides. Consequently, they exhibit negligible specific interactions with the hydrogel polymer network. Hydrogel uptake and release follow Fick’s second law with almost identical diffusion coefficients in the uptake and release directions. To interpret our data, we implement a Large Pore Effective Medium (LPEM) model taking into account hydrodynamic drag, steric obstruction, and the distribution of mesh sizes available for solute transport. All necessary parameters are measured independently. In all cases, a priori- predicted diffusion coefficients by LPEM theory display excellent agreement with experiment. In contrast to the nonspecific interacting dextrans, cationic avidin protein exhibits near irreversible adsorption with incomplete loading even after 6 days and incomplete release even after two weeks. Avidin protein uptake and release rates clearly highlight the significance of solute-specific adsorption in understanding solute transport rates in hydrogels. Despite its importance, little attention has been given to how solute-specific interactions affect solute uptake in and release from hydrogels. We measure and theoretically predict partition and diffusion coefficients for prototypical water-soluble drugs in hydrogels where solute-specific binding is pronounced. Hydrogel composition is varied by adjusting the ratio of monomer constituents, 2-hydroxyethyl methacrylate (HEMA) and methacrylic acid (MAA). Partition and diffusion coefficients are obtained through two-photon confocal microscopy and UV/Vis-absorption spectrophotometry upon back extraction. The studied prototypical drugs all exhibit specific adsorption to nonionic MAA and HEMA moieties characterized by greater-than-unity partition coefficients and smaller effective diffusion rates. Conversely, none of the prototypical drugs displayed specific interactions with anionic MAA moieties. To predict equilibrium partition coefficients, we express the partition coefficient as a product of the hydrogel water content and individual enhancement factors for size-exclusion, nonspecific electrostatic interactions, and specific adsorption. Again, all necessary parameters are obtained independently. To predict effective diffusion coefficients, we extend LPEM for specific-solute adsorption and impose local equilibrium. As with the non-interacting dextran solutes, predicted partition and diffusion coefficients are in good agreement with experiment. A framework is now available to predict solute partitioning and diffusion in solute-hydrogel systems that exhibit specific interactions. The developed theories for solute partitioning are further extended for direct application to silicone-hydrogel contact-lens materials. Silicone hydrogels (SiHy) are microphase-separated materials with silicone domains for oxygen transport and hydrophilic-polymer domains for aqueous-solute transport. Equilibrium silicone-hydrogel water and solute uptake are measured and predicted with an extended partitioning theory assuming that water and aqueous solutes reside only in the hydrophilic-polymer phase, whereas oleophilic solutes partition primarily into the silicone microphase. Excellent agreement is found between theory and experiment. Significantly, our development provides estimation of partitioning properties in silicone hydrogels based solely on synthesis formulation chemistry. Finally, we deduce compositional properties of a laminated soft contact lens, DAILIES TOTAL 1® (delefilcon A) through measurement of fluorescent solute partition coefficients. Measured partition coefficients and solute-partitioning theory establish (1) the silicone-hydrogel core of the laminated lens is structurally similar to that of a non-laminated commercial SiHy soft contact lens, O2OPTIXTM, (2) the laminated-lens surface-gel layers are ~10 µm in thickness, (3) the laminated-lens surface-gel layers are of higher water content than the core, and (4) the surface-gel layers of the laminated lens are anionic, whereas the core is nonionic. Importantly, with solute-uptake properties known, our proposed solute-partitioning theory provides a means to elucidate hydrogel physico-chemical properties.

Cellular membranes are complex assemblies and a clear understanding of the physical interactions during their function is of paramount importance. Here, we perform two separate studies for a better understanding of the interactions between membrane compartments and other biomolecules. In the first study, we developed a coupler to integrate a high sensitivity spectrometer with an epi-fluorescence microscope to measure fluorescence spectra of small area samples (400 micrometer squared). We applied our measurements on standard samples, performed three corrections on them and after a linear demixing process, the percentage of FRET efficiency was obtained. The development of this method will be advantageous in future single cell studies for detecting population heterogeneity. In the second study, we investigated the dynamics of membrane lipids in a supported lipid bilayer. Single particle tracking total internal reflection fluorescence microscopy (TIRF) was used to study the lateral mobility of phosphatidylinositol phosphate (PIP) lipids with and without an adsorbed polycationic polymer, quaternized polyvinylpyridine (QPVP). Diffusion coefficients were determined with Brownian and anomalous models. Our results indicate a decrease in diffusion coefficient of the lipids in the presence of QPVP in comparison to its absence, revealing their interaction.

Magnesium, being lightweight, offers potential to be developed into extensive structural applications. The transportation segment has particular interest in Mg and Mg alloy for applications where reduced vehicle weight is proportional to increased fuel efficiency. Aluminum and zinc are two of the most common alloying elements in commercial Mg alloys. They improve the physical properties of Mg through solid solution strengthening and precipitation hardening. Diffusion plays a key role in the kinetics of and microstructural development during solidification and heat treatment. However, there is limited diffusion data available for Mg and Mg alloys. In particular, because Al is monoisotopic, tracer diffusion data is not available. Interdiffusion of Mg solid solution with Zn also does not exist in literature. The diffusional interaction of Al and Zn in Mg solid solution at temperatures ranging from 623 – 723K was examined using solid-to-solid diffusion couple method. The objective of this thesis is two-fold: first, is the examination of interdiffusion in the Mg solid solution phase of the binary Mg-Al and Mg-Zn systems; second, is to explore non-conventional analytical methods to determine impurity diffusion coefficients. The quality of diffusion bonding was examined by optical microscopy and scanning electron microscopy with X-ray energy dispersive spectroscopy, and concentration profiles were determined using electron probe microanalysis with pure standards and ZAF matrix correction. Analytical methods of concentration profiles based on Boltzmann-Matano analysis for binary alloys are presented along with compositional dependent interdiffusion coefficients. As the iv concentration of Al or Zn approaches the dilute ends, an analytical approach based on the Hall method was employed to estimate the impurity diffusion coefficients. Zinc was observed to diffuse faster than Al, and in fact, the impurity diffusion coefficient of Al was smaller than the self-diffusion coefficient of Mg. In the Mg solid solution with Al, interdiffusion coefficients increased by an order of magnitude with an increase in Al concentration. Activation energy and pre-exponential factor for the average effective interdiffusion coefficient in Mg solid solution with Al was determined to be 186.8 KJ/mole and 7.69 x 10-1 m2/sec. On the other hand, in the Mg solid solution with Zn, interdiffusion coefficients did not vary significantly as a function of Zn concentration. Activation energy and pre-exponential factor for the average effective interdiffusion coefficient in Mg solid solution with Zn was determined to be 129.5 KJ/mole and 2.67 x 10-4 m2/sec. Impurity diffusion coefficients of Al in Mg was determined to have activation energy and pre-exponential factor of 144.1 KJ/mole and 1.61 x 10-4 m2/sec. Impurity diffusion coefficients of Zn in Mg was determined to have activation energy and preexponential factor of 109.8 KJ/mole and 1.03 x 10-5 m2/sec. Temperature and compositiondependence of interdiffusion coefficients and impurity diffusion coefficients are examined with respect to reported values in literature, thermodynamic factor, Φ, diffusion mechanisms in hexagonal close packed structure, and experimental uncertainty

Abstract: Single-particle tracking (SPT) is a method used to study the diffusion of various molecules within the cell. SPT involves tagging proteins with optical labels and observing their individual two-dimensional trajectories with a microscope. The analysis of this data provides important information about protein movement and mechanism, and is used to create multistate biological models. One of the challenges in SPT analysis is the variety of complex environments that contribute to heterogeneity within movement paths. In this thesis, we explore the limitations of current methods used to analyze molecular movement, and adapt analytical methods used in animal movement analysis, such as correlated random walks and first-passage time variance, to SPT data of leukocyte function-associated antigen-1 (LFA-1) integral membrane proteins. We discuss the consequences of these methods in understanding different types of heterogeneity in protein movement behaviour, and provide support to results from current experimental work.

The sequence-dependent curvature and flexibility of DNA is critical for many biochemically important processes. However, few experimental methods are available for directly probing these properties at the base-pair level. One promising way to predict these properties as a function of sequence is to model DNA with a set of base-pair parameters that describe the local stacking of the different possible base-pair step combinations. In this dissertation research, we develop and study a computational model for predicting the diffusion coefficients of short, relatively rigid DNA fragments from the sequence and the base-pair parameters. We focus on diffusion coefficients because various experimental methods have been developed to measure them. Moreover, these coefficients can also be computed numerically from the Stokes equations based on the three-dimensional shape of the macromolecule. By comparing the predicted diffusion coefficients with experimental measurements, we can potentially obtain refined estimates of various base-pair parameters for DNA. Our proposed model consists of three sub-models. First, we consider the geometric model of DNA, which is sequence-dependent and controlled by a set of base-pair parameters. We introduce a set of new base-pair parameters, which are convenient for computation and lead to a precise geometric interpretation. Initial estimates for these parameters are adapted from crystallographic data. With these parameters, we can translate a DNA sequence into a curved tube of uniform radius with hemispherical end caps, which approximates the effective hydrated surface of the molecule. Second, we consider the solvent model, which captures the hydrodynamic properties of DNA based on its geometric shape. We show that the Stokes equations are the leading-order, time-averaged equations in the particle body frame assuming that the Reynolds number is small. We propose an efficient boundary element method with a priori error estimates for the solution of the exterior Stokes equations. Lastly, we consider the diffusion model, which relates our computed results from the solvent model to relevant measurements from various experimental methods. We study the diffusive dynamics of rigid particles of arbitrary shape which often involves arbitrary cross- and self-coupling between translational and rotational degrees of freedom. We use scaling and perturbation analysis to characterize the dynamics at time scales relevant to different classic experimental methods and identify the corresponding diffusion coefficients. In the end, we give rigorous proofs for the convergence of our numerical scheme and show numerical evidence to support the validity of our proposed models by making comparisons with experimental data.

The uptake and reaction of methanol at the air-liquid interface of 0-96.5 wt% sulfuric acid (SA) solutions has been observed directly using vibrational sum frequency generation spectroscopy (VSFG) and Raman spectroscopy. Evidence for the formation of methyl hydrogen sulfate (MHS) was obtained by the presence of a new peak in the 800 cm-1 region, not present in either the neat methanol or concentrated sulfuric acid spectra. This peak is attributed to the singly bonded OSO symmetric stretch of MHS. The maximum yield of MHS with a large SA excess is shown to be (95±5)% at –(15±2)°C. No evidence was found to suggest formation of dimethyl sulfate. As the concentration of SA increases from 0–96.5 wt%, the SFG spectra shift from that of methanol to that of methyl hydrogen sulfate. The surface is saturated with a mixture of the three methyl compounds after 15 minutes, although the relative amounts of MeOH, MeOH2+, and MHS vary with SA concentration. Uptake occurred on a much longer timescale, suggesting that uptake of methanol by sulfuric acid solutions is diffusion-limited. The diffusion coefficients for methanol into 0–96.5 wt% sulfuric acid solutions were measured by passing MeOH vapor in N2over the SA solutions and monitoring the uptake using Raman spectroscopy. The value obtained for methanol into water, D = (0.7±0.2) x 10-5cm2/s, is in agreement with values found in the literature. The values of D in 39.2-96.5 wt% SA range from (1–2.7) x 10-6cm2/s with the maximum value occurring for the 59.5 wt% SA solution. This may be due to the speciation of MeOH in the SA solutions or to speciation of the SA solutions. The organization of 1-butanol and 1-hexanol, at air-liquid interfaces was investigated using VSFG. There is evidence for centrosymmetric structures at the surface of pure butanol and hexanol. At most solution surfaces, butanol molecules organize in all-trans conformations. In contrast, the spectrum of 0.052 M butanol in 59.5 wt% sulfuric acid solution possesses a significant number of gauche defects. Relative to surface butanol, surface hexanol chains are significantly more disordered at the surface of the solutions.

A new method was developed to measure the effectiveness of water in reducing the release of radon emanating from 226 Ra-bearing sand into air. Fick’s law on diffusion was used to model the transport of radon in water including the impact associated with radioactive decay. A multi-region, one-dimensional, steady-state transport model was used to analyze the movement of radon through a sequential column of air, water and air. An effective diffusion coefficient was determined by varying the thickness of the water column to predict the transport of 222 Rn through particular thickness of water. A one-region, one-dimensional transient diffusion equation was developed to investigate the build up of radon at the end of the water column until a steady-state, equilibrium condition was achieved. This build up with time is characteristic of the transport rate of radon in water and established the basis for estimating the effective diffusion coefficient for 222 Rn in water. The results suggest that convective forces other than molecular diffusion impact the transport of 222 Rn through the water barrier. An effective diffusion coefficient is defined that includes effects of molecular diffusion and convection to describe the transport of radon in water. Several experimental arrangements were evaluated to examine the influence of physical parameters on the radon transport. The effective diffusion coefficients measured in these experiments are 6.8×10 -4 ± 28% and 3.5×10 -4 ± 34% cm 2  sec -1 for the steady-state and transient diffusion approaches, respectively. Water barriers ranging in thickness from 30 – 50 cm reduce the amount of radon released from the radium-bearing source material by a factor of 0.3 – 0.1, respectively.

The viscosity and diffusion coefficient of polystyrene (PS) in decahydronaphthalene (DHN) were measured in the presence of CO₂ to investigate the effect of CO₂ on the transport properties of polymers in solution. The viscosity of 1-15 wt% PS in DHN was measured, using a moving piston viscometer. The effects of CO₂ pressure (0 to 3000 psi), polymer concentration (1-15 wt%), temperature (33-150°C), and molecular weight (126 to 412 kDa) on viscosity were investigated. Viscosity measurements of PS in DHN showed the viscosity increase with increasing concentration was described by the Martin equation. Addition of 30-40 wt% CO₂ resulted in the maximum viscosity reduction for all temperatures, polymer concentrations, and molecular weights. Viscosity reduction was greatest for high molecular weight polymer, high polymer concentrations, and low temperatures. At the highest CO₂ pressures, the viscosity of all polymer solutions converged to approximately 1-3 cp. The viscosity of PS/DHN/SF₆ was also measured. The viscosity reduction with SF₆ was approximately the same as that with CO₂. In addition, the diffusion coefficient of 0.5- 1.25 wt% 412,000 M[subscript n] PS in DHN was measured from 25-150°C. The diffusion coefficient results were extrapolated to zero concentration to determine the infinite dilution diffusion coefficient, D₀. The hydrodynamic radius was calculated from D₀. The hydrodynamic radius increased with temperature, indicating an increase in solvent quality of DHN with increasing temperature. Upon addition of CO₂ to 0.75-1 wt% 412,000 M[subscript n] PS in DHN, the diffusion coefficient increased, approximately doubling in value. The decrease in viscosity and increase in diffusion coefficient with CO₂ show that CO₂ is effective as a facilitator of improved transport.

Hot section components in gas turbines can be NiCoCrAlY-coated to provide the component with an Al reservoir that maintains a protective oxide layer on its surface. Over the service life of the component, the coatings degrade by composition and phase changes due to oxidation/hot-corrosion, and multicomponent interdiffusion from and into the superalloy substrate. In this study, the rate of Al interdiffusion into selected Ni-base superalloys using various diffusion couples of two-phase NiCoCrAlY (beta + gamma) and single beta-phase NiAl with the selected alloys is measured. The diffusion couples were examined with an emphasis on the composition-dependence of Al interdiffusion. Microstructural analysis of the NiCoCrAlY vs. superalloys couples is performed to examine the dependence of coatings lifetime on the superalloy composition. The beta-NiAl diffusion couples were analyzed to determine the integrated, apparent and average effective interdiffusion coefficient as a function of superalloy's composition. Concentration profiles were obtained by EPMA of the NiAl vs. superalloy diffusion couples. Findings of this study show that the lifetimes of NiCoCrAlY are heavily dependent on superalloy compositions. The rate of interdiffusion in the diffusion couples is affected by the refractory precipitate phase microstructure structures in the interdiffusion zones as well as by component interactions. The results of the beta-NiAl diffusion couples show that increasing concentrations of Cr, Mo and Ti in the superalloy increase the Al effective interdiffusion coefficient into the superalloy, while increasing concentrations of Al, Ta and W reduce it. Thus NiCoCrAlY-superalloy systems may be designed to produce optimal microstructures in the interdiffusion zone and minimize Al interdiffusion by consideration of these diffusional interactions.

Soil moisture determination using a multisensor capacitance probe: a laboratory calibration