Diffusion of One‐ and Two‐Component Fluids into Polymeric Membranes

The starting point of this paper is a model of the non‐Fickian diffusion of a simple fluid into a polymeric medium that has been introduced in El Afif and Grmela (2002). The model is extended to a mixture consisting of two simple fluids and one polymeric medium. The effects of the polymeric medium on diffusion are included in the formulations through the use of relative momenta playing the role of internal variables. In ternary mixtures, new phenomena arise due to cross‐coupling effects. As a consequence, the single diffusion Deborah number in binary mixtures becomes a 2 × 2 matrix in ternary mixtures. Two specific examples for which experimental data are available are investigated in detail.     

Enhancing electrocatalytic N2 reduction via tailoring the electric double layers

The electrocatalytic nitrogen reduction reaction (NRR) for NH₃ synthesis is still far from being practical and competitive with the common Haber–Bosch process. The rational design of highly selective NRR electrocatalyst is therefore urgently needed, which requires a deep understanding of both the electrode–electrolyte interface and the mass transport of reactants. Here, we develop a theoretical framework that includes electric double layer (EDL), mass transport, and the NRR kinetics. This allows us to evaluate the roles of near-electrode environment and N₂ diffusion on the NRR selectivity and activity. The EDL, as the immediate reaction environment, remarkably impedes the diffusion of N₂ to the cathode surface at high electrode potentials, which explains experimental observations. This article also gives microscopic insights into the interplay between N₂ diffusion and reaction activity under the nano-confinement, providing theoretical guidance for future design of advanced NRR electrocatalytic systems.     

Kinetics and Mechanism of Copper Transfer by Hydroxy-Oximes in Three-Phase Liquid Systems

Abstract

A model of copper-facilitated transport in a three-liquid-phase pertraction system is proposed. The model takes into account the diffusional transport of copper species (ions and complexes) in all three liquids as well as the kinetics of chemical reactions. The latter, according to the model suggested, occurs in narrow reaction zones: aqueous layers adjacent to the donor membrane and acceptor-membrane interfaces. Experiments were carried out in a two-compartment agitated diffusion cell with a bulk liquid membrane: a 5% (vol) solution of the commercial copper extractant LIX 860 in C₁₁-C₁₃ normal paraffins. The donor solution used contained 1 g/L copper and the acceptor liquid was a 4.5 N aqueous solution of sulfuric acid. On the basis of experimentally obtained pertraction curves and the model suggested, it was found that the overall rate of the process is controlled by the diffusion of copper ions in the donor phase as well as by copper-complex decomposition. By applying an optimization procedure, the mass transfer coefficients and the rate constants were evaluated under the experimental conditions of this study.     

Effects of ceramic types on evolution of micrometer-sized features during sintering

In this work, ZnO and ZrO₂ ridges with 2 μm size are created based on a centrifuge-aided micromolding approach and then sintered with different time. Characterization of feature morphology, fidelity, grain size, relative density, and linear shrinkage has been conducted. The densification mechanisms for both ZnO and ZrO₂ are controlled by grain-boundary diffusion, but their grain growth mechanisms are dominated by gas diffusion and surface diffusion respectively. The sintering behavior for the bulk can be described with a N_g/N_b factor at 36, while for the features, a smaller N_g/N_b factor (15 for ZnO and 8 for ZrO₂) is needed. Attributed to their sintering mechanism difference, the grains in the ZnO features have a faster growth rate than those in the bulk, while the grains in the ZrO₂ features have a similar growth rate to those in the bulk. ZnO has a much faster grain growth behavior, leading to ridge fidelity loss and severe ridge destruction, while ZrO₂ has a much slower grain growth rate, resulting in high ridge fidelity and strong resistance to ridge destruction.     

Enhanced surface morphology and transport property of poly(N‐vinylcarbazole)/gold nanocomposite Langmuir–Schaefer films

A composite of poly(N‐vinylcarbazole) (PVK) containing gold nanoparticles (GNPs) was synthesized via simple solid‐state in situ bulk polymerization of N‐vinylcarbazole in the presence of GNPs at a high temperature. Both PVK and PVK–GNP composites were characterized by Fourier transform infrared (FTIR) and UV–vis spectroscopy. The surface morphology of the composites was studied by scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy (TEM). Thermal stability was identified via thermogravimetric analysis. The composites were fabricated into films using the Langmuir–Schaefer process. The enhancement in the characteristics of room temperature I–V, pressure–area isotherms, and photoelectrochemical behaviors was observed in the composite films. Results suggest that a charge transfer process occurs across the hybrid at the interface of the PVK–GNP composites. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Effect of silane coupling agent on swelling behaviors and mechanical properties of thermosensitive hybrid gels

A series of organic–inorganic hybrid thermosensitive gels with three different structures were prepared from N‐isopropylacrylamide (NIPAAm), and N, N′‐methylenebisacrylamide (NMBA) and tetraethoxysilane (TEOS) [N‐IPN]; NIPAAm, 3‐(trimethoxysilyl) propyl methacrylate (TMSPMA) as coupling agent and TEOS [NT‐IPN]; and NIPAAm, TMSPMA, and TEOS [NT‐semi‐IPN] by emulsion polymerization and sol–gel reaction in this study. The effect of different gel structures and coupling agent on the swelling behavior, mechanical properties, and morphologies of the present gels was investigated. Results showed that the properties of the gels would be affected by the gel networks such as IPN or semi‐IPN and with or without existence of TMSPMA as the bridge chain between networks. The NT‐semi‐IPN gel had higher swelling ratio and faster diffusion rate because poly(NIPAAm) moiety in the semi‐IPN gels was not restricted by NMBA network. However, the IPN gels such as N‐IPN and NT‐IPN had good mechanical properties and lower swelling ratio, but had a poor thermosensitivity due to the addition of coupling agent, TMSPMA, into the gel system that resulted in denser link between organic and inorganic components. The morphology showed that IPN gels had partial aggregation (siloxane domain) and showed some denser phases. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Molecular thermodynamic understanding of transport behavior of CO2 at the ionic liquids-electrode interface

The transport behavior of CO₂ at the ionic liquids (ILs)-electrode interface was revealed from the thermodynamic view via molecular dynamics simulations. The hopping and self-diffusive mechanisms were identified in the interfacial and bulk region, and thereafter a hopping-diffusion model was developed to evaluate the transport resistance of CO₂ from bulk to the interface. Meanwhile, the vibrational spectrum and entropy change of CO₂ at the interface were calculated using the thermodynamic analysis method. For ILs with the same cation ([Emim]⁺), both transport resistance and entropy decrease follow the order: [BF₄]⁻ < [AC]⁻ < [NO₃]⁻, indicating [BF₄]⁻ possesses the faster CO₂ transport efficiency across the electrical double layer. Furthermore, the methyl substitution effect on transport and thermodynamic properties was clarified, indicating the coupling relation between the transport process and thermodynamic advantage. These findings can lay the ground for the molecule design of ILs-electrode interface in the applications in the chemical engineering field.     

Silicon-29 solid-state nuclear magnetic resonance spectroscopy of composite interfaces

The types of structures and bonds that are formed with silicons in the composite interface were studied using ²⁹Si cross-polarization/magic angle sample spinning (CP/MAS) nuclear magnetic resonance spectroscopy (NMR). The change in mobility of silane coupling agent bonded to silica, as compared with bulk hydrolyzed silane coupling agent, can be monitored by the change in line width and the shift of resonances to higher fields, as well as by the change in the silicon-proton cross-polarization time T_SiH. In the silane coupling agent-matrix resin interface, the T_SiH values reflect the change in mobility as a function of the concentration and degree of hydrolysis of the silane coupling agent. It has been demonstrated that quantitative measurements of T_SiH can be used to investigate relative mobilities.     

Experimental determination of residual stress in silicon nitride diffusion bonds obtained by high-energy X-ray diffraction

High resolution X-ray scanning diffractometry has been used to study the residual strain in binary metal/ceramic (Ni/Si₃N₄) and ceramic/ceramic (Si₃N₄/Ni thin film/Si₃N₄) diffusion bonds. Bonds were fabricated by simultaneous high temperature heating and uniaxial pressing. The axial and radial strain profiles have been determined along selected lines perpendicular to the bonding interface inside the ceramic bodies. The X-ray experiments have been done at the energy of 60 keV, which assured a very small absorption, and therefore, strain fields have been measured in the ceramic bulk. Strains showed higher values near the interface that decreased with the distance.     

Dynamic shear in continuous-flow rotating-disk catalytic reactors with stress-sensitive kinetics based on Curie's theorem in non-equilibrium thermodynamics

Stress-sensitive response is simulated in a modified parallel-disk reactor that implements steady and unidirectional dynamic shear in the creeping flow regime. Reactants chemisorb on the surface of the rotating plate, and catalytic sites are replenished from the bulk fluid via radial and tangential flow accompanied by transverse diffusion in the z-direction toward the active surface. Chemical reaction is enhanced by viscous shear at the interface between the bulk fluid and the rotating plate. This heterogeneous catalytic rotational reactor is modeled via radial convection and axial diffusion with angular symmetry in cylindrical coordinates. The reaction/diffusion boundary condition on the surface of the rotating plate accounts for stress-sensitive reactant consumption via the zr- and zΘ-elements of the velocity gradient tensor at the catalytic surface. Linear transport laws in chemically reactive systems that obey Curie's theorem predict the existence of cross-phenomena between scalar reaction rates and the magnitude of the second-rank velocity gradient tensor, selecting only those elements of ?v experienced by reactants that are chemisorbed on the surface of the rotating plate. Stress sensitivity via the formalism of irreversible thermodynamics introduces a zeroth-order contribution to heterogeneous consumption rates that must be quenched when reactants or active vacant sites are not present on the surface of the rotating plate. Rotating-disk reactor simulations are presented for simple 1st-order, simple 2nd-order, and complex heterogeneous stress-free kinetics, where the latter considers Langmuir-type dissociative adsorption of one of the reactants. Accurate design of rotating-disk reactors must consider stress-sensitivity when the shear-rate-based Damköhler number (i.e., ratio of the stress-dependent zeroth-order consumption rate relative to the rate of reactant diffusion toward the active surface) is greater than its threshold value which increases at higher stress-free Damköhler numbers. Modulated rotation of the catalytically active plate demonstrates that these rotating-disk reactors must operate above a threshold for the stress-sensitive Damköhler number, identified under steady shear conditions, before dynamic shear has a distinguishable effect on reactor performance.     

The use of rotating electrodes in the study of the different processes relevant in electrochemical engineering

The authors present recent results illustrating the developments in Electrochemical Engineering arising from the use of the rde system as devised by Levich since 1942. It is known, indeed, that when a fast electrochemical reaction is used, the control by mass transport from the bulk solution to the interface of the overall current, yields a way of studying hydrodynamics in the boundary layer thanks to the coupling due to convective diffusion.The examples presented below pertain in particular to boundary layer turbulence (measurement of eddy diffusivity and study of the local hydrodynamic drag reduction) as well as to convective diffusion in non Newtonian fluids with a prospect of application to Bioelectrochemistry.The authors underline also the improvements achieved in the methodology by describing recent non steady state methods.     

Oxygen transport kinetics of MIEC membranes coated with different catalysts

Oxygen permeation through mixed ionic‐electronic conducting membrane may be controlled by oxygen bulk diffusion and/or oxygen interfacial exchange kinetics. In this article, we chose BaCe_0.05Fe_0.95O_3‐δ (BCF) as a representative to study the oxygen transport resistances of the membrane coated with different porous catalysts, including BCF itself, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) and Sm_0.5Sr_0.5CoO_3‐δ (SSC). The oxygen transport resistances of bulk, gas‐solid interfaces of feed‐side and sweep‐side of the catalyst‐coated membranes can be separately obtained through a linear regression of experimental data according to an oxygen permeation model. The three resistances of the membrane coated with BCF catalyst are smaller than those of the membrane coated with BSCF and SSC catalysts, although BSCF catalyst itself has the fastest bulk diffusion and interfacial exchange kinetics. The catalytic activities of BSCF and SSC catalysts on BCF membranes are impacted by the transport kinetics of catalysts, microstructure of catalyst layers, and cationic inter‐diffusion between the membrane and catalysts. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2803–2812, 2016     

Characterization of an anisotropic hydrogel tissue substrate for infusion testing

Artificial tissue models that capture specific transport properties are useful for investigating physical phenomena important to drug delivery. In this study, an in vitro tissue model was developed and characterized with the goal of mimicking aligned tissue. An anisotropic porous medium was developed by the construction of a 1% agarose hydrogel implanted with different volume fractions (～ 5, 10, and 20%) of 10‐μm‐diameter glass fibers. The developed substrate was able to capture anisotropic transport after the direct infusion of a macromolecular tracer, Evans blue albumin (EBA). To further characterize the test substrate, the diffusion tensor of water was measured by diffusion tensor imaging, and the ratios of the diffusivities in the directions parallel and perpendicular to the glass fibers were 1.16, 1.20, and 1.26 for 5, 10, and 20% fiber volume fractions, respectively. The hydraulic conductivity was estimated by the measurement of pressure gradients across samples under controlled microflow conditions in the direction parallel to implanted fibers. The hydraulic conductivities at various hydrogel concentrations without fibers and in a 1% hydrogel with various fiber volume fractions were measured; for example, K_∥ = 1.20 × 10^?12 m⁴ N^?1 s^?1 (where K_∥ is the conductivity component in the direction parallel to the glass fibers) for 20% fiber volume fractions. Also, EBA distributions were fit to porous medium transport models to estimate hydraulic conductivity in the direction perpendicular to glass fibers. The estimated ratio of directional hydraulic conductivity, K_∥/K_? (where K_? is the conductivity component in the direction perpendicular to the glass fibers), ranged from approximately 3 to 5, from 6 to 10, and from 40 to 90 for 5, 10, and 20% fiber volume fractions, respectively. These agarose hydrogel models provided convenient media for quantifying infusion protocols at low flow rates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Study of transport of pure and mixed CO2/N2 gases through polymeric membranes

The permeations of pure CO₂ and N₂ gases and a binary gas mixture of CO₂/N₂ (20/80) through poly(dimethylsiloxane) (PDMS) membrane were carried out by the new permeation apparatus. The permeation and separation behaviors were characterized in terms of transport parameters, namely, permeability, diffusion, and solubility coefficients which were precisely determined by the continuous‐flow technique. In the permeation of the pure gases, feed pressure and temperature affected the solubility coefficients of CO₂ and N₂ in opposite ways, respectively; increasing feed pressure positively affects CO₂ solubility coefficient and negatively affects N₂ solubility coefficient, whereas increasing temperature favors only N₂ sorption. In the permeation of the mixed gas, mass transport was observed to be affected mainly by the coupling in sorption, and the coupling was analyzed by a newly defined parameter permeation ratio. The coupling effects have been investigated on the permeation and separation behaviors in the permeation of the mixed gas varying temperature and feed pressure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 179–189, 2000     

Active transport membrane for anions. III. Effect of crown ether and tautomeric functional groups

Membranes derived from copolymers having N-hydroxyalkyl groups were prepared, and the active transport for anions through the membranes was investigated in the presence of a crown ether which suppresses the diffusion of counter ions. The maximum of the concentrated anions in one side cell was maintained for a long time by the addition of a crown ether in the cell, and a concentration of anions up to 50% was attained. Various N-hydroxyalkyl groups of different lengths were incorporated in copolymers with styrene, and the transport for anions was investigated in order to establish the carrier mechanism of the tautomeric functional groups. Longer-length alkyl moieties bearing N-hydroxyalkyl groups led to a decrease in the active transport for anions, which supports a carrier mechanism for the membranes.     

Synthesis and Interfacial Activity of Novel Heterogemini Sulfobetaines in Aqueous Solution

Three new heterogemini sulfobetaines and their chloride salts were synthesised. The interfacial activities of the obtained chlorides in aqueous solution were studied by equilibrium and dynamic surface tension measurements. The critical micelle concentration, surface excess concentration, minimum area per surfactant molecule and standard Gibbs energy of adsorption as well as micelle lifetime and diffusion coefficient were determined. The adsorption properties and micelle lifetime of these compounds significantly depend on the length of alkyl chain. The critical micelle concentration decreases with increasing chain length of the compounds considered. The values of the diffusion coefficient of N‐alkyl‐N‐methyl‐N‐(3‐sulfopropyl)‐6‐(N‐alkyl‐N‐methylamino)hexylammonium chloride tend to decrease as the concentration is increased.     

Active transport membrane for chlorine ion

A specific functional group that could interact with ions was introduced in a synthetic membrane to achieve an active transport of ions. One way to synthesize the active transport membrane was to introduce a functional group which has a tautomerism upon pH changes in an aqueous solution. A polymer having pendent N-hydroxyethyl amide groups was synthesized to form a membrane, and the membrane was fixed in a cell as a partition film, in which one side of the solution was adjusted to be acidic and the other side basic. It was then possible to transport chlorine ion through the membrane owing to the carrier functions caused by tautomerism of the N-hydroxyethyl amide group from the acidic to the basic sides. The transport of the chlorine ion was not dependent on diffusion control.     

Transport of Thorium(IV) Across a Supported Liquid Membrane Containing N,N,N′,N′-Tetraoctyl-3-oxapentanediamide (TODGA) as the Extractant

The transport behavior of Th⁴⁺ was investigated from a feed containing 3.0 M HNO₃ into a receiver phase containing 0.1 M oxalic acid across a PTFE flat sheet supported liquid membrane (SLM) which contained TODGA (N,N,N′,N′-Tetraoctyl-3-oxapentanediamide) in n-dodecane as the extractant. Effects of the nature of the strippant, extractant concentration, Th concentration in the feed, and feed acidity on the transport rates were investigated. The transport behavior apparently depended on the rate of extraction of the metal ion at the feed-membrane interface and was not diffusion controlled. Influence of Th concentration on flux was also investigated. Transport mechanism was elucidated and the diffusion coefficient was calculated to be 2.13 × 10^?7 cm²/s. Solvent extraction studies at varying feed acidity and TODGA concentration were also carried out.     

SURFACTANT ADSORPTION TO SPHERICAL PARTICLES: THE INTRINSIC LENGTH SCALE GOVERNING THE SHIFT FROM DIFFUSION TO KINETIC-CONTROLLED MASS TRANSFER

When a drop or bubble of radius b is formed in surfactant solution, surfactant adsorbs, diffuses from solution, and desorbs to establish the equilibrium surface concentration. The transport coefficients for diffusion, adsorption, and desorption are fundamental parameters. However, the transport mechanisms that control the interface far from equilibrium are highly context dependent. Thus, no surfactant has universal “diffusion-controlled” transport. Here we identify a new length scale, R _D-K , that depends on surfactant physicochemistry, and which ranges from roughly 15 to 65 microns. For drops or bubbles with b?R _D-K , mass transfer is kinetically controlled. For b?R _D-K , mass transfer is diffusion controlled. Simulations of adsorption to quiescent spherical interfaces support the importance of R _D-K in determining the controlling transport mechanism for surfactant solutions with concentrations below the critical micelle concentration (CMC). While the case of surfactant adsorbing to a bubble is discussed in detail, the arguments presented are quite general and should apply to adsorption of any solute to any spherical particle.