Dynamics of diffusion with reversible binding in microscopically heterogeneous membranes: General theory and applications to dermal penetration

Essentially all biological membranes and tissues exhibit microscopic heterogeneity in the form of cellular, lamellar or other organization, and molecular diffusion in these materials is frequently slowed by binding to elements of the microstructure (“trapping”). This paper addresses situations where binding is describable as a linear reversible process at the microscale, with forward (“on”) and reverse (“off”) rate constants k_f(x) and k_r(x) that vary with position. Very commonly it is tacitly assumed that the macroscopically observable binding behavior should follow the same rate law with the substitution of appropriate effective (tissue-average) rate constants and . This assumption is probed theoretically for spatially periodic microstructures using a judicious application of numerical calculations and asymptotic analysis to prototypical one-dimensional transport problems. We find that smooth microscopic variations produce an anomalous macroscopic exchange between free and bound solute populations that is not well described by a single pair of forward and reverse rate constants, i.e., violates the usual paradigm. In contrast, discontinuous variations (as in two-phase composite media) are evidently well described by the usual paradigm. For the latter case we derive simple and general algebraic equations giving and , and generalize them to any three-dimensional unit cell representing the tissue microstructure. Validity of the formulas is demonstrated with reference to a concrete example describing molecular diffusion through the stratum corneum (barrier) layer of skin, comprising lipid (intercellular) and corneocyte (cellular) phases. Our analysis extends coarse-graining (homogenization, effective transport) theory for irreversible trapping systems to the reversible case.     

Development and performance test of a thermo-denuder for separation of volatile matter from submicron aerosol particles

In this study we designed and evaluated a home-made thermo-denuder (TD) both experimentally and numerically. Sodium chloride (NaCl) particles, toluene gas, and carbon black particles were used for the performance evaluation of the TD. The TD was evaluated for various set-point air temperatures and particle sizes using the following three parameters: the temperature profile, penetration efficiency, and gas adsorption efficiency. At , the temperature was nearly uniform, remaining within of the set-point temperature, in the heating section and decreased to the temperature of ambient air in the cooling section. The particle penetration efficiencies were 93–96% at and 58–67% at for particle sizes of 20–60 nm. The gas adsorption efficiency was nearly unity until the breakthrough time of 65 h, and the total amount of toluene adsorbed by activated carbon particles was 72 mg-toluene/g-activated carbon particles. From size distribution measurements of dry carbon black and toluene enriched carbon black particles, the mode diameter measured at the set-point temperature of was found to be 48.6 nm, which agreed with the one obtained from the dry particle measurements. The overall number concentration obtained after particle losses were compensated was lower than that measured without using the TD by 35.6%, which was caused by gas adsorption in the TD.     

Quantifying desorption and rearrangement rates of carbon monoxide on a PEM fuel cell electrode

A simple procedure to quantify the rates of carbon monoxide (CO) desorption from, and simultaneous rearrangement on, supported platinum fuel cell electrode (Pt on Vulcan XC-72R) is reported. The surface coverage of CO on Pt electrode in equilibrium with bulk CO was measured from the anodic peaks in the CO stripping voltammogram. The decline in these surface coverages due to desorption and rearrangement, once CO was replaced by N₂ in the gas phase was recorded and used in conjunction with a kinetic model to quantify the respective rates. Two distinct CO oxidation peaks observed in the voltammogram due to the oxidation of two distinct ad-species, namely weakly and strongly adsorbed CO ( and ), were baseline corrected and deconvoluted using a bimodal Gaussian distribution. Saturation surface coverage of decreased with increasing temperature, while the opposite was true for . Rearrangement from to was faster than the desorption rate of either CO species. The desorption rate of was at least an order of magnitude lower than that of molecules at all temperatures studied. The activation energies for desorption of and were estimated to be 24.08 and 27.99 kJ/mol, respectively. The activation energy for rearrangement from to was 35.23 kJ/mol and that from to was 27.55 kJ/mol.     

Oxidation of FGD-CaSO3 and effect on soil chemical properties when applied to the soil surface

Use of high-sulfur coal for power generation in the United States requires the removal of sulfur dioxide (SO₂) produced during burning in order to meet clean air regulations. If SO₂ is removed from the flue gas using a wet scrubber without forced air oxidation, much of the S product created will be sulfite (). Plants take up S in the form of sulfate (). Sulfite may cause damage to plant roots, especially in acid soils. For agricultural uses, it is thought that in flue gas desulfurization (FGD) products must first oxidize to in soils before crops are planted. However, there is little information about the oxidation of in FGD product to under field conditions. An FGD-CaSO₃ was applied at rates of 0, 1.12, and 3.36 Mg ha⁻¹ to the surface of an agricultural soil (Wooster silt loam, Oxyaquic Fragiudalf). The in the surface soil (0-10 cm) was analyzed on days 3, 7, 17, 45, and 61. The distribution of and Ca in the 0-90 cm soil layer was also determined on day 61. Results indicated that in the FGD-CaSO₃ rapidly oxidized to on the field surface during the first week and much of the and Ca moved downward into the 0-50 cm soil layer during the experimental period of two months. It is safe to grow plants in soil treated with FGD-CaSO₃ if the application is made at least three days to several weeks before planting.     

Kinetic analysis on LiFePO4 thin films by CV, GITT, and EIS

LiFePO₄ thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average values calculated from CV results were in the order of 10⁻¹⁴ cm² s^–1. The values obtained by GITT, and EIS techniques were in the range of 10^–14–10^–18 cm² s^–1, 10^–14–10^–18 cm² s^–1, respectively. The values obtained by the two methods show a minimum point at x ∼ 0.5 for Li_1−xFePO₄. However, the overpotential values of the LiFePO₄ thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li_1–xFePO₄. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li_1–xFePO₄ which constitutes two phase, i.e., LiFePO₄ and FePO₄ in this region.     

A mechanism study of chloride and sulfate effects on Hg reduction in sulfite solution

This paper studied the effects of sulfate and chloride ions on bivalent mercury (Hg²⁺) absorption and reduction behaviors in a simulated WFGD system. The aqueous mercuric ion-sulfite system reduction behaviors were monitored and investigated using a UV-visible spectrum. Thereafter, the mechanism of Hg²⁺ reduction in the presence of sulfate and chloride ions was proposed. Experimental results revealed that both sulfate and chloride ions had inhibition effects on aqueous Hg²⁺ reduction to Hg⁰. The inhibition was assumed due to the formation of (in the presence of ) and / (in the presence of Cl⁻). And it was found that complex was more stable than in excess of Cl⁻. The formation of the above-mentioned complexes in the presence of and Cl⁻ would damp the formation of HgSO₃, whose decomposition was assumed to be the key step of Hg²⁺ reduction.     

Incorporating primary amine pendant groups into copolymers via nitroxide mediated polymerization

Primary amine incorporation as pendant groups on copolymers was studied via nitroxide mediated polymerization (NMP) using the BlocBuilder family of unimolecular initiators. When using the neat BlocBuilder initiator, a protected amine monomer, N-(t-BOC-aminopropyl)methacrylamide (tBOC-MAm), was copolymerized with styrene (ST) at 110 °C or terpolymerized with ST and methyl methacrylate (MMA) at 90 °C. When copolymerized with ST, tBOC-MAm incorporation into copolymers was low compared to the feed composition, despite good control of the molecular weight distribution (polydispersity ). tBOC-MAm/MMA/ST terpolymerizations indicated better tBOC-MAm incorporation but high and poor “living character” (loss of linear number average molecular weight versus monomer conversion X relationship at X > 0.3 and poor ability to re-initiate a second batch of monomer). In contrast, using a protected form of BlocBuilder attached to oligomeric ST or MMA/ST-based macroinitiators, feed compositions f_AST,0 containing 4-aminostyrene (AST) non-protected monomer up to 0.10, resulted in good primary amine incorporation and linear M_n versus X up to X = 0.4. Higher f_AST,0 resulted in side-reactions and poorer control of the polymerization process. Thus, for the two primary amine monomers studied, in protected or non-protected forms, controlled polymerizations with good incorporation of amine-containing monomer are more readily approached when using AST in conjunction with a protected BlocBuilder-terminated oligomeric initiator, provided the AST feed composition is not excessively high.     

Hydrogen adsorption on functionalized graphene

The hydrogen adsorption on basal graphite planes functionalized by hydrogen atoms is studied by molecular modeling and numerical simulation at temperatures of 77 K and 293 K up to high pressure. At 77 K and pressure of 1 MPa, on such an adsorbing surface, the excess hydrogen physisorption is estimated equal to . At 293 K and 30 MPa, the excess physisorption reaches . A comparison between the hydrogen adsorption properties of functionalized graphite basal planes and plain graphite basal planes is presented for materials exhibiting similar porosities.     

Flashback propensity of syngas fuels

This paper presents experimental measurements of the critical velocity gradient and flashback behavior of H₂-CO and H₂-CH₄ mixtures. Effects of H₂ concentration, external excitation, and swirl on the flashback behavior for flames of these fuel mixtures are discussed. For H₂ concentration burner and scaling studies the critical velocity gradient (g_F), defined as the ratio of the square of the laminar burning velocity to the thermal diffusivity of the mixture , was used to quantify the flashback propensity of the flames. The critical velocity gradient of both H₂-CH₄ and H₂-CO flames changed nonlinearly with the increase in H₂ contents in the mixture. The critical velocity gradient (g_F) of 5-95% and 15-85% H₂-CO mixtures somewhat agreed with the scaling relation and yielded an average c value of 0.04. Similarly, values of a 25%H₂-75%CH₄ for different burner diameters were also fitted using the scaling relation yielding an average c value of 0.044. The g_F values of 25-75% H₂-CO mixture showed non-linear variation with the ratio (especially for ), and at a lower ratios burner diameter had small effect on critical velocity gradient measurements. The opposite trend was observed for a 25-75% H₂-CH₄ mixture showing non-linear variation at a lower ratios (for ) and having less effect at higher ratios. It was also determined that the effect of external excitation on the flashback propensity of H₂-CO flames with more than 5% H₂ was not significant. Flashback through two mechanisms and their dependence on combustor parameters were also identified for swirl stabilized H₂-CO flames.     

Estimate of solid flow rate from pressure measurement in circulating fluidized bed

A method is described to estimate solid mass flow rate based on measurement of pressure drop in horizontal section of circulating fluid bed (CFB). A theoretical model was derived based on momentum balance equation and used to predict the solids flow rate. Several approaches for formulating such models are compared and contrasted. A correlation was developed that predicts the solids flow rate as a function of pressure drop measured in the horizontal section of piping leading from the top of the riser to the cyclone, often referred to as the cross-over. Model validation data was taken from literature data and from steady state, cold flow, CFB tests results of five granular materials with various sizes and densities in which the riser was operated in core-annular and dilute flow regimes. Experimental data were taken from a 0.20 m ID cross-over piping and compared to literature data generated in a 0.10 m ID cross-over pipe. The solids mass flow rate data were taken from statistically designed experiments over a wide range of Froude number , load ratio , Euler number , density ratio , Reynolds number , and Archimedes number . Several correlations were developed and tested to predict the solids mass flux based on measuring pressure drop in the horizontal section of CFB. It was found that load ratio is a linear function of the Euler number and that each of these expressions all worked quite well (R² > 95%) for the data within the range of conditions from which the coefficients were estimated.     

A comparative study of PtCo, PtCr, and PtCoCr catalysts for oxygen electro-reduction reaction

A comparative study of carbon supported Pt₃₀Co₇₀, Pt₃₀Cr₇₀, and Pt₃₀Co₃₀Cr₄₀ catalysts for oxygen electro-reduction reaction (ORR) activity was performed. In alloy catalysts synthesized via NaBH₄ reduction, more than a 3-fold improvement was observed in ORR specific activity compared with that of Pt/C catalyst, while mass activities did not show significant improvement. After annealing at 900 °C under reducing conditions, the ORR specific activities of the alloy catalysts increased to give a relative ORR catalytic activity ordering of PtCo/C-900 (2800 μA ) > PtCoCr/C-900 (1770 μA ) > PtCr/C-900 (871 μA ) > Pt/C-900 (393 μA ) > Pt/C (334 μA ). On the other hand, the ORR mass activity followed an order of PtCr/C-900 (140 mA ) > PtCoCr/C-900 (111 mA ) > PtCo/C-900 (84.1 mA ). Cyclic voltammetry results suggest that incorporation of Cr resulted in a large electrochemically active surface area producing higher mass activity in the PtCr/C-900 catalyst although it showed the lowest specific activity among the alloy catalysts. The intermediate EAS and ORR activity values of the PtCoCr/C-900 catalyst suggest that the characteristics of the PtCo/C-900 (low mass and high specific activities) and PtCr/C-900 (high mass and low specific activities) are combined by alloying of Pt with both Co and Cr.     

Anion exchanged polymerized ionic liquids: High free volume single ion conductors

In this study, we investigate the isolated effect of anion type on the chemical, thermal, and conductive properties of imidazolium-based polymerized ionic liquids (PILs). PILs with various anions at constant average chain length were prepared by ion exchange with a water-soluble PIL precursor, (poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (poly(MEBIm-Br)). NMR, IR, and elemental analysis confirm that anion exchange of ploy(MEBIm-Br) with bis(trifluoromethanesulfonyl) imide (TFSI), tetrafluoroborate (BF₄), trifluoromethanesulfonate (Tf), and hexafluorophosphate (PF₆) in water resulted in nearly fully exchanged PILs. As a function of anion type, the glass transition temperature plays a dominant role, but not the sole role in determining ion conductivity. Other factors affecting ionic conductivity include the size and symmetry of the anion and dissociation energy of the ion pair. Both the Vogel-Fulcher-Tammann (VFT) and Williams-Landel-Ferry (WLF) equations were employed to investigate the temperature dependent ionic conductivities. The (9.03) and (168 K) values obtained from the WLF regression of these PILs greatly deviate from the classical WLF values originally obtained from the mechanical relaxation of uncharged polymers ( = 17.44,  = 51.6 K) and the WLF values obtained from the conductive properties of other polymer electrolytes. This suggests that the fractional free volume (f (T_g) = B/(2.303)) and Vogel temperature (T₀ = T_g − ) are strong functions of ion concentration, where high free volume allows for ion mobility at temperatures farther below the glass transition temperature of the polymer.     

Carbon dioxide absorption with aqueous potassium carbonate promoted by piperazine

Many commercial processes for the removal of carbon dioxide from high-pressure gases use aqueous potassium carbonate systems promoted by secondary amines. This paper presents thermodynamic and kinetic data for aqueous potassium carbonate promoted by piperazine. Research has been performed at typical absorber conditions for the removal of CO₂ from flue gas.Piperazine, used as an additive in 20- potassium carbonate, was investigated in a wetted-wall column using a concentration of at 40-80°C. The addition of piperazine to a potassium carbonate system decreases the CO₂ equilibrium partial pressure by approximately 85% at intermediate CO₂ loading. The distribution of piperazine species in the solution was determined by proton NMR. Using the speciation data and relevant equilibrium constants, a model was developed to predict system speciation and equilibrium.The addition of piperazine to potassium carbonate increases the rate of CO₂ absorption by an order of magnitude at 60°C. The rate of CO₂ absorption in the promoted solution compares favorably to that of MEA. The addition of piperazine to potassium carbonate increases the heat of absorption from 3.7 to . The capacity ranges from 0.4 to for PZ/K₂CO₃ solutions, comparing favorably with other amines.     

Activated carbon for hydrogen purification by pressure swing adsorption: Multicomponent breakthrough curves and PSA performance

In this work, multicomponent breakthrough experiments (binary H₂-CO₂, ternary H₂-CO₂-CO and five-component H₂-CO₂-CO-CH₄-N₂) were performed under different operating conditions in activated carbon extrudates to validate the mathematical model. A 10 steps one-column VPSA experiment was also performed. These experiments allow experimental validation of adsorption equilibrium predicted by the multicomponent extension of the Virial isotherm and a fixed-bed mathematical model. In the VPSA experiment, a 99.981% hydrogen purity stream (with 63 ppm of CO contamination) was obtained with a hydrogen recovery of 81.6% and an adsorbent productivity of .The mathematical model was also employed to assess the effect of operating conditions and the influence of step times and pressure equalizations in the PSA unit. It was verified that high-purity hydrogen (>99.99%) can be obtained using this adsorbent with recoveries higher than 75% and unit productivities of .     

Liquid mixing in gas-liquid two-phase flow by meandering microchannels

The influence of the channel radius on the mass transfer in rectangular meandering microchannels (width and height of ) has been investigated for gas-liquid flow. Laser induced velocimetry measurements have been compared with theoretical results. The symmetrical velocity profile, known from the straight channel, was found to change to an asymmetrical one for the meandering channel configuration. The changes in the secondary velocity profile lead to an enhanced radial mass transfer inside the liquid slug, resulting in a reduced mixing length. In the investigated experimental range (superficial gas velocity and superficial liquid velocity ) the mixing time was reduced eightfold solely due to changes in channel geometry. An experimental study on the liquid slug lengths, the pressure drop and their relation to the mass transfer have also been performed. Experimental results were validated by a simulation done in Comsol Multiphysics^®. To obtain information for higher velocity rates, simulations were performed up to . These velocity variations in the simulation indicate the occurrence of a different flow pattern for high velocities, leading to further mass transfer intensification.     

Effect of oxidizing agents on selenate formation in a wet FGD

In a coal combustion process, a considerable amount of selenium is captured in the wet FGD, where it is oxidized from selenite to selenate , which is difficult to remove. Diethyl-p-phenylene-diammonium (DPD) absorptiometric analysis and ion chromatography identified peroxodisulfate ion as the dominant oxidizing agent in the FGD liquor. Selenite was easily oxidized to selenate in the presence of and the oxidation was accelerated as the temperature increased. Addition of Mn²⁺ ion was found to be effective in controlling selenate formation. When Mn²⁺ ion was added, oxidized not selenite to selenate but rather Mn²⁺ to MnO₂, which captured some dissolved selenite.