Sorption-desorption Kinetics Research Articles

Role of active surface in processes of hydrogen sorption-desorption by intermetallic compounds

Abstract The influence of surface composition on hydrogen activation in the process of liquid-phase hydrogenation of organic compounds was determined by the investigation of intermetallic hydrides LaNi 5-x Cu x H n ( x = 1–4) and La 1-x Ce x Ni 5 H n ( x = 0.15–0.35). The complex of physicochemical methods showed that the formation dynamics and the nature of the active surface layer are determined by both the preliminary processing conditions and the reaction medium impact. It is shown that by varying the reaction medium composition it is possible to give a useful rise in the preferential segregation of one of the alloy components and to create active centers with definite composition on the surface. The investigation of the dependence of hydride catalytic activity on intermetallic nature, hydride forming pressure, sorption capacity and hydride surface composition allows us to draw the conclusion that surface center composition has a decisive influence on both hydrogen sorption-desorption kinetics and organic compound hydrogenation.

Microemulsion electrokinetic chromatography: comparison with micellar electrokinetic chromatography

The fundamental characteristics of microemulsion electrokinetic chromatography (MEEKC) were studied in comparison with micellar electrokinetic chromatography (MEKC). A microemulsion consisting of heptane-sodium dodecyl sulphate-butanol-buffer (pH 7.0) (0.81:1.66:6.61:90.92) was mainly employed. The separation selectivity of MEEKC was compared with that of MEKC with SDS micelles by using three different test mixtures. The microemulsion showed a stronger affinity to non-polar compounds than the SDS micelle. The migration-time window in MEEKC was easily extended owing to an increase in the electrophoretic mobility of the microemulsion by increasing the SDS fraction in the microemulsion. The efficiency in MEEKC was also compared with that in MEKC. The plate heights in MEEKC were higher than, but less than double, those in MEKC. The effect of microheterogeneity was not significant but the effect of sorption-desorption kinetics seemed more serious in MEEKC than in MEKC.

Experimental evaluation of the separation efficiency in capillary electrophoresis using open tubular and gel-filled columns.

The band dispersion phenomena in capillary zone electrophoresis (CZE) using untreated and surface-treated open tubular and gel-filled capillaries were experimentally evaluated, with emphasis on small capillary diameters (10-100 microns). Laser-induced fluorescence detection was used for high-sensitivity detection of the isoindoles originated from model amino acids. The plots of plate height vs electric field strength were generated for different column radii and compared with a theoretical model for CZE. In addition to the diffusion-controlled band dispersion in the relatively low electric field range, adsorptive interactions between a solute and the capillary wall may play a certain role in band-broadening. The sorption-desorption kinetics become important with increasing electric field strength. Thermal effects appear to contribute little to band-broadening in relatively small capillaries (less than 50-microns i.d.) within normal operating voltages (less than 30 kV), but could become significant in capillaries with larger bores (greater than 75-microns i.d.). With gel-filled capillaries of small diameters (less than 50-microns i.d.), diffusion processes can be minimized. In addition, thermal effects do not appear critical in such columns at reasonable voltages.

Heat and mass transfer in hydrogen sorption-desorption in metal-hydride power-plant components

A mathematical model which describes the dynamics of hydride-accumulator operation, taking account of the laws of heat and mass transfer and the reaction kinetics of hydrogen sorption-desorption, is developed. Estimates of the accuracy of the approximate analytical solutions are given. A generalization of the quasi-steady method of solving the Stefan problem is obtained.

Study of ultramicroporous carbons by high-pressure sorption. Part 5.—CH4isotherm and diffusion kinetics

Sorption–desorption kinetics and the isotherm for CH4 in as-received TCM 128 ultramicroporous carbon fibre are reported at 35°C up to 15 atm. The CH4 isotherm exhibits a similar hysteresis to that seen for N2 and CO2 on the carbon but at a much lower pressure. The low-pressure hysteresis might reflect adsorption in restricted regions composed of constrictions analogous to the tiny constrictions which were the cause of the hystereses seen with N2 and CO2. For the larger CH4, however, some of the regions that were easily accessbile to N2 may be behaving as ‘restricted’ to CH4 owing to a molecular-sieving phenomenon. Analysis of the diffusion kinetics indicates that the large CH4 molecules cause a slow barrier build-up over a few constrictions in series at the outer pores of the carbon. The desorption kinetic response, which is much faster than the adsorption, follows the Fickian model and thus reveals diffusion coefficients of the order 10–13 cm2 s–1.

Study of ultramicroporous carbons by high-pressure sorption. Part 2.—Nitrogen diffusion kinetics

Sorption–desorption kinetics for nitrogen in the as-received TCM 128 ultramicroporous carbon fibre are reported at 35 °C over a wide range of pressures. The nitrogen sorption kinetics at low pressures follow the Fickian model. As the pressure increases, the deviations from the model become more pronounced, and at sufficiently high pressure a sigmoid kinetic response shape is observed which is indicative of a non-Fickian diffusion process. An additional timescale must be active to account for the non-Fickian transport behaviour. This second timescale process may correspond to adsorbate surface rearrangements leading to locally time-dependent clearing of constrictions at a rate with a characteristic kinetic constant. The nitrogen desorption kinetics are found to be less affected by the non-Fickian transport, thus leading to higher apparent diffusion coefficients for the same average pressures. At high pressures and very long sorption times, slow protracted uptake becomes apparent into restricted regions. The kinetics for this process can be described by a barrier model and are sufficiently slow to allow treatment independent of the processes occurring in the more open pore system.

Moisture sorption of polyelectrolyte complex between poly(acrylic acid) and poly(4‐vinylpyridine)

AbstractSuccessive differential sorptions have been measured for the system polyelectrolyte complex of poly(acrylic acid)/poly(4‐vinylpyridine) + water vapor. The sorption data revealed that the sorption process of water vapor in the complex is controlled not only by diffusion mechanism but also by relaxation mechanism of polymer chains. It has been considered that the complex is composed of a loosely crosslinked, slightly ionized, and relatively homogeneous network structure. The sorption–desorption kinetics have also been investigated for the region of intermediate and high vapor pressures. The interval sorption–desorption curves demonstrated that the complex has the nature of the hysteresis effect in sorptions. It has been concluded that the appearance of sorption hysteresis is due to the depression of mobility of polymer chains resulting from crosslinks between carboxyl groups and pyridine rings.

Molecular Filtration of Hydrocarbon Isomers Through Polymer/[Liquid Crystal] Composite Membranes

Abstract Ultrathin membranes of a polymer/(liquid crystal) mixture were prepared by spreading a single drop of a casting solution on the water surface. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the solution. In the case of a liquid crystalline state above the crystal-nematic phase transition temperature, T KN, the polymer (liquid crystal) composite membrane follows Henry's law for the sorption isotherm of hydrocarbon gases and, also, Fickian sorption for the sorption-desorption kinetics. These results indicate that hydrocarbon gases permeate through a homogeneous medium composed of liquid crystalline molecules. Therefore, the permeability coefficients of hydrocarbon gases can be controlled by the dimensions of the channels through which the gas molecules diffuse. The channel for diffusion is generated by thermal or fluctuating molecular motion which opens up the intermolecular distance between liquid crystalline mol...

Significance and estimation of chromatographic parameters

An analytical solution to the equations providing a mechanistic description of linear chromatography is used to examine the predicted effects of different parameters. The mechanisms accounted for are convection, axial dispersion, mass transfer to and within the particles, and sorption—desorption kinetics. Intraparticle rate limitations, i.e., slow intraparticel diffusin and sorptions kinetics, are found to produce similar chromatographic results, making it difficult to distinguish their effects. In both cases the resulting peaks are symmetric, in contrast to the pronounced tailing which is shown to arise from secondary sorption. Calculation of plate heights from the model results indicates that moment determinations are susceptible to appreciable error; estimation of mechanistic parameters should therefore be performed using the time-domain results. The utility of several limiting approximations is demonstrated using both direct comparison with the analytical solution as well as order-of-magnitude estimates of the characteristic times of the different processes in the column.

Hygrothermal Aging Effects in Cured Epoxies

The effect of relatively severe hygrothermal aging on the sorption and also the diffusion of water at 40°C has been studied for a number of epoxy networks comprising tetraglycidyl 4,4′-diaminodiphenyl methane, and the curing agent 4,4′-diaminodiphenyl sulphone, 4,4′-diaminodiphenyl methane and 2,2′-dichloro-4,4′-diaminodiphenyl methane. Aging at high temperatures is accompanied by a pronounced increase in sorptive capacity at 40°C consistent with a significant expansion of the network. Dry annealing of the aged network at sub- Tg temperatures collapses the expanded structure to recover the sorptive capacity of the unaged network. Preliminary studies of the sorption-desorption kinetics for aged networks indicate that the effective diffusion coefficient is not markedly affected.

Direct measurement of liquid chromatographic sorption—desorption kinetics and the kinetic contribution to band broadening

The kinetics of sorption—desorption in an octadecylsilica, ion pairing chromatographic system have been directly measured for the first time, via the pressure-jump relaxation kinetics method. The sorption rate constant was found to be 1.4 · 10 9 1/mol s. The rates of sorption—desorption were found to correlate with differences in the chromatographic efficiency of the stationary phase studied, with more efficient phases showing more rapid sorption—desorption equilibration times. This indicates that, at least for the systems studied here, that the dynamics of the primary sorption—desorption process play a significant role in determining the performance of the octadecylsilicas as liquid chromatographic stationary phases.

The sorption and diffusion of water in halogen-containing epoxy resins

The effect of halogen containing substituents on the sorption and transport of water in the epoxy system, tetraglycidyl 4,4′ diaminodiphenyl methane cured with 4,4′ diaminodiphenyl sulphone, has been studied at 40°C, 50°C and 60°C. The introduction of halogen containing groups reduces the sorption over the whole of the activity range, the effect being greatest for the fluorine-containing sample. Sorption-desorption kinetics exhibit many of the features observed for the unsubstituted network including some element of non-Fickian behaviour. Diffusion coefficients are greatest for the fluorine-containing sample and have a relatively weak concentration dependence for all the samples.

Non-equilibrium model of liquid column chromatography : II. Explicit solutions and non-ideal conditions

the analysis begun in the previous paper of a general model for liquid chromatography in a column is continued here. Explicit solutions are obtained for small-zone and large-zone non-equilibrium chromatography models both without and with diffusion. The effects on the moments of the elution profile of a distribution function which characterizes molecular heterogeneity (such as molecular size) or bead non-uniformities are analyzed. A first-order correction to the mean value of the elution profile when sorption—desorption kinetics are concentration dependent is derived. Numerical simulations of the elution profile indicate the following. (1) The peak and mean may differ by as much as a factor of two for slow mass transfer ( k 1 ≲ 0.01). Since the mean is uniquely detemined by the equilibrium constant but the peak is not, the use of the peak to characterize the equilibrium constant for broad asymmetric profiles may lead to serious errors. (2) When the rate of mass transfer from the void to penetrable volumes becomes comparable to u/ h, a second peak will develop in the elution profile. This happens even for a completely homogeneous population under conditions, and is caused by molecules that traverse the column without penetrating beads. The dispersion of this peak is therefore determined entirely by effects other than mass transfer. (3) In the non-linear regime ( i. e., when mass transfer rates are concentration dependent), the equilibrium constant is, in general, no longer uniquely determined by the mean. Uniqueness is, however, obtained in the limit as both mass transfer coefficients become very small, with their ratio remaining moderate.

Difficulties associated with the measurement of the diffusion coefficient of solvating liquid or vapor in semicrystalline polymer. II. Sorption–desorption kinetics

The diffusion coefficient of a solvating liquid or vapor in a semicrystalline membrane can be measured in a number of ways. In a previous paper, we discussed methods based on permeation rates. Here, we consider sorption–desorption kinetics at near-ambient temperatures. Anomalies are identified and possible causes discussed in the light of new experiments, mostly involving p-xylene and polyethylene.

Film growth and proton sorption-desorption on passive iron in neutral solution

Iron in the passive state in neutral boric-acid—borate solution was anodically oxidized or cathodically reduced in the potential of passivity and oxygen evolution. It is shown that the potential of passivated iron is not a single-valued function of the surface film thickness but is controlled by only a small amount of charge, equivalent to less than 1 per cent of the film thickness, which can be associated with protons sorbed in the outermost layer of the oxide. The rate of proton sorption at constant potential decreases with increasing the amount of proton sorbed, according toia = Aa exp (−ΔQa/Ba), which is of the same form as the equation for the steady growth of the surface oxide on iron. Constant Ba depends on the film thickness QT, and a kinetic parameter ba = dBa/dQT is a function of the potential at which proton is sorbed. The parameter ba decreases with potential, extrapolating to zero at the oxygen-evolution potential, suggesting a correlation between the sorption—desorption kinetics of proton in the outermost layer of the surface oxide and the onset of oxygen evolution.

Theoretical Basis for Kinetic Effects in Gas-Solid Chromatography

LATERAL diffusion in the gas phase as well as sorption–desorption kinetics may both influence the plate height of gas-solid chromatography. While the complicated nature and alignment of adsorbing surfaces in a column cannot be fully described mathematically, a model is presented here which allows for the fundamental diffusion and reaction processes of gas-solid chromatography. The theory is written in a general form which can be readily extended to more complex phenomena. The adsorption isotherm is assumed linear, a condition recently made practical by highly sensitive detectors.