Ion transport by viscous gas flow through capillaries

The effects of a number of experimental parameters on the efficiency of ion transport by viscous gas flow through narrow capillaries have been studied. Both electrospray and corona ion sources were used. The experimental data are consistent with ions loss to the walls of the capillary, which initially is caused mainly by space-charge expansion, but later is caused by diffusion. These processes can result in severe discrimination against low mass ions. The extent of ion loss may be calculated by using a simple model for radial diffusional loss in long cylinders, with an exponential decay of the ion density along the transport capillary. However, such a simple model underestimates ion loss by ignoring the effects of space-charge, turbulent flow, and rapid decay of higher radial diffusion modes (enhanced loss of ions that enter the capillary close to the wall). In contrast, Monte Carlo simulations showed that the effect of the parabolic velocity profile, under laminar flow conditions, is to increase the transmitted ion current, sometimes by several orders of magnitude, relative to the predictions of the simple diffusion model. After considering all these factors, the transmitted current from a corona was well reproduced by using mobility values for ions formed in such discharges. However, the measured transmitted current from an electrospray source was much too high. To explain this, it was necessary to assume that about 2% of the electrospray current is carried by aerosol particles with radii in the 10-25-Å range. Finally, it is argued that in glass capillaries wall charging may explain why the transmitted ion current is observed to be very similar to that in metal capillaries.     

The role of attractive forces in viscous liquids

We present evidence from computer simulation that the slowdown of relaxation of a standard Lennard-Jones glass-forming liquid and that of its reduction to a model with truncated pair potentials without attractive tails are quantitatively and qualitatively different in the viscous regime. The pair structure of the two models is however very similar. This finding, which appears to contradict the common view that the physics of dense liquids is dominated by the steep repulsive forces between atoms, is characterized in detail, and its consequences are explored. Beyond the role of attractive forces themselves, a key aspect in explaining the differences in the dynamical behavior of the two models is the truncation of the interaction potentials beyond a cutoff at typical interatomic distance. This leads us to question the ability of the jamming scenario to describe the physics of glass-forming liquids and polymers.     

The transition from inertial to viscous flow in capillary rise

We investigate the initial moments of capillary rise of liquids in a tube. In this period both inertia and viscous flow losses balance the pressure generated by the meniscus curvature (capillary pressure). It is known that the very first stage is purely dominated by inertial forces, where subsequently the influence of viscosity increases (visco-inertial flow). Finally the effect of inertia vanishes and the flow becomes purely viscous. In this study we derive the times and meniscus heights at which the transition between the time periods occur. This is done in an attempt to provide a method to determine a priori which terms of the momentum balance are relevant for a given problem. Analytic solutions known from previous literature are discussed and the time intervals of their validity compared. The predicted transition times and the calculated heights show good agreement with experimental results from literature. The results are also discussed in dimensionless form and the limitations of the calculations are pointed out.     

Viscous flow and non-linear phenomena in non-equilibrium thermodynamics of membrane transport Part 2 — The phenomenological equations

Phenomenological equations are deduced which give the flows of matter, volume, charge and heat for a discontinuous system in which inertial terms in the viscous equations of motion are not negligible. It is found that the equations contain terms in the powers of the first-order affinities, with corresponding phenomenological coefficients. These coefficients can all be derived from first-order coefficients, however, and thus refer to a system which is close to equilibrium. The theory agrees with well-documented specific hydrodynamic calculations, but generalizes these in the framework of now equilibrium thermodynamics.     

Anisotropic gas transport in a bilayer membrane in the free molecular flow regime

Asymmetric phenomena associated with gas transport in the free molecular flow in multilayer membranes have been investigated. Bilayer track membranes have been examined. A model describing anisotropic gas transport across a multilayer membrane has been constructed and analyzed. The interaction parameters characterizing the effect of the geometry of the inner surface of the pores on the gas flow through the membrane have been determined.     

The role of lipophilicity in transmembrane anion transport

The transmembrane anion transport activity of a series of synthetic molecules inspired by the structure of tambjamine alkaloids can be tuned by varying the lipophilicity of the receptor, with carriers within a certain log P range performing best.     

Systematic investigation of theories of transport in the Lennard-Jones fluid

Three kinetic theories of transport are investigated for the single-species Lennard-Jones model fluid. Transport coefficients, including diffusion, shear, and bulk viscosity, are calculated from these theories for the Lennard-Jones fluid across the fluid regions of the phase diagram. The results are systematically compared against simulation. It is found that for each transport property considered, there is at least one theoretical result based on approximations that have been systematically derived from a first-principles starting point that is quantitatively useful over a wide range of densities and temperatures. To the authors' knowledge, this article constitutes the first such compendium of results for the Lennard-Jones model fluid that has been assembled.     

Generalization of membrane reflection coefficients for nonideal,nonisothermal, multicomponent systems with external forces and viscous flow

The notion of reflection coefficients is generalized from dilute ideal solutions to apply to virtually any kind of solution and any kind of membrane whose properties are not affected by the solution. The crucial points in the generalization are the selection of a suitable definition of partial osmotic pressure and the inclusion of separative viscous flow as a transport mechanism (necessary to obtain the correct semipermeability limit). The latter can lead to loss or concealment of Onsager reciprocity, so that the reflection coefficients for volume flow and for solute fluxes are not necessarily equal. Two choices of reference state are presented: the traditional choice of zero reflection coefficient for solvent volume flow, and a more symmetric choice of an average reflection coefficient equal to zero. Several examples are worked out for binary and ternary solutions and compared with results from experiments and from model calculations. Thermal gradients are included for completeness, but play no essential role in the reflection coefficients. The development is given entirely in terms of differential equations of transport, and problems and inconsistencies associated with the use of finite-difference equations are briefly discussed.     

Mass and momentum transport in extra-luminal flow (ELF) membrane devices for blood oxygenation

In this paper, we report on the characterisation of transport in membrane modules for blood oxygenation where blood is circulated outside hollow fibre membranes arranged in double layer cross-laid mats at an angle with respect to the main direction of blood flow. The effect of design and operating variables on module performance was investigated with respect to oxygen transfer into water, as gaseous oxygen and water are circulated counter-currently, respectively inside the membrane lumen and through the membrane assembly.Increasing water flow rates and membrane angles enhanced oxygen transfer across the membrane and resulted in robust operation but also in high pressure drops.Module pressure drop and oxygen transfer rate were correlated to module geometry, fibre packing density, water flow rate and membrane angle with respect to the main direction of the liquid flow in non-dimensional equations that can be used by membrane module manufacturers for the design of optimal ELF blood oxygenators. The results suggest that an optimum membrane angle exists, beyond which module operation is not convenient in terms of energy.     

Improved charge transport in dye-sensitized solar cells employing viscous non-volatile electrolytes

Dye-sensitized solar cells (DSSCs) employing a viscous non-volatile electrolyte were prepared by utilizing anatase TiO₂ nanorods (synthesized via oriented attachment) as a photoanode material. One promising way to enhance the photovoltaic performance of DSSCs employing viscous electrolytes is to increase ion conductivity by increasing the salt concentration. This is accompanied by an acceleration of the charge recombination reaction and the limiting of the overall conversion efficiency. The results showed that a TiO₂ nanorod electrode enables more favorable electron transport than a conventional nanoparticle-based electrode due to the improved electron diffusion length and the large intrinsic surface area.     

Analytical theories of transport in concentrated electrolyte solutions from the MSA

Ion transport coefficients in electrolyte solutions (e.g., diffusion coefficients or electric conductivity) have been a subject of extensive studies for a long time. Whereas in the pioneering works of Debye, Hückel, and Onsager the ions were entirely characterized by their charge, recent theories allow specific effects of the ions (such as the ion size dependence or the pair association) to be obtained, both from simulation and from analytical theories. Such an approach, based on a combination of dynamic theories (Smoluchowski equation and mode-coupling theory) and of the mean spherical approximation (MSA) for the equilibrium pair correlation, is presented here. The various predicted equilibrium (osmotic pressure and activity coefficients) and transport coefficients (mutual diffusion, electric conductivity, self-diffusion, and transport numbers) are in good agreement with the experimental values up to high concentrations (1-2 mol L(-1)). Simple analytical expressions are obtained, and for practical use, the formula are given explicitly. We discuss the validity of such an approach which is nothing but a coarse-graining procedure.     

Electrical pumping in carrier-mediated membrane transport

A model of carrier-mediated pumping induced by electrochemical (redox) reactions is presented. The model is compared with published data for the facilitated transport of nitric oxide in a formamide membrane containing dissolved ferrous and ferric chlorides wherein the flux of nitric oxide is augmented by diffusion of the reversible complex, (NO—Fe²⁺. Passing a current through the membrane drives the reduction of ferric ions at the cathode and the oxidation of ferrous ions at the anode, coupling the charge and mass fluxes within the membrane. Our results indicate that this electrically powered, carrier-mediated membrane can pump permeant up to a concentration 0(10) times greater than that in the feed.     

Electrochemical coupling in carrier-mediated membrane transport

The possibility of controlling permeant flux through facilitated-transport membranes by passage of electric current is examined theoretically. The major effects observed are due to the role of the electrodes as sources and sinks for the species which participate in the electrode reactions. If those species also participate in the homogeneous chemical reactions, the passage of current can profoundly influence the membrane permeability. Order of magnitude increases in permeant flux are predicted, as is the pumping of permeant against its overall concentration difference. Predicted energy consumption is competitive with that of conventional separation processes.     

Uniform directional alignment of single-walled carbon nanotubes in viscous polymer flow

In this work, we probed the effects of shear flow on the alignment of dispersed single-walled carbon nanotubes in polymer solutions. Two different systems were compared: Single-walled carbon nanotubes dispersed using an anionic surfactant and single-walled carbon nanotubes dispersed using an anionic surfactant and a weakly binding polymer. It was determined that the addition of the weakly binding polymer increased the degree of dispersion of the carbon nanotubes and the ability to induce their alignment when subjected to shear forces.     

The role of flow injection analysis in speciation studies

An overview of the features of flow injection analysis (FIA) which make it a very useful technique for speciation studies is presented. Special emphasis is paid on pre- and post-column coupling as these arrangements allow major speciation problems to be solved.