Residual aqueous ozone determination by gas diffusion reverse flow injection analysis

A novel system based on reverse flow injection analysis with a gaseous diffusion step (GD-r-FIA) has been developed for the analysis of ozone. It includes an automatic microburet injection system. The ozone diffuses through a microporous membrane of polyvinylidene difluoride (PVDF) from the donor stream to the acceptor stream containing nitrite ions. The nitrite concentration in the acceptor solution decreases due to the ozone reduction reaction. In this way, a simple indirect measurement of the ozone concentration can be performed using the Griess–Ilosvay reaction for the nitrite ion. This correlates with the decrease in absorbance of the azoic dye formed with the ozone concentration in the donor stream. The system has been optimised by investigating the effect of the nitrite concentration in the acceptor stream on the diffusion flow. The optimum nitrite concentration was set at 0.250 ppm with a flow rate of 1.5 ml/min. The efficiency of the ozone diffusion through the membrane was only 4.4%. This affects the average sensitivity, which is low (0.0092±0.0012 AU/ppm), although the detection limit is similar to that obtained with other reported methods (0.03 ppm). The main advantage of the system reported here is that it has a linear range that is an order of magnitude broader than those observed for other GD-FIA systems. This is especially useful for continuous monitoring systems, since the residual ozone concentration is normally between 0.05 and 5.0 ppm. Additionally, using the reverse flow injection analysis (FIA) technique minimises chemical consumption and residue generation. Finally, the stability of the ozone solution and the repeatability and reproducibility of the method have been studied.     

Measuring the transverse concentration gradient between adjacent laminar flows in a microfluidic device by a laser-based refractive index gradient detector

A detection scheme that probes the refractive index gradient (RIG) between adjacent laminar flows in microfluidic devices has been developed and evaluated. The behavior of low Reynolds number flows has been well documented and shows that molecular transport (mixing) between adjacent laminar flows occurs by molecular diffusion between the flow boundaries. A diode laser has been used to probe the transverse concentration gradient at a selected position along a microchannel. The concentration gradient is affected by the transverse diffusion from a flow with analyte into a flow initially without analyte. To optimize sensitivity, the RIG is probed at a position in which molecular diffusion across the boundary of the two flows has been minimal, i.e. just after the flow initially without analyte merges with the flow initially containing the analyte at a given concentration. The RIG formed causes the laser beam, impinging orthogonal to the RIG through the microchannel, to be deflected. The angle of deflection is then monitored on a position sensitive detector (PSD). Currently, this detection scheme is demonstrated to provide quantitative detection of sucrose, as a test analyte, with a concentration limit of detection (LOD) of 96 ppm (w/v) or 280 muM, corresponding to 1.3x10(-5) DeltaRI units using 3sigma baseline noise. A dynamic range of 96 ppm to 50% sucrose is obtained. This detection method provides universal detection selectivity for microfluidic analysis systems that are becoming increasingly useful in monitoring chemical systems, particularly for the polymer, pharmaceutical and life sciences fields. For a larger molecular weight analyte with a smaller diffusion coefficient, lower concentration and RI LODs were achieved since detection sensitivity is a function of analyte diffusion. For example, for the polymer poly (ethylene glycol) with a molar mass of 11 840 g mol(-1), the LOD was experimentally determined to be 56 ppm (4.7 muM), equivalent to a RI LOD of 4.5x10(-6) DeltaRI (3sigma). The detection limit for proteins was also found to be favorable. For example, with the current configuration, ribonuclease A (RNAse) had a LOD of 46 ppm (3.4 muM), and bovine serum albumin (BSA) had a LOD of 54 ppm (780 nM).     

Diffusion in polyacrylate aqueous systems at 25°C. Role of reference frame choice in interpreting diffusion coefficient data

Diffusion coefficients have been measured for the binary systems sodium polyacrylate-water and polyacrylic acid-water at 25°C as a function of concentration. Diffusion coefficients have been also measured for the ternary system sodium chloride-sodium polyacrylate-water at constant NaCl concentration and varying polyacrylate concentration. The experimental results have been compared with some limit expressions, available in literature, for the four D _ik diffusion coefficients of systems containing two electrolytes with a common ion. The ternary system shows strong interaction between flows: as the polyelectrolyte concentration, C₂, approaches zero, the main diffusion coefficient D₂₂ and the cross coefficient D₂₁ approach zero, while the cross coefficient D₁₂ reach quite high values. The water motion during the diffusion process is also discussed.     

Application of free‐volume theory to self diffusion of solvents in polymers below the glass transition temperature: A review

Theories based on free‐volume concepts have been developed to characterize the self and mutual‐diffusion coefficients of low molecular weight penetrants in rubbery and glassy polymer‐solvent systems. These theories are applicable over wide ranges of temperature and concentration. The capability of free‐volume theory to describe solvent diffusion in glassy polymers is reviewed in this article. Two alternative free‐volume based approaches used to evaluate solvent self‐diffusion coefficients in glassy polymer‐solvent systems are compared in terms of their differences and applicability. The models can correlate/predict temperature and concentration dependencies of the solvent diffusion coefficient. With the appropriate accompanying thermodynamic factors they can be used to model concentration profiles in mutual diffusion processes that are Fickian such as drying of coatings. The free‐volume methodology has been found to be consistent with molecular dynamics simulations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Tracer diffusion of Co2+ ions in agar gel containing multi-electrolyte systems

Tracer diffusion coefficients of cobalt ions have been measured in the supporting medium containing multi-electrolyte systems of alkali bromides. The electrolyte concentration was varied between 10^–6-0.1M at 25°C and the diffusion coefficients were determined by zone-diffusion technique using agar gel medium. The trend in the theoretical values of diffusion coefficients is accounted for by considering the relative contribution of mobility function, ionic strength as well as ion size parameter to the theoretical value in different systems. While the deviations between theoretical and experimental values of diffusion coefficients are explained on the basis of various co-occurring effects in ion-gel-water system.     

Relation between hydraulic permeability and diffusion in homogeneous swollen membranes

The question of viscous flow versus molecular diffusion mechanisms for pressure-induced liquid transport through membranes is critically examined for the specific case of homogeneous swollen membranes. It is shown that previous attempts to compute diffusion coefficients from hydraulic permeabilities for such systems have used an equation which is grossly in error. It estimates diffusion coefficients which are orders of magnitude too high and often exceed self-diffusion coefficients. This has frequently led to the conclusion that viscous flow predominates. The origins of the errors in this equation are indicated, and a substitute equation is developed which gives diffusion coefficients well below that for self-diffusion when applied to literature data. As a result it is concluded that molecular diffusion is the dominant mechanism in homogeneous systems.     

Diffusion of Alkyltriphenylphosphonium Bromides in Aqueous Micellar Solutions

Dynamic light scattering, conductometry, and capillary viscometry have been used to study aqueous micellar solutions of dodecyl-, tetradecyl-, and hexadecyltriphenylphosphonium bromides in a wide range of concentrations covering the first and second critical micelle concentrations (CMC₁ and CMC₂). It has been shown that the concentration curves for the diffusion coefficients of the ionic surfactants increase above CMC₁ and, then, pass through a maximum. As the alkyl chain length increases, the slopes of the concentration curves within the range of the linear growth in the diffusion coefficient rise, the height of the maximum increases, and its position shifts toward lower concentrations. The obtained results have been explained in terms of a theory previously developed for ideal micellar systems. It has been shown that the mobility factor plays the predominant role in the range of the linear increase in the diffusion coefficient and the effect of the viscosity of a micellar surfactant solution is enhanced with a rise in its concentration.     

The Conductance Percolation and Droplets Dimension of AOT in Alkanol Systems

The conductance behaviors of AOT in alkanol (hexanol, heptanol, octanol, and decanol) reverse microemulsions have been investigated. The percolation phenomenon induced by water is observed in the water/AOT/decanol system at 15°C and 30°C, and the water/AOT/octanol system at 15°C. The percolation phenomenon of water/AOT/alkanol systems is discussed from the interaction between the hydroxy group of alkanol and the polar group of AOT, droplets diffusion coefficient, and the rate constant for droplets collision. The droplets size and diffusion coefficient of the water/AOT/alkanol systems have also been studied by modifying the water concentration. The results show that hydrodynamic diameter of droplets decreases and diffusion coefficient increases with the increasing of water content, which may be explained by the polarity of alkanol phase.     

Conductometric determination of ammonium by a multisyringe flow injection system applying gas diffusion

A multisyringe flow injection system (MSFIA) coupled to a gas-diffusion cell has been developed for the conductometric determination of ammonium in different water samples. Operation strategies, membrane, reagent concentrations, and flow rates have been studied to optimize the sensitivity of detection and to fit the required working range. The proposed MSFIA system has been compared with former FIA and SIA systems using gas diffusion. The system was applied to the determination of ammonium in water samples of different matrices in order to evaluate its performance. These samples were coastal waters, pond waters, and compost aqueous extracts. Good recoveries of 102?±?13% were obtained and no significant differences with the reference methods were found. The system can be used for a wide concentration range of ammonia, from 0.075 to 360?mg?L^?1, without sample dilution and with a precision better than 2% of RSD. The throughput of the method was 32 injections per hour.     

Statistical theory of turbulent mass transfer in electrochemical systems

The paper presents an overview of the statistical theory of turbulent mass transfer in electrochemical systems and some new results which generalize the previously obtained relations for the flows of complex geometry. The developed theory does not use traditional semi-empirical hypotheses and analogies, but directly addresses to the solving of the critical for turbulent transfer the closure problem. The mathematical procedure for solving of the closure problem makes use of new equations for the correlations between concentration and velocity fluctuations in different space points and at different time moments; the dumping of turbulent pulsations in the viscous sublayer allows to neglect high order moments and obtain a closed equation for the turbulent mass flux. In general, the relation between the turbulent mass flux and the mean concentration gradient is non-local. Using available experimental information, the non-local equation for the turbulent mass flux is reduced to the traditional local one and the functional form of the turbulent diffusion coefficient is obtained. It is demonstrated that Reynolds analogy cannot been used for the prediction of the turbulent diffusivity. Applications of the developed theory to chemical engineering and to electrochemical flow diagnostics (prediction of flow characteristics using limiting diffusion current measurements) are discussed.     

Correlation between thermal diffusion and solvent self-diffusion in semidilute and concentrated polymer solutions

We have performed measurements of thermal diffusion coefficients DT and solvent self-diffusion coefficients Dss in semidilute to concentrated polymer solutions. Solutes of different glass transition temperatures and solvents of different solvent qualities have been used. The investigated systems are in detail: poly(dimethyl-siloxane) in toluene, tristyrene in toluene, polystyrene in toluene, polystyrene in tetrahydrofuran, polystyrene in benzene, and polystyrene in cyclohexane. The thermal diffusion data are compared to our data and literature data for solvent self-diffusion coefficients. In all systems the concentration dependence of DT closely parallels the one of Dss which may be viewed as a local probe for friction on a length scale of the size of one polymer segment. This identifies local friction as the dominating parameter determining the concentration dependence of DT. Solvent quality, in contrast, has no influence on DT.     

Diffusion coefficients and complex equilibria in solution-IV Experimental determination and manipulation of diffusion data

An experimental method for effectively measuring the change of diffusion current of a metal ion in the presence of increasing concentration of a complexing ligand is described and applied to five metal-ligand systems. The method has been applied to cases of reversible and irreversible electrochemical behaviour and is shown to provide reliable formation-curve data. The systems selected have formation constants which range over some ten orders of magnitude and in most cases the values obtained agree satisfactorily with those reported in the literature. In one case equilibrium data have been calculated for the first time.     

Phase structure of silanol-modified ethylene-vinyl acetate copolymers

For (ethylene-vinyl acetate copolymer)-tetraethoxysilane and (ethylene-vinyl acetate copolymer)-polydimethylsiloxane systems, the solubility of components has been studied in a wide range of temperatures and compositions. Phase diagrams have been constructed, pair interaction parameters have been calculated, and diffusion coefficients and activation energies of diffusion have been estimated. The temperature and concentration ranges of a change in solubility related to chemical interactions between the components have been revealed, and the structure of the modified copolymers has been studied. On the basis of the kinetic data on the movement of isoconcentration planes in diffusion zones of component mixing, time intervals corresponding to the onset of the chemical reaction between the components have been determined and the apparent activation energies of the process have been computed. Complex-shaped binodal and boundary curves have been interpreted within the framework of the classical Flory-Huggins theory.     

利用Schliteren效应考察流动注射分析系统中扩散作用对溶液分散的影响

本文利用FIA系统中四氯化碳、葡萄糖、氯化钠和盐酸溶液的Schlieren效应响应曲线，考察了扩散作用对溶液分散行为即对响应曲线形状、样品运行时间、保留时间及基线-基线峰宽的影响。通过分析表明，实验结果与理论是一致的     

Estimation of concentration-dependent diffusion coefficient in pressure-decay experiment of heavy oils and bitumen

Molecular diffusion has been considered to be an underlying mechanism for many of oil recovery processes like miscible and immiscible gas injection projects. Reliable estimation of the molecular diffusion coefficient as a transport property is therefore important in studying the performance of such systems. Interpretation of pressure-decay data has been traditionally used to estimate the molecular diffusion coefficient and usually to simplify the interpretation, its concentration dependency has been neglected. A pressure-decay model with concentration-dependent diffusion coefficient leads to a non-linear problem in which an analytical solution is difficult if not impossible to obtain. In this study, we used the Heat Integral Method (HIM) to solve the non-linear diffusion problem as a forward model. Using that forward model, we have developed a simple methodology for estimating the diffusion coefficient regardless of the form of function used for the concentration dependency of the molecular diffusion. Three different forms of functions for diffusion coefficient were considered. In its simplest form, the diffusion coefficient is set to be a constant value. In the two other forms, the diffusion coefficient was evaluated as a concentration-dependent two parameter equation using exponential and power-law functions, respectively. The proposed methodology is verified and tested using direct numerical solutions of the non-linear diffusion problem. Many numerical examples with a wide range of input parameters demonstrate the effectiveness of the proposed approach.     

Measuring flux distributions for diffusion in the small-numbers limit

For the classical diffusion of independent particles, Fick's law gives a well-known relationship between the average flux and the average concentration gradient. What has not yet been explored experimentally, however, is the dynamical distribution of diffusion rates in the limit of small particle numbers. Here, we measure the distribution of diffusional fluxes using a microfluidics device filled with a colloidal suspension of a small number of microspheres. Our experiments show that (1) the flux distribution is accurately described by a Gaussian function; (2) Fick's law, that the average flux is proportional to the particle gradient, holds even for particle gradients down to a single particle difference; (3) the variance in the flux is proportional to the sum of the particle numbers; and (4) there are backward flows, where particles flow up a concentration gradient, rather than down it. In addition, in recent years, two key theorems about nonequilibrium systems have been introduced: Evans' fluctuation theorem for the distribution of entropies and Jarzynski's work theorem. Here, we introduce a new fluctuation theorem, for the fluxes, and we find that it is confirmed quantitatively by our experiments.     

The self‐diffusion of macromolecules in binary blends of poly(ethylene glycol)

The concentration and molecular mass dependencies of the self‐diffusion coefficients were obtained for higher molecular mass component in binary blends of the homopolymer poly(ethylene glycol) (PEG) by a nuclear magnetic resonance method with pulsed magnetic field gradient. The shape of the diffusion decay and its dependence on the diffusion observation time in binary PEG blends have been investigated. The experimental results were explained by hypothesizing the existence of cluster formation in polymer melts and polymer blends and the possibility of molecular exchange between clusters. The entanglement time in such systems was evaluated. Copyright © 1999 John Wiley & Sons, Ltd.     

Phase equilibrium and diffusion in binary systems based on nonyl acrylate, acrylic acid, and their homo- and copolymers

The phase equilibrium and diffusion in systems based on nonyl acrylate, acrylic acid, and their homopolymers and copolymers of different compositions have been studied in the range of 20–100°C by the microinterference technique. For the systems with the limited solubility of components, the phase diagrams have been constructed. It has been established that, with a rise in the content of carboxyl groups in the copolymers, the diffusion coefficients and the thermodynamic compatibility of the monomers decrease. It is suggested that the nonlinear pattern of a variation in the diffusion coefficient with the content of acrylic acid in the copolymers is predominantly associated with a change in the type of hydrogen bonds and the formation of carboxyl dimers upon the transition to poly(acrylic acid). The concentration of carboxyl groups may serve as a parameter indicating variations in thermodynamic compatibility and diffusion constants of the monomers with the intensity and type of hydrogen bonds in the polymers under study. The presence of hydrogen bonds and their type are estimated on the basis of the IR spectra of the homo- and copolymers of interest.     

Flow activation free energy in cholesteric lyotropic liquid crystals

The flow activation free energy ( ΔG *) is the minimum energy necessary to induce a flow in a system that is at rest. It can be calculated from the system viscosity using the Andrade--Eyring theory. In the present work the flow activation free energy of cholesteric lyotropic liquid crystals was studied as a function of D-(+)-mannose concentration added to a nematic mesophase. The results obtained showed that all the systems can be characterized as Newtonian, but a smooth time-dependent effect can be observed mainly in systems with a lower chiral inductor concentration. It has also been observed that an increase of cholestericity leads to a decrease of ΔH * values until a limit is reached at 1 mol % of inductor. From a positive ΔS * variation it was possible to verify the existence of a locally less organized transient state during the process of micellar diffusion. When the cholesterization process was considered, the increase of the inductor concentration leads to a decrease of ΔS * and consequently to a system, as a whole, more orderly, possibly as a result of the restriction of movement caused by the presence of chiral interactions.     

Phase equilibrium and mutual diffusion in the poly(sulfone)–dimethylsulfoxide system

Phase equilibrium and mutual diffusion in the poly(sulfone) (PSF)–dimethylsulfoxide (DMSO) system have been studied at temperatures from 20 to 110°C over a wide solution composition range. The phase diagram for this system has been obtained and the effect of water on boundary concentrations has been studied. It is shown that the presence of water in DMSO has a considerable effect on the binodal curve. Thus, increasing the water content up to 1.3% by weight results in the displacement of the upper critical solution temperature by 30°C and the widening of the two-phase region. The kinetic regularities in mutual dissolution of components have been investigated. The concentration dependencies of the mutual diffusion coefficients in the systems studied are presented. The effect of moisture in DMSO on the mutual diffusion coefficients appears near the phase transition in the system. © 1994 John Wiley & Sons, Inc.