Investigation of oxygen diffusion into contact lenses using electron spin resonance spectroscopy

Oxygen permeability is the most important parameter of contact lenses, as lack of oxygen causes corneal edema and threatens the vision of the patient. This study was unique in that it used an electron spin resonance (ESR) technique to determine the oxygen diffusion coefficient (D) of contact lenses. Although there are many methods and techniques for investigating oxygen diffusion into contact lenses, ESR was used for the first time in this study. The ESR technique is based on the scavenging of radicals produced in lenses by oxygen. As a contact lens is not a paramagnetic substance, it cannot give an ESR spectrum. But it does produce an ESR spectrum after γ irradiation. When a vacuum‐irradiated contact lens is exposed to air, the radicals trapped in the lens are transformed into peroxide radicals by the addition of molecular oxygen to the free radicals, and the ESR spectrum begins to change with time. This effect can be used as a tool to measure oxygen uptake in irradiated contact lenses. The oxygen diffusion coefficient of a contact lens was determined from changes in ESR signal intensity varying with time. The diffusion coefficients of oxygen for a contact lens were determined for rapid decay [(1.5 + 0.4) × 10^?8 cm²/s] and slow decay [(1.3 + 0.3) × 10^?9 cm²/s] in this study. These values are in agreement with the D values given in the literature for polymeric materials used for contact lenses. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2937–2941, 2006     

Determination of diffusion coefficient of oxygen into polymers by using electron spin resonance spectroscopy. II. Poly(vinyl acetate)

Electron spin resonance spectroscopy has been used to follow the kinetics of radical decay during postirradiation oxidation of poly(vinyl acetate). The oxygen uptake of γ-irradiated spherical poly(vinyl acetate) has been determined from changes in the intensity of the ESR signal. The diffusion coefficient (D) of oxygen determined by this technique showed an inverse relation with dose. The value of the diffusion coefficient of oxygen obtained in this study, after correcting for dose and size effects is 4.5 × 10^?8 cm²/s, and is in very good agreement with the literature value of D = 5.1 × 10^?8 cm²/s.     

Kinetics and diffusion processes in colour removal from effluent using wood as an adsorbent

The rates of adsorption of a basic dye, Astrazone Blue, and an acidic dye, Telon Blue, on wood have been studied. The rate controlling step is mainly intraparticle diffusion, although a small resistance due to a boundary layer is experienced. The activation energies for the adsorption of Astrazone Blue and Telon Blue on wood are 16.8 kJ mol^?1 and 9.6 kJ mol^?1, respectively. The diffusion coefficients vary from 6×10^?13 cm² s^?1 to 18×10^?13 cm² s^?1 for Astrazone Blue at 18°C and from 3 × 10^?13 cm² s^?1 to 8 × 10^?13 cm² s^?1 for Telon Blue at 18°C. The variation in diffusivities is attributed to boundary layer effects.     

Studies on molecular transport of n‐alkanes through poly(tetrafluoroethylene‐co‐propylene) elastomeric membrane

Molecular transport of a series of n‐alkanes through commercial TFE elastomer (FA 150L) has been studied in the temperature range 30–50 °C using sorption‐gravimetric method. The Fickian diffusion equation was used to calculate the diffusion coefficients, which were dependent on the size of the alkanes and temperature. The diffusion coefficients at 30°C varied from 4.53 × 10^?8 cm²/s (n‐heptane) to 0.18 × 10^?8 cm²/s (n‐hexadecane). The liquid concentration profiles have also been computed using analytical solution of Fick's equation with the appropriate initial and boundary conditions and these were presented as a function of penetration depth of molecular migration and time of immersion. These results have been discussed in terms of molecular size of alkanes as well as temperature. In all the liquid penetrants, the transport phenomenon was found to follow the anomalous behavior. From the temperature dependence of diffusion and permeation coefficients, the Arrhenius activation parameters have been estimated. These parameters do not exhibit any systematic variation with the size of the penetrants. The resulting low diffusion coefficients, contribute to the superior barrier performance of the membrane, is due, in part, to the high glass transition temperature of Aflas? TFE elastomer. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2228–2235, 2006     

Analysis of intraparticle diffusion on adsorption of crystal violet on bentonite

Abstract

In the present work, kinetics of crystal violet (CV) adsorption on bentonite was studied by pore volume and surface diffusion model (PVSDM), surface diffusion model (SDM), and pore volume diffusion model (PVDM). The adsorption decay curves were obtained in batch system using different adsorbent dosages. The PVDM model did not interpreted the kinetic adsorption since the calculated value of D_p equal to 5.64?×?10^?7 cm² s^?1 predicted a slower adsorption than that obtained by the experimental data. The PVSDM results indicates that the intraparticle diffusion is predominantly due to surface diffusion (93%) and the pore volume diffusion can be negligible. Once the surface diffusion was the limiting step, the estimation with one (D_s) and two (D_sq and α) parameters were tested in the SDM model. The statistical analysis revealed that the one-parameter SDM model was most appropriate to predict the CV adsorption on bentonite. The optimal values of Ds ranged from 6.19?×?10^?10 to 6.49?×?10^?10 cm² s^?1, and decrease with the adsorbent dosage.     

On the determination of diffusivities of volatile hydrocarbons in semi‐solid bitumen

Residual volatile hydrocarbons in bitumen constitute a potential source of air pollution. Diffusivities of volatile components in bitumen are needed to assess the extent of environmental damages that could result from bitumen spill or working loss of vapour to the atmosphere. Knowledge of solvent dispersion is also required in the recovery of viscous bitumen by solvent displacement. This paper discussed the de‐coupled transfer model developed by Tang and Zhang and its limiting solution. Fu and Phillips' diffusion data were re‐interpreted based on the limiting solution with delay time correction. The diffusion coefficients of hexane iso‐hexane, cyclo‐hexane and toluene at 25°C were found to be 8.6 × 10^?8 cm²/s, 6.3 × 10^?8 cm²/s, 2.4 × 10^?8 cm²/s and 6.8 × 10^?8 cm²/s, respectively. Improper use of the limiting solution could lead to 25% over‐estimates of diffusion coefficients.     

Adsorption of dyes onto bagasse pith using a solid diffusion model

The adsorption of four dyes (Basic Blue 69, Basic Red 22, Acid Blue 25, and Acid Red 114) onto bagasse pith has been studied using an agitated batch adsorber. The variables studied were initial dye concentration and pith mass. A mathematical model has been developed based on external mass transfer and solid-phase diffusion. The model has been used to generate theoretical concentration–time decay curves, and these results were adjusted to experimental data by a best fit approach. The external mass transfer coefficients are 2.0 × 10^?3, 1.5 × 10^?3, 8.0 × 10^?4, and 5.0 × 10^?4 cm s^?1 and the solid diffusivities are 1.1 × 10^?8, 1.0 × 10^?8, 6.0 × 10^?9, and 3.0 × 10^?9 cm² s^?1 for Basic Blue 69, Basic Red 22, Acid Blue 25, and Acid Red 114.     

Anion exchange of uranium from aqueous sulphuric acid solutions: Diffusion kinetics

The ion exchange kinetics of uranium sorption from acidic sulphate solutions onto weak and strong base resins has been studied. In order to facilitate experimentation and analysis, isotopic redistribution experiments of uranyl sulphate ions (with U-235 as a tracer) under conditions of particle diffusion control have been performed. Also, self-diffusion experiments for sulphate ions (with S-35 as a tracer) and counter diffusion measurements involving uranyl sulphate-sulphate exchange have been performed. The ion exchange of uranium is a slow reaction and appears to be particle diffusion controlled at all solution conditions of practical interest. For a weak base resin (Relite MG-1), the self-diffusion coefficient of uranyl sulphate ions changes with particle size, increasing from 1.49 to 4.88 × 10^?9 cm²/s as the particle diameter is reduced from 0.146 to 0.054 cm. With a strong base resin (Dowex-1×8), the self-diffusion coefficient of uranyl sulphate ions remains reasonably constant with varying particle diameter at a value of 1.76 × 10^?9 cm²/s. Sulphate ion self-diffusion coefficients were also constant with particle size (2.50 × 10^?7 cm²/s) and the counter exchange diffusivity lies between the values for the individual species; a typical value is 1.53 × 10^?8 cm²/s. It is argued that resin matrix structure has a significant effect on uranium kinetics and that a knowledge of the relevant diffusion coefficients will provide a rational basis for the design of ion exchange processes and plant equipment.     

Kinetics of aluminium deposition from aluminium chloride—alkali chloride melts

The kinetics of aluminium deposition from NaClAlCl₃ and NaClKClAlCl₃ melts (c_AlCl3 < 0.4 mol%) was studied by linear sweep voltammetry and potential step amperometry. The reduction of AlCl₃ on tungsten and aluminium electrodes was found to be diffusion controlled. The diffusion coefficients of AlCl₃ were: 3.5 × 10^?5 cm² s^?1 at 820°C in NaClAlCl₃, 2.7 × 10^?5cm²s^?1 at 825°C, and 2.1 × 10^?5cm²s^?1 at 705°C in KClNaClAlCl₃. The rate constant for AlCl₃ reduction at these conditions was found to be in the order of 0.2 cm s^?1, in good agreement with extrapolated literature data.     

Effect of polymer–clay interaction on solvent transport behavior of fluoroelastomer–clay nanocomposites and prediction of aspect ratio of nanoclay

Diffusion and sorption of methyl ethyl ketone and tetrahydrofuran through fluoroelastomer‐clay nanocomposites were investigated in the temperature range of 30–60°C by swelling experiments. Slightly non‐Fickian transport behavior was found for these nanocomposites, having variation of type of nanoclay and loading. Different transport parameters depend on the size and shape of the penetrant molecules. The results were used to study the effect of nanoclay on the solvent transport‐properties of nanocomposites and their interactions with solvents. The diffusion coefficient of methyl ethyl ketone at 30°C for neat rubber was 1.43 × 10^?8 cm² s^?1, while those of the unmodified and the modified clay filled samples at 4 phr loading were 0.24 × 10^?8 and 0.50 × 10^?8 cm² s^?1, respectively. At 8 and 16 phr loading of the unmodified clay, it was found to be 0.44 × 10^?8 and 0.64 × 10^?8 cm² s^?1, respectively. The samples were also reswelled after deswelling. Surprisingly, transport behavior became Fickian on reswelling. Interestingly, ratio of diffusion coefficients of the filled system to the neat system was found to be almost same for the first time swelling and reswelling experiments. The results showed that better polymer‐clay interaction in the case of the unmodified‐clay filled nanocomposites is responsible for enhanced solvent‐resistance property. From the permeation data, for the first time, aspect ratio of nanoclays in different composites was calculated and found to have good correlation with the morphology data obtained from transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Behavior of sulfonated poly(ether ether ketone) in ethanol–water systems

The behavior of sulfonated poly(ether ether ketone) (sPEEK) membranes in ethanol–water systems was studied for possible application in direct ethanol fuel cells (DEFCs). Polymer membranes with different degrees of sulfonation were tested by means of uptake, swelling, and ethanol transport with dynamic measurements (liquid–liquid and liquid–gas systems). Ethanol permeability was determined in an liquid–liquid diffusion cell. For membranes with an ion‐exchange capacity (IEC) between 1.15 and 1.75 mmol/g, the ethanol permeability varied between 5 × 10^?8 and 1 × 10^?6 cm²/s, being dependent on the measuring temperature. Ethanol and water transport in liquid–gas systems was tested with pervaporation as a function of IEC and temperature. Higher IEC accounted for higher fluxes and lower water/ethanol selectivity. The temperature had a large effect on the fluxes, but the selectivity remained constant. Furthermore, the membranes were characterized with proton conductivity measurements. The proton diffusion coefficient was calculated, and a transition in the proton transfer mechanism was found at a water number of 12. Membranes with high IEC (>1.6 mmol/g) exhibited larger proton diffusion coefficients in ethanol–water systems than in water systems. The membrane with the lowest IEC exhibited the best proton transport to ethanol permeability selectivity. The use of sPEEK membranes in DEFC systems depends on possible modifications to stabilize the membranes in the higher conductive region rather than on modifications to increase the proton conductivity in the stable region. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

On the measurement of liquid phase diffusivities for slightly soluble solids

A procedure is developed to measure diffusion coefficients in the liquid phase for slightly soluble solids. Transient concentration profiles for the dissolution of a planar solid surface are obtained by means of a Mach-Zehnder interferometer. Fitting of these profiles to Fick's second law then provides a measure of the diffusivity, even when it is concentration dependent. Application of the technique to the water-benzoic acid system yields a diffusion coefficient at 25°C which is 1.3 × 10^?5 cm²/sec at infinite dilution and which decreases slowly as the concentration approaches saturation. The initial value for acetanilide in water is 1.1 × 10^?5 cm²/sec at 25°C and again the diffusivity decreases as the concentration increases.     

Determination of diffusion coefficient of oxygen into polymers by using electron spin resonance spectroscopy. I. Poly(methyl methacrylate)

A simple technique has been proposed for the measurement of the diffusion coefficient of oxygen into polymeric spheres. The technique is based on the scavenging of radicals produced by high energy radiation by oxygen. The diffusion coefficient of oxygen determined for poly(methyl methacrylate) has shown an inverse dependency on the dose received. Diffusion coefficient determined for low doses (D = 3.4 × 10^?8 cm²/s) as well as that determined after extrapolation to zero dose (D₀ = 3.7 × 10^?8 cm²/s) are in excellent agreement with the values reported in the literature.     

Ion Exchange in Selected Low Rank Coals. Part II: Kinetics

ABSTRACT

The kinetics of ion exchange in a subbituminous coal have been investigated. The rate determining step in the exchange processes was determined to be intraparticle diffusion for particle sizes of 9,500 × 4,000 μm, 425 × 250 μm and 180 × 125 μm. The kinetics of exchange for the 180 × 125 μm particles was very rapid and approached film diffusion control. A relatively simple mathematical model developed by Helfferich (1) describe adequately ion exchange kinetics. The model was effective in predicting the extent of exchange as a function of time and the time required to reach complete exchange. Interdiffusion coefficients at 298 K obtained from the model for exchange of H for Na ⁺, Mg²⁺and Ca²⁺ranged from 2.0 × 10^10?6to 2.6 × 10^?8cm²/s with the values decreasing in the order:

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Diffusion of water and caprolactam in nylon 6 melts

The diffusions of both caprolactam and water from nylon 6 melts have been measured using a semiinfinite geometry. The process involving diffusion and chemical reaction was analyzed utilizing Higbie's penetration model and known kinetic data. The caprolactam and water diffusion coefficients are, respectively, 8 × 10^?8 and 2.5 × 10^?4 cm²/sec. Using the data, molecular weight profiles were obtained for both semiinfinite and cylindrical geometries as a function of melt depth and time.     

Sorption studies of propylene in polypropylene. Diffusivity in polymer particles formed by different polymerization processes

Propylene was polymerized in gas phase and liquid phase by using a novel nonporous Ziegler–Natta‐catalyst system. The polymer particles formed at different polymerization times were used for sorption measurements. In both cases it was found that the effective diffusion coefficient is increasing with increasing size of polymer particles and the effective diffusion coefficients of polymer particles formed by liquid‐phase polymerization are larger than those of polymer particles produced by gas‐phase polymerization. The effective diffusion coefficients of polymer particles are in the range of 2 × 10^?11 to 1.6 × 10^?10 m²/s with activation energies from 34 to 22 kJ/mol. The analyzed polymer particles have average diameters between 250 and 875 μm. The solubility of propylene in polypropylene particles can be described by the law of Henry at conditions studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2642–2648, 2006     

ESR studies of solvent penetration into polymer-blend-like material: Particles formed from poly(methyl methacrylate) and polyisobutylene

Solvent penetration into nonaqueous dispersions (NAD) of poly(methyl methacrylate) (PMMA) sterically stabilized by PIB were studied by ESR spectroscopy. These colloidal particles were exposed to dilute solution of spin probe 3-carbomoyl 2,2,5,5-tetramethyl 3-pyrolin-1-yloxy and the bimodal distribution of ESR spectra of this probe molecules were carefully monitored as a function of time. Fickian type diffusion of spin probes into the particles was observed and a spherical model was employed to obtain the diffusion coefficients (D) in various solvents. D values were found around 10^?15 cm²/s, and interpreted as the parameter, inversely proportional to the apparent viscosity of the environment inside the colloid particle. Maximum amount of diffused solvent molecules (M_∞) into the PMMA particle was found inversely proportional to polymer-solvent interaction through the solubility parameters. The interconnected network of PIB was found to be responsible for the penetration of spin-probe (i.e., solvent) molecules into the glassy PMMA phase and/or PIB-PMMA interface.     

The interaction of hydrogen sulfide with lead- and barium–cadmium–zinc-stabilized poly(vinyl chloride)

Poly(vinyl chloride) polymers stabilized with tribasic lead sulfate discolor upon exposure to hydrogen sulfide gas as a result of lead sulfide formation. The discoloration occurs for samples in both cord and sheet forms and is shown to be a function of total H₂S exposure, reaching a limiting value that is determined by the amount of lead stabilizer used in the polymer formulation. The permeation and diffusion constants for H₂S through PVC stabilized with tribasic lead sulfate and with a liquid Ba–Cd–Zn formulation are found to be P_Pb = (6.0 ± 0.2) × 10^?9, P_BaCdZn = (5.2 ± 0.2) × 10^?9 (both in cm³ gas?cm film/cm² area?sec?cm Hg), D_Pb = (1.3 ± 0.2) × 10^?7 cm²/sec, and D_BaCdZn = (6.4 ± 0.6) × 10^?8 cm²/sec, all measured at 21°C. The stabilizing efficiencies of the formulations were assessed by HCl evolution measurements, which show that exposure to H₂S decreases the initial polymer stability for both Pb-stabilized and Ba–Ca–Zn-stabilized formulations. Protection of stabilized PVC formulations from diffusing hydrogen sulfide is thus advisable for long-term stability as well as for color integrity.     

Bending-beam study of water sorption by thin poly(methyl methacrylate) films

A bending-beam technique has been employed to study the effects of film thickness (7–55 μm) and rate of cooling during film preparation (～ 6°C/h to a dry ice quench) on sorption characteristics of water by poly(methyl methacrylate) films coated on thin fused quartz beams (? 84 μm thick). In each experiment, the curvature of a polymer coated beam exposed to liquid water was monitored as a function of time by a low power laser pointer. With the use of a transport model which considers the sorption process as the linear superposition of contributions from Fickian diffusion and a first-order polymer molecular relaxation process, the beam curvature data were analyzed to determine the governing transport kinetics and associated transport parameters such as water diffusion coefficient and relaxation rate constant. From curvature analysis for thin films (7–13 μm in thickness), it was found that water diffusion proceeds at early times in a Fickian-like manner with a diffusion constant of 2–4 × 10^?9 cm²/s. At later times, significant relaxation contributions lead to non-Fickian diffusion behavior, an effect that is more pronounced as the film thickness or sample cooling rate decreases. In addition, sorption of water was found to reduce the film stress (initially tensile at ? 10⁸ dyn/cm²) at a rate that increases with sample cooling rate. The high initial film stress not present in free-standing films may account for the relatively higher diffusion coefficient (～ 2 × 10^?8 cm²/s) found here for very thick (55 μm) PMMA coatings. Because the bending-beam technique uses coated samples, it is especially well suited for studying penetrant transport into polymer coatings.     

Translational Diffusion and Interaction of a Photoreceptor and Its Cognate Transducer Observed in Giant Unilamellar Vesicles by Using Dual‐Focus FCS

In order to monitor membrane–protein binding in lipid bilayers at physiological protein concentrations, we employed the recently developed dual‐focus fluorescence correlation spectroscopy (2fFCS) technique. In a case study on a photoreceptor consisting of seven transmembrane helices and its cognate transducer (two transmembrane helices), the lateral diffusion for these integral membrane proteins was analyzed in giant unilamellar vesicles (GUVs). The two‐dimensional diffusion coefficients of both separately diffusing proteins differ significantly, with D=2.2×10^?8 cm² s^?1 for the photoreceptor and with D=4.1×10^?8 cm² s^?1 for the transducer. In GUVs with both membrane proteins present together, we observed significantly smaller diffusion coefficients for labelled transducer molecules; this indicates the presence of larger diffusing units and therefore intermolecular protein binding. Based on the phenomenological dependence of diffusion coefficients on the molecule's cylindrical radius, we are able to estimate the degree of membrane protein binding on a quantitative level.