Observed Diffusion Coefficients Research Articles

Restricted diffusion of bicarbonate and hypophosphite ions modulated by transport in suspensions of red blood cells

The central requirement for the determination of transport rates by NMR is to be able to distinguish between the intra- and extracellular species. We present two methods for characterizing fast transmembrane transport that are based on the pulsed field gradient NMR technique. The first method relies on the transported solute having different diffusion coefficients on either side of the membrane and the subsequent modulation of the observed diffusion coefficients in the presence of transmembrane transport. The second method, the absorbing wall technique, relies on attenuation of the spin-echo signal from the observed species diffusing into the “absorbing wall.” The absorbing wall condition was established by addition of Mn 2+ to the extracellular fluid. For these experiments the transport of both bicarbonate and hypophosphite ions across the membrane of the human red blood cell was studied.

Experimental Measurements of the Diffusion Coefficient of 212Pb

Knowledge of the diffusion coefficient of Rn progeny is necessary for assessing the radiation exposure resulting from exposure to Rn and its progeny. The diffusion coefficient for 220Rn progeny was determined in ambient air by two independent methods, measuring deposition using a cylindrical tube or screens. A sampling train consisting of a diffusion tube and a screen-type diffusion battery was used for the experimental study. A range of flow rates and relative humidities was investigated. For 35% less than or equal to RH less than or equal to 85%, results from the two systems agree with each other. The diffusion coefficient of 212Pb was 0.036 +/- 0.002 cm2 s-1 and 0.037 +/- 0.004 cm2 s-1 for the tube and screen penetration methods, respectively. In low humidity air (RH less than 30%), a linear relationship between the diffusion coefficient of 212Pb and relative humidity was observed. The observed diffusion coefficient is strongly affected by the amount of material agglomerated onto the 212Pb atom. Further studies on the effects of trace gases and organics are required to fully understand the results.

Role of solvent dynamics on kinetics of electro-oxidation of ferrocene at platinum electrode

The one-electron oxidation of ferrocene to ferricenium ion was used as a model system to study the role of solvent dynamics on the kinetics of heterogeneous electrochemical electron-transfer reactions. Similar types of polar solvents (methanol to heptan-1-ol) with systematically changing dielectric properties were used. Diffusion coefficients were determined using the rotating-disc electrode method. Standard rate constants for the electrooxidation of ferrocene were obtained from cyclic voltammograms. The observed diffusion coefficients were found to follow the Stokes–Einstein theory. An inverse correlation was found between the experimental and theoretical values of the standard rate constants when the theoretical results were obtainded by considering the electron-transmission coefficient, ke1= 1, and the solvent dependence was taken into account only in the exponential factor. However, the experimental rate constants in different solvents agreed well with the rate constants obtained from the theoretical expression only when the latter was modified to take into account the solvent dependence [in terms of the dielectric and the dynamic (relaxation properties)] in the electron-transmission coefficient term at the interface as well as in the frequency factor.

Hydrogen Fluoride Diffusion Through a Polyimide Membrane

Hydrogen fluoride has been shown to diffuse through polyimide membranes from 1.30 to 5.00 μm thick. Transport occurs by diffusion of the associated species , or the ion pair , rather than by the ionic components H+ and F−. diffusion can be suppressed by introduction of fluoride salts, which presumably form the bulky bifluoride anion, a species too large to pass through polyimide membranes. Accelerated diffusion is observed at low pH. Observed diffusion coefficients for various concentrations of are .

Solute diffusion in dilute liquid-metal alloys

Abstract The theory of diffusion recently developed by the author for pure liquid metals in the framework of a pseudopotential formulation is extended to dilute liquid alloys. It is shown that when the theory predicts very short mean free paths of the solute atoms, the observed diffusion coefficients can be simply explained in terms of Stokes' law, as required by the fact that in the above case the atoms behave as if they were embedded in a classical continuum.

Determination of diffusion coefficient of titanium ion in TiO 2SiO 2 wet gel prepared from metal alkoxides during leaching

It was found that the radial gradient refractive index ( r-GRIN) glass rod could be produced by the leaching of wet gel prepared by a sol-gel method from metal alkoxides and heat-treatment of the gel. The concentration gradient of the doped component resulting from its diffusion in the wet gel was studied. In order to estimate the exact diffusion coefficient of the titanium ion in the binary TiO 2SiO 2 wet gel, its value was measured by using two methods. The observed diffusion coefficient by colorimetric analysis was about 1.1 × 10 −6 cm 2/s, while the value obtained from analysis of EPMA was about 1.2 × 10 −6 cm 2/s, and both values were nearly equal to each other. The activation energy of titanium ion diffusion in the wet gel was estimated to be about 11 kcal/mol.

Diffusive transport through a medium of static traps

The diffusion coefficient and equilibrium concentrations of particles in solution and in traps are considered for hopping in and out of traps. The spherical traps are static, randomly distributed, allowed to overlap and of arbitrary size and concentration. The author's recent calculation of the trapping rate is used to relate the observed fraction of time spent in solution to the microscopic dwell time within a trap. The observed diffusion coefficient D net depends on more than just the relative times spent in traps and in solution, contrary to simple theory. If particle is stuck at the surface until it escapes, D net is reduced because of correlation; if it is free to wander within the trap before escaping, D net is substantially enhanced. Simulation data are presented in support of the semi-quantitative physical arguments.

The use of the fluorescence photobleaching recovery technique to study the self-assembly of tubulin

Fluorescently labeled microtubule-associated proteins or poly- l-lysine (13,000 MW) were prepared by reaction with fluorescein isothiocyanate. The labeled compounds were used as probes of the assembly of calf brain tubulin using fluorescence photobleaching recovery techniques which allow measurement of the diffusion coefficient and percentage mobility of the fluorescent probe. When unfractionated tubulin (defined as material containing tubulin and microtubule-associated proteins) was polymerized at room temperature or 37°C, either probe could be incorporated into microtubules, since the observed diffusion coefficient (∼1.7 × 10 −8 cm 2/s) was much slower than that for either probe free in solution. The microtubules formed in the presence of labeled microtubule-associated proteins were free to diffuse while those formed in the presence of labeled polylysine were partially immobilized. Thus the fluorescence photobleaching recovery technique can be used to measure crosslinking of microtubules as well as assembly or interactions with other structures. When unfractionated tubulin was incubated with labeled polylysine in the presence of Ca 2+ at room temperature, the observed diffusion coefficient (∼5.1 × 10 −8 cm 2/s) probably represents the formation of rings of tubulin. The effect of mild and vigorous shearing, of cholchicine, and of different Mg 2+ concentrations on the properties of the system were examined.

Nuclear-magnetic-resonance study of crystalline tellurium and selenium.

Nuclear-magnetic-resonance measurements of $^{125}\mathrm{Te}$ and $^{77}\mathrm{Se}$ have been carried out in single crystals of tellurium and selenium, respectively, between room temperature and the melting point ${T}_{m}$. In particular, both the time evolution of the nuclear magnetization in the laboratory frame as well as in the rotating frame, following both broadband and site-selective excitations, have been investigated in detail. In tellurium, the Zeeman spin-lattice relaxation is determined by three different mechanisms: (1) Below 300 K, spin-lattice relaxation is governed by a two-phonon (Raman) process; (2) in the temperature range between about 300 and 450 K, the influence of conduction electrons overcoming the gap energy of 0.30 eV becomes important; (3) above 450 K, the spin-lattice relaxation is due to mobile charged vacancies. From the high-temperature data, the formation energy of a charged vacancy was found to be 0.66 eV. In selenium, on the other hand, the Zeeman spin-lattice relaxation time is essentially caused by a two-phonon (Raman) process over the entire temperature range. In either system, the phonon-induced spin-lattice relaxation shows the same dependence on the crystal orientation relative to the direction of the external field. For T\ensuremath{\ge}0.74${T}_{m}$ in tellurium as well as in selenium, a diffusional contribution to the rotating-frame relaxation rate is observed, arising from fluctuations in the nuclear dipole and chemical shift interaction due to atomic self-diffusion. However, from these data alone the correlation time of atomic motion could be determined only with a relative large error. In contrast, from site-selective excitation experiments the correlation time was extracted directly with a high degree of accuracy without any assumption regarding the nature of the nuclear spin interactions. The observed correlation times are discussed in terms of Mehrer's model for monovacancy diffusion via nearest- and next-nearest-neighbor jumps. Diffusivities in tellurium deduced from this model are in good agreement with the tracer data obtained recently by Mehrer et al. In contrast, the observed diffusion coefficients in selenium are about a factor of 20 smaller than the tracer diffusion coefficients measured some years ago by Br\"atter and Gobrecht.

Actin polymerization. The effect of brevin on filament size and rate of polymerization.

Fluorescent probes covalently bound to actin or to the actin binding protein, brevin, have been utilized to provide information about actin filaments formed in the presence of brevin as well as about the effect of brevin on the rate of polymerization. At actin to brevin ratios of 10:1 to 100:1, the observed diffusion coefficients of filaments, as measured by fluorescence photobleaching recovery using rhodamine-labeled actin or fluorescein-labeled brevin, are similar to those calculated from theoretical considerations for rigid rods. At lower brevin concentrations, the observed diffusion coefficients for actin filaments are lower than predicted, indicating that the filament structure is closer to that observed in the absence of brevin where filaments are immobilized due to interactions between them. The fluorescein-labeled brevin was found to be about as effective in influencing actin polymerization as unlabeled brevin. Using pyrene-labeled actin, we show that brevin binds 2 mol of monomeric actin. We conclude that at sufficiently high brevin concentration there is one brevin molecule per actin filament. From measurements of the initial rate of polymerization at 5.9 microM actin in the presence of brevin, we calculate both the apparent elongation rate constant and dissociation rate constant from one end (presumably the slow-growing end) of the actin filament. The former is highly dependent on Mg2+ concentration while the latter is not.

Diffusion in aqueous solutions of sulfuric acid

Diffusion of sulfuric acid in aqueous solution has been measured by a conductimetric technique at 25 °C over the concentration range from 0.00035 to 0.2 mol dm−3. At the lower end of this range, dissociation of bisulfate to sulfate and hydrogen ions produces a sharp increase in the rate of diffusion of the sulfuric acid component. Expressions are developed to predict the concentration dependence of the diffusion coefficient in terms of the mobilities of the diffusing ions and the degree of bisulfate dissociation. The treatment includes electrophoretic corrections that are derived from Onsager–Fuoss theory for the unsymmetrical, multi-ion electrolyte. At low concentrations where theory applies the observed diffusion coefficients are lower than the predicted values.

DIFFUSION OF DYES IN CLOTH ROLL AND IN AQUEOUS SOLUTION

In order to study the dyeing behavior of textile assemblies, the diffusion coefficients of dyes in cotton cloth roll, namely a piece of cotton cloth wrapping around a glass tube and in aqueous solution were determined.Two acid dyes were used and the dye bath was not stirred.The results obtained are as follows:(a) The concentration distribution in cotton cloth roll was in good agreement with Fick's law curve.(b) The activation energies of diffusion were about 5 kcal/mol both in cotton cloth roll and in water.(c) Each diffusion coefficient in cotton cloth roll and in aqueous solution decreased with an increase in NaCl concentration in the dye bath.Then, the observed diffusion coefficients could be explained reasonably by assuming the pore model which has been accepted for diffusion of ionic dyes into cellulosic substrate: the calculated values agreed well with the experimental values.

Nuclear magnetic resonance study of self-diffusion in solid selenium

The self-diffusion in crystalline selenium is investigated by means of nuclear spin relaxation under selective spin excitation between 350 K and the melting point (492 K). Diffusion coefficients down to 10 −19 cm 2 sec −1 have been observed. The diffusion process is attributed to a monovacancy mechanism, but the temperature dependence of the measured diffusion coefficients indicates also a small additional contribution to the monovacancy process near the melting point ( T > 460 K). For temperatures below 400 K, pipe diffusion along dislocations occurs. Generally, the observed diffusion coefficients are about a factor of 20 smaller than the tracer diffusion coefficients as measured by Brätter and Gobrecht.

Diffusivity of tritium in Li-Al alloys

The diffusion coefficients of tritium in Li-Al alloys were measured. The observed diffusion coefficients were expressed by the following equations: D = 8.5 × 10 −5 exp[ −43.9± 14.8(kJ/mol)/RT] (cm 2/s ), D = 9.3 × 10 −5 exp[ −48.5± 15.0(kJ/mol)/RT] (cm 2/s ), and D = 5.3 × 10 −5 exp[ −62.7± 13.8(kJ/mol)/RT] (cm 2/s ) for 0.02, 0.26 and 1.12% Li-Al alloys, respectively. Results showed that the diffusion coefficients as well as the apparent activation energy for diffusion depend upon the lithium concentration in the alloy.

A quasi-elastic laser light scattering study of tubulin and microtubule protein from bovine brain

Quasi-elastic light scattering has been used to characterize the oligomeric properties of solutions of glycerol-cycled bovine microtubule protein, and the properties of the 30 S oligomeric species and 6 S tubulin heterodimer prepared by gel filtration on Sepharose 6B. It is shown that in dimer preparations, as little as 0.04% by number of 30 S rings would account for the difference between an observed mean diffusion coefficient D 20, W = 3.1 × 10 −7 cm 2 s −1 and the value of D 20, W = 5.1 × 10 −7 cm 2 s −1 calculated for tubulin dimer of M rel 100,000. The 30 S ring has an observed diffusion coefficient of D 20, W = 0.49 × 10 −7 cm 2 s −1. These values are not changed significantly by the presence of 4 m-glycerol, indicating the persistence of 6 S and 30 S forms for dimer and ring, respectively. Mixtures of ring and dimer components of this preparation behave as a non-interacting two-component system, indicating the absence of substantial re-equilibration between the species at 5 °C and pH 6.5. The effect of salt on ring and microtubule protein samples indicates partial dissociation, consistent with the formation of additional intermediate oligomeric forms. In quasi-elastic light scattering measurements adapted to kinetic studies, changes in the oligomeric composition of microtubule protein are detected in the early stages of the reversible assembly process at pH 6.5. A 25% decrease in scattered light intensity, without significant change in mean diffusion coefficient, indicates the lability of the ring oligomeric structures, which undergo partial transformation to alternative oligomeric species under these assembly conditions.

Mass-Transfer Characteristics within Microbial Systems

The diffusion coefficient within a microbial system was determined by both diffusion and kinetic studies on the basis of Fick's model and an effectiveness factor model, in order to predict the mass-transfer characteristics of this system. It was found to be dependent on the molar weight and C/N ratio of the compounds and substrates used in the diffusion study. On the otherhand, the transitional film thickness from a reaction limited case to a diffusion limited one was found to be about 150 µ in a kinetic study. The observed diffusion coefficient in the diffusion study was of the same order as that of the kinetic study.

Chemical and Physical Treatment Effects on Solid‐Liquid Extraction of Apple Tissue

ABSTRACTVarious experimental treatments have been selectively applied to apple tissue in an effort to determine whether a solid‐liquid extraction process could be utilized for manufacturing single strength beverage juice. Vacuum impregnation of enzymatic and chemical solutions showed a positive effect on accelerating the rate of diffusion in the tissues studied. The greatest overall effect on increasing the rate of diffusion was caused by a pretreatment with radio‐frequency energy. Observed diffusion coefficients were 4.11 × 10‐6 cm2/sec with this treatment, as compared to 1.20 × 10‐6 cm2/sec for the control group of samples. Pretreatment techniques are suggested to enhance the rate of diffusion in apple tissue that may be of commercial value.

Effects of multiple membranes on measurements of cell surface dynamics by fluorescence photobleaching.

Fluorescence photobleaching recovery curves on a pair of membranes at various separations were calculated from a detailed knowledge of the variation in relative illumination areas and intensities as well as in relative contributions to the collected intensity with membrane separation. The observed diffusion coefficients were found to be relatively insensitive to membrane separation in all cases. Only for membranes with very different mobilities are there significant differences in fractional recoveries. Systematic variations in fractional recoveries with cell thickness may be indicative of differential mobility in the apical and basal membranes.

Radiation induced oxidative degradation of polymers—I

Abstract The γ-ray induced oxidation of polyethylene and ethylene-propylene copolymer films has been studied to obtain technological information on reducing the period in the radiation resistance testing of polymer materials. The polymers were irradiated under pressurized oxygen atmosphere (0.21∼10 atm) at high dose rate (0.5∼1 Mrad/h) in order to accelerate the oxidative degradation. The depth of oxidation region in the film was estimated by the gel fraction measurement. The depth was well agreed with the oxygen penetration region, which was calculated using the observed diffusion coefficient and solubility constant of oxygen in the film and specific rate of the oxygen consumption during irradiation. It was found that the depth of oxidation region was proportional to the square root of [oxygen pressure/does rate] and that the testing period could be reduced by higher dose rate irradiation at higher oxygen pressure.

Isothermal binary and ternary diffusion in the system (C2H5)4 NBr–KBr–H2O

Isothermal free diffusion in the binary systems KBr–H2O, (C2H5)4NBr–H2O and in the ternary system (C2H5)4NBr–KBr–H2O has been studied at various compositions with a precision single-lens Gouy interferometer. For the binary systems, the observed diffusion coefficients have been compared with those of similar compounds and, in the case of (C2H5)4NBr, have been compared with theoretical values predicted from activity coefficient behaviour for which a parameter-fitting least-squares approach has been used to determine the activity coefficient variation with concentration. Ternary coefficients have been analysed qualitatively in terms of ionic attractions in the solution and have been compared with the binary coefficients at similar concentrations. The Onsager Reciprocal Relation has been verified for one composition of the ternary system, but the calculations differ from previous tests in that the more versatile Scatchard approach to multicomponent solutions has been used rather than Harned's rule.