Oxygen diffusion in poly(dimethyl siloxane) using fluorescence quenching. I. Measurement technique and analysis

The transport properties of oxygen in poly(dimethyl siloxane) have been measured using the method of quenching of fluorescence. This paper discusses the uniqueness of this method and its use in measuring the diffusion coefficient of oxygen in unfilled PDMS. The results show (1) a large value for the diffusion coefficient of oxygen in pure PDMS at 25°C, D = 3.55 × 10^?5 cm²/s, (2) a low value of the acitivation energy, E_D = 4.77 kcal/mole, which was not temperature dependent in the ranges evaluated, and (3) a large value of the preexponential term, D₀ = 0.115 cm²/s. The diffusion coefficient was found to be independent of both the oxygen concentration and fluorophor concentration in the pressure and temperature ranges used in these experiments. The import of these experiments lies in their application to a unique biomedical oxygen sensor which is fast, sensitive, and does not consume oxygen.     

Surface heterogeneity and surface area from linear inverse gas chromatography. Application to glass fibers

A new method of measuring surface heterogeneity of non-porous adsorbents by inverse gas chromatography (IGC) is developed. In contrast with the methods described in the literature which are based on nonlinear chromatography (thermodynamics of adsorption) this method is based on linear chromatography (kinetics of adsorption). The mass balance equation of linear chromatography with axial diffusion term and the Langmuir kinetic equation on an open (non-porous) heterogeneous surface are solved by the method of Laplace transforms. The expressions for four semi-invariants of an elution profile are obtained. These are linear combinations of the moments of the distribution of sites in residence time of adsorbed molecule with coefficients determined from elution profile of non-adsorbing gas. Four semi-invariants of elution profiles of benzene and methane on a column packed with E-glass fiber were determined experimentally at temperatures around 100 degrees C. The mean, the standard deviation of the normal distribution in adsorption energy as well as the sticking coefficient and pre-exponential factor of the Frenkel equation are determined for benzene on E-glass. A method of the specific surface determination from linear IGC is proposed.     

Polymer structure and the compensation effect of the diffusion pre-exponential factor and activation energy of a permeating solute

In the present work, the relation between the pre-exponential factor and the apparent activation energy of diffusion, ln D(0) = alpha + betaE(D), so-called compensation effect, is re-examined and critically discussed for diffusion of gases in rubbery and glassy polymers. In principle, the above equation could be derived from the enthalpy-entropy compensation in the framework of the transition state theory. However, one should consider the influence of the jump length term contained in the pre-exponential factor, which may be affected by permeating species and polymer properties. We found that parameter alpha depends on penetrant size and polymer properties, such as local chain mobility and free volume. This can be interpreted by the fact that the jump length is affected by both penetrant and polymer properties. Finally, methods for estimating the jump length are discussed.     

氧气在聚丙烯内吸附和扩散的分子模拟

采用巨正则Monte Carlo和分子动力学模拟相结合的方法研究了氧气在不同聚合度的聚丙烯内的吸附和扩散. 模拟结果表明, 随聚丙烯聚合度的增加, 聚丙烯对氧气的吸附量逐渐增加, 而氧气在聚丙烯内的扩散系数减小; 当聚合度增大到一定程度时, 吸附量和扩散系数都趋于一稳定值. 随温度的升高, 氧气在聚丙烯内的吸附量减少, 而扩散系数增大. 本文还应用自由体积理论探讨了氧气在聚合物内扩散的机理, 发现氧气在聚丙烯内以空穴形式存在的自由体积之间扩散, 即氧气先在一个空穴内不停振动, 然后通过聚丙烯链段运动形成的通道跳跃到下一个空穴来完成扩散. 结果表明, 较高聚合度的聚合物材料在常温及低温下使用对于其在食品包装材料中的应用是有利的, 这为食品包装材料行业相关产品的应用开发提供了一定的指导和依据.     

Diffusion of coadsorbed CO and D on Ni(lOO)

Pulsed laser heating has been used to desorb CO(a) and D(a), separately adsorbed and coadsorbed, from a 325 μm radius spot on Ni(100). Refilling by diffusion is temperature dependent, is slower for D(a) than for CO(a), and is slower for both when coadsorbed. From temperature-dependent data, both activation energies and pre-exponential factors are calculated. The importance of pre-exponential factor changes is stressed.     

Quaternary diffusion coefficients of NaCl−MgCl2−Na2SO4 −H2O synthetic seawaters by least-squares analysis of taylor dispersion profiles

Equations are developed to calculate the nine inter diffusion coefficients of four-component solutions by least-squares analysis of the refractive index profiles across Taylor dispersion peaks. The pre-exponential weighting factors and the eigenvalues of the diffusion coefficient matrix are used as the fitting parameters instead of the diffusion coefficients. This simplifies the fitting equations and speeds up the convergence. The analysis is tested on dilute aqueous solutions of sucrose + glycine + urea, and then used to determine the interdiffusion coefficients of aqueous NaCl+MgCl₂+Na₂SO₄ solutions at compositions corresponding to synthetic seawaters of salinities 5, 15, 25 and 35. The results are used to predict the concentration profiles that develop when seawater interdiffuses with fresh water.     

Measurement of the eddy diffusion term in chromatographic columns. I. Application to the first generation of 4.6mm I.D. monolithic columns

The corrected heights equivalent to a theoretical plate (HETP) of three 4.6mm I.D. monolithic Onyx-C(18) columns (Onyx, Phenomenex, Torrance, CA) of different lengths (2.5, 5, and 10 cm) are reported for retained (toluene, naphthalene) and non-retained (uracil, caffeine) small molecules. The moments of the peak profiles were measured according to the accurate numerical integration method. Correction for the extra-column contributions was systematically applied. The peak parking method was used in order to measure the bulk diffusion coefficients of the sample molecules, their longitudinal diffusion terms, and the eddy diffusion term of the three monolithic columns. The experimental results demonstrate that the maximum efficiency was 60,000 plates/m for retained compounds. The column length has a large impact on the plate height of non-retained species. These observations were unambiguously explained by a large trans-column eddy diffusion term in the van Deemter HETP equation. This large trans-rod eddy diffusion term is due to the combination of a large trans-rod velocity bias (?3%), a small radial dispersion coefficient in silica monolithic columns, and a poorly designed distribution and collection of the sample streamlets at the inlet and outlet of the monolithic rod. Improving the performance of large I.D. monolithic columns will require (1) a detailed knowledge of the actual flow distribution across and along these monolithic rod and (2) the design of appropriate inlet and outlet distributors designed to minimize the nefarious impact of the radial flow heterogeneity on band broadening.     

Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.610(-10) to 1.810(-9) m(2)/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.     

Asymmetric membrane formation via immersion precipitation method. I. Kinetic effect

The kinetic effect of the phase inversion process on the membrane morphology is explored, with emphasis on the diffusion coefficient of the nonsolvent as a measure of the solvent/nonsolvent exchange rate. The diffusion coefficient is closely related to the nonsolvent tolerance of the polymer solution, which was estimated from a pseudo-ternary phase diagram of the following system: polymer: polysulfone; solvent system: a mixture of the solvent 1-methyl-2-pyrrolidinone and a solvent additive (formic acid, water or ethanol); and nonsolvent: ethanol. Regardless of the kind of solvent additive employed, when the diffusion coefficient of the nonsolvent is high for a given gelation medium, then the membrane consists of a smooth, defect-free surface and macrovoid-free cross section, and is highly permeable to oxygen. However, using a polymer solution with a low diffusion coefficient results in a membrane of a rather defective morphology. Therefore, it is concluded that the diffusion coefficient of the nonsolvent is a crucial parameter in controlling membrane morphology.     

Solubility of oxygen in glucose solutions

A knowledge of the solubility of oxygen in glucose-containing solutions is essential for the determination of the kinetics of the glucose oxidase-catalysed glucose oxidation. The enzyme glucose oxidase was used in a new glucose sensor. Combination of data for the dynamic viscosity and density from the literature and data from measurements with a rotating disc electrode (RDE) for hydrogen peroxide and hydroquinone showed that the factor ηD (η = dynamic viscosity; D = diffusion coefficient) remains constant in solutions with a glucose concentration ranging from 0 to 1 M. Assuming that this is also valid for oxygen, the diffusion coefficient of oxygen in glucose solutions was calculated and the solubility of oxygen was determined with RDE measurements. At both 25 and 37°C the relationship between the solubility of oxygen and the glucose concentration is a second-degree polynomial.     

氧气在碱溶液中结构与扩散系数的分子动力学模拟

采用分子动力学方法对不同温度(25-120 ℃)及碱浓度(1:100-1:5, 摩尔比)下NaOH和KOH溶液中的氧气进行了模拟. 本文考察了NaOH及KOH溶液中溶剂-溶剂、氧气-溶剂及氧气-溶质的径向分布函数, 并采用爱因斯坦方程从均方位移曲线中计算得到了氧气及溶质离子的扩散系数. 结果显示随着碱浓度的升高, 氧气扩散系数逐渐减少; 在相同条件下, 氧气在NaOH溶液中扩散系数小于在KOH溶液中的扩散系数; 溶质离子扩散系数的变化规律与氧气一致. 通过与现有实验结果对比, 发现了分子动力学方法的可靠性及用于研究实验受限领域的优越性.     

Electrical properties of niobium-doped titanium dioxide. 2. Equilibration kinetics

The present work reports isothermal gas/solid equilibration kinetics for Nb-doped TiO(2) (0.65 atom %) at elevated temperatures (1073-1298 K) within narrow ranges of oxygen activity spanning between 10(-13) Pa and 75 kPa. The equilibration kinetics were monitored using electrical conductivity measurements. The kinetic data were used to determine the chemical diffusion coefficient (D(chem)). D(chem) as a function of p(O(2)) exhibits a complex dependence, which is considered in terms of defect disorder and the related concentrations of electronic charge carriers. The activation energy of D(chem) in the p(O(2)) range 10 Pa < p(O(2)) < 75 kPa varies in the range 88.0-98.2 kJ/mol. It is important to note that the chemical diffusion coefficient in strongly reduced conditions [p(O(2)) = 10(-9) Pa] exhibits a negative temperature dependence of D(chem) (-67.2 kJ/mol). This finding indicates that under these conditions transport in a chemical potential gradient is consistent with metallic charge transport.     

Processes of luminescence quenching in study of molecular oxygen diffusion in glassy matrices

The methods have been developed for studying oxygen mobility by donor quenching in solutions with uniform concentration and in geminate pairs in a solid phase. It is shown that the kinetics of luminescence variations with time is described by a diffusion model. Oxygen mobility has been studied at low temperatures (77 K) in hard polymer matrices. squalane. glasses of low-molecular hydrocarbons and alcohols. The methods can be used to measure very low oxygen mobility with a diffusion coefficient of about 10(-23) cm2 s(-1).     

Electrochemically induced oxygen spillover and diffusion on Pt(111): PEEM imaging and kinetic modelling

Electrochemically induced oxygen spillover and diffusion in the Pt(O(2))|YSZ system is investigated in a combined experimental and theoretical study. The spreading of spillover oxygen is imaged by photoelectron emission microscopy (PEEM) on dense and epitaxial Pt(111) thin film electrodes prepared by pulsed laser deposition (PLD). Two different models are used to obtain surface diffusion coefficients from the experimental data, (i) an analytical solution of Fick's 2nd law of diffusion, and (ii) a numerical reaction-diffusion model that includes recombinative desorption of O(2) into the gas phase. The resulting diffusion coefficient has an activation energy of 50 kJ mol(-1) and a preexponential factor of 0.129 cm(2) s(-1) with an estimated uncertainty of ±20% for the activation energy and ±50% for the absolute value. The Fickian model slightly overpredicts diffusion coefficients due to the neglect of oxygen desorption. Experimental and theoretical results and limitations are discussed and compared to previous work.     

A comparison of diffusion coefficients for ternary mixed micelle solutions measured by macroscopic gradient and dynamic light scattering techniques

Taylor dispersion is used to measure ternary mutual diffusion coefficients (D(ik)) for aqueous solutions of decylsulfobetaine (SB10) (1) + dodecylsulfobetaine (SB12) (2), SB10 (1) + SB14 (2), and SB12 (1) + SB14 (2) mixed zwitterionic micelles. Cross-coefficient D(21) for the coupled flow of surfactant 1 produced by a concentration gradient in surfactant 2 is relatively small for these solutions, but D(12) reaches values as large as the main D(ii) coefficients. The results are interpreted by using the equation D(ik) = partial differential(C(i)D(i))/ partial differentialC(k) to relate the ternary mutual diffusion coefficients to the concentration-weighted average diffusion coefficients D(i) of the micellar and free-monomer forms of the surfactants. The macroscopic-gradient Taylor measurements are compared with diffusion coefficients measured by dynamic light scattering (DLS), which monitors microscopic concentration fluctuations. At most compositions, the intensity autocorrelation function G(tau) is a single exponential decay in D((2)), the smaller eigenvalue of the mutual diffusion coefficient matrix. A contribution from D((1)) is identified at high solute fractions of surfactant 1. The DLS results are consistent with contributions to G(tau) from uncoupled fluctuations in the concentrations of eigencomponents defined as the linear combinations of surfactants 1 and 2 that diagonalize the D(ik) matrix. A procedure for the rapid and convenient DLS measurement of ternary mutual diffusion coefficients, including the cross-coefficients for coupled diffusion, is suggested, using the Onsager reciprocal relation together with the eigenvalues and pre-exponential factors from G(tau).     

Cation self-diffusion in LaCoO(3) and La(2)CoO(4) studied by diffusion couple experiments

Reaction kinetics between dense, polycrystalline pellets of La2O3 and CoO were investigated at temperatures of 1370-1673 K and oxygen partial pressures of 40 Pa - 50 kPa. At high oxygen partial pressures, single phase LaCoO3 was formed. The growth of the LaCoO3 phase followed the parabolic rate law. The location of Pt markers demonstrated that diffusion of Co3+ cations in LaCoO3 dominated over diffusion of La3+. The diffusion coefficient of Co3+ was determined from the parabolic rate constant, and an activation energy of (250 +/- 10) kJ mol-1 was found. The diffusion coefficient of Co3+ in LaCoO3 decreased with decreasing oxygen partial pressure. At the lowest oxygen partial pressure investigated, two product phases, LaCoO3 and La2CoO4, were observed. The diffusion coefficient of Co cations in La2CoO4 was estimated. Results were discussed in relation to cation diffusion in other LnBO3 oxides (B = Cr3+, Mn3+, Fe3+). A correlation between diffusion of the B cation and the melting point was found for LnBO3 materials.     

Temperature dependence of the viscosity of dilute poly(β-naphthyl methacrylate) solutions

The temperature dependence of the actual viscosity of dilute poly(β-naphthyl methacrylate) solutions was described by an Arrhenius expression according to Moore's treatment. The apparent activation energy of flow was found to be lower for solutions in benzene, toluene and tetralin than for solvent. In dioxane, a good solvent, the activation energy was nearly constant and close to that for the solvent. The pre-exponential terms were, in all cases, higher for solutions than for the solvents. The decrease in activation energy and increase in pre-exponential term are largely dependent on solvent power: the maximum effect is found in the poorest solvent. This behaviour is discussed in connection with the temperature dependence of the Mark-Houwink and of the Fox-Flory constants obtained from intrinsic viscosity data: the differences in the activation energy of flow and in the pre-exponential term between the solvent and the solutions are related to variation of coil expansion with temperature.     

Simulation and modelling of transient permeation of organic solvents through polymer films in the case of a concentration dependent diffusion coefficient

The measured transient permeation kinetics of acetic acid from a water-acetic acid mixture through poly(vinyl alcohol) films could not be accounted for by the Fick law with a constant diffusion coefficient. A new calculation procedure was developed on the basis of simulation results of the Fickian kinetics in which the diffusion coefficient was assumed to increase exponentially with the local permeant concentration. A fast and reliable fitting procedure, which was set up on a Personal Computer, involves an iterative numerical adjustment of (a) the value of the limiting diffusion coefficient D^* (i.e., difusion of the permeant in dry polymer) using the onset part of the permeation rate followed by (b) the value at the upstream face of the plasticization term (argument of the exponential function) using the shape of the experimental curve. The values obtained from fitting the model to the transient kinetics showed that the limiting diffusion coefficient increases drastically, but the plasticization term changes little, with increasing temperature.     

聚苯硫醚及其碳纤维复合材料结晶动力学的研究

用膨胀计法研究了聚苯硫醚及其碳纤维复合材料的结晶动力学行为。发现其碳纤维复合材料对Avrami方程有较明显的偏离。聚苯硫醚具有中等的结晶速率,与PE、PMO等高聚物比较,它的σσ_c值、堆砌系数K_c以及结晶动力学方程中的前置指数G_0值较低。G_0是决定高聚物结晶速率的主要因素,ρ_a/ρ_c(值较小的高聚物有可能出现较高的结晶速率。     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium manganites

Porous _963-1 (x = 0, 0.4, 0.6) materials are synthesized using a ceramic technique. The method of isotopic oxygen exchange with gas-phase analysis was used to study the kinetics of oxygen exchange in La_1-x Sr _x MnO_3±δ at 600–850°C and at oxygen pressure of 0.13–9.33 kPa. The values of oxygen interphase exchange rate, diffusion coefficient, and contributions of three exchange types are calculated. It is found that the rate of oxygen interphase exchange rate and diffusion coefficient grow at an increase in the concentration of strontium. The values of effective activation energies of oxygen exchange and diffusion decrease at an increase in the strontium content. Oxygen diffusion and its exchange mechanism are considered as related to the defect structure of oxides in the La_1-x Sr _x MnO_3±δ system.