Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. II. Diffusion of Ar, Kr, Xe, and CH4 through methanol

We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their T(g). The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas are used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for temperature programmed desorption experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH(4) as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.     

Formation of supercooled liquid solutions from nanoscale amorphous solid films of methanol and ethanol

Molecular beam techniques are used to create layered nanoscale composite films of amorphous methanol and ethanol at 20 K. The films are then heated, and temperature programed desorption and infrared spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that the initially unmixed amorphous layers completely intermix to form a deeply supercooled liquid solution after heating above T(g). Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. The desorption rates from the supercooled and crystalline phases are then used to derive the binary solid-liquid phase diagram. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram when thermodynamically favored crystallization occurs. In those cases, the finite lifetime of the metastable solutions results in the precipitation of crystalline solids. Finally, in very thick films at temperatures and compositions where a stable liquid should exist, we unexpectedly observe deviations from ideal solution behavior. Visual inspection of the sample indicates that these apparent departures from ideality arise from dewetting of the liquid film from the substrate. We conclude that compositionally tailored nanoscale amorphous films provide a useful means for preparing and examining deeply supercooled solutions in metastable regions of the phase diagram.     

Probing the mobility of supercooled liquid 3-methylpentane at temperatures near the glass transition using rare gas permeation

We study the diffusivity of three-methylpentane (3MP) using the permeation of inert gases (Ar, Kr, Xe) through the supercooled liquid created when initially amorphous overlayers are heated above T(g). We find that the permeation rates for all of the gases have non-Arrhenius temperature dependences that are well described by the Vogel-Fulcher-Tamman equation. Comparison with the literature viscosity shows that the Stokes-Einstein equation breaks down at temperatures approaching T(g). The fractional Stokes-Einstein equation, D ∝ (T∕η)(n), does fit the permeation data, albeit with different values of n for each gas. There is qualitative agreement with the Stokes-Einstein equation in that the permeation rate decreases with increasing radius of the rare gas probe, but the small differences in radii significantly underestimate the observed differences in the permeation rates. Instead the permeation rates are better correlated with the rare gas-3MP interaction energy than with the atomic radius.     

Comparative study of temperature dependent orientational relaxation in a model thermotropic liquid crystal and in a model supercooled liquid

Recent optical Kerr effect experiments have revealed a power law decay of the measured signal with a temperature independent exponent at short-to-intermediate times for a number of liquid crystals in the isotropic phase near the isotropic-nematic transition and supercooled molecular liquids above the mode coupling theory critical temperature. In this work, the authors investigate the temperature dependence of short-to-intermediate time orientational relaxation in a model thermotropic liquid crystal across the isotropic-nematic transition and in a binary mixture across the supercooled liquid regime in molecular dynamics simulations. The measure of the experimentally observable optical Kerr effect signal is found to follow a power law decay at short-to-intermediate times for both systems in agreement with recent experiments. In addition, the temperature dependence of the power law exponent is found to be rather weak. As the model liquid crystalline system settles into the nematic phase upon cooling, the decay of the single-particle second-rank orientational time correlation function exhibits a pattern that is similar to what has been observed for supercooled liquids.     

Analysis of the visible absorption spectrum of I2 in inert solvents using a physical model

Absorption spectra of I(2) dissolved in n-heptane and CCl(4) are analyzed with a quantum gas-phase model, in which spectra at four temperatures between 15° and 50 °C are least-squares fitted by bound-free spectral simulations to obtain estimates of the excited-state potential energy curves and transition moment functions for the three component bands--A ← X, B ← X, and C ← X. Compared with a phenomenological band-fitting model used previously on these spectra, the physical model (1) is better statistically, and (2) yields component bands with less variability. The results support the earlier tentative conclusion that most of the ~20% gain in intensity in solution is attributable to the C ← X transition. The T-dependent changes in the spectrum are accounted for by potential energy shifts that are linear in T and negative (giving red shifts in the spectra) and about twice as large for CCl(4) as for heptane. The derived upper potentials resemble those in the gas phase, with one major exception: In the statistically best convergence mode, the A potential is much lower and steeper, with a strongly varying transition moment function. This observation leads to the realization that two markedly different potential curves can give nearly identical absorption spectra.     

Water sorption and permeation in chitosan films: Relation between gas permeability and relative humidity

The water sorption of chitosan has been studied at 20 °C. Water transport is governed by a Fickian process for relative humidities lower than 0.4, and in that range of partial pressures, the diffusion coefficient is concentration‐dependent. At a higher activity, anomalous diffusion is observed. The sorption isotherm is well described by the Guggenheim‐Anderson‐de Boer (GAB) model, and the clustering phenomenon observed at high relative pressures can be studied with the parameters of this model. The water permeability coefficient greatly increases with the relative pressure, and the water plasticization effect leads to a loss of the gas barrier properties under wet conditions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3114–3127, 2001     

Kinetic model of liquid B2O3 gasification in a hydrocarbon combustion environment: I. Heterogeneous surface reactions

As part of an ongoing program to model hydrocarbon assisted boron combustion, a kinetic model has been developed to describe gasification of the ubiquitous boron oxide coating that inhibits particulate boron ignition. This model includes homogeneous gas phase oxidation reactions, multi-component gas phase diffusion, heterogeneous surface reactions, and oxide vaporization. The kinetic processes are treated using a generalized kinetics code, with embeded sensitivity analysis, for the combustion of a one-dimensional (particle radius), spherical particle. This article presents the heterogeneous surface reactions used to describe oxide gasification and presents selected model results for a spherical boron oxide droplet which illustrate the dependence of the oxide gasification rates on the ambient temperature and particle diameter.     

Kinetic model of the decomposition of a PET fibre cloth in an inert and air environment

The kinetics of the global thermal decomposition of a PET fibre cloth have been studied in three different atmospheres with nitrogen, 20 and 10% oxygen, using thermogravimetric analysis (TGA) between room temperature and 1050 K. Three different heating rates were used at each atmosphere condition. Moreover, several isothermal experiments in nitrogen and 20% oxygen have been carried out. For the pyrolysis of an old PET fibre cloth, the kinetic model comprises three independent reactions. The combustion kinetic model considers that pyrolysis of two fractions of the initial material occurs regardless of the oxygen concentration and then a combustion of the forming char and the third fraction starts. One set of parameters can explain all the isothermal and dynamic experiments at the three different heating rates used.     

Quantitative analysis of trace bulk oxygen in silicon wafers using an inert gas fusion method.

This paper describes a method for removing oxide film from the surface of silicon wafers using an inert gas fusion impulse furnace and precise determination of bulk oxygen within the wafer. A silicon wafer was cut to about 0.35 g (6 x 13 x 2 mm) and dropped into a graphite crucible. The sample was then heated for 40 s at 1300 degrees C. The wafer's oxide film was reduced by carbon and removed as carbon monoxide. The treated silicon sample was taken out of the graphite crucible and maintained again with the holder of the oxygen analyzer. The graphite crucible was then heated to 2100 degrees C. The treated silicon sample was dropped into the heated graphite crucible and the trace bulk oxygen in the wafer was measured using the inert gas fusion infrared absorption method. The relative standard deviations of the oxygen in silicon wafer samples with the removed surface oxide film were determined to be 0.8% for 9.8 x 10(17) atoms/cm3, and 2.7% for 13.0 x 10(17) atoms/cm3.     

Surfactant in the gaseous phase I. Formation of foams and thin liquid films

A new possibility of obtaining foams and single thin liquid films by supplying a surfactant through the gaseous phase is presented. Argon saturated with vapors ofn-amyl orn-decyl alcohol was passed through the water phase and formation of the foam was measured as a function of time, gas flow rate, and volume of the water phase. Formation and properties of microscopic thin liquid films were measured as a function of the time of their contact with the surfactant vapors. Two possible mechanisms of foam formation by supplying surfactant via the gaseous phase are discussed. A theoretical model of surfactant dissolution is presented and its predictions are compared with the experimental results. It is shown that foam formation resulted mainly from dissolution of the surfactant vapors in the water phase.     

One-dimensional model for water and aqueous solutions. I. Pure liquid water

Two simplified one-dimensional models for waterlike particles are studied. One is referred to as the primitive model which is a simplified version of a model introduced by Ben-Naim in 1992 [Statistical Thermodynamics for Chemists and Biochemists (Plenum, New York, 1992)]. The second, referred to as the primitive cluster model, is a simplified version of the model used by Lovett and Ben-Naim in 1969 [J. Chem. Phys. 51, 3108 (1969)]. The two models are shown to be nearly equivalent and both exhibit some of the most characteristic behavior of liquid water. It is argued that a key feature of the molecular interactions--the correlation between the strong binding energy and low local density--is essential for the manifestation of the anomalous behavior of liquid water. It is also essential for the understanding of the outstanding behavior of liquid water.     

Molecular organization in two-dimensional films of liquid I. Langmuir films of binary mixtures of liquid crystals with a crystalline mixtures terminal cyano group

The Langmuir films of two liquid crystal materials, 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT), and of their mixtures have been studied by recording surface pressure-area isotherms and Brewster angle microscopy (BAM) images. The pure liquid crystals revealed very different characters of the surface pressure-area isotherms indicating different organization of the molecules and different molecular interactions in the monolayer at the water-air interface. The surface pressure-area isotherms of Langmuir films formed from 8CB/5CT mixtures give evidence for phase separation of the components over the whole range of molar fractions. Similar conclusions have been drawn on the basis of BAM image analysis.     

Molecular organization in two-dimensional films of liquid I. Langmuir films of binary mixtures of liquid crystals with a crystalline mixtures terminal cyano group

The Langmuir films of two liquid crystal materials, 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT), and of their mixtures have been studied by recording surface pressure-area isotherms and Brewster angle microscopy (BAM) images. The pure liquid crystals revealed very different characters of the surface pressure-area isotherms indicating different organization of the molecules and different molecular interactions in the monolayer at the water-air interface. The surface pressure-area isotherms of Langmuir films formed from 8CB/5CT mixtures give evidence for phase separation of the components over the whole range of molar fractions. Similar conclusions have been drawn on the basis of BAM image analysis.     

Microporous hollow fibers for gas absorption : I. Mass transfer in the liquid

Microporous hollow fiber modules offer a larger area per volume between gas and liquid than that commonly encountered in packed towers. This larger area can be sustained at very low flows, where packed towers will not be loaded, and at very high flows, where packed towers will flood. As a result, the modules offer the potential of faster mass transfer. This potential can be compromised by the resistance to mass transfer of the membrane itself, a resistance which is increased if the liquid wets the membrane. The results presented in this two-part series show when the advantage of the increased area is greater than the disadvantage of the membrane resistance. In this part, a theory for the operation of hollow fiber membrane modules is developed, and mass transfer coefficients in the liquid phase are investigated.     

Kinetic study of gas phase olefin polymerization with a TiCl4/MgCl2 catalyst. II. Kinetic parameter estimation and model building

Monomer transport and polymerization kinetics are two key phenomena in olefin polymerization with heterogeneous transition metal catalysts. To have a better understanding of these interrelated kinetics and diffusion phenomena, a quantitative calculation of the monomer diffusion directly from experimental study is essential. In this work, a novel temperature-perturbation technique is developed to systematically study the kinetic and diffusion limitations in catalyzed gas phase olefin polymerization. A physical model of the particle growth mechanism as well as its mathematical representation is presented and the diffusion limitations occurring in the system at high temperature are characterized and quantitatively analyzed. Finally, the practical implications of the results of this study on the operation of industrial scale polyolefin reactors are examined. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2075-2096, 1997     

Kinetic theory of radio frequency quadrupole ion traps. I. Trapping of atomic ions in a pure atomic gas

A kinetic theory based on the Boltzmann equation is developed for the trapping of atomic ions in a radio-frequency quadrupole ion trap containing enough neutral atoms that ion-neutral collisions cannot be ignored. The collisions are treated at the same level of sophistication and detail as is used to deal with the time- and space-dependent electric fields in the trap. As a result, microscopic definitions are obtained for the damping and stochastic forces that originate from such collisions. These definitions contrast with corresponding phenomenological terms added ad hoc in previous treatments to create damped Mathieu and Langevin equations, respectively. Furthermore, the theory indicates that either collisional cooling or heating of the ions is possible, depending upon details of the ion-neutral mass ratios and interaction potential. The kinetic theory is not dependent on any special assumptions about the electric field strengths, the ion-neutral interaction potentials, or the ion-neutral mass ratio. It also provides an ab initio way to describe the ion kinetic energies, temperatures, and other properties by a series of successive approximations.     

Kinetic and chemical studies of polymer cross-linking using thermal gravimetry and hyphenated methods. Degradation of polyvinylchloride

The chemistry and kinetics of polyvinylchloride (PVC) degradation have been studied using thermal gravimetry and hyphenated spectroscopic methods to identify condensed phase and vapour phase products. A method of kinetic modelling has been devised to estimate Arrhenius parameters for the sequential and competing processes which comprise the thermal decomposition, and to plot concentrations of decomposition intermediates which make up the TG curve. MS and GC-MS have been used to relate the composition of volatile materials to the TG curve and FTIR has related this to changes in the condensed phase. The chemical studies have highlighted a char forming mechanism based on hydrogen disproportionation which results in carbon enrichment and the formation of aliphatic hydrocarbons.     

Bioanalysis of the neuropeptide des-enkephalin-gamma-endorphin by high-performance liquid chromatography with on-line sample pretreatment using gel permeation and solid-phase isolation.

A bioanalytical method is described that allows the determination of a number of beta-endorphin-related peptides. The method is based on the application of fluorescence detection after high-performance liquid chromatography followed by post-column derivatization with o-phthaldialdehyde. Concentrations exceeding 10-25 ng/ml could be determined by using conventional fluorescence detection, whereas lower concentrations demand the use of laser-induced fluorescence detection. The sample pretreatment includes the use of on-line gel permeation, on-line solid-phase isolation and heart cutting of a peak from reversed-phase gradient elution. The sample pretreatment procedure does not discriminate between the dodecapeptide des-enkaphalin-gamma-endorphin (DE gamma E) and its metabolites in order to obtain similar recoveries for all components. The final chromatographic phase system is based on ion-pair formation, which permits the separation of DE gamma E from its metabolites and degradation products. The optimized procedure allows the determination of these peptides in plasma at concentration levels down to about 1 ng/ml, demanding a sample volume of 1 ml.