Douglas Barlow - Academia.edu (original) (raw)

Papers by Douglas Barlow

Recently, molecular dynamics simulations have predicted that concentric layers of gaseous carbon ... more Recently, molecular dynamics simulations have predicted that concentric layers of gaseous carbon dioxide particles will appear in carbon nanotubes. We show in this letter how this effect can be predicted analytically by considering the potential field generated by the pore wall. The layer potential expression thus derived can be used to reproduce the essential features of a particular MD study of gaseous carbon dioxide within a (40, 40) carbon nanotube and confirm, from an energetic point of view, that an outer gaseous layer will be stable. With a closed form expression for the layer potential known, we are able to derive formulas for quantities typically of interest in a Lennard-Jones analysis, such as minimum energy, equilibrium position and the location of zero potential.

International Journal of Chemical Kinetics

Journal of Crystal Growth, 2021

The carbon disulfide-methanol liquid-liquid critical point is studied using a MonteCarlo simulati... more The carbon disulfide-methanol liquid-liquid critical point is studied using a MonteCarlo simulation of classical Stockmayer particles. A low energy configuration for the segregatedtwo component system is determined using standard Monte Carlo methods then a modifiedGibbs ensemble is employed to study the effect of transferring particles from one phase toanother. Rather than use the model for the entropy of mixing in the Gibbs ensemble, which isof the regular solution type, a semi-quasi-chemical model is used which involves an interactionenergy. We are able to simulate the mixing of the two components as the temperature approachesthe critical temperature from below. Further, a method is given whereby the simulation resultscan be used to predict the critical temperature.

International Journal of Chemical Kinetics, 2021

Journal of Applied Physics, 2020

Using a recently reported method for the statistical representation of gaseous diffusion within a... more Using a recently reported method for the statistical representation of gaseous diffusion within a cylindrical pore, we report here on an analysis of situations that describe fast diffusion within carbon nanotubes. It is proposed that if gaseous flow properties of the tube, in the highly rarefied situation, are due to there being only specular particle–wall reflections, then these particles can transit the tube via self-diffusion. On comparing this self-diffusive flux with Knudsen transport diffusion, our model predicts that enhanced diffusion is indeed possible in the carbon nanotube. Depending upon the statistical nature of the particle–wall scattering phenomenon, the enhancements are predicted be three to four times that of classical transport diffusion and, for certain conditions, the enhancement factor can be greater than 4.

We propose here that the intermediate nucleation phase identified in a certain caseof protein cry... more We propose here that the intermediate nucleation phase identified in a certain caseof protein crystal growth actually consists of two distinct parts; a low density andhigher density phase. A theory for crystal growth is utilized to study the formationand growth of each phase. Within the framework of this theory the low density phaseis shown to obey a forth order kinetic law while the high density phase is zeroth order.The combination of these two phases is shown to be a good match for X-ray diffraction data which is indicative of its presence. The crystal growth rate is then givenin terms of the kinetic behavior of the intermediate nucleation phase. From this, thecrystal radius is estimated and shown to compare favorably with reported size data.A method is proposed for determining the conditions that lead to protein crystals oflargest possible size.

Aps Texas Sections Fall Meeting Abstracts, Oct 1, 2003

Physical Review E, 2011

The mixture of isobutyric acid and water has a consolute point at a temperature of 25.75 °C and m... more The mixture of isobutyric acid and water has a consolute point at a temperature of 25.75 °C and mole fraction 0.1148 isobutyric acid. When charcoal is added to this mixture, the concentration of isobutyric acid is reduced by adsorption. We have measured the action of charcoal on solutions of isobutyric acid and water as a function of isobutyric acid mole fraction at temperatures of 25.85 and 32.50 °C. At the higher temperature, the specific adsorption density (y(2)(α)/m) satisfies the Freundlich equation (y(2)(α)/m)=KX(2)(1/n), where y(2)(α) is the mass of isobutyric acid adsorbed, m is the mass of charcoal, X(2) is the equilibrium mole fraction of isobutyric acid, n is the Freundlich index, and K=K(T) is an amplitude that depends upon the temperature T. At 25.85 °C, a critical endpoint is located at an isobutyric acid mole fraction X(2)(ce)=0.09. When compared with the Freundlich equation at this temperature, a plot of the specific adsorption density as a function of X(2) in the vicinity of the critical-endpoint composition assumes a shape which is reminiscent of the derivative of a Dirac delta function. Using critical-point scaling theory, we show that this divergent pattern is consistent with the principle of critical point universality.

Molecular Physics, 2003

The molecular architecture of proteins can be determined by analysing the X-ray diffraction patte... more The molecular architecture of proteins can be determined by analysing the X-ray diffraction patterns of their crystals. The technology of X-ray crystallography has reached the point, however, where the determination of the structure of a given crystal is controlled by the limited availability of the crystals themselves. Proteins can often be crystallized from pH buffered aqueous solutions of strong electrolytes.

Molecular Physics, 2008

Results of a density functional study for the molecules Bi2Te3 and Bi4Te6 are reported here. For ... more Results of a density functional study for the molecules Bi2Te3 and Bi4Te6 are reported here. For Bi2Te3, calculations yield eight stable conformations. For Bi4Te6, eight stable isomers are identified. Equilibrium geometries, adiabatic ionisation potentials, atomisation energies, and vibrational bands are estimated. The lowest energy conformations in both cases are clusters of Cs symmetry with all Te atoms two-fold coordinated and all Bi atoms three-fold coordinated. The predicted low energy conformation for Bi4Te6 has alternating rows of Bi and Te atoms. This molecule seems a reasonable precursor to solid bismuth telluride, which has alternating Bi and Te layers.

This dissertation contains theoretical work motivated by recently reported experimental data for ... more This dissertation contains theoretical work motivated by recently reported experimental data for Te and Hg0.84Zn0.16Te. These data show a constant temperature viscosity relaxation in molten Te and Hg 0.84Zn0.16Te and a constant temperature thermal diffusivity relaxation in molten Te. The first chapter gives a detailed description of the experimental data along with a discussion of relevant related topics from the literature. Then, in Chapter 2, modeling these relaxations as equilibrium fluctuations, the Green-Kubo relations are used to arrive at correlation functions for the viscosity in Te and Hg0.84Zn0.16Te and the thermal diffusivity in Te. In Chapter 3 an existing model is considered which suggested that the relaxation in Hg0.84Zn0.16Te was due to Te polymerization. This model assumed the melt contained no polymer before the onset of polymerization. An alternate approach is considered where there is an initial non-zero polymer distribution present before polymerization. A new ti...

Journal of Physics and Chemistry of Solids, 2013

Abstract At atmospheric pressure, many of the II–VI semiconducting alloys are known to undergo a ... more Abstract At atmospheric pressure, many of the II–VI semiconducting alloys are known to undergo a zinc-blende to wurtzite solid–solid transition. Experimental values for these transitions temperatures have only been reported for two alloys. We show here that chemical potentials for one of the components in a solid solution with the other can be used to estimate the transition temperature. The non-ideal behavior of the solvent component is addressed via an activity coefficient which is determined using the quasi-chemical model. The chemical potentials for each case, zinc-blende and wurtzite are then taken to be equal at the transition temperature. Predicted transition temperatures are reported here for ZnS, CdS, ZnSe, ZnTe, and CdSe. In the case of ZnS and CdS these values agree with the experimental. For ZnSe, ZnTe, and CdSe the values are above their respective melting points. This result suggests that there is no complete bulk transition below the melting point for these three alloys but calculations predict that the two phases are nearly isoenergetic.

Journal of Crystal Growth, 2009

A closed form solution is found for the Buyevich-Mansurov theory of the intermediate stage of cry... more A closed form solution is found for the Buyevich-Mansurov theory of the intermediate stage of crystal growth. Using a common model for the growth rate, expressions for the time-dependent relative supersaturation and the distribution function for crystal sizes are found. With these results, the rate of homogeneous nucleation is established and need not be given as a boundary condition. The formula for the relative supersaturation is compared with experimental data for the crystallization of lysozyme and canavalin. The theory accurately describes the kinetics of the supersaturation decay during crystallization for both of these proteins.

Journal of Crystal Growth, 2004

More than 75 years ago, von Weimarn summarized his observations of the dependence of the average ... more More than 75 years ago, von Weimarn summarized his observations of the dependence of the average crystal size on the initial relative concentration supersaturation prevailing in a solution from which crystals were growing. Since then, his empirically derived ''rules'' have become part of the lore of crystal growth. The first of these rules asserts that the average crystal size measured at the end of a crystallization increases as the initial value of the relative supersaturation decreases. The second rule states that for a given crystallization time, the average crystal size passes through a maximum as a function of the initial relative supersaturation. Using a theory of nucleation and growth due to Buyevich and Mansurov (J. Crystal Growth 104 (1990) 861), we calculate the average crystal size as a function of the initial relative supersaturation. We confirm the von Weimarn rules for the case where the nucleation rate is proportional to the third power or higher of the relative supersaturation.

Infrared Physics & Technology, 2004

Abstract An existing method is utilized to provide a new theoretical estimate for certain II–VI p... more Abstract An existing method is utilized to provide a new theoretical estimate for certain II–VI pseudobinary phase diagrams. This requires no knowledge of experimental liquidus or solidus data except for the melting temperatures of the pure binary end members. In this method, the liquid is treated as a ternary regular solution and the interaction parameter can be calculated with knowledge of the electronegativities, energy of vaporization and molar volumes of the pure elements. The solid is treated as a binary regular solution where the solid interaction parameter, and therefore the enthalpy of mixing, depend only on the lattice mismatch of the pure II–VI compounds. This method is shown to work well with the Cd 1− x Zn x Te alloy but to fail with Hg 1− x Cd x Te and Hg 1− x Zn x Te. We propose here that the above method fails, in these two cases, because it gives the incorrect value for the Hg–Cd and Hg–Zn interaction parameters. An alternative way to compute these interaction parameters is proposed. With this change, we are able to calculate Hg 1− x Cd x Te and Hg 1− x Zn x Te pseudobinary phase diagrams that show improved agreement with reported experimental data.