Thomas R. Rybolt

Rare gas–graphite interaction potentials

The interaction of neon, argon, krypton, and xenon with the graphitized carbon black P33 (2700) is examined using the virial coefficient treatment of physical adsorption. Adsorption isotherm data in the low coverage, Henry’s law region are used to carry out this analysis. A modified Buckingham potential is selected to model the gas–solid interaction potential. A best fitting technique is used to make an unambiguous selection of the interaction potential parameters. A cubic power law decay is found to be the proper choice for the attractive portion of the potential, while the appropriate screening parameters for the exponential, repulsive portion of the potential are determined to be 0.0181, 0.0196, 0.0198, and 0.0215 (nm) for Ne, Ar, Kr, and Xe, respectively. The adatom–adsorbent internuclear separations are 0.307, 0.333, 0.336, and 0.366 (nm) and the gas–solid interaction energies are 384, 1113, 1467, and 1928 (°K) for Ne, Ar, Kr, and Xe, respectively. A surface area of 10.8±0.3 m2g−1 is obtained for the...

Superhigh Surface Area Determination of Microporous Carbons

Abstract The high resolution N2 adsorption isotherms of pitch-based activated carbon fibers, activated mesocarbon microbeads (a-MCMB), and a high surface area carbon powder(Super Sorb), which have apparent BET surface area greater than a classical upper limit of the surface area for carbonous materials, were measured. The true surface area of these high surface area carbons was determined by the subtracting pore effect (SPE) method with the aid of the high resolution αs-plots. The BET surface area depended sensitively on the relative pressure region and the routine BET analysis using the data of the relative pressure region of 0.1-0.3 gave the seriously overestimated surface area.

Gas—solid chromatography and virial analysis of chlorofluorocarbon adsorption on a microporous carbon

Abstract Gas-solid chromatography was used to determine the second gas-solid virial coefficients in the temperature range 373–512 K for ethane, propane, chloromethane, dichloromethane, fluoromethane, chlorodifluoromethane (Freon 22), and dichlorodifluoromethane (Freon 12), with Super Sorb, a microporous carbon adsorbent. The temperature dependence of the second gas-solid virial coefficients of these adsorbates was used in conjunction with a Lennard-Jones and Devonshire cell model to determine the effective structural parameters of cavity radius and number of cavities per gram of adsorbent, as well as gas-cavity interaction energies. The interaction energies were correlated with adsorbate boiling points and energetic additivity rules based on molecular structure including the number of atoms of different types in the molecule and the dipole moment of the molecule.

Gas—solid chromatography and virial analysis of hydrocarbon adsorption on 13X zeolite

Abstract Gas-solid chromatography was used to determine the second gas-solid virial coefficients in the temperature range 343–597 K for ethane, propane, n-butane, 2-methylpropane, n-pentane, 2-methylbutane, 2,2-dimethylpropane, n-hexane, 2,2-dimethylbutane, and cyclohexane with 13X zeolite. The temperature dependence of the second gas-solid virial coefficients of these 10 hydrocarbons along with the available methane data was used in conjunction with a Lennard-Jones and Devonshire cell model to determine the 13X zeolite structural parameters of cavity radius and number of cavities per gram of adsorbent, as well as gas-cavity interaction energies. The 13X structural parameters were compared to X-ray crystallographic values. The interaction energies were found to be linearly dependent on the number of carbon atoms in the adsorbate hydrocarbon with the more highly branched molecules having somewhat lower energies. The interaction energies were correlated with hydrocarbon boiling points and energetic additivity rules on the basis of molecular structure.

High-temperature physical adsorption of argon, krypton, and xenon on hexagonal boron nitride

Abstract The interaction of argon, krypton, and xenon with hexagonal boron nitride was examined utilizing a two-surface gas—solid virial analysis. A precision volumetric apparatus was used to obtain 10 adsorption isotherms in the temperature range 273–368°K. The low coverage, Henrys law isotherm data were used to obtain values for the second gas—solid virial coefficients for ArBN, KrBN, and XeBN. The boron nitride adsorbent consists of higher energy edge sites and basal plane sites. A two-surface virial coefficient treatment was coupled with a graphical fitting technique to separate the effects of the two surface sites. Values of the inert gas-edge and inert gas-basal plane interaction energy are reported for each of the three adsorbates. Also, the surface area for the two surfaces and fraction of edge sites are reported. The results are compared with previous studies for these systems.

Correlations of Critical Properties with Computed Molecular Surface Areas for 11 Homologous Series of 118 Organic Compounds.

Abstract A simple and convenient method for predicting critical constants (Tc, Pc, and Vc) for members of a homologous series was developed from computed molecular surface areas obtained from a molecular modeling program (CAChe version 3.5). The best linear correlations are obtained when Vc and T c P c 0.5 are plotted against computed molecular surface area (SA) and when Tc and Pc are plotted against the natural logarithm of surface area, In SA, for 11 homologous series with a total of 118 organic compounds. The series included n-alkanes, n-alkyl primary amines, n-alkyl primary alcohols, acetate esters, n-alkyl primary thiols, secondary alcohols, n-alkylbenzenes, 2-methylalkanes, perfluoroalkanes, methyl esters, and ethyl esters. For C2C19 n-alkanes, the calculated Tc (K) values agreed well with the literature critical temperatures (R = 1.000) and with other previously described correlations. Similar Tc correlations were found with the other 10 homologous series. The correlation between T c P c 0.5 and SA for the 11 homologous series is particularly striking and suggests a strong relationship between molecular surface area and intermolecular attractions.

A comparison of two surface virial analyses of monatomic gases on graphite and boron nitride

Abstract By examining monatomic gases (argon, krypton, and xenon) adsorbed on P33 graphite and hexagonal boron nitride, a two-surface model to represent gas-solid interaction systems was developed. Second gas-solid virial coefficients were used in conjunction with a Lennard-Jones (16,3) potential to determine the gas-solid interaction energy, the surface area, and the percentage area of the higher energy surface for both gas-P33 and gas-boron nitride systems. The analyses indicated that P33 graphite is best characterized as a single-surface solid, while boron nitride is best characterized as a two-surface solid. This two-surface virial approach was able to clearly distinguish the two types of surfaces using the temperature dependence of Henrys law adsorption data, but without any specific prior assumptions of differences in surface structures.

Binding energies for alkane molecules on a carbon surface from gas-solid chromatography and molecular mechanics.

Gas-solid chromatography was used to determine B(2s) (gas-solid virial coefficient) values for 12 alkanes (10 branched and 2 cyclic) interacting with a carbon powder (Carbopack B, Supelco). B(2s) values were determined by multiple size variant injections within the temperature range of 393 to 623 K with each alkane measured at 5 or 6 different temperatures. The temperature variations of the gas-solid virial coefficients were used to find the experimental adsorption energy or binding energy values (E( *)) for each alkane. A molecular mechanics based, rough-surface model was used to calculate the molecule-surface binding energy (E(cal)( *)) using augmented MM2 parameters. The surface model consisted of three parallel graphene layers with each layer containing 127 interconnected benzene rings and two separated nanostructures each containing 17 benzene rings arranged in a linear strip. As the parallel nanostructures are moved closer together, the surface roughness increases and molecule-surface interactions are enhanced. A comparison of the experimental and calculated binding energies showed excellent agreement with an average difference of 3.8%. Linear regressions of E( *) versus E(cal)( *) for the current data set and a combined current and prior alkane data set both gave excellent correlations. For the combined data set with 18 linear, branched and cyclic alkanes; a linear regression of E( *)=0.9848E(cal)( *) and r(2)=0.976 was obtained. The results indicate that alkane-surface binding energies may be calculated from MM2 parameters for some gas-solid systems.

Statistical thermodynamics of aerosols and the gas–solid Joule–Thomson effect

Due to the adsorption of a gas by a solid, it is expected that an aerosol created by dispersing a fine powder in a gas would have unique thermodynamic properties not found in pure or mixed gases. The virial equation of state associated with an aerosol dusty gas is obtained from statistical thermodynamic considerations. In the theoretical model presented here, the aerosol is considered to be a two component fluid made up of solid particles and gas molecules. The aerosol virial equation of state is used to derive an expression for the Joule–Thomson effect associated with a gas–solid dispersion. The magnitude of the gas–solid Joule–Thomson effect is expressed in terms of gas and gas–solid virial coefficients. Previous adsorption data for an argon–porous carbon system is used to obtain gas–solid virial coefficients and to predict the magnitude of the gas–solid Joule–Thomson effect. A significant enhancement of the Joule–Thomson effect is predicted for gas–solid systems which display a strong interaction. For ...

The gas-solid Joule-Thomson Effect

The magnitude of interaction of a gas with a solid is expected to cause an aerosol created by dispersing a fine powder in a gas to have unique thermodynamic properties not found in pure or mixed gases. The virial equation of state associated with a gas-solid aerosol or dusty gas is obtained from statistical thermodynamic considerations. The theoretical model of the aerosol is that of a two component fluid made up of solid particles and gas molecules. The aerosol virial equation of state is used to derive an expression for the Joule-Thomson effect associated with a gas-solid dispersion. The magnitude of the gas-solid Joule-Thomson effect is expressed in terms of gas-gas and gas-solid virial coefficients. Previous adsorption data for an argon-porous carbon system is used to obtain gas-solid virial coefficients and to predict the significant enhancement of the Joule-Thomson effect is predicted for gas-solid systems which display strong interactions. An apparatus was constructed to disperse a fine powder in a flowing gas and measure the thermal changes associated with a pressure drop across a glass orifice. The gas-solid Joule-Thomson effect was examined for 12 different gas-solid systems at a temperature of 302 K. Measurements for a series of aerosols composed of four different carbon adsorbents with argon, nitrogen and carbon dioxide confirm the effect and pose new questions. magnitude of the gas-solid Joule-Thomson effect. A