Chromatographic measurements and molecular statistical calculations of thermodynamic characteristics of adsorption of aromatic and polycyclic hydrocarbons on graphitized thermal carbon black

Intermolecular Interactions in Gas Chromatography on Carbon Black Coated with Monolayers of Hydrocarbons with Different Electronic Structures

The adsorption and chromatographic properties of surfaces obtained by the adsorptive modification of thermally graphitized carbon black (TGCB) with hydrocarbons differing in composition and electronic structure (n-dotriacontane and coronene) were studied by gas chromatography (GC). The contributions towards the retention of methylene groups in homologous series of n-alkanes, alkylbenzenes, ethers, esters and n-alcohols were estimated for TGCB coated with monolayers of the above-mentioned hydrocarbons. The thermodynamic characteristics of the adsorption process were also measured for molecules of the various classes of organic compounds described. In addition, differences in the mechanism of chromatographic retention on n-alkane monolayers, when the latter may exist as solid-like, transitional or liquid-like states, and on coronene monolayers were studied. The influence of the electronic configuration of the carbon atoms in the modifier molecules on the adsorption energy of the various organic compounds studied has been demonstrated. The possibility of employing GC methods for demonstrating phase transitions in long-chain alkane monolayers adsorbed on carbon black is discussed. Furthermore, the possibility of controlling the retention selectivity during GC by TGCB modification with adsorbed monolayers of hydrocarbons is presented.

Studies of adsorption of polychlorocarbons on carbon blacks by gas chromatography

Adsorption properties of carbon black before and after modification to polyhalogenohydrocarbons, including polyorganochlorine pesticides, have been investigated. It has been shown that graphitised thermal carbon blacks and carbochroms possess high adsorption capacity relative to the above compounds and can be used as supporting adsorbents to analyse these airborne chemicals. Modification raises the selectivity of adsorbents to the compounds being investigated and lowers retention volumes and heats of adsorption, which makes it possible to carry out thermal desorption of the accumulated species at lower temperatures.

Gas Chromatographic Characterization of Silica Bonded with a Phenylpolysiloxane Film

A method of synthesizing modified silica with a dense grafted phenylpolysiloxane layer is described. Gas chromatography at zero surface coverage has been used for investigating the adsorption properties of both modified and initial silicas. It was shown that the thermodynamic parameters for retention (retention volume, Va; heat, q; and entropy, ΔS0, of adsorption) and the Kovats retention indices I for different classes of organic compounds are essentially changed with surface modification. The increase in Va and q for n-alkanes on phenyl-bonded silica was interpreted as due to penetration of the solutes into the grafted phenylpolysiloxane layer. The significant reduction in retention parameters for all compounds capable of specific interaction, including the formation of hydrogen bonds, on the phenyl-bonded silica relative to the initial silica has allowed conclusions to be drawn regarding the weak role of residual silanol groups in the retention process. These conclusions have been confirmed by a comparison of the chromatographic properties of phenyl-bonded silica and the polymethylphenylsiloxanes OV-17 and OV-25.

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.

Evaluation of molecular mechanics calculated binding energies for isolated and monolayer organic molecules on graphite

The calculated molecule-surface binding energy, E(cal)( *), for physical adsorption was determined using molecular mechanics MM2 parameters for a model graphite surface and various organic molecules. The results for E(cal)( *) were compared to published experimental binding energy values, E( *), from gas chromatography (GC) or thermal desorption (TD). The binding energies from GC were for isolated molecules in the Henry's law region of adsorption, and the binding energies from TD were for molecules in monolayer coverage on a highly oriented pyrolytic graphite (HOPG). A simple desorption model was used to allow the calculation of monolayer coverage to include both molecule-surface and molecule-molecule interactions and then the results were compared to experimental values. For the 14 TD organic adsorbates (polyaromatic hydrocarbons, alcohols, benzene, substituted benzenes, methane, chloroalkanes, N,N-dimethylformamide, and C(60) Buckyball), the experimental versus calculated binding energies were E( *)=1.1193E(cal)( *) and r(2)=0.967. The GC E( *) values were also well correlated by calculated E(cal)( *) values for a set of 11 benzene and methyl substituted benzenes and for another set of 10 alkanes and haloalkanes. The TD E(cal)( *) mechanics computation provides a useful comparison to the one for GC data since adsorbate-adsorbate interactions as well as adsorbate-surface must be considered.

Correlations of Adsorption Energies with Physical and Structural Properties of Adsorbate Molecules

The purpose of this work is to find a suitable method to correlate Henry's law adsorption energies such as the isosteric heat or the gas-solid interaction energy. Available adsorption energies for a total of 63 gases distributed among five adsorbents-a zeolite, a silica gel, a carbon black, and two microporous carbons adsorbents were correlated with the boiling point, a critical constant ratio involving critical temperature and critical pressure, the log of the octanol-water partition coefficient, and molecular structural energy. In the structural approach each gas molecule is considered to be composed of a series of structural units that each make a fixed contribution to the total molecular adsorption energy. Multiple regression analyses were done for each of the five adsorbent systems using the four main correlation methods. The method based on the structural units of a molecule and its dipole moment was found to give a 0.99 correlation coefficient for all five gas-solid systems. This structural method was superior to correlations based on boiling points, critical constants, partition coefficients, and various combinations of other physical properties that were also examined. Since there has been a variety of interest in either predicting the energy of adsorption or extent of adsorption for specific adsorbate-adsorbent pairs, our correlation results could be useful in future quantitative structure-activity relationship (QSAR) correlations of adsorption. Copyright 1997 Academic Press. Copyright 1997Academic Press

Correlation between the retention of cardiac glycosides in reversed-phase high-performance liquid chromatography with a diphenylsilyl stationary phase, the structure of their molecules and their biological activity

The separation of mixtures of cardiac glycosides by reversed-phase high-performance liquid chromatography on silica gel with chemically grafted diphenylsilyl groups using water-ethanol as the eluent was carried out. It is shown that the configuration and conformation of the glycoside molecules, and the hydrophilic properties of their aglycones and glycones, influence the separation. The hydrophilic properties of the aglycones are more important than those of the glycones. The glycosides with more hydrophilic aglycones have higher biological activity. This is probably related to the easier transport of these glycosides to the receptor.