Basal Face Of Graphite Research Articles

Adsorption of haloadamantanes on the basal face of graphite

The graphite surface exhibits different structural selectivity to isomeric haloadamantanes depending on the kind and number of halogen atoms in the molecule. In the series of fluoro-, chloro-, bromo-, and iodoadamantanes, the selectivity of separation of the respective isomers on columns packed with graphitized thermal soot increases. The equilibrium adsorption characteristics of haloadamantanes (adsorption equilibrium constants, heats and entropies of adsorption), calculated within the framework of molecular-statistical theory, agree well with the experimental values and allow comprehensive description of the adsorbed state of these compounds on the graphite surface. The results of molecular-statistical calculations allow evaluating the possibility of haloadamantane separation on columns with graphitized thermal soot under the conditions of equilibrium gas-adsorption chromatography.

Thermodynamic characteristics of the adsorption of isomeric molecules of fluoroadamantanes on the surface of the basal face of graphite

The thermodynamic characteristics of adsorption (TCA) for isomeric molecules of mono-, di-, tri-, and tetrafluoroadamantanes were determined for the first time experimentally and by molecular statistics on the surface of graphitized thermal black (GTB). The parameters of atom-atom potential function of pair intermolecular interaction (φ(r)) for F atoms included in fluoroadamantanes with C atoms on the basal face of graphite were calculated for the first time within an atom-atom approximation of the semi-empirical molecular statistical theory of adsorption. The adsorption non-equivalence of F atoms in various positions of an adamantane framework, a consequence of the mutual effect of atoms in a framework molecule, was determined. On the basis of the definite TCA values, the conclusion was drawn as to the possibility of isolation of isomeric fluoroadamantanes under the conditions of gas-adsorption chromatography on GTB.

Molecular statistical calculation of thermodynamic characteristics of adsorption of O-, S-, and Se-containing heteroadamantanes on graphitized thermal carbon black

The thermodynamic characteristics of adsorption (TCA) on the basal face of graphite have been calculated in terms of the semiempirical molecular statistical theory of adsorption for molecules of O-, S-, and Se-containing heteroadamantanes of different structure and isostructural cyclohexane derivatives. The influence of the nature, number, and position of heteroatoms in the adamantane framework on the TCA values was studied in detail, which made it possible to predict the retention of the compounds considered on the surface of graphitized thermal carbon black under the conditions of equilibrium gas adsorption chromatography. The introduction of each subsequent heteroatom into a polyheteroadamantane molecule makes a non-additive contribution to the TCA values. The contributions of various fragments to the total adsorption energy were calculated by the TCA of isostructural heterocyclohexane molecules, and the most preferential orientations of the framework molecules of the adsorbates relative to the planar graphite surface were determined.

Calculation of henry constants for the adsorption of isomeric phenylenediamines on graphitized thermal carbon black

Henry constants for the adsorption of o- and p-phenylenediamines on the surface of graphitized thermal carbon black within the temperature range 433–479 K were calculated by the molecular statistical method. The parameters of the atom-atom potential function of intermolecular interaction between the nitrogen atom in aniline and isomeric phenylenediamines and the carbon atom of the basal face of graphite were determined. It was shown that an intramolecular H bond influenced the geometry and adsorption properties of o-phenylenediamine.

Experimental and molecular statistical study of alkyladamantane adsorption on graphitized thermal carbon black

Experimental measurements and molecular statistics methods were used to calculate thermodynamic characteristics of adsorption (TCA) of alkyladamantanes of different structure on the surface of hydrogen-treated graphitized thermal carbon black (GTCB) Carbopack C HT. High selectivity of separation of isomeric alkyladamantanes on GTCB is due to specific features of their geometry and electronic structure. The influence of the spatial structure of adsorbates on the TCA was established. Special attention was given to analysis of the heat capacities of adsorption. The best agreement between the experimental TCA and those calculated by the molecular statistics methods is achieved when the “cage effect“ in the adamantane ring is taken into account. The corresponding corrections are introduced into the parameters of the atom-atomic potential function of pair intermolecular interaction of the Me groups with the carbon atoms of the basal face of graphite, which are localized in the nodal positions of the lattice. The critical parameters for a large group of alkyladamantanes were calculated in terms of the additive group Lydersen method. A set of various topological indices used further for constructing structure-retention correlations on GTCB was obtained.

Adsorption of tetramethylsilane molecules on the basal face of graphite

The thermodynamic adsorption characteristics of tetramethylsilane (TMS) molecules on graphitized carbon black (GCB) were for the first time determined experimentally and by molecular statistics methods. The potential function parameters of pair molecular interactions (φ(r)) between Si and C atoms on the basal face of graphite were calculated in the atom-atom approximation of the semiempirical molecular-statistical theory of adsorption. The contributions of Si and C atoms to thermodynamic adsorption characteristics are compared for the example of nonspecific adsorption of TMS and isostructural neopentane molecules on the flat surface of GCB.

Numerical modeling of polymolecular adsorption of argon on a model amorphous oxide

Using the Monte Carlo method for the grand canonical ensemble, we have calculated the Ar adsorption isotherms in the range 77–107 K on an inhomogeneous surface with random close packing of spheres (the Bemal model), representing oxygen ions in a model oxide. The isotherm at 77 K is very close to the standard Ar adsorption isotherm on nonporous silica. The difference involves the weak step character of the calculated isotherm. The adsorbate on the model oxide at 77 K has a layered structure, reminiscent of the structure of Ar on the basal face of graphite, but significantly more extended. The atomic area of Ar depends on the method for determination of the capacity of the monolayer and lies within the range 0.1–0.13 nm2.

Heat of adsorption of water vapors and benzene on high-silicon zeolite of the faujasite type

1. The isotherms and differential heats of adsorption of water and benzene vapors on ultrahigh-silicon zeolite of the faujasite type (faujasil), obtained by the original method of substitution of Al atoms by Si atoms in the reaction of zeolite NaY with SiCl4, were investigated. 2. The isotherms and heats of adsorption indicate the existence of hydroxyl groups and a small volume of mesopores in faujasil. 3. The presence of hydroxyl groups in the zeolite masks the overall hydrophobic character of the surface of the adsorbing cavities. With the exception of the initial high heats (up to filling of 0.15 mmole/g), the entire sorption volume (17.5 mmole/g) is filled with water with almost zero “pure heat,” and the entropy of the adsorbate does not differ from the entropy of a normal liquid. 4. The pronounced bend in the benzene adsorption isotherm, the increase in the heat of adsorption with an increase in the filling, the passage of the heat through a relatively high maximum, and the deep entropy minimum in completion of sorption indicate a high degree of chemical and geometric homogeneity of the adsorbing cavities in faujasil. 5. The heats of adsorption of benzene on the basal face of graphite and on faujasil with zero filling are almost the same, ∿43 kJ/mole. Volume filling of the micropores creates favorable orientations of the molecules for the maximum adsorbate-adsorbate interaction and the heats increase to ∿60 kJ/mole, while the heats only increase to ∿45 kJ/mole in planar orientation of the molecules on graphite.

Wetting of graphite (0001) by carbon monoxide. A stepwise adsorption isotherm study

Sets of stepwise adsorption isotherms of carbon monoxide on exfoliated graphite have been measured volumetrically for coverages ranging from one to five monolayers, at temperatures surrounding the transition temperature between the α and β forms of the bulk crystalline adsorbate. From the analysis of the phase diagram representing the condensation pressures vs. temperature of both the successive monolayers and the bulk crystal phases, it is concluded that the basal face of graphite is incompletely wet by the low-temperature form α-CO and perfectly wet by the high temperature form β-CO. Since the molecules are orientationally ordered in α-CO and disordered in β-CO (plastic phase), this investigation supports the point of view, suggested by previous studies, that orientational order within a crystal phase is generally an obstacle to its spreading over a plane surface. The previous observations of changes in wetting regimes at triple points are briefly discussed in parallel with the present results.

Melting of the first monolayer of Ar adsorbed on the basal face of graphite

Melting of the first monolayer of Ar adsorbed on the basal face of graphite

Melting of the first monolayer of Ar adsorbed on the basal face of graphite

We have measured adsorption isotherms of Ar on graphite from 67 to 77 K in the range of coverages at which the monolayer freezes. The transition appears continuous and shows some similarity with a λ transition. The large increase of the isothermal compressibility at freezing seems to indicate that it cannot be accounted for by the Kosterlitz-Thouless-Halperin-Nelson-Young theory.

Fluid-registered solid transition in a monolayer of carbon monoxide adsorbed on the basal face of graphite

Adsorption isotherms of CO on the basal face of graphite have been measured, showing a first order transition within the dense monolayer. There is high a priori probability that this transition is the analogue of the fluid √3 × √3 R 30° transition of the isoelectronic molecule N 2 adsorbed on the same substrate. The comparison between the thermodynamic properties of both transitions, using in particular a corresponding states law scaling, very strongly supports this conjecture.

Fluid-registered solid transition in a monolayer of carbon monoxide adsorbed on the basal face of graphite

Fluid-registered solid transition in a monolayer of carbon monoxide adsorbed on the basal face of graphite

Transition from fluid to registered solid of the krypton monolayer adsorbed on the basal face of graphite

Precise measurements of adsorption isotherms of Kr on graphite in the coverage range corresponding to the transition between the fluid and the registered monolayer have been made. This transition is shown to remain first order up to a temperature of 115 K, contrarily to earlier studies which suggested 98 K. A critical analysis is made of theories recently proposed for this transition.

Critical temperatures of two-dimensional condensation in the first adlayer of noble gases on the cleavage face of graphite

A set of adsorption isotherms of Ar on the basal face of graphite gives for the critical temperature of two-dimensional condensation in the first adlayer a value of 59\ifmmode\pm\else\textpm\fi{}1 K, significantly different from that previously accepted. This value, together with those obtained for Kr and Xe, agrees well with the prediction of a virial expansion truncated after the fourth coefficient, suggesting that the liquid phases of the first monolayers of noble gases adsorbed on graphite are not significantly perturbed by the surface structure.

Effect of the form and the parameters of intermolecular interaction potential on calculated values. Henry constant for adsorption of noble gases on graphitized black and the specific surface area determined therefrom

The effect of the form and the parameters of the atom-atom potential of intermolecular interaction, as well as that of the approximations used in its summation over the atoms of a solid, on the values of the Henry constant K 1 for the adsorption of noble gases on the basal face of graphite has been examined. At fixed values of the depth and of the position of the atom-atom potential minimum, K 1 values are only slightly sensitive to the form of this potential, if its summation is performed taking into account the layer structure of graphite. Disregarding the laminated structure of the graphite lattice leads to highly underestimated values of K 1. The values of the parameter c 1 of the attractive forces of the potential (6, 8, exp) for the intermolecular interaction of the atoms of noble gases with the carbon atom of graphite have been determined, using experimental values of K 1 for the adsorption of these gases on graphitized thermal carbon blacks. The corresponding changes in internal energy, entropy and heat capacity have been calculated. The effect of the form and the parameters of the potential function φ of the intermolecular interaction of a monoatomic molecule with the basal face of graphite on the energy φ o in the minimum of the potential curve φ( z ), and on the product sz o of the specific surface area s , and the equilibrium distance between the molecule and the surface z o, has been studied. φ o and sz o were determined from experimental values of the Henry adsorption constant K 1 measured at different temperatures. From the experimental values of K 1 for the adsorption of noble gases, the value of s for graphitized thermal carbon black Sterling PT, 2700 (P-33), has been determined. This value of s does not essentially depend on the adsorbate and lies within the limits of error of the s value, determined for this carbon black by the BET method. When determining s from K 1, it is necessary to take into account the structure of the solid so as to choose the form of potential φ correctly. Values of z o should be determined from the atom-atom potential function of the adsorbate-adsorbent intermolecular interaction.

Molecular-statistical calculation of retention volumes in gas adsorption chromatography

The retention volumes, V R , for a series of hydrocarbons in gas adsorption chromatography on the basal face of graphite at zero surface coverage were calculated by a semi-emperical molecular-statistical method. The potential function of the interaction between a molecule and a surface is assumed to be equal to the sum of the potential functions of the interaction between atoms of the molecule and atoms of the adsorbent. The potential functions of the interaction between the C and H atoms of saturated hydrocarbon molecules and the C atoms of the graphite lattice (the atom-atom potential functions) were estimated by methods according to the theory of the intermolecular interactions and were corrected using experimental adsorption data for methane, ethane and propane on graphitized thermal carbon blacks. The calculated values of V R for all the saturated hydrocarbon molecules considered ( n-butane, isobutane, n-pentane, neopentane, cyclopentane, n-hexane, and cyclohexane) are in good agreement with the experimental values of V R on graphitized thermal carbon blacks. The calculations of V R for unsaturated and aromatic hydrocarbon molecules made it possible to estimate the difference between the potential functions of interaction of the C atoms of saturated, unsaturated, and aromatic molecules with graphite.

The adsorption energies of CO2, SO2, (CH3)2CO and (C2H5)2O on graphite

1. The adsorption energies of isolated molecules of different structure have been calculated, i.e., molecules of CO2, SO2, (CH3)2CO, and (C2H52O, on the surface of the basal face of graphite. The two terms in the potential of the electro-kinetic (dispersion) forces, the potential of the electro-static induction forces and the potential of the repulsive forces were calculated and were found to be close to the values of the differential heats of adsorption on graphitized carbon blacks, measured at small coverages of the surface. 2. The calculated mutual interaction energies of adsyrbate molecules for molecules of CO2, SO2, (CH32CO and (C2H52O in a dense monolayer for different packings with consideration of the energy of electro-kinetic, electrostatic-dipolar, electrostatic quadrupolar and repulsive interaction forces, are found to be in agreement with the experimental values of the rise in the heat of adsorption with coverage of the monolayer.