Molecular-statistical calculation of the thermodynamic characteristics of adsorption of saturated and unsaturated hydrocarbons on graphitized thermal carbon black

Abstract

Virial equations for the internal energy, entropy and heat capacity of an adsorbed substance and for their changes on adsorption are derived using the grand partition function. Molecular-statistical calculations were carried out of Henry's constant and the changes in internal energy, entropy and heat capacity on adsorption of a series of n-alkanes, isoalkanes, cycloalkanes, alkenes, alkynes and conjugated dienes onto graphitized thermal carbon black (GTCB), at zero surface coverage and various temperatures. The potential function of the intermolecular interaction of a molecule with an adsorbent Φ was assumed to be equal to the sum of ϕ the atom-atom potential functions. The functions ϕ were estimated using the approximate theory of intermolecular interactions and corrected by comparison with the experimental data for adsorption on GTCB of a few representative hydrocarbons from each homologous series. The calculated thermodynamic characteristics of adsorption of all the hydrocarbons investigated, except the rather strained cyclopropane, are in good agreement with the experimental data. The calculations showed that the intermolecular interaction of a carbon atom in the hydrocarbon molecule with a carbon atom in graphite depends on the valence state of the carbon atom in the molecule. The intermolecular interaction of a carbon atom in graphite with a carbon atom forming a double or triple bond in a hydrocarbon molecule is ∼ 7 and 27 % greater respectively than with a carbon atom in a single bond. The conjugation of valence bonds in alkadienes has little effect on their intermolecular interaction with graphite.