A. V. Kiselev

Adsorption equilibria and the energy of adsorption forces

1. The adsorption energy of nonpolar molecules is calculated by a method which takes into consideration three terms in the potential of the dispersive forces by means of constants calculated on the basis of the polarizabilities and magnetic susceptibilities, Other factors taken into consideration in the development of the method are inductive potential (through the mean polarizability of the adsorbate and the mean electrostatic field of the adsorbent) and the repulsion potential (through an exponential constant calculated from the individual constants of adsorbent and adsorbate and with summation of all of the interactions of the given energy center of the molecule of adsorbate over all of the centers of the lattice of the adsorbent). The exponential repulsion constant was determined from the minimum condition of the summed energy of all of the interactions at the equilibrium distance from the adsorbent surface. 2. The calculated values of the energy of adsorption of noble gases, nitrogen and thirteen hydrocarbons of various structures (normal and isomeric alkanes, an alkene, cyclanes and aromatics) on graphite are close to the measured heats of adsorption on graphitized carbon blacks. 3. The calculated values of the energy of adsorption of n-alkanes, benzene and toluene on magnesia are likewise close to the measured heats of adsorption. 4. In the case of adsorption on graphite the contributions of the first, second and third terms of the energy of the dispersive forces and the absolute value of the repulsion energy constitute 90–95, 5–10, 0.5–1 and 35–40%, respectively, of the total energy of the dispersive forces for the investigated adsorbates.

Calculation op the adsorption energy of nonpolar molecules on graphite

The details of an earlier theoretical calculation of the energy of adsorption of simple and complex non-polar molecules on graphite were presented. The power sums of type\(\Sigma r_{i\bar j} ^n \) were calculated for different distances of the adsorbate molecule from the external base plane of graphite, at n=6, 8, 10 and 12, as were also the exponential sums of type\(\Sigma e - ^{r_{ij} /} \rho \), at δ=0.28 A. Using approximate quantum mechanics equations, we calculated the constants of three terms of the energy of dispersive attraction, and also the constants of repulsion from the equilibrium condition. The additive scheme was used to calculate the potential curves and equilibrium adsorption energies of complex molecules.

The adsorption and heat of adsorption of normal alcohols on graphitized carbon black

1. The adsorption isotherms and the differential heats of adsorption of the vapors of water, methanol, ethanol, n-propanol, and n-butanol on graphitized channel carbon black have been determined; the heat of adsorption of water vapor is much less than the heat of condensation, and the heats of adsorption of the alcohols increase with increase in the length of the carbon chain. 2. The values of the heats of adsorption and the areas corresponding to a molecule of the alcohols in the compact monolayer on the surface of graphitized carbon black indicate that the alcohol molecules are arranged parallel to the surface and form hydrogen bonds with one another. 3. The standard thermodynamic characteristics of the adsorption of n-alcohols on graphitized channel black have been determined and it has been shown that the state of a substance in the adsorption layer in these cases is close to the associated state in the liquid. 4. On going from adsorption on graphitized carbon black to adsorption on the original (oxidized) carbon black, and more particularly to adsorption on silica gel with a hydrated surface, there is a sharp increase in the localization of methanol molecules.

Adsorption and heat of adsorption of pyridine and benzene vapor on graphitized carbon black

1. Measurements have been made of the adsorption and differential heat of adsorption isotherms of pyridine vapor on TI thermal black, graphitized at 3000°. On going from benzene to pyridine, the mutual attraction of the molecules in the adsorbed layer is increased, which causes a change in form of the adsorption isotherm at small fillings from convex (benzene) to concave (pyridine), and an increase in the maximum on the heat of adsorption isotherm. 2. The adsorption of benzene and pyridine on graphitized carbon black is, apparently, predominantly localized. The calculated values of adsorption entropy of the benzene and pyridine molecules on graphitized carbon black are quite close. 3. A calculation of the potential energy of adsorption of benzene and pyridine on graphite gave satisfactory agreement with the measured heats of adsorption.

Isotherms and heats of adsorption of neopentane and carbon tetra chloride vapor on graphitized carbon black

1. Measurements have been made of the adsorption isotherms and differential heat of adsorption of neopentane and carbon tetrachloride on thermal black graphitized at 3000°. 2. The large sphere-like molecules of these substances are adsorbed in a predominantly nonlocalized way, and interact strongly with one another, which causes a marked increase in the heat of adsorption as the monolayer is being filled and a strongly concave initial portion of the adsorption isotherm. 3. Free rotations or rotational oscillations of these molecules are possible in the adsorbed layer with small amounts of surface filling. 4. A discussion is given of different possible ways of defining the standard state of the adsorbed layer on a homogeneous surface, and values are given for the corresponding thermodynamic quantities of the adsorptive systems studied.

The adsorption energies of water, alcohols, ammonia and methylamine on graphite

1. The adsorption energy of water, normal and isomeric alcohols, ammonia and methylamine molecules on the surface of the basal face of graphite has been calculated. In the calculation, the two terms in the potential of the electro-kinetic (dispersion) forces and the potential of the electrostatic (induced) attractive forces have been considered and the potential of the repulsive forces has been calculated in the form of its relation with the distance. 2. The values of the adsorption energies of isolated molecules of water, alcohols, ammonia and methylamine are significantly less than the measured heat of adsorption. Calculation of the energy of association of these molecules in the adsorption layer and using the energy of formation of hydrogen bonds, led to agreement between the calculated values of the adsorption energy and the heats of adsorption.

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.

Heat of adsorption of benzene vapor on carbon blacks thermodynamics of the process and the adsorption forces

1. With the aid of an automatic calorimeter having constant heat exchange and an adsorption apparatus with a capillary microburet, determinations were made of differential heats of adsorption and the adsorption isotherms for benzene vapor on carbon black, both graphitized and in its original oxidized state. Much of the surface of graphitized carbon is homogeneous and is filled at a constant heat of adsorption. The adsorption isotherm is satisfactorily described by the Langmuir and BET equations. 2. The relation of the free energy, total energy, and entropy of adsorption to the surface coverage was investigated. The respective standard values for adsorption of vapor and for wetting were determined. 3. A theoretical calculation of the adsorption energy of benzene was carried out by means of a summation of the dispersional interactions with the 100 nearest atoms of the graphite lattice, account being taken of interactions with the remaining atoms with the aid of an integral formula. The calculated value of the adsorption energy is very close to the measured heat of adsorption. 4. Comparison are made between the isotherms and heats of adsorption of benzene and hexane vapors. As requried by theoretical calculation, in the unimolecular region the heat of adsorption of benzene is considerably less than that of hexane.

Potential energy of adsorption of the sphere-like molecules CH4, C(CH3)4 and CCl4 on graphite

Theoretical calculations of the potential energy of the dispersion interaction of sphere-like molecules of methane, neopentane, and carbon tetrachloride with the basal plane of the graphite lattice have shown that it is a good idea to divide the neopentane molecule into links, while it is better to divide the carbon tetrachloride molecule, which contains large chlorine atoms, into individual atoms.

Molecular statistical calculation of the change in the thermodynamic functions when CH4 is adsorbed on graphite

The theoretically calculated variation in the potential energy of methane molecules with distance from the graphite surface, and the approximate model of their motion along the surface as being nonlocalized adsorption with harmonic oscillations perpendicular to the surface have been used to make a molecular statistical calculation of the changes in the thermodynamic functions of methane when it is adsorbed on the basal plane of graphite at small surface filling. The theoretically calculated change in the total energy of methane at small surface fillings is very nearly the same as the heat of adsorption found by experiment.