Of gases on solids

The adsorption of gases on solid surfaces proceeds to such an extent that approximately 10 7 gr. is present per cm.2 in the equilibrium state. This is of the same order of magnitude as the strength of the limiting capillary layer of a liquid ( 184), hence it is not improbable, as suggested by Faraday (9) (1884), that the adsorbed gas is sometimes present in the liquid state. The adsorbed amount increases with the pressure and diminishes with rise of temperature. The first effect does not follow a law of simple proportionality, as in the case of the absorption of gases by liquids, rather the adsorbed amount does not increase so rapidly, and the equation ... 

In considering the adsorption of gases on solid surfaces we will suppose that m gr. adsorbent take up x c.c. gas, reduced to N.T.P. Then, since the solid adsorbent has a spongy structure,... 

A.R. Miller, The Adsorption of Gases on Solids, Cambridge Univ. Press, 1949... 

The adsorption of gases on solids can be classified into physical and chemical adsorption. Physical adsorption is accompanied by a low enthalpy of adsorption, and the adsorption is reversible. The adsorption/desorption characteristics are in these cases often described by adsorption isotherms. On the other hand, chemical adsorption or segregation involves significantly larger enthalpies and is generally irreversible at low temperatures. It is also often accompanied by reconstruction of the surface due to the formation of strong ionic or covalent bonds. 

This isotherm finds use mainly in the study of the adsorption of gases on solids however, it can be useful in the study of adsorption of pollutants from aqueous systems, particularly onto solid phases. The heterogeneous nature of a solid surface (i. e., soils, sediments, suspended solids) would obviously invalidate the first assumption (i.e., a, above) used in developing the relationship. The third assumption (i. e., c, above) also would be invalid in a situation where one is dealing with multi-layer adsorption. 

The Elovich model was originally developed to describe the kinetics of heterogeneous chemisorption of gases on solid surfaces [117]. It describes a number of reaction mechanisms including bulk and surface diffusion, as well as activation and deactivation of catalytic surfaces. In solid phase chemistry, the Elovich model has been used to describe the kinetics of sorption/desorption of various chemicals on solid phases [23]. It can be expressed as [118] ... 

The Langmuir equation (Eq. 5.1), derived originally to describe the adsorption of gases on solids, assumes that the adsorbed entity is attached to the surface at specific, homogeneous, localized sites, forming a monolayer. It is also assumed that the heat of adsorption is constant over the entire monolayer, that there is no lateral interaction between adsorbed species, that equilibrium is reached, and that the energy of adsorption is independent of temperature ... 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

An interesting phenomenon about adsorption of gases on solids and ion exchange of ions on resins is swelling. Some porous solids expand on exposure to the vapors of adsoiptives. 

Physical adsorption of gases on solids is virtually always enthalpically driven (Att< 0). Entropically driven adsorption can exist but usually the entropy of molecules on a surface is much lower than in the gas phase. Vibrational, rotational, and also translational degrees of freedom are restricted on surfaces. 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

A number of additional equations are often used to describe reaction kinetics in soil-water systems. These include the Elovich equation, the parabolic diffusion equation, and the fractional power equation. The Elovich equation was originally developed to describe the kinetics of gases on solid surfaces (Sparks, 1989, 1995 and references therein). More recently, the Elovich equation has been used to describe the kinetics of sorption and desorption of various inorganic materials in soils. According to Chien and Clayton (1980), the Elovich equation is given by... 

The London forces are predominant in the adsorption of gases on solid substances such as carbon. In the adsorption on ionic lattices such as salt layers (CaF2 in electric lamps), silicic acid and aluminium oxide the adsorption of the first layer however depends mainly on polarization by electrostatic forces, in which isolated ions, corners and edges will give a larger heat of adsorption than a perfect crystal surface. In adsorption in multimolecular layers the Van der Waals-London energy is, however, predominant as in the cohesion energy. 

For the physical adsorption of gases on solids the attraction between the molecules and the surface is almost the exclusive driving force. Thermodynamically this means that such gas adsorption is exothermic. Usually the enthalpy of adsorption per molecule depends on 0 because of heterogeneity (upon filling an adsorbent with adsorbate the "highest energetic" parts are covered first) and because, with increasing 0, lateral interaction also increases (this contribution may be attractive or repulsive). 

Physical adsorption of gases on solids is virtually always enthalplcally driven < 0. Entropically driven adsorption can exist but it is very unlikely... 

H.C. van Ness, Adsorption of Gases on Solids. Review of the Role of Thermodynamics in Chemistry and Physics of Interfaces, Am. Chem. Soc. Publication (1971) p. 121. (Review, also covers mixtures of gases.)... 

W.A. Steele. The Physical Adsorption of Gases on Solids in Adv. Colloid Interface Set 1 (1967) 3. (Older but not outdated review many examples.)... 

E. L. Fuller Jr. and K.A. Thompson, Chemical and Geometric Factors in Physical Adsorption/ Desorption of Gases on Solids, Langmuir, 3 (1987) 171. 

---