Physical adsorption energetics

This chapter introduces the topic of adsorption, giving examples of both physical adsorption and chemical adsorption, and discusses the similarities and differences between the two. The standard nomenclature of surface science is given from within this context. The energetics of adsorption are explained in terms of the enthalpies of bond formation A/Tadsr Next, isotherms are discussed. 

Physical adsorption, or van der Waals adsorption, results from a relatively weak interaction between the solid and the gas. The forces responsible for adsorption are dispersion forces (characterized by London see 3.3.1) and/or electrostatic forces (Coulombic see 3.3.2) if either the gas or the solid is polar in nature. Physical adsorption is reversible hence all the gas adsorbed by physical adsorption can be desorbed by evacuation at the same temperature. Chemical adsorption is a result of a more energetic interaction between the solid and the gas than that of physical adsorption. Reversal of chemical adsorption using a vacuum requires elevated temperature, and even that may not be sufficient. Physical adsorption, being of more interest in gas-solid flows, is the focus of the following sections. 

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). 

Bottani, E.J. and Tascon, J.M.D. (2004). Energetics of physical adsorption of gases and vapors on carbons. Chem. Phys. Carbon, 29, 209-423. 

Study of chemical reactions is a simple matter, i.e., to determine whether a bond exists or not, since covalent bonds have rather fixed geometries. Adhesion begins with physical adsorption, but the process becomes complicated by ever-changing molecular geometries. Nevertheless, after a molecular mechanics calculation, we have the conformers of oligolignols in energetically favorable positions to interact with the cellulose macromolecular model. 

Sokolowska, Z. (1989). On the physical adsorption on geometrically and energetically heterogeneous solid surface. Z. Phys. Chem., (Leipzig), 270, 1113-1120. 

L Energetic Considerations Physical Adsorption versns Chemisorption... 

FIGURE 9.4. Although physical adsorption and chemisorption may be energetically quite different, the stronger chemisorption phenomenon must be preceeded by a physical adsorption process. 

Diamond Synthesized at Low Pressure, S. N. Marinkovic Energetics of Physical Adsorption of Gases and Vapors on Carbons,... 

Jaroniec, M., and A. W. Marczewski. 1984. Physical adsorption of gases on energetically heterogeneons solids I. Generalized Langmuir equation and its energy distribution. Monatshefte fur Chemie-Chemical Monthly 115, no. 8-9 997-1012. doi 10.1007/ BF00798768. 

Physical adsorption of inert gases on metals is studied at temperatures between 10 and 78 K. At too high a temperature the adsorbed layer boils off the surface. At too low a temperature an adsorbed gas molecule does not migrate on the surface after striking it the surface structure is random and does not anneal to reflect the energetics of adsorbate-substrate interaction. In the temperature range for which surface equilibration occurs, the structure of the surface layer is independent of the inert gas adsorbed and of the metal surface exposed. Whether Xe is adsorbed on graphite,... 

The adsorption isotherm results from the controlled physical adsorption of a gas or vapor or solute from a solution into an activated carbon. This process of physical adsorption is the way in which the adsorptive (gas, vapor or solute) enters into the porosity. The close proximity of carbon atoms to each other, as they constitute the boundary or surface of the porosity, results in these gas adsorbate molecules being retained within the porosity. The retention is the result of enhanced van der Waals forces (dispersion forces) within the porosity. This process of physical adsorption is exceedingly dynamic. Energetically, an adsorbed state is more stable than the gaseous state of the adsorptive at 273 K and 0.1 MPa of pressure. 

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