Adsorption and interaction forces of reagents

Adsorption can be considered to be a process of selective partitioning of the adsorbent species to the interface in preference to the bulk and is the result of interactions of such species with the surface species on the solid. The interactions responsible for adsorption can be either physical or chemical in nature. Adsorption in solution is a more complicated phenomenon than that in gas owing to the competition for adsorption sites by the solvent species as well. 

Equilibrium Concentration of Solute in Solution Fig. 4.1. Classification of adsorption isotherm (after Giles et al., 1960). 

Adsorption isotherms are commonly used to describe adsorption processes and these represent a functional relationship between the amount adsorbed and the activity of the adsorbate at a constant temperature. The shape of the adsorption isotherm gives useful information regarding the mechanisms of the adsorption process. A classification of adsorption phenomena based on the shape of the isotherms is given by Giles et al. (1960) as shown in Fig. 4.1. Mainly four major classes of isotherms have been identified based on the initial part of the isotherms (a) S-type isotherm with a convex shaped initial portion where adsorption rate increases with adsorption density and is indicative of vertical orientation of adsorbed molecules at the surface (b) L-type (Langmuir type) isotherm, characterized by a concave initial region, represents systems in which the solvent is relatively inert and adsorption rate decreases with adsorption density. This is usually indicative of molecules adsorbed flat on the surface or ions vertically adsorbed with strong intermolecular attraction. 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

The adsorption density, which is the amount of adsorbate removed from the solution to the interface can be mathematically expressed as  

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First, it is important to recognize that the chemical character of a surface, even for those faces that are not photosensitive, can change the rates and reaction paths of chemical reactions taking place thereon [74]. The effect of the surface on the diffusional motion of adsorbates, whether they are stable species or reactive intermediates, depends sensitively on the adsorptive forces that bind the reagent to the reaction site. When a substrate of interest is adsorbed, surface confinement changes the electronic distribution in the molecule and, hence, both the accessible trajectories for interaction with another reagent and the molecule s electron density. 

In this chapter, the interaction forces responsible for adsorption at solid-Uquid interfaces and the microstructure of the adsorbed layer that influences the flotation and flocculation processes are discussed. Surface and bulk inta actions between the flotation reagents and dissolved mineral species and their solution equilibria are described using multi-pronged experimental and diagrammatic schemes. Electrochemical equilibria of mineral-flotation agent are also discussed. 

In principle, the diffusion steps (a) and (e) could be studied through molecular dynamics simulations as long as rehable forces fields are available to describe the zeolite structure and its interaction with the substrates. Also, if the adsorption takes place without charge transfer between the reagents/products and the zeolite, steps (b) and (d) could also be investigated either by molecular dynamics or Monte Carlo simulations. Step (c) however can only be followed by quantum mechanical techniques because the available force fields cannot yet describe the breaking and formation of chemical bonds. 

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