Solvent adsorption Gibbs free energy

Therefore, pure nanoporous carbons exhibit low values when used for SPE of explosives (Figure 5, NA)9 and narcotics (Figure 4).13 The polarity and molecular size of solutes and solvents affect adsorption/desorption, as well as the distribution of different liquids in different pores, because these parameters govern changes in the Gibbs free energy in the adsorption layers. For instance,... 

A = The thermodynamic change associated with converting a concentration of solvent in the bulk vapor state (not adsorbed) to the adsorbed state. This is called the Gibbs free energy of adsorption and is computed as... 

Assumption 5 In the definition of the isotherm, the convention is adopted that the solvent (if pure) or the weak solvent (in a mixed mobile phase) is not adsorbed [8]. Riedo and Kov ts [9] have given a detailed discussion of this problem. They have shown that the retention in liquid-solid i.e., adsorption) chromatography can best be described in terms of the Gibbs excess free energy of adsorption. But it is impossible to define the surface concentration of an adsorbate without defining the interface between the adsorbed layer and the bulk solvent. This in turn requires a convention regarding the adsorption equilibrium [8,9]. The most convenient convention for liquid chromatography is to decide that the mobile phase (if pure) or the weak solvent (if the mobile phase is a mixture) is not adsorbed [8]. Then, the mass balance of the weak solvent disappears. If the additive is not adsorbed itself or is weakly adsorbed, its mass balance may be omitted [30]. 

The solute and solvent molecules present in any solution have different intensities of attractive force fields, and also have different molecular volumes and shapes. A concentration difference between the surface region and the bulk solution occurs because the molecules that have the greater fields of force tend to pass into the interior, and those with the smaller force fields remain at the surface. The Gibbs surface layer of a solution is more concentrated in the constituents that have smaller attractive force fields, and thus whose intrinsic surface free energy is smaller than the interior. As we stated in Section 3.3, this concentration difference of one constituent of a solution at the surface is termed adsorption. In qualitative terms, if the solution has a smaller surface tension than its pure solvent, the solute is concentrated in the surface layer indicating a positive adsorption according to... 

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