Total and Excess Adsorption

Adsorbent nonpermeability is an important condition, since it essentially states that all processes occurs in the liquid phase. Since adsorption is related to the adsorbent surface, it is possible to consider the analyte distribution between the whole liquid phase and the surface. Using surface concentrations and the Gibbs concept of excess adsorption [20], it is possible to describe the adsorption from binary mixtures without the definition of adsorbed phase volume. 

Adsorption is the process of analyte accumulation on the surface under the influence of the surface forces. Determination of the total amount of the analyte adsorbed on the surface requires the definition of the volume where this accumulation is observed, usually called the adsorbed layer volume (U ). In chromatographic systems, adsorbents have large surface area, and even very small variation in the adsorbed layer thickness lead to a significant variation on the adsorbed layer volume. There is no uniform approach to the definition of this volume or adsorbed layer thickness in the literature [14,21,22]. 

Another approach to the expression of the analyte adsorbed on the surface is based on the consideration of the surface specific quantity which has been accumulated on the surface in excess to the equilibrium concentration of the same analyte in bulk solution. This allows avoiding an introduction of any model of adsorbed layer as shown later, it is a fruitful approach for the description of HPLC retention. 

Equation (2-44) allows for the calculation of the surface-specific excessively adsorbed amount from the original analyte concentration (before adsorption) and the equilibrium analyte concentration (after adsorption equilibrium is established). 

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