Influence of Experimental Conditions on Equilibrium Isotherms

The equilibrium isotherms depend on the parameters that characterize the physical state of the system and the chemical potential of the compounds studied in this system selected, i.e., its temperature and pressure, the chemical compositions of the mobile and the solid phases. The last of these parameters is far more difficult to investigate than that of the other three. Suffice it to say that important variations of the isotherm parameters have been observed for different brands of Cig-bonded silica. However, the best isotherm model accormting for the adsorption data measured on these different brands remained the same [61] while the colmnn to column reproducibility of the data was excellent for at least one brand and probably for several others [11,121,122], with relative standard deviations for the parameters being of the order of a few percent [123]. 

The influence of the pressure on the adsorption equilibrixun constant derives from the classical fundamental relationship of thermodynamics for a system at constant temperature and pressure 

In RPLC, the influence of pressure on the chromatographic behavior is related to the hydrophobic interactions involved in the retention mechanism and to the change upon adsorption in the numbers of acetonitrile and water molecules in the solvent shells of the protein molecule and of the bonded layer. The importance of the changes in the retention factor and the saturation capacity with a change in the average column pressure will thus depend on the retention mode used and will vary with the hydrophobicity of the molecule [128]. In RPLC, it is larger with polymeric than with monomeric bonded phases [126]. 

The relationship between the equilibrium constant and temperature is given by van t Hoff equation 

The strong dependence of the isotherm parameters on the composition of the mobile phase conJfirms that adsorption of small molecules on chemically bonded Cis silica is more complex than is usually beUeved. Most prior work has been based on data acquired under linear conditions, that is, at infinite dilution. Chro-matographers have long ignored the way in which consideration of the whole equilibrium isotherm and its modeling may inform on the detail of the retention mechanisms involved [131]. 

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