Solvent Strength in Liquid-Solid Chromatography

Knowing that the solvent and the solute compete for active sites of the adsorbent, it is easily understood that the more the mobile phase interacts with the adsorbent the less a solute molecule is retained. Therefore, the major factor determining product retention in adsorption chromatography is the relative polarity of the solvent compared with the solute. 

Values in the table only serve as a guide because the relative polarity can change, depending on the type of sample and adsorbent. 

An eluotropic series is a tool to adjust the retention of sample components. If the retention of a product has to be reduced, a solvent of higher ranking in the eluotropic series is chosen. Due to increased competition between product and solvent molecules for the active sites on the adsorbent, the retention factor of the product will automatically decrease. The same holds true for the opposite situation. 

According to Snyder (1968) it is possible to calculate the solvent strength of binary mixtures using Equation 3.8. 

Hb describes the molecular cross-section occupied on the adsorbent surface by the solvent molecule b in units of 0.085 nm (which is 1 /6 of the area of an adsorbed benzene molecule, corresponding to the effective area of an aromatic carbon atom on the adsorbent surface), while the parameter a describes the activity parameter of the adsorbent surface. The adsorbent activity parameter is a measure of the adsorbent s ability to interact with adjacent molecules of solute or solvent and is constant for a given adsorbent. For silica gel a value of 0.57 is given by Poole (2003). The molar solvent ratio is calculated by Equation 3.9 with the molar volumes of the molecules (Equation 3.10). 

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