Eluotropic Series of Solvents for TLC

Abstract

The easiest way to vary the relative adsorption of the sample (and its Rf value, migration rate, respectively) is to change the solvent: strong (polar) eluents decrease adsorption, and weak (no polar) eluents increase it. When benzene, for instance, is used as solvent on silica gels or alumina layers, the ethers and esters are found on top of the chromatogram (with high Rf values), ketones and aldehydes are approximately in the center (medium Rf values), and the alcohols are below them (low Rf values), whereas the acids remain at the starting point (Rf 1⁄4 0). Thus the separation sequence follows the polarities of the compounds. The dielectric constant may be taken as an indication of solvent polarity, but the interfacial tension between solvents and polar adsorbents, approximated by the interfacial tension between the solvent and water, has been suggested as a fundamental basis for correlating solvent strength. The physical factors that determine solvent strength in a given adsorption system have long been understood in general terms. Solvent strength can be interpreted in terms of the following basic contributions: 1) interactions between solvent molecules and a sample molecule in solution; 2) interactions between solvent molecules and a sample molecule in the adsorbed phase; and 3) interactions between an adsorbed solvent molecule and the adsorbent.