The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Mono-layer Adsorption > Page 44
Above a concentration of 40%v/v, the layer of chloroform covering the surface of the silica is almost complete and the properties of the interactive surface is no longer changing significantly with the mobile phase composition. Alkane mixtures with chlorinated hydrocarbons, aromatic hydrocarbons and weakly polar solvents such as the alkyl ethers all appeared to give isotherms of the type shown in figure 18.
The same process of solvent adsorption occurs with reverse phases only dispersive substance are held strongly on the surface as opposed to polar materials on silica gel. The adsorption isotherms are Langmuir in form and a monolayer coating of the solvent is deposited on the hydrocarbon chains bonded to the surface. The adsorption isotherms of acetic and propionic acid are shown in figure 19. It is seen that the acids exhibit the same type of curve as chloroform on silica gel. It is also apparent that when the solvent mixture contains about 10 %w/v of acid the reverse phase is covered with carboxyl groups from the adsorbed acid.
Figure 19. Adsorption Isotherms of Acetic and Propionic Acid on a Reveres Phase