Why are silica and alumina used as stationary phases in liquid-solid chromatography?

In liquid-solid chromatography, silica and alumina are utilized as stationary phases because they function as polar adsorbents.12

These materials are effective because they possess the ability to retain polar compounds through the mechanism of adsorption. In this process, separation is based on the chemical properties of different classes of compounds, such as distinguishing between amines, alcohols, esters, and acids.21

Because silica and alumina are polar, they are typically paired with a nonpolar or slightly polar solvent as the mobile phase. This configuration is known as normal-phase chromatography, where the polar stationary phase interacts with the sample components, and the mobile phase facilitates their movement through the column. If a highly polar mobile phase were used instead, the solutes would be swept from the bed too rapidly to achieve effective separation.21