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Silica and alumina are both polar adsorbents so the more polar components in the mixture to be separated are retained more strongly on the stationary phase and are therefore eluted from the column last. Silica is recommended for most compounds, but as it is slightly acidic, it preferentially retains basic compounds.
The solvent soaks into the layer of silica, and after it gets up an inch or so it hits your line of stuff. ... Right down near the bottom, not far up from your original streak, will be the most polar stuff, especially any basic amines – silica gel is mildly acidic, so the amines will stick to it very tightly indeed
It is a technique in which the stationary phase is solid adsorbents like silica gel and activated alumina powder and the mobile phase is a liquid. ... The principle of active compound separation depends on the activity of adsorbents and polarity of the solvent.
Silica gel and alumina are highly polar so they interact strongly with polar compounds and solvents, and weakly with nonpolar molecules. The stationary phase is loaded into the column as a slurry with the solvent and is then packed by flowing solvent through the stationary phase.