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.

Application

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