The surface of silica (glass and sand are essentially all silica) is polar because it is covered with silanols (Si-OH). The -OH is a polar functional group (think alcohols), and serves as both a Hydrogen-bond acceptor and donor. Thus, there are many possible interactions with alcohols, amines, acids, and carbonyl compounds. Also, silanols are acidic, having a pKa about about 4.5 (similar to many weak organic acids). So, at a pH above about 5, the surface of silica has a negative charge. At a pH above 7, the silica starts to dissolve. All of these things contribute to silica being considered “polar.”
When bare silica is using in LC, these polar interactions occur with the compounds being separated and the mobile phase. When using less polar solvents like hexane, this is call “normal phase chromatography.” When used with a polar mobile phase like water and acetonitrile, the operating mode is known as aqueous normal phase or hydrophilic liquid chromatography (HILIC).
If you react the silanols and replace them with a chain of carbon atoms (e.g., C18), the surface loses its polarity and is now hydrophobic. When used with a more polar mobile phase, this is called “reversed phase chromatography.”
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