Silica - analyte interactions

Physical adsorption usually takes place through the surface attraction and repulsion forces:

- Dispersive (hydrophobic) interactions.
- London-type dispersion forces from induced dipole - induced dipole attractions;
- Interactions between induced or permanent dipoles of molecules and the electric field of the adsorbent surface;
- Charge transfer between the adsorbed molecule and the adsorbent surface (non-bonding resonance state).

It is important to note, that in HPLC the surface interactions are always competitive. Adsorbent surface is in the permanent contact with the eluent. Eluent molecules are interact with the surface adsorption sites according to their structure, polarity and ionisability. Analyte molecules will adsorb onto the silica surface (and so retained) only if their interaction is stronger than that of eluent molecules.

Each HPLC mode possess its specific type of interactions.

Reversed-phase chromatography mainly utilize non-specific hydrophobic (dispersive) interactions. Some analytes might have slight specificity due to the interactions with residual silanols or with active groups of attached ligands.

Normal phase mode mainly utilize polar interactions (dipole - dipole, dipole - induced dipole).

Ion-exchange chromatography employ an ionic interactions of ionizible analytes with ion-exchange adsorption centers on the adsorbent surface.

Chemical modification of the silica surface

Chemical modification of the silica surface relates to all processes that lead to a change in the chemical composition of the surface. Two basic types of chemical modification can be distinguished:

- physical treatment (thermal or hydrothermal) leads to the change in the concentration and ratio of silanol and siloxane groups on the surface.
- chemical treatment; chemically bonded organic ligands significantly change adsorption properties of the silica surface.