With the increase in the concentration of organic solvent, water interacts more strongly with the polar surface of stationary phase. Hence, organic solvent cannot interact with the residual silanols and are left unrecovered thereby causing water molecules getting adsorbed on them. The adsorption of water increases as the proportion of water in the mobile phase decreases.[17] When the concentration of water in the mobile phase is lower than 20%, water adsorption can be multilayer in nature, and it can create an excess of adsorbed water in comparison with the concentration of water in the eluent. HILIC separation involves not only partition but also hydrogen donor interactions between neutral species, weak electrostatic interactions when the high organic solvents are used and dipole-dipole interactions …show more content…
These groups cannot be removed due to steric effects of the bonded ligands are hence partially ionized. These residual silanols may influence the separation of polar analytes because of hydrogen bond interactions and dipole-dipole interactions between their basic groups and ionized residues. Water molecules that are adsorbed on the stationary phase suppresses the ionization and thereby leads to negative charge on the silica gel. This negative layer causes negative electric field in the mobile phase. The attraction of positive charges create an electrical double layer.
The extent to which each mechanism dominates is dependent on the actual type of stationary phase used and the buffer conditions, including the level and type of organic solvent, the type and concentration of salt, and the pH