Different bonded phases for reversed-phase separations exhibit different combinations of possible solute-stationary phase interactions. For bonded phase, the variety of separation mechanisms will influence the overall selectivity. This predominance depend on the properties of the analyte, and the applied chromatographic conditions. The most significant interactions between solute and stationary phases that contribute to column selectivity are as below.

• Hydrophobic interaction is a dominant retention mechanism for all reversed-phase columns and the most significant interaction of alkyl phases. For a given phase, retention time is proportional to the hydrophobicity of the molecule.

• Hydrogen-bonding capacity of a phase generally involves the interaction of a basic solute group with an acidic group within the stationary phase, possibly from unbonded silanol groups.

• π-π interactions are observed between an aromatic or unsaturated solute and an aromatic stationary phase.

• Steric selectivity is a measure of the accessibility of solutes to the stationary phase. Larger solute molecules may be excluded from the stationary phase.

• Dipole-dipole interactions, between a dipolar solute group and a dipolar group in the stationary phase are most important in the case of cyano and PFP bonded columns.

• Cation-exchange interactions may occur between a cationic solute and an ionised silanol within the stationary phase.

A general summary of these interactions for typical reversed-phase bonded phases is given below. Different interactions may be dominant for different analytes and interaction strengths will vary amongst different manufacturers' bonded phases. However, this table gives a useful indication of likely interaction strengths.

Source Galak