Bonded Phases - Ionic Interactions
In order to consider the different types of interactions that take place on the surface of a reversed phase, it will be necessary to extend the discussion on intermolecular forces. There are largely four types of interactive forces that will be germane to LC and they are ionic forces, polar forces, dispersive forces and chemical forces. Some include a fifth type of molecular interaction that has been given the term, hydrogen bonding, However, from a physical chemical point of view, hydrogen bonding is merely an example of very strong polar forces.
Ionic interactions stem from permanent electrical charges that reside on molecules, e.g. organic acids, bases and salts. Ionic interactions are utilized in ion exchange chromatography. An ion exchange group can be attached to the silica gel surface but an ion exchange agent can often be adsorbed from a solution in the mobile phase onto the surface of a reverse phase and act as an adsorbed ion exchanger.
Polar interactions arise from permanent or induced dipoles on molecules and do not result from a net charge on the molecule as with ionic interactions. Alcohols, ketones etc. are examples of polar substances having permanent dipoles. Aromatic hydrocarbons such as benzene or toluene are examples of polarizable substances with no permanent dipoles.
A polarizable substance is one that itself can be polarized to provide a pair of opposite charges on the molecule by its proximity to a molecule with a permanent dipole and consequently, electrical interaction can occur between the induced charges and the permanent charges. Silica gel exhibits strong polar interactions with solutes and solvents as a result of the highly polar surface silanol groups that have very strong permanent dipoles.
Dispersive interactions are not easy to describe and though electrical in nature, they result from charge fluctuations rather than permanent electric charges or dipoles on the molecule. Examples of purely dispersive interactions are the molecular forces that occur between hydrocarbon molecules. n-Heptane is not a gas at ambient temperatures due to the collective result of all the dispersive interactions that hold the molecules together as a liquid. The interactions of solutes or solvents with a reverse phase (aside from any silanol groups that may be present) are exclusively dispersive in nature.