The Mechanism of Chromatographic Retention - Chromatographic Interactions > Page 2

The net effect of solute-phase interaction on retention is determined by both the magnitude of the interactive forces and the probability of them occurring. It will be seen later that this is important when mixed phases are used, where the probability of interaction with one particular component of the phase will depend on its relative concentration. In fact, the effective use of mixed phases to control retention and, thus, ensure resolution depends on manipulating the composition of the mixture to adjust the probability of specific solute/solvent interactions. This will be discussed in detail when mixed phases are considered.

Solute/phase interactions result from three basic types of intermolecular force, all of which are electrical in nature. Although it is theoretically possible that magnetic and even gravimetric forces may be also present, they will have no significance compared with those of electrical origin. The three types of molecular interactive force are dispersive, polar and ionic giving rise to dispersive interactions, polar interactions and ionic interactions. Polar forces have been further divided into sub groups ranging from 'strong dipole-dipole interactions' (hydrogen bonding) to 'weak dipole-dipole interactions ((p)-(p) interactions). This type of division is questionably useful as it tends to 'confuse' more than 'explain' when dealing with chromatographic retention. Division into the two groups, dipole-dipole interactions and dipole-induced dipole interactions, however, is appropriate, as it describes two physically different types of polar interaction that is very pertinent to chromatographic retention. However, the arbitrary division of polar forces into numerous groups of different strength has not proved to be particularly helpful. No significant attempt has been made to partition dispersive or ionic interactions and such division is certainly not necessary when dealing with chromatographic retention. Most molecular interactions will consist of a mixture of at least two different types of interaction; the only type of interaction that can occur in isolation is dispersive. Consequently, dispersive interactions will be the first to be considered.