Ionic Forces

Polar compounds possessing dipoles, have no net charge. In contrast, ions possess a net charge and consequently can interact strongly with ions having an opposite charge. Ionic interactions are exploited in ion exchange chromatography where the counter ions to the ions being separated are situated in the stationary phase.

In a similar manner to polar interactions, ionic interactions are always accompanied by dispersive interactions and usually, also with polar interactions. Nevertheless, in ion exchange chromatography, the dominant forces controlling retention usually result from ionic interactions. Ionic interaction is depicted diagramatically in figure 10.

Figure 10. Ionic and Dispersive Interactions

A molecule can have many interactive sites comprised of the three basic types, dispersive, polar and ionic. Large molecules (for example biopolymers) may have hundreds of different interactive sites throughout the molecule and the interactive character of the molecule as a whole will be determined by the net effect of all the sites. If the dispersive sites dominate, the overall property of the molecule will be dispersive which the biotechnologists call "hydrophobic" or "lyophobic". If dipoles and polarizable groups dominate in the molecule, then the overall property of the molecule will be polar, which the biotechnologist call "hydrophilic" or lyophilic". These terms are not based on physical chemical argument but have evolved largely in the discipline of biology .

Hydrophobic and Hydrophilic Interactions

The term "hydrophobic interaction" implies some form of molecular repulsion, which, of course, outside the Van der Waals radii of a molecule, is impossible. The term "hydrophobic force", literally meaning "fear of water" force, is an alternative to the well-established term, dispersive force. The term may have been provoked by the immiscibility of a dispersive solvent such as n-heptane with a very polar solvent such as water.

n-heptane and water are immiscible, not because water molecules repel n-heptane molecules, they are immiscible because the forces between two n-heptane molecules and the forces between two water molecules are much greater than the forces between a n-heptane molecule and a water molecule. Thus, water molecules and n-heptane molecules interact very much more strongly with themselves than with each other.