Ion Pairs Principles

In order to distribute the solute molecules in a nonpolar environment, it has to be uncharged. However, if an ion of opposite charge is present in enough concentration the two ions may be distributed as a pair in the same way. An ion being double charged may be distributed together with two ions of single charge or one double charged, the only prerequisite being electroneutrality of the pair (Figure 1).

The equilibrium for a simple 1: 1 ion pair may be given as:

and is characterized by an extraction constant:

The nonpolar environment may be a liquid (mobile or stationary), a surface or a micelle in the liquid. For simplicity the principle for a liquid-liquid chromato-graphic system with aqueous mobile phase will be presented. Here the retention factor for the solute, kHB#, is equal to:

where DHB# is the distribution coefficient of the solute between the aqueous mobile phase and the organic stationary phase and Vs and Vm are the volumes of the two phases.

The distribution coefficient is defined as:

i.e., the ratio of the total concentration of solute in organic phase over the total concentration in aqueous phase. For the ion pair, DHb+ may be expressed as:

Thus, the retention factor will depend on the extraction constant of the ion pair and the concentration of counterion in the aqueous mobile phase. The magnitude of the extraction constants depends on the hy-drophobicity of the solute and the counterion, the

[1] Figure 1 Distribution of charged compounds to a nonpolar phase together with a counterion.

kind of interaction forces between the two ions, and the physiochemical properties of the organic phase.

For protolytic solutes, the retention will also depend on the pH of the aqueous phase, as the charge of the solute is pH dependent. At a pH where an amine is uncharged, its distribution coefficient is:

where KD(B) is the distribution constant of B.

Considering a more general expression for the distribution coefficient for a basic compound over the entire pH range in the presence of a counterion, the equation is as follows:

where K(HB# ) is the acid dissociation constant of HB + defined as:

Equation [6] means that the retention of a solute may be made up of two parts: the retention as an ion pair and the retention in an uncharged form:

k = k uncharged

In chromatographic systems with solid stationary phases, the retention involves distribution with a solid surface. The limited capacity of the surface and the possibility to have competition for the limited adsorption sites have to be considered in the equation but the models otherwise resemble each other to a large extent.

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