Liquid Chromatography - Molecular Interactions in the Mobile Phase 1

Molecular Interactions in the Mobile Phase

The 'elutive' capacity of the mobile phase (as opposed to the 'retentive' capacity of the stationary phase) depends on the strength of the different interactions that can take place in the mobile phase and the probability of a particular interaction occurring. Purnel and Laub (34) experimentally demonstrated in GC, that for a stationary phase consisting of a pair of non-associating liquids, the distribution coefficient of a solute was linearly related to the volume fraction of either liquid. This relationship indicated that the volume fraction of a solvent in a liquid mixture determined the probability of interaction.

Much the same as the partial pressure of gas determines the probability of collision. This relationship was challenged by a number of worker who also demonstrated that for certain solvent pairs this linear relationship broke down. However, it was also shown (35,36) that the nonlinear relationship occurred when there was strong association between the components of the mixture which resulted in a ternary mixture containing the two components and the associate of the two components the concentration of which depended on the equilibrium constant and the experimental conditions.

Thus, for a mobile phase consisting of a binary mixture of solvents, as the retention volume will be inversely proportion to the elutive capacity of the mobile phase it will be also inversely proportional to the volume fraction of either component providing there is no strong association between the components. This was experimentally demonstrated by Katz et al. (35) who employed a liquid/liquid distribution system using water and a series of immiscible solvent mixtures as the two phases and measured the absolute distribution coefficient of a solute for different mixtures. The solute they used was n-pentanol and the immiscible solvent consisted of mixtures of n-heptane and chloroheptane, n-heptane and toluene and n-heptane and heptyl acetate. The two phase system was thermostatted at 25oC and, after equilibrium had been established, the concentration of solute in the two phases was determined by GC analysis. The results they obtained are shown in figure 46. It is seen that linear relationships between solvent composition and distribution coefficient was obtained for all three solvent mixtures simulating the results that Purnell and Laub obtained in their gas chromatography experiments.