Liophilic ions acetonitrile

The liophilic ions present in the mobile phase at given concentration Cup will be distributed between the acetonitrile adsorbed layer and mobile phase. This distribution could be represented by the distribution constant, f iip(cMeCN), which is the function of the acetonitrile content. 

For acetonitrUe/water systems it was found that acetonitrile forms thick adsorbed layer on the surface of hydrophobic bonded phase, while methanol adsorption from water formed a classical monomolecular adsorbed layer [166]. The thick adsorbed layer of acetonitrile provides a suitable media for the adsorption of liophilic ions on the stationary phase adding an electrostatic component to the retention mechanism, while monomolecular adsorption of methanol should not significantly affect adsorption of ions. 

The suggested phenomenological model describes the retention of PFe ions on different reversed-phase columns very well. Average deviation of calculated values from experimentally measured values is on the level of 1%, which confirms that indeed a superposition of several processes govern the retention of liophilic ions in acetonitrile/water systems. Experimental values along with the theoretical curves are shown in Figure 4-53. 

Hexafluorophosphate retention dependencies similar to the one shown in Figure 4-56 [169] were observed on different stationary phases, but only when acetonitrile was used as an organic eluent component. If acetonitrile was substituted with methanol, the effect of the increase of PFe retention with the increase of organic concentration disappears. This indicates that liophilic ions show strong dispersive interactions with acetonitrile and have little affinity to the hydrophobic adsorbent surface—as opposed to the amphiphilic ions, which... 

Figure 4-54. Schematic of the retention mechanism of basic analyte on reversed-phase material in water/acetonitrile elnent in the presence of liophilic ions (PFe ). See color plate.

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