Liophilic ions

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. 

The nature of the analyte interactions with liophilic ions could be electrostatic attraction, ion association, or dispersive-type interactions. Most probably all mentioned types are present. Ion association is essentially the same as an ion-pairing used in a general form of time-dependent interionic formation with the average lifetime on the level of 10 sec in water-organic solution with dielectric constant between 30 and 40. With increase of the water content in the mobile phase, the dielectric constant increases and approaches 80 (water) this decrease the lifetime of ion-associated complexes to approximately 10 sec, which is still about four orders of magnitude longer than average molecular vibration time. 

We only assume the existence of specific interactions of positively charged basic analyte and liophilic ions. In our system the following equilibria coexist ... 

In the contrast to the irreversible adsorption of amphiphilic ions on the reversed-phase surface, the liophiUc ions shows relatively weak interactions with the alkyl chains of the bonded phase. Liophilic means oil-loving. These liophilic ions are usually small inorganic ions and they possess an important ability for dispersive type interactions. They are (a) characterized by significant delocalization of the charge, (b) primarily symmetrical, (c) usually spherical in shape, and (d) absence in surfactant properties. 

The adsorption of amphiphilic ions was experimentally confirmed about 30 years ago, while the actual interaction of the small liophilic ions with hydrophobic stationary phase in reversed-phase conditions was found only recently [165]. 

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.

Overall, liophilic ions (usually small ions capable for dispersive interactions) provide a useful means for selective alteration of the retention of basic analytes. Influence of these ions on the column properties is fully reversible, and equilibration requires minimal time (usually less than an hour, or about 10 to 20 column volumes). On the other hand, the mechanism of their effect is very complex and is dependent on the type of organic modifier used and on the concentration applied. Theoretical description and mathematical modeling of this process is a subject for further studies. 

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