Lipophilic counter ions

Type and Concentration of Lipophilic Counter Ions in the Mobile Phase... 

Most often Pj values are 3 to 5 decades smaller than the corresponding values the differences can be larger, e.g. for salicylic acid (19) which contains two acidic groups (Figure 25) [463], but also much smaller, e.g. for chenodesoxycholic acid, which forms micelles [462], or in the presence of lipophilic counter ions, e.g. for salicylic acid in the presence of tetrabutyl-ammonium bromide (20) (Figure 26) [463]. Lipophilicity and polarizability of the counterion have a significant influence on the partitioning of ion pairs e.g. [465 — 468]). 

Ion pair or ion association TLC has been carried out on C-18 layers in a mobile phase containing a lipophilic counter-ion salt and buffered to a pH value at which the analyte is ionized with a charge opposite to the counterion. In some cases, the ion pair reagent was precoated onto the stationary phase. Ion pair TLC was used for the resolution of alkaloids using di(2-ethylhexyl)orthophosphoric acid and other organic and inorganic acid reagents (135). 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

Takacs-Novak, K. Szasz, G., Ion-pair partition of quaternary ammonium drugs The influence of counter ions of different lipophilicity, size, and flexibility, Pharm. Res. 16, 1633-1638 (1999). 

There are two main factors that influence the selectivity of a sensor limits in discrimination of an interfering ion and upper limits in stability constant of an analyte-ionophore complex. While an ideal ionophore does not form complexes with interfering ions, too strong complexation with the primary ion leads to a massive extraction of analyte into membrane phase coupled with a coextraction of sample counter-ions, known as Donnan exclusion failure. In such cases, at high activities and lipophilicities of sample electrolytes, fli(org) increases and a breakdown of membrane permselectivity prevents the Nemst equation to hold. 

Figure 5.21 — Mechanisms involving an optical change in flow-through metal ionophore-based sensors. The ionophore (I), indicator (In) and hydrophobic counter-ion (X) are in the lipophilic phase (shaded area), which can be a solid (1), a layer (2) or a membrane (3-6). The analyte (A) can be a cation (1-4), an anion (6) or a neutral species (5). The dotted arrow indicates the origin of the optical change, which is always related to the indicator. For details, see text.

The mechanism of IPC was hotly debated for years and continues as a matter of debate. The retention model of Bidlingmeyer [15,16] is more comprehensive than the ion-pair and dynamic ion exchange models. Lipophilic IPRs, due to their adsorbo-philic nature, dynamically adsorb onto the alkyl-bonded apolar surface of the stationary phase, forming a primary charged ion layer and, together with counter ions... 

The octanol-water partition coefficient for surfactants can not be determined using the shake-flask or slow stirring method because of the formation of emulsions. In addition, the surfactant molecules will exist in the water phase almost exclusively as ions, whereas they will have to pair with a counter-ion in order to be dissolved in octanol. Therefore, experimental determination of K w does not characterize the partition of ionic surfactants (Tolls, 1998). On the other hand, it has been shown that the bioconcentration of anionic and non-ionic surfactants increases with increasing lipophilicity (Tolls, 1998). Tolls (1998) showed that for some surfactants, an estimated log Kow value using LOGKOW could represent the bioaccumulation potential however, for other surfactants some correction to the estimated log Kow value using the method of Roberts (1989) was required. These results illustrate that the quality of the relationship between log Kow estimates and bioconcentration depends on the class and specific type of surfactants involved. Therefore, the classification of the bioconcentration potential based on log Kow values should be used with caution. 

The addition of a counter-ion to the acid or base results in the formation of an ion pair. This counter-ion is provided with more lipophilic properties, thus influencing the chromatographic behaviour in a more defined manner. 

Some ionised drug molecules can traverse the lipophilic gut membrane by combining with an ion of opposite charge (a counter ion) to form an ion pair. The ion pair, although composed of two ionic species, behaves as a neutral molecule with a high partition coefficient and can cross biomembranes effectively Quaternary ammonium compounds, which are charged at all values of pH, may be absorbed into the body in this way ... 

In the presence of counter ions, lyso-phospholipid, phospholipids or other lipophilic molecules, the CMC of mixed bile salt micelles is reduced (Carey,... 

The separation between the internal and external solutions of the specific electrode is achieved using a porous and hydrophobic disc (of 3 mm diameter). This disc is saturated with an organic solvent that is immiscible with water on either side and contains also an ion-carrier called an ionophore (Figure 19.4). Hence, the membrane behaves as a lipophile immobilized liquid. The counter ion is a molecule... 

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