Ions, hydrophobic

FIG. 9 Proposed model of the transfer of (a) a hydrophilic ion and (b) a hydrophobic ion across the O/W interface. (From Ref 49. Copyright 1998 American Chemical Society.)... 

Similarly to the induction or the inhibition of the current oscillation at a biomembrane with a sodium channel, the current oscillation at the liquid membrane (without any channel proteins) caused by the transfer of Na can be induced by acetylcholine ion, Ach, or inhibited by such rather hydrophobic ions as alkylammonium and glutamate ions. 

The current oscillation observed by applying AFwi-wi = —0-48 V was inhibited when one of the rather hydrophobic ions such as TEA, TBA+ or glutamate ion was added to Wl to more than 10 BSA, was added to W2. 

Flewelling, R. F., Hubbell, W. L., Hydrophobic ion interactions with membranes. Thermodynamic analysis of tetraphenylphosphonium binding to vesicles, Biophys. J. 1986, 49, 531— 540. 

To date, there have been only very few studies on the sorption of environmentally relevant HIOCs to membranes. In contrast, there exists a large body of literature on ionogenic drug partitioning (see, e.g. [3-5]), and the partitioning of hydrophobic ions [6], and the use of fluorescent membrane probes [7], This literature review attempts to relate the findings from these related scientific fields to environmental chemistry. 

This section describes the nucleophilic reactions—acyl transfer reactions mostly—promoted by micelles and polysoaps. The nucleophiles are imidazoles, oxyanions and thiols, the same catalytic groups found ubiquitously in the enzyme active site. These nucleophiles are remarkably activated in the anionic form in the presence of cationic micelles and cationic polysoaps. These results are explained by the concept of the hydrophobic ion pair (Kunitake et al.,... 

In previous sections, numerous examples of anion activation by cationic micelles and polysoaps were presented. The extent of rate augmentation— 102—lO -fold—cannot be rationalized in terms of concentration effects alone. We believe that these observations are explained most reasonably by the concept of the hydrophobic ion pair (Kunitake et al., 1976a). According to this concept, anionic reagents are activated probably due to desolvation when they form ion pairs with an ammonium moiety in a hydrophobic microenvironment. The activation of anionic species in the cationic micellar phase... 

The ease of formation of hydrophobic ion pairs, and hence the rate acceleration, will be determined by the hydrophobic and electrostatic interactions between the anionic and cationic species. Lapinte and Viout (1974) found that the nucleophilic order OH- > CN > C6H50- in water was completely reversed in CTAB micelles hydrophobic phenoxide ion is activated better by the micelle. The micellar binding of phenols and phenoxides was determined by Bunton and Sepulveda (1979). Similarly, hydrophobic hydroxamates are activated much better than their hydrophilic counterparts. In the same vein, the extent of activation correlates approximately with the hydrophobic nature of aqueous aggregates as estimated by Amax of methyl orange (Table 7) and of picrate ion (Bougoin et al., 1975 Shinkai et al., 1978f Table 5). 

The formation of hydrophobic ion pairs may be envisaged as in Fig. 6. This may be considered as a microscopic counterpart of phase transfer catalysis. 

Fig. 6 Formation of a hydrophobic ion pair a comparison with phase transfer catalysis...

As an extension of the research on hydrophobic ion pairs, ion pairs such as tetraethylammonium hydroxamate [63] have been prepared and their nucleophilic reactivity estimated in organic solvents (Shinkai and Kunitake, 1976d Shinkai et al., 1979a). The ion pair showed very high nucleophilicity toward... 

As mentioned repeatedly, a variety of anionic reagents are highly activated in the hydrophobic microenvironment of cationic micelles and polysoaps. The range of anionic reagents studied in the past includes imidazole, hydroxide, thiolates, oximates, hydroxamates, carboxylates and carbanions. Polyanionic coenzymes are similarly activated. These results can be interpreted in a unified way by the concept of hydrophobic ion pairs, and the major source of activation seems to be concentration and desolvation of the anionic reagent in the... 

Also attracting growing attention is the phase coexistence curve characteristic of ionic systems it plays a role in some ionic solution phenomena, although examples in aqueous solutions are not known at this time. Other new features are the intense concentration dependence - at low concentration - of certain of the Hamed coefficients that characterize mixed electrolyte solutions and the evidence for a solvent-separated state of the hydrophobic bond, the attractive force between hydrophobic ions, even those of zero charge, in water. 

The selection of the counter-ion and its concentration are important for the separation of ionic compounds in reversed-phase and ion-exchange liquid chromatography. The addition of hydrophobic ions is an especially powerful method and several surfactants can be used as hydrophobic counter-ions. The theoretical column efficiency of ion-pair liquid chromatography is much better than that of an ion-exchange column, and the regeneration of a column is much faster. Thus, if we can control ion-pair liquid chromatography, we can solve a separation problem. (The important background sources in this area are listed at the end of the chapter.)... 

The addition of a surfactant counter-ion reduced the retention factor at low pH due to the surface modification of the stationary phase material. Covering the surface of the stationary phase with the surfactant reduces the hydro-phobicity of the stationary phase material. The addition of the tetrabutyl-ammonium counter-ion increased the retention factor at high pH. The pKa of the indole acetate was 5.15 without surfactant, 4.85 with octyl sulfate ion, and 5.60 with tetrabutylammonium ion. That is, the addition of a same-charged hydrophobic ion reduced the pKa value, and the addition of the counter-ion increased the pKa value. The difference in the pKa value on the addition of surfactant is not constant it is affected by the kind of ion and the concentration. It is difficult to estimate the pKa change. 

Plastic film membranes can also contain fixed ion-exchange groups. Jyo and coworkers [79] chloromethylated Amberlite XAD-2 (cross-linked styrene-divinylbenzene copolymer of the macroreticular type) and formed quaternary ammonium groups in the product by treatment with dimethyltetradecylamine. They converted the substance into the chloride, nitrate or perchlorate form and saturated it with nitrobenzene. The presence of hydrophobic ion-exchange sites... 

Consequently, an ISE for nitrate for example, a strongly hydrophilic ion, must have a strongly hydrophobic ion-exchanger ion. This conclusion has been demonstrated experimentally for a series of NO3 ISEs based on tetra-alkyl-ammonium salts with long alkyl chains [161] (see fig. 7.2). It was found that, in the studied series of substances, the tetradodecylammonium ion which is... 

In recent years, experience has shown that a large number of hydrophobic ions could act as ion-exchanger ions in liquid membranes and that almost all water soluble ions not unusually hydrophilic, such as Li, may produce a response in such an ISE. These facts have led to an extraordinary increase in publications in this field, although relatively few of these ISEs have actually been used to solve analytical problems. This field, which is becoming rather confused, will not be discussed in this book and the reader is referred to reviews [103-105]. 

To widen the potential window, even more hydrophobic ions should be used in the organic phase, for example tetraphenylarsonium [15] or crystal violet [21 ] cations and l,2-7r-dicarbalkylcobaltate(III) anions [13]. 

Meyer, J.D., J.E. Matsnnra, J.A. Rnth, E. Shelter, S.T. Patel, J. Bansch, E. McGonigle, and M.C. Manning, Selective precipitation of interleukin-4 using hydrophobic ion pairing a method for improved analysis of proteins formulated with large excesses of human serum albumin. Pharm Res, 1994. 11(10) 1492-5. 

In the chromatography technique, proteins bind differentially to solid matix supports or media with various functional groups to provide hydrophobic, ion-exchange, and affinity interactions. Some of the matrices used for intermediate purification that provide sufficient flow rate for large-scale purification are listed in Table 4.13. Some of the functional groups attached to matrix supports and examples of proteins purified by these matrix supports are fisted in Table 4.14. The chromatography technique should provide high capacity and selectivity. The matrix material must withstand multiple purification cycles with minimum loss of efficiency. 

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