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Lipophilic ions

In concluding this section we hope we have shown that there is a clear need for more elaborated studies to understand how lipophilic ions interact with biological membranes - an aspect that may look deceptively simple, but which is not yet completely understood. [Pg.425]

The conductivity of membranes that do not contain dissolved ionophores or lipophilic ions is often affected by cracking and impurities. The value for a completely compact membrane under reproducible conditions excluding these effects varies from 10-8 to 10 10 Q 1 cm-2. The conductivity of these simple unmodified membranes is probably statistical in nature (as a result of thermal motion), due to stochastically formed pores filled with water for an instant and thus accessible for the electrolytes in the solution with which the membrane is in contact. Various active (natural or synthetic) substances... [Pg.451]

The sensor layer consists of a selective ionophore (e.g. valinomycin for potassium), a lipophilic anionic site (borate) and the cationic PSD. Before interaction with potassium, a lipophilic ion pair between the cationic PSD and borate anion is formed in the polymer layer. When valinomycin (also contained in the layer) selectively extracts potassium into the layer, then the positively charged valinomycin-potassium complex forms an ion pair with... [Pg.311]

Another approach for ion-sensing (here copper and zinc) is based on the water-soluble ligand Zincon. Its lipophilic ion pair with quaternary... [Pg.314]

While ionophore-free membranes based on classical ion exchangers are still in use for the determination of lipophilic ions, such sensors often suffer from insufficient selectivity, as it is governed solely by the lipophilicity pattern of ions, also known for anions as the Hofmeister sequence. This pattern for cations is Cs+ > Ag+ >K+ > NH > Na+ > Li+ > Ca2+ > Mg2+ and for anions CIOT > SCN- > I > Sal- > N03- > Br > N02- > Cl- > OAc- HC03- > SO - > HPO4. While the ion exchanger fixes the concentration of hydrophilic analyte ions in the membrane on the basis of the electroneutrality condition within the membrane, the second key membrane component is the ionophore that selectively binds to the analyte ions. The selectivity of... [Pg.102]

Lipophilic ion exchangers traditionally used for polymeric membrane preparation are the anionic tetraphenylborate derivatives and the cationic tetraalkylammonium salts. The charges on both lipophilic ions are localized on a single (boron or nitrogen) atom, but the steric inaccessibility of the charged center, due to bulky substituents, may inhibit ion-pair formation in the membrane and provide, when necessary, non-specific interactions between ionic sites and sample ions. [Pg.123]

Smejtek, P. and Wang, S. (1991). Domains and anomalous adsorption isotherms of dipalmitoylphosphatidylcholine membranes and lipophilic ions pentachlorophenolate, tetraphenylborate, and dipicrylamine, Biophys. J., 59, 1064-1073. [Pg.264]

Bychkova and Shvarev [16] recently prepared nanosensors (0.2-20 pm) for sodium, potassium and calcium using the precipitation method. Similarly to the previous works, the plasticized poly(vinyl chloride) included a phenoxazine chro-moionophore, a lipophilic ion exchanger and a cation-selective ionophore. The dynamic range of the very selective sensors was 5 x 10 4-0.5 M for sodium, 1 x 10 5-0.1 M for potassium and 2 x 10 4 - 0.05 M for calcium. As was demonstrated by Bakker and co-workers [45] a particle caster can be used can be used for preparation of much larger beads (011 pm). [Pg.210]

Subsequent ion exchange of the metal cation with the quaternary ammonium ion catalyst provides a lipophilic ion pair (step 2), which either reacts with the requisite alkyl electrophile at the interface (step 3) or is partitioned into the electrophile-containing organic phase, whereupon alkylation occurs and the catalyst is reconstituted. Enantioselective PTC has found apphcation in a vast number of chemical transformations, including alkylations, conjugate additions, aldol reactions, oxidations, reductions, and C-X bond formations." ... [Pg.336]

A small amount of an anionic or cationic dye is added to an aqueous solution of the analyte, which is a lipophilic cationic or anionic compound. A small amount of coloured lipophilic ion pair is formed and this is extracted into a small amount of... [Pg.63]

This procedure is more widely used in pharmacopoeial assays than the dye extraction procedure. Excess potassium iodide is added to an aqueous solution of the analyte, which is a lipophilic cation. A lipophilic ion pair is formed between the cation and the iodide ion and is then removed by extraction into an organic phase such as chloroform. The excess iodide remaining in the aqueous phase is then titrated in concentrated HCl (> 4 M) with potassium iodate. The iodate oxidises iodide to E, which immediately reacts with Cl to give ICl resulting in the following equation ... [Pg.64]

Ion pair extraction provides a standard method for estimating ionic surfactants either colorometrically or titrimetrically. For example a cationic surfactant such as cetrimide can be estimated by pairing it with a lipophilic anionic dye such as bromocresol purple. The ion pairing creates a coloured lipophilic ion pair, which can be extracted into an organic solvent such as chloroform and a quantitative measurement of the colour extracted can be made spectrophotometrically. This type of assay is described in the BP for Clonidine Injection and Benzhexol Tablets. [Pg.317]

There are, of course, some limitations on the use of PTC. First of all, this approach is restricted to reactions of anions. Among reactions of anions there are some cases in which lipophilic ion pairs cannot be formed because of high hydrophilicity of the anions. On the other hand, generation of organic anions, particularly carbanions from weak acids, is somewhat restricted because of limited basicity of alkali hydroxides. Typically, CH acids of pKa value exceeding 24 cannot be efficiently deprotonated under PTC conditions. [Pg.233]

One way to estimate the electrical potential difference across the mitochondrial membrane is to measure the concentrations of a lipophilic ion after the ion has come to equilibrium. The larger the electrical potential difference (At//), the larger the ratio of the concentrations on the two sides. Such measurements indicate respiring mitochondria generate a At// on the order of —0.15 V, inside negative. [Pg.319]

Cadogan et al. [73] deposited polypyrrole onto platinum from a variety of solutions and found the resultant films to be anion sensitive but very unselec-tive. When polypyrrole was deposited from NaN03 solution, however, the resultant electrode gave near-Nernstian responses to nitrate ion in the range 0.5-0.0005 M with selectivities of about 20 over other lipophilic ions such as iodide and perchlorate [74]. Similar electrodes could be fabricated via a screen-... [Pg.108]

A tetraruthenated porphyrin was electropolymerised onto glassy carbon and used to catalyse the oxidation of nitrite to nitrate, with the resultant current giving a selective measure of the concentration of nitrite ion [81]. As an alternative method, soluble poly(3-octyl thiophene) [82] was cast along with tridodecylmethylammonium chloride onto glassy carbon, to give electrodes with superior selectivity over PVC-based membranes to lipophilic ions such as bromide or nitrate. [Pg.110]

For the transmembrane transfer of ions containing hydrophobic substituents the model was proposed that takes into account the variations of dielectric properties across the membrane. According to this model [194-200] the lipophilic ions are adsorbed at the minima of the potential energy near to the membrane // water interface (see Fig. 6b). The transfer of the ions across the membrane is considered to be monomolecular reaction of the ion s surmounting of the hydrophobic barrier in the center of the membrane with the first order rate constant k,. [Pg.38]

The above results indicate that in order to maintain the high rate of transmembrane electron transfer, it is necessary to provide efficient neutralization of the arising polarization. For this purpose lipophilic ions and proton carriers were successfully used (see Table 1). These compounds are known to act as the uncouplers of the mitochondrial oxidative phosphorylation and are able to remove the gradients of electric fields across lipid membranes. [Pg.41]

Hassan and Rizk developed potentiometric dipyridamole sensors based on lipophilic ion-pair complexes and native ionic polymer membranes [18]. The sensors are based on the use of the ion-association complexes of dipyridamole cation with tetraphenylborate and reineckate counteranions as ion-exchange sites in plasticized PVC matrices. A plasticized native polymer (carboxylated polyvinyl chloride) can also be used. These sensors exhibit linear and near-Nernestian responses for 10 mM-1 pM... [Pg.251]

Before the advent of soft ionization techniques, the analysis of bile acids was long and tedious and needed large sample quantities. First, the bile acids had to be extracted from the biological fluid and separated by lipophilic ion exchange chromatography into four classes ... [Pg.382]

Lipophilic ions tends to gather together at the surface of water, and hence are surface-active species that lower the surface tension of the solvent, at variance with small and strongly hydrated kosmotropic ions. [Pg.6]

Thomlinson [78] was the first chromatographer to point out that the classical electrostatic ion-pair concept did not hold for IPRs that were usually bulky hydrophobic ions he also emphasized that in the interfacial region between the mobile and the stationary phases, the dielectric constant of the medium is far lower than that of the aqueous phase. Chaotropes that break the water structure around them and lipophilic ions that produce cages around their alkyl chains, thereby disturbing the ordinary water structnre, are both amenable to hydrophobic ion-pairing since they are both scarcely hydrated. The practical proof of such ion-pairing mode can be found in References 80 and 81 many examples of such pairing modes are reported in the literature [79-86],... [Pg.17]

Lipophilic ions first adsorb at the surface of the stationary phase, and the dynamically generated charge sites provide an ion exchange character that explains the retention of oppositely charged analytes [7-11]. This retention mechanism does not explain the contribution of solute hydrophobicity to retention because it should not be relevant if retention is only charge driven. It can be speculated that both mechanisms act and the extent to which one is more significant than the other depends on the experimental set-up and the nature of the IPR [12]. [Pg.30]

The model was recently tested to determine whether it was able to model analyte retention in the presence of novel and unusual IPRs (see Chapter 7) such as chaotro-pic salts and ionic liquids. Chaotropes that break the water structure around them and lipophilic ions (classical IPRs and also ionic liquids) that produce cages around their alkyl chains, thereby disturbing the ordinary water structure, are both inclined to hydrophobic ion-pairing since both are scarcely hydrated. This explains the success of the theory, that is predictive in its own right, when neoteric IPRs are used [64]. Recently a stoichiometric model (vide supra) was put forward to describe retention of analytes in the presence of chaotropic IPRs in eluents [18] but its description of the system is not adequate [64]. [Pg.44]

Cecchi T. Use of lipophilic ion adsorption isotherms to determine the surface area and the monolayer capacity of a chromatographic packing, as well as the thermodynamic equilibrium constant for its adsorption. J. Chromatogr. A. 2005,1072, 201-206. [Pg.52]


See other pages where Lipophilic ions is mentioned: [Pg.618]    [Pg.423]    [Pg.93]    [Pg.148]    [Pg.102]    [Pg.116]    [Pg.218]    [Pg.2]    [Pg.15]    [Pg.94]    [Pg.451]    [Pg.269]    [Pg.244]    [Pg.177]    [Pg.319]    [Pg.728]    [Pg.266]    [Pg.14]    [Pg.30]    [Pg.331]    [Pg.42]    [Pg.181]    [Pg.15]    [Pg.100]   
See also in sourсe #XX -- [ Pg.5 ]




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Type and Concentration of Lipophilic Counter Ions in the Mobile Phase

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