Ionization ionogens

High sorption capacities with respect to protein macromolecules are observed when highly permeable macro- and heteroreticular polyelectrolytes (biosorbents) are used. In buffer solutions a typical picture of interaction between ions with opposite charges fixed on CP and counterions in solution is observed. As shown in Fig. 13, in the acid range proteins are not bonded by carboxylic CP because the ionization of their ionogenic groups is suppressed. The amount of bound protein decreases at high pH values of the solution because dipolar ions proteins are transformed into polyanions and electrostatic repulsion is operative. The sorption maximum is either near the isoelectric point of the protein or depends on the ratio of the pi of the protein to the pKa=0 5 of the carboxylic polyelectrolyte [63]. It should be noted that this picture may be profoundly affected by the mechanism of interaction between CP and dipolar ions similar to that describedby Eq. (3.7). 

Proposition 3 Most of the un-ionized aluminium halide becomes complexed with unreacted monomer the resulting complex is not an initiator. The concentration of free aluminium halide is thereby reduced so much, that the rate of the ionogenic reaction (ii) (the formation of A1X2+) becomes negligibly small, and there is thus no further initiation. This accounts for the limited yields which are generally found in this type of polymerisation, and which had defied plausible explanation. 

Electrolyte concentration and pH have profound effect on electrostatic interactions and consequently on the retention behavior of ionogenic sample components in RPC. In agreement with the results of a detailed theoretical treatment (207), which is summarized in Section V,B,2,b. ionization results in a decrease of the retention factor although exceptions... 

Figure 2.11. Percent of ionogenic (ionizable) species present for weak acids and bases when solution pH is 2 units above or below the acid dissociation constant.

Some highly hydrophobic weak acids and bases exhibit substantial hydro-phobicity even in the ionized state. For highly hydrophobic ionogenic organic compounds, not only is transfer of the neutral species between the aqueous phase and the immiscible phase important, but the transfer of the hydrophobic, ionized, organic species as free ions or ion pairs may also be significant [37]. Mathematically, this is described by refining the n-octanol/ water partition coefficient, as defined in equation (2.7), to reflect the pH-dependent distribution between water (W) and K-octanol (O) of chemical X in both the ionized and nonionized forms. If chemical X is a weak acid, HA, the distribution ratio is... 

For example, the ratio of the n-octanol/watcr distribution coefficient of the nondissociated species to that of the ionic species is nearly 10,000 for 3-methyl-2-nitrophenol, but only about 1000 for pentachlorophenol because of the greater significance of the hydrophobicity of the ionized form of pentachlorophenol. The logarithm of the -octanol/water distribution coefficient of pentachlorophenol as the phenolate is about 2 (determined at pH 12, and 0.1 M KC1), which indicates significant distribution of the ionized form into the n-octanol phase [8,37], Extraction of such highly hydrophobic ionogenic organic compounds can result from mixed-mode mechanisms that incorporate both the hydrophobic and ionic character of the compound. 

If the compound is ionogenic (or ionizable) in aqueous solution (as discussed earlier), there may be an electrostatic attraction between the... 

The benzhydryl chlorides and BC13 react with formation of ion pairs (ionization constant, Ki) which dissociate to give the free ions (dissociation constant, KD). Because paired and free diarylcarbenium ions show only slightly different UV-visible spectra, [41], spectrophotometric measurements allow the determination of the total carbocation concentration. On the other hand, only free ions are detected by conductometric analysis, and a combination of both methods allows the determination of Ki and Kd using the theory of binary ionogenic equilibria [42,43]. 

The retention of ionogenic analytes in RP-HPLC is obviously dependent on the eluent pH. The pH value is an important optimization parameter because it controls the extent of ionization of the solute and hence the magnitude of electrostatic interactions. Varying the acidity of the mobile phase can lead to extreme changes in selectivity. Many interdependent parameters can be modulated in IPC and their optimization requires a theory-driven procedure. 

Solutions of non-electrolytes contain neutral molecules or atoms and are nonconductors. Solutions of electrolytes are good conductors due to the presence of anions and cations. The study of electrolytic solutions has shown that electrolytes may be divided into two classes ionophores and ionogens [134]. lonophores (like alkali halides) are ionic in the crystalline state and they exist only as ions in the fused state as well as in dilute solutions. Ionogens (like hydrogen halides) are substances with molecular crystal lattices which form ions in solution only if a suitable reaction occurs with the solvent. Therefore, according to Eq. (2-13), a clear distinction must be made between the ionization step, which produces ion pairs by heterolysis of a covalent bond in ionogens, and the dissociation process, which produces free ions from associated ions [137, 397, 398]. 

The ionization of an ionogen can therefore be regarded as a coordinative interaction between substrate and solvent [281]. The polarization of the covalent bond to be ionized can occur via a nucleophilic attack of the EPD solvent on the electropositive end of the bond, or by an electrophilic attack of an EPA solvent on the electronegative end. Both attacks can, of course, also occur simultaneously. The following examples are illustrative. 

Another remarkable example of the solvent effect on the ionization of ionogens is the Friedel-Crafts intermediate antimony pentachloride/4-toluoyl chloride. It can exist as two distinct well-defined adducts depending on the solvent from which it is recrystallized, the donor-acceptor complex (2) or the ionic salt (3) [159],... 

Other nice examples of well-studied solvent-dependent ionization equilibria of ionogens are azidocycloheptatriene tropylium azide [282, 283] and (triphenylcyclo-propen-l-yl) (4-nitrophenyl)malononitrile ("2a) triphenylcyclopropenium dicyano(4-nitrophenyl)methide (3a), the latter being one of the first examples of direct heterolysis of a weak carbon-carbon bond to a carbocation and carbanion in solution [284],... 

The ionization of an ionogen and its subsequent dissociation according to Eq. (2-13) can be further elaborated. Between the ion pair immediately formed on heterolysis of the covalent bond and the independently solvated free ions, there are several steps of progressive loosening of the ion pair by penetration of solvent molecules between the ions. At least four varieties of ion interactions representing different stages of dissociation have been postulated [96, 134, 138, 141] cf. Eq. (2-19) and Fig. 2-14. 

In solution, ions are produced by the heterolysis of covalent bonds in ionogens. This ionization reaction is favored by solvents due to their cooperative EPD and EPA properties (c/ Section 2.6). In the gas phase, however, ionization of neutral molecules to form free ions is rarely observed because this reaction is very endothermic. For example, in order to ionize gaseous H—Cl into H and Cl , an energy of 1393 kJ/mol (333 kcal/ mol) must be provided. This considerably exceeds the 428 kJ/mol (102 kcal/mol) needed to homolytically cleave H—Cl into hydrogen and chlorine atoms. Thus, for the creation of isolated ions in the gas phase, energy must be supplied by some means other than solvation with EPD/EPA solvents. The most widely used method is ionization by elec-... 

Not only reaction rates, but also the positions of chemical equilibria can be influenced by liquid crystals as reaction media. A nice example is the ionization equilibrium of chloro-tris(4-methoxyphenyl)methane according to Ar3C-Cl Ar3C+ -I-C1 , which is more shifted in favour of the nearly planar triarylcarbenium ion in nematic liquid crystals as compared to in an isotropic reaction medium [868], Obviously, the discshaped carbenium ion fits better into the rod-Hke nematic phase than the tetrahedral covalent ionogen, which distorts the internal structure of the nematic liquid crystal. 

Because different forms of analyte usually show different affinity to the stationary phase, secondary equilibria in HPLC column (ionization, solvation, etc.) can have a significant effect on the analyte retention and the peak symmetry. HPLC is a dynamic process, and the kinetics of the secondary equilibria may have an impact on apparent peak efficiency if its kinetics is comparable with the speed of the chromatographic analyte distribution process (kinetics of primary equilibria). The effect of pH of the mobile phase can drive the analyte equilibrium to either extreme (neutral or ionized) for a specific analyte. Concentration and the type of organic modifier affect the overall mobile phase pH and also influence the ionization constants of all ionogenic species dissolved in the mobile phase. 

For the separation of ionogenic (ionizable) solutes, the variations of mobile-phase pH can lead to extreme changes in selectivity. The mobile-phase (eluent) pH affects the ionization of ionogenic species and consequently their HPLC retention. However, the pH of the aqueous phase is not equivalent to the pH of the aqueous/organic eluent, and consequently the variation of the mobile-phase composition leads to the variation in pH under both isocratic and gradient conditions [58-60]. Therefore the pH shift of the mobile phase upon the addition of the organic modiher is imperative for a proper description of the... 

The fixed ionogenic groups are completely ionized at all times. 

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