Dipolar ion exchangers

Porath I, Fornstedt N (1970) Group fractionation of plasma proteins on dipolar ion exchangers. J Chrom A 51 479 89... 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

Although Certify is a mixed mode sorbent with C8 and sulfonic acid moieties, the authors rationalized that the hydrophobic retention on this sorbent is more dominant and caused the nonretention of certain drugs during methanol wash. The weak WCX ion exchanger was also excluded for similar reasons. Both the mixed mode strata-X-C and the ion exchange sorbent SCX were found to be most amenable for the derivatization-based GC/MS analysis and both yielded pure extracts. However, the yields were consistently lower with strata-XC than SCX and the authors hypothesized that this was due to the inability of the 5% ammonia/methanol eluent to completely disrupt the hydrophobic and dipolar interactions between the analytes and XC. 

In the homogeneous mechanism, the reaction is assumed to start by protonation of one of the reactants, either ester (mechanisms denoted as Aac1 and Aac2 [397,398]) or, less frequently, alcohol (mechanism Aal1). It seems likely that protonation of reactants is an important step in esterification catalysed by ion exchangers, too. This follows from all that has been said above about the effect of the acidic properties of ion exchangers on their catalytic activity and is further supported by the effect of the dielectric constant of solvents (Fig. 18), which indicates that the reaction mechanism involves a positive ion and a dipolar molecule [454]. 

Ion exchange from organic solvents and mixed organic aqueous solvents offers interesting possibilities for the extraction and separation of metals because of the different nature of the solvation processes in these systems. Only cation solvation is significant in dipolar, aprotic, organic... 

Ions are also attracted and repulsed by the polar ends of a dipolar molecule. These forces can be referred to as ion-dipole forces. In addition to ion exchange chromatography, they are also important in aqueous LC systems where a polar stationary phase can interact with ionic solutes. 

Samsonov [15] studied the direct sorption of ALA and other dipolar ions by SDV-3 ion exchanger resin at pH = 7. The enthalpy and entropy components of these sorptions were obtained from the isotherm dependence on temperature. It was found that the transformation of the resin from the hydrogen to the amino acid form was accompanied by a rise in the system s entropy. The thermodynamic-based description of the exchange of a-amino acids with hydrogen on three ion-exchange resins at pH < pK .co2 were determined as well. However, precise descriptions of the experimental measurements and the calculations program used were not given. 

The electron spin echo modulation technique detects directly the coordination environment around a paramagnetic ion by observing the dipolar coupling to nuclei of weakly coordinated ligands. This technique has been used extensively by Kevan, for example, to examine transition metal ion exchanged and substituted zeolite materials [19]. 

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