Ionic atmosphere polarization

The most important parameters of the ionic atmosphere are the charge density Qv r) and the electrostatic potential /(r) at the various points. Each of these parameters is understood as the time-average value. These values depend only on distance r from the central ion, not on a direction in space. For such a system it is convenient to use a polar (spherical) coordinate system having its origin at the point where the central ion is located then each point can be described by a single and unique coordinate, r. 

A new theory of electrolyte solutions is described. This theory is based on a Debye-Hiickel model and modified to allow for the mutual polarization of ions. From a general solution of the linearized Poisson-Boltzmann equation, an expression is derived for the activity coefficient of a central polarized ion in an ionic atmosphere of non-spherical symmetry that reduces to the Debye-Hiickel limiting laws at infinite dilution. A method for the simultaneous charging of an ion and its ionic cloud is developed to allow for ionic polarization. Comparison of the calculated activity coefficients with experimental values shows that the characteristic shapes of the log y vs. concentration curves are well represented by the theory up to moderately high concentrations. Some consequences in relation to the structure of electrolyte solutions are discussed. 

The conventional viewpoint, which assumes that the ionic atmosphere is spherically symmetric, does not take account of the inevitable effects of ionic polarization. From an analysis of the general solution (19), however, it is evident that the ionic atmosphere must be spherically symmetric for nonpolarizable ions, and the DH model is therefore adequate. (Moreover, in very dilute solution polarization effects are negligibly small, and it does not matter whether we choose a polarizable or unpolarizable sphere for our model.) But once we have made the realistic step of conferring a real size on an ion, the ion becomes to some extent polarizable, and the ionic cloud is expected to be nonspherical in any solution of appreciable concentration. Accordingly, we base our treatment on this central hypothesis, that the time-average picture of the ionic solution is best represented with a polarizable ion surrounded by a nonspherical atmosphere. In order to obtain a value for the potential from the general solution of the LPBE we must first consider the boundary conditions at the surface of the central ion. 

The constant Doo vanishes as expected because the potential VL(r,0,Laplace potential, VL, results from all the multipoles induced in the surface of the central ion by the structured ionic atmosphere, and vanishes at infinite dilution as required. 

The allowance for polarization in the DH model obviates the need for separation of long-range and short-range attractive forces and for inclusion of additional repulsive interactions. Belief in the necessity to include some kind of covolume term stems from the confused analysis of Onsager (13), and is compounded by a misunderstanding of the standard state concept. Reference to a solvated standard state in which there are no interionic effects can in principle be made at any arbitrary concentration, and the only repulsive or exclusion term required is that described by the DH theory which puts limits on the ionic atmosphere size and hence on the lowering of electrical free energy. The present work therefore supports the view of Stokes (34) that the covolume term should not be included in the comparison of statistical-mechanical results with experimental ones. 

If the dipoles are so tightly correlated that all move together, we have essentially one molecule with an enormous dipole moment, as in the low-frequency response of a single ferroelectric domain. The response is likely to be strongly non-linear at moderate field intensities. Another example is a polypeptide molecule coiled into a solid polar bar, and in this case it may be possible to uncoil the molecule and study it in a less corrdated motion. In the most interesting situation of biological material, polar long-chain molecules are dispersed in an ionic fluid, and the total moment associated with one molecule is a sum of more or less correlated permanent dipoles with a certainly-correlated ionic atmosphere. 

Table 8). This permits the interpretation of experimental data by using the electro-optical properties of flexible-chain polymers in terms of a worm-like chain model However, EB in solutions of polyelectrolytes is of a complex nature. The high value of the observed effect is caused by the polarization of the ionic atmosphere surrounding the ionized macromolecule rather than by the dipolar and dielectric structure of the polymer chain. This polarization induced by the electric field depends on the ionic state of the solution and the ionogenic properties of the polymer chain whereas its dependence on the chain structure and conformation is slight. Hence, the information on the optical, dipolar and conformational properties of macromoiecules obtained by using EB data in solutions of flexible-chain polyelectrolytes is usually only qualitative. Studies of the kinetics of the Kerr effect in polyelectrolytes (arried out by pulsed technique) are more useful since in these... 

Numerous data are available on the Kerr effect in solutions of ionogenic chain molecules with secondary structures particularly of polypeptides displaying a helical conformation A high EB value in these solutions is ensured by both the polarization of ionic atmospheres and the character of the rigid secondary structure of the molecules. It is not always possible to distinguish between the effect of the structural order and the ionic state of the medium on the electro-optical properties of rigid polyelectrolyte solutions. Thus it is difficult to determine their molecular structure. 

The polarization of ionic atmosphere around a polyelectrolyte molecule has been experimentally studied by dielectric and electro-optical techniques in a wide concentration range. In a solution with no added salt, the polyions have an extended conformation and may entangle with each other. This complicates the interpretation of the counterion polarization, since it has been theoretically analyzed for solutions with no interactions between the... 

In the theory, all ions are considered to be spherically symmetrical with the charge on the ion taken to be at the centre of the ion. One ion is taken to be a central reference ion with all the other ions which make up the ionic atmosphere arranged around it. There is no preferred direction for this arrangement and this implies that the ionic atmosphere is spherically symmetrical around the central reference ion. Polar coordinates are therefore used and the centre of the central reference ion is taken as the origin of the coordinate system. It is also the position of the charge on the central reference ion. 

The work presented in this chapter was performed in moderately polar media [primarily tetrahydrofuran (THF)], in which electrostatic interactions between ions are considerably larger than the thermal energy, kT. Therefore, all results are interpreted in terms of specific ion-pairing effects rather than the bulk ionic atmosphere relaxation. 

A counterion polarization mechanism has been proposed to explain the electric induction of conformational changes in polyelectrolyte complexes such as the (U A U) triple helix. In accordance with this idea, the external electric field shifts the ionic atmosphere of the (U A U) complex and thereby induces a dipole moment. At the negative pole of the induced macrodipole, the screening by the ion cloud of the negative phosphate residues is reduced. This, in turn, causes repulsion between the ends of the polyanions and leads finally to the unwinding of the triple helix. It later came to our attention that a polarization mechanism had already been proposed for strand separation of DNA by Poliak and Rein. " ... 

The calculated values of are considerably closer to the experimental values when polarization is taken into account. Hence it should be considered that the critical field strength is determined not only by the properties of the ionic atmosphere of the particles, but also by their polarization. 

Equation (15) takes into account the change of the reactant electronic state and of its intramolecular nuclear configuration, involving the transfer of the heavy particles, the change of the solvent polarization near the reactants and the effect of the reactants on the structure of the solvent contacting them, the approach of the reactants one to another and the separation of the products, the change of the ionic atmosphere around the reacting particles, and finally the effect of the electrode properties and the electric field on the probability of the elementary act and the rate of the electrochemical processes as a whole. 

We have implicitly allowed the friction coefficients to be independent of the magnitude and the nature of applied forces, that is to say these coefficients are completely defined by the equilibrium properties of the solution as shown for example by Bearman for self-diffusion processes in binary liquid solutions [14]. Nevertheless, for ionic solutions polarization effects resulting from the application of an external field of forces may give rise to distorted ionic atmospheres and the identification of a unique interaction parameter in electrical and self-diffusion processes becomes questionable. However, it has been proved that as far as polyelectrolytes are concerned, the perturbation of the counter-ion distribution with respect to the equilibrium situation is fairly small despite the high polarizability of polyelectrolyte solutions [18]. Moreover, linear forces - fluxes relations have usually been reported from experimental investigations and for both polyelectrolyte and pure salt solutions electrical and self-diffusion determinations have led to nearly identical frictional parameters [19-20]. The friction model might therefore be used with confidence as long as systems not too far from equilibrium are concerned. 

Kikuchi et al. have observed that the initial attack of amine occurs at the carbonyl carbon, resulting in the formation of an ionic intermediate 26. This reaction is very sensitive to the solvent polarity. Under nitrogen atmosphere, intermediate 27 is further aminated to give 28. Oxidation of 27 and 28 gives 23 and 24, respectively. Oxidation in nitrobenzene, however, results in dealkylation products. In the presence of air and triethylamine, decomposition of aminoanthraquinones occurs. 

Table 6.10 reports the main areas of application of the various ionisation methods and the principal ions detected. A breakdown of MS techniques applied to various types of analytes is as follows thermally stable, low-MW Cl, El thermally instable, low-MW APCI (FLA, LC-MS), ESI and high-MW DCI, FD, FAB, LD, ESI (FLA, LC-MS, CZE-MS). Soft ionisation techniques such as FL, FAB and LD are useful for the detection of non-volatile, sometimes oligomeric, polymer additives. Recent developments in ionisation techniques have allowed the analysis of polar, ionic, and high-MW compounds, previously not amenable to mass-spectrometric analysis. Figure 6.4 shows the applicability of various atmospheric pressure ionisation techniques in terms of molar mass and polarity. 

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