Binding ionic-atmosphere

Oppositely charged ions are attracted to each other by electrostatic forces and so will not be distributed uniformly in solution. Around each ion or polyion there is a predominance of ions of the opposite charge, the counterions. This cloud of counterions is the ionic atmosphere of the polyion. In a dynamic situation, the distribution of counterions depends on competition between the electrostatic binding forces and the opposing, disruptive effects of thermal agitation. 

Not all ions are mobile within the ionic atmosphere of the polyion. A proportion are localized and site-bound-a concept apparently first suggested by Harris Rice (1954). Localized ion binding is equivalent to the formation of an ion-pair in simple electrolytes. Experimental evidence comes mainly from studies on monovalent counterions. 

The systems of the first class afford the closest approach to a simple barrier penetration process, and perhaps they more readily respond to a theoretical analysis. It can reasonably be supposed that for these systems orbital overlap for the two ions is small, so that the frequency of the electronic transition is small, and there is no substantial binding between the two exchanging centers. A model of this kind presumably corresponds to the weak overlap cases as defined and discussed by Marcus (8 ). In attempting to calculate the rates of these reactions, besides the problem of the shape and height of the barrier for the electron transfer, electrostatic interaction of the reactants must be dealt with and the energy necessary to distort the solvent and ionic atmosphere about each ion to make the enei of the electron equal at the two sites. Different workers have emphasized different ones of these factors, and serious differences of opinion are recorded. 

A rational description of ionic atmosphere binding is provided by the Poisson-Boltzmann equation and the cylindrical cell model. Figure 1 is an example of such computations and shows the variation of the local concen-... 

The observable properties of a polyelectrolyte depend upon the distribution of small ions in its neighborhood. This distribution is affected by two types of "binding" (20, 21). The first involves the binding of counterions to specific sites of the macroion, i.e., "site-binding" (20). The second involves the binding of counterions anywhere in the vicinity of the macroion, i.e., "ionic-atmosphere-binding". The site-binding of phosphates to nonpolar amides is unique. 

In effect, the amount and sign of the difference between measures of ionic-atmosphere-binding and site-binding, i.e.. 

A bit of explanation is required here for those readers unfamiliar with the condensation concept, a key notion to describe polyelectrolytes. Consider as here a polyanion. If the charges are brought closer to one another, on the average, below a critical distance their mutual repulsion is such that — in order to continue to obey first principles electrostatics such as the Poisson equation — they screen themselves with an atmosphere of counterions. This atmospheric condensation, which can coexist with ionic binding at the individual sites, boosts the local concentration of counterions in the space surrounding the polyelectrolyte by as much as three orders of magnitude. The nmr measurements analyzed here focus on these water hydration molecules coordinated to condensed sodium counterions, next to the surface of the tactoids (see Fripiat s chapter). 

The AH , term results from the part of the binding process in which the Na+ ions, electrostatically bound to the polyion (atmospheric binding), are replaced by the monovalent surfactant cations. This contribution is assumed to be small in comparison with the other two and can be neglected. When the surfactant micelles form in the presence of a hydrophilic polyelectrolyte, the third contribution, AH in Eq. 4 may be assumed to be much smaller than the second term, since only pine electrostatic interactions are expected to act between hydrophilic polyion and ionic micelle. However, the AH, term in the case of interaction between a hydrophobic polyelectrolyte and surfactant micelle may not be neglected. To confirm this, it is instructive to... 

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