Counterion site binding

From all of this discussion it is apparent that, as Manning (1979) said, the binding between counterion and polyion can range from atmospheric to covalent site binding. 

Strauss, U. P. Leung, Y. P. (1965). Volume changes as a criterion for site binding of counterions by polyelectrolytes. Journal of the American Chemical Society, 87, 1476-80. 

The problems of the constancy of a and the site of reaction are closely linked. It is very convenient to assume that the charge on the micellar head groups is extensively neutralized by counterions which bind specifically to the micellar surface. In this way micellar stability is associated with a balance between hydrophobic attractions between apolar groups and coulombic repulsions of the ionic head groups which will be reduced by favorable interactions with the counterions in both the Stem and the diffuse Gouy-Chapman layers. It is the behavior of the counterions which is important in considerations of their chemical reactivity. 

Rare earth element complexation by PAA and HA is fast (5). However, most of the initial interactions are labile or weak stronger interactions take over as the metal reside longer on the polyelectrolyte. Weak and strong interactions are indicated by both ion exchange and ligand exchange dissociation experiments. It is difficult to infer site binding versus counterion condensation from these methods, and such a distinction must await methods that probe more directly the molecular character of the different interactions. 

Gueron, M. Weisbuch, G. Polyelectrolyte theory. I. Counterion accumulation, site-binding, and their insensitivity to polyelectrolyte shape in solutions containing finite salt concentrations. Biopolymers 1980 19 353-382. 

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. 

The application of ultrasonic methods to electrolytic solutions is convenient to study the mechanism of ion pair formation [15,16] the extension of polyelectrolytes gives information on the existence of site binding depending strongly on charge density and the nature of the counterions [17]. 

The same sequences of perturbation are obtained if the specific volumes of polyelectrolytes are measured [18]. These ultrasonic experiments are the best proof of site binding and the importance of water perturbation at the origin of the ion selectivity nevertheless it is a global measurement and only a spectroscopic experiment would be convenient to estimate the number of associated counterions. 

Hinderberger D, Spiess HW, Jeschke G. Dynamics, site binding, and distribution of counterions in polyelectrolyte solutions studied by electron paramagnetic resonance spectroscopy. J Phys Chem B 2004 108 3698-3704. 

M. Gueron and G. Weisbuch, Biopolymers, 19, 353 (1980). Polyelectrolyte Theory. I. Counterion Accumulation, Site-Binding, and Their Insensitivity to Polyelectrolyte Shape in Solutions Containing Finite Salt Concentrations. 

P. J. Heath and J. M. Schurr, Macromolecules, 25,4149 (1992). Counterion Condensation. Effects of Site Binding, Fluctuations in Nearest-Neighbors Interactions, and Bending. 

On the other hand t and A r contain the rate constants of the equilibrium under study and, for bimolecular reactions, the concentrations of the species involved in the reaction [4b, 8]. Had counter-ion site binding been a one step process the determination of t would have provided a direct estimation of the lifetime of the bound counter-ions and, therefore, of the exchange rate between bound and free counterions. On the other hand the study of as a function of concentration would have permitted to obtain informations on the distribution of bound counterions between those bound with and without dehydration. Unfortunately, as will be shown in Section 4, site binding is a multistep process involving at least two equilibria. All of the unknown quantities involved in such a process (four rate constants, two volume changes and the concentrations of the species) cannot be obtained from ultrasonic absorption data alone. Independent measurements become necessary. For this purpose we have measured the density d of the polyelectrolyte solutions from which can be obtained the apparent molal volume FcP of the polyelectrolyte CP (C counterion, P polyion) according to ... 

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