Effective size of the ions

Complexation by reagents other than the monomer usually has several consequences the reduction of the charge density by the complexant and the increase in the effective size of the ion will both increase the KD of the propagating ion-pair so that y/p increases and also the unpaired ion is now encumbered by something different from the solvent molecules which solvate it in the absence of complexant, and one cannot predict generally whether the balance of these effects will augment or diminish the propagation, transfer, and termination rates. 

The hydrated radius of an ion is the effective size of the ion plus its tightly bound sheath of water molecules, which are attracted by the positive or negative charge of the ion (Figure 12-3). The large species Li(H20) does not have as much access to the resin as the smaller species K(H20). ... 

Eaker and Porath studied the elution of amino acids on Sephadex 6-10 with particular focus on the eluant ionic strength and composition (e.g. acetic acid or pyridine content) (11). They described the gel as a "weak exchanger," thus accounting for effective separations of mixtures of up to seven amino acids. They noted that K ec increases with ionic strength, and attributed this effect to variations in the "effective size" of the ion, which includes the "electrical double layer and the hydration layer." In addition to discussing the effects of ionic sites on gel and solute, Eaker and Porath also discussed the "aromatic adsorption" of solutes with coplanar II electron systems, and the hydrophobic adsorption of partly apolar solutes (accompanied by a positive enthalpy change). They pointed out that such "short range" effects require "intimate contact" between solute and gel and are unlikely to occur in the presence of electrical repulsion. 

Because the electrons in a metal become delocalized, it may seem strange that the hard sphere size of the ions cores consistent with the measured density of the solid is larger than the ionic diameter used to calculate the size of ionic compoimds. The explanation is that a delocalized electron is not completely lost from the ion core as it is when transferred to an electronegative atom. The delocalized electrons are shared among all of the ion cores so the effective size of the ion core is less diminished when its valence electrons delocalized. 

In all cases, the activity coefficients of the electrolytes first sharply decrease, due to the strong electrostatic interactions between cations and anions. After a certain concentration of salt, a classical ion specific effect appears the decrease is much smaller with further increase of salt concentration or even there is a change to a significant increase. Very roughly and qualitatively speaking, this phenomenon is attributed to the hydration of the ions and also due to the increasing repulsion between the hydrated ions. As can be seen in Fig. 2(a), for a given concentration, the values of the activity coefficients increase in the series Cs < Rb < K+ < Na < Li+ < H+. This behaviour is classically explained by an increase of the effective size of the ions in the same direction. Effective means that the first hydration shell is considered to be part of the ion so that the series is just opposite to the series of the sizes of the bare ions. 

Figure 19-4 contrasts the effective sizes of the halide ions. Each of these dimensions is obtained from the examination of crystal structures of many salts involving the particular halide ion. The effective size found for a given halide ion is called its ionic radius. These radii are larger than the covalent radii but close to the van der Waals radii of neutral atoms. 

As far as the anions are concerned, some form of homo-conjugation is the most common variant. For example, the poor efficiency of many oxo-acids AH as initiators is (partly) due to the formation of the homo-conjugate anion A2H". Evidently, because the effective size of the A2H ion is greater and its charge density less than for A, its ion-pair formation constant will be smaller. 

However, when adsorption of ionic species takes place on solid electrodes, it is difficult to decide what particular characteristic—surface heterogeneity, transfer of charge, lateral interactions, displacement of adsorbed solvent, size of the ions, etc.—is dominant in the process or which one can be neglected. Nonetheless, would it not be possible to include all these effects in a single isotherm It is possible, although not easy. In the following sections we will introduce the development of one isotherm for ionic adsorption where many of these distinctive characteristics of ionic adsorption are considered. 

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