Associated ions

Multiphoton ionization. Occurs when an atom or molecule and its associated ions have energy states in which they can absorb the energy in two or more photons. 

A criterion for the presence of associated ion pairs was suggested by Bjerrum. This at first appeared to be somewhat arbitrary. An investigation by Fuoss,2 however, threw light on the details of the problem and set up a criterion that was the same as that suggested by Bjerrum. According to this criterion, atomic ions and small molecular ions will not behave as strong electrolytes in any solvent that has a dielectric constant less than about 40. Furthermore, di-divalent solutes will not behave as strong electrolytes even in aqueous solution.2 Both these predictions are borne out by the experimental data. 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

As already mentioned, the criterion of complete ionization is the fulfilment of the Kohlrausch and Onsager equations (2.4.15) and (2.4.26) stating that the molar conductivity of the solution has to decrease linearly with the square root of its concentration. However, these relationships are valid at moderate concentrations only. At high concentrations, distinct deviations are observed which can partly be ascribed to non-bonding electrostatic and other interaction of more complicated nature (cf. p. 38) and partly to ionic bond formation between ions of opposite charge, i.e. to ion association (ion-pair formation). The separation of these two effects is indeed rather difficult. 

When isomeric ions were produced by ternary association in the flow tube, thus allowing the potential surface to be accessed, the chemistry was more complicated. The gas with which the ions associated was added upstream and sufficient time was allowed for the association reaction to proceed before the reactant gas was added. When the associated ions are initially produced, they will be stable against dissociation if ternary collisions with the He remove sufficient energy to take them below the dissociation limits. However, they will still be internally excited and this excitation needs to be removed before the reactivity is probed. Again, the bulk of the evidence suggests that this de-excitation has occurred before the reactant gas is added. In a few cases there is some indication of residual excitation (see Section... 

Neither the electronic absorption nor the emission spectrum of Re2Cl8 changes in the presence of the quenchers, and no evidence for the formation of new chemical species was observed in flash spectroscopic or steady-state emission experiments. The results of these experiments suggest that the products of the quenching reaction form a strongly associated ion pair, Re2Cl8 D+. 

The salting-in effect may be used to increase the solubility of a drug substance through the formation of associated ion pairs, most commonly making use of anionic countering (hydrochloride being the most popular). Detailed reviews of pharmaceutical salts have been published, which contain extensive tables of anions and cations acceptable for pharmaceutical use [44,47]. These articles also describe useful processes for the selection of the most desirable salt... 

Associated ions in this context means an association species held together (albeit transiently) via electrostatic interactions. 

In the preceeding section mention was made of ion association (ion-pairing) which, for the purposes of this paper, will refer to coulombic entities with or without cosphere overlap. Experimental support for ion-pairing has come from sound attenuation (2). Raman spectroscopy (2) and potentiometry (2, 2). Credibility has resulted from the model of Fuoss (2) applied by Kester and Pytkowicz (2). 

Thus, in a medium of low dielectric constant the ions will undergo ion association. Associated ions, such as ion pairs of 1 1 electrolytes will not contribute to the conductivity of the solution at low field strengths. Furthermore, Coulomb s law explains why ions of equal charge but of different size are associated to a different degree in a medium of given dielectric constant a compound consisting of big ions is more dissociated than one with small ions cesium hydroxide is a stronger base than potassium hydroxide. On the other hand, various halides of the alkali metal ions do not obey this law 2>. 

Ionization of a covalent compound may be defined as the process leading to the formation of solvated ions independent of their presence as associated ions or as free entities (Fig. 6). In a medium of low dielectric constant the formation of associated ions is favored. It is therefore conceivable to consider the overall process of ionization as consisting of two steps, i.e., the formation of associated ions due to cation-coordination and anion-solvation and the dissociation of the associated ions in solution as a dielectric effect. 

The equilibrium constant for the formation of associated ions may be termed ionization constant K on... 

The equilibrium constant for the formation of free ions from associated ions may be termed dissociation constant A iss since the process represents the dissociation of associated ions. Aoiss is a function of the dielectric constant of the medium. The reciprocal value is also termed association constant ATass f°r the association of free ions. 

It has been pointed out, that the process of the formation of associated ions in a medium of e = 1 will be favored n>13) a) by increase in donicity of D,... 

It has been pointed out, that the dielectric constant is an important quantity in determining the extent of dissociation of associated ions. The dissociation constants Aj)iss of quaternary ammonium salts may be considered as a qualitative indication of the dielectric properties of a solvent. For example, in a medium of e 10 A diss 10 4 and in a medium of e 35 Aoiss 10 2 ... 

The dielectric properties of the solvent have also an influence on the ionization constant of an incompletely ionized substrate. By the process of ion dissociation the concentration of associated ions is decreased this results because the latter are in equilibrium with non-ionized species and the ionization equilibrium will be restored by the formation of additional associated ions. 

Taken together these results suggest that the quaternary ammonium ions must not form closely associated ion pairs in acetone solutions. In this more polar solvent, it appears as if the reactants are present as solvent separated ion pairs and that the rate of reaction is, consequently, not effected by the structure of the quaternary ammonium ion. In methylene chloride solutions, where theory predicts tighter ion pairs (61), the ions must be intimately associated in either (or both) the ground state and the transition... 

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