Ionic model, oversimplification

Ionic crystals may be viewed quite simply in terms of an electrostatic model of lattices of hard-sphere ions of opposing charges. Although conceptually simple, this model is not completely adequate, and we have seen that modifications must be made in it. First, the bonding is not completely ionic with compounds ranging from the alkali halides, for which complete ionicity is a very good approximation, to compounds for which the assumption of the presence of ions is rather poor. Secondly, the assumption of a perfect, infinite mathematical lattice with no defects is an oversimplification. As with all models, the use of the ionic model does not necessarily imply that it is true , merely that it is convenient and useful, and if proper caution is taken and adjustments are made, it proves to be a fruitful approach. 

Electrostatic Models. For polar reactions it is possible to develop an electrostatic model. This is necessarily an oversimplification, though it does allow the gross features of such systems to be delineated. Thus, when a complex and the free ligand both react with the same gross mechanism, it is possible to estimate the electrostatic contribution to the change in entropy of activation when the reaction is polar or ionic. Following Laidler (21) we can estimate the electrostatic contribution to the entropy of activation as ... 

Reactions when Y is a solvent molecule are referred to as solvolysis reactions of which hydrolysis [E<(. (XVI. 1.2)] is a special example involving H2O. The dual role of solvent as ionizing agent and as nucleophilic agent is what causes the kinetic difficulties. It is the feeling of the author that much of the controversy in the interpretation of solvolysis reactions can be attributed to the fundamental complexity of the molecular systems in contrast to the oversimplification of the models which have been presented to explain them. In fact, until the ecpiilibrium theory of ionic systems shows considerable progress, it is doubtful if the kinetic data will have much fundamentar interpretation. 

This two-state model, which assumes that surfactant exists either as monomers or micelles, is almost certainly an oversimplification. The mass action model assumes an equilibrium between monomers, n-mers and micelles, with the proviso that the bulk of the surfactant is present as monomers or micelles. In other words micelliza-tion is considered to occur over a narrow range of surfactant concentration, at least for aqueous micelles [1,2,23]. The thermodynamics of micellization have been discussed in terms of the hydrophobic interactions and the electrostatic interactions of the head groups, and, for ionic micelles, of the counterions with the ionic head groups [18,22,24],... 

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