Hydration of ions in solution

The question of the hydration of ions in solution (p. 98) is essentially the same as the question of the formation of complex hydrates in the solid state. 

It should be noted that the above definitions of pH are based on an assumption that the solution is behaving in an ideal nature meaning that the thermodynamic activity is equal to concentration (e.g., what happens when the dilution is infinite). However, as the concentration increases, ionic attraction and incomplete hydration results in a decrease in the effective concentration (or the activity). This activity is defined as the "apparent concentration" of an ionic species, which is due to the attraction that ions exert on one another, and the incomplete hydration of ions in solutions that are too concentrated. The lower the concentration, the less is the interaction. At infinite dilution, activity coefficients approach unity. 

Owing to the experimental difficulties, in situ determination of SAM composition is nearly absent. The only data of this kind were obtained by in situ X-ray reflectivity measurements of the binding of Cd + to a SAM of mercaptohexade-canoic acid (MHA), HOOC(CH2)i5SH, as a function of the solution pH [28]. Complexation of the metal ion by the carboxylate-terminated SAM depends on the protonation equilibrium of —COOH groups in the SAM and hydration of ion in solution. A series of X-ray reflectivity measurements were performed with SAMs prepared on single-crystal Au(lll) substrates in 1 mM CdCl2 solutions of variable pH from 3.9 to 8.5. 

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