Ions. bare entropy

The process of contact adsorption can be viewed in the following way (Fig. 6.90) First, a hole of area of at least 1U —where r, is the radius of the bare ion—is swept free of water molecules in order to make room for the ion. At the same time, the ion strips itself of part of its solvent sheath and then jumps into the hole. During this process, the involved particles—electrode, ion, water molecules—break old attachments and make new ones (change of enthalpy, AH) and also exchange freedoms and restrictions for new freedoms (change of entropy, AS). 

The association of bare, non-solvated ions undoubtedly proceeds without any activation energy and without increase of the entropy of activation. Therefore, the respective ka > 10u M-1s 1. Ions surrounded by tight solvation shells form loose, solvent separated ion-pairs on their encounters, and again it is unlikely that their formation requires activation energy or negativ entropy of activation, i.e., most probably the pertinent ks > 10u M-1s-1. Temperature dependence of ka is determined by temperature dependence of viscosity given by a formal activation energy of 2-3 kcal/mol. 

A similar change was observed by Grenthe and Williams [Gr 67] in a study of the protonation of the acetate ion in water and in a 95% methanol-water mixture. The heat of protonation of the acetate ion changed from AH = —0.22 (in water) to AH= —2.12 in the methanol-water mixture, and the entropy change increased from dS = 6.0 to AS = 1A. The change in the medium had barely any effect on the protonation of the amino group. 

The larger mass, M, of the solvated ion than that of the bare one yielded the first term (Section 4.4.3, with = 0.360 Iz, I/(r,/nm) for aqueous solutions), and (26.0JK" mol" for water) was the molar translational entropy of the liquid solvent that did not participate in the solvent structural effects. The supposition that sodium ions were indifferent with respect to the water structure making and structure breaking was then made [50] ... 

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