Previous studies have established that t h e extended coordination model of solvation can satisfactorily account for t h e variation in t h e transfer enthalpies of solutes in mixed-solvent systems. However, the model parameter relating to t h e solute-induced disruption of the solvent structure shows a marked dependence on the nature of t h e mixed solvent. This result is not consistent with the underlying model of t h e solvation process.In t h e present paper we report t h e transfer enthalpies of tetrabutylammonium bromide, for which this dependence is large, into a series of highly aqueous mixed solvents. Analysis of these in terms of the extended coordination model confirms both t h e model's ability to account for the experimental data, and the variability of the structural disruption parameter. The possible origins of this latter result are considered in detail.We have recently reported the enthalpies of transfer, At He, of several solutes from water to aqueous organic solvent mixtures.'-' These data were considered in terms of the extended coordination model of ~olvation.~.' These studies have revealed the existence of a transition in the solvating properties of the aqueous systems at some critical composition, x;, which depends on the organic cosolvent. They also showed that the extent to which the solutes disrupt the solvent structure, as measured by the model parameter (an + BN), varied
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