1.10.10: Gibbs Energies- Solutions- Cosphere-Cosphere Interactions

For neutral solutes in aqueous solutions, the solvent plays a key role in determining the form and magnitude of solute-solute interactions. One description of these interactions uses the Gurney model for solute cospheres.[1] Cosphere-cosphere interaction, involving the solvent, feeds back to the properties of the solute; i.e. Gibbs - Duhem communication. Two limiting cases are identified.

1. The hydration characteristics (i.e. solute-solvent interaction) of the two solute molecules, \(\alpha\) and \(\beta\) are quite different such that in the region of overlap their structural influence on water-water interactions are incompatible/antagonistic. Thus the pairwise Gibbs energy interaction parameter \(\mathrm{g}_{j j}\) is positive; \(\gamma_{j} > 1\) and \(\ln \left(\gamma_{\mathrm{j}}\right)>0\). The intuitive idea is that incompatability is synonymous with repulsion (almost human behaviour-- a dangerous anthropomorphic argument).
2. In the second case, the hydration cospheres of the two solute molecules α and β are similar to the extent that in the region of cosphere overlap there is an enhancement of water-water interactions and hence attraction. Thus the pairwise Gibbs energy interaction parameter \(\mathrm{g}_{j j}\) is negative; \(\gamma_{j} < 1\) and \(\ln \left(\gamma_{j}\right)<0\). Where solute molecules α and β are apolar this attraction is called hydrophobic bonding/attraction.

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