1.10.35: Gibbs Energies- Liquid Mixtures- Immiscibility
For a given binary liquid mixture (at defined \(\mathrm{T}\) and \(\mathrm{p}\)) characterised by a plot of excess Gibbs energy \({\mathrm{G}_{\mathrm{m}}}^{\mathrm{E}}\) against mole fraction, \({\mathrm{G}_{\mathrm{m}}}^{\mathrm{E}}\) can be positive. Indeed if \(\left[\mathrm{G}_{\mathrm{m}}^{\mathrm{E}} / \mathrm{R} \, \mathrm{T}\right]\) strongly exceeds 0.5, the mixture is partially miscible. That is to say the liquid comprises two liquid phases having different mole fraction compositions.
A fascinating variety of patterns emerge in the context of partial miscibilities.
- Some binary liquid mixtures are completely miscible but become partially miscible with increase in temperature. The corresponding miscibility curve has a minimum at a Lower Critical Solution Temperature, LCST. For example in the case of 2-butoxyethanol + water , the LCST is at \(322.2 \mathrm{~K}\) where \(x\left(\mathrm{H}_{2}\mathrm{O}\right)=0.942\) [1]. In fact all commonly quoted examples of this class of systems have water as one component. A fascinating example concerns propionitrile+ polystyrene mixtures. The miscibility curves indicate that the LCST occurs at negative pressures; in effect when the mixture is ‘stretched’ [2].
- Many binary liquid mixtures (e.g/ phenol + water has (at ambient pressure).are partially miscible, becoming completely miscible on raising the temperature. The miscibility curve has a maximum at an Upper Critical Solution Temperature, UCST. At ambient pressure a small number of liquid mixtures exhibit both UCST and LCST. In other words the miscibility plot forms a closed loop.
Partial miscibility plots also show deuterium isotope effects. In the case of \(\mathrm{CH}_{3}\mathrm{CN}+\mathrm{H}_{2}\mathrm{O} \left(\text{component } 2 = \mathrm{~CH}_{3}\mathrm{CN}\right)\) the UCST is \(272.10 \mathrm{~K}\) at \(x_{2}=0.38\) [2].
Footnotes
[1] A. Imre and W.A. Van Hook, J. Polym Sci.; Part B; Polymer Physics,1994, 32 ,2283.
[2] M. J. Blandamer, M. J. Foster and D. Waddington, Trans. Faraday Soc.,1970, 66 ,1369.