1.22.10: Volume: Aqueous Binary Liquid Mixtures

Typically Aqueous Mixtures

For these systems \(\left[\mathrm{V}_{2}^{\infty}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) is negative. But this pattern is not unique to aqueous systems. The unique feature is the decrease in \(\left[\mathrm{V}_{2}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) with increase in \(\mathrm{x}_{2}\) at low \(\mathrm{x}_{2}\) [1]. In fact with increase in hydrophobicity of chemical substance 2, the decrease is more striking and the minimum in \(\left[\mathrm{V}_{2}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) occurs at lower \(\mathrm{x}_{2}\). At mole fractions beyond \(\mathrm{x}_{2}\left[\mathrm{~V}_{2}(\mathrm{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) increases with increase in \(\mathrm{x}_{2}\). Many explanations have been offered for this complicated pattern. The following is one explanation.

The negative \(\left[\mathrm{V}_{2}^{\infty}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) is accounted for in terms of a liquid clathrate in which part of the hydrophobic group ‘occupies’ a guest site in the liquid water ‘lattice’. The decrease in \(\left[\mathrm{V}_{2}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) is accounted for in terms of an increasing tendency towards a liquid clathrate hydrate structure. With increase in \(\mathrm{x}_{2}\) there comes a point where there is insufficient water to construct the liquid clathrate host. Hence \(\left[\mathrm{V}_{2}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) increases [2,3].

Typically Non-Aqueous Systems

Although \(\left[\mathrm{V}_{2}^{\infty}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\) is negative no minimum is observed in \(\left[\mathrm{V}_{2}(\operatorname{mix})-\mathrm{V}_{2}^{*}(\lambda)\right]\).

Footnotes

[1] See for example; fluoroalcohols(aq); C. H. Rochester and J. R. Symonds, J. Fluorine Chem.,1974,4,141.

[2] F. Franks, Ann. N. Y. Acad. Sci.,1955,125,277.

[3] For many binary aqueous mixtures the patterns in volume related properties often identify transition points at ‘structurally interesting compositions’; G. Roux, D. Roberts, G. Perron and J. E. Desnoyers, J. Solution Chem.,1980,9,629.