1.18.5: Liquid Mixtures: Binary: Pseudo-Excess Properties
At defined \(\mathrm{T}\) and \(\mathrm{p}\), a thermodynamic (molar) property \(\mathrm{P}\) of an ideal binary liquid mixture (e.g. volume) can be expressed as a function of the mole fraction composition using equation (a).
\[\mathrm{P}(\mathrm{mix} ; \mathrm{id})=\mathrm{x}_{1} \, \mathrm{P}_{1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{P}_{2}^{*}(\ell) \nonumber \]
Here \(\mathrm{P}_{1}^{*}(\ell)\) and \(\mathrm{P}_{2}^{*}(\ell)\) are the properties of the two pure liquids at the same \(\mathrm{T}\) and \(\mathrm{p}\). In many cases equation (a) is taken as a pattern on which to base a description of other properties of liquid mixtures; e.g. relative permitivities, surface tensions and viscosities. There is often no thermodynamic basis for this description although it has to be admitted that such an equation has an intuitively attractive form. In the next stage the difference between measured property \(\mathrm{P}(\mathrm{mix})\) and \(\mathrm{P}(\text { mix } ; \mathrm{id})\) leads to a defined pseudo-excess property. \(\mathrm{P}^{\mathrm{E}}\).
For the sake of completeness, the use of molar changes on mixing is recommended in the present context. Thus,
\[X_{m}(n o-\operatorname{mix})=x_{1} \, X_{1}^{*}(\ell)+x_{2} \, X_{2}^{*}(\ell) \nonumber \]
Then by definition at common temperature and pressure,
\[\Delta_{\text {mix }} X_{m}(\operatorname{mix})=X_{m}(\operatorname{mix})-X_{m}(n o-m i x) \nonumber \]