1.7.22: Compressions- Isothermal- Liquid Mixtures Binary- Compressibilities

The isothermal compressibility of a given binary liquid mixture having ideal thermodynamic properties is related to the isothermal compressions of the liquid components using equation (a) [1].

\[\mathrm{K}_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})=\frac{\mathrm{K}_{\mathrm{T} 1}^{*}(\ell)+\mathrm{x}_{2} \,\left[\mathrm{K}_{\mathrm{T} 2}^{*}(\ell)-\mathrm{K}_{\mathrm{T} 1}^{*}(\ell)\right]}{\mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \,\left[\mathrm{V}_{2}^{*}(\ell)-\mathrm{V}_{1}^{*}(\ell)\right]}\]

The excess compression for a given binary liquid mixture is defined by equation (b).

\[\mathrm{K}_{\mathrm{Tm}}^{\mathrm{E}}=\mathrm{K}_{\mathrm{Tm}}(\mathrm{mix})-\mathrm{K}_{\mathrm{Tn}}(\mathrm{mix} ; \mathrm{id})\]

Or,

\[\mathrm{K}_{\mathrm{Tm}}^{\mathrm{E}}=\mathrm{K}_{\mathrm{Tm}}(\mathrm{mix})-\left[\mathrm{x}_{1} \, \mathrm{K}_{\mathrm{T} 1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{K}_{\mathrm{T} 2}^{*}(\ell)\right]\]

The isothermal compressibilities of ideal and real binary liquid mixtures are defined by equations (d) and (e) respectively.

\[\kappa_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})=-\frac{1}{\mathrm{~V}(\operatorname{mix} ; \mathrm{id})} \,\left(\frac{\partial \mathrm{V}(\mathrm{mix} ; \mathrm{id})}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

\[\kappa_{\mathrm{T}}(\operatorname{mix})=-\frac{1}{\mathrm{~V}(\operatorname{mix})} \,\left(\frac{\partial \mathrm{V}(\operatorname{mix})}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

For a given binary liquid mixture we can define an excess compressibility using equation (f).

\[\kappa_{\mathrm{T}}^{\mathrm{E}}=\kappa_{\mathrm{T}}(\operatorname{mix})-\kappa_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})\]

Then

\[\begin{aligned}
&\kappa_{\mathrm{T}}^{\mathrm{E}}(\mathrm{mix})=-\frac{1}{\mathrm{~V}(\mathrm{mix})} \,\left(\frac{\partial \mathrm{V}(\mathrm{mix})}{\partial \mathrm{p}}\right)_{\mathrm{T}} \\
&+\frac{1}{\left[\mathrm{x}_{1} \, \mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{V}_{2}^{*}(\ell)\right]} \,\left(\frac{\partial\left[\mathrm{x}_{1} \, \mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{V}_{2}^{*}(\ell)\right]}{\partial \mathrm{p}}\right)_{\mathrm{T}}
\end{aligned}\]

A similar equation was used by Moelwyn-Hughes and Thorpe [3]. They introduced the concept of a compressibility of the excess volume.

\[\Delta \kappa_{\mathrm{T}}(\operatorname{mix})=-\frac{1}{\Delta \mathrm{V}(\operatorname{mix})} \,\left(\frac{\partial \Delta \mathrm{V}(\mathrm{mix})}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

In publications by Prigogine and by Moelwyn-Hughes and Thorpe the analysis was taken a step further to facilitate analysis of experimental results. However approximations were made in both treatments. An exact formulation was given by Missen [4] in terms of volume fractions of both components in the corresponding having ideal thermodynamic properties, \(\phi_{1}(\text { mix;id })\) and \(\phi_{2}(\text { mix;id })\). Hence,

\[\kappa_{\mathrm{T}}^{\mathrm{E}}(\operatorname{mix})=-\frac{1}{\mathrm{~V}_{\mathrm{m}}(\operatorname{mix})} \,\left[\left(\frac{\partial \mathrm{V}_{\mathrm{m}}^{\mathrm{E}}}{\partial \mathrm{p}}\right)_{\mathrm{T}}+\mathrm{V}_{\mathrm{m}}^{\mathrm{E}} \, \kappa_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})\right]\]

A partial compressibility was defined by Moelwyn-Hughes [5]. For liquid 1 in a binary liquid mixture at defined \(\mathrm{T}\) and \(\mathrm{p}\), the partial compressibility is defined by equation (j).

\[\kappa_{T_{1}}(\operatorname{mix})=-\frac{1}{V_{1}(\operatorname{mix})} \,\left(\frac{\partial V_{1}(\operatorname{mix})}{\partial p}\right)_{T}\]

Similarly for component 2,

\[\kappa_{\mathrm{T} 2}(\operatorname{mix})=-\frac{1}{\mathrm{~V}_{2}(\operatorname{mix})} \,\left(\frac{\partial \mathrm{V}_{2}(\operatorname{mix})}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

The excess compressibility of a given binary liquid mixture \(\mathrm{K}_{\mathrm{T}}^{\mathrm{E}}(\operatorname{mix})\) was defined in equation (f). Hence,

\[\begin{aligned}
\kappa_{\mathrm{T}}^{\mathrm{E}}(\operatorname{mix})=& \phi_{1}(\operatorname{mix}) \, \kappa_{\mathrm{T} 1}^{\mathrm{E}}(\operatorname{mix})+\phi_{2}(\operatorname{mix}) \, \kappa_{\mathrm{T} 2}^{\mathrm{E}}(\operatorname{mix}) \\
&+\left[\phi_{1}(\operatorname{mix})-\phi_{1}(\operatorname{mix} ; \mathrm{id})\right] \, \kappa_{\mathrm{T} 1}^{*}(\ell)+\left[\phi_{2}(\operatorname{mix})-\phi_{2}(\operatorname{mix} ; \mathrm{id})\right] \, \kappa_{\mathrm{T} 2}^{*}(\ell)
\end{aligned}\]

It may be noted that ‘true’ partial properties can also be defined for the isothermal compressibility [6]. Then the properties introduced in equations (j) and (k) would be termed specific partial isothermal compressions [6].

It is also possible to formulate a set of equations incorporating rational activity coefficients for the two components of the binary liquid mixture. We start with the equation for the partial molar volume of component 1.

\[V_{1}(\operatorname{mix})=V_{1}^{*}(\ell)+R \, T \,\left(\frac{\partial \ln \left(f_{1}\right)}{\partial p}\right)_{T}\]

\[K_{T 1}(\operatorname{mix})=K_{T 1}^{*}(\ell)-R \, T \,\left(\frac{\partial^{2} \ln \left(f_{1}\right)}{\partial p^{2}}\right)_{T}\]

Similarly

\[\mathrm{K}_{\mathrm{T} 2}(\operatorname{mix})=\mathrm{K}_{\mathrm{T} 2}^{*}(\ell)-\mathrm{R} \, \mathrm{T} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{2}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}\]

Therefore

\[\mathrm{K}_{\mathrm{Tm}}(\operatorname{mix})=\mathrm{K}_{\mathrm{Tm}}(\operatorname{mix} ; \mathrm{id})-\mathrm{R} \, \mathrm{T} \,\left[\mathrm{x}_{1} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{1}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}+\mathrm{x}_{2} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{2}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}\right]\]

The two liquid components are characterised by their molar excess properties.

\[\mathrm{K}_{\mathrm{T} 1}^{\mathrm{E}}(\operatorname{mix})=-\mathrm{R} \, \mathrm{T} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{1}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}\]

and

\[\mathrm{K}_{\mathrm{T} 2}^{\mathrm{E}}(\operatorname{mix})=-\mathrm{R} \, \mathrm{T} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{2}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}\]

Therefore

\[\mathrm{K}_{\mathrm{Tm}}^{\mathrm{E}}(\mathrm{mix})=-\mathrm{R} \, \mathrm{T} \,\left[\mathrm{x}_{1} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{1}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}+\mathrm{x}_{2} \,\left(\frac{\partial^{2} \ln \left(\mathrm{f}_{2}\right)}{\partial \mathrm{p}^{2}}\right)_{\mathrm{T}}\right]\]

Also

\[\mathrm{K}_{\mathrm{T} 1}^{\mathrm{E}}=-\left(\frac{\partial \mathrm{V}_{1}^{\mathrm{E}}}{\partial \mathrm{p}}\right)_{\mathrm{T}} \text { and } \mathrm{K}_{\mathrm{T} 2}^{\mathrm{E}}=-\left(\frac{\partial \mathrm{V}_{2}^{\mathrm{E}}}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

In other words

\[\mathrm{K}_{\mathrm{Tm}}^{\mathrm{E}}=-\left(\frac{\partial \mathrm{V}_{\mathrm{m}}^{\mathrm{E}}}{\partial \mathrm{p}}\right)_{\mathrm{T}}\]

Isothermal compressions of liquid mixtures can be directly measured [7]. Hamann and Smith [8] report measurements using binary liquid mixtures at \(303 \mathrm{~K}\) and two pressures. Hamann and Smith define excess isothermal molar compressions \(\mathrm{K}_{\mathrm{T}}^{\mathrm{E}}(\phi)\) in terms of volume fraction weighted isothermal compressions of the pure liquids. The volume fractions are defined as follows.

\[\phi_{1}=\mathrm{x}_{1} \, \mathrm{V}_{1}^{*}(\ell) /\left[\mathrm{x}_{1} \, \mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{V}_{2}^{*}(\ell)\right]\]

\[\phi_{2}=x_{2} \, V_{2}^{*}(\ell) /\left[x_{1} \, V_{1}^{*}(\ell)+x_{2} \, V_{2}^{*}(\ell)\right]\]

Then

\[\mathrm{K}_{\mathrm{T}}^{\mathrm{E}}(\phi)=\mathrm{K}_{\mathrm{Tm}}(\mathrm{mix})-\left[\phi_{1} \, \mathrm{K}_{\mathrm{T} 1}^{*}(\ell)+\phi_{2} \, \mathrm{K}_{\mathrm{T} 2}^{*}(\ell)\right]\]

For most binary aqueous mixtures \(\mathrm{K}_{\mathrm{T}}^{\mathrm{E}}(\phi)\) is negative, plots of \(\mathrm{K}_{\mathrm{T}}^{\mathrm{E}}(\phi)\) against \(\phi_{2}\) being smooth curves. The minima in aqueous mixtures containing \(\mathrm{THF}\) and propanone the minima are near 0.4 and 0.6 respectively [0].

Footnotes

[1] For a binary liquid mixture having ideal thermodynamic properties,

\[\mathrm{V}_{\mathrm{m}}(\operatorname{mix} ; \mathrm{id})=\mathrm{x}_{1} \, \mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{V}_{2}^{*}(\ell)\]

Then

\[\mathrm{K}_{\mathrm{Tm}}(\operatorname{mix} ; \mathrm{id})=\mathrm{x}_{1} \, \mathrm{K}_{\mathrm{T} 1}^{*}(\ell)+\mathrm{x}_{2} \, \mathrm{K}_{\mathrm{T} 2}^{*}(\ell)\]

But

\[\kappa_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})=\frac{\mathrm{K}_{\mathrm{Tm}}(\operatorname{mix} ; \mathrm{id})}{\mathrm{V}_{\mathrm{m}}(\operatorname{mix} ; \mathrm{id})}\]

Then,

\[\kappa_{\mathrm{T}}(\operatorname{mix} ; \mathrm{id})=\frac{\mathrm{K}_{\mathrm{T} 1}^{*}(\ell)+\mathrm{x}_{2} \,\left[\mathrm{K}_{\mathrm{T} 2}^{*}(\ell)-\mathrm{K}_{\mathrm{T} 1}^{*}(\ell)\right]}{\mathrm{V}_{1}^{*}(\ell)+\mathrm{x}_{2} \,\left[\mathrm{V}_{2}^{*}(\ell)-\mathrm{V}_{1}^{*}(\ell)\right]}\]

[2] I. Prigogine, The Molecular Theory of Solutions, North Holland, Amsterdam, 1957, p.18.

[3] E. A. Moelwyn-Hughes and P. L. Thorpe, Proc. R. Soc. London, Ser. A,1964,278A, 574.

[4] R. W. Missen, Ind. Eng. Chem. Fundam., 1969,8,81.

[5] E. A. Moelwyn-Hughes, Physical Chemistry, Pergamon, London, 2nd. Edn., 1965, .817

[6] J. C. R. Reis, J. Chem. Soc. Faraday Trans.,1998,94,2385.

[7] J. E. Stutchbury, Aust. J. Chem.,1971,24,2431.

[8] S. D. Hamann and F. Smith, Aust. J. Chem.,1971,24,2431.

[9] For a detailed report on the properties of liquid mixtures see G. M. Schneider, Pure Appl. Chem.,1983,55,479 ; and references therein.