1.14.21: Donnan Membrane Equilibria

A given experimental system comprises two compartments, I and II, separated by a membrane. The two compartments contain aqueous solutions at common temperature and pressure. The experimental system is set up by placing in compartment I an aqueous salt solution; e.g. \(\mathrm{NaCl}(\mathrm{aq})\) having concentration \(\mathrm{c}_{1} \mathrm{~mol dm}^{-3}\). However compartment II contains a salt, \(\mathrm{R}^{+}\mathrm{Cl}^{-} (\mathrm{aq})\), concentration \(\mathrm{c}_{2} \mathrm{~mol dm}^{-3}\). The membrane is permeable to both \(\mathrm{Na}^{+} and \(\mathrm{Cl}^{-}\) ions but not to \(\mathrm{R}^{+}\) cations. The sodium and chloride ions spontaneously diffuse across the membrane until the two solutions are in thermodynamic equilibrium. We represent the equilibrium system as follows where | | represents the membrane.

[I]

\[ \mathrm{Na}^{+}\left(\mathrm{c}_{1}-\alpha\right)^{\mathrm{cq}} \mathrm{Cl}^{-}\left(\mathrm{c}_{1}-\alpha\right)^{\mathrm{eq}}||[\mathrm{II}] \mathrm{Na}^{+}(\alpha)^{\mathrm{eq}} \mathrm{R}^{+}\left(\mathrm{c}_{2}\right)^{\mathrm{eq}} \mathrm{Cl}^{-}\left(\mathrm{c}_{2}+\alpha\right)^{\mathrm{eq}} \nonumber \]

The solutions on both sides are electrically neutral. A thermodynamic analysis is somewhat complicated if account is taken of the role of ion-ion interactions. However the essential features of the argument are revealed if we identify the activities of the ions as equal to their concentrations.

Hence at equilibrium at fixed \(\mathrm{T}\) and \(\mathrm{p}\),

\[\begin{aligned}
&{\left[\mu^{0}\left(\mathrm{Na}^{+} ; \mathrm{aq}\right)+\mathrm{R} \, \mathrm{T} \, \ln \left\{\left(\mathrm{c}_{1}-\alpha\right)_{\mathrm{Na}}^{\mathrm{eq}} / \mathrm{c}_{\mathrm{r}}\right\}\right]_{\mathrm{I}}} \\
&+\left[\mu^{0}\left(\mathrm{Cl}^{-} ; \mathrm{aq}\right)+\mathrm{R} \, \mathrm{T} \, \ln \left\{\left(\mathrm{c}_{1}-\alpha\right)_{\mathrm{Cl}}^{\mathrm{eq}} / \mathrm{c}_{\mathrm{r}}\right\}\right]_{\mathrm{I}}= \\
&{\left[\mu^{0}\left(\mathrm{Na}^{+} ; \mathrm{aq}\right)+\mathrm{R} \, \mathrm{T} \, \ln \left\{\alpha_{\mathrm{Na+}}^{\mathrm{eq}} / \mathrm{c}_{\mathrm{r}}\right\}\right]_{\mathrm{II}}} \\
&+\left[\mu^{0}\left(\mathrm{Cl}^{-} ; \mathrm{aq}\right)+\mathrm{R} \, \mathrm{T} \, \ln \left\{\left(\mathrm{c}_{2}+\alpha\right)_{\mathrm{Cl-}}^{\mathrm{eq}} / \mathrm{c}_{\mathrm{r}}\right\}\right]_{\mathrm{II}}
\end{aligned} \nonumber \]

But

\[\left(c_{1}-\alpha\right)_{\mathrm{Na+}}^{\mathrm{eq}}=\left(\mathrm{c}_{1}-\alpha\right)_{\mathrm{Cl}-}^{\mathrm{eq}} \nonumber \]

Or,

\[\left[\left(c_{1}-\alpha\right)_{\mathrm{Na}+}^{\mathrm{eq}}\right]^{2}=\alpha_{\mathrm{Na}+}^{\mathrm{eq}} \,\left(\mathrm{c}_{2}+\alpha\right)^{\mathrm{eq}} \mathrm{CI}^{-} \nonumber \]

Then [1],

\[\frac{\alpha^{\mathrm{eq}}}{\mathrm{c}_{1}}=\frac{\mathrm{c}_{1}}{2 \, \mathrm{c}_{1}+\mathrm{c}_{2}} \nonumber \]

The latter is Donnan’s Equation. The ratio \(\left(\alpha^{\mathrm{eq}} / \mathrm{c}_{1}\right)\) tends to be smaller the larger is \(\mathrm{c}_{2}\). This conclusion is confirmed by experiment.

We have simplified the algebra by writing \(\mathrm{R}^{+} \mathrm{Cl}^{-}\) as the salt in compartment II. In practice the Donnan equilibrium finds major application where salt \(\mathrm{RCl}\) is a macromolecule [2-4].

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