Wetting-nonwetting transition at the liquid-air interface of methanol-cyclohexane-water mixtures

In mixtures of methanol and cyclohexane, a layer of the lower methanol-rich phase (a) intrudes between the upper cyclohexane-rich phase (b) and air (c). As water is added, the layer disappears. In the literature, this effect is interpreted as a wetting-nonwetting transition near a critical point. We suggest instead considering the transition as being caused by the effect of water on the surface tensions ${\sigma }_{\mathrm{ac}}$ and ${\sigma }_{\mathrm{bc}}$ of the two liquid phases against air: In the water-free mixture one finds $w_{\sigma }$≡(${\sigma }_{\mathrm{bc}}$-${\sigma }_{\mathrm{ac}}$)/${\sigma }_{\mathrm{ab}}$>1. Consequently, the b/c interface is thermodynamically unstable, and the system minimizes its free energy by placing a layer of phase a between phase b and air. As water is added, both ${\sigma }_{\mathrm{ac}}$ and ${\sigma }_{\mathrm{ab}}$ increase steeply, whereas ${\sigma }_{\mathrm{bc}}$ is less affected, which makes $w_{\sigma }$ decrease. As soon as $w_{\sigma }$b/c interface becomes stable and the system gradually disposes with the layer of phase a.