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 σac and σbc of the two liquid phases against air: In the water-free mixture one finds w_σ{≡}(σbc-σac)/σab>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 σac and σab increase steeply, whereas σbc is less affected, which makes w_σ decrease. As soon as w_σ1, the b/c interface becomes stable and the system gradually disposes with the layer of phase a.