Experimental study of interfacial phase transitions in three-component surfactant systems

In a three-phase equilibrium system, the middle phase either wets or does not wet the interface between the upper and lower phases. In this work, we perform a systematic experimental search for wetting transitions between wetting and nonwetting regimes in three-component water--n-alkane--Cᵢ{E}ⱼ$ systems, where ${C}ᵢEⱼ denotes the surfactant polyoxyethylene alcohol Cᵢ{H}₂ᵢ₊₁$(${OC}₂H₄{)}ⱼ$OH. It is found that the water--n-hexadecane--${C}₆E₂ and water--n-octadecane--C₆{E}₂systems exhibit a wetting transition lying at least 10^∘ C below the upper critical temperature, and one system, water--n-tetradecane--{C}₆E₂, exhibits a wetting transition lying 4.3^∘{} C below the upper critical temperature. Measurements of the contact angles as a function of temperature indicate that the wetting transition is of first order.