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–${\mathrm{C}}_{\mathit{i}}$ ${\mathrm{E}}_{\mathit{j}}$ systems, where ${\mathrm{C}}_{\mathit{i}}$ ${\mathrm{E}}_{\mathit{j}}$ denotes the surfactant polyoxyethylene alcohol ${\mathrm{C}}_{\mathit{i}}$ ${\mathrm{H}}_{2\mathit{i}+1\mathrm{}}$(${\mathrm{OC}}_2$ ${\mathrm{H}}_4$ ${)}_{\mathit{j}}$OH. It is found that the water–n-hexadecane–${\mathrm{C}}_6$ ${\mathrm{E}}_2$ and water–n-octadecane–${\mathrm{C}}_6$ ${\mathrm{E}}_2$ systems exhibit a wetting transition lying at least 10° C below the upper critical temperature, and one system, water–n-tetradecane–${\mathrm{C}}_6$ ${\mathrm{E}}_2$, 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.