Characterization of a two-aqueous phase system containing a nonionic surfactant

A surfactant-containing two-aqueous phase system is composed of a coacervate (surfactant-rich) phase and a dilute one (surfactant-poor). This work aims at characterizing such a two-phase system by using a recently developed optical analyzer: Turbiscan LAb Expert (R). This optical device allows investigating the behavior of concentrated dispersed media through light transmission or back-scattering measurements at constant temperature. Above the cloud point of the Triton X114 solution, two distinct phenomena were observed and followed as a function of time: phase separation, with interface formation, and phase clarification, during which very small drops of one phase join the other after the interface has appeared. The process of phase separation, rather fast, appears to be composed of three steps: (i) induction period with no distinct phases; (ii) rapid migration of droplets and interface appearance; (iii) slow displacement of the interface up to its equilibrium position. As regards clarification, the Turbiscan diagrams reveal different behaviors according to the proximity of the cloud point (T-c). About 5 degrees C above T-c, the lighter (dilute) phase begins to clarify as the interface appears, the coacervate remaining turbid. In fact, the time required for complete phase clarification, i.e., corresponding to maximum (equilibrium) transmission intensity value, is 10-18 h. At 10 degrees C above the cloud point, the two phases do not reach equilibrium after two days. The coacervate phase then shows an oscillatory behavior with a characteristic period of approximately 25 h. At higher temperatures, i.e., farther from the critical point, a chaotic behavior can be observed.