Temperature-Induced Reversible-Phase Transition in a Surfactant-Free Microemulsion

Microemulsion represents an important class of the colloidal system, though the development of stimuli-responsive microemulsion is still in its infancy. Here, we demonstrated the temperature responsiveness of a conventional surfactant-free microemulsion composed of n-octanol as nonpolar phase, ethanol as amphi-solvent, and water as polar phase for the first time. In the single-phase region of the phase diagram, the pre-ouzo zone was confirmed by dynamic light scattering (DLS), and the type of microemulsion was confirmed via the conductivity and polarity probe methods. The effects of temperature on the phase behavior and droplet size of the n-octanol–water–ethanol microemulsion system were systemically evaluated by the ternary phase diagram and DLS techniques. The results showed that the area of single-phase increases upon increasing temperature, but the area of pre-ouzo zone decreases accompanied by a decrease in the droplet size. Moreover, the critical point gradually draws close to the n-octanol corner with increasing temperature. When one formulation is far away from the demixing border, the droplet size can be reversibly and precisely regulated by changing temperature. When one formulation is located on the vicinity of the boundary, a minor variation in temperature can lead to a prominent phase transition between Winsor IV (high temperature) and Winsor II (low temperature). Such a temperature-responsive microemulsion can be used as a microreactor for Knoevenagel condensation. The reaction was carried out at 35 °C, and the product was collected from the water phase by simple filtration at 25 °C.