Tuning phase transitions and realization of special thermodynamic states in alcohol–water mixtures by the addition of ions

We demonstrate with the aid of visual investigations that multiply reentrant liquid–liquid transitions can be obtained in ethyl alcohol (E) – water (W) mixtures, by the addition of ions. Multiplicity of reentrance can be tuned by changing the concentration, size, or the structure forming ability of the ions added. This is a route to the realization of various thermodynamic states, such as special critical points (SCPs), in the vanishing limit of reentrance. We show that in E+W+ ion systems, one can achieve a plait point, a critical double point, a double critical point, and a critical inflection point, in addition to a line of critical points. We adduce evidence for the presence of supramolecular structures and the associated nanoscopic length scale in these systems, by small angle X-ray scattering experiments. Structuring gives rise to the intriguing phase behavior and renders these systems potentially significant to study the role of supramolecular structuring on the approach to asymptotic Ising criticality near SCPs.