Light scattering investigations in a ternary liquid mixture: Quest for a critical inflection point

We identify the existence of both a double critical point (DCP) and a critical double point (CDP) in a ternary liquid mixture consisting of ethanol plus water plus potassium carbonate. This system has a potential to yield a critical inflection point (CIP), a point at which a DCP and its associated CDP merge. We present measurements of osmotic compressibility $(\chi _T)$ near to and away from the DCP and the CDP. The approach to DCP is signalled by the variation of loop size $(\Delta T)$ from 35 to 2 K. The convexity of the coexistence surface was probed along different thermodynamic paths. The data for $\chi _T$ are quantified better by the mean-field or classical behavior i.e., critical exponent, $\gamma=1$ than by 3D Ising behavior $(\gamma = 1.24)$. The universal value of $\gamma(= 1)$ is recovered for any distance to DCP or CDP by invoking a field variable that is third order in temperature $(^tULU)$ in lieu of the usual field variable (i.e., reduced temperature, t). There is a lack of symmetry between the approaches to the upper and to the lower convex paths near to and away from DCP. One of the unusual features of our findings is a rather large limiting value of the correlation length amplitude $(\xi_0\sim 0.3- 0.8\hspace{2mm} nm)$ for the upper convex paths. We rationalize our observations in terms of the geometric picture and the phenomenological theories of phase transitions. The fortuitous similarity between this solvophobic system and the truly ionic fluids is highlighted