Effect of potassium thiocyanate on the phase diagram of Methyl Ethyl Ketone + secondary Butyl Alcohol + Water: Search for the Quadruple Critical Point

Abstract

The effect of addition of small amounts of the ionic impurity Potassium Thiocyanate (KSCN) on the tunnel-like region in the phase diagram of Methyl Ethyl Ketone (MEK) + secondary Butyl Alcohol (sBA) + Water (W) is investigated in detail. The KSCN weight fraction range, 0 ≤ XKSCN ≤ 0.0318, results in a systematic decrease in the diameter of the critical-tunnel from ΔT = 71.416 K at XKSCN = 0, to ΔT = 5.275 K at XKSCN = 0.0315. For 0.03190 ≤ XKSCN ≤ 0.03195 the line of critical points end abruptly at specific values of water weight fractions (XW) resulting in the formation of several critical end points. For XKSCN ≥ 0.0320 the critical-tunnel splits into several pairs of parabolic critical lines, each carrying double critical points (DCPs) at their vertices, with one-phase region in-between. The distance between the DCPs corresponding to the low- and high-MEK regions can be made to approach or recede from each other depending on the KSCN weight fraction. The QCP in this system is pushed into a hypothetical region due to curvature effects of the coexistence surface in MEK + W + sBA + KSCN.