Phase Behavior of Benzyltributylphosphonium Salts in Aromatic Hydrocarbons or Aqueous Solutions

Abstract The solubility of benzyltributylphosphonium salt (BTBPX: X = Cl, Br, or I) in aromatic hydrocarbons was examined as a function of the temperature. The solubility curve had specific features, which were characterized by critical values corresponding to the Krafft point and critical micelle concentration. A slight difference in the oil structure exerted a significant effect on the solubility behavior. The Krafft boundary as well as a liquid–liquid immiscibility gap, on the other hand, featured the solubility behavior of BTBPX in an aqueous solution. The shape of the gap was assumed to be a closed loop and the region of the gap was sharply dependent on the concentration of the added metal halide (MX). The phase behavior of the three- or four-component systems, composed of BTBPX, benzene, water, and MX, has been examined in detail. According to their constituent, composition, and temperature, the system afforded a single- to four-phase state form. There were at least three kinds of liquid phases: a BTBPX-rich phase (M), an aqueous solution phase (W), and an oil solution phase (O). When these three liquid phases coexist, the system affords an O–M–W three-liquid-phase equilibrium state. The three-liquid phase may be converted to O–M or M–W two-liquid-phase, depending on the temperature and MX concentration in the W phase. In the presence of excess MX (S), BTBPCl gave an O–M–W–S four-phase state, while BTBPBr or BTBPI never formed a four-phase state. The phase behavior of the four-component systems is discussed based on phase diagrams of fewer component systems.