The role of critical points and binodal surfaces in emulsion type, HLB, and the phase inversion temperature: Evidence from the cyclohexane/Water/ i-C 9H 19C 6H 4(OC 2H 4) 9.2OH/Temperature diagram

Abstract

A trigonal prismatic phase diagram has been constructed for the system cychlohexane/water/ i-C 9H 19C 6H 4(OCH 2CH 2) 9.2OH, 10°C < T < 90°C., with emphasis on the region of three coexisting liquids and the contiguous two-phase regions. The lower and upper critical endpoint temperatures for the three-phase region and the binodal surfaces that bound the two-phase regions are located. Examination of the phase diagram shows that, theoretically, there are not two kinds of liquid-liquid emulsions ( O W and W O ), but four. The temperature—composition regions over which “oil-in-water” (“ O W ”) and “water-in-oil” (“ W O ”) emulsions, respectively, have been reported to form are then compared to the phase diagram. It is found that conjugate pairs of phases on one of the binodals always formed one type of emulsion, whereas conjugate phases on the other binodal always formed a second emulsion type. Examination of the literature indicates that this is a general correlation valid for a wide variety of systems, conditions, and methods of emulsion preparation. Consideration of the interfacial tension explains how this correlation might obtain and why (apparently) only two of the four hypothetical emulsion types are observed. Comparison of the phase and emulsion behavior also shows that requirements must be met for the phase (i.e., emulsion) inversion temperature (PIT) to be unique and shows when and why emulsions invert in response to changes of temperature and/or composition. Thus, the comparison establishes the phase behavior that a correct HLB system must be able to predict.