Excess Enthalpies (HE) for 2-Butoxyethanol/Water and Determination of Liquid-Liquid Equilibrium Phase Boundaries from Fits to HE of Redlich-Kister, Pade, and Critical-Scaling Equations.

Abstract

Excess enthalpies (HE) for the binary n-butanol/water measured by isothermal flow calorimetry at 30 and 50°C, and nonionic amphiphile 2-butoxyethanol/water at 10 different temperatures (from 48.5 to 70°C) reported in the literature were analyzed for the determination of phase boundaries. HE exhibited S-shape behavior in the former system and U-shape behavior in the latter system. When the HE data in the single-phase were fitted by semi-empirical polynomials and critical-scaling equations, plots of specific HE vs. weight fraction provided more accurate fitting with fewer parameters than conventionally drawn molar HE vs. mole fraction plots. This was due to the enhanced symmetry of specific HE vs. weight fraction plots. Liquid-liquid equilibrium phase boundaries between the single- and two-phase regions were determined from HE. The phase boundary points were obtained as the intersections of the curve and the straight line, which describe the composition dependence of HE for the single- and two-phase regions, respectively. When Redlich-Kister (RK) and Pade polynomials were employed, the phase boundary points could be determined for the n-butanol/water but not for the 2-butoxyethanol/water. These results imply that correct phase boundaries may not be obtained with the semi-empirical polynomials when HE behavior is of the U-type. However, when the critical-scaling equations for HE of binary mixtures were used, the phase boundaries were obtained accurately, irrespective of the type of HE data.