Excess enthalpies (H-E) for 2-butoxyethanol/water and determination of liquid-liquid equilibrium phase boundaries from fits to H-E of Redlich-Kister, Pade, and critical-scaling equations

Abstract

Excess enthalpies (H-E) for the binary n-butanol/water measured by isothermal flow calorimetry at 30 and 55 degreesC, and nonionic amphiphile 2-butoxyethanol/water at 10 different temperatures (from 48.5 to 70 degreesC) reported in the literature were analyzed for the determination of phase boundaries. H-E exhibited S-shape behavior in the former system and U-shape behavior in the latter system. When the H-E data in the single-phase were fitted by semi-empirical polynomials and critical-scaling equations, plots of specific H-E vs. weight fraction provided more accurate fitting with fewer parameters than conventionally drawn molar H-E vs, mole fraction plots. This was due to the enhanced symmetry of specific H-E vs. weight fraction plots. Liquid-liquid equilibrium phase boundaries between the single- and two-phase regions were determined from H-E. The phase boundary points were obtained as the intersections of the curve and the straight line, which describe the composition dependence of H-E for the single- and two-phase regions, respectively. When Redlich-Kister(RK) and Fade 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 H-E behavior is of the U-type, However, when the critical-scaling equations for H-E of binary mixtures were used, the phase boundaries were obtained accurately, irrespective of the type of H-E data.