Critical behavior and interfacial tension of mixtures of propylene glycol and ethylene glycol oligomers

Abstract

This investigation presents an analysis of the critical behavior of mixtures of oligomers of propylene glycol, PG₁₇, and ethylene glycol, EG_n, withn=3, 4, 5, 6.4, 8.7, 12.1 and 22.1. The critical coordinates,φ _c andT _c were determined from the phase diagrams. The critical compositions compare very well with the Huggins-Flory predictions. The interaction parameterχ _n is around one for EG₃, EG₄ and EG₅ and it increases up to two for the higher oligomers. The break in the interaction parameter also corresponds to a minimum in the critical temperature. The phase diagrams and the interfacial tension were used to get the critical exponents β and μ, respectively. The data were analysed with two approaches. First, from the temperature dependence of the length of the tie-lines and of the interfacial tension up to the upper critical solution temperature, UCST. Second, with the data at 30°C using the critical temperature of the systems as the variable. The first method led to 〈β〉=0.39±0.05 in good agreement with the result of the second method, 〈β〉=0.37±0.04. The exponents for the interfacial tension, μ, determined with the first method for PG₁₇ with EG_6.4, EG_8.7 and EG_12.1 are μ=1.66±0.11, 1.46±0.25 and 1.73±0.18, respectively. The second method led to 〈μ〉=1.17±0.14. The critical exponents are compared to mean field and ising-3D predictions.