CRITICAL-BEHAVIOR AND INTERFACIAL-TENSION OF MIXTURES OF PROPYLENE-GLYCOL AND ETHYLENE-GLYCOL OLIGOMERS

This investigation presents an analysis of the critical behavior of mi xtures of oligomers of propylene glycol, PG(17), and ethylene glycol, EG(n), with n = 3, 4, 5, 6.4, 8.7, 12.1 and 22.1. The critical coordin ates, phi(c), and T-c, were determined from the phase diagrams. The cr itical compositions compare very well with the Huggins-Flory predictio ns. The interaction parameter chi(12) is around one for EG(3), EG(4) a nd EG(5) and it increases up to two for the higher oligomers. The brea k in the interaction parameter also corresponds to a minimum in the cr itical temperature. The phase diagrams and the interfacial tension wer e used to get the critical exponents beta and mu, respectively. The da ta were analysed with two approaches. First, from the temperature depe ndence of the length of the tie-lines and of the interfacial tension u p to the upper critical solution temperature, UCST. Second, with the d ata at 30 degrees C using the critical temperature of the systems as t he variable. The first method led to [beta] = 0.39 +/- 0.05 in good ag reement with the result of the second method, [beta] = 0.37 +/- 0.04. The exponents for the interfacial tension, mu, determined with the fir st method for PG(17), with EG(6.4), EG(8.7) and EG(12.1) are mu = 1.66 +/- 0.11, 1.46 +/- 0.25 and 1.73 +/- 0.18, respectively. The second m ethod led to [mu] = 1.17 +/- 0.14. The critical exponents are compared to mean field and Ising-3D predictions.