Coexistence curve and diameter of the system ethylene glycol mono-isobutyl ether + water

Abstract

A coexistence-curve measurement was made for the system ethylene glycol mono-isobutyl ether + water, which has a closed-loop coexistence curve. The measurement was made at the lower critical solution temperature. The diameter of the coexistence curve was analysed for several composition variables. For the volume fraction and the mole fraction the critical exponent µ for the diameter was nearly the same and very small, µ= 0.65. This indicates an incorrect order parameter in view of the relationship µ= 2β. The correct order parameter was found from the behaviour of the diameter. A range variation analysis for the composition difference shows that the value of β for the coexistence curve is almost constant irrespective of the choice of the composition variable. The recommended critical exponent is β= 0.332 ± 0.002, which is in good agreement with the value of β for the system polystyrene + methylcyclohexane obtained using the same experimental apparatus. A deviation from simple scaling was not explained with the correction terms based on the Wegner expansion.