Fits of scaling and UNIQUAC equations to liquid-liquid equilibrium (LLE) phase compositions of the binary amphiphile 2-butoxyethanol (1) + water (2) measured by the phase volume method

The liquid-liquid coexistence curve of the binary nonionic amphiphile 2-butoxyethanol (1) + water (2) was determined around the lower consolute solution temperature, Tc, by phase volume measurements. The phase volume method uses the linear relationship between the phase volume and the component concentration according to the lever rule. The method is much more convenient to use for low-pressure phase studies than other methods. Moreover, the results show that this method yields equilibrium compositions as accurate and precise as other methods. The coexistence curve of the binary system is fitted to (critical-)scaling equations. The scaling equations have a valid region that is limited near the critical point. This region is extended from AT = |T - Tc| = 8 K to AT = 20 K by defining a new order parameter and incorporating this parameter into the equations. These new equations yield excellent fits to the liquid-liquid equilibrium data of the binary system. When compared to UNIQUAC, the scaling equations with a new order parameter are slightly better than UNIQUAC. In addition, they give correct information on the critical points, which are usually predicted erroneously by UNIQUAC.