Consider a binary liquid mixture for which the excess Gibbs energy is given by G^E/RT = Ax₁x₂. What is the minimum value of A for which liquid/liquid equilibrium is possible?

To the xy diagram provided in Fig. 12.23. This diagram shows xy curves both for ethanol(1)/ethyl acetate(2) and for chloroform(1)/tetrahydrofuran(2), both at a constant pressure of 1 bar. The curves are intentionally unlabeled. 

0.9 0.8 0.7 0.6 E 0.5 0.4 0.3 0.2 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1