The liquid–liquid coexistence curves of {x 1-octyl-3-methylimidazolium hexafluorophosphate + (1 − x) 1-butanol} and {x 1-octyl-3-methylimidazolium hexafluorophosphate + (1 − x) 2-butanol} in the critical region

• Coexistence curves of binary RTIL/butanol solutions were measured.
• Critical behaviors of RTIL/butanol solutions were studied.
• Asymmetry of the coexistence curves were analyzed by the complete scaling.
• Heat capacity contribution to the diameter asymmetry was found to be important.

Coexistences curves were reported for the liquid–liquid phase transition of binary solutions of the room temperature ionic liquid (RTIL) 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]) in 1-butanol and 2-butanol. The critical amplitude B and the critical exponent β corresponding to the coexistence curve were deduced and the values of β were found to be consistent with the 3D-Ising value. The experimental results are also analyzed to examine the asymmetry of the diameters for the coexistence curves. These asymmetries were discussed by the complete scaling theory.