The liquid–liquid coexistence curves of {x nitrobenzene + (1 − x) 2-methylpentane} and {x nitrobenzene + (1 − x) isooctane} in the critical region

•Coexistence curves of binary nitrobenzene/branched alkane solutions were measured.•Landau–Ginsburg–Wilson type model was tested.•Asymmetry of the coexistence curves was analyzed.•Heat capacity contribution to the diameter asymmetry was found to be important.

The liquid–liquid coexistence curves for {x nitrobenzene + (1 − x) 2-methylpentane} and {x nitrobenzene + (1 − x) isooctane} were measured and the critical exponents and the critical amplitudes are deduced. The critical exponents were found to be consistent with the 3D-Ising value. The experiment results are analyzed to examine the dependences of the width and the asymmetry of the coexistence curves on the molar volume of the branched alkane. These dependences were also discussed through the Landau–Ginsburg–Wilson type model and the complete scaling theory.