(Liquid+liquid) phase equilibria of binary mixtures 1-butyl-3-methylimidazolium tetrafluoroborate and tert-butanol or 1,2-butanediol

•(Liquid+liquid) equilibria of binary RTIL/alcohol solutions were measured.•Critical exponent was found to belong to 3D-Ising universality.•Asymmetry of the coexistence curves was analyzed.•RPM-rescaled critical parameters were calculated.•The dependences of critical parameters on the permittivity of alcohol were discussed. Precise (liquid+liquid) coexistence curves of binary mixtures {1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4])+tert-butanol or RS-1,2-butanediol} have been measured. Critical exponent β and critical amplitude B̂0Z were deduced from the (liquid+liquid) equilibria data in the critical regions, and it was found that the critical exponent β coincided well with the 3D-Ising value. The complete scaling theory was applied to describe the asymmetry behavior of the diameter of the coexistence curves, which suggested that the heat capacity related term showed significant importance. The reduced critical temperature and density were calculated by the restricted primitive model (RPM), which were used to discuss the driven force of phase separation. Moreover, the dependences of the critical temperature, the critical mole fraction, the reduced critical temperature and the reduced critical density on the permittivity of alcohol for a series of binary solutions of RTIL in alcohols are discussed.