Liquid–Liquid Equilibria, Equilibrium Phase Densities, and Refractive Indices for the Quaternary Mixtures Containing 2‑Propanol or 2‑Methyl-2-Propanol of Fuel Oxygenate at T = 298.15 and 318.15 K

Two quaternary systems in regard to water + 2-propanol + 2,2,4-trimethylpentane + 1,4-dimethylbenzene and water + 2-methyl-2-propanol + methylcyclohexane + 1,4-dimethylbenzene were carried out to determine the data on liquid–liquid equilibrium (LLE) concerning the temperatures with 298.15 and 318.15 K and a pressure of 0.1 MPa. The gas–liquid chromatography was applied to obtaining the equilibrium compositions over each phase. Increasing the concentration of the alcohol enhances the water solubility in the hydrocarbon-abundant phase. The distribution of the alcohol between two equilibrium phases was analyzed, and a greater portion of 2-methyl-2-propanol was found to hydrocarbon-abundant phase. Each binary pair of energy parameters associated with the nonrandom two-liquid (NRTL) and universal quasichemical activity coefficient (UNIQUAC) models was established through the experimental data of LLE, and the average deviation from this calculation was less than 0.013. Based on the Hand and Othmer–Tobias equations, the correlation of these LLE data was satisfactorily deployed. Equilibrium density and refractive index in the coexistent solutions were assessed, and their relations with the concentration of 2-propanol/2-methyl-2-propanol were illustrated.