Effect of compositional variability due to thermal instability on critical phenomena in n-hexane + IL mixture

Abstract

Isochoric heat capacity of binary n-hexane + IL (1-octyl-3-methylimidazolium chloride, abbreviated as [C8C1im]Cl) along the fixed isochore of 255.13 kg∙m−3 as a function of temperature near the critical point of pure solvent (n-hexane) have been measured. The measurements were performed for the selected concentration of 0.0089 mol fraction of IL in the three-, two-, and one-phase regions over a temperature range from (308 to 527) K at pressures to 4 MPa. The measurements were performed using a high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter. The combined expanded uncertainty of the density (ρ), temperature (T), concentration (x), and isochoric heat capacity (CV) measurements at the 0.95 confidence level with a coverage factor of k = 2 is estimated to be 0.0015, 15 mK, 0.0001, and 0.03, respectively. The possibility of the chemical reactions occurring during isochoric heat capacity measurements at high temperatures has been discussed. The onset of the effects of thermal decomposition on the isochoric heat capacity of binary n-hexane + [C8C1im]Cl mixture was found at temperatures above approximately 420 K. The addition of n-hexane into the IL made little change in thermal stability of IL. The liquid–vapor phase transition parameters (TS=509.88 ± 0.05 K, ρS=255.13 kg∙m−3, and PS = 3.1175 MPa) have been measured using the technique based on isochoric heat capacity abrupt behavior. Addition of the IL (0.0089 mol fraction) increases the phase transition temperature of pure n-hexane by 2.17 K, while the phase transition pressure increases by 0.0734 MPa.