Liquid Phase Behavior of Ionic Liquids with Alcohols:  Experimental Studies and Modeling

Abstract

Ionic liquids (ILs) have been suggested as potential "green" solvents to replace volatile organic solvents in reaction and separation processes due to their negligible vapor pressure. To develop ILs for these applications, it is important to gain a fundamental understanding of the factors that control the phase behavior of ionic liquids with other liquids. In this work, we continue our study of the effect of chemical and structural factors on the phase behavior of ionic liquids with alcohols, focusing on pyridinium ILs for comparison to imidazolium ILs from our previous studies. The impact of different alcohol and IL characteristics, including alcohol chain length, cation alkyl chain length, anion, different substituent groups on the pyridinium cation, and type of cation (pyridinium vs imidazolium) will be discussed. In general, the same type of behavior is observed for pyridinium and imidazolium ILs, with all systems studied exhibiting upper critical solution temperature behavior. The impacts of alcohol chain length, cation chain length, and anion, are the same for pyridinium ILs as those observed previously for imidazolium ILs. However, the effect of cation type on the phase behavior is dependent on the strength of the cation-anion interaction. Additionally, all systems from this study and our previous work for imidazolium ILs were modeled using the nonrandom two-liquid (NRTL) equation using two different approaches for determining the adjustable parameters. For all systems, the NRTL equation with binary interaction parameters with a linear temperature dependence provided a good fit of the experimental data.

Reliable computation of binary parameters in activity coefficient models for liquid–liquid equilibrium

“…The final three examples involve modeling of binary LLE in ionic liquid (IL) systems, using both NRTL and eNRTL. Using the parameter estimation method described here, our colleagues Crosthwaite et al [27] have modeled several complete phase diagrams for IL-alcohol systems, and have shown that it is possible to correlate binary LLE data for these systems using NRTL with binary parameters having a linear temperature dependence. However, it does not appear that NRTL is a good predictive model in this context [28].…”

Structural effects of polyethers and ionic liquids in their binary mixtures on lower critical solution temperature liquid-liquid phase separation

“…This is a common way of measuring the temperature at which liquid-liquid phase separation occurs. [19][20][21] The phase separation temperature is also called the cloud point. We repeated turbidity measurements at least three times, and confirmed that the phase transition temperature for each measurement is within 0.2 1C.…”

“…An increase in the miscibility of imidazolium ILs with longer alkyl chains was also observed in other binary mixture systems, alcohol/IL systems and polymethacrylate/IL systems. [19][20][21][28][29][30][31][32][33][34][35][36] LCST phase separation of binary mixtures is governed by the magnitude of negative enthalpy and negative entropy changes of mixing (vide infra). A large decrease in the latter results in lower phase separation temperatures.…”

“…Such a tendency in miscibility is the same as that observed in alcohol/IL binary systems. [19][20][21] The nature of ILs is largely influenced by the anion structures; therefore, the choice of an anion is another factor that determines the LCST phase behavior.…”

“…[16][17][18] Brennecke and co-workers have examined the phase behavior of IL/alcohol mixtures thoroughly and revealed their upper critical solution temperature behavior. [19][20][21] Other binary systems such as IL/haloalkane, 22,23 IL/CO 2 , 24,25 IL/ IL 26,27 and IL/synthetic polymer [28][29][30][31][32][33][34][35][36] systems have also exhibited unique phase behavior. This unique phase behavior has not only given us a better understanding of ILs but has also highlighted their potential for use in several applications, for example, in separation and purification processes and stimuli-responsive polymers.…”