•The alkalinity and physicochemical properties of four azole-based PILs were measured.•The CO2 absorption behavior of azole-based PILs were investigated.•The effect of the alkalinity and physicochemical properties on CO2 absorption behavior was discussed.•The CO2 absorption mechanism was proposed and enthalpy of absorption was calculated.Recently, chemical fixation of CO2 catalyzed by protic ionic liquids (PILs) has been successfully achieved, suggesting that CO2 absorption behavior plays an important role on CO2 activation and transformation. For this reason, the influence of the alkalinity and physicochemical properties on CO2 absorption behavior of four azole-based PILs, namely [DBNH][Pyr], [DBNH][Im], [DBUH][Pyr], and [DBUH][Im] was discussed in this work. The alkalinity of PILs was determined by potentiometric titration, and free space of PILs was evaluated by thermal expansion coefficient and refractive index. Solubility parameter of PILs was obtained from the activation energy for viscous flow. The results show that PILs with [Pyr]− exhibits stronger alkalinity and larger free space, which can help to improve CO2 capture capacity. Whereas PILs with larger cation size [DBUH]+ exerts higher viscosity, resulting in slower absorption rate of CO2. Furthermore, the CO2 absorption capacity is inversely proportional to the solubility parameter of PILs. The absorption mechanism proved by FT-IR and 13C NMR spectra indicates that [Pyr]− or [Im]− can react with CO2 to form carbamate. Accordingly, considering the CO2 capture capacity and absorption rate, [DBNH][Pyr] may be a good candidate for efficient CO2 capture and activation with low enthalpy of absorption of −38.2 kJ mol−1.

•Viscosity of ternary mixtures containing IL was estimated from their binary subsystems.•Eyring-UNIQUAC, Eyring-NRTL, and Eyring-Wilson equations were employed and compared.•The AARD% of 13 ternary mixtures obtained from the Eyring-Wilson equation is 4.89%.Viscosity of the mixture containing ionic liquid (IL) is important for engineering design and application. The modeling of viscosity of the ternary mixtures containing IL is still in its infancy. The aim of this work is to evaluate the predictive ability of the semi-theoretical viscosity equations based on Eyring’s absolute rate theory and excess Gibbs free energy model for the ternary mixtures containing IL in terms of the viscosity of their binary subsystems. Three viscosity equations namely Eyring-UNIQUAC, Eyring-NRTL, and Eyring-Wilson equations are employed and extended to estimate the viscosity of ternary mixtures of IL with two molecular solvents using the interaction parameters obtained from their binary subsystems. The results were compared and discussed. The correlation results of the viscosity for 30 binary subsystems with 444 data points show that the Eyring-Wilson equation has more applicability and precision than those of the Eyring-UNIQUAC and Eyring-NRTL equations. Accordingly, the prediction results of the viscosity for 13 ternary mixtures containing IL with 813 data points indicate that the Eyring-Wilson equation has much better predictive ability than that of the Eyring-UNIQUAC and Eyring-NRTL equations. Moreover, the Eyring-Wilson equation can be used to predict the ternary viscosity at other temperature such as 313.15 K using the interaction parameters obtained from their binary subsystems at 298.15 K.

•Density and viscosity of binary mixtures of [TMG]IM with water were measured at T = (293.15 to 313.15) K.•Electrical conductivity and refractive index of binary mixtures of [TMG]IM with water and alcohols were determined.•Excess properties of the mixtures were calculated and fitted to Redlich–Kister equation.•Intermolecular interactions of [TMG]IM + water mixture were analyzed and compared to those of [TMG]IM + alcohol mixture.In this work, densities and viscosities of aqueous solution of 1,1,3,3-tetramethylguanidine imidazolide ([TMG]IM) ionic liquid were measured at temperatures from T = (293.15 to 313.15) K. Volumetric properties including excess molar volumes VE, apparent molar volumes, partial molar volumes, and excess partial molar volumes were calculated from the experimental density. Viscosity deviations Δη, Gibbs energy and excess Gibbs energy of activation for viscous flow were deduced from the viscosity. Moreover, electrical conductivities κ and refractive indices of binary mixtures of [TMG]IM with water and alcohols were determined at 298.15 K. The results show that VE values of {[TMG]IM (1) + water (2)} mixture are negative over the whole composition range, while Δη and refractive index deviations ΔnD values exhibit positive deviations, indicating that the hydrogen bonding interaction between [TMG]IM and water is dominant. The absolute values of VE and Δη of the mixture decrease with the increasing temperature. The VE, Δη, and ΔnD can be fitted by the Redlich–Kister equation with satisfactory results. The concentration-dependent κ of the studied systems are fitted to Casteel–Amis equation, showing that the κ values of {[TMG]IM (1) + water (2)} mixture are much higher than those of {[TMG]IM (1) + ethanol (2)} mixture.

The effect of C2–H of alkylimidazolium tetrafluoroborate ionic liquids on the interaction and vapor–liquid equilibrium (VLE) of the ethyl acetate + ethanol mixture was studied using spectroscopy and the COSMO-RS method. Concentration-dependent 1H NMR chemical shifts of ethyl acetate + ethanol + [Bmim]BF4 or [Bmmim]BF4 (xIL = 0.1 and 0.3) systems show that the interaction between [Bmim]BF4 and ethanol is much stronger than that between [Bmmim]BF4 and ethanol. Moreover, the experimental and predicted VLE demonstrate that the improvement of relative volatility of ethyl acetate to ethanol by [Bmim]BF4 is better than that by [Bmmim]BF4. Also, σ-profile obtained from COSMO-RS method indicates that the hydrogen bonding donator ability of [Bmim]+ is greater than that of [Bmmim]+. Therefore, it can be deduced that the acidic C2–H plays an important role in the interaction differentiation of the ionic liquids and their effect on the VLE of the ethyl acetate + ethanol system, resulting from the interaction of the acidic C2–H with the ethanol that is much stronger than with the ethyl acetate.

•Densities and viscosities of binary mixtures of [HDBU]IM and [BDBU]IM with water were measured.•Excess molar volumes and viscosity deviations were calculated and fitted to Redlich–Kister equation.•Other volumetric properties and excess Gibbs free energy of activation for viscous flow were deduced.•The intermolecular interactions between water and [HDBU]IM or [BDBU]IM were analyzed and compared.Densities and viscosities of binary mixtures of 8-hydrogen-1,8-diazabicyclo[5,4,0]-undec-7-enium imidazolide ([HDBU]IM) and 8-butyl-1,8-diazabicyclo[5,4,0]-undec-7-enium imidazolide ([BDBU]IM) ionic liquids (ILs) with water were measured at temperatures from T = (293.15 to 313.15) K. Excess molar volumes VE and viscosity deviations Δη of the mixtures were calculated to study the intermolecular interactions and structural factors between ILs and water. The results show that the VE values of the two mixtures are negative over the whole composition range, while the Δη values have positive deviations, indicating that the hydrogen bonding interactions between IL and water are dominant in the mixtures. Moreover, the absolute values of VE (|VE|) of {[HDBU]IM (1) + H2O (2)} system are larger than those of {[BDBU]IM (1) + H2O (2)} system at the same condition, indicating that the hydrogen bonding interactions between [HDBU]IM and water are stronger than those between [BDBU]IM and water. Both |VE| and Δη values of the mixtures decrease with the increasing temperature, resulting from the decreasing the hydrogen bonding interactions between IL and water. Other derived properties of the studied systems, such as the apparent molar volumes, partial molar volumes, excess partial molar volumes, Gibbs free energy of activation for viscous flow, and excess Gibbs free energy of activation for viscous flow were also calculated from the experimental values.

1,1,3,3-Tetramethylguanidinium imidazole ([TMG][IM]) ionic liquid was synthesized and its absorption and desorption of CO2 were investigated, as well as the effect of reaction temperature, N2, and moisture on capture of CO2 and the absorption mechanism. The results show that the maximum molar ratio of CO2 to [TMG][IM] is to achieve 1.01 at 30 °C under atmosphere pressure with a high absorption rate. The capture capability is not obviously affected by the moisture of CO2, but decreases with temperature increasing from 30 to 50 °C. The absorption mechanism might be that CO2 reacts with the imidazole anion of [TMG][IM] and then a new carbamate group is formed, which is confirmed by IR and NMR spectra. The enthalpy of CO2 absorption for [TMG][IM] is −30.3 kJ mol−1. The captured CO2 can be readily released at 65 °C by bubbling N2 and recycled with little loss of its capture capability. Considering the efficient and reversible process with [TMG][IM], this method has great potential for the capture of CO2.

The correlation model of gas–liquid interfacial tension (surface tension) of nonaqueous mixtures containing ionic liquids (ILs) is still in its infancy. The purpose of this work is to develop a modified Hildebrand–Scott equation based on local composition concept and the group surface area parameters of UNIFAC model as well as to evaluate the capability of this model. The surface tension of 33 IL–cosolvent binary systems were correlated by the modified Hildebrand–Scott equation with two energy parameters and an overall average relative deviation of only 0.92%. The ILs in the study included imidazolium-based, pyridinium-based, and isoquinolinium-based ones, and the cosolvents included methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile, and tetrahydrofuran. Using the energy parameters obtained from the surface tension of a given IL–cosolvent binary system at 298.15 K, the surface tension of the same system at other different temperatures was predicted by the modified Hildebrand–Scott equation, with an overall average relative deviation of 1.06%.

Values of the density and viscosity of the pure ionic liquid n-butylammonium nitrate (N4NO3) and its binary mixtures with methanol, ethanol, 1-propanol, and 1-butanol were measured at temperature ranging from T = (293.15 to 313.15) K. The thermal expansion coefficient, molecular volume, standard entropy, and lattice energy of N4NO3 were deduced from the experimental density results. The temperature dependence of the viscosity of N4NO3 was fitted to the fluidity equation. Excess molar volumes VE and viscosity deviations Δη for the binary mixtures were calculated and fitted to the Redlich–Kister equation with satisfactory results. Both excess molar volumes and viscosity deviations show negative deviations for (N4NO3 + alcohol) mixtures. The effect of the temperature and the size of the alcohol on the excess molar volumes and viscosity deviations are discussed and analysed. Other derived properties, such as the apparent molar volume, partial molar volume, excess partial molar volume, thermal expansion coefficient, and excess thermal expansion coefficient of the above-mentioned systems were also calculated.Highlights► Densities and viscosities of (N4NO3 + alcohols) mixtures were measured. ► Coefficient of thermal expansion, molecular volume, standard entropy, and lattice energy were obtained. ► Excess molar volumes, viscosity deviations, and partial molar volumes were calculated. ► Redlich–Kister polynomial was used to correlate the excess properties. ► The intermolecular interactions between N4NO3 and alcohols were analysed.

•Alkanolamine-based dual functional ionic liquids were synthesized for CO2 capture.•The CO2-absorption molar ratio of [Na(MDEA)2][Pyra] is as high as 0.75 at T = 353.2 K.•CO2 can react with a quasi-aza-crown ether cation and pyrazolide anion simultaneously.•The ILs could be used to capture CO2 at relatively high temperature.Six dual functional ionic liquids (DFILs) with metal chelate cation and pyrazolide anion were synthesized by reacting alkanolamine with pyrazole sodium salt, and characterized by 1H NMR, 13C NMR, FT-IR, and ESI–MS spectra. Thermal decomposition temperature and viscosity were determined. The behavior of CO2 absorption for these DFILs was evaluated at T = 353.2 K. Absorption mechanism and the influence of temperature and N2 on the CO2 absorption were investigated. The results show that the CO2 capacity of the DFILs consisting of alkanolamine with pyrazole sodium in a molar ratio of 2:1 is larger than that of the DFILs in a molar ratio of 1:1. The CO2 absorption mechanism obtained from FT-IR and 13C NMR spectra indicates that CO2 could react with a quasi-aza-crown ether cation and pyrazolide anion simultaneously, resulting in improved capacity of CO2 absorption. According to the saturated CO2 absorption capacity at 333.2–363.2 K and van’t Hoff’s equation, the enthalpy of CO2 absorption for three DFILs is derived. Furthermore, the DFILs were carried out for five cycles with no obvious loss in the absorption capacity of CO2, indicating that these DFILs may be good candidates for efficient CO2 capture at relatively high temperature.