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Journal of Chemical & Engineering Data Article temperature and pressure of interest. The gas was introduced into the gas reservoir and the pressure was recorded with the pressure transducers. Then an amount of gas was introduced into the equilibrium cell and dissolved in the solvent. It was considered to have reached equilibrium if the pressure remained a constant for 4 h, and the corresponding solubility pressure was measured with the pressure transducers. The moles of gas dissolved in the liquid phase were determined from the pressure change in the gas reservoir, the equilibrium pressure, and the volume of the equilibrium cell. This was based on the assumption that the vapor pressures of the solvents were negligible. For ChCl/urea, as a type of IL, the vapor pressure is negligible. For ChCl/urea with water, it was also assumed that the water content in the vapor phase is very low and can be neglected, that is, only CO2 was assumed to exist in the vapor phase. More discussions on this assumption were given in section 3.3. The uncertainties of the CO2 solubility measurement consist of the system errors of pressure, temperature and the volumes of gas reservoir, and equilibrium cell. The precision of pressure transducers is 0.075%, the accuracy of temperature and volume measurements is 0.1 K and 0.5 mL. The overall uncertainty for the measured solubility of CO2 was estimated to be within ±1%. 3. RESULTS 3.1. Density of (ChCl/Urea + Water). The densities of {ChCl/urea (1) + water (2)} were measured at temperatures from 298.15 K to 333.15 K at atmospheric pressure. The measured experimental densities are listed in Table 1. The densities decrease linearly with increasing temperature at certain water content as shown in Figure 1. With the increase of mole fraction of water (x2), the densities illustrate a polynomial decrease. The densities of ChCl/urea (i.e., the mole fraction of water x is zero) were measured by Clocirlan et al.17 and in this work. 2 The comparison shows that the experimental densities measured in this work agree well with those in the literature17 as shown in Figure 1a, and the average relative deviation in density is less than 0.16%. The effect of water on the densities of ChCl/urea was also studied by Yadav et al.,18 Su et al.,19 and Leron et al.20 The comparison at 303.15 K and 333.15 K is illustrated in Figure 1b. Again, the experimental data measured in this work agrees well with those from others’ work. The excess molar volume (VE) of {ChCl/urea (1) + water (2)} was calculated from the density data using the equation E It can also be equivalently expressed as ⎡⎤ VE = ⎢(x1M1 + x2M2)⎥ − x1M1 − x2M2 ⎢ρ⎥ρρ ⎣m⎦12 (2) where x is the mole fraction in the liquids phase, V is the liquid molar volume, ρ is the liquid density, M is the molecular weight, and the subscripts 1, 2, and m stand for ChCl/urea, water, and their mixture, respectively. The calculated VE is also listed in Table 1. The variations of VE at different temperatures over the entire mole fraction range are shown in Figure 2. Similar distributions were found compared to the VE of other solvents in the literature.21−26 The values of the excess molar volume are negative and the minimum value was found at x2 ≈ 0.65 within the studied Figure 2. Excess molar volume VE of {ChCl/urea (1) + water (2)}. Symbols: ■, 298.15 K; □, 303.15 K; ●, 308.15 K; ○, 313.15 K; ▲, 318.15 K; △, 323.15 K; ▼, 328.15 K; ▽, 333.15 K. Curves: correlations. temperature range. The negative values indicate that the intermolecular interactions between ChCl/urea and water are strong and cause a volume contraction. The strong intermolecular interactions lead to a lower vapor pressure of {ChCl/urea (1) + water (2)} compared to pure water, which is consistent with the data reported in the literature.12 It is also observed that the VE increases with increasing temperature, which means that the molecular bonding becomes weak with increasing temperature. Redlich−Kister equation27 was used to fit the excess properties (VE) 4 VE = x1x2 ∑An(x1 − x2)n V=V−xV−xV (1) m1122 where n is the number of the estimated parameters, ai is the lineally/polynomial coefficient to represent Ai, and T is the absolute temperature. The fitted parameters are listed in Table 2 with the fitting error of 0.02%. 3.2. Viscosity of (ChCl/Urea + Water). The viscosities of {ChCl/urea (1) + water (2)} were measured at temperatures from 298.15 K to 333.15 K and at atmospheric pressure. The results are listed in Table 3 and illustrated in Figure 3. The temperature effect on the viscosity of ChCl/urea is significant, 28−30 which is also similar to other pure ILs, significantly decreases with increasing temperature. The viscosity is very high at low temperatures. For example, at 298.15 K, the viscosity of ChCl/urea is 1571 mPa·s. When the temperature increases up to 333.15 K, the viscosity of ChCl/ urea decreases to 107.7 mPa·s. The viscosity of {ChCl/urea (1) + water (2)} decreases dramatically with increasing water content. Figure 3a shows that the increase of temperature decreases the viscosity, but it is not as notable as those with lower water contents. For example, at 298.15 K, when the mole fraction of water increases to 0.1525 (the mass fraction of water is 3.61%), the viscosity decreases to 323.9 mPa·s that is only one-fifth of the viscosity compared to pure ChCl/urea (1571 mPa·s). With further increase in water content, the viscosity keeps decreasing, but not as notable as those at higher ChCl/urea concentrations. n=0 (3) A =∑aTi 1 ni i=0 (4) that is, the viscosity 3346 dx.doi.org/10.1021/je500320c | J. Chem. Eng. Data 2014, 59, 3344−3352PDF Image | CO2 Separation with Ionic Liquids
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