## 3 Results and Discussion  
### Density and Excess Molar Volume of the Binary Mixtures  
The measured densities of pure MDEA, DMEA, and DEEA in the temperature range from 293.15 K to 353.15 K under atmospheric pressure are listed in Table 3. A comparison of measured densities of pure amines in this study with available literature data indicates that the instrument was calibrated properly prior to all experiments. The density of the aqueous amine mixtures was measured in the temperature range from 293.15 K to 343.15 K under atmospheric pressure. In the Appendix, the measured densities are presented in Tables 1-3 under different mass fractions, mole fractions, and temperatures. Density of pure and aqueous amine mixtures decreases with increasing temperature. For the MDEA + H\({}_{2}\)O mixtures, the density starts to increase with MDEA concentration from \(x_{1}=0\) and reach a maximum value and then decreases. A shift of maximum from \(x_{1}=0.3\) at 293.15 K to \(x_{1}=0.28\) at 353.15 K was observed due to the influence from temperature. The density of DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures continues to decrease from \(x_{1}=0\), and a minimum was observed at \(x_{1}=1\). A comparison of measured density of aqueous mixtures with literature from authors [PERSON] et al. (2018) and [PERSON] et al. (2018) is included in the supplementary materials. The comparison showed a good agreement between measured densities with available density data in the literature. The excess molar volume \(V^{E}\) of a binary mixture is given by Equations (1) and (2) and is a property that can be used to fit density data of a binary mixture. The sign of \(V^{E}\) carries information of intermolecular interactions and molecular structure of the molecules in a mixture. The excess molar volume \(V^{E}\) becomes negative when the intermolecular interaction between unlike molecules are stronger than that in like molecules ([PERSON] and [PERSON], 2017; [PERSON], 2017). Further, \(V^{E}\) is negative when the molecules are efficiently packed in the solution ([PERSON] and [PERSON], 2017). For the mixtures having weak intermolecular interactions such as dispersion forces have positive deviation for \(V^{E}\)([PERSON] and [PERSON], 2017). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**T/K**} & \multicolumn{2}{c}{**MDEA**} & \multicolumn{2}{c}{**DMEA**} & \multicolumn{2}{c}{**DEEA**} \\ \cline{2-7}  & **This Work** & **Literature** & **This Eork** & **Literature** & **This Work** & **Literature** \\ \hline
293.15 & 1040.6 & & 887.9 & 887.5  \\  & 1035.9 & 1033.1 & 1032.0  \\  & 1029.3 & 1029.0  \\  & & 870.3  & 866.4 & 867.3  \\  & & 857.6 & 851.6 & 846.8 & 846.5  \\  & & 839.6 & 832.3 & 827.4 & 827.2  \\  & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 3: Measured density (\(\rho\)/kg\(\cdot\)m\({}^{-3}\)) of pure amines MDEA, DMEA, and DEEA. \[V^{E}=V-\left(x_{1}V_{1}^{0}+x_{2}V_{2}^{0}\right) \tag{1}\]  
\[V^{E}=\left[\frac{x_{1}M_{1}+x_{2}M_{2}}{\rho}\right]-\frac{x_{1}M_{1}}{\rho_{1} }-\frac{x_{2}M_{2}}{\rho_{2}} \tag{2}\]  
The calculated \(V^{E}\) for MDEA + H\({}_{2}\)O, DMEA + H\({}_{2}\)O, and DEEA + H\({}_{2}\)O are shown in Tables A1-A3, respectively. The density of pure H\({}_{2}\)O was taken from the reference [26]. A negative deviation for \(V^{E}\) was observed for the whole range of amine concentration with a minimum in H\({}_{2}\)O-rich region at \(x_{1}\) of 0.36, 0.38, and 0.38 for MDEA, DMEA, and DEEA, respectively. This indicates the existence of strong intermolecular interactions like H-bonds among unlike molecules and efficient packing of molecules in the mixtures. Figure 1a,b compares the variation of \(V^{E}\) between three different mixtures at 293.15 K and 353.15 K.  
A negative \(V^{E}\) or a volume contraction in the systems further reveals that considered tertiary amines and H\({}_{2}\)O are completely miscible (polar organic solvent + H\({}_{2}\)O) systems [27].