## 3 Results and Discussion
### Viscosity of the Binary Mixtures
\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 Work** & **Literature** & **This Work** & **Literature** \\ \hline
293.15 & 100.72 & & 3.89 & & 4.95 & \\
298.15 & 75.90 & 77.19  & 3.39 & & 4.17 & 4.02  \\
303.15 & 57.82 & & 2.96 & & 3.54 & 3.31  \\
308.15 & 44.62 & & 2.59 & & 3.01 & \\
313.15 & 34.89 & 34.11  & 2.28 & 2.24  & 2.58 & 2.41  \\
318.15 & 27.67 & & 2.01 & & 2.24 & \\
323.15 & 22.22 & & 1.79 & 1.76  & 1.95 & \\
328.15 & 18.10 & & 1.60 & & 1.71 & \\
333.15 & 14.89 & 14.30  & 1.43 & 1.41  & 1.51 & 1.44  \\
338.15 & 12.38 & & 1.29 & & 1.35 & \\
343.15 & 10.38 & 9.85  & 1.17 & 1.16  & 1.21 & \\
348.15 & 8.78 & & 1.05 & & 1.09 & \\
353.15 & 7.48 & 7.12  & 0.96 & 0.96  & 0.98 & 0.93  \\
358.15 & 6.43 & & 0.87 & & 0.90 & \\
363.15 & 5.56 & & 0.80 & & 0.82 & \\ \hline \hline \end{tabular}  
* Literature references: \({}^{\rm a}\) [PERSON] et al. [31], \({}^{\rm b}\) [PERSON] et al. [33], \({}^{\rm c}\) [PERSON] et al. [32].  
\end{table} 
Table 6: Measured viscosity (\(\eta\)/mPa-s) of pure amines MDEA, DMEA, and DEEA. consisting of one adjustable parameter \(G_{12}\) that is beneficial to correlate dynamic viscosity of binary mixtures using pure component viscosities. The model for a binary mixture is given as follows. \[ln(\eta)=\sum_{i=1}^{i=2}x_{i}ln(\eta_{i})+x_{1}x_{2}G_{12} \tag{18}\]  
The sign of \(G_{12}\) gives similar information as \(\eta^{E}\) for the liquid mixtures. Accordingly, \(G_{12}\) is negative for systems in which dispersion forces are dominant and \(G_{12}\) become positive as the strength of the intermolecular interaction increases [37]. The dynamic viscosity model proposed by [PERSON] [40] based on the theory of absolute reaction rate provides another approach to examine the molecular interaction in a binary mixture. For a liquid mixture, the viscosity is represented according to the [PERSON]'s model as follows:  
\[\eta=\frac{hN_{A}}{V}\textit{exp}\left(\frac{\Delta G^{*}}{RT}\right) \tag{19}\]  
Accordingly, excess free energy of activation for viscous flow \(\Delta G^{E*}\) is defined as follows using pure component viscosities and molar volumes. \[ln(\eta V)=ln(\eta V)_{ideal}+\frac{\Delta G^{E*}}{RT} \tag{20}\]  
\[ln(\eta V)=\sum_{i=1}^{i=2}x_{i}ln\left(\eta_{i}V_{i}^{0}\right)+\frac{\Delta G ^{E*}}{RT} \tag{21}\]  
[PERSON] et al. [41] discussed the applicability of the sign of \(\Delta G^{E*}\) as in viscosity deviation \(\eta^{E}\) to understand the types of intermolecular interactions. It has been reported by authors [41; 42; 43; 44] that the positive \(\Delta G^{E*}\) indicates strong interactions like H-bond and negative \(\Delta G^{E*}\) signifies weak molecular interactions like dispersion forces. The \(ln(\eta_{\gamma})\) shown in Equation (22) is similar to the term \(x_{1}x_{2}G_{12}\) in the [PERSON] [39] model. Figure 4a-c illustrate the variation of \(\eta^{E}\), \(ln(\eta_{\gamma})\) and \(\Delta G^{E*}\) with amine concentration in different mixtures at 293.15 K.  
\[ln(\eta)=ln\left(\eta_{\gamma}\right)+\sum_{i=1}^{i=2}x_{i}ln(\eta_{i}) \tag{22}\]  
Figure 4a shows that \(\eta^{E}\) was negative for low MDEA concentrations indicating the presence of weak intermolecular interactions like weak dipole and dispersion forces. As MDEA concentration increases, \(\eta^{E}\) becomes positive and shows a maximum in the amine-rich region, signifying the existence of strong intermolecular interactions like H-bonds among unlike molecules. The DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures showed positive deviation for \(\eta^{E}\) for the whole amine concentrations, revealing that the intermolecular interactions between those amines and H\({}_{2}\)O are stronger than interactions between like molecules. For DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures, the maximum \(\eta^{E}\) was observed in the H\({}_{2}\)O rich region.