## 3 Results and Discussion
### Viscosity of the Binary Mixtures
The highest positive deviation for \(\eta^{E}\) was reported by MDEA + H\({}_{2}\)O mixtures around \(x_{1}\) = 0.6 The DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures reached their highest \(\eta^{E}\) around \(x_{1}\) = 0.35. DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures showed a similar behavior for \(\eta^{E}\), while MDEA + H\({}_{2}\)O mixtures showed deviations indicating the influence of (-OH) groups in MDEA on viscosity of aqueous mixtures. The calculated \(G_{12}\) and \(\Delta G^{E*}\) showed positive deviations for all considered aqueous amine mixtures for the whole range of amine concentrations. This shows that the considered aqueous amine mixtures are having strong intermolecular interactions among unlike molecules for the whole range of amine concentrations. The MDEA + H\({}_{2}\)O mixtures showed a highest peak around \(x_{1}\) = 0.35, while DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures showed peaks around \(x_{1}\) = 0.3 and \(x_{1}\) = 0.25, respectively. Figure 4c illustrates that DEEA + H\({}_{2}\)O mixtures have a higher \(\Delta G^{E*}\) in H\({}_{2}\)O rich region than MDEA +  
H\({}_{2}\)O mixtures and \(\Delta G^{E*}\) of MDEA + H\({}_{2}\)O showed higher values than DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O for the amine rich region. However, \(\Delta G^{E*}\) does not show a large deviation among the different amine mixtures as in \(\eta^{E}\). This can be assumed due to the influence of molecular packing in the mixtures on viscous flow. The slope of the excess free energy of activation for viscous flow \(\Delta G^{E*}\) against temperature (\(T\)) carries the information about the excess entropy of activation for viscous flow \(\Delta S^{E*}\) as given in Equation (23). The plot of \(\Delta G^{E*}\) versus temperature (\(T\)) was linear in the temperature range from 293.15 K to 363.15 K at a certain mole fraction for the mixtures studied. \[\Delta S^{E*}=-\left[\frac{\partial\left(\Delta G^{E*}\right)}{\partial T}\right] \tag{23}\]  
Figure 5 illustrates the excess entropy of activation for viscous flow \(\Delta S^{E*}\) for MDEA + H\({}_{2}\)O, DMEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures over the whole range of concentrations. Equation (23) was adopted to calculate \(\Delta S^{E*}\) for temperature range 293.15-363.15 K. Similar to the \(\Delta G^{E*}\) variation with the mole fraction, \(\Delta S^{E*}\) increases with the increase of mole fraction up to a maximum and then  
decreases. The peaks were observed around \(x_{1}\) = 0.38 for MDEA + H\({}_{2}\)O and DEEA + H\({}_{2}\)O mixtures, while DMEA + H\({}_{2}\)O showed a peak around \(x_{1}\) = 0.32. The \(\Delta S^{E*}\) of DEEA + H\({}_{2}\)O mixtures was higher than that of MDEA + H\({}_{2}\)O and DMEA + H\({}_{2}\)O mixtures for the whole range of amine concentration. The measured viscosities from 293.15 K to 363.15 K of the mixtures were fitted to the empirical correlation shown in Equation (22). The Redlich-Kister model [28] is a good candidate to correlate excess properties in a binary mixtures. In order to acquire a good accuracy in data fit, a higher degree polynomial is required with a large number of fitting parameters. A simplified lower degree polynomial was suggested as given in Equation (24). Similar work have been reported by [PERSON] et al. [45] for the viscosity of MEA + H\({}_{2}\)O mixtures. \[ln\Big{(}\eta_{\gamma}\Big{)}=\Big{(}A_{1}+A_{2}T+A_{3}T^{2}+A_{4}x_{1}+A_{5} Tx_{1}^{2}+A_{6}x_{1}^{3}\Big{)}x_{1}x_{2} \tag{24}\]  
Table 7 lists the calculated parameters of Equation (24) for different mixtures. The correlations provide acceptable accuracies for use in engineering calculations. For MDEA + H\({}_{2}\)O mixtures, AMD shows a relatively high deviation as 6.4 mPa-s, but measured viscosity is as high as 114.261 mPa-s.  
[PERSON] [46] developed a semiempirical model based on [PERSON]'s theory of absolute reaction rates to represent kinematic viscosities in a binary mixture. The [PERSON] [46] three-body model considered interactions among three molecules that are all in one plane.