Binary System of Polyethylene Glycol 200 (1) + 3-Dimethylamino-1-propylamine (2) for CO2 Absorption: Thermophysical Properties and Spectroscopic Study.

As the concentration of CO2 in the atmosphere keeps increasing, the development of a highly efficient CO2 absorbent is highly desired. In this work, a binary mixture system of polyethylene glycol 200 (PEG 200) (1) + 3-dimethylamino-propylamine (DMAPA) (2) was used for CO2 absorption. Considering the importance of thermophysical properties to binary solutions, the densities and viscosities of the PEG 200 (1) + DMAPA (2) mixture were measured at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and atmospheric pressure over the entire composition range. Based on the density and viscosity data, the excess properties and viscous flow thermodynamic parameters were calculated, respectively. To obtain the coefficients and to estimate the standard deviations between the experimental and calculated quantities, the excess molar volume (V m E), the viscosity deviation (Δη), and the excess Gibbs free energies of activation for viscous flow (ΔG* E) were fitted to the Redlich-Kister equation. Furthermore, based on the results of UV-vis, FTIR, and 1H NMR spectra, the intermolecular interaction of PEG 200 and DMAPA was discussed. Particularly, a strong intermolecular bonding is formed when the molar ratio of PEG 200 to DMAPA is about 1:2 because of the excess molar volume (V m E). On that account, a mixture of PEG 200 and DMAPA in a ratio of 1:2 was used for studying CO2 absorption, and a CO2 absorption of about 0.19 g per gram of absorbent was achieved at room temperature and atmosphere.

Introduction

The extensive use of fossil fuels and the rapid increase of CO2 content in the atmosphere have caused the greenhouse effect and the negative consequences of global climate changes, and this has attracted widespread attention.[1,2] Experts generally deem CO2 capture and utilization (CCU) to be one of the most effective methods to reduce global CO2 emissions.[3,4] Various methods have been proposed to capture CO2, such as solid adsorption,[5,6] membrane separation,[7,8] and organic solvent scrubbing.[9−11] The organic solvent scrubbing method involves either physical or chemical solvents according to the strength of the solvent–solute interaction. Representative amine scrubbing is the most commonly used commercial technology for CO2 capture through chemical absorption.[12,13] However, the loss of amine by volatilization easily causes secondary pollution in the process of CO2 removal. Especially, there are also some shortcomings, such as equipment corrosion, relatively high cost, and serious loss of efficiency. To improve the process of amine absorption of CO2, the selection of a suitable solvent is important. Thus, it is the focus of this study to find an organic solvent that can reduce the volatilization of amine, has better CO2 uptake, and causes less environmental damage. Previous studies[14−17] have shown that addition of certain organic solvents, such as alcohol and ethylene glycol derivatives, to combine with amines, has dramatically reduced the volatilization of amines, and the resulting amine–alcohol binary system showed superior CO2 absorption capacity. Polyethylene glycol 200 (PEG 200) has been widely studied for CO2 absorption because it has many good properties.[18,19] Especially, it has reasonable characteristics to be labeled as a green solvent such as being nonvolatile, noncombustible, nontoxic to humans, animals, and aquatic organisms, and can be biodegraded by bacteria in soil and sewage. These properties make it suitable for replacing the previously used organic auxiliary solvent of alcohol for CO2 capture.

In the course of our study on the capture of CO2,[20] we found that 3-dimethylamino-1-propylamine (DMAPA) can efficiently absorb CO2 through intramolecular proton transfer. However, the problem of high volatility of DMAPA is still difficult to solve. Inspired by CO2 organic binding solutions (superbase plus butanol),[21] we propose to prepare a hydrogen bond-mixture solution of PEG 200 plus DMAPA for CO2 absorption. The systematic study of CO2 absorption processes for the PEG 200 (1) + DMAPA (2) system includes the following three steps: (1) determining the density, viscosity, and excess properties of the PEG 200 (1) + DMAPA (2) mixture, (2) obtaining the gas–liquid equilibrium data for the gas mixture of CO2 + N2 at various concentrations of PEG 200 (1) + DMAPA (2), and (3) determining the spectral properties of the PEG 200 (1) + DMAPA (2) + CO2 system. To the best of our knowledge, studies on the thermodynamic properties of the binary system of PEG 200 (1) + DMAPA (2), such as density (ρ), viscosity (η), and excess properties, over a wide experimental temperature range are very lacking. Herein, the present work mainly focuses on investigating the density and viscosity data at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K for the whole composition range. Based on the above experimental results, the excess molar volume (VmE), viscosity deviation (Δη), and the excess Gibbs free energies of activation for viscous flow (ΔG*E) were calculated. According to the results of excess properties, the CO2 uptake of the PEG 200 (1) + DMAPA (2) system was also studied. These results can be used to obtain important basic data for potential industrial applications. Furthermore, the possible intermolecular interaction of PEG 200 with DMAPA was also investigated using UV–vis, FTIR, and 1H NMR spectroscopic techniques.

Results and Discussion

Density and Viscosity

The density of PEG 200 (1) + DMAPA (2) mixtures with different compositions at atmospheric pressure and T = (298.15, 303.15, 308.15, 313.15, and 318.15) K was determined. The results are shown in Table and Figure , where the mole fraction of PEG 200 is expressed by x1.

Table 1

Density Data for the Binary System of PEG 200 (1) + DMAPA (2) as a Function of PEG 200 Mole Fraction (x1) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and Atmospheric Pressurea

	ρ/(g·cm–3)
x1	T = 298.15 K	T = 303.15 K	T = 308.15 K	T = 313.15 K	T = 318.15 K
0.0000	0.8098	0.8042	0.8023	0.7949	0.7938
0.0262	0.8323	0.8296	0.8265	0.8197	0.8216
0.0537	0.8585	0.8532	0.8476	0.8441	0.8435
0.0827	0.8807	0.8731	0.8700	0.8677	0.8672
0.1133	0.9008	0.8973	0.8917	0.8890	0.8879
0.1455	0.9205	0.9158	0.9119	0.9083	0.9075
0.1796	0.9394	0.9358	0.9302	0.9278	0.9267
0.2157	0.9573	0.9522	0.9473	0.9445	0.9434
0.2541	0.9746	0.9714	0.9664	0.9610	0.9600
0.2948	0.9907	0.9867	0.9822	0.9784	0.9774
0.3381	1.0053	1.0012	0.9966	0.9933	0.9922
0.3844	1.0192	1.0164	1.0116	1.0071	1.0062
0.4339	1.0328	1.0295	1.0251	1.0215	1.0206
0.4869	1.0455	1.0417	1.0378	1.0314	1.0308
0.5438	1.0566	1.0546	1.0497	1.0460	1.0450
0.6052	1.0690	1.0661	1.0618	1.0574	1.0560
0.6714	1.0791	1.0767	1.0718	1.0684	1.0659
0.7433	1.0900	1.0860	1.0811	1.0788	1.0781
0.8214	1.1001	1.0958	1.0926	1.0851	1.0832
0.9066	1.1064	1.1043	1.1013	1.0975	1.0964
1.0000	1.1213	1.1175	1.1139	1.1090	1.1057

Standard uncertainties u for each variable are u (T) = 0.01 K, u (p) = ± 5%, and u (x1) = 0.0001 and the combined expanded uncertainty is uc (ρ) = ± 0.02%, with a 0.95 level of confidence (k ≈ 2).

Figure 1

Changes in experimental densities with molar fraction for PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K.

It can be seen from Figure that the density of mixtures increases with the increase of the mole fraction of PEG 200 at the same temperature. In the same sample, the density of the mixtures decreases with the increase of temperature. The density of the binary solution can be calculated using eq )[22]where x1, M1 and x2, M2 represent the molar fraction and relative molecular weight of pure PEG 200 and pure DMAPA, respectively. V1 and V2 are the molar volumes of pure PEG 200 and DMAPA, respectively. A represents the fitting parameters.

The average absolute deviation was calculated using eq (22)where Yexp. is the experimental density value. Ycal. is the calculated density value. n is the number of experiments. The relative deviation of density of the PEG 200 (1) + DMAPA (2) mixture is within ±0.002 and is plotted in Figure .

Figure 2

Difference in density (ρcal. – ρexp.) values between the experimental data and calculated values for the binary system of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K.

The viscosity of the PEG 200 (1) + DMAPA (2) mixture at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K was measured with an Ubbelohde viscometer. The viscosity data are listed in Table and plotted in Figure . From Figure , one can find that the viscosity decreases gradually with the increase of temperature. This phenomenon indicates that the molecular energy of the binary system solution tends to increase with the increase of temperature. With the increase of the mole fraction of PEG 200, the viscosity of mixtures increases first and then decreases at the same temperature. When the mole fraction of PEG 200 is greater than 0.66, the viscosity increases more slowly. Particularly, when the mole fraction of PEG 200 is 0.8214, the viscosity reaches the maximum value and then begins to decrease.

Table 2

Viscosity Data for the Binary System of PEG 200 (1) + DMAPA (2) as a Function of PEG 200 Mole Fraction (x1) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and Atmospheric Pressurea

	η/(mPa·s)
x1	T = 298.15 K	T = 303.15 K	T = 308.15 K	T = 313.15 K	T = 318.15 K
0.0000	1.06	1.04	1.02	1.00	0.99
0.0262	1.66	1.58	1.54	1.53	1.50
0.0537	2.28	2.13	2.02	1.98	1.96
0.0827	3.30	3.03	2.84	2.73	2.66
0.1133	4.67	4.23	3.89	3.69	3.57
0.1455	6.58	5.93	5.34	4.96	4.71
0.1796	9.14	8.01	7.11	6.54	6.15
0.2157	12.34	10.66	9.24	8.42	7.80
0.2541	16.22	13.85	11.88	10.78	9.79
0.2948	20.78	17.68	15.29	13.26	12.16
0.3381	25.74	21.52	17.98	15.93	14.35
0.3844	30.91	25.71	21.34	18.79	16.66
0.4339	36.11	30.09	24.68	21.69	19.20
0.4869	40.91	33.78	27.87	24.36	21.38
0.5438	45.25	37.32	30.76	26.68	23.44
0.6052	49.05	40.19	33.04	28.96	25.33
0.6714	51.42	42.45	35.55	30.51	26.73
0.7433	53.61	44.18	36.37	31.81	27.79
0.8214	55.52	45.63	37.25	32.74	28.75
0.9066	55.51	46.04	37.84	33.25	29.67
1.0000	54.56	45.65	37.59	30.60	27.29

Standard uncertainties u for each variables are u (T) = ± 0.01 K, u (p) = 5%, and u (x1) = 0.0001, and the combined expanded uncertainty is uc (η) = ± 3%, with a 0.95 level of confidence (k ≈ 2).

Figure 3

excess molar volume (VmE) with the mole fraction for the binary system of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K. The symbols represent the experimental values, and the solid curves represent the calculated values.

The viscosity can be calculated using eq (23−25)where x1 and x2 denote the mole fractions of PEG 200 and DMAPA, respectively. η is the viscosity of mixtures, and η1 and η2 represent the viscosity values of pure PEG 200 and DMAPA, respectively. The relative deviation of viscosity between the experimental value and the calculated value is plotted in Figure . It was found that the relative deviation of viscosity data is within ±0.65.

Figure 4

Changes in experimental viscosities with the molar fraction for PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K.

Excess Property

The excess molar volume (VmE) of the binary system can be calculated using eq (23−25)where ρ represents the density of the mixtures; x1, ρ1, and M1 and x2, ρ2, and M2 are the mole fraction, density, and relative molecular weight of pure PEG 200 and DMAPA, respectively.

Figure and Table show that the VmE in all composition ranges is negative at the experimental temperatures. That is, the volume of the mixed solution is less than the sum of the volumes before the two liquids are mixed. With the same composition, the VmE increases with the increase of temperature. The VmE of the binary solution mixture is mainly related to the chemical action, physical properties, and structural properties of the components. The negative VmE values of mixtures indicate that there is a certain interaction force between PEG 200 and DMAPA, which makes the intermolecular distance smaller and the volume to shrink. Particularly, when the molar ratio of DMAPA and PEG 200 is about 2:1, the VmE is the lowest, meaning that the intermolecular bonding is the strongest at this concentration.where η represents the viscosity of the PEG 200 (1) + DMAPA (2) system. η1 and η2 represent the viscosity of pure PEG 200 and DMAPA, respectively. x1 and x2 represent the mole fraction of PEG 200 and DMAPA, respectively. Figure and Table show the dependence of viscosity deviations on composition and temperature. It can be seen from each Δη curve of the PEG 200 (1) + DMAPA (2) mixture that the value of Δη is not always negative but also shows a positive maximum and negative minimum at around x1 ≈ 0.6067 and x1 ≈ 0.1158, respectively. The curves become more and more flat, and the absolute values decrease at elevated temperatures.

Figure 5

Figure 5Difference in the viscosity deviation (ηcal. – ηexp.) values between the experimental data and calculated values for the binary system of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K.

Table 3

Excess Molar Volumes (VmE) for the Binary System of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) Ka

	VmE/(cm3·mol–1)
x1	T = 298.15 K	T = 303.15 K	T = 308.15 K	T = 313.15 K	T = 318.15 K
0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
0.0262	–1.7083	–2.1642	–1.9843	–2.1180	–2.6013
0.0537	–3.8424	–3.9372	–3.3991	–4.0538	–4.1461
0.0827	–5.2871	–5.0583	–4.9254	–5.7380	–5.8736
0.1133	–6.3670	–6.7161	–6.2447	–7.0076	–7.0726
0.1455	–7.3049	–7.5030	–7.2899	–7.9086	–8.0298
0.1796	–8.0756	–8.4267	–7.9954	–8.7758	–8.8577
0.2157	–8.6536	–8.7912	–8.4734	–9.1898	–9.2909
0.2541	–9.0955	–9.4720	–9.1634	–9.5216	–9.6386
0.2948	–9.3248	–9.5820	–9.3503	–9.9103	–10.0429
0.3381	–9.2877	–9.5269	–9.2876	–9.8840	–10.0395
0.3844	–9.0934	–9.4964	–9.2356	–9.6581	–9.8447
0.4339	–8.7954	–9.1128	–8.9158	–9.4385	–9.6484
0.4869	–8.3083	–8.5222	–8.4152	–8.5180	–8.7788
0.5438	–7.5121	–7.9618	–7.7111	–8.1737	–8.3938
0.6052	–6.8057	–7.0975	–6.9522	–7.2794	–7.4595
0.6714	–5.6654	–6.0137	–5.7823	–6.2147	–6.2528
0.7433	–4.5201	–4.6007	–4.3817	–4.9268	–5.2759
0.8214	–3.1235	–3.1243	–3.1621	–2.8816	–3.0499
0.9066	–0.9768	–1.2871	–1.3667	–1.5997	–1.9304
1.0000	0.0000	0.0000	0.0000	0.0000	0.0000

The viscosity deviation (Δη) of the binary system was calculated according to eq ).[23−25]

Figure 6

Viscosity deviation (Δη) with the mole fraction for the binary system of PEG 200 (1) + DMAPA (2) T = (298.15, 303.15, 308.15, 313.15, and 318.15) K. The symbols represent experimental values, and the solid curves represent the values calculated from eq .

Table 4

Viscosity Deviation (Δη) for the Binary System of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K

	Δη/(mPa·s)
x1	T = 298.15 K	T = 303.15 K	T = 308.15 K	T = 313.15 K	T = 318.15 K
0.0000	0.00	0.00	0.00	0.00	0.00
0.0262	–0.80	–0.62	–0.44	–0.28	–0.15
0.0537	–1.65	–1.30	–0.97	–0.63	–0.43
0.0827	–2.18	–1.70	–1.21	–0.72	–0.51
0.1133	–2.45	–1.86	–1.28	–0.67	–0.41
0.1455	–2.26	–1.60	–1.00	–0.35	–0.12
0.1796	–1.52	–1.04	–0.48	0.22	0.43
0.2157	–0.26	0.00	0.33	1.03	1.13
0.2541	1.57	1.48	1.57	2.26	2.12
0.2948	3.95	3.49	3.49	3.54	3.41
0.3381	6.59	5.40	4.59	4.92	4.46
0.3844	9.29	7.53	6.26	6.41	5.55
0.4339	11.85	9.70	7.80	7.85	6.79
0.4869	13.80	11.02	9.04	8.95	7.58
0.5438	15.10	12.02	9.85	9.58	8.14
0.6052	15.62	12.16	9.89	10.04	8.42
0.6714	14.44	11.46	9.97	9.63	8.08
0.7433	12.78	9.98	8.17	8.81	7.26
0.8214	10.52	7.95	6.19	7.43	6.16
0.9066	5.95	4.56	3.66	5.42	4.84
1.0000	0.00	0.00	0.00	0.00	0.00

The excess Gibbs free energies of activation for viscous flow (ΔG*E) values were calculated from the viscosity data using eq (26)where R is the general constant of the gas, T is the absolute temperature, V, V1, and V2 are the molar volumes of the binary mixtures, pure PEG 200, and DMAPA, respectively, η, η1, and η2 are the absolute viscosity of the binary mixtures, pure PEG 200, and DMAPA, respectively. The values of ΔG*E are given in Table and are plotted in Figure .

Table 5

Excess Gibbs Free Energy of Activation of Viscous Flow (ΔG*E) for the Binary System of PEG (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K

	ΔG*E/(kJ·mol–1)
x1	T = 298.15 K	T = 303.15 K	T = 308.15 K	T = 313.15 K	T = 318.15 K
0.0000	0.00	0.00	0.00	0.00	0.00
0.0262	0.84	0.78	0.78	0.77	0.86
0.0537	1.31	1.24	1.19	1.19	1.27
0.0827	1.92	1.83	1.77	1.76	1.77
0.1133	2.47	2.35	2.27	2.25	2.26
0.1455	3.00	2.88	2.77	2.73	2.69
0.1796	3.47	3.30	3.18	3.13	3.09
0.2157	3.85	3.68	3.52	3.47	3.40
0.2541	4.15	3.96	3.80	3.77	3.67
0.2948	4.37	4.19	4.07	3.94	3.88
0.3381	4.48	4.28	4.09	4.04	3.94
0.3844	4.49	4.29	4.11	4.07	3.94
0.4339	4.40	4.23	4.03	4.01	3.89
0.4869	4.20	4.03	3.87	3.86	3.73
0.5438	3.91	3.75	3.61	3.60	3.48
0.6052	3.52	3.36	3.24	3.28	3.17
0.6714	3.01	2.89	2.83	2.85	2.75
0.7433	2.43	2.32	2.25	2.33	2.24
0.8214	1.76	1.68	1.60	1.74	1.68
0.9066	0.95	0.91	0.87	1.03	1.02
1.0000	0.00	0.00	0.00	0.00	0.00

Figure 7

Excess Gibbs free energies of activation for the viscous flow (ΔG*E) with the mole fraction for the binary system of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K. The symbols represent experimental values, and the solid curves represent the values calculated from eq .

As shown in Figure , the values of ΔG*E are all positive in the whole concentration range. ΔG*E can be used as a reliable standard to detect the interaction between different molecules.[27] Here, the results of ΔG*E show that there is an interaction between PEG 200 and DMAPA.

The polynomial regression values of VmE, Δη, and ΔG*E were calculated using the Redlich Kister eq where Q represents VmE, Δη, and ΔG*E. x1 represents the mole fraction of PEG 200 and A is the polynomial regression coefficient. In order to study the fitting efficiency of VmE, Δη, and ΔG*E, the standard deviations between the calculated value and the experimental value were acquired from eq where Z represents VmE, Δη, or ΔG*E, and N and m are the number of experimental points and the number of parameters retained in the respective equations. The calculated values A and σ and the fitting degree(R2) are shown in Table .

Table 6

Coefficients of the Redlich–Kister Equation and Standard Deviations for VmE/(cm3·mol–1), Δη/(mPa·s), and ΔG*E/(kJ·mol–1) for the Binary System of PEG 200 (1) + DMAPA (2) at Different Temperatures; R2 is the Degree of Fitting

T/(K)	property	A3	A2	A1	A0	σ	R2
	VmE/(cm3·mol–1)	–32.8245	21.1379	–12.4724	16.3136	0.1487	0.9981
298.15	Δη/(mPa·s)	53.57	60.38	–63.77	3.84	0.72	0.9953
	ΔG*E/(kJ·mol–1)	16.52	–9.70	2.45	2.28	0.06	0.9993
	VmE/(cm3·mol–1)	–33.7457	21.6664	–11.7893	15.0121	0.1296	0.9987
303.15	Δη/(mPa·s)	42.93	45.03	–52.19	4.16	0.55	0.9958
	ΔG*E/(kJ·mol–1)	15.86	–9.33	2.21	2.21	0.05	0.9994
	VmE/(cm3·mol–1)	–33.0189	22.2396	–10.7541	9.7849	0.0948	0.9991
308.15	Δη/(mPa·s)	35.80	36.07	–40.87	–0.49	0.35	0.9969
	ΔG*E/(kJ·mol–1)	15.24	–8.66	2.20	1.46	0.05	0.9989
	VmE/(cm3·mol–1)	–34.4147	21.8592	–14.3372	18.8922	0.1476	0.9980
313.15	Δη/(mPa·s)	34.45	32.45	–22.57	16.16	0.45	0.9953
	ΔG*E/(kJ·mol–1)	15.07	–8.00	3.17	2.27	0.06	0.9992
	VmE/(cm3·mol–1)	–34.6386	21.4404	–14.8585	16.5294	0.2882	0.9963
318.15	Δη/(mPa·s)	29.11	24.10	–17.83	18.60	0.44	0.9931
	ΔG*E/(kJ·mol–1)	14.47	–7.79	3.44	1.75	0.10	0.9983

The apparent molar volumes Vφ,1 and Vφ,2 of the binary mixture were obtained from eqs and 10,[28] and the results are listed in Table .where ρm is the density of the mixture, and x1, ρ1, and M1 and x2, ρ2, and M2 are the mole fraction, density value, and relative molecular weight of pure PEG 200 and DMAPA, respectively. Table shows that the apparent molar volumes of binary solutions Vφ,1 and Vφ,2 increase with the increase of temperature at the same composition. At a certain temperature, the apparent molar volume of the binary solution Vφ,1 increases and that of Vφ,2 decreases with the increase of the mole fraction of PEG 200.

Table 7

Apparent Molar Volumes Vφ,1 and Vφ,2 for the Binary System of PEG 200 and DMAPA at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K

	Vφ,1(PEG)/(cm3·mol–1)					Vφ,2(DMAPA)/(cm3·mol–1)
x1	T/K = 298.15	303.15	308.15	313.15	318.15	T/K = 298.15	303.15	308.15	313.15	318.15
0.0000						126.19	127.06	127.35	128.54	128.72
0.0262	113.12	128.99	147.16	187.20	175.73	124.42	124.85	125.33	126.41	126.10
0.0537	106.83	121.15	136.83	146.44	148.31	122.10	122.91	123.80	124.35	124.45
0.0827	114.44	127.55	132.93	137.13	137.95	120.38	121.56	122.05	122.43	122.50
0.1133	122.15	126.55	133.55	136.94	138.26	118.95	119.51	120.40	120.84	121.01
0.1455	128.17	132.57	136.30	139.85	140.57	117.56	118.31	118.95	119.55	119.67
0.1796	133.41	136.07	140.37	142.26	143.14	116.24	116.83	117.77	118.18	118.37
0.2157	138.25	141.42	144.52	146.32	147.02	115.03	115.90	116.75	117.25	117.44
0.2541	142.56	144.27	146.91	149.79	150.38	113.84	114.42	115.32	116.30	116.50
0.2948	146.73	148.57	150.62	152.37	152.87	112.77	113.54	114.40	115.13	115.34
0.3381	150.90	152.52	154.37	155.73	156.15	111.92	112.75	113.69	114.39	114.60
0.3844	154.71	155.67	157.39	159.00	159.33	111.13	111.73	112.81	113.81	114.02
0.4339	158.09	159.11	160.52	161.67	161.95	110.30	111.09	112.17	113.04	113.26
0.4869	161.30	162.39	163.49	165.33	165.52	109.57	110.60	111.65	113.39	113.57
0.5438	164.55	165.07	166.35	167.29	167.57	109.18	109.80	111.32	112.45	112.78
0.6052	167.12	167.82	168.82	169.85	170.21	108.26	109.33	110.87	112.45	112.99
0.6714	169.93	170.44	171.51	172.24	172.81	108.02	109.08	111.25	112.75	113.91
0.7433	172.28	173.08	174.06	174.53	174.66	107.27	109.59	112.41	113.77	114.15
0.8214	174.56	175.36	175.95	177.35	177.72	106.62	110.29	113.02	119.43	121.14
0.9066	177.29	177.64	178.16	178.82	179.01	111.35	114.79	119.84	126.24	128.10
1.0000	178.82	179.65	180.11	180.59	180.69

Table 9

Gibbs Free Energy Changes (ΔG*/kJ·mol–1), Enthalpy of Activation (ΔH*), and Entropy of Activation (ΔS*) for the Viscous Flow of the Binary System PEG 200 (1) + DMAPA (2) at Different Concentrations

			ΔG*/(kJ·mol–1)
x1	ΔH*/(kJ·mol–1)	ΔS*/(J·mol–1·K–1)	T/K = 298.15	303.15	308.15	313.15	318.15
0.0000	1.51 ± 0.087	–215.52 ± 0.28	65.77	66.85	67.92	69.00	70.08
0.0262	2.83 ± 0.99	–214.73 ± 3.23	66.89	67.89	68.96	70.03	71.19
0.0537	5.07 ± 1.18	–209.77 ± 3.84	67.66	68.64	69.65	70.72	71.87
0.0827	7.80 ± 0.92	–203.70 ± 3.00	68.58	69.53	70.52	71.57	72.65
0.1133	10.07 ± 1.08	–199.00 ± 3.50	69.45	70.38	71.33	72.36	73.44
0.1455	12.86 ± 0.86	–192.61 ± 2.79	70.32	71.24	72.16	73.15	74.18
0.1796	15.17 ± 1.13	–187.60 ± 3.66	71.15	72.02	72.92	73.89	74.91
0.2157	17.64 ± 1.22	–181.89 ± 3.95	71.92	72.77	73.62	74.58	75.57
0.2541	19.26 ± 1.20	–178.82 ± 3.88	72.62	73.45	74.29	75.25	76.20
0.2948	20.91 ± 0.97	–175.52 ± 3.15	73.28	74.11	74.97	75.83	76.81
0.3381	22.68 ± 1.35	–171.45 ± 4.37	73.85	74.65	75.43	76.35	77.29
0.3844	23.95 ± 1.21	–168.90 ± 3.92	74.35	75.14	75.91	76.83	77.74
0.4339	24.64 ± 1.26	–168.05 ± 4.10	74.79	75.59	76.34	77.25	78.16
0.4869	25.06 ± 1.16	–167.88 ± 3.77	75.15	75.94	76.71	77.62	78.52
0.5438	25.60 ± 1.10	–167.11 ± 3.58	75.47	76.25	77.03	77.92	78.82
0.6052	25.55 ± 1.27	–168.13 ± 4.13	75.73	76.51	77.28	78.20	79.10
0.6714	25.37 ± 0.82	–169.45 ± 2.65	75.92	76.72	77.54	78.41	79.32
0.7433	25.49 ± 1.10	–169.62 ± 3.57	76.10	76.90	77.68	78.60	79.50
0.8214	25.40 ± 1.43	–170.45 ± 4.63	76.27	77.07	77.83	78.77	79.69
0.9066	24.56 ± 1.45	–173.60 ± 4.70	76.37	77.18	77.96	78.90	79.86
1.0000	27.63 ± 1.18	–163.61 ± 3.85	76.41	77.25	78.04	78.78	79.74

The partial molar volume of the mixed solution was calculated using equation and 12(28)where V1and V2 are the molar volumes of the pure components and VmE is the excess molar volume of the mixture. The calculated data are listed in Table The partial molar volumes of V1® and V2® increase with the increase of the mole fraction of PEG 200, while V1® and V2® do not change significantly with temperatures.

Table 8

Partial Molar Volumes V1® and V2® for the Binary System of PEG 200 and DMAPA at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K

	V1(PEG)/(cm3·mol–1)					V2(DMAPA)/(cm3·mol–1)
x1	T/K = 298.15	303.15	308.15	313.15	318.15	T/K = 298.15	303.15	308.15	313.15	318.15
0.0000	107.04	106.83	110.07	104.60	106.46	126.19	127.06	127.35	128.54	128.72
0.0262	116.10	115.48	118.29	114.01	115.05	122.82	123.21	123.74	124.69	124.43
0.0537	124.08	123.88	126.50	122.89	124.02	119.43	120.15	121.07	121.43	121.60
0.0827	132.04	132.22	133.97	131.26	132.09	117.12	118.13	118.67	118.86	119.01
0.1133	139.62	139.27	140.95	139.23	139.92	115.58	115.98	116.85	117.12	117.36
0.1455	146.53	146.40	147.47	146.70	147.16	114.57	115.09	115.68	116.18	116.37
0.1796	152.76	152.54	153.56	153.28	153.65	114.11	114.45	115.22	115.68	115.90
0.2157	158.30	158.36	159.07	159.36	159.62	114.20	114.73	115.33	116.00	116.22
0.2541	163.06	162.95	163.51	164.54	164.73	114.75	115.01	115.56	116.76	116.96
0.2948	167.11	167.18	167.59	168.65	168.82	115.79	116.16	116.59	117.74	117.94
0.3381	170.49	170.65	171.06	172.19	172.35	117.31	117.69	118.09	119.37	119.57
0.3844	173.12	173.19	173.65	174.94	175.11	119.15	119.37	119.77	121.35	121.56
0.4339	175.00	175.23	175.68	176.77	176.97	121.16	121.48	121.82	123.39	123.63
0.4869	176.29	176.69	177.13	178.47	178.68	123.35	123.80	124.09	126.09	126.36
0.5438	177.22	177.45	178.12	178.89	179.18	125.77	126.04	126.52	128.11	128.51
0.6052	177.53	177.98	178.61	179.30	179.66	127.98	128.50	128.94	130.52	131.05
0.6714	177.91	178.35	179.10	179.51	180.02	130.56	131.16	131.72	133.00	133.73
0.7433	178.09	178.83	179.55	179.76	179.92	133.28	134.35	134.84	135.82	136.21
0.8214	178.41	179.22	179.65	180.68	180.97	136.90	138.35	138.29	140.06	140.40
0.9066	179.30	179.74	180.11	180.68	180.78	142.95	144.51	143.68	145.31	144.91
1.0000	178.82	179.65	180.11	180.59	180.69	151.91	154.62	152.19	154.99	153.27

Thermodynamic Parameters

The VmE of the PEG 200 (1) + DMAPA (2) mixture is negative, which proves that there is an interaction between PEG 200 and DMAPA. This intermolecular interaction can also be explained by the thermodynamics of viscous fluids. To obtain the kinematic viscosity of the PEG 200 (1) + DMAPA (2) system, the thermodynamic parameters of the viscous fluid of the PEG 200 (1) + DMAPA (2) system were calculated, including the activation enthalpy ΔH*, activation entropy ΔS*, and activation Gibbs free energy ΔG* using eqs and 14(29)where v, h, NA, M, and R, respectively, represent the kinematic viscosity, Planck constant, Avogadro constant, average molar mass, and molar gas constant of the binary system. From 13 and 14, the following formula can be obtained:

R ln(νM/hNA) on the left side of eq was found to be in a linear relationship with 1/T on the right side, when a plot was constructed by taking 1/T as the abscissa and R ln(νM/hNA) as the ordinate. The fitting results are shown in Figure . The slope is the ΔH* value and the intercept is the −ΔS* value, and the results are shown in Table . The activation Gibbs free energy ΔG* can also be calculated using eq , and the results obtained are shown in Table and plotted in Figure .

Figure 8

Plots of R ln(νM/hNA) against 1/T for PEG 200 (1) + DMAPA (2) at different temperatures, and the molar fractions corresponding to lines are as follows: (A: x1 = 0.0000 ∼ U: x1 = 1.0000).

Figure 9

Gibbs free energies of activation (ΔG*) with the mole fraction for the viscous flow of PEG 200 (1) + DMAPA (2) at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and atmospheric pressure. The symbols represent experimental values, and the solid curves represent the values calculated from eq .

It was found from Table that the value of ΔG* over the range of components is positive at measured temperatures and ΔH* increases from 1.51 to 27.63 kJ·mol–1·K–1. The results indicate that the viscous flow in pure DMAPA is easier than that in pure PEG 200 or the binary solution. At the same time, ΔS* increases from −215.52 to −163.61 J·mol–1·K–1 with the increase of the mole fraction of PEG 200. The negative ΔS* indicates that the self-association binding force of PEG 200 and DMAPA is stronger than that of DMAPA alone.

Spectral Properties

UV–Vis Spectra

The preceding results may be related to the hydrogen bond and interaction between PEG 200 and DMAPA. The UV–vis spectra of the PEG 200 (1) + DMAPA (2) mixture were measured and are shown in Figure . PEG 200 was used as a reference solvent. The absorption peak of DMAPA was found at about 234 nm, which is attributed to N electrons undergoing the n → σ* electron transition. The peak red-shifted from 234 to 237 nm with the increase of the DMAPA concentration. This phenomenon is mainly due to the self-association hydrogen bond between PEG 200 molecules was broken, and the new hydrogen bond was formed between PEG 200 and DMAPA through the formation of (CH3)2N···HOCH2CH–O– and H2N···HOCH2CH–O-. Particularly, the ability of DMAPA molecules to form a hydrogen bond with PEG 200 molecules was reinforced with the increase of the DMAPA concentration, which will make the nitrogen atom in DMAPA molecules interact more easily with hydroxyl on PEG 200, leading to the red shift of the absorption peak.

Figure 10

UV–vis spectral changes with increasing DMAPA concentration in the binary system of PEG 200 (1) + DMAPA (2).

FTIR Spectra

To further study the interaction between DMAPA and PEG 200, the FTIR spectra of PEG 200, DMAPA, and the binary system of PEG 200 (1) + DMAPA (2) at various concentrations were obtained. In the FTIR spectra shown in Figure , the stretching vibration peak of the O–H bond in PEG 200 molecules appears at 3422 cm–1.[30] Interestingly, the absorption peak moves to a low wavenumber of 3356 cm–1 with an increase of the DMAPA concentration; meanwhile, the bending vibration band of the hydroxyl group in PEG 200 moved from 1104 cm–1 to a lower frequency of 1100 cm–1 with the increase of the DMAPA concentration. These results suggest that the intermolecular hydrogen bonds in PEG 200 are broken gradually, and new hydrogen bonds are formed. The absorption band of hydroxyl groups in PEG 200 moves to a lower frequency, which is due to the vibrational properties of hydroxyl groups in PEG 200, further indicating the hydrogen bonding and interaction of the hydroxyl hydrogen atoms in PEG 200 with nitrogen atoms in DMAPA.

Figure 11

FTIR spectra of (a) PEG 200, (b) PEG 200 + DMAPA (2:1 mole ratio), (c) PEG 200 + DMAPA (1:1 mole ratio), (d) PEG 200 + DMAPA (1:2 mole ratio), and (e) DMAPA.

1H NMR Spectra

The 1H NMR spectra of PEG 200, DMAPA, and the binary system of PEG 200 (1) + DMAPA (2) with various concentrations were also compared. Figure shows that the chemical displacement of methyl hydrogen in DMAPA is 2.158 ppm. With the increase of the PEG 200 concentration, the chemical displacement gradually moves to a low field of 2.000 ppm; meanwhile, the chemical shifts of other methylene groups in DMAPA also move to the lower field. The fact is that the electron cloud density of protons on DMAPA becomes thinner due to hydrogen bonding interactions between DMAPA and PEG 200. The most reasonable form of hydrogen bonding was proposed as the formation of (CH3)2N···HOCH2CH–O– and H2N···HOCH2CH–O–. The results are in well accordance with the FTIR and UV–vis results.

Figure 12

1H NMR spectra of (a) PEG 200, (b) PEG 200 + DMAPA (2:1 mole ratio), (c) PEG 200 + DMAPA (1:1 mole ratio); (d) PEG 200 + DMAPA (1:2 mole ratio), and (e) DMAPA.

CO2 Absorption

Based on the above experimental results, the PEG 200 plus DMAPA mixture was used to measure CO2 absorption. Figure a shows the relationship between CO2 uptake and time at room temperature and atmospheric pressure. The absorption equilibrium of PEG 200 + DMAPA (1:1 mole ratio), PEG 200 + DMAPA (1:2), and PEG 200 + DMAPA (2:1) was all achieved within 45 min. In contrast, the absorption capacity of CO2 of PEG 200 + DMAPA (1:2) is quite large, reaching 0.19 g CO2 per gram solvent. Interestingly, when the molar ratio of PEG 200 and DMAPA is 1:2, the intermolecular bonding is the most strongest based on the results obtained for VmE of the mixture. This intermolecular hydrogen bonding can reduce the volatility of amines, which will be further discussed later. Considering the importance of CO2 removal efficiency in practical industrial processes, we investigated low-concentration CO2 removal under environmental conditions. As shown in Figure b, it was found that CO2 was completely removed in the first 40 min using PEG 200 + DMAPA (1:2) as the absorbent, then the removal efficiency was reduced, and the final absorption reaches saturation in about 60 min.

Figure 13

(a) Comparison of CO2 absorption processes by the amide-based binary system of PEG 200 (1) + DMAPA (2) as a function of time at 25 °C and 1.0 bar; (b) CO2 (10 wt %) + N2 removal efficiency as a function of time at 25 °C and 1.0 bar in PEG 200 plus DMAPA (1:2 mole ratio).

Conclusions

In summary, the density and viscosity of the PEG 200 (1) + DMAPA (2) binary solution in the whole concentration range of T = (298.15, 303.15, 308.15, 313.15, and 318.15) K at atmospheric pressure were reported. The density and viscosity data have been used to compute the parameter of excess properties and viscous flow thermodynamics. The VmE values were negative, Δη is not always negative, it also shows a positive maximum and a negative minimum at around x1 ≈ 0.6067 and x1 ≈ 0.1158; meanwhile, the ΔH*, ΔS*, and ΔG* values were calculated. Based on the results of VmE, it was found that a stronger intermolecular bonding is formed. A mixture of PEG 200 and DMAPA in a ratio of 1:2 was used for investigating CO2 absorption at a pressure of 1 bar, and a CO2 absorption of about 0.19 g per gram of the absorbent at room temperature was achieved. In addition, the UV–vis, FTIR, and 1H NMR results indicate that there are hydrogen bonding and interactions of hydroxyl hydrogen atoms in PEG 200 with nitrogen atoms in DMAPA leading to the formation of (CH3)2N···HOCH2CH–O– and H2N···HOCH2CH–O–. These results are helpful to understand and optimize the CO2 absorption capacity of the binary solution.

Experimental Section

Materials

Analytical grade DMAPA (≥99.0%) and PEG 200 (≥99.0%) were purchased from Shanghai Titan Technology Co., Ltd., China. They were dried over 4 Å molecular sieves and degassed by ultrasonication just before the experiment. The water contents of PEG 200 and DMAPA were determined by the Carl Fischer method to be 0.21 and 0.07 wt %, respectively. Moreover, double-distilled water and HPLC grade ethanol were used to calibrate the pycnometer and the Ubbelohde viscometer, respectively. All specifications of chemical samples are shown in Table S1.

Instrumentation

All measurements of mass were performed on an electronic balance with an accuracy of ±0.1 mg (Sartorius BS224S). The uncertainty of mole fraction was estimated to be ±0.0001. The UV–vis spectra were recorded on a Shimadzu UV-3600 plus spectrometer. FTIR spectra were recorded on a Nicolet 50 FTIR spectrometer. 1H NMR spectra were recorded on a JNM-ECZ-400 spectrometer. All spectral experiments of PEG 200 (1) + DMAPA (2) were performed at room temperature and atmospheric pressure.

Measurements

The density of the sample was measured by the pycnometer method. Briefly, the pycnometer containing the binary solvent solution was placed in a constant temperature water bath at a desired temperature for 30 min, and then weighed at least three times to obtain the average value. The uncertainty of density measurement was estimated to be ±0.02%. The viscosity of the sample was measured using an Ubbelohde viscometer. First, the Ubbelohde viscometer was calibrated with double-distilled water and HPLC grade ethanol, and the viscometer constant was calculated. A stopwatch was used to measure the time when the liquid flows through the capillary of the viscometer and this was repeated at least six times for each sample to obtain the average value. The uncertainty of the viscosity value was ±3%. In order to determine the reliability of the experiment, the experimentally measured values of density and absolute viscosity of DMAPA and PEG 200 are compared with those reported in the literature,[30−35] as shown in Table S2. The agreement between the experimental and literature values was found to be satisfactory.

Absorption of CO2

The bubble weight method was used in the experiment of CO2 absorption. In short, CO2 absorption was carried out at room temperature at a pressure of 1 bar. As shown in Figure S1, a known amount of absorbent was placed in a glass tube (inner diameter: 10 mm; length: 200 mm). The gas containing CO2 is bubbled into the absorption solution through a long stainless-steel needle. During gas absorption, the gas solubility was determined by weighing the tube on an electronic analytical balance (Sartorius BS224S). To ensure balance is achieved, the weighing procedure was repeated until the mass remains constant between successive measurements. For diluted CO2 absorption, in a typical procedure, about 2.0 g of absorbent was added to a U-shaped absorption tube with an inner diameter of 8 mm, and then the absorption tube was placed in a 25 °C water bath. The CO2 gas (10 wt %) was controlled by a mass flow meter to control the flow rate to 30 mL·min–1. CO2 was absorbed by the absorbent when it passed through the U-shaped tube. Then, the concentration of the CO2 tail gas was detected using the flue gas analyzer (LY800-CO2, Shenzhen Liye Industrial Co., LTD. China) and the change in CO2 concentration was recorded online using a computer.