Surface Tension of Aqueous Binary Mixtures of 2-(Methylamino)ethanol and 2-(Ethylamino)ethanol and Aqueous Ternary Mixtures of These Amines with Triethanolamine or N-Methyldiethanolamine from (293.15 to 323.15) K Estrella Álvarez, † Diego Gómez-Díaz, ‡ M. Dolores La Rubia, § and José M. Navaza* ,‡ Department of Chemical Engineering, ETSEI, University of Vigo, Rúa Maxwell s/n, E-36310, Vigo, Spain, Department of Chemical Engineering, ETSE, University of Santiago de Compostela, Rúa Lope Gómez de Marzoa s/n, E-15706, Santiago de Compostela, Spain, and Department of Chemical, Environmental and Materials Engineering, EPS, University of Jaén, Paraje Las Lagunillas s/n, E-23071, Jaén, Spain The surface tension of aqueous solutions of MAE (2-(methylamino)ethanol) and EAE (2-(ethylamino)ethanol) were determined at (293.15 to 323.15) K over the entire range of concentrations of binary mixtures with water. Also, ternary mixtures of previous systems with two amines commonly employed in carbon dioxide capture processes (TEA, triethanolamine, and MDEA, N-methyldiethanolamine) were studied. Equations were used to fit the experimental data at different compositions and temperatures. Introduction Surface tension has great importance 1 regarding the behavior and hydrodynamics in gas–liquid systems, and due to aqueous solutions of amines that are employed in carbon dioxide capture, the value of surface tension of these liquid phases could play an important role in this kind of operation. The influence of surface tension upon bubble size has been widely studied in the literature, 2 and in different research works, surface tension of the liquid phase was modified by the addition of surfactants producing modifications upon the bubble’s diameter when surface tension decreases. 3 Similar behaviors have also been observed when different solutes that produce a decrease in surface tension were employed. 4 Nowadays, a great part of research regards characterization of aqueous solutions of amines for their use in carbon dioxide capture and is centered on the use of new amines 5 that contribute interesting characteristics such as suitable physical properties, high reaction rate, or low degradation in regeneration processes. Also, a mixture of amines in aqueous solutions has been considered as a new alternative to increase the capture rate of acid gases by chemical absorption with the aim of a second amine in aqueous solutions that produces an enhancement effect. 6 For this reason, the knowledge of surface tension for these ternary mixtures could be important to understand the behavior of the liquid phases on absorption processes. Experimental Section All amines have been supplied by Merck, with a purity of >98 % for 2-(methylamino)ethanol (CAS Registry No. 109- 83-1), 2-(ethylamino)ethanol (CAS Registry No. 110-73-6), and N-methyldiethanolamine (CAS Registry No. 105-59-9). The purity for triethanolamine (CAS Registry No. 102-71-6) was >99 %. All liquid mixtures were prepared by mass using an analytical balance (Kern 770) with a precision of ( 10 -4 g. The maximum uncertainty of the sample preparation in mole fraction was ( 0.0006. Surface tension was determined by a Krüss K-11 tensiometer using the Wilhelmy plate method. The plate employed was a commercial platinum plate supplied by Krüss. The platinum plate was cleaned with water and acetone and flame-dried before each measurement. The uncertainty of the measurement was ( 0.04 mN · m -1 . In general, each surface tension value reported was an average of ten measurements. Before surface tension measurements were made, the samples were stirred in a thermostatted vessel that was closed to prevent evaporation. Surface tension measurements were carried out in the range of (293.15 to 323.15) K with 5 K steps. The measurement vessel was connected to a thermostat-cryostat bath (Selecta Frigiterm) controlled to ( 0.1 K. Table 1 shows the surface tension values for pure water obtained in the present work and the comparison with literature values to ensure that the experimental procedure for surface tension determination contributes suitable values. Results and Discussion The experimental values obtained for surface tension of aqueous binary mixtures of MAE and EAE over the entire * To whom correspondence should be addressed. Department of Chemical Engineering, University of Santiago de Compostela, Santiago de Compos- tela, E-15782, Galicia, Spain. E-mail: eqnavaza@usc.es. † University of Vigo. ‡ University of Santiago de Compostela. § University of Jaén. Table 1. Comparison of Results with Literature Data for the Pure Water Surface Tension σ/mN · m -1 T/K this work lit. (ref) 293.15 72.75 72.75 7 298.15 72.01 71.95 8 72.01 9 71.98 10 303.15 71.21 71.20 10 308.15 70.42 70.40 10 70.41 9 313.15 69.52 69.60 10 318.15 68.84 68.77 10 68.79 9 323.15 67.92 67.94 10 J. Chem. Eng. Data 2008, 53, 318–321 318 10.1021/je700536m CCC: $40.75  2008 American Chemical Society Published on Web 11/09/2007