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Surface thermodynamics of aqueous solutions of alkylethanolamines
Fluid Phase Equilibria 182 (2001) 325–336
Surface thermodynamics of aqueous solutions of alkylethanolamines Y. Maham, A.E. Mather∗ Department of Chemical and Materials Engineering, University of Alberta, Alberta, Edmonton, Canada T6G 2G6 Received 4 September 2000; received in revised form 21 November 2000; accepted 22 November 2000
Abstract The surface tension of aqueous solutions of methyldiethanolamine and dimethylethanolamine has been measured at temperatures from 298.15 to 328.15 K over the whole range of concentrations. The surface tensions of these aqueous binary mixtures show more concentration dependence on the water-rich side. Addition of a small amount of alkylethanolamine reduces the surface tension of water drastically. This effect is related to the presence of hydrophobic groups such as (–CH3 ) which tend to remain more on the water surface. The thermodynamics of the water surface, the surface entropy and surface enthalpy have been calculated for six (alkylethanolamine + water) mixtures. The surface thermodynamic properties of these mixtures are classified in two categories: those aqueous solutions of ethanolamines with an end hydrophobic group and those without. The surface entropy of the category without an end hydrophobic group is similar to the water surface while the surface properties of the category with an end hydrophobic group are much different from the water surface. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluids; Interfacial tension; Surface entropy; Surface enthalpy; Mixture; Data
1. Introduction This is a continuation of our collection of physical properties of pure and aqueous mixtures of alkylalkanolamines. Bulk properties of pure liquid alkylalkanolamines: heat capacities [1], thermal conductivity [2], densities and viscosities [3] and of binary aqueous mixtures, excess molar properties of (monoethanolamine, MEA + H2 O), (monomethylethanolamine, MMEA + H2 O) and (dimethylethanolamine, DMEA + H2 O) mixtures at 298.15 K [4], the volumetric properties of (MEA + H2 O), (diethanolamine, DEA + H2 O) and (triethanolamine, TEA + H2 O) mixtures at 298.15–353.15 K [5], of (methyldiethanolamine, MDEA + H2 O) and (ethyldiethanolamine, EDEA + H2 O) mixtures at 298.15– 353.15 K [6], and of (dimethylethanolamine, DMEA + H2 O) and (diethylethanolamine, DEEA + H2 O) Abbreviations: DEA, diethanolamine (NH(C2 H4 OH)2 ); DMEA, dimethylethanolamine (((CH3 )2 NC2 H4 OH)2 ); MDEA, methyldiethanolamine ((CH3 NHC2 H4 OH); MEA, monoethanolamine (NH2 C2 H4 OH); TEA, triethanolamine (N(C2 H4 OH)3 ) ∗ Corresponding author. Tel.: +1-780-492-3957; fax: +1-780-492-2881. E-mail address: [email protected] (A.E. Mather). 0378-3812/01/$20.00 © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 ( 0 1 ) 0 0 3 9 1 - 0
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mixtures at 293.15–313.15 K [7], thermodynamic properties such as densities, apparent molar volume have been reported. Isobaric specific heat capacities of (MEA + H2 O) mixtures [8], transport properties of (DEA + H2 O) and (MDEA + H2 O) mixtures at 298.15–353.15 K [9], excess molar enthalpies of (MEA + H2 O), (MMEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K [4] of (DEA + H2 O), (MDEA + H2 O) and (TEA + H2 O) mixtures at 298.15 K [10], of (monoethylethanolamine, MEEA + H2 O), (diethylethanolamine, DEEA + H2 O) and (n-propylethanolamine, n-PEA + H2 O) and (2-amino-2-methyl-1-propanol, AMP+H2 O) mixtures at 298.15 K [11] and the excess molar enthalpies of (diethanolamine, DEA + H2 O), (methyldiethanolamine, MDEA + H2 O), (ethyldiethanolamine, EDEA + HO) and (n-butyldiethanolamine, n-BDEA + H2 O) mixtures [12] were reported at 298.15–338.15 K. The surface properties of (MEA + H2 O) and 2-amino-2-methyl-1-propanol, (AMP + H2 O) mixtures and ternary mixtures of water with these ethanolamines at 298.15–323.15 K [13], and of (DEA + H2 O) and (TEA+H2 O) mixtures at 298.15–323.15 K [14] and of (MDEA+H2 O) and ternary mixtures of water with these ethanolamines at 298.15–323.15 K [15] were studied. In this report, we are interested in the surface properties of (methyldiethanolamine, MDEA+H2 O) and (dimethylethanolamine, DMEA+H2 O) mixtures. 2. Experimental section 2.1. Materials Dimethylethanolamine [(CH3 )2 NCH2 CH2 OH, DMEA, 99%] and methyldiethanolamine [CH3 N(CH2 CH2 OH)2 , MDEA, 99%] were both obtained from Aldrich Chemicals. These compounds were used as received, after confirmatory analysis by titration with standard hydrochloric acid. Mixtures of these compounds with nano-pure distilled water were made by mass, with care being taken to minimize exposure to air (carbon dioxide). 2.2. Apparatus The capillary-rise technique was used to measure the surface tension [16]. This consists of a capillary about 20 cm long which was immersed in the solution to measure the surface tension and the test tube containing the solution was placed inside a water bath. The temperature of the bath was controlled to ±0.05 K. The height of the liquid inside the capillary was read by a cathetometer. The calibration of the capillary tube has been done carefully with distilled water, monoethanolamine and its aqueous mixtures [13,19] with different concentrations at four temperatures from 298.15 to 328.15 K. From the calibration a constant of 0.1315 ± 0.0008 for the capillary tube has been obtained which was multiplied by the height of each reading for different solutions. The precision of the cathetometer is ±0.005 cm. The solutions were poured in the test tube and their surface tension was measured by changing the temperature of the water bath. The accuracy of the results is ±2%. 2.3. Results The surface tension of (MDEA + H2 O) and (DMEA + H2 O) mixtures is given from 298.15 to 328.15 K in Tables 1 and 2, respectively. The composition dependence of the surface tension for (MEA + H2 O)
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Table 1 Surface tension (γ , mJ m−2 ) of (methyldiethanolamine, MDEA + water) mixtures at temperatures from 298.15 to 328.15 K x2
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
Temperature (K) 298.15
308.15
318.15
328.15
72.0 59.1 55.0 51.3 48.4 44.7 41.8 40.4 39.7 39.3 38.6 38.3
70.4 57.7 54.0 50.3 47.8 44.1 41.4 39.8 39.1 38.8 38.0 37.7
68.8 56.6 53.1 49.6 47.0 43.5 40.9 39.3 38.6 38.1 37.5 37.1
67.1 55.5 52.3 48.7 46.3 42.8 40.2 38.7 38.0 37.6 36.9 36.6
[13], (DEA + H2 O) [14], (TEA + H2 O) [14], (MDEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K is shown in Fig. 1. The surface tensions of aqueous solutions of MEA, DEA and TEA (ethanolamines without any end alkyl group) are similar to each other, with a very small difference between MEA and DEA solutions and a slightly bigger difference between DEA and TEA mixtures. This shows the small effect of the presence of an extra (–C2 H4 OH) group of the ethanolamine on the surface tension. Contrary to the (–C2 H4 OH) group, the (–CH3 ) group has a much bigger effect on the surface tension of these aqueous mixtures and its effect becomes much larger with the number of the methyl groups on the ethanolamine molecules. The drop in the surface tension of the water by addition of ethanolamine is also much bigger for the ethanolamine with a methyl group. The same behavior of the surface tension against Table 2 Surface tension (γ , mJ m−2 ) of (dimethylethanolamine, DMEA + water) mixtures at temperatures from 298.15 to 328.15 K x2
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
Temperature (K) 298.15
308.15
318.15
328.15
72.0 53.2 47.8 44.7 42.4 37.8 35.7 34.5 33.4 32.7 32.0 31.5
70.4 52.0 46.5 43.5 41.4 36.8 34.8 33.5 32.4 31.8 31.3 30.8
68.8 50.5 45.3 42.3 40.5 36.0 34.0 32.9 31.8 31.1 30.6 30.0
67.1 49.3 44.1 41.2 39.4 35.3 33.3 32.0 31.1 30.4 29.8 29.2
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Fig. 1. Surface tension of the aqueous solutions: ( ), (MEA + H2 O) [12]; ( ), (DEA + H2 O) [14]; ( ), (TEA + H2 O) [14]; ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures at 298.15 K.
the concentration of organic compounds can be seen in the surface tension of (alcohol + water) [17,21] and (organic acid + water) [18] mixtures. This could be explained with a much larger concentration of ethanolamine on the water surface than in the bulk (Gibbs adsorption) [19]. The variations of the surface tensions for both (MDEA + H2 O) and (DMEA + H2 O) mixtures with temperature are linear in the temperature range of 298.15–328.15 K (Fig. 2). This linear variation of the surface tension with temperature has been shown by Jasper [20] and Vázquez et al. for aqueous mixtures of alkylalkanolamines [13,14] and alcohols [17], and organic acids [18]. The temperature dependence of the surface tension of the alkylalkanolamines is presented as γ = K1 + K2 T
(1)
where γ is the surface tension, K1 and K2 are fitting coefficients and T is the temperature in K. These coefficients for (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Table 3. 2.4. Surface thermodynamics Thermodynamic properties of the surface of these aqueous solutions are obtained by the following equations [19,21]:
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Fig. 2. The temperature variation of the surface tension for the (MDEA + H2 O) mixtures. Each line represents a constant mole fraction of MDEA in water.
∂γ S =− ∂T
s
(2) C,P
and the surface enthalpy ∂γ Hs = γ − T ∂T C,P
(3)
The surface entropy values are obtained by multiplying the slopes of the linear plots by minus one Eq. (2). Considering the error in the experimental measurements, the surface entropies and enthlapies have an uncertainty of about 3%. We have also calculated the surface entropy for the mixtures (MEA +H2 O) [13], (DEA + H2 O) [14], (TEA + H2 O) [14] using the data from the literature. The composition dependence of these aqueous systems is shown in Fig. 3. It is quite interesting that the surface entropy for the three aqueous mixtures of MEA, DEA and TEA remains practically constant at +0.16 mJ m−2 K−1 and the surface entropy of (MDEA + H2 O) and (DMEA + H2 O) mixtures varies in a different way. The surface entropy of above two systems decreases rapidly at low concentration (x2 < 0.2) and it remains practically constant for the rest of the solutions. The surface entropy values for the (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Table 4. The surface enthalpies of these mixtures are obtained by using Eq. (3). The enthalpy values for (MEA + H2 O), (DEA + H2 O) and (TEA + H2 O) mixtures and for (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Tables 5 and 6, respectively. The surface enthalpies of all five systems are shown in Fig. 4. There are small differences between monoethanolamine
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Table 3 Surface tension parameters (K1 , mJ m−2 ) and (K2 , mJ m−2 K−1 ) from linear fitting of the surface tension of the binary mixtures of (methyldiethanolamine, MDEA + water) and (dimethylethanolamine, DMEA + H2 O) in the temperature range of 298.15–328.15 K x2
K1
K2
(MDEA + H2 O) 0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8118 1.0000
120.6 94.5 81.8 76.6 69.6 63.5 57.7 57.1 56.4 56.6 55.3 55.3
−0.163 −0.119 −0.090 −0.086 −0.071 −0.063 −0.053 −0.056 −0.056 −0.058 −0.056 −0.057
(DMEA + H2 O) 0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
120.6 92.6 84.4 79.6 71.9 62.5 59.5 58.6 55.7 55.3 53.8 54.5
−0.163 −0.132 −0.123 −0.117 −0.099 −0.083 −0.080 −0.081 −0.075 −0.076 −0.073 −0.077
and diethanolamine solutions and a little larger difference between diethanolamine and triethanolamine mixtures (same as Fig. 1). In all three Figs. 1, 3 and 4, the end alkyl group has a dominant effect on the surface thermodynamic properties of these aqueous solutions. The surface properties of the pure water, alcohols [17], acids [18] and alkylalkanolamines at 298.15 K are given in Table 7. These three surface thermodynamic properties reflect well the nature of these surfaces. Water has the highest values for all three properties and all three surface properties drop by adding an alkyl group. The size of this alkyl group is a dominant factor in reducing all three properties. 2.5. Surface binding Connors and Wright [24] have developed a model for the binary aqueous solution of the organic molecules. The bulk and surface phases are considered separately. The organic molecules on the surface are divided in “free” and “bound” molecules to the water surface sites. There are some occupied and some unoccupied sites on the water surface. The fraction of occupied sites is given by the following
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331
Fig. 3. Surface entropy values: ( ), (MEA + H2 O); ( ), (DEA + H2 O); ( ), (TEA + H2 O); ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures.
Table 4 The surface entropy (Ss , mJ m−2 K−1 ) of the (MDEA + H2 O) and (DMEA + H2 O) mixtures at temperatures from 298.15 to 328.15 K xMDEA
Ss
xDMEA
Ss
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
0.16 0.12 0.09 0.09 0.07 0.06 0.05 0.05 0.06 0.06 0.06 0.06
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
0.16 0.13 0.12 0.12 0.10 0.08 0.08 0.08 0.07 0.07 0.07 0.07
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Table 5 The surface enthalpies (Hs , mJ m−2 ) of the (MEA + H2 O), (DEA + H2 O) and (TEA + H2 O) mixtures at 298.15 Ka xMEA
Hs
xDEA
Hs
xTEA
Hs
0.000 0.015 0.032 0.049 0.069 0.112 0.164 0.228 0.307 0.407 0.541 0.726 1.000
120 115 114 112 111 109 107 106 104 103 102 100 98
0.000 0.019 0.041 0.068 0.102 0.146 0.204 0.285 0.407 0.606 1.000 – –
120 115 111 109 107 106 104 103 102 100 96 – –
0.000 0.013 0.029 0.049 0.074 0.108 0.153 0.22 0.326 0.521 1.000 – –
120 114 109 106 103 101 99 98 97 96 95 – –
a These surface enthalpy values were obtained by using the surface tension values from [13] for (MEA + H2 O) and [14] for (DEA + H2 O) and (TEA + H2 O) mixtures.
equation: f
F =
Kx2s f 1 + Kx2s
(4)
where K is the binding site constant, x2s is the mole fraction of the organic free molecule on the water surface. Connors and Wright [24], also assumed that the number of the binding sites on the surface is proportional to the number of water molecules. By defining partition coefficients; P1 = x1s /x1B and f P2 = x2s /x2B and using x1B + x2B = 1, they were able to derive the following equation: bx1B P2 x2B (5) x2s = 1 + 1 − ax1B f
Table 6 The surface enthalpy (Hs , J m−2 ) of (MDEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K xMDEA
Hs
xDMEA
Hs
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
121 95 82 77 70 63 58 57 56 57 56 55
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
121 93 84 80 72 63 60 59 56 55 54 54
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333
Fig. 4. Surface enthalpy values: ( ), (MEA + H2 O); ( ), (DEA + H2 O); ( ), (TEA + H2 O); ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures.
Table 7 The surface thermodynamics of pure organic compoundsa Compounds
γ at 298.15 K (mJ m−2 )
Ss (dγ /dT = −Ss ) (mJ m−2 K−1 )
Hs = γ −T(dγ /dT) (mJ m−2 )
H2 O CH3 OH C2 H5 OH n-C3 H7 OH i-C3 H7 OH t-C4 H9 OH HCOOH CH3 COOH C2 H5 COOH NH2 C2 H4 OH (MEA) NH(C2 H4 OH)2 (DEA) N(C2 H4 OH)3 (TEA) CH3 N(C2 H4 OH)2 (MDEA) (CH3 )2 NC2 H4 OH (DMEA)
72 22 23 22 21 20 37 27 26 49 47 46 38 31
0.16 0.09 0.09 0.08 0.10 0.11 0.11 0.10 0.10 0.16 0.16 0.16 0.06 0.07
121 50 50 46 51 52a 69 57 56 98 96 95 55 54
a
At 303.15 K.
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where a=
KP2 1 + KP2
(6)
b=
kKP1 1 + KP2
(7)
and
and b/a = kP1 /P2 . It was also assumed that the surface tension of a mixture is the sum of the pure compound with taking care of the concentrations of each on the surface phase. They found the following formula relates the surface tension to the mole fraction of the components 1 and 2. bx1B γ = γ1 − 1 + P2 x2B (γ1 − γ2 ) (8) 1 − ax1B And finally, by assuming that P2 (the partition function of organic compound) is constant and equal to 1, the following equation is obtained: bx1 γ = γ1 − 1 + x2 (γ1 − γ2 ) (9) 1 − ax1 where γ , γ 1 , γ 2 are surface tension of the mixture, of water and of the organic compound, x1 , and x2 are mole fractions of water and organic compound and a and b are the fitting coefficients. Eq. (9), can be rearranged to the following form by introducing a reduced surface tension as γred =
γ1 − γ γ1 − γ2
(10)
and by defining the quantity R = γ red /x2 , Eq. (9) can be rearranged as x1 1 ax1 = − R−1 b b
(11)
This equation applies to all the (alkylalkanolamines + water) mixtures discussed in this paper and also to other organic aqueous solutions. The binding constant K can be obtained by a K= (12) 1−a It was claimed by Connors and Wright [24] that Eq. (4) applies to whole range of concentration of aqueous solutions. This equation has been tested for aqueous solutions of MEA, DEA, TEA, MDEA, DMEA, methanol, ethanol, 1-propanol, 2-propanol and t-butanol mixtures. The aqueous solutions of t-butanol have been studied by Glinski et al. [21–23] and they found a maximum for the surface entropy and a minimum for surface enthalpy in the concentration range of x2 = 0.01–0.02. This was related to the aggregate formation in this range of concentration, which behaves in a different way from the rest of the concentration range. We believe that there is the same kind of aggregate for the alkylalkanolamines with alkyl groups but on a different scale compared with t-butanol aqueous solution. The surface tension parameters for aqueous solutions of alcohols and alkanolamines are given in Table 8. The “a” and “b”
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Table 8 The surface tension parameters of the aqueous organic compounds at 298.15 K Organic compounds
a
b
K
Methanol [17] Ethanol [17] n-Propanol [17] i-Propanol [17] t-Butanol [21] Monoethanolamine [13] Diethanolamine [14] Triethanolamine [14] MDEAa DMEAa
0.866 0.957 0.993 0.980 0.995 0.946 0.956 0.958 0.949 0.965
0.845 0.925 0.980 1.004 0.951 0.628 0.667 0.951 0.957 0.940
6.5 23.3 141.9 57.8 199.0 17.5 21.7 22.8 18.6 27.6
a
This work.
were obtained by fitting Eq. (11) to the data. The binding constant K, for the association of the organic compounds with the surface region varies as a function of the size of the hydrophobic group present on the organic molecule. This effect is more pronounced in the case of alcohol with one hydrophilic (–OH) and one hydrophobic (Cn H2n+1 ) groups. The K values for methanol (CH3 OH) and t-butanol ((CH3 )3 COH) are 6.5 and 199.0, respectively. The variation of K becomes much smaller for the alkylalkanolamine solutions due to the present of two hydrophilic (–NH2 , –OH) groups. These two polar groups will push the alkylalkanolamines to the bulk rather than keeping them on the water surface. 3. Conclusion Surface tension measurements are a good way to obtain surface thermodynamic properties. The comparison of the values of the surface entropy and surface enthalpy for the aqueous solutions of these alkylethanolamines, helps us to conclude that the end alkyl group is a dominant factor on the surface property of the aqueous solutions. We need more data to add more light on the competition between the hydrophobic and hydrophilic groups on the water surface. List of symbols a constant value in Eq. (5) b constant value in Eq. (5) Hs surface enthalpy (mJ m−2 ) k ratio of b/a K binding constant in Eq. (7) K1 and K2 constant values in Eq. (1) R ratio of reduced surface tension to mole fraction of ethanolamine in the binary solutions Ss surface entropy (mJ m−2 K−1 ) T temperature (K) Greek letters γ surface tension (mJ m−2 )
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Surface thermodynamics of aqueous solutions of alkylethanolamines Y. Maham, A.E. Mather∗ Department of Chemical and Materials Engineering, University of Alberta, Alberta, Edmonton, Canada T6G 2G6 Received 4 September 2000; received in revised form 21 November 2000; accepted 22 November 2000
Abstract The surface tension of aqueous solutions of methyldiethanolamine and dimethylethanolamine has been measured at temperatures from 298.15 to 328.15 K over the whole range of concentrations. The surface tensions of these aqueous binary mixtures show more concentration dependence on the water-rich side. Addition of a small amount of alkylethanolamine reduces the surface tension of water drastically. This effect is related to the presence of hydrophobic groups such as (–CH3 ) which tend to remain more on the water surface. The thermodynamics of the water surface, the surface entropy and surface enthalpy have been calculated for six (alkylethanolamine + water) mixtures. The surface thermodynamic properties of these mixtures are classified in two categories: those aqueous solutions of ethanolamines with an end hydrophobic group and those without. The surface entropy of the category without an end hydrophobic group is similar to the water surface while the surface properties of the category with an end hydrophobic group are much different from the water surface. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluids; Interfacial tension; Surface entropy; Surface enthalpy; Mixture; Data
1. Introduction This is a continuation of our collection of physical properties of pure and aqueous mixtures of alkylalkanolamines. Bulk properties of pure liquid alkylalkanolamines: heat capacities [1], thermal conductivity [2], densities and viscosities [3] and of binary aqueous mixtures, excess molar properties of (monoethanolamine, MEA + H2 O), (monomethylethanolamine, MMEA + H2 O) and (dimethylethanolamine, DMEA + H2 O) mixtures at 298.15 K [4], the volumetric properties of (MEA + H2 O), (diethanolamine, DEA + H2 O) and (triethanolamine, TEA + H2 O) mixtures at 298.15–353.15 K [5], of (methyldiethanolamine, MDEA + H2 O) and (ethyldiethanolamine, EDEA + H2 O) mixtures at 298.15– 353.15 K [6], and of (dimethylethanolamine, DMEA + H2 O) and (diethylethanolamine, DEEA + H2 O) Abbreviations: DEA, diethanolamine (NH(C2 H4 OH)2 ); DMEA, dimethylethanolamine (((CH3 )2 NC2 H4 OH)2 ); MDEA, methyldiethanolamine ((CH3 NHC2 H4 OH); MEA, monoethanolamine (NH2 C2 H4 OH); TEA, triethanolamine (N(C2 H4 OH)3 ) ∗ Corresponding author. Tel.: +1-780-492-3957; fax: +1-780-492-2881. E-mail address: [email protected] (A.E. Mather). 0378-3812/01/$20.00 © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 ( 0 1 ) 0 0 3 9 1 - 0
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mixtures at 293.15–313.15 K [7], thermodynamic properties such as densities, apparent molar volume have been reported. Isobaric specific heat capacities of (MEA + H2 O) mixtures [8], transport properties of (DEA + H2 O) and (MDEA + H2 O) mixtures at 298.15–353.15 K [9], excess molar enthalpies of (MEA + H2 O), (MMEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K [4] of (DEA + H2 O), (MDEA + H2 O) and (TEA + H2 O) mixtures at 298.15 K [10], of (monoethylethanolamine, MEEA + H2 O), (diethylethanolamine, DEEA + H2 O) and (n-propylethanolamine, n-PEA + H2 O) and (2-amino-2-methyl-1-propanol, AMP+H2 O) mixtures at 298.15 K [11] and the excess molar enthalpies of (diethanolamine, DEA + H2 O), (methyldiethanolamine, MDEA + H2 O), (ethyldiethanolamine, EDEA + HO) and (n-butyldiethanolamine, n-BDEA + H2 O) mixtures [12] were reported at 298.15–338.15 K. The surface properties of (MEA + H2 O) and 2-amino-2-methyl-1-propanol, (AMP + H2 O) mixtures and ternary mixtures of water with these ethanolamines at 298.15–323.15 K [13], and of (DEA + H2 O) and (TEA+H2 O) mixtures at 298.15–323.15 K [14] and of (MDEA+H2 O) and ternary mixtures of water with these ethanolamines at 298.15–323.15 K [15] were studied. In this report, we are interested in the surface properties of (methyldiethanolamine, MDEA+H2 O) and (dimethylethanolamine, DMEA+H2 O) mixtures. 2. Experimental section 2.1. Materials Dimethylethanolamine [(CH3 )2 NCH2 CH2 OH, DMEA, 99%] and methyldiethanolamine [CH3 N(CH2 CH2 OH)2 , MDEA, 99%] were both obtained from Aldrich Chemicals. These compounds were used as received, after confirmatory analysis by titration with standard hydrochloric acid. Mixtures of these compounds with nano-pure distilled water were made by mass, with care being taken to minimize exposure to air (carbon dioxide). 2.2. Apparatus The capillary-rise technique was used to measure the surface tension [16]. This consists of a capillary about 20 cm long which was immersed in the solution to measure the surface tension and the test tube containing the solution was placed inside a water bath. The temperature of the bath was controlled to ±0.05 K. The height of the liquid inside the capillary was read by a cathetometer. The calibration of the capillary tube has been done carefully with distilled water, monoethanolamine and its aqueous mixtures [13,19] with different concentrations at four temperatures from 298.15 to 328.15 K. From the calibration a constant of 0.1315 ± 0.0008 for the capillary tube has been obtained which was multiplied by the height of each reading for different solutions. The precision of the cathetometer is ±0.005 cm. The solutions were poured in the test tube and their surface tension was measured by changing the temperature of the water bath. The accuracy of the results is ±2%. 2.3. Results The surface tension of (MDEA + H2 O) and (DMEA + H2 O) mixtures is given from 298.15 to 328.15 K in Tables 1 and 2, respectively. The composition dependence of the surface tension for (MEA + H2 O)
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Table 1 Surface tension (γ , mJ m−2 ) of (methyldiethanolamine, MDEA + water) mixtures at temperatures from 298.15 to 328.15 K x2
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
Temperature (K) 298.15
308.15
318.15
328.15
72.0 59.1 55.0 51.3 48.4 44.7 41.8 40.4 39.7 39.3 38.6 38.3
70.4 57.7 54.0 50.3 47.8 44.1 41.4 39.8 39.1 38.8 38.0 37.7
68.8 56.6 53.1 49.6 47.0 43.5 40.9 39.3 38.6 38.1 37.5 37.1
67.1 55.5 52.3 48.7 46.3 42.8 40.2 38.7 38.0 37.6 36.9 36.6
[13], (DEA + H2 O) [14], (TEA + H2 O) [14], (MDEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K is shown in Fig. 1. The surface tensions of aqueous solutions of MEA, DEA and TEA (ethanolamines without any end alkyl group) are similar to each other, with a very small difference between MEA and DEA solutions and a slightly bigger difference between DEA and TEA mixtures. This shows the small effect of the presence of an extra (–C2 H4 OH) group of the ethanolamine on the surface tension. Contrary to the (–C2 H4 OH) group, the (–CH3 ) group has a much bigger effect on the surface tension of these aqueous mixtures and its effect becomes much larger with the number of the methyl groups on the ethanolamine molecules. The drop in the surface tension of the water by addition of ethanolamine is also much bigger for the ethanolamine with a methyl group. The same behavior of the surface tension against Table 2 Surface tension (γ , mJ m−2 ) of (dimethylethanolamine, DMEA + water) mixtures at temperatures from 298.15 to 328.15 K x2
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
Temperature (K) 298.15
308.15
318.15
328.15
72.0 53.2 47.8 44.7 42.4 37.8 35.7 34.5 33.4 32.7 32.0 31.5
70.4 52.0 46.5 43.5 41.4 36.8 34.8 33.5 32.4 31.8 31.3 30.8
68.8 50.5 45.3 42.3 40.5 36.0 34.0 32.9 31.8 31.1 30.6 30.0
67.1 49.3 44.1 41.2 39.4 35.3 33.3 32.0 31.1 30.4 29.8 29.2
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Fig. 1. Surface tension of the aqueous solutions: ( ), (MEA + H2 O) [12]; ( ), (DEA + H2 O) [14]; ( ), (TEA + H2 O) [14]; ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures at 298.15 K.
the concentration of organic compounds can be seen in the surface tension of (alcohol + water) [17,21] and (organic acid + water) [18] mixtures. This could be explained with a much larger concentration of ethanolamine on the water surface than in the bulk (Gibbs adsorption) [19]. The variations of the surface tensions for both (MDEA + H2 O) and (DMEA + H2 O) mixtures with temperature are linear in the temperature range of 298.15–328.15 K (Fig. 2). This linear variation of the surface tension with temperature has been shown by Jasper [20] and Vázquez et al. for aqueous mixtures of alkylalkanolamines [13,14] and alcohols [17], and organic acids [18]. The temperature dependence of the surface tension of the alkylalkanolamines is presented as γ = K1 + K2 T
(1)
where γ is the surface tension, K1 and K2 are fitting coefficients and T is the temperature in K. These coefficients for (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Table 3. 2.4. Surface thermodynamics Thermodynamic properties of the surface of these aqueous solutions are obtained by the following equations [19,21]:
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Fig. 2. The temperature variation of the surface tension for the (MDEA + H2 O) mixtures. Each line represents a constant mole fraction of MDEA in water.
∂γ S =− ∂T
s
(2) C,P
and the surface enthalpy ∂γ Hs = γ − T ∂T C,P
(3)
The surface entropy values are obtained by multiplying the slopes of the linear plots by minus one Eq. (2). Considering the error in the experimental measurements, the surface entropies and enthlapies have an uncertainty of about 3%. We have also calculated the surface entropy for the mixtures (MEA +H2 O) [13], (DEA + H2 O) [14], (TEA + H2 O) [14] using the data from the literature. The composition dependence of these aqueous systems is shown in Fig. 3. It is quite interesting that the surface entropy for the three aqueous mixtures of MEA, DEA and TEA remains practically constant at +0.16 mJ m−2 K−1 and the surface entropy of (MDEA + H2 O) and (DMEA + H2 O) mixtures varies in a different way. The surface entropy of above two systems decreases rapidly at low concentration (x2 < 0.2) and it remains practically constant for the rest of the solutions. The surface entropy values for the (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Table 4. The surface enthalpies of these mixtures are obtained by using Eq. (3). The enthalpy values for (MEA + H2 O), (DEA + H2 O) and (TEA + H2 O) mixtures and for (MDEA + H2 O) and (DMEA + H2 O) mixtures are given in Tables 5 and 6, respectively. The surface enthalpies of all five systems are shown in Fig. 4. There are small differences between monoethanolamine
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Table 3 Surface tension parameters (K1 , mJ m−2 ) and (K2 , mJ m−2 K−1 ) from linear fitting of the surface tension of the binary mixtures of (methyldiethanolamine, MDEA + water) and (dimethylethanolamine, DMEA + H2 O) in the temperature range of 298.15–328.15 K x2
K1
K2
(MDEA + H2 O) 0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8118 1.0000
120.6 94.5 81.8 76.6 69.6 63.5 57.7 57.1 56.4 56.6 55.3 55.3
−0.163 −0.119 −0.090 −0.086 −0.071 −0.063 −0.053 −0.056 −0.056 −0.058 −0.056 −0.057
(DMEA + H2 O) 0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
120.6 92.6 84.4 79.6 71.9 62.5 59.5 58.6 55.7 55.3 53.8 54.5
−0.163 −0.132 −0.123 −0.117 −0.099 −0.083 −0.080 −0.081 −0.075 −0.076 −0.073 −0.077
and diethanolamine solutions and a little larger difference between diethanolamine and triethanolamine mixtures (same as Fig. 1). In all three Figs. 1, 3 and 4, the end alkyl group has a dominant effect on the surface thermodynamic properties of these aqueous solutions. The surface properties of the pure water, alcohols [17], acids [18] and alkylalkanolamines at 298.15 K are given in Table 7. These three surface thermodynamic properties reflect well the nature of these surfaces. Water has the highest values for all three properties and all three surface properties drop by adding an alkyl group. The size of this alkyl group is a dominant factor in reducing all three properties. 2.5. Surface binding Connors and Wright [24] have developed a model for the binary aqueous solution of the organic molecules. The bulk and surface phases are considered separately. The organic molecules on the surface are divided in “free” and “bound” molecules to the water surface sites. There are some occupied and some unoccupied sites on the water surface. The fraction of occupied sites is given by the following
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331
Fig. 3. Surface entropy values: ( ), (MEA + H2 O); ( ), (DEA + H2 O); ( ), (TEA + H2 O); ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures.
Table 4 The surface entropy (Ss , mJ m−2 K−1 ) of the (MDEA + H2 O) and (DMEA + H2 O) mixtures at temperatures from 298.15 to 328.15 K xMDEA
Ss
xDMEA
Ss
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
0.16 0.12 0.09 0.09 0.07 0.06 0.05 0.05 0.06 0.06 0.06 0.06
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
0.16 0.13 0.12 0.12 0.10 0.08 0.08 0.08 0.07 0.07 0.07 0.07
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Table 5 The surface enthalpies (Hs , mJ m−2 ) of the (MEA + H2 O), (DEA + H2 O) and (TEA + H2 O) mixtures at 298.15 Ka xMEA
Hs
xDEA
Hs
xTEA
Hs
0.000 0.015 0.032 0.049 0.069 0.112 0.164 0.228 0.307 0.407 0.541 0.726 1.000
120 115 114 112 111 109 107 106 104 103 102 100 98
0.000 0.019 0.041 0.068 0.102 0.146 0.204 0.285 0.407 0.606 1.000 – –
120 115 111 109 107 106 104 103 102 100 96 – –
0.000 0.013 0.029 0.049 0.074 0.108 0.153 0.22 0.326 0.521 1.000 – –
120 114 109 106 103 101 99 98 97 96 95 – –
a These surface enthalpy values were obtained by using the surface tension values from [13] for (MEA + H2 O) and [14] for (DEA + H2 O) and (TEA + H2 O) mixtures.
equation: f
F =
Kx2s f 1 + Kx2s
(4)
where K is the binding site constant, x2s is the mole fraction of the organic free molecule on the water surface. Connors and Wright [24], also assumed that the number of the binding sites on the surface is proportional to the number of water molecules. By defining partition coefficients; P1 = x1s /x1B and f P2 = x2s /x2B and using x1B + x2B = 1, they were able to derive the following equation: bx1B P2 x2B (5) x2s = 1 + 1 − ax1B f
Table 6 The surface enthalpy (Hs , J m−2 ) of (MDEA + H2 O) and (DMEA + H2 O) mixtures at 298.15 K xMDEA
Hs
xDMEA
Hs
0.0000 0.0246 0.0473 0.0756 0.1012 0.1990 0.3108 0.4220 0.4902 0.5891 0.8174 1.0000
121 95 82 77 70 63 58 57 56 57 56 55
0.0000 0.0234 0.0494 0.0754 0.0992 0.2034 0.2994 0.3936 0.4888 0.5973 0.8075 1.0000
121 93 84 80 72 63 60 59 56 55 54 54
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333
Fig. 4. Surface enthalpy values: ( ), (MEA + H2 O); ( ), (DEA + H2 O); ( ), (TEA + H2 O); ( ), (MDEA + H2 O) and ( ), (DMEA + H2 O) mixtures.
Table 7 The surface thermodynamics of pure organic compoundsa Compounds
γ at 298.15 K (mJ m−2 )
Ss (dγ /dT = −Ss ) (mJ m−2 K−1 )
Hs = γ −T(dγ /dT) (mJ m−2 )
H2 O CH3 OH C2 H5 OH n-C3 H7 OH i-C3 H7 OH t-C4 H9 OH HCOOH CH3 COOH C2 H5 COOH NH2 C2 H4 OH (MEA) NH(C2 H4 OH)2 (DEA) N(C2 H4 OH)3 (TEA) CH3 N(C2 H4 OH)2 (MDEA) (CH3 )2 NC2 H4 OH (DMEA)
72 22 23 22 21 20 37 27 26 49 47 46 38 31
0.16 0.09 0.09 0.08 0.10 0.11 0.11 0.10 0.10 0.16 0.16 0.16 0.06 0.07
121 50 50 46 51 52a 69 57 56 98 96 95 55 54
a
At 303.15 K.
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where a=
KP2 1 + KP2
(6)
b=
kKP1 1 + KP2
(7)
and
and b/a = kP1 /P2 . It was also assumed that the surface tension of a mixture is the sum of the pure compound with taking care of the concentrations of each on the surface phase. They found the following formula relates the surface tension to the mole fraction of the components 1 and 2. bx1B γ = γ1 − 1 + P2 x2B (γ1 − γ2 ) (8) 1 − ax1B And finally, by assuming that P2 (the partition function of organic compound) is constant and equal to 1, the following equation is obtained: bx1 γ = γ1 − 1 + x2 (γ1 − γ2 ) (9) 1 − ax1 where γ , γ 1 , γ 2 are surface tension of the mixture, of water and of the organic compound, x1 , and x2 are mole fractions of water and organic compound and a and b are the fitting coefficients. Eq. (9), can be rearranged to the following form by introducing a reduced surface tension as γred =
γ1 − γ γ1 − γ2
(10)
and by defining the quantity R = γ red /x2 , Eq. (9) can be rearranged as x1 1 ax1 = − R−1 b b
(11)
This equation applies to all the (alkylalkanolamines + water) mixtures discussed in this paper and also to other organic aqueous solutions. The binding constant K can be obtained by a K= (12) 1−a It was claimed by Connors and Wright [24] that Eq. (4) applies to whole range of concentration of aqueous solutions. This equation has been tested for aqueous solutions of MEA, DEA, TEA, MDEA, DMEA, methanol, ethanol, 1-propanol, 2-propanol and t-butanol mixtures. The aqueous solutions of t-butanol have been studied by Glinski et al. [21–23] and they found a maximum for the surface entropy and a minimum for surface enthalpy in the concentration range of x2 = 0.01–0.02. This was related to the aggregate formation in this range of concentration, which behaves in a different way from the rest of the concentration range. We believe that there is the same kind of aggregate for the alkylalkanolamines with alkyl groups but on a different scale compared with t-butanol aqueous solution. The surface tension parameters for aqueous solutions of alcohols and alkanolamines are given in Table 8. The “a” and “b”
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Table 8 The surface tension parameters of the aqueous organic compounds at 298.15 K Organic compounds
a
b
K
Methanol [17] Ethanol [17] n-Propanol [17] i-Propanol [17] t-Butanol [21] Monoethanolamine [13] Diethanolamine [14] Triethanolamine [14] MDEAa DMEAa
0.866 0.957 0.993 0.980 0.995 0.946 0.956 0.958 0.949 0.965
0.845 0.925 0.980 1.004 0.951 0.628 0.667 0.951 0.957 0.940
6.5 23.3 141.9 57.8 199.0 17.5 21.7 22.8 18.6 27.6
a
This work.
were obtained by fitting Eq. (11) to the data. The binding constant K, for the association of the organic compounds with the surface region varies as a function of the size of the hydrophobic group present on the organic molecule. This effect is more pronounced in the case of alcohol with one hydrophilic (–OH) and one hydrophobic (Cn H2n+1 ) groups. The K values for methanol (CH3 OH) and t-butanol ((CH3 )3 COH) are 6.5 and 199.0, respectively. The variation of K becomes much smaller for the alkylalkanolamine solutions due to the present of two hydrophilic (–NH2 , –OH) groups. These two polar groups will push the alkylalkanolamines to the bulk rather than keeping them on the water surface. 3. Conclusion Surface tension measurements are a good way to obtain surface thermodynamic properties. The comparison of the values of the surface entropy and surface enthalpy for the aqueous solutions of these alkylethanolamines, helps us to conclude that the end alkyl group is a dominant factor on the surface property of the aqueous solutions. We need more data to add more light on the competition between the hydrophobic and hydrophilic groups on the water surface. List of symbols a constant value in Eq. (5) b constant value in Eq. (5) Hs surface enthalpy (mJ m−2 ) k ratio of b/a K binding constant in Eq. (7) K1 and K2 constant values in Eq. (1) R ratio of reduced surface tension to mole fraction of ethanolamine in the binary solutions Ss surface entropy (mJ m−2 K−1 ) T temperature (K) Greek letters γ surface tension (mJ m−2 )
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