Journal of Molecular Liquids 182 (2013) 7–13

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Journal of Molecular Liquids

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Volumetric properties of some water + monoalkanolamine systems between 303.15and 323.15 K

Faisal Islam Chowdhury a, Muhammad A.R. Khan a, Muhammad A. Saleh b,1, Shamim Akhtar a,⁎a Department of Chemistry, University of Chittagong, Chittagong 4331, Bangladeshb Research Laboratory, Department of Chemistry, University of Chittagong, Chittagong 4331, Bangladesh

⁎ Corresponding author. Tel.: +880 1712 090 205.E-mail address: shamim3332000@yahoo.com (S. Ak

1 Deceased.

0167-7322/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.molliq.2013.03.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 September 2012Received in revised form 6 March 2013Accepted 10 March 2013Available online 25 March 2013

Keywords:DensityExcess molar volumeHydrophobicityMonoalkanolamines

Density (ρ) of aqueous solutions of monomethylethanolamine (MEA), monoethylethanolamine (EEA),dimethylethanolamine (DMEA), dimethylpropan-1-olamine (DMPA-1) and dimethylpropan-2-olamine(DMPA-2) is measured at a 5 K interval between 303.15 and 323.15 K in the range of 0 ≤ x2 ≤ 1, where x2 isthe mole fraction of alkanolamines. The ρ of the monoalkanolamines is assumed to be related to respectivestrength of association. Excess molar volumes (VE

m) were negative for all systems in the whole range of compo-sition and at all temperatures with minima occurring at x2 ~ 0.35 to 0.40. The depth of minima varied as,W + DMPA-2 > W + DMEA > W + DMPA-1 > W + EEA > W + MEA. Negative VE

m were explained bylarge size difference as well as hydrophobicity effects. Thermal expansivity (α) followed as, W +DMPA-2 > W + DMEA > W + DMPA-1 > W + EEA > W + MEA, while its excess values were found toform distinct minima. The ρ vs. x2 isotherms have been correlated with polynomials, while VE

m and αE were fittedto the Redlich–Kister equation. In addition to intermolecular H-bonding, interstitial accommodation, steric effectsetc. ‘hydrophobicity’of the alkanolamines is suggested toplay themost vital role for the systemsunder investigation.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

This is a continuation of our study on the physico-chemical proper-ties of pure alkanolamines and their aqueous solutions. Very recentlywe have reported the volumetric properties of aqueous solutions ofsome diethanolamines: diethanolamine (DEA), methyldiethanolamine(MDEA), ethyldiethanolamine (EDEA) and n-butyldiethanolamine(BDEA) [1].

Some of the unique pieces ofworks on volumetric properties of aque-ous solutions of monoalkanolamines that need to bementioned here areas follows. Recently, Narayanaswamy et al. have reported on volumetric,viscosity, refractometric and surface properties for DMPA-1 + water[18]. Subsequently, Li et al. [2] reported on volumetric properties ofW + MEA, Maham et al. [3,4] also studied the volumetric properties ofaqueous solutions of monoethanolamine (EA), EEA and DMEA and den-sities, excess molar volumes and partial molar volumes of W + EA,W + DEA andW + triethanolamine (TEA).Moreover, volumetric prop-erties of aqueous solution of 2-amino-2-methyl-1-propanol (AMP) andn-propylethanolamine (PEA) were also reported by Mather et al. [5]. Inother reports partial molar volumes, excessmolar volumes and adiabaticcompressibility ofW + EAwere studied by Hawrylak et al. [6]; viscosityof W + EEA and W + diethylethanolamine (DEEA) by Lee and Lin [7];densities of W + EA by Page and his coworkers [8]. Furthermore,

htar).

rights reserved.

volumetric properties of W + DEEA were measured by Barbas et al. [9]and Lampreia et al. [10]. Also, Lebrette et al. [11] measured volumetricproperties of aqueous solutions of EEA and DEEA.

Nevertheless, data on physico-chemical properties of aqueous solu-tions of monoethanolamines are still inadequate. Not only that, data ofDMPA-1 and DMPA-2 and their aqueous solutions are yet to be reported.Furthermore, all those reported works dealt with volumetric propertiesof different monoalkanolamines separately or in series, which do not be-long to the series of alkyl, substituted monoalkanolamines. The series ofMEA, EEA, DMEA, DMPA-1 andDMPA-2 could be used to examine the ef-fect of size and number of alkyl groups attached to N-atom and alsobranching effect of the alkanol portion of monoalkanolamines on volu-metric properties. Therefore, with the aim of getting new data and fillingthis gap, we have measured densities and calculated excess molarvolumes of the aqueous solutions of MEA, EEA, DMEA, DMPA-1 andDMPA-2 in thewhole range of composition at different temperatures be-tween 303.15 and 323.15 K. All these may provide us an opportunity toexamine the effects due to difference in the size as well as the number ofalkyl groups attached to the N-atom and relevant n branching in thealkanol part alongwith variation in the position of\OH group in alkanolportion of alkanolamines. Andhence,we can get a deeper insight into theintermolecular interactions between such self-associated structures.

2. Experimental

All the liquids used to prepare the binary liquid mixtureswith quoted purities are: monomethylethanolamine (98%+),

8 F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

monoethylethanolamine (>97%), dimethylpropanol-1-amine (≥98%)and dimethylpropanol-2-amine (>98%) were procured from Merck-Schuchardt, except the dimethylethanolamine (99.5%+) from AldrichChemical Co. Ltd. Water used in preparation of all solutions wasdegassed and distilled twice. The liquids were used after distillationand kept under molecular sieves (4 Å) for all at least 2 weeks prior touse.Water contents determined by a Karl–Fisher titrator (Metler ToledoDL 31, Switzerland) for were found to be 0.4862, 0.1687, and0.5240 mol.kg−1 for EEA, DMPA-1 and DMPA-2, respectively. As ameasure of purity check, experimental densities of pure liquids werefurther compared with available literature values [2,3,11,12,18], whichshowed satisfactory good agreement as in Table 1.

Densities were measured by using a 5 cm3 pycnometer (MBL) andweighing was done by the Mettler Toledo, SAG285 balance with anaccuracy of ±0.00001 g. During all measurements a thermostaticallycontrolled water bath (Thermo Haake DC10 Thermostat) was usedwhichwas capable ofmaintaining temperature constant up to±0.05°K.

Frommeasured density data, excessmolar volume,VEm was estimated

by the following equation,

VEm ¼ x1M1 þ x2M2

ρ− x1M1

ρ1þ x2M2

ρ2

� �ð1Þ

where, ρ is themeasured density of themixture; ρ1,M1, V1 and x1 are thedensity, molar mass, molar volume and mole fraction of component 1and ρ2, M2, V2 and x2 are the corresponding quantities of component 2in the mixtures.

3. Results and discussion

The densities of the systems W + MEA, W + EEA, W + DMEA,W + DMPA-1 and W + DMPA-2 in the whole range of compositionat 5 different temperatures (between 303.15 and 323.15 K) are

Table 1Experimental and literature densities (ρ/g cm−3), of pure methylethanolamine (MEA),ethylethanolamine (EEA), dimethylethanolamine (DMEA), dimethylpropanol-1-amine(DMPA-1) and dimethylpropanol-2-amine (DMPA-2) at different temperatures.

Sample T/K ρ (g.cm−3)

This work Literature

MEA 303.15 0.9323 0.93226a, 0.93238b

308.15 0.9283313.15 0.9243 0.92442aa, 0.92610b

318.15 0.9204323.15 0.9165 0.91648a

EEA 303.15 0.9091 0.909401c, 0.90877d

308.15 0.9049 0.905405c

313.15 0.9008 0.901388c, 0.90097d

318.15 0.8968 0.897344c

323.15 0.8926 0.893275c

DMEA 303.15 0.8785 0.87835b

308.15 0.8741313.15 0.8696 0.86986b

318.15 0.8652323.15 0.8607

DMPA-1 303.15 0.8760 0.87646e

308.15 0.8719313.15 0.8678 0.86842e

318.15 0.8637323.15 0.8596 0.86029e

DMPA-2 303.15 0.8425308.15 0.8376313.15 0.8326 –

318.15 0.8278323.15 0.8230

a Li et al. [2].b Maham et al. [3].c Alvarez et al. [12].d Lebrette et al. [11].e Narayanaswamy et al. [18].

displayed as in Table 2. Fig. 1 represents comparative variation of ρvalues for W + MEA, W + EEA, W + DMEA, W + DMPA-1 andW + DMPA-2 at 303.15 K, as a function of mole fraction of the re-spective alkanolamine. For all systems ρ values are fitted with poly-nomial equations of the following form:

ρ=g cm−3 ¼Xni¼0

aixi2 ð2Þ

where, x2 is the mole fraction of alkanolamine, ai the regression coeffi-cient and n is the degree of polynomial. Each system values of ρ fittedto Eq. (2) well for n = 5. The coefficients ai of Eq. (2) and relevant aver-age absolute deviations (AAD/%) for the density of all the five systems atvarious temperatures (between 303.15 and 323.15 K) obtained by leastsquares method are listed in Table 3.

Measured densities of the systems are comparedwith those availablein literatures—W + MEAof Li et al. [2];W + EEA of Lebrette et al. [11];W + DMEA of Maham et al. [3] and W + DMPA-1 of Narayanaswamyet al. [18]. All were found to be in good agreement as displayed in Fig. 1.

As Fig. 1 shows, different variation patterns of ρ with respect to x2were observed at the initial stage, but afterwards ρ against composi-tion follows more or less a similar trend. The curve for W + MEA arefound to show an S-shaped behavior passing through a minimum atx2 = ~0.05 followed by a maximum at x2 ~ 0.10 and then a slowerdecrement with increasing MEA up to x2 ~ 0.20 occurred. Nearly asimilar behavior was also observed by Li et al. [2], Maham et al. [3]and Lebrette et al. [11] at this composition range for W + MEA.

For the other systems (W + EEA, W + DMEA, W + DMPA-1 andW + DMPA-2), above said behavior is not so prominent. Neverthe-less, passing out of the initial stage, variation follows essentially asimilar trend as ρ decreases almost consistently forming a slightlyconvex curvature at increasing x2. Moreover, the rate of decrementof ρ for W + DMPA-1 in the amine-rich region is relatively lowercompared to those of the rest.

According to Maham et al. [3], due to addition of ethanolamine inwater, a ‘non-random distribution’ of water and/or ‘micelle formation’may occur at the initial stage, whereby molecules of water andalkanolamine form a more compact aggregation than in other zones. Be-sides, at higher temperatures there is every possibility of a competitionbetween this molecular organization and thermal agitation, which is ev-idently clearer in these systems at higher temperatures. So, it can be spec-ulated that themicelle formation occurs at the initial range only at lowertemperatures, i.e., in the water-rich region of aqueous MEA solution.

As Fig. 1 also shows, densities of pure alkanolamines vary as:MEA > EEA > DMEA > DMPA-1 > DMPA-2, which is in the reverseorder of molar masses. Possibly, size and steric hindrance of N-alkylgroups, chain length of alkyl moiety as well as position of the \OHgroup in the alkyl chain can affect the self-association of the amines,and hence, also their ρ values. Presumably, densities of all themonoalkanolamines are related to the strength of self-association. On ex-amination of their structures, one can easily anticipate that, amongst allamines of this particular series, MEA is the most associated one due toits least steric effect. The obvious consequence of such a steric effect isalso reflected in the observedorder ofρ for the othermonoalkanolamines.

The position of\OHgroup in the alkyl chain can be considered as an-other important factor for molecular association. Pang et al. [13] men-tioned that, from a structural point of view 2-propanol is larger than1-propanol. In other words, 2-propanol is more structured than1-propanol, and hence, there is more void space in the former than inthe latter. As a result, observed ρ of 1-propanol is higher than that of2-propanol. In accordance, variation in ρ of DMPA-1 and DMPA-2 aswell as their aqueous solutions in the study is also quite conceivable —

in the whole range, ρ is higher for W + DMPA-1 compared toW + DMPA-2.

Table 2Experimental densities (ρ/g cm−3), and excess molar volumes, VE

m (cm3 mol−1) for the systems of water (x1) + methylethanolamine (x2), water + ethylethanolamine (x2),water + dimethylethanolamine (x2), water + dimethylpropanol-1-amine (x2) and water + dimethylpropanol-2-amine (x2) at different molar ratios and at differenttemperatures.

T/K 303.15 308.15 313.15 318.15 323.15

x2 ρ VEm ρ VE

m ρ VEm ρ VE

m ρ VEm

Water (x1) + methylethanolamine (x2)0.0000 0.9957 0.000 0.9941 0.000 0.9922 0.000 0.9903 0.000 0.9881 0.0000.0130 0.9949 −0.051 0.9929 −0.048 0.9913 −0.054 0.9891 −0.053 0.9868 −0.0530.0238 0.9948 −0.104 0.9930 −0.106 0.9910 −0.107 0.9888 −0.109 0.9865 −0.1110.0339 0.9946 −0.152 0.9927 −0.153 0.9905 −0.154 0.9882 −0.154 0.9858 −0.1560.0844 0.9951 −0.420 0.9925 −0.415 0.9898 −0.412 0.9869 −0.408 0.9840 −0.4050.1894 0.9934 −0.904 0.9897 −0.887 0.9862 −0.875 0.9825 −0.861 0.9787 −0.8470.3001 0.9854 −1.171 0.9814 −1.151 0.9775 −1.137 0.9736 −1.122 0.9696 −1.1050.4014 0.9758 −1.222 0.9719 −1.209 0.9680 −1.199 0.9640 −1.187 0.9601 −1.1750.4480 0.9711 −1.189 0.9671 −1.176 0.9631 −1.165 0.9592 −1.152 0.9551 −1.1390.5016 0.9667 −1.166 0.9626 −1.152 0.9586 −1.141 0.9546 −1.130 0.9507 −1.1220.5474 0.9622 −1.084 0.9581 −1.069 0.9542 −1.067 0.9503 −1.059 0.9464 −1.0540.6004 0.9579 −1.009 0.9541 −1.008 0.9502 −1.006 0.9462 −1.000 0.9423 −0.9920.7021 0.9503 −0.813 0.9463 −0.809 0.9424 −0.806 0.9385 −0.805 0.9345 −0.8000.7975 0.9439 −0.589 0.9399 −0.585 0.9360 −0.587 0.9320 −0.584 0.9281 −0.5780.9048 0.9373 −0.284 0.9333 −0.279 0.9293 −0.274 0.9253 −0.268 0.9213 −0.2641.0000 0.9323 0.000 0.9283 0.000 0.9243 0.000 0.9204 0.000 0.9165 0.000

Water (x1) + ethylethanolamine (x2)0.0132 0.9939 −0.078 0.9921 −0.079 0.9902 −0.080 0.9881 −0.081 0.9857 −0.0820.0332 0.9932 −0.233 0.9903 −0.215 0.9880 −0.215 0.9856 −0.215 0.9830 −0.2130.0499 0.9917 −0.339 0.9894 −0.338 0.9868 −0.335 0.9841 −0.332 0.9813 −0.3310.0998 0.9893 −0.687 0.9856 −0.665 0.9822 −0.652 0.9789 −0.642 0.9753 −0.6300.1506 0.9840 −0.942 0.9801 −0.917 0.9763 −0.900 0.9724 −0.880 0.9685 −0.8650.2011 0.9774 −1.108 0.9735 −1.088 0.9695 −1.070 0.9656 −1.052 0.9615 −1.0360.2982 0.9654 −1.307 0.9615 −1.296 0.9573 −1.278 0.9533 −1.263 0.9492 −1.2490.4029 0.9535 −1.361 0.9494 −1.349 0.9453 −1.339 0.9413 −1.328 0.9373 −1.3250.4458 0.9492 −1.353 0.9451 −1.343 0.9410 −1.336 0.9370 −1.327 0.9330 −1.3270.5051 0.9440 −1.336 0.9399 −1.332 0.9358 −1.329 0.9318 −1.324 0.9277 −1.3200.5529 0.9398 −1.288 0.9357 −1.282 0.9316 −1.283 0.9275 −1.272 0.9235 −1.2750.6002 0.9359 −1.223 0.9317 −1.216 0.9276 −1.210 0.9236 −1.211 0.9195 −1.2070.6508 0.9316 −1.102 0.9274 −1.098 0.9233 −1.099 0.9193 −1.098 0.9153 −1.1060.7049 0.9278 −1.001 0.9237 −1.000 0.9196 −1.000 0.9155 −0.996 0.9114 −0.9970.7985 0.9214 −0.745 0.9172 −0.749 0.9132 −0.753 0.9092 −0.758 0.9051 −0.7640.9014 0.9154 −0.445 0.9113 −0.454 0.9072 −0.464 0.9032 −0.467 0.8991 −0.4711.0000 0.9091 0.000 0.9049 0.000 0.9008 0.000 0.8968 0.000 0.8926 0.000

Water (x1) + dimethylethanolamine (x2)0.0135 0.9931 −0.111 0.9912 −0.112 0.9893 −0.114 0.9872 −0.115 0.9848 −0.1170.0238 0.9913 −0.196 0.9893 −0.197 0.9871 −0.198 0.9845 −0.193 0.9826 −0.2060.0340 0.9901 −0.290 0.9879 −0.291 0.9856 −0.292 0.9832 −0.294 0.9806 −0.2970.0500 0.9883 −0.436 0.9858 −0.434 0.9832 −0.434 0.9805 −0.433 0.9776 −0.4330.1003 0.9828 −0.866 0.9793 −0.854 0.9759 −0.846 0.9724 −0.838 0.9687 −0.8310.1993 0.9670 −1.419 0.9629 −1.403 0.9586 −1.389 0.9545 −1.374 0.9503 −1.3670.2947 0.9506 −1.660 0.9464 −1.652 0.9420 −1.639 0.9377 −1.626 0.9334 −1.6220.3489 0.9421 −1.718 0.9378 −1.710 0.9335 −1.702 0.9291 −1.692 0.9248 −1.6910.4004 0.9344 −1.720 0.9301 −1.710 0.9258 −1.708 0.9215 −1.702 0.9171 −1.7030.4530 0.9273 −1.687 0.9229 −1.678 0.9186 −1.681 0.9143 −1.676 0.9100 −1.6850.5040 0.9208 −1.620 0.9165 −1.615 0.9121 −1.615 0.9079 −1.618 0.9035 −1.6280.6047 0.9095 −1.408 0.9051 −1.406 0.9007 −1.414 0.8964 −1.414 0.8920 −1.4240.7044 0.8997 −1.107 0.8954 −1.117 0.8910 −1.127 0.8865 −1.112 0.8820 −1.1110.8062 0.8913 −0.761 0.8870 −0.772 0.8827 −0.789 0.8783 −0.789 0.8739 −0.8000.9046 0.8845 −0.398 0.8801 −0.401 0.8756 −0.410 0.8714 −0.420 0.8671 −0.4421.0000 0.8785 0.000 0.8741 0.000 0.8696 0.000 0.8652 0.000 0.8607 0.000

Water (x1) + dimethylpropanol-1-amine (x2)0.0132 0.9914 −0.104 0.9896 −0.104 0.9876 −0.106 0.9854 −0.107 0.9832 −0.1090.0250 0.9886 −0.209 0.9865 −0.208 0.9836 −0.195 0.9819 −0.210 0.9794 −0.2090.0339 0.9886 −0.329 0.9864 −0.330 0.9841 −0.331 0.9816 −0.331 0.9788 −0.3280.0478 0.9846 −0.427 0.9820 −0.421 0.9793 −0.418 0.9764 −0.414 0.9735 −0.4110.0968 0.9762 −0.844 0.9728 −0.832 0.9693 −0.817 0.9657 −0.803 0.9619 −0.7870.1512 0.9661 −1.191 0.9620 −1.165 0.9581 −1.146 0.9541 −1.128 0.9499 −1.1040.2009 0.9566 −1.403 0.9525 −1.380 0.9483 −1.358 0.9442 −1.337 0.9400 −1.3150.2997 0.9400 −1.654 0.9357 −1.628 0.9315 −1.609 0.9273 −1.591 0.9230 −1.5690.3537 0.9318 −1.693 0.9276 −1.675 0.9233 −1.656 0.9192 −1.642 0.9151 −1.6300.4037 0.9250 −1.695 0.9208 −1.680 0.9166 −1.664 0.9124 −1.650 0.9083 −1.6370.4574 0.9184 −1.657 0.9142 −1.647 0.9100 −1.634 0.9058 −1.623 0.9017 −1.6130.4990 0.9134 −1.589 0.9093 −1.585 0.9051 −1.575 0.9011 −1.571 0.8969 −1.5580.6012 0.9032 −1.390 0.8990 −1.384 0.8949 −1.381 0.8908 −1.377 0.8868 −1.3750.7274 0.8925 −1.015 0.8886 −1.029 0.8845 −1.032 0.8804 −1.028 0.8763 −1.0190.8081 0.8870 −0.748 0.8829 −0.754 0.8787 −0.747 0.8747 −0.753 0.8707 −0.753

(continued on next page)

9F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

0.82

0.85

0.87

0.90

0.92

0.95

0.97

1.00

1.02

0.00 0.20 0.40 0.60 0.80 1.00

x2

ρ / g

cm

-3

Fig. 1. Comparative diagram for densities (ρ) as a function of mole fraction ofmonoethanolamine (x2) at 303.15 K of the systems: W + MEA (◊), W + EEA (□),W + DMEA (△), W + DMPA-1 (○) and W + DMPA-2 (∗). The full lines representfitting values with Eq. (2). ♦: W + MEA, ■: W + EEA, ▲: W + DMEA and ●:W + DMPA-1 represent the work of Li et al. [2], Lebrette et al. [11], Maham et al. [3]and Narayanaswamy et al. [18], respectively.

Table 2 (continued)

T/K 303.15 308.15 313.15 318.15 323.15

x2 ρ VEm ρ VE

m ρ VEm ρ VE

m ρ VEm

Water (x1) + dimethylpropanol-1-amine (x2)0.9112 0.8806 −0.342 0.8766 −0.361 0.8725 −0.367 0.8685 −0.372 0.8645 −0.3841.0000 0.8760 0.000 0.8719 0.000 0.8678 0.000 0.8637 0.000 0.8597 0.000

Water (x1) + dimethylpropanol-2-amine (x2)0.0129 0.9919 −0.170 0.9901 −0.173 0.9881 −0.175 0.9859 −0.178 0.9834 −0.1780.0249 0.9895 −0.342 0.9873 −0.343 0.9843 −0.332 0.9825 −0.349 0.9798 −0.3500.0330 0.9870 −0.437 0.9844 −0.435 0.9805 −0.407 0.9792 −0.437 0.9763 −0.4370.0500 0.9856 −0.713 0.9827 −0.709 0.9797 −0.709 0.9766 −0.708 0.9733 −0.7060.0997 0.9752 −1.318 0.9711 −1.302 0.9670 −1.289 0.9630 −1.280 0.9588 −1.2710.1507 0.9615 −1.735 0.9569 −1.715 0.9523 −1.699 0.9477 −1.684 0.9431 −1.6690.2021 0.9480 −2.028 0.9432 −2.006 0.9384 −1.990 0.9336 −1.975 0.9288 −1.9580.2501 0.9360 −2.193 0.9312 −2.176 0.9262 −2.160 0.9214 −2.148 0.9164 −2.1290.3013 0.9247 −2.309 0.9198 −2.293 0.9149 −2.282 0.9100 −2.274 0.9053 −2.2680.3507 0.9149 −2.360 0.9099 −2.343 0.9049 −2.335 0.9000 −2.326 0.8952 −2.3190.4013 0.9060 −2.371 0.9009 −2.352 0.8959 −2.341 0.8909 −2.329 0.8859 −2.3160.4578 0.8967 −2.305 0.8918 −2.300 0.8868 −2.296 0.8820 −2.295 0.8770 −2.2810.5049 0.8900 −2.233 0.8850 −2.224 0.8801 −2.225 0.8752 −2.223 0.8703 −2.2230.6009 0.8777 −1.979 0.8728 −1.978 0.8679 −1.983 0.8630 −1.984 0.8582 −1.9870.7054 0.8667 −1.617 0.8618 −1.618 0.8568 −1.619 0.8519 −1.615 0.8470 −1.6120.7955 0.8582 −1.183 0.8533 −1.190 0.8484 −1.197 0.8435 −1.198 0.8386 −1.2000.8905 0.8504 −0.665 0.8455 −0.668 0.8405 −0.680 0.8356 −0.676 0.8307 −0.6640.9564 0.8458 −0.317 0.8409 −0.317 0.8360 −0.325 0.8311 −0.325 0.8263 −0.3251.0000 0.8425 0.000 0.8376 0.000 0.8326 0.000 0.8278 0.000 0.8230 0.000

10 F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

As the comparative diagram (Fig. 1) exhibits, ρ of aqueous solutionsof monoalkanolamines vary as: W + MEA > W + EEA > W +DMEA > W + DMPA-1 > W + DMPA-2. In Maham et al's study [3],aqueous solutions of ethanolamine (EA), MEA and DMEA also showeda similar variation trend of ρ against composition. With that, this orderof variation may be related to the relative strength of self- as well ascross-associations, and it is also related to the steric effect as mentionedabove. The variation in steric effect for various ethanolamines, therefore,can be suggested to depend as:

2ð\CH3Þ > \CH2CH3 > \CH3ðdepending on N� substituted alkyl groupsÞ

\CH2CH2CH2\ > \CH2CH2\ðdepending on chain length of alkanolsÞ

\CHOH\ > \CH2OHðdepending on branching in alkanolsÞ:

Excess molar volumes, VEm as a function of mole fraction of the cor-

responding alkanolamines(x2) for aqueous solutions of MEA, EEA,DMEA, DMPA-1 and DMPA-2 at different temperatures (303.15–323.15 K) are as presented in Table 2. Fig. 2 compares the VE

m valuesat 303.15 K. The observed VE

m for all systems are fitted well with aRedlich–Kister polynomial of the following form:

VEm=cm

3:mol−1 ¼ x2 1−x2ð Þ

Xni¼1

Ai 1−2x2ð Þi�1 ð3Þ

where, x2 is the mole fraction of alkanolamine, Ai (cm3 mol−1) is thecoefficient of Redlich–Kister Eq. (3) and n represents the degree ofpolynomial. For each system, values of VE

m are observed to fit well toEq. (3) at n = 5. The coefficients, Ai, of Eq. (3) and relevant standarddeviation, σ (cm3 mol−1), for VE

m obtained by least squares methodfor all the five systems at temperatures between 303.15 and323.15 K are listed in Table 4.

The VEm of the systems W + DMEA and W + DMPA-1 agreed well

with the data of Maham et al. [3] and Narayanaswamy et al. [18], re-spectively. The values for all these three systems are negative in thewhole range of composition and at all temperatures, and follow asimilar trend for other aqueous alkanolamine solutions [4,14,15].

Like in others, magnitudes of VEm for all the aqueous solutions of

these alkanolamines are also significantly large.Usually, negative VE

m is the result of volume contraction in mixtures,and hence, it can be explained by the large differences inmolar volumesof the components. According to Pal and Sing [16], volume contractionis particularly due to the ability of alkanolamines with the\OH groupsto form intermolecular H-bonds with water molecules through O\H…O\H and/or H\N…H\O linkages. However, another interpretation forthe marked change in VE

m may also be assigned due to accommodationof solute molecules within the lattice/void space of three dimensionalnetworks of water. For the present systems, sharp changes in VE

mare found to occur in the water-rich region as the monoethanolaminesare added and minima having significantly large depth occur atabout x2 ~ 0.40 for W + MEA and W + EEA. Those for the other

Table 4Fitting coefficients, Ai (cm3 mol−1), of Redlich–Kister polynomial Eq. (3) and thevalues of σ for VE

m (cm3 mol−1) for water (x1) + methylethanolamine (x2), water +ethylethanolamine (x2), water + dimethylethanolamine (x2), water +dimethylpropanol-1-amine (x2) and water + dimethylpropanol-2-amine (x2) systems at differenttemperatures.

T/K A1 A2 A3 A4 A5 σ

Water (x1) + methylethanolamine (x2)303.15 −4.600 −2.343 −1.352 1.466 2.592 0.009308.15 −4.554 −2.223 −1.272 1.300 2.505 0.009313.15 −4.525 −2.108 −1.217 1.120 2.433 0.007318.15 −4.486 −2.001 −1.191 0.951 2.436 0.007323.15 −4.452 −1.911 −1.019 0.815 2.193 0.007

Water (x1) + ethylethanolamine (x2)303.15 −5.316 −1.858 −1.587 0.286 0.502 0.022308.15 −5.289 −1.821 −1.483 0.537 0.403 0.021313.15 −5.275 −1.730 −1.232 0.621 −0.022 0.020318.15 −5.250 −1.637 −1.074 0.643 −0.261 0.018323.15 −5.249 −1.549 −0.878 0.676 −0.517 0.015

Water (x1) + dimethylethanolamine (x2)303.15 −6.464 −3.378 −1.720 0.579 1.626 0.018308.15 −6.444 −3.258 −1.787 0.534 1.706 0.015313.15 −6.458 −3.118 −1.646 0.512 1.435 0.013318.15 −6.456 −3.121 −1.224 0.709 0.738 0.015323.15 −6.495 −3.111 −0.806 0.953 −0.090 0.014

Water (x1) + dimethylpropanol-1-amine (x2)303.15 −6.347 −3.273 −1.904 0.025 1.712 0.016308.15 −6.324 −3.116 −1.666 0.118 1.226 0.015313.15 −6.288 −2.998 −1.501 0.103 1.029 0.017318.15 −6.266 −2.894 −1.221 0.123 0.557 0.013323.15 −6.236 −2.846 −0.884 0.320 −0.002 0.012

Water (x1) + dimethylpropanol-2-amine (x2)303.15 −8.924 −4.017 −3.222 −0.826 0.989 0.029308.15 −8.897 −3.876 −3.125 −0.877 0.891 0.027313.15 −8.888 −3.813 −3.057 −0.688 0.841 0.031318.15 −8.892 −3.706 −2.674 −0.872 0.243 0.023323.15 −8.879 −3.594 −2.533 −1.022 0.183 0.023

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.00 0.20 0.40 0.60 0.80 1.00

x2

Fig. 2. Comparison of excess molar volumes (VEm) for the systems: W + MEA (◊),

W + EEA (□), W + DMEA (△), W + DMPA-1 (○) and W + DMPA-2 (∗) as a functionof mole fraction of monoethanolamine (x2) at 303.15 K. The solid lines represent fittingvalues of Eq. (3). ▲: W + DMEA and ●: W + DMPA-1 represent the works of Mahamet al. [3] and Narayanaswamy et al. [18].

Table 3Fitting coefficients, ai (g cm−3), of polynomial Eq. (2) and average absolute deviations (AAD/%) for ρ (g cm−3) for water (x1) + methylethanolamine (x2), water + ethylethanolamine(x2), water + dimethylethanolamine (x2), water + dimethylpropanol-1-amine (x2) andwater + dimethylpropanol-2-amine (x2) systems at different temperatures.

T/K a0 a1 a2 a3 a4 a5 AADa %

Water (x1) + methylethanolamine (x2)303.15 0.9946 0.0326 −0.2529 0.0677 0.2518 −0.1616 0.017308.15 0.9930 0.0173 −0.2253 0.0641 0.2229 −0.1439 0.015313.15 0.9913 0.0035 −0.2004 0.0584 0.2022 −0.1309 0.012318.15 0.9894 −0.0115 −0.1625 0.0203 0.2124 −0.1280 0.012323.15 0.9873 −0.0235 −0.1431 0.0289 0.1726 −0.1060 0.010

Water (x1) + ethylethanolamine (x2)303.15 0.9876 −0.1196 −0.1031 0.3249 −0.2745 0.0774 0.009308.15 0.9932 −0.0532 −0.3492 0.7508 −0.6237 0.1870 0.007313.15 0.9915 −0.0760 −0.2704 0.6232 −0.5250 0.1575 0.006318.15 0.9897 −0.0986 −0.1837 0.4669 −0.3920 0.1144 0.007323.15 0.9876 −0.1196 −0.1031 0.3249 −0.2745 0.0774 0.006

Water (x1) + dimethylethanolamine (x2)303.15 0.9945 −0.0973 −0.3452 0.7423 −0.5816 0.1658 0.011308.15 0.9930 −0.1232 −0.2528 0.5834 −0.4499 0.1236 0.012313.15 0.9913 −0.1466 −0.1762 0.4665 −0.3656 0.1002 0.009318.15 0.9895 −0.1698 −0.0909 0.3147 −0.2362 0.0580 0.014323.15 0.9875 −0.1932 −0.0013 0.1475 −0.0867 0.0068 0.006

Water (x1) + dimethylpropanol-1-amine (x2)303.15 0.9945 −0.1928 −0.0306 0.3072 −0.2954 0.0931 0.009308.15 0.9931 −0.2216 0.0778 0.1185 −0.1389 0.0430 0.013313.15 0.9912 −0.2445 0.1585 −0.0110 −0.0416 0.0152 0.012318.15 0.9895 −0.2718 0.2677 −0.2120 0.1345 −0.0434 0.011323.15 0.9875 −0.2981 0.3769 −0.4163 0.3105 −0.1008 0.007

Water (x1) + dimethylpropanol-2-amine (x2)303.15 0.9946 −0.1668 −0.5891 1.7181 −1.7175 0.6035 0.017308.15 0.9931 −0.2030 −0.4489 1.4632 −1.4980 0.5315 0.012313.15 0.9911 −0.2349 −0.3181 1.2138 −1.2751 0.4562 0.020318.15 0.9896 −0.2674 −0.2087 1.0464 −1.1573 0.4255 0.009323.15 0.9875 −0.2964 −0.0963 0.8440 −0.9865 0.3710 0.009

a AAD=% ¼ 100=nð Þ∑nk¼1 ρcal

k −ρ expk

��� ���=ρ expk .

11F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

three systems (W + DMEA, W + DMPA-1 and W + DMPA-2) arefound to be at about x2 = 0.37, and the depth of minima varies as:W + DMPA-2 > W + DMEA ≥ W + DMPA-1 > W + EEA > W +MEA. A similar trend and variation in the depth of minima forthe solutions W + EA, W + MEA and W + DMEA and foralkyl-substituted diethanolamines (MDEA and EDEA) also werefound by Maham et al. [3,14]. Thus, the effects due to the effect ofsize and number of alkyl groups attached to N-atom of alka-nolamines are reflected in the same order as the depth of minimaof VE

m occurred for the aqueous solutions. That means, the largerthe alkyl group the more negative the Vm

E , and also, the more numberof alkyl groups (\CH3) the more negative the value of Vm

E . However,occurrence of deep minima may additionally be attributed to hydro-phobicity of the hydrocarbon moiety attached to N-atom of thealkanolamines, and thus, the more hydrophobicity the more thedepth of minima in VE

m vs. x2 curves. Again, magnitudes of VEm of

W + DMPA-1 are slightly higher (in minima) than that ofW + DMEA. That is, the effect of a\CH2\ group (in alkanol chains)of N-substituted dialkylalkanolamines on VE

m of their aqueous solu-tions is insignificant. However, significant effect for the same isfound for monoalkyl-substituted alkanolamines. This is due to largesteric effect of double methyl groups attached to the N-atompredominating over the \CH2\ groups in alkanol chain. Anotherinference may also be drawn as that, regarding VE

m the effect ofnonlinearity is higher than linearity in the chain. Furthermore, italso reveals that, as the depth of minima in VE

m vs. x2 forW + DMPA-2 is higher than those for W + DMPA-1, compactness ofalkanolamines with a linear alkanol is lower than that of alkanolamines

3.50

5.50

7.50

9.50

11.50

13.50

0.00 0.20 0.40 0.60 0.80 1.00x2

Fig. 4. Comparison of thermal expansivities (α) for the systems: W + MEA (◊),W + EEA (□), W + DMEA (△), W + DMPA-1 (○) and W + DMPA-2 (∗) as a functionof mole fraction of monoethanolamine (x2) at 303.15 K. The solid lines represent fittingvalues of Eq. (2).

-2.50

-2.30

-2.10

-1.90

-1.70

-1.50

-1.30

-1.10

300.00 305.00 310.00 315.00 320.00 325.00T / K

Fig. 3. Comparison of excess molar volumes (VEm) for the systems of W + MEA (◊),

W + EEA (□), W + DMEA (△), W + DMPA-1 (○) and W + DMPA-2 (∗) as a functionof temperature (T), each at the composition of maximum, x2 ≈ 0.4.

12 F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

having a nonlinear alkanol-moiety. From another study [17] onW + alkanols, |VmE | was found to be larger for W + propanol-2 thanthat of W + propanol-1. This is also in a good correspondence withthe present systems.

Comprehensively, effect of alkyl group(s) attached to N ofalkanolamines, length of alkanol alkyl chains, and position of the\OH group etc. all contribute negatively towards VE

m, and should fol-low the trends:

2ð\CH3Þ > \CH2CH3 > \CH3ðattached to N atom of alkanolaminesÞ

\CH2CH2\ > \CH2CH2CH2\ðalkyl chain of alkanol moietyÞ

\CHOH\ > \CH2OHðbranching in alkanolsÞ:

Table 5Experimental expansivities (α/K−1), excess expansivities (αE/K−1) of the systems wdimethylethanolamine (x2), water + dimethylpropanol-1-amine (x2) and water + dimeth

W (x1) + MEA (x2) W (x1) + EEA (x2) W (x1) + DMEA (

x2 α × 104 αE × 104 x2 α × 104 αE × 104 x2 α × 104

0.0000 3.833 0.000 0.0000 3.833 0.000 0.0000 3.8330.0130 4.005 0.110 0.0132 4.108 0.205 0.0135 4.1540.0238 4.167 0.221 0.0332 5.093 1.084 0.0238 4.4970.0339 4.428 0.435 0.0499 5.293 1.196 0.0340 4.7960.0844 5.633 1.402 0.0998 7.059 2.697 0.0500 5.4650.1894 7.397 2.671 0.1506 7.934 3.303 0.1003 7.1880.3001 8.045 2.796 0.2011 8.203 3.303 0.1993 8.6830.4014 8.123 2.396 0.2982 8.495 3.080 0.2947 9.1410.4480 8.280 2.333 0.4029 8.600 2.630 0.3489 9.2730.5016 8.324 2.125 0.4458 8.614 2.416 0.4004 9.3590.5474 8.248 1.832 0.5051 8.664 2.152 0.4530 9.3990.6004 8.241 1.576 0.5529 8.748 1.982 0.5040 9.4720.7021 8.343 1.197 0.6002 8.831 1.814 0.6047 9.6480.7975 8.438 0.843 0.6508 8.776 1.491 0.7044 9.9190.9048 8.602 0.501 0.7049 8.908 1.336 0.8062 9.8321.0000 8.551 0.000 0.7985 8.872 0.803 0.9046 9.928

0.9014 8.937 0.322 1.0000 10.221.0000 9.138 0.000

That is to say, the larger the size of substituted alkyl groups and thegreater the number of substituted alkyl groups in the alkanolamine de-rivatives, the more should be the hydrophobicity of solutes.

As Fig. 2 shows, minima of VEm vs. x2 curves for aqueous solutions of

dialkyl substituted alkanolamines shift progressively towards thewater-rich region compared to those for the monoalkyl-substitutedones. This is also an indication of their relevant hydrophobic effects. Itmay be concluded that, the more alkyl groups there are attached to Nof the alkanolamines, the more the hydrophobicity, and hence, shiftingof minima preferably becomes more towards the water-rich region.

It has also been observed that, VEm becomes less negative as the tem-

perature increases. Considering VEm as a function of T at their minima

nearly at x2 = 0.4 for all the systems, their variations are found to be

ater (x1) + methylethanolamine (x2), water + ethylethanolamine (x2), water +ylpropanol-2-amine (x2) for different molar ratios.

x2) W (x1) + DMPA-1 (x2) W (x1) + DMPA-2 (x2)

αE × 104 x2 α × 104 αE × 104 x2 α × 104 αE × 104

0.000 0.0000 3.833 0.000 0.0000 3.833 0.0000.234 0.0132 4.171 0.264 0.0129 4.307 0.3720.512 0.0250 4.664 0.691 0.0249 4.871 0.8400.746 0.0339 4.975 0.952 0.0330 5.438 1.3431.312 0.0478 5.652 1.552 0.0500 6.256 2.0272.714 0.0968 7.388 3.013 0.0997 8.432 3.8093.576 0.1512 8.407 3.727 0.1507 9.613 4.5853.423 0.2009 8.772 3.814 0.2021 10.24 4.8073.209 0.2997 9.121 3.609 0.2501 10.56 4.7472.966 0.3537 9.084 3.270 0.3013 10.64 4.4142.669 0.4037 9.160 3.066 0.3507 10.89 4.2812.416 0.4574 9.158 2.764 0.4013 11.22 4.2001.948 0.4990 9.117 2.489 0.4578 11.14 3.6751.582 0.6012 9.174 1.974 0.5049 11.17 3.3320.844 0.7274 9.218 1.311 0.6009 11.28 2.6840.310 0.8081 9.286 0.927 0.7054 11.53 2.1010.000 0.9112 9.208 0.272 0.7955 11.53 1.388

1.0000 9.433 0.000 0.8905 11.68 0.7880.9564 11.71 0.2871.0000 11.76 0.000

0.00

1.00

2.00

3.00

4.00

5.00

0.00 0.20 0.40 0.60 0.80 1.00

x2

Fig. 5. Comparison of excess thermal expansivities (αE) for the systems of W + MEA(◊), W + EEA (□), W + DMEA (△), W + DMPA-1 (○) and W + DMPA-2 (∗) as afunction of mole fraction of monoethanolamine (x2) at 303.15 K.

13F.I. Chowdhury et al. / Journal of Molecular Liquids 182 (2013) 7–13

almost linear as in Fig. 3. This type of temperature dependences fre-quently occurs for different water + alkanolamine solutions also. Thisis a common phenomenon for different W + alkanolamine solutions.In fact, H-bonds due to both self-association and cross-association ofthe components decrease with the increase of temperature leading to acertain positive contribution to Vm

E , and hence, overall VEm becomes less

negative.Thermal expansivity (α) is defined as α = (1 / Vm) (∂Vm / ∂T),

and it ultimately reduces to (4)

α ¼ − ∂ ln ρ =∂Tð Þ: ð4Þ

Again, considering additive rules, its excess property follows:

αE ¼ x1α1 þ x2α2: ð5Þ

Here, all the termshave their usual significances. The values ofα andαE for the systems are as tabulated in Table 5. As Fig. 4 comparesthe curves of α as a function of mole fraction of the respectivealkanolamines at 303.15 K, all the systems are observed to show analo-gous variation pattern;α rising sharply up to x2 = 0.25 and afterwardsalmost flattened. For pure alkanolamines as well as their aqueous sys-tems, α varied as: W + DMPA-2 > W + DMEA > W + DMPA-1 >W + EEA > W + MEA, in a wide range of concentration. While, αE

values were all positive and as Fig. 5 shows, the curves of αE vs. x2 forall the systems were asymmetric and formed sharp maxima nearly atx2 = 0.2. These observations further lead to demonstrate that,irrespective of size and shape, all the alkanolamine molecules preferto act as structuremakers forwater, especially in thewater-rich regions.This may evidently be related to the dominating hydrophobicity effectover all other factors, which is quite strong as well in the whole rangeof temperature studied.

4. Conclusion

Densities for the systems W + MEA, W + EEA, W + DMEA,W + DMPA-1 and W + DMPA-2 have been measured in the rangeof 0 ≤ x2 ≤1 at temperatures between 303.15 and 323.15 K. VE

mwere largely negative in the whole range of compositions. It was con-sidered as the result of volume contraction in the mixture. For all thesystems sharp changes and deep minima in VE

m are found to occur inthe water-rich region, which has been interpreted in terms of hydro-phobicity of hydrocarbon-moieties attached to N-atom of the alka-nolamines. The hydrophobicity of the alkanolamines followed theorder: W + DMPA-2 > W + DMEA ≥ W + DMPA-1 > W +EEA >W + MEA. H-bonding, large size differences as well as interstitial ac-commodation of solute molecules in water lattice were also consideredas important factors. The values of αE were found to be all positive andformed asymmetric curves of αE against x2 with sharp maxima nearx2 = 0.2 for all the systems, which further lead to designate all thealkanolamines as strong structure makers towards water at alltemperatures.

Acknowledgment

The authors gratefully acknowledge the financial grant by theMinistry of Science, Information and Communication Technology,Peoples Republic of Bangladesh, for the project “Physical Propertiesand Molecular Interactions in Liquid Systems”.

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