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Thermodynamic properties of aqueous mixtures of hydrophilic compounds 2. Aminoethanol and its methyl derivatives
M-1320 J. Chem. Thermodynamics 1982, 14. 145-156
Thermodynamic properties of aqueous mixtures of hydrophilic compounds 2. Aminoethanol and its methyl derivatives H. TOUHARA, S. OKAZAKI, K. IKARI, and K. NAKANISHI”
F. OKINO,
H. TANAKA,
Department of Industrial Chemistry, Kyoto University, Kyoto 606. Japan (Received 26 March
1981; in revised.form
22 June 1981)
The vapour pressures of water+2-aminoethanol. +N-methyl-2-aminoethanol. and +N.Ndimethyl-2-aminoethanol at 298.15 and 308.15 K were measured over the whole composition range by a static method. The excess enthalpies and densities of the same mixtures at 298.15 K were also measured with an isothermal displacement calorimeter and a pyknometer. The thermodynamic excess functions: GE, HE, TSE, and VE were calculated. Except for aminoethanol-rich region in (water +N,N-dimethyl-2-aminoethanol), where GE is slightly positive, the signs and relative magnitudes of molar excess functions were 0 > GE > TSE > HE, Y’- < 0. These are in accordance with general characteristics of (water +a highly hydrophilic compound). However, hydrophobicity is clearly seen for dilute aqueous solutions where the partial molar volume of aminoethanols exhibits a minimum and the partial molar enthalpy decreases rapidly to a limiting value at infinite dilution. This tendency increases with the introduction of methyl groups into 2-aminoethanol.
1. Introduction The work described herein is part of our continuing efforts to compile thermodynamic quantities for aqueous mixtures of rather hydrophilic compounds. Since one can study such mixtures over the whole composition range, the results might be useful for testing theories on complex liquid mixtures. It is also interesting to see the effect of hydrophobicity, present even in hydrophilic compounds. on thermodynamic excess functions of aqueous mixtures. We have already studied the excess functions of (glycol+ water)“‘2’ and (pyridine + water). (3) Here we report thermodynamic excess functions: GE. H”, VE, and TSE for aqueous mixtures of aminoethanol and its methyl derivatives. The following three compounds have been used : 2-aminoethanol, N-methyl-2aminoethanol, and NJ-dimethyl-2-aminoethanol. The hydrogen bonding and hydrophobic interactions in their aqueous mixtures serve as a model for those in ” To whom correspondence should be addressed. 0021-9614/82/020145+
12 $01.00/O
F 1982 Academic Press Inc. (London) Limited
H. TOUHARA
146
ET AL
biopolymers. Although it is still in its infancy, the present work is the first step of our research program on the molecular interactions in biologically important aqueous mixtures.
2. Experimental The samples of aminoethanols used were specially prepared 2-aminoethanol for liquid scintillation, extra pure N-methyl-2-aminoethanol, and guaranteed N,Ndimethyl-2-aminoethanol. Since no suitable drying agent had been suggestedJ4’ they were purified simply by repeated distillations with a column of about 30 theoretical plates under a reduced nitrogen atmosphere. These compounds are highly hygroscopic and the distillates were stored in sealed glass vessels unless used immediately. The sample of water was de-ionized, fractionally distilled, and degassed three times by freezing in (dry ice+methanol) and thawing under vacuum. More complete degassing was done for any sample used for vapour-pressure measurement. Purities of aminoethanols were checked by g.1.c. The peak due to contamination of water did not disappear completely even by repeated distillation. The mass fraction of water in the samples was determined by the Karl-Fischer titration and found to decrease after distillation from 0.0011 to 0.00044 for 2-aminoethanol, from 0.0014 to 0.00029 for N-methyl-2-aminoethanol, and from 0.00057 to 0.00013 for N,Ndimethyl-2-aminoethanol. Some physical properties of purified aminoethanols at 298.15 K are shown in table 1. Reliable densities and vapour pressures are limited in the literature.‘4’ The vapour-pressure apparatus, pyknometer, and the Pulfrich refractometer were the same as those described in our previous papers. (3, 5, The excess enthalpies were measured with the isothermal dilution calorimeter’@ used previously for the study of (glycol + water). (*) However some improvements have been made in the calorimeter. Figure 1 is a schematic diagiam of the improved mixing vessel of the calorimeter. In contrast with the previous one, no Viton O-ring is necessary in the new vessel because the contacting surfaces between the Dewar flask and the lid have been lapped well enough to assure non-leakage. This improvement made it possible to use such corrosive materials as amines safely in the calorimeter. The lid of the new vessel had a taper, so that we could fill the vessel with mercury leaving no bubble in it and that there was no need to turn the vessel upside down during the sample-filling procedure. No other changes were made in the operational procedure.(6) TABLE
1. Densities
p, refractive
indices
n,, and vapour
pressures
p of
pure fluids
This work 2-aminoethanol
d(g.cm-3) nD PlkPa
1.01130 1.4516 0.065
N-methyl-2-aminoethanol
gEamm3’
0.93589 0.123
NJ-dimethyl-2-aminoethanol
pl(g.cm-‘) t&Pa
0.88221 0.852
at 298.15 K Literature 1 .01159 W’ 0.046 +’
AQUEOUS
MIXTURES
OF AMINOETHANOLS
147
FIGURE 1. Schematic diagram of the improved mixing vesselof the isothermal dilution calorimeter. A, Silvered Dewar flask with window ; B, lid of Teflon ; C, 2-blade glass propeller with a sealed magnet bar; D, mercury cup of Teflon; E, 2 kR thermistor; F, 25 R manganin wire heater in stainless steel ; G, stainlesssteel flange cemented to Dewar flask; H, mouth for the first component injection, sealed with Teflon screw through Viton rubber; I, Teflon feed tube for the second component with hypodermic syringe at the end ; J. stainless-steel heat sink ; K, cooling module.
Calibration of the improved calorimeter has been done by using (dichloromethane + p-dioxane) at 298.15 K as a standard. The internal consistency of the results of these calibration measurements was within &- 1.5 per cent (the rootmean-square deviation err was 0.0141). This was not good by comparison with the results of Murakami and Benson (0, = 0.0032)“’ and Touhara et al. (a, = 0.0036),(6’ but was comparable with that of Winterhalter and Van Ness (6, = O.O135).‘8’ Although the internal consistency was not fully satisfactory, the HE values themselves were in reasonably good agreement with those of these investigators. The composition of the equilibrium liquid phase in the vapour-pressure measurements was determined either by refractive-index measurements in the case of the water-rich region of (water + 2-aminoethanol) or density measurements for others. The curve of density against mole fraction for (water + 2-aminoethanol) has a maximum and only the 2-aminoethanol-rich region was used as a calibration curve for composition determination. That for (water +N-methyl-2-aminoethanol) shows a peculiar behaviour in the water-rich region and the composition determination in this
H. TOUHARA
148
ET AL
region was done after changing the composition quantitatively to the N-methyl-2aminoethanol-rich region, where the density changes linearly with composition by dilution with N-methyl-2-aminoethanol. No such procedure was necessary for (water +N,N-dimethyl-2-aminoethanol) because its curve of density against mole fraction had no extreme value. For use as a calibration curve for the water-rich region of ((1 -x)H,O +xNH,CH,CH,OH), the refractive indices n, were measured over the whole composition range at 298.15 K. The results were fitted to the following polynomial equation with a root-mean-square deviation of 2.0 x 10m4: n, = 1.33248 +0.40878x-0.65794x2
+0.58553.x3 -0.23904x4
+ 0.02176.~“.
(1)
This equation was used to convert n, to x in the water-rich region. The error in the determination of the equilibrium composition was less than f 0.002 for (water + 2aminoethanol) and ~0.0002 for the two others. 3. Results VAPOUR
PRESSURE
The total vapour pressures p of { (1 - x)H,O + xNH,CH,CH,OH}, { (1 - x)H,O + xCH,NHCH,CH,OH}, and ((1 -x)H,O+x(CHa),NCH,CH,OH) at 298.15 and 308.15 K are given in table 2, together with molar excess Gibbs free energies GE. Except for x $ 0.5 in the third mixture the composition dependence of the total vapour pressure shows a small negative deviation from Raoult’s law. Our literature survey indicates that very little is available and only for ((1 -x)H,O + xNHzCH2CH20H} at 298.15 K. (9) Danilov reported that p = 1.76 kPa at x = 0.25, 1.033 kPa at x = 0.50, and 0.483 kPa at x = 0.75, respectively. While the value at x = 0.25 is lower than ours, the other two values coincide fairly well with ours. No vapour pressures are available for the other two mixtures. MOLAR
EXCESS
GIBBS
FREE ENERGIES
From the measured isothermal total vapour pressures given in table 2, molar excess Gibbs free energies GE were calculated by the Barker method.“” In this calculation, the composition dependence of GE was assumed to be expressed by the polynomial equation : GE/(J.mol-‘)
= x(1-x)
t
A,(l-2xr-‘.
(2)
n=l
Here the coefficients A, are temperature dependent. The values of the molar volume K’ and of the second virial coefficient B,, for pure fluids are necessary in the calculation. Since no Bii values for aminoethanols are found in the literature, we assumed them to be zero. Because of the smallness of p, the effect of gas imperfection is negligible in the calculation of GE. In fact, even if Bii is assumed to be, say, 5000 cm3. mol- ‘, variation in calculated GE from that with Bii = 0 is less than 1 per cent.
AQUEOUS TABLE
2. Vapour
pressures
MIXTURES p and molar
plkpa
0.0378 0.0580 0.0956 0.1561 0.1900 0.2413 0.2958 0.4026 0.5225 0.6044 0.6983 0.8135 0.8918 1
0 0.0721 0.1434 0.1985 0.2816 0.3026 0.3522 0.4690 0.5439 0.5984 0.7 155 0.8164 0.8755 0.9126 1
3.168 2.933 2.634 2.404 1.999 1.915 1.680 1.307 1.041 0.876 0.637 0.440 0.323 0.267 0.123
0 0.0243 0.0509 0.0998 0.2001 0.3002 0.3993 0.4985 0.6001 0.6994 0.7987 0.8971 0.8974 0.9472 0.9530 1
3.168 3.106 3.049 2.921 2.616 2.337 2.072 1.855 1.636 1.448 1.256 1.091 1.083 0.976 0.997 0.852
excess Gibbs
GE/(J-mol-‘) 298.15 K
(I3.168 3.092 2.989 2.837 2.592 2.421 2.170 1.937 1.425 0.965 0.741 0.524 0.347 0.212 0.065
0
x)H,O+
149
OF AMINOETHANOLS free energies
GE at 298.15
PlkPa
and 308.15 K GE/(J .mol-
308.15 K
xH,NCH,CH,OH 0 -- 127.0 ~- 192.6 - 308.8 ---475.1 -- 555.1 ---655.9 - 735.2 -. 806.9 - 768.3 -. 684.7 --. 549.5 - 350.7 - 206.3 0
5.623 5.466 5.284 4.989 4.590 4.316 3.873 3.454 2.643 1.821 1.407 0.984 0.631 0.407 0.135
0 - 127.5 -. 191.3 - 301.6 -455.1 -527.7 -619.1 -691.6 - 762.3 - 737.6 - 666.6 - 542.5 - 347.3 - 200.9 0
5.624 5.216 4.754 4.342 3.669 3.541 3.128 2.477 2.016 I .759 1.253 0.867 0.644 0.535 0.260
- 168.6 - 309.0 - 394.6 - 483.2 - 498.0 --521.3 - 520.9 -489.2 - 455.7 - 364.1 - 268.0 - 200.5 -- 151.2 0
5.624 5.542 5.444 5.213 4.772 4.301 3.866 3.473 3.088 2.738 2.401 2.058 2.032 1.835 1.843 1.589
0 -~ 5.6 -- 12.5 - 26.2 --51.3 -63.6 -59.2 -40.1 -11.7 15.8 33.2 31.0 31.0 20.0 18.2 0
(1 -x)H,O+xCH,HNCH,CH,OH
(1 - x)H*O
0 - 209.9 - 378.1 478.2 --579.5 ~~.595.9 --621.1 -614.1 -- 571.2 - 526.6 405.4 -. 282.8 -. 203.3 -. 149.1 0
0
+x(CH,),NCH,CH,OH 0 -21.3 -42.9 -77.9 - 129.2 - 152.6 - 150.4 -.-. 127.2 -88.7 -45.5 -7.6 12.5 12.5 11.7 11.0 0
’)
150
H. TOUHARA
ET AL
The results of iterative calculation indicated that the present total vapour pressures could be fitted satisfactorily to equation (2) with four coefficients. The GE and A, values based on the iterative calculations are given in tables 2 and 3. respectively. The root-mean-square deviation err of the vapour pressures is also included in table 3. TABLE
3. Coefficients
of equation
Al (1 - x)H,O GE/(J
mol - ’ )
@/(J.
mol-‘)
298.15 K 308.15 K 298.15 K
- 3135.6 - 2999.1 - 9294.0
298.15 K 308.15 K 298.15 K
- 2391.5 - 2040.7 - 10218.2
298.15 K 308.15 K 298.15 K
- 507.0 159.0 - 10809.5
(2) and similar
A2
A3
polynomial
4
equation
A5
0,
u
+ xH,NCH,CH,OH 1231.3 942.9 3381.9
448.0 354.1 77.8
- 504.2 - 80.7 -429.1
0.0153 0.0084 0.0153
9.45
- 148.6 - 248.1 - 2279.7
- 490.6 - 627.8 894.1
0.0170 0.0170 0.0128
10.61
- 19.0 - 164.0 2601.9
0.0101 0.0163 0.0211
19.7
(1 -x)H,O+x(CH,)HNCH,CH,OH GE/(J.mol-‘) HE/(J.mol-‘)
(1 - x)H,O GE/(J.mol-‘) HE/(J.mol-‘)
MOLAR
EXCESS
1097.4 808.0 5522.4
+x(CH,),NCH,CH,OH 602.3 500.0 6473.7
225.0 285.7 - 1080.7
4440.2
ENTHALPIES
The molar excess enthalpies HE for the three mixtures were measured at 298.15 K and the results are given in table 4. All the three mixtures exhibit fairly exothermic mixing; the minimum value of HE is about - 2.4 kJ. mol- ’ at x z 0.4 for ((1 -x)H,O+xNH,CH,CH,OH}, -2.75 kJ.mol-’ at x z 0.37 for {(1-x)H,O + xCH,NHCH,CH,OH), and -2.96 kJ.mol-’ at x z 0.34 for ((1 -x)H,O + x(CH,),NCH,CH,OH}. The experimental points were fitted to an equation of the same form as equation (2) with four coefficients by the method of least squares. The coefficients A,, standard deviation cr, and the root-mean-square deviation err are shown in table 3. The deviations are reasonable for the first and second mixtures in view of the facts that aminoethanols are highly viscous and that it takes rather a long time to mix the two components well. For the third mixture, however, the result is barely satisfactory. The measurements of excess enthalpy for this mixture were extremely difficult because of the high viscosity of N,N-dimethyl-2-aminoethanol and the large density difference between the two components preventing rapid and effective mixing. MOLAR
EXCESS
ENTROPIES
Molar excess entropies SE can be calculated from the GE and HE values given above. Thus thermodynamic excess functions, GE, HE, and TSE for the present three mixtures are plotted as a function of x in figure 2. All the excess functions are negative and the
AQUEOUS
MIXTURES
TABLE
-HF J,mol-’
x
4. Molar
-HE
OF AMINOETHANOLS
excess enthalpies
x
J,mol-’
71.9 151.0 269.3 381.2 497.5 632.7 784.0 945.0 1057.3 1193.3 1332.3
0.1433 0.1664 0.1835 0.2005 0.2160 0.2165 0.2276 0.2285 0.2387 0.2449 0.2657
1479.0 1606.2 1717.3 1816.4 1900.0 1885.8 1944.8 1966.7 2013.5 2024.2 2110.7 (1 -x)H,O+
0.0036 0.0096 0.0181 0.0281 0.0393 0.0524 0.0672 0.0811 0.0967
0.0186 0.0371 0.0445 0.0580 0.0703 0.0824 0.0961 0.1122 0.1291
64.0 172.8 320.7 494.1 669.4 865.7 1071.3 1266.4 1465.1
0.1138 0.1305 0.1482 0.1620 0.1742 0.1869 0.1931 0.2029 0.2254
1659.3 1824.3 1977.2 2082.5 2166.5 2244.5 2311.5 2363.3 2443.1
405.6 684.5 940.7 1177.0 1377.1 1564.4 1751.0 1948.6 2110.4
0.1463 0.1634 0.1773 0.1942 0.2103 0.2262 0.2427 0.2682 0.2982
2270.0 2400.0 2485.3 2574.5 2636.2 2685.5 2736.0 2825.8 2882.1
0.2867 0.3077 0.3302 0.3527 0.3738 0.3970 0.4200 0.4443 0.4694 0.4945 0.5224
at 298.15
K
-HE
-HE
J.mol-’
(1 -x)H,O+xH,NCH,C 0.0059 0.0128 0.023 1 0.0339 0.0443 0.0578 0.0730 0.0878 0.0998 0.1146 0.1309
151
’
J.mol
.x
-’
:H,OH 2185.6 2249.4 2306.5 2349.4 2373.3 2390.1 2394.9 2385.0 2362.1 2326.5 2276.8
0.5475 0.5715 0.5968 0.6213 0.6478 0.6733 0.6945 0.7174 0.7400 0.7609 0.7829
2219.1 2147.8 2074.1 1994.7 1900.0 1806.8 1710.9 1603.0 1490.8 1390.5 1274.4
0.8102 0.8340 0.8566 0.8791 0.9025 0.9342 0.959 1 0.9808
I 130.5 999.5 X66.0 733.7 594.6 402.4 24x .o 116.3
0.5 192 0.5500 0.5822 0.6142 0.6428 0.6729 0.7032 0.7328 0.7630
2495.0 2396.6 2280.9 2150.2 2016.3 1867.0 1710.2 1549.6 1388.1
0.7970 0.8332 0.8639 0.8950 0.9232 0.9460 0.9645
1211.0 1006.1 821.2 638.9 465.2 322.4 219.2
0.5672 0.5952 0.6244 0.6536 0.6813 0.7121 0.7492 0.7734 0.8072
2438.5 2307 .O 2164.6 2ooo.o 1824.4 1653.1 1487.7 1338.7 1152.3
0.83X1 0.8814 0.9259 0.9602 0.9850
971.1 716.8 453.3 345.x 105.6
w(CH,)HNCH,CH,OH 0.2422 0.2683 0.2989 0.3313 0.3611 0.3939 0.4259 0.4601 0.4889
2528.0 2610.8 2680.3 2732.6 2754.7 2753.1 2718.5 2683.3 2591.1
(1 -u)H,O+x(CHj),NCHzCH,OH 0.3184 0.3430 0.3693 0.3950 0.4248 0.4532 0.4800 0.5106 0.5411
2938.7 2963.6 2933.3 2911.2 2887.3 2852.5 2794.7 2686.9 2559.4
0 (iF / -2
TsE
Cc)
a 0 x
FIGURE 2. Thermodynamicexcess (I -x)H,O+x(CH,)HNCH,CH,OH:
x
HE 0.5
I
.Y
functions at 298.15 K for (a). (I -x)H,0+xH,NCH,CH20H; (c), (1 -.x)H,O+.x(CH,),NCH,CH,OH.
(bb.
152
H. TOUHARA
mixing process is enthalpy dominant. (( 1 - s)H?O +s(CH,),NCH,CH,OH] DENSITIES
AND
MOLAR
EXCESS
ET .4L
The only one exception is region of large x for where G” is slightly positive. VOLUMES
The densities p of the three mixtures were measured at 298.15 K and the results together with the molar excess volumes VE are given in table 5. Except for the regions TABLE
x
~ P g.crn-j
5. Densities
-VE
x
cm-‘.mol-’
(1 -x)H,O+xH,NCH,CH,OH 0 0.0238 0.0560 0.0936 0.1403 0.1874 0.2891 0.3999 0.5140 0.6059 0.7013 0.8129 0.8993 1
0.99707 0.99989 1.00374 1.00820 1.01340 1.01766 1.02360 1.02565 1.02467 1.02263 1.02007 1 .01679 1.01426 1.01130
TABLE
p and molar
0 0.0087 0.0166 0.0250 0.0612 0.0984 0.1408 0.1996 0.2967 0.3472 0.4008 0.4487 0.4868 0.5563 0.5921 0.6869 0.7410 0.7948 0.9100 1
6. Coefficients
of equation
(1 - x)H,O
4 A, A4 4 4 A, 43 4 or CT
P __ g.crnm3
+
-2.095 11.581 - 62.856 108.9 -76.165 18.614
0.0027 O.OCNMi
0.99707 0.99656 0.99616 0.99611 0.99666 0.99783 0.99797 0.99589 0.98850 0.98392 0.97857 0.97395 0.97051 0.9643 1 0.96130 0.95405 0.95024 0.94681 0.94024 0.93589
VE at 298.15
-VE
K
P -~g.crnm3
x
cm3.mol-’
(1 -x)H,D+x(CH,)HNCH,CH,OH 0 0.033 0.088 0.163 0.268 0.368 0.543 0.640 0.642 0.583 0.486 0.331 0.189 0
xH,NCH,CH,OH
Al
excess volumes
-VE cm3.mol-’
(1 -x)H,D+x(CH3)*NCH,CH,OH 0 0.033 0.065 0.140 0.293 0.503 0.717 0.948 1.157 1.203 1.198 1.171 1.140 1.044 0.982 0.794 0.668 0.539 0.240 0
(3) for the molar
0 0.0249 0.0500 0.0789 O.looo 0.1501 0.2003 0.2519 0.3018 0.3495 0.3974 0.4501 0.5003 0.5498 0.5992 0.6496 0.6998 0.7502 0.7993 0.8507 0.9003 0.9498 1 excess volumes
(1 -x)H,O+ x(CH,)HNCH,CH,OH -5.198 35.625 - 295.241 873.807 - 1245.108 925.689 - 336.747 44.516 0.0047 0.0022
0.99707 0.99276 0.99036 0.98788 0.98550 0.97834 0.97005 0.96122 0.95281 0.94513 0.93796 0.93072 0.92438 0.91844 0.91311 0.90813 0.90342 0.89927 0.89541 0.89161 0.88842 0.88514 0.88221 at 298.15 K
(1 -x)H,O+ x(CH,),NCH,CH,OH - 7.874 30.317 -405.378 1648.855 - 3104.196 2899.555 - 1019.860 - 247.427 202.048 0.0037 0.0034
0 0.203 0.440 0.698 0.868 1.196 1.430 1.588 1.674 1.705 1.701 1.662 1.596 1.495 1.383 1.254 1.098 0.947 0.779 0.587 0.416 0.203 0
AQUEOUS
MIXTURES
153
OF AMINOETHANOLS
of small x for {(l-x)H,O+xNH,CH,CH,OHi and ((I-zc)H,O + xCH,NHCH,CH,OH), these p values were used as the calibration curves for the determination of equilibrium liquid-phase compositions in the vapour-pressure measurements. As shown in figure 3, all the three mixtures exhibit a large volume contraction upon mixing; VE is negative for the whole composition range.
-7 2 Gs
-1.0
w7 k
-2.0“4 0 FIGURE 3. Molar xCHjHNCH,CH,OH;
0.2
0.4
0.6
excess volumes at 298.15 K for (l-x)H,O 0. x(CH,),NCH,CH,OH.
0.8
1
+: 0. xH,NCH,CH,OH;
3,
The molar excess volumes VE were fitted to an equation of the type VE/(cm3.mo1-I)=
x(1-x)
i
A,(1-.u)‘k-1)12,
(3)
k=l
with six or seven coefficients by the method of least squares. The coefficients are listed in table 6. PARTIAL
MOLAR
QUANTITIES
In the present measurements of excess functions, the accuracy and precision of GE obtained are at best of usual standard, whereas HE and VE are accurate enough to permit the evaluation of the composition dependence of partial molar excess enthalpy HE and partial molar excess volume vE. Figure 4 shows the evaluated HiE and YE as functions of x for the three mixtures at 298.15 K. Hereafter HF and VT refer to the partial molar excess quantities for water, and Hf and Vt refer to those for an point in aminoethanol. In accordance with an inflection VE against x, the partial molar volume of aminoethanol takes a minimum in the water-rich region. This behaviour had already been observed for (water+ 2aminoethanol).‘“’ The magnitude of the volume contraction, and the position of the
154
H. TOUHARA
ET .1L
.Y
x
FIGURE 4. Partial molar excess (a), enthalpies and (b), volumes xH,NCH,CH,OH; B, xCH,HNCH,CH,OH; C, x(CH,),NCH,CH,OH.
at 298.15 K for (1 -x)H,O
+ : A.
minimum, change regularly with the introduction of methyl groups into aminoethanol ; more specifically, the minimum becomes deeper and shifts to the water-rich region as the size of the aminoethanol and accordingly the hydrophobicity increases, On the other hand, no minimum is found in HE against x and the decrease in H; at smaller x near infinite dilution of aminoethanol is increasingly rapid when the hydrophobicity of the aminoethanol becomes large. This kind of partial-molarquantity behaviour is similar to that familiar in (water + a monohydric alcohol).” ‘)‘
4. Discussion As is shown in figures 2 and 3, the sign and relative magnitude functions for the present mixtures are water + 2-aminoethanol water +N-methyl-2-aminoethanol water +N,N-dimethyl-2-aminoethanol aminoethanol-rich region water-rich region
0> GE > TSE >HE,
of molar excess
VE <0
GE > 0 > HE > but z I-SE, VE < 0 0 > GE > TSE > but z HE, VE < 0.
Except for the aminoethanol-rich region of (water +N,N-dimethyl-2-aminoethanol), this is a typical example of general characteristics in the excess-function behaviour of highly hydrophilic (or typically non-aqueous)” 3, aqueous mixtures.‘i4’ If we compare the three mixtures with one another, we observe that the hydrophobicity of an aminoethanol increases rapidly with the introduction of a
AQUEOUS MIXTURES
155
OF AMINOETHANOLS
methyl group, and (water +N,N-dimethyl-2-aminoethanol) is only moderately hydrophilic in nature. Yet the present three aqueous mixtures are enthalpy controlled and the energetic stabilization due to a strong hydrogen-bonding interaction between unlike components is predominant over the contribution from the entropy changes and this leads to a negative deviation from ideality. The fact that this tendency decreases rapidly with the introduction of methyl groups indicates the predominant role of N . . . HO hydrogen bonding in the unlike interaction. The negative contribution to HE should increase in the sequence: 2-amino< N-methyl-2-aminoethanol as judged ethanol z N,N-dimethyl-2-aminoethanol from the basicity of the aminoethanol. This sequence is inconsistent with that of the HE's: 2-aminoethanol < N-methyl-2-aminoethanol experimental < N,N-dimethyl-2-aminoethanol. This discrepancy may be ascribed to the difference in the self-association of the aminoethanols. The vapour pressures of the aminoethanols studied here do not parallel their molecular size; the sequence is as follows : N,N-dimethyl-2-aminoethanol 9 N-methyl-2-aminoethanol > 2-aminoethanol. This means that, due to the decrease in the proton-accepting ability of the amino group in N,N-dimethyl-2-aminoethanol the degree of selfassociation in aminoethanols may be expected to be in the sequence; 2-aminoethanol > N-methyl-2-aminoethanol s N,N-dimethyl-2-aminoethanol. The above discussion reveals that a strong hydrogen bonding between water and an aminoethanol is the most important factor in determining the excess-function behaviour of (water +an aminoethanol). Yet, the evidence for the influence of hydrophobic interaction is clearly seen both in V,” and H; at x 4 1. The existence of a distinct minimum in Vf is the most significant evidence for the hydrophobicity of aminoethanol. Furthermore, the fact that, while (HT),,, values are almost common to all the aminoethanols, there is much difference in (H;),,, values, is further evidence. Thus, the present mixtures should be a good model for the study of hydrophobic interaction in biologically important molecules. We are currently engaged in a precise determination of enthalpy and density virials in very dilute solutions and we shall show here only a rough estimate of excess partial molar enthalpy and volume of both components each at infinite dilution in table 7. TABLE 7. Partial molar excess enthalpies and volumes at infinite dilution at 298.15 K W:L, kJ.mol-’ (1 -x)H,O+ xH,NCH,CH,OH x(CH,)HNCH,CH,OH x(CH,),NCH,CH,OH
-6.1 -6.2 -6.3
W:L, kJ.mol-’ - 12.5 - 18.5 - 25.4
cm-1 cm’.mol-’ -1.3 -2.7 -4.1
CV,“L, ____cm-‘~mol~’ --____ - 1.9 - 3.7 - 7.5
We thank Mr J. Abe for his cooperation in an earlier stage of this work. We also thank Professor N. Watanabe for encouragement.
156
H. TOUHARA
ET
AL.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Nakanishi, K.; Kato, N.; Maruyama, M. J. Phys. Chem. 1%7, 71, 814. Matsumoto, Y. ; Touhara, H. ; Nakanishi, K. ; Watanabe, N. J. Chem. Thermodynamics 1977, 9, 801. Abe, J.; Nakanishi, K.; Touhara, H. J. Chem. Thermodynamics 197% 10, 483. Riddick, J. A.; Bunger, W. B. Organic Solvents 3rd edition. Wiley-Interscience: New York. 1970. Nakanishi, K. ; Wada, H.; Touhara, H. J. Chem. Thermodynamics 1975, 7, 1125. Touhara, H.; Ikeda, M.; Nakanishi, K.; Watanabe, N. J. Chem. Thermodynamics 1975, 7, 887. Murakami, S.; Benson, G. C. J. Chem. Thermod.vnamics 1969, 1, 559. Winterhalter, D. R.; Van Ness, H. C. J. Chem. Eng. Data 1966, 11, 189. Danilov, B. B.; Martinson, I. G.; Ravdeli, A. A. ID. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 1974,17, 140. Barker, J. A. Aust. J. Chem. 1953, 6, 207. Kaulgud, M. V.; Patil, K. J. J. Phys. Chem. 1976, 80, 138. Franks, F.; Ives, D. J. G. Q. Rev. (London) 1966, 20, 1. Franks, F. In Hydrogen-Bonded Solvent Systems.Covington, A. K.; Jones, P.: editors. Taylor and Francis : London. 1968. Rowlinson, J. S. Liquids and Liquid Mixtures 2nd edition. Butterworths: London. 1969.
Thermodynamic properties of aqueous mixtures of hydrophilic compounds 2. Aminoethanol and its methyl derivatives H. TOUHARA, S. OKAZAKI, K. IKARI, and K. NAKANISHI”
F. OKINO,
H. TANAKA,
Department of Industrial Chemistry, Kyoto University, Kyoto 606. Japan (Received 26 March
1981; in revised.form
22 June 1981)
The vapour pressures of water+2-aminoethanol. +N-methyl-2-aminoethanol. and +N.Ndimethyl-2-aminoethanol at 298.15 and 308.15 K were measured over the whole composition range by a static method. The excess enthalpies and densities of the same mixtures at 298.15 K were also measured with an isothermal displacement calorimeter and a pyknometer. The thermodynamic excess functions: GE, HE, TSE, and VE were calculated. Except for aminoethanol-rich region in (water +N,N-dimethyl-2-aminoethanol), where GE is slightly positive, the signs and relative magnitudes of molar excess functions were 0 > GE > TSE > HE, Y’- < 0. These are in accordance with general characteristics of (water +a highly hydrophilic compound). However, hydrophobicity is clearly seen for dilute aqueous solutions where the partial molar volume of aminoethanols exhibits a minimum and the partial molar enthalpy decreases rapidly to a limiting value at infinite dilution. This tendency increases with the introduction of methyl groups into 2-aminoethanol.
1. Introduction The work described herein is part of our continuing efforts to compile thermodynamic quantities for aqueous mixtures of rather hydrophilic compounds. Since one can study such mixtures over the whole composition range, the results might be useful for testing theories on complex liquid mixtures. It is also interesting to see the effect of hydrophobicity, present even in hydrophilic compounds. on thermodynamic excess functions of aqueous mixtures. We have already studied the excess functions of (glycol+ water)“‘2’ and (pyridine + water). (3) Here we report thermodynamic excess functions: GE. H”, VE, and TSE for aqueous mixtures of aminoethanol and its methyl derivatives. The following three compounds have been used : 2-aminoethanol, N-methyl-2aminoethanol, and NJ-dimethyl-2-aminoethanol. The hydrogen bonding and hydrophobic interactions in their aqueous mixtures serve as a model for those in ” To whom correspondence should be addressed. 0021-9614/82/020145+
12 $01.00/O
F 1982 Academic Press Inc. (London) Limited
H. TOUHARA
146
ET AL
biopolymers. Although it is still in its infancy, the present work is the first step of our research program on the molecular interactions in biologically important aqueous mixtures.
2. Experimental The samples of aminoethanols used were specially prepared 2-aminoethanol for liquid scintillation, extra pure N-methyl-2-aminoethanol, and guaranteed N,Ndimethyl-2-aminoethanol. Since no suitable drying agent had been suggestedJ4’ they were purified simply by repeated distillations with a column of about 30 theoretical plates under a reduced nitrogen atmosphere. These compounds are highly hygroscopic and the distillates were stored in sealed glass vessels unless used immediately. The sample of water was de-ionized, fractionally distilled, and degassed three times by freezing in (dry ice+methanol) and thawing under vacuum. More complete degassing was done for any sample used for vapour-pressure measurement. Purities of aminoethanols were checked by g.1.c. The peak due to contamination of water did not disappear completely even by repeated distillation. The mass fraction of water in the samples was determined by the Karl-Fischer titration and found to decrease after distillation from 0.0011 to 0.00044 for 2-aminoethanol, from 0.0014 to 0.00029 for N-methyl-2-aminoethanol, and from 0.00057 to 0.00013 for N,Ndimethyl-2-aminoethanol. Some physical properties of purified aminoethanols at 298.15 K are shown in table 1. Reliable densities and vapour pressures are limited in the literature.‘4’ The vapour-pressure apparatus, pyknometer, and the Pulfrich refractometer were the same as those described in our previous papers. (3, 5, The excess enthalpies were measured with the isothermal dilution calorimeter’@ used previously for the study of (glycol + water). (*) However some improvements have been made in the calorimeter. Figure 1 is a schematic diagiam of the improved mixing vessel of the calorimeter. In contrast with the previous one, no Viton O-ring is necessary in the new vessel because the contacting surfaces between the Dewar flask and the lid have been lapped well enough to assure non-leakage. This improvement made it possible to use such corrosive materials as amines safely in the calorimeter. The lid of the new vessel had a taper, so that we could fill the vessel with mercury leaving no bubble in it and that there was no need to turn the vessel upside down during the sample-filling procedure. No other changes were made in the operational procedure.(6) TABLE
1. Densities
p, refractive
indices
n,, and vapour
pressures
p of
pure fluids
This work 2-aminoethanol
d(g.cm-3) nD PlkPa
1.01130 1.4516 0.065
N-methyl-2-aminoethanol
gEamm3’
0.93589 0.123
NJ-dimethyl-2-aminoethanol
pl(g.cm-‘) t&Pa
0.88221 0.852
at 298.15 K Literature 1 .01159 W’ 0.046 +’
AQUEOUS
MIXTURES
OF AMINOETHANOLS
147
FIGURE 1. Schematic diagram of the improved mixing vesselof the isothermal dilution calorimeter. A, Silvered Dewar flask with window ; B, lid of Teflon ; C, 2-blade glass propeller with a sealed magnet bar; D, mercury cup of Teflon; E, 2 kR thermistor; F, 25 R manganin wire heater in stainless steel ; G, stainlesssteel flange cemented to Dewar flask; H, mouth for the first component injection, sealed with Teflon screw through Viton rubber; I, Teflon feed tube for the second component with hypodermic syringe at the end ; J. stainless-steel heat sink ; K, cooling module.
Calibration of the improved calorimeter has been done by using (dichloromethane + p-dioxane) at 298.15 K as a standard. The internal consistency of the results of these calibration measurements was within &- 1.5 per cent (the rootmean-square deviation err was 0.0141). This was not good by comparison with the results of Murakami and Benson (0, = 0.0032)“’ and Touhara et al. (a, = 0.0036),(6’ but was comparable with that of Winterhalter and Van Ness (6, = O.O135).‘8’ Although the internal consistency was not fully satisfactory, the HE values themselves were in reasonably good agreement with those of these investigators. The composition of the equilibrium liquid phase in the vapour-pressure measurements was determined either by refractive-index measurements in the case of the water-rich region of (water + 2-aminoethanol) or density measurements for others. The curve of density against mole fraction for (water + 2-aminoethanol) has a maximum and only the 2-aminoethanol-rich region was used as a calibration curve for composition determination. That for (water +N-methyl-2-aminoethanol) shows a peculiar behaviour in the water-rich region and the composition determination in this
H. TOUHARA
148
ET AL
region was done after changing the composition quantitatively to the N-methyl-2aminoethanol-rich region, where the density changes linearly with composition by dilution with N-methyl-2-aminoethanol. No such procedure was necessary for (water +N,N-dimethyl-2-aminoethanol) because its curve of density against mole fraction had no extreme value. For use as a calibration curve for the water-rich region of ((1 -x)H,O +xNH,CH,CH,OH), the refractive indices n, were measured over the whole composition range at 298.15 K. The results were fitted to the following polynomial equation with a root-mean-square deviation of 2.0 x 10m4: n, = 1.33248 +0.40878x-0.65794x2
+0.58553.x3 -0.23904x4
+ 0.02176.~“.
(1)
This equation was used to convert n, to x in the water-rich region. The error in the determination of the equilibrium composition was less than f 0.002 for (water + 2aminoethanol) and ~0.0002 for the two others. 3. Results VAPOUR
PRESSURE
The total vapour pressures p of { (1 - x)H,O + xNH,CH,CH,OH}, { (1 - x)H,O + xCH,NHCH,CH,OH}, and ((1 -x)H,O+x(CHa),NCH,CH,OH) at 298.15 and 308.15 K are given in table 2, together with molar excess Gibbs free energies GE. Except for x $ 0.5 in the third mixture the composition dependence of the total vapour pressure shows a small negative deviation from Raoult’s law. Our literature survey indicates that very little is available and only for ((1 -x)H,O + xNHzCH2CH20H} at 298.15 K. (9) Danilov reported that p = 1.76 kPa at x = 0.25, 1.033 kPa at x = 0.50, and 0.483 kPa at x = 0.75, respectively. While the value at x = 0.25 is lower than ours, the other two values coincide fairly well with ours. No vapour pressures are available for the other two mixtures. MOLAR
EXCESS
GIBBS
FREE ENERGIES
From the measured isothermal total vapour pressures given in table 2, molar excess Gibbs free energies GE were calculated by the Barker method.“” In this calculation, the composition dependence of GE was assumed to be expressed by the polynomial equation : GE/(J.mol-‘)
= x(1-x)
t
A,(l-2xr-‘.
(2)
n=l
Here the coefficients A, are temperature dependent. The values of the molar volume K’ and of the second virial coefficient B,, for pure fluids are necessary in the calculation. Since no Bii values for aminoethanols are found in the literature, we assumed them to be zero. Because of the smallness of p, the effect of gas imperfection is negligible in the calculation of GE. In fact, even if Bii is assumed to be, say, 5000 cm3. mol- ‘, variation in calculated GE from that with Bii = 0 is less than 1 per cent.
AQUEOUS TABLE
2. Vapour
pressures
MIXTURES p and molar
plkpa
0.0378 0.0580 0.0956 0.1561 0.1900 0.2413 0.2958 0.4026 0.5225 0.6044 0.6983 0.8135 0.8918 1
0 0.0721 0.1434 0.1985 0.2816 0.3026 0.3522 0.4690 0.5439 0.5984 0.7 155 0.8164 0.8755 0.9126 1
3.168 2.933 2.634 2.404 1.999 1.915 1.680 1.307 1.041 0.876 0.637 0.440 0.323 0.267 0.123
0 0.0243 0.0509 0.0998 0.2001 0.3002 0.3993 0.4985 0.6001 0.6994 0.7987 0.8971 0.8974 0.9472 0.9530 1
3.168 3.106 3.049 2.921 2.616 2.337 2.072 1.855 1.636 1.448 1.256 1.091 1.083 0.976 0.997 0.852
excess Gibbs
GE/(J-mol-‘) 298.15 K
(I3.168 3.092 2.989 2.837 2.592 2.421 2.170 1.937 1.425 0.965 0.741 0.524 0.347 0.212 0.065
0
x)H,O+
149
OF AMINOETHANOLS free energies
GE at 298.15
PlkPa
and 308.15 K GE/(J .mol-
308.15 K
xH,NCH,CH,OH 0 -- 127.0 ~- 192.6 - 308.8 ---475.1 -- 555.1 ---655.9 - 735.2 -. 806.9 - 768.3 -. 684.7 --. 549.5 - 350.7 - 206.3 0
5.623 5.466 5.284 4.989 4.590 4.316 3.873 3.454 2.643 1.821 1.407 0.984 0.631 0.407 0.135
0 - 127.5 -. 191.3 - 301.6 -455.1 -527.7 -619.1 -691.6 - 762.3 - 737.6 - 666.6 - 542.5 - 347.3 - 200.9 0
5.624 5.216 4.754 4.342 3.669 3.541 3.128 2.477 2.016 I .759 1.253 0.867 0.644 0.535 0.260
- 168.6 - 309.0 - 394.6 - 483.2 - 498.0 --521.3 - 520.9 -489.2 - 455.7 - 364.1 - 268.0 - 200.5 -- 151.2 0
5.624 5.542 5.444 5.213 4.772 4.301 3.866 3.473 3.088 2.738 2.401 2.058 2.032 1.835 1.843 1.589
0 -~ 5.6 -- 12.5 - 26.2 --51.3 -63.6 -59.2 -40.1 -11.7 15.8 33.2 31.0 31.0 20.0 18.2 0
(1 -x)H,O+xCH,HNCH,CH,OH
(1 - x)H*O
0 - 209.9 - 378.1 478.2 --579.5 ~~.595.9 --621.1 -614.1 -- 571.2 - 526.6 405.4 -. 282.8 -. 203.3 -. 149.1 0
0
+x(CH,),NCH,CH,OH 0 -21.3 -42.9 -77.9 - 129.2 - 152.6 - 150.4 -.-. 127.2 -88.7 -45.5 -7.6 12.5 12.5 11.7 11.0 0
’)
150
H. TOUHARA
ET AL
The results of iterative calculation indicated that the present total vapour pressures could be fitted satisfactorily to equation (2) with four coefficients. The GE and A, values based on the iterative calculations are given in tables 2 and 3. respectively. The root-mean-square deviation err of the vapour pressures is also included in table 3. TABLE
3. Coefficients
of equation
Al (1 - x)H,O GE/(J
mol - ’ )
@/(J.
mol-‘)
298.15 K 308.15 K 298.15 K
- 3135.6 - 2999.1 - 9294.0
298.15 K 308.15 K 298.15 K
- 2391.5 - 2040.7 - 10218.2
298.15 K 308.15 K 298.15 K
- 507.0 159.0 - 10809.5
(2) and similar
A2
A3
polynomial
4
equation
A5
0,
u
+ xH,NCH,CH,OH 1231.3 942.9 3381.9
448.0 354.1 77.8
- 504.2 - 80.7 -429.1
0.0153 0.0084 0.0153
9.45
- 148.6 - 248.1 - 2279.7
- 490.6 - 627.8 894.1
0.0170 0.0170 0.0128
10.61
- 19.0 - 164.0 2601.9
0.0101 0.0163 0.0211
19.7
(1 -x)H,O+x(CH,)HNCH,CH,OH GE/(J.mol-‘) HE/(J.mol-‘)
(1 - x)H,O GE/(J.mol-‘) HE/(J.mol-‘)
MOLAR
EXCESS
1097.4 808.0 5522.4
+x(CH,),NCH,CH,OH 602.3 500.0 6473.7
225.0 285.7 - 1080.7
4440.2
ENTHALPIES
The molar excess enthalpies HE for the three mixtures were measured at 298.15 K and the results are given in table 4. All the three mixtures exhibit fairly exothermic mixing; the minimum value of HE is about - 2.4 kJ. mol- ’ at x z 0.4 for ((1 -x)H,O+xNH,CH,CH,OH}, -2.75 kJ.mol-’ at x z 0.37 for {(1-x)H,O + xCH,NHCH,CH,OH), and -2.96 kJ.mol-’ at x z 0.34 for ((1 -x)H,O + x(CH,),NCH,CH,OH}. The experimental points were fitted to an equation of the same form as equation (2) with four coefficients by the method of least squares. The coefficients A,, standard deviation cr, and the root-mean-square deviation err are shown in table 3. The deviations are reasonable for the first and second mixtures in view of the facts that aminoethanols are highly viscous and that it takes rather a long time to mix the two components well. For the third mixture, however, the result is barely satisfactory. The measurements of excess enthalpy for this mixture were extremely difficult because of the high viscosity of N,N-dimethyl-2-aminoethanol and the large density difference between the two components preventing rapid and effective mixing. MOLAR
EXCESS
ENTROPIES
Molar excess entropies SE can be calculated from the GE and HE values given above. Thus thermodynamic excess functions, GE, HE, and TSE for the present three mixtures are plotted as a function of x in figure 2. All the excess functions are negative and the
AQUEOUS
MIXTURES
TABLE
-HF J,mol-’
x
4. Molar
-HE
OF AMINOETHANOLS
excess enthalpies
x
J,mol-’
71.9 151.0 269.3 381.2 497.5 632.7 784.0 945.0 1057.3 1193.3 1332.3
0.1433 0.1664 0.1835 0.2005 0.2160 0.2165 0.2276 0.2285 0.2387 0.2449 0.2657
1479.0 1606.2 1717.3 1816.4 1900.0 1885.8 1944.8 1966.7 2013.5 2024.2 2110.7 (1 -x)H,O+
0.0036 0.0096 0.0181 0.0281 0.0393 0.0524 0.0672 0.0811 0.0967
0.0186 0.0371 0.0445 0.0580 0.0703 0.0824 0.0961 0.1122 0.1291
64.0 172.8 320.7 494.1 669.4 865.7 1071.3 1266.4 1465.1
0.1138 0.1305 0.1482 0.1620 0.1742 0.1869 0.1931 0.2029 0.2254
1659.3 1824.3 1977.2 2082.5 2166.5 2244.5 2311.5 2363.3 2443.1
405.6 684.5 940.7 1177.0 1377.1 1564.4 1751.0 1948.6 2110.4
0.1463 0.1634 0.1773 0.1942 0.2103 0.2262 0.2427 0.2682 0.2982
2270.0 2400.0 2485.3 2574.5 2636.2 2685.5 2736.0 2825.8 2882.1
0.2867 0.3077 0.3302 0.3527 0.3738 0.3970 0.4200 0.4443 0.4694 0.4945 0.5224
at 298.15
K
-HE
-HE
J.mol-’
(1 -x)H,O+xH,NCH,C 0.0059 0.0128 0.023 1 0.0339 0.0443 0.0578 0.0730 0.0878 0.0998 0.1146 0.1309
151
’
J.mol
.x
-’
:H,OH 2185.6 2249.4 2306.5 2349.4 2373.3 2390.1 2394.9 2385.0 2362.1 2326.5 2276.8
0.5475 0.5715 0.5968 0.6213 0.6478 0.6733 0.6945 0.7174 0.7400 0.7609 0.7829
2219.1 2147.8 2074.1 1994.7 1900.0 1806.8 1710.9 1603.0 1490.8 1390.5 1274.4
0.8102 0.8340 0.8566 0.8791 0.9025 0.9342 0.959 1 0.9808
I 130.5 999.5 X66.0 733.7 594.6 402.4 24x .o 116.3
0.5 192 0.5500 0.5822 0.6142 0.6428 0.6729 0.7032 0.7328 0.7630
2495.0 2396.6 2280.9 2150.2 2016.3 1867.0 1710.2 1549.6 1388.1
0.7970 0.8332 0.8639 0.8950 0.9232 0.9460 0.9645
1211.0 1006.1 821.2 638.9 465.2 322.4 219.2
0.5672 0.5952 0.6244 0.6536 0.6813 0.7121 0.7492 0.7734 0.8072
2438.5 2307 .O 2164.6 2ooo.o 1824.4 1653.1 1487.7 1338.7 1152.3
0.83X1 0.8814 0.9259 0.9602 0.9850
971.1 716.8 453.3 345.x 105.6
w(CH,)HNCH,CH,OH 0.2422 0.2683 0.2989 0.3313 0.3611 0.3939 0.4259 0.4601 0.4889
2528.0 2610.8 2680.3 2732.6 2754.7 2753.1 2718.5 2683.3 2591.1
(1 -u)H,O+x(CHj),NCHzCH,OH 0.3184 0.3430 0.3693 0.3950 0.4248 0.4532 0.4800 0.5106 0.5411
2938.7 2963.6 2933.3 2911.2 2887.3 2852.5 2794.7 2686.9 2559.4
0 (iF / -2
TsE
Cc)
a 0 x
FIGURE 2. Thermodynamicexcess (I -x)H,O+x(CH,)HNCH,CH,OH:
x
HE 0.5
I
.Y
functions at 298.15 K for (a). (I -x)H,0+xH,NCH,CH20H; (c), (1 -.x)H,O+.x(CH,),NCH,CH,OH.
(bb.
152
H. TOUHARA
mixing process is enthalpy dominant. (( 1 - s)H?O +s(CH,),NCH,CH,OH] DENSITIES
AND
MOLAR
EXCESS
ET .4L
The only one exception is region of large x for where G” is slightly positive. VOLUMES
The densities p of the three mixtures were measured at 298.15 K and the results together with the molar excess volumes VE are given in table 5. Except for the regions TABLE
x
~ P g.crn-j
5. Densities
-VE
x
cm-‘.mol-’
(1 -x)H,O+xH,NCH,CH,OH 0 0.0238 0.0560 0.0936 0.1403 0.1874 0.2891 0.3999 0.5140 0.6059 0.7013 0.8129 0.8993 1
0.99707 0.99989 1.00374 1.00820 1.01340 1.01766 1.02360 1.02565 1.02467 1.02263 1.02007 1 .01679 1.01426 1.01130
TABLE
p and molar
0 0.0087 0.0166 0.0250 0.0612 0.0984 0.1408 0.1996 0.2967 0.3472 0.4008 0.4487 0.4868 0.5563 0.5921 0.6869 0.7410 0.7948 0.9100 1
6. Coefficients
of equation
(1 - x)H,O
4 A, A4 4 4 A, 43 4 or CT
P __ g.crnm3
+
-2.095 11.581 - 62.856 108.9 -76.165 18.614
0.0027 O.OCNMi
0.99707 0.99656 0.99616 0.99611 0.99666 0.99783 0.99797 0.99589 0.98850 0.98392 0.97857 0.97395 0.97051 0.9643 1 0.96130 0.95405 0.95024 0.94681 0.94024 0.93589
VE at 298.15
-VE
K
P -~g.crnm3
x
cm3.mol-’
(1 -x)H,D+x(CH,)HNCH,CH,OH 0 0.033 0.088 0.163 0.268 0.368 0.543 0.640 0.642 0.583 0.486 0.331 0.189 0
xH,NCH,CH,OH
Al
excess volumes
-VE cm3.mol-’
(1 -x)H,D+x(CH3)*NCH,CH,OH 0 0.033 0.065 0.140 0.293 0.503 0.717 0.948 1.157 1.203 1.198 1.171 1.140 1.044 0.982 0.794 0.668 0.539 0.240 0
(3) for the molar
0 0.0249 0.0500 0.0789 O.looo 0.1501 0.2003 0.2519 0.3018 0.3495 0.3974 0.4501 0.5003 0.5498 0.5992 0.6496 0.6998 0.7502 0.7993 0.8507 0.9003 0.9498 1 excess volumes
(1 -x)H,O+ x(CH,)HNCH,CH,OH -5.198 35.625 - 295.241 873.807 - 1245.108 925.689 - 336.747 44.516 0.0047 0.0022
0.99707 0.99276 0.99036 0.98788 0.98550 0.97834 0.97005 0.96122 0.95281 0.94513 0.93796 0.93072 0.92438 0.91844 0.91311 0.90813 0.90342 0.89927 0.89541 0.89161 0.88842 0.88514 0.88221 at 298.15 K
(1 -x)H,O+ x(CH,),NCH,CH,OH - 7.874 30.317 -405.378 1648.855 - 3104.196 2899.555 - 1019.860 - 247.427 202.048 0.0037 0.0034
0 0.203 0.440 0.698 0.868 1.196 1.430 1.588 1.674 1.705 1.701 1.662 1.596 1.495 1.383 1.254 1.098 0.947 0.779 0.587 0.416 0.203 0
AQUEOUS
MIXTURES
153
OF AMINOETHANOLS
of small x for {(l-x)H,O+xNH,CH,CH,OHi and ((I-zc)H,O + xCH,NHCH,CH,OH), these p values were used as the calibration curves for the determination of equilibrium liquid-phase compositions in the vapour-pressure measurements. As shown in figure 3, all the three mixtures exhibit a large volume contraction upon mixing; VE is negative for the whole composition range.
-7 2 Gs
-1.0
w7 k
-2.0“4 0 FIGURE 3. Molar xCHjHNCH,CH,OH;
0.2
0.4
0.6
excess volumes at 298.15 K for (l-x)H,O 0. x(CH,),NCH,CH,OH.
0.8
1
+: 0. xH,NCH,CH,OH;
3,
The molar excess volumes VE were fitted to an equation of the type VE/(cm3.mo1-I)=
x(1-x)
i
A,(1-.u)‘k-1)12,
(3)
k=l
with six or seven coefficients by the method of least squares. The coefficients are listed in table 6. PARTIAL
MOLAR
QUANTITIES
In the present measurements of excess functions, the accuracy and precision of GE obtained are at best of usual standard, whereas HE and VE are accurate enough to permit the evaluation of the composition dependence of partial molar excess enthalpy HE and partial molar excess volume vE. Figure 4 shows the evaluated HiE and YE as functions of x for the three mixtures at 298.15 K. Hereafter HF and VT refer to the partial molar excess quantities for water, and Hf and Vt refer to those for an point in aminoethanol. In accordance with an inflection VE against x, the partial molar volume of aminoethanol takes a minimum in the water-rich region. This behaviour had already been observed for (water+ 2aminoethanol).‘“’ The magnitude of the volume contraction, and the position of the
154
H. TOUHARA
ET .1L
.Y
x
FIGURE 4. Partial molar excess (a), enthalpies and (b), volumes xH,NCH,CH,OH; B, xCH,HNCH,CH,OH; C, x(CH,),NCH,CH,OH.
at 298.15 K for (1 -x)H,O
+ : A.
minimum, change regularly with the introduction of methyl groups into aminoethanol ; more specifically, the minimum becomes deeper and shifts to the water-rich region as the size of the aminoethanol and accordingly the hydrophobicity increases, On the other hand, no minimum is found in HE against x and the decrease in H; at smaller x near infinite dilution of aminoethanol is increasingly rapid when the hydrophobicity of the aminoethanol becomes large. This kind of partial-molarquantity behaviour is similar to that familiar in (water + a monohydric alcohol).” ‘)‘
4. Discussion As is shown in figures 2 and 3, the sign and relative magnitude functions for the present mixtures are water + 2-aminoethanol water +N-methyl-2-aminoethanol water +N,N-dimethyl-2-aminoethanol aminoethanol-rich region water-rich region
0> GE > TSE >HE,
of molar excess
VE <0
GE > 0 > HE > but z I-SE, VE < 0 0 > GE > TSE > but z HE, VE < 0.
Except for the aminoethanol-rich region of (water +N,N-dimethyl-2-aminoethanol), this is a typical example of general characteristics in the excess-function behaviour of highly hydrophilic (or typically non-aqueous)” 3, aqueous mixtures.‘i4’ If we compare the three mixtures with one another, we observe that the hydrophobicity of an aminoethanol increases rapidly with the introduction of a
AQUEOUS MIXTURES
155
OF AMINOETHANOLS
methyl group, and (water +N,N-dimethyl-2-aminoethanol) is only moderately hydrophilic in nature. Yet the present three aqueous mixtures are enthalpy controlled and the energetic stabilization due to a strong hydrogen-bonding interaction between unlike components is predominant over the contribution from the entropy changes and this leads to a negative deviation from ideality. The fact that this tendency decreases rapidly with the introduction of methyl groups indicates the predominant role of N . . . HO hydrogen bonding in the unlike interaction. The negative contribution to HE should increase in the sequence: 2-amino< N-methyl-2-aminoethanol as judged ethanol z N,N-dimethyl-2-aminoethanol from the basicity of the aminoethanol. This sequence is inconsistent with that of the HE's: 2-aminoethanol < N-methyl-2-aminoethanol experimental < N,N-dimethyl-2-aminoethanol. This discrepancy may be ascribed to the difference in the self-association of the aminoethanols. The vapour pressures of the aminoethanols studied here do not parallel their molecular size; the sequence is as follows : N,N-dimethyl-2-aminoethanol 9 N-methyl-2-aminoethanol > 2-aminoethanol. This means that, due to the decrease in the proton-accepting ability of the amino group in N,N-dimethyl-2-aminoethanol the degree of selfassociation in aminoethanols may be expected to be in the sequence; 2-aminoethanol > N-methyl-2-aminoethanol s N,N-dimethyl-2-aminoethanol. The above discussion reveals that a strong hydrogen bonding between water and an aminoethanol is the most important factor in determining the excess-function behaviour of (water +an aminoethanol). Yet, the evidence for the influence of hydrophobic interaction is clearly seen both in V,” and H; at x 4 1. The existence of a distinct minimum in Vf is the most significant evidence for the hydrophobicity of aminoethanol. Furthermore, the fact that, while (HT),,, values are almost common to all the aminoethanols, there is much difference in (H;),,, values, is further evidence. Thus, the present mixtures should be a good model for the study of hydrophobic interaction in biologically important molecules. We are currently engaged in a precise determination of enthalpy and density virials in very dilute solutions and we shall show here only a rough estimate of excess partial molar enthalpy and volume of both components each at infinite dilution in table 7. TABLE 7. Partial molar excess enthalpies and volumes at infinite dilution at 298.15 K W:L, kJ.mol-’ (1 -x)H,O+ xH,NCH,CH,OH x(CH,)HNCH,CH,OH x(CH,),NCH,CH,OH
-6.1 -6.2 -6.3
W:L, kJ.mol-’ - 12.5 - 18.5 - 25.4
cm-1 cm’.mol-’ -1.3 -2.7 -4.1
CV,“L, ____cm-‘~mol~’ --____ - 1.9 - 3.7 - 7.5
We thank Mr J. Abe for his cooperation in an earlier stage of this work. We also thank Professor N. Watanabe for encouragement.
156
H. TOUHARA
ET
AL.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Nakanishi, K.; Kato, N.; Maruyama, M. J. Phys. Chem. 1%7, 71, 814. Matsumoto, Y. ; Touhara, H. ; Nakanishi, K. ; Watanabe, N. J. Chem. Thermodynamics 1977, 9, 801. Abe, J.; Nakanishi, K.; Touhara, H. J. Chem. Thermodynamics 197% 10, 483. Riddick, J. A.; Bunger, W. B. Organic Solvents 3rd edition. Wiley-Interscience: New York. 1970. Nakanishi, K. ; Wada, H.; Touhara, H. J. Chem. Thermodynamics 1975, 7, 1125. Touhara, H.; Ikeda, M.; Nakanishi, K.; Watanabe, N. J. Chem. Thermodynamics 1975, 7, 887. Murakami, S.; Benson, G. C. J. Chem. Thermod.vnamics 1969, 1, 559. Winterhalter, D. R.; Van Ness, H. C. J. Chem. Eng. Data 1966, 11, 189. Danilov, B. B.; Martinson, I. G.; Ravdeli, A. A. ID. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 1974,17, 140. Barker, J. A. Aust. J. Chem. 1953, 6, 207. Kaulgud, M. V.; Patil, K. J. J. Phys. Chem. 1976, 80, 138. Franks, F.; Ives, D. J. G. Q. Rev. (London) 1966, 20, 1. Franks, F. In Hydrogen-Bonded Solvent Systems.Covington, A. K.; Jones, P.: editors. Taylor and Francis : London. 1968. Rowlinson, J. S. Liquids and Liquid Mixtures 2nd edition. Butterworths: London. 1969.