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Fluid Phase Equilibria, 40 1988) 279-288
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
279
LIQUID LIQUID COEXISTENCE CURVES FOR BINARY SYSTEMS
DONACIANO BERNABE, ASCENCION ROMERO-MARTINEZ and ARTURO TRl JO*
Xnstituto Mexican0 de1 PetrHeo, Subdirecci6n de LB. P., Eje Central Lizaro Grdenas 152,
07730 MPxico, D. F MPxico)
(Received May 11, 1987; accepted in final form September 29, 1987)
ABSTRACT
Bernabe, D., Romero-Martinez, A. and Trejo, A., 1988. Liquid-liquid coexistence curves for
binary systems. Fluid Phase Equilibria, 40: 279-288.
The liquid-liquid coexistence curves of polar+non-polar binary systems have been
determined experimentally. The polar compounds studied were ethanenitrile, methanol and
N-methylpyrrolidone, whereas the non-polar compounds were chosen from the n-alkane
series. The upper critical solution temperature for each set of mixtures increases with
increasing n-alkane chain length, and the critical composition of the polar component also
increases in this fashion.
INTRODUCTION
The experimental investigation of phase equilibria in fluid mixtures has
revealed a wide variety of phase behaviour. The importance of such investi-
gations extends into the fields of both industrial applications and science; this
is demonstrated by the continuous flow of publications in this field. In
particular, a knowledge of liquid-liquid equilibria (LLE) is of the utmost
importance for the design, operation and optimization of different kinds of
separation equipment. Equally important is the information on the interac-
tions between unlike molecules, which may be derived from LLE data of
carefully selected mixtures through the use of theories of the liquid state or
solution models.
There exist several possible types of liquid-liquid coexistence curves,
which have been studied and discussed in the literature. It is well known that
many binary liquid mixtures that form a single, homogeneous phase will
separate into two liquid phases when cooled below a characteristic temper-
* Author to whom correspondence should be addressed.
037%3812/88/$03.50
0 1988 Elsevier Science Publishers B.V.
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ature, i.e. the mutual miscibility of the components decreases with de-
creasing temperature. The characteristic temperature is the maximum of the
coexistence curve and is called the upper critical solution temperature
(UCST). In other cases, the mutual miscibility of the components decreases
with increasing temperature; thus, the coexistence curve shows a minimum
corresponding to the lower critical solution temperature (LCST). Some other
systems present closed coexistence curves, hence possessing both upper and
lower critical points.
Binary systems that present liquid-liquid coexistence curves with UCS
phenomena are more abundant than any of the other behaviours mentioned
above. This is verified from the wealth of experimental data compiled by
Francis (1961) and by Sorensen and Arlt (1979/1980). Upper critical
solution phenomena characterize binary mixtures formed by a polar com-
pound and a non-polar compound, where specific interactions between
unlike pairs of molecules are absent.
In this report of our work on the LLE of binary mixtures, we present
experimental results of the coexistence curve for systems formed by
ethanenitrile, methanol or N-methylpyrrolidone as the polar compound, and
an n-alkane containing between 5 and 13 carbon atoms as the non-polar
compound. Previous work on the liquid-liquid phase behaviour of this type
of system included measurements of the upper coexistence temperature for
26 mixtures composed of n-alkanenitriles and n-alkanes (McLure et al.,
1982); the binary mixtures studied contained ethanenitrile, propanenitrile,
or n-butanenitrile, and an n-alkane containing between 5 and 18 carbon
atoms.
TABLE 1
Sources, grades and estimated purity of materials used in the liquid-liquid determinations
Substance Source and grade Estimated purity (mol W)
Ethanenitrile
Methanol
N-Methylpyrrolidone
N-Pentane
n-Hexane
n-Heptane
n-Nonane
n-Undecane
n-Tridecane
= h.p.1.c.
b h.p.1.c.
L.R.
d L.R.
e pure
a h.p.1.c.
e pure
e pure
e pure
99.9
99.8
98.0
99.0
99.0
99.6
99.0
99.0
99.0
a Baker. b Merck. Matheson Colleman & Bell. d Sigma. e Phillips Petroleum.
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EXPERIMENTAL
aterials
The source, grade and estimated purity of each substance used are listed
in Table 1. All compounds were further purified by placing them in contact
with a drying agent. The polar compounds were dried with a molecular
sieve, whereas the n-alkanes were dried with sodium.
4
375
x
:
35
325
3
I I
1
I
I
I
I
1
I
Fig. 1. Experimental coexistence temperature-composition curves for ethanenitrile (l)+ n-
alkane (2) systems.
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Coexistence temperatures
The liquid-liquid coexistence curves of the three sets of binary systems
were determined using the sealed-tube technique (Trejo, 1979; McLure et
al., 1982). The samples were prepared by weight in Pyrex glass tubes. Two
stainless-steel ball-bearings were placed inside the sample tube for stirring of
the mixture during measurements. Each sample was thoroughly degassed by
several freeze-pump-thaw cycles before being flame sealed under vacuum.
375
350
5
h-
3
300
275
I 1
I
I I
I I
0.5 I
Xl
Fig. 2. Experimental coexistence temperature+zomposition curves for N-methylpyrrolidone
(l)+ n-alkane (2) systems.
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The measurement of temperature was carried out with a Hewlett-Packard
quartz thermometer, model 2804A. The samples were brought into thermal
equilibrium in a water bath for temperatures up to 340 K and a ethylene
TABLE 2
Experimental coexistence temperature-composition data for ethanenitrile 1) + n-alkane 2)
systems
T 6)
Xl T 6) 4
n-Pentane
329.11
336.19
339.12
339.36
340.26
340.49
340.48
340.20
339.57
338.11
336.11
328.44
321.34
312.27
n-Heptane
328.37
338.26
348.58
349.32
351.68
353.74
356.94
356.84
357.30
356.89
356.43
356.05
353.43
348.93
341.76
336.56
333.33
332.87
321.94
0.2146
0.2972
0.3819
0.3894
0.4644
0.4862
0.5457
0.6093
0.6526
0.6975
0.7316
0.8013
0.8407
0.9008
0.1591
0.2181
0.3153
0.3231
0.3669
0.4071
0.5522
0.5705
0.6250
0.6738
0.7202
0.7391
0.7977
0.8382
0.8494
0.8921
0.9060
0.9076
0.9303
n-Hexane
319.80
332.51
341.75
341.96
342.07
344.56
345.98
348.29
348.34
348.51
348.65
348.51
348.31
347.22
342.32
333.74
313.56
n-Tridecane
340.82
358.28
363.45
372.10
372.60
386.68
398.98
394.67
396.27
398.57
399.16
399.46
399.25
397.14
384.76
0.1465
0.2050
0.2955
0.2979
0.2996
0.3460
0.3808
0.5013
0.5096
0.5764
0.6000
0.6111
0.6564
0.7098
0.7996
0.8531
0.9123
0.1873
0.2791
0.2886
0.3401
0.3471
0.4825
0.5096
0.5702
0.6108
0.6648
0.7527
0.8050
0.8505
0.8944
0.9465
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glycol bath for measurements above 340 K. The coexistence temperatures
were determined by observing the onset of mixing, marked by the presence
of opalescence, after the samples had been subjected to changes in temper-
ature with heating rates of about 0.03 K mm-. Repeated measurements of
the temperature at which the liquid-liquid meniscus of the same sample
disappeared gave the same value within fO.O1 K. Thus, the coexistence
temperatures reported here are a mean of several determinations for each
one of the compositions studied.
RESULTS AND DISCUSSION
The experimental coexistence temperature-composition data are given in
Tables 2-4 for ethanenitrile, methanol and N-methylpyrrolidone with n-al-
TABLE 3
Experimental coexistence temperature-composition data for methanol (1) + n-alkane (2)
systems
T (K)
XI
T K)
XI
n-Pentane
276.89
283.71
287.49
288.29
288.68
289.28
289.20
289.19
288.67
288.35
283.92
279.93
269.99
n-Nonane
329.61
340.88
349.83
354.54
355.78
355.94
355.75
355.22
354.36
341.44
0.100
0.220
0.313
0.374
0.413
0.438
0.480
0.491
0.565
0.600
0.702
0.760
0.803
0.192
0.281
0.395
0.511
0.604
0.659
0.700
0.751
0.803
0.902
n-Heptane
322.18
326.79
327.69
328.57
329.03
329.17
328.83
328.53
326.71
325.10
319.97
314.69
n-Undecane
348.11
356.56
364.34
370.32
374.44
376.13
376.16
375.78
374.94
370.59
363.15
0.277
0.376
0.394
0.449
0.508
0.555
0.587
0.661
0.701
0.745
0.826
0.852
0.200
0.310
0.395
0.499
0.596
0.698
0.750
0.803
0.851
0.900
0.920
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TABLE 4
Experimental coexistence temperature-composition data for N-methylpyrrolidone (1) + n-
alkane (2) systems
T W
x,
n-Pentane
309.12
319.45
323.76
324.14
324.18
324.20
323.14
324.36
323.54
319.32
309.51
290.72
n-Nonane
318.66
327.16
332.36
332.98
333.05
332.95
332.85
332.66
332.07
325.87
306.62
0.1030
0.1999
0.3000
0.3502
0.3791
0.3990
0.4323
0.4501
0.4960
0.5980
0.7010
0.8012
0.2011
0.2996
0.4003
0.5015
0.5350
0.5692
0.6044
0.6492
0.6891
0.7970
0.8938
n-Undecane
318.94
331.61
337.48
340.28
341.17
341.42
341.36
341.10
338.18
322.22
n-Heptane:
312.69
317.62
321.05
324.35
325.13
325.41
325.74
325.73
324.34
320.82
l
0.1622
0.2006
0.2518
0.3061
0.3502
0.3978
0.4442
0.4958
0.6063
0.7037
0.1938
0.3016
0.4030
0.4925
0.5580
0.5954
0.6493
0.6981
0.7960
0.9004
kanes, respectively. Figures 1-3 show the solubility curves for the three sets
of systems. From these results it is evident that the solubility of the
n-alkanes decreases as their chain length increases, hence the upper critical
solution temperatures (UCST) for a given set of systems increases as the
chain length of the n-alkane increases; the critical composition (X,C) of the
polar component also increases in this fashion. This behaviour may also be
observed if, instead of referring the changes to the variation of the n-alkane
chain length, any other convenient property is used (e.g. molar volume or
boiling temperature). Table 5 contains UCST and Xc values for all the
systems studied.
Our UCST values for ethanenitrile with C,, C, and C, agree reasonably
well with the upper coexistence temperatures of McLure et al. (1982) and
with the UCST of Zieborak and Olszewski (1956a). No reported values of
the UCST for ethanenitrile with C,, were found in the literature. The UCST
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273-
0
Fig. 3. Experimental coexistence temperature-composition curves for methanol (l)+n-alkane
(2) systems.
value
determined here for methanol with C, agrees well with the value
quoted by Francis (1961) and it is slightly higher than the values reported by
Kiser et al. (1961) and by Zieborak et al. (1956b). Our value of the UCST
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TABLE 5
Comparison of upper critical solution temperatures (UCST) and critical compositions (X,C)
for the systems studied in this work
System This work
UCST (K)
Ethanenitrile
+n-C,
340.21
n-C,
348.80
n-C,
357.35
n-G
399.83
Methanol
+ n-C,
288.32
n-C,
329.27
n-C9
355.88
n-C,,
376.04
N-methylpyrrolidone
+ n-C,
324.35
n-C,
325.76
n-C,
333.19
n-C,,
341.38
Literature
X,c UCST (K) X,E
0.5094
341.2 a 0.687
=
0.5557
350.2 a 0.714
d
0.5972
358.0 a, 351.8 0.736
, 0.63 b
0.7907
_
_
0.461
287.40 , 287.90 , 0.502
c
287
0.551
324.4 , 324.2 d,e 0.601
, 0.54 e
0.660
_
0.737
376 d.e _
0.3478
_ -
0.4659
_ _
0.5438
- _
0.6148
- _
a McLure et al. (1982). b Zieborak and Olszewski (1956a). Kiser et al. (1961). d Francis
(1961). e Zieborak et al. (1956b).
for methanol with C, is 5 K higher than the values given by Kiser et al.
(1961), Francis (1961) and Zieborak et al. (1956b). No comparison is carried
out for the UCST with C, since no data were available from the literature.
Our UCST with C,, agrees very well with the value reported by Zieborak et
al. (1956b). It was not possible to compare our UCST values for N-methyl-
pyrrolidone + n-alkanes due to lack of reported data in the literature for this
set of systems.
The compositions given in Table 5 for ethanenitrile + n-alkane systems by
McLure et al. (1982) correspond to equivolume mixtures, thus a direct
comparison with X,C values of this work is not possible. Our X,C values for
methanol with C, and C, are lower than those reported by Kiser et al.
(1961), although our value with C, agrees with that reported by Zieborak et
al. (1956b). As for the UCST values, no reported d t of critical composi-
tions are available in the literature for N-:methylpyrrolidone + n-alkane
systems.
LIST OF SYMBOLS
LLE liquid-liquid equilibria
UCST upper critical solution temperature
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LCST lower critical solution temperature
T temperature (K)
X,
mole fraction of component
x
critical composition of component i
REFERENCES
Francis, A.W., 1961. Critical solution temperature. Am. Chem. Sot., No. 31, Adv. Chem. Ser.,
Washington, DC.
Kiser, RW., Johnson, G.D. and Shetlar, M.D., 1961. Solubilities of various hydrocarbons in
methanol. J. Chem. Eng. Data, 6: 338-341.
McLure, LA., Trejo, A., Ingham, P.A. and Steele, J.F., 1982. Phase equilibria for binary
n-alkanenitrile-n-alkane mixtures. I. Upper liquid-liquid coexistence temperatures for
ethanenitrile, propanenitrile and n-butanenitrile with some
C5 Cl8
n-alkanes. Fluid Phase
Equilibria, 8: 271-284.
Sorensen, J.M. and Arlt, W., 1979/1980. Liquid-liquid equilibrium data collection, DE-
CHEMA Chemistry Data Series, Vol V, 3 parts. Frankfurt.
Trejo, A., 1979. A thermodynamic study of polar+non-polar fluid mixtures. Ph.D. Thesis,
University of Sheffield, Sheffield.
Zieborak, K. and Olszewski, K., 1956a. Heteropolyazeotropic systems. II. Acetonitrile-n-
paraffinic hydrocarbons. Bull. Acad. Pol. Sci. Cl. III, 4: 823-827.
Zieborak, K., Maczynska, 2. and Maczynski, A., 1956b. Heteropolyazeotropic systems. I.
System methanol-n-paraffinic hydrocarbons. Bull. Acad. Pol. Sci. Cl. III, 4: 153-157.