Solvent Characterization by Gas-Liquid Partition Coefficients of Selected Solutes Lutz Rohrschneider Chemische Werke Huls AG, 4370 Marl, Germany
A method is given for the determination of gas-liquid partition coefficients by gas chromatographic headspace analysis with two different columns. The partition coefficients of n-octane, toluene, ethanol, methyl ethyl ketone, dioxane, and nitromethane are specified for eighty solvents. The correlations of these data with solvent polarity, solubility parameter, and the molecular volume of the solvents are discussed. The measured data reference to the aromatics selectivity of extraction solvents and the solubility for polymers in diverse solvents.
the solvent characteristics. In this method, the partition coefficients of selected substances are used for characterization of the solvents. Corresponding to the characterization of gas chromatographic stationary liquids, the solubility of other substances should be capable of being predicted from the partition coefficients of the standard substances. Principles of t h e Method. The gas-liquid partition coefficient
K
=
concentration of t h e solute in t h e solvent/ concentration of the solute in the gas phase = C,/C,
A characterization of solvents is required in order to describe their solvent properties for several reasons. To use solvents for technical purposes, there is a lack of data characterizing the solubility for numerous substances. The influence of solvents on reaction rates and the band positions of molecular spectra is also a result of solvent properties. The use of solvents in chromatographic separations makes it necessary to obtain complete information of their solvent properties which determine the retention behavior. Only the knowledge of solvent characteristics makes it possible to select the right liquid phase for a chromatographic system. In many cases only one parameter was taken for the characterization of solvents. In fact, very often it was found that there is only one parameter for determining the solubility of a variety of solutes. Based on this Hildebrand ( I ) stated his theory of regular solutions in which the solvent is characterized by the solubility parameter 6. This parameter runs parellel with the solvent polarity, which is characterized by many different values. In a summarizing report, Reichardt (2) has given polarity data for several solvents. He introduced a new polarity scale, the ET value, calculated from the band-shift of pyridinium-N-phenol-betaine in each of the solvents. The solvent strength t o of Snyder (3) is a value, which characterizes the mobile phase in adsorption chromatography. It correlates quite well with the ET value. In gas chromatography, it is possible to determine the solvent power of the stationary liquid very accurately for a large number of substances. By experience, it may be stated that the polarity of the stationary liquid is the most important parameter. Distinct effects additionally contribute to the solvent behavior. It was possible to characterize the stationary liquids by 5 or 10 data (4, 5 ) and to calculate the retention data of many other substances from them. To study volatile solvents which cannot be examined as stationary liquids, a method was developed to measure
(1) I . H . Hildebrand and R . L. Scott, "Regular Solutions," Prentice Hall, Englewood Cliffs, N.J.. 1962 ( 2 ) C. Reichardt, Fortschr. Chern. Forsch.. 11, 1 (1968). ( 3 ) L. R . Snyder. "Principies of Adsorption Chromatography,'' Marcel Dekker. New York. N.Y., 1968. (4) L. Rohrschneider, J. Chrornatogr., 22, 6 (1966). ( 5 ) W . 0. McReynolds, J. Chromatogr. Sci.. 8, 685 (1970).
of a substance distributed between the gas phase and the solvent in a sealed flask can be determined by gas chromatographic analysis of the gas phase. If the amount G of the solute, the volume Vi of the solvent and the volume Vg of the gas phase is known, the concentration of the solute Cg in the gas phase is related to the partition coefficient K. The concentration C, can be determined by headspace analysis of the gas phase in the flask. The amount G is distributed between both phases: G = VlC1 + V,C,. For Ci = KC,, it follows G = C, (VlK V,) and
+
The concentration in the gas phase is proportional to the peak height h with a substance specific correction factor f . Thus the concentration in the gas phase reads Cg = hf. The partition coefficient equals to K = G / ( h f VL) - V,/ Vi. If V , and VLare known, the quotient G / f must be determined separately. The gas-liquid partition coefficent K can be calculated from the physical data of solvent and solute if the vapor pressure p and the activity coefficient a of the solute and the molecular-weight M L and the density of the solvent p are known: [Littlewood (S)] K = RTp/paMi. Using a known partition coefficient K,, the value of ( G / f ) ,can be found for the solute in the corresponding solvent, ( G l f ) , = Vg/VL K,h,V,. If ( C / o , is known for one standard substance of the test mixture, K , can be calculated from the peak heights h, in all solvents. The values ( G / f ) n for the other components can be determined from the peak heights hn of a sample directly injected using the same analysis conditions G n l f n = h,( G , / f , ) / h , . Selection of S t a n d a r d Substances. The selection of standard test substances was made by using the knowledge in the characterization of stationary liquids in gas chromatography ( 4 ) . From experience, representative compounds of each of the most important functional group classes were selected. The standard substances should easily be determined in the gas phase of the sample flasks together with the other substances.
+
(6) A. 8.Littlewood, "Gas Chromatography," 2nd ed.. Academic Press, New York. N.Y.. 1970, p 5 0 .
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7 , JUNE 1973
1241
Table I . Solvent Properties
Polymer solubility soluble nonsoluble
+
-
Solvent 1. Carbon disulfide 2. Cyclohexane 3. Triethylamine 4 Ethyl ether 5 Ethyl bromide 6 n-Hexane 7 Isooctane 8. Tetrahydrofuran 9. Isopropyl ether 10. Toluene 11. Benzene 12. p-Xylene 13. Chloroform 14. Carbontetrachloride 15. Butyl ether 16. Methylene chloride 17. Decane 18. Chlorobenzene 19. Bromobenzene 20. Fluorobenzene 21. 2,g-Lutidine 22. Squalane 23. Hexafluoro benzene 24. Ethoxy benzene 25. 2-Picoline 26. Ethylene chloride 27. Ethyl acetate 28. lodo benzene 29. Methyl ethyl ketone 30. Bis( 2-ethoxyethy1)ether 31. Met hoxybenzene 32. Octanol 33. Cyclohexanone 34. teff-Butanol 35. Tetramethylguanidine 36. lsopentanol 37. Pyridine 38. 1,4-Dioxane 39. Butanol 40. Isopropanol 41. Propanol 42. Phenyl ether 43. Acetone 44. Benzonitrile 45. Tetramethylurea 46. Benzylether 47. Acetophenone 48. Hexamethyl phosphoric acid triamide 49. Ethanol 50. Quinoline 51. Nitrobenzene 52. rn-Cresol 53. N,N-Dimethylacetamide 54. Acetic acid 55. Nitroethane 56. Methanol 57. Benzyl alcohol 58. Dimethylformamide 59. Tricresyl phosphate 60. Methoxyethanol 61. Nonylphenoloxet hylate 62. N-Methyl-2-pyrrolidone 63. Acetonitrile 1242
Molecular volume
Solubility parameter
VM
619,
ET121
t013!
9.82 8.19 7.42 7.53
32.6 31.2 33.3 34.6
0.15 0.04
8.91 7.27
...
0.38 0.37
30.9
0.01
37.4 34.0 33.9 34.5
0.45 0.28 0.29 0.32 0.26 0.40 0.18
60.6 108.4 139.6 104.4 75.0 131.1 165.1 81.2 142.0 106.6 89.2 123.6 80.4 96.9 170.0 64.4 195.5 102.0 105.3 94.4 116.8 525.3 115.0 126.5 97.7 78.9 98.1 111.6 89.9
.*. 108.6 158.0 103.8 94.2
... 7.06 8.93 9.16 8.83 9.16 8.55 7.76 9.88 7.74 9.67 9.87 9.11
...
...
8.91 10.13 9.45
... ...
37.2
...
...
...
...
...
... ...
... 41.9 38.1 37.9 41.3
10.30 10.42
..*
...
10.58
... 12.78
... ... ... ...
...
...
...
10.22
...
... 0.42 0.30
36.4
11.9 10.62 10.13 11.60 11.44 12.18 10.10 9.62
96.6 52.7
...
...
37.5 37.5 38.1
...
...
175.0 58.6 118.1 102.2 105.6 92.3 57.1 71.3 40.6 103.5 77.3 31 1.4 75.7
... 39.1 32.5 33.4 41.1
... ...
109.0 80.6 85.5 91.7 76.8 75.1 160.0 73.8 10.3 190.2 117.1
Solvent polarity
... ,..
... ... ...
...
...
0.49
0.58
...
...
...
... . . e
...
40.8 43.9 39.3 47.0 40.2 36,O
0.71 0.56
50.2
...
48.6 50.7 35.3 42.2 42.0 41 .O
... 9
.
I
40.9 51.8 39.4 42.0
...
...
...
...
... 0.82
... 0.56
...
... *..
... ... 0.88
... ... .
.
I
...
13.01
43.7 51.2
..,
...
...
14.50 12.05 11.79 ..,
55.5
0.95
50.8 43.8
...
... ... ...
12.11
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1 .o
...
... ... ...
42.2 46.0
0.65
t
.
.
...
... ...
Polystyrene
-
Selectivity S , KTO I / Koctsn e 0.62 0.29 0.34 0.51 1.06 0.36 0.27 1.23 0.46
++ + + + + + + ++ + +-+ + ++
0.95 0.67 1.47 0.80 0.54 1.76 0.41 1.06 1.20 1.59 1.28 0.51 2.32 1.52 1.75 2.62 1.76 1.60 2.02 1.73 1.78 0.81 2.26 0.72 2.34 0.82 2.75 2.67 1.11 0.93 1.15 2.22 2.58 2.80 3.77 2.37 3.04
+-
-
r
+ + + + + + + + +
-
+ .t
-
+ +-
-
+ + + -
...
3.52 1.76 2.75 3.28 2.55 5.35 2.77 5.30 2.45 3.24 6.66 2.63 3.73 2.22 8.63 6.38
Table I (Continued)
Polymer solubility soluble - nonsoiuble
+
64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81.
Molecular volume
Solubility parameter
Solvent
VM
6191
ET(?)
Aniline Methyiformamide Cyanoethylmorpholine Butyrolactone Nitromethane Dodecafluoroheptanol Formylmorpholine Propylene carbonate Dimethyl sulfoxide Tetrafluoropropanol Tetrahydrothiophene1,l-dioxide Triscyanoethoxypropane Oxydipropionitrile Diethylene glycol Triethylene glycol Ethylene glycol Formamide Water
91.9 62.0
... 9.93
44.3 54.1
Solvent polarity to13 1
... ...
...
...
... ...
...
...
53.8
12.90
46.3
0.64
... ...
...
... ...
85.3 70.9
...
...
46.5 45.0
... ... ...
...
...
...
...
...
...
...
... ...
... ...
... 95.2 133.4 55.8 39.7 18.02
...
t
.
.
... ... 14.24
0.6
53.8 53.8 56.3 56.6 63.1
... 17.05
... 23.53
... ... ...
Polystyrene
Selectivity S, KTollKOctane
+-
6.29 5.36 6.52 13.46 9.96 4.48 11.02 10.53 14.42 7.39
++ + -
20.46 14.62 20.28 6.1 7 10.69
...
Table II. Gas-Liquid Partition Coefficients of the Standard Substances 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
Solvent
Octane
Toluene
Ethanol
Carbon disulfide Cyclohexane Triethylamine Ethyl ether Ethyl bromide n-Hexane Isooctane Tetrahydrofuran Isopropyl ether Toluene Benzene p-Xylene Chloroform Carbon tetrachloride Butyl ether Methylene chloride Decane Chlorobenzene Bromobenzene Fluorobenzene 2,6-Lutidine Squalane Hexafluorobenzene Ethoxy benzene 2-Picoline Ethylene chloride Ethyl acetate lodobenzene Methyl ethyl ketone Bis(2-ethoxyethy1)ether Methoxybenzene Octanol Cyclohexanone teff-Butanol Tetramethylguanidine lsopentanol Pyridine
14,300 13,500 13,300 13,200 11,100 9,900 9,800 8,870 8,800 8,705 8,310 7,620 7,150 7,030 6,920 6,302 6,302 5,972 4,808 4,567 3,805 3,512 3 3 12 3,043 2,945 2,852 2,852 2,852 2,535 2,466 2,466 2,339 2,225 2,172 2,027 1,982 1,982
8,900 3,900 4,450 6,760 11,800 3,520 2,680 10,800 4,020
143 80 690 2,114
... 7,930 5,120 10,500 5,620 3,750 11,100 2,571 6,261 5,762 7,245 4,875 1,775 8,181 4,609 5,174 7,459 5,020 4,567 5,121 4,260 4,224 1,895 5,020 1,561 4,738 1,631 5,452
... 82 61 2,168 719 273 320 268 779 149 368 632 62 265 248 336 3,681 42
M.E.-ketone 661 355 677 1,003 5,777 355 31 0 2,136 786 1,339 1,880 1,260 9,632 997 592
... 265 1,901 1,600 2,339 1,516 184
... 337 4,705 559 1,279 216 1,878 1,384 431 2,168 1,690 6,051 19,715 3,527 5,294
1,371
5,035 4,442 1,224 1,174 1,783 639 1,747 1,600 907 2,404
Dioxane 2,654 1,137 1,692 2,616 8,907 9 74 886 6,141 1,725 3,956 6,597 3,632 35,646 4,342 1,586 3,368 820 5,237 4,812 6,361 3,870 640 4,342 3,708 4,685
... 4,946 4,140 4,812 3,235 5,087 1,518 4,239 2,869 4,046 1,953 5,936
Nitromethane 264 126 458 1,170 2,115 135 118 5,379 719 1,452 1,994 1,191 2,686 384 490 4,994 114 1,468 1,327 2,460 2,739 84 1,408 1,812 3,778 4,369 5,179 1,031 6,994 4,369 2,739 41 1 5,379 705 7,362 603 5,594
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1243
Table I I (Continued) Solvent
38 1,4-Dioxane 39 Butanol 40 Isopropanol 41, Propanol 42. Phenyl ether 43. Acetone 44. Benzonitrile 45. Tetramethylurea 46. Benzyl ether 47. Acetophenone 48. Hexamethyl phosphoric aciid triamide
49. Ethanol 50. Quinoline 51. Nitrobenzene 52. m-Cresol 53. N, N-Dimethylacetamide 54. Acetic acid 55. Nitroethane 56. Methanol 57. Benzyl alcohol 58. Dimethyl forrnamide 59. Tricresyl phosphate 60. Methoxyethanol 61. Nonylphenoloxethylate 62. N-Methyl-2-pyrrolidone 63. Acetonitrile 64. Aniline 65. Methylforqarnide 66. Cyanoethylmorpholine 67. Butyrolactone 68. Nitromethane 69. Dodecafluoroheptanol 70. Formyl morphoiine 71. Propylenecarbonate 72. Dimethyl sulfoxide 73. Tetrafluoropropanol 74. Tetrahydrothiophene1 , 1 -dioxide 75. Triscyanoethoxypropane 76. Ox ydi prop ion itrile 77. Diethylene glycol 78. Triethylene glycol 79. Ethylene glycol 80. Formamide 81. Water
Octane
Toluene
Ethanol
1,900 1,823 1,688 1,657 1,628 1,599 1,519 1,401 1,301 1,282
5,070 2,016 1,566 1,902 3,620 4,121 4,260 5,282 3,089 3,899
1,878 4,758 ... 4,705 230
1,264 1,264 1,247 1,152 891 776 750 714 608 581 577 566 526 48 1 459 41 8 356 285 235 208 205 187 170 168 121 119
4,447 2,221 3,424 3,782 2,270 4,155 2,075 3,782 1,487 1,881 3,840 1,487 1,961 1,067 3,960 2,666 2,240 1,528 1,532 2,799 2,041 837 1,874 1,769 1,745 880
38,539 ... 2,918 424 12,284 9,211 7,061 834
1,248 775 1,010 240 263 105 138
61 .O 53.0 49.8 38.9 24.6
M.E.-ketone
Dioxane
Nitromethane
1,729 1,039 1,318 1,085 980 2,838 2,307 1,840 1,009 1,646
2,195 2,403 3,490 5,237 5,936 4,140 3,225 4,946
3,527 6,778 463 2,821 651 7,368 1,761 1,837 5,647 936 2,286 1,125 6,051 2,644 1,205 7,061 8,560
1,747 1,490 1,416 1,729 24,091 2,035 3,331 2,935 2,247 2,247 2,163 603 1,371 380 1,662 2,935 3,685 1,516 853 1,901 2,886 26,282 1,121 1,543 1,270 28,437
3,632 2,695 3,708 5,396 99,028 4,685 17,820 7,422 3,870 8,907 4,946 1,241 3,956 1,049 4,342 8,097 13,707 3,632 2,280 5,087 9,898 162,000 3,235 4,451 3,870 71,298
3,778 7,362 8,745 13,994 8,229
1,407 662 794 1,407 1,140 2,644 3,527 5,647
997 649 1,039 221 201 270 857 469
3,870 1,953 3,490 699 699 1,081 3,295 5,396
7,362 4,236 6,358 927 869 743 2,739 1,105
... 926 7,636 356 1,054
...
t
.
.
, . .
5,594 759 858 903 1,180 8,800 4,994 9,994 1,861 4,112 15,724 1,499 2,911 3,883 2,973 13,202 3,778 8,377 2,851 2,084 13,994 1,452 5,179 1,394 12.607 13,327 5,594 5,594 3,494 9,994
...
Table 1 1 1 . Peak Heights of the Standard Substances in Dimethylformamide and Coefficient of Variation, V = 100 a/mean of 17 Measurements Time after sample injection [ h ]
Octane Toluene Ethanol Methyl ethyl ketone Dioxane Nitromethane
1244
2
1345 830 71.5 450 220 62
3
4
6
1365 840 70 450 220 62
1335 830 71 440 215 63
1380 860 74 465 225 65
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
10
1360 850 71 450 220 62.5
15
18
1370 860 74 460 220 64
1360 850 70 450 220 62
V%
h1.3 f1.4 f1.8 f1.7 f1.8 f1.6
Table IV. Comparison of Measured and Calculated Partition Coefficients
K P, 25 "C
Solute
n-Octane Toluene
(Torr) 14.0 28.47
Methyl ethyl ketone
90.22
Solvent
a
Acetonitrile Acetonitrile Squalane Methyl ethyl ketone Ethanol Water Hexane Ethanol Water
61.3 4.51 0.73 1.55 7.6 26.2 5.0 2.62 3.48
Ethanol 59.27 Activity coefficients of 0 .Locke, J. Chromatogr., 35, 24 (1968) and Pierotti (8). Vapor
Density pg/ml Mol w t , Ml 0.783 41.05 0.783 41.05 0.8048 422.83 0.805 72.10 46.05 0.785 1 18.02 0.660 86.17 0.785 46.07 1 18.02 pressure data of Hoy ( 9 ) .
Calcd 413 2763 1704 4649 1465 437 316 1341 5004
Measd 418 2666 1775 51 21 2221 469 355 1440 5647
_ _ . -
Class of compound
Standards
Paraffins Aromatics Alcohols Carbonyl compounds Ethers Nitro compounds
n-Octane Toluene Ethyl alcohol Methyl ethyl ketone Dioxane Nitro methane
EXPERIMENTAL T h e concentrations Cr in the gas phase of t h e sample flasks .were measured with the automatic headspace analyzer F 40 of Perkin-Elmer (7). In this apparatus, there are thirty sample flasks in a constant temperature controlled water b a t h a t 25 "C. I n pre-programmed time distances, samples are taken out of the gas phases in t h e flasks and are transferred to t h e gas chromatographic separating system consisting of a column and a flame ionization detector. S a m p l e F l a s k s . The volume of t h e glass sample flasks used was 13.4 ml. They were sealed with a pressure tight rubber stopper. Columns. As t h e solvent or a foreign component often disturbed the determination of t h e peak height of a standard substance, two columns of different polarity were used for t h e gas chromatographic separation. Column A
Stationary
Reoplex
Diethylhexylsebacate
15%
15%
Em bacel60-
Embacel6010 mesh
liquid
Amount of
B
st. liq.
Support Length Temperature Flow velocity
10 mesh 2m 115 "C
4m 100 "C
36.4 ml/
32.6 ml/
min/H2
min/H2
per. T h e test mixture, 5 pl, was injected by a syringe through t h e rubber stopper into the solvent. The sample flasks were placed in the temperature controlled water bath. After two t o fifteen hours, t h e gas phases were analyzed automatically. All of the headspace samples were separated in two runs with the two separating columns. T h e chromatograms were registered on a recorder with two channels of different sensitivity. I n order to determine the polymer solubility CQ. 0.1 gram of the polymers was added to 1 ml of solvent. Two days later the samples were tested for solution. T h e results are given in Table I. The only two solvents for polyamide-12 were rn-cresol and dodecafluoroheptanol.
RESULTS To obtain the partition coefficients by the peak heights, the values G L / f Lhad to be first determined on both columns for each standard substance. The activity-coefficient for n-octane in squalane is cy = 0.72 (25 "C) (8) while the vapor pressure of n-octane amounts to 14.0 Torr a t this temperature (9). With a molecular weight of squalane of 422.83 and a density of 0.8048 g/ml a t 25 "C, the partition coefficient can be calculated as follows: 62370 x 298.2 x 0.8048 RT6 K=-= 3512 14.0 x 0.72 x 442.83 paM, The peak height of n-octane on squalane was h = 26.0 mm (sensitivity = ys). Hence, the standard-factor of noctane is calculated to Golfo = 11.4/2 3512 26 - 2 = 182605. The standard factors for the other substances were each determined by a gas chromatographic analysis of the test mixture on both columns. With these standard factors the partition coefficients were calculated by the peak heights. The mean of the two partition coefficients for a solvent with both columns was taken, if the difference did not exceed 10%. In case of greater differences, the measurements were repeated until the coincidence was sufficient. The partition coefficients determined are given in Table TI.
+
R e a g e n t s . T h e solvents and t h e components for the test mixture were normal laboratory reagents. Nearly all of t h e m had to be purified by distillation in order t o remove foreign components. T h e solvents were later dried with molecular sieves a s small amounts of water can influence the solvent characteristics of the samples. Polymers. Pure polystyrene and polyamide-12 a n d two samples of polyvinyl chloride were used. A medium impact PVC (I) with a content of styrene-butadiene-ester polymerizate a n d a plasticized PVC (11) with 44% dioctyl phthalate as plasticizer were also used. M e a s u r e m e n t s . Two ml of t h e solvent were placed into the sample flasks which were then closed with a pressure tight stop-
Accuracy. The accuracy of the determined partition coefficients depends on the repeatability of the sample amounts placed in the flasks, on the capacity of the rubber stopper for absorbing the solvent as well as the components of the test mixture, and on the purity of the solvents used. In order to test the magnitude of error, several determinations were made a t different time distances between standard sample addition and analysis. It was found t h a t the rubber stopper is not quite inert but t h a t
(7) D. Jentzsch, H. Kruger, G . Lebrecht, G . Dencks, and J . Gut, Frese-
(8) G . I. Pierotti, C. H . Deai. and E. L. Derr, Document NO. 5782, American Documentation institute, Washington, D . C . , 1958. (9) K. L. Hoy. J . Paint Techno/. 42, 76 1970.
n i u s ' z . A n d . Chem., 236, 96 (1968).
DISCUSSION
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 7, J U N E 1973
1245
4.0-
lg K
5.0-
Toluene
lg K MEK
0.0
0
3.5-
e
30-
3.0-
'
ZC-
lg K
$5lg K O c t a n e
in the presence of the solvent, sufficient repeatable results are obtained (Table 111). If the partition coefficient 'Is very small, the rubber stopper causes greater errors because the amount of absorbed substances is proportional to the high concentration in the gas phase. Furthermore, the stopper causes errors if it has a considerable absorbing capacity for the solvent. In this case, the measurements should be made soon after filling the sample flasks. Both errors are of the greatest consequences to the partition coefficient of n-octane and toluene. The standard deviation is caused mainly by different sample amounts. A number of partition coefficients are capable of being calculated from known data of the pure substances. The summary of the results is shown in Table IV. As partition coefficients in great dilution measured in conventional ways also have a certain error, the reason for the differences is not limited to the method reported here. In our estimations, the error of the given partition coefficients should not exceed 15% relative with 95% safety. Solvent Properties. The determined partition coefficients give a good insight into the solvent properties of the examined solvents for paraffins, aromatics, alcohols, substances with dipole moments like methyl ethyl ketone, electron doners such as dioxane and the pseudo-acid nitromet hane. Activity Coefficients. The activity coefficient of the solute in the respective solvent can be calculated from the partition coefficient a = RTp/KpML. Ml = molecular weight of the solvent, p = density of the solvent, and p = vapor pressure of the solute. For example, the activity coefficient for methyl ethyl ketone in acetophenone is a=
62370 x 298.2 x 1.026 1424 x 90.22 x 124.14
=
1.196
Because of the limited accuracy of the method, the calculation of precise activity coefficients is possible only in case of favorable conditions. Correlation with the Solubility P a r a m e t e r 6. If the 6 values are compared with the partition coefficients of noctane and toluene which are mainly kept in the solvent by dispersion forces, there is no precise correlation between the partition coefficients and the solubility parameter. Thus, the nonpolar solvents, carbon disulfide and ethyl bromide, have in fact a good solubility for n-octane but have relative big solubility parameters of 10 and 8 g, respectively. Since the limiting activity coefficient of the solute a. increases with the difference of the solubility parameters 1246
ANALYTICAL CHEMISTRY, V O L . 45, NO. 7 , JUNE 1973
..
* . e . .
e
e
. '
.... . .. .'.. . .. .*
* e
Nitromethane
I
I
I
I
I
I
2.0
25
3.0
3.5
40
4.5
Table V. Solvent Selectivity for Aromatics/Paraffins Solvent
Dimethylformamide N-Methyl-2-pyrrolidone Butyrolactone Nitromethane Formylmorpholine Propylene carbonate Dimethylsulfoxide Tetrahydrothiophene1 , l-dioxide Oxydipropionitrile
SelectivityS, Koctane
KtoilKoctane
577 459 208 205 170 168 121
6.7 8.6 13.5 10.0 11.0 14.4
61 50
20.5 20.3
10.5
between the solvent 62 and the solute 61: RT In cyo = V ~ l ( 6l 62)2, Solvents with a greater solubility parameter such as carbon disulfide and carbon tetrachloride ought to show poor solubility for n-octane (61 = = 7.5). The solubility and likewise the partition coefficient, however, are influenced not only by the solubility parameter but also by the molecular volume of the solvent, K = RT/(P1aV,vl).The value K is a reciprocal to the molecular volume V M of L the solvent ( V M L = Ml/p). Table I shows that substances with small molecular volumes (CS2 and C2HsBr have a remarkable solubility for n-octane combined with a relative great solubility parameter. The ratio of two eartition coefficients in a solvent is on the contrary independent of the solvent's molecular volume K I / K z = (P2az)/(P1a1). It can be shown that the ratio of the two partition coefficients in regular solutions depends only on the solubility parameter of the solvent. The aromatics selectivity S = Ktoluene/Koctane, which is a suitable scale for the dispersive component of the solubility parameter 6 d ( I O ) is shown in Table I. O L E F I N S A N D AROMATICS SELECTIVITY Extraction solvents for aromatics, olefins, and diolefins (butadiene) are important for commercial separations. Gas chromatographic experience has shown that stationary liquids having a great selectivity between aromatics and paraffins do indeed separate olefins from paraffins quite well (11). For a qualified extraction solvent, a great solubility for the substances to be separated together with a great selectivity expressed by different partition coefficients for n-octane and toluene are desired. In Table V, (10) R. F. Blanks a n d I . M . Prausnitz Ind. Eng. Chem., f u n d a m . . 3, 1 1964. (11) L. Rohrschneider. Fresenius'Z. Anal. Chem.. 211, 18 (1965).
some outstanding extraction solvents for aromatics are given with their selectivity S = K t o l u e n e / K o c t a n e and the partition coefficient K o c t a n e . One of the solvents, N methylpyrrolidone, preferred for the commercial extraction of butadiene, exhibits great selectivity together with a remarkable solubility of n-octane. POLYMER SOLVENTS To prove the utility of the determined data, four different polymers, polystyrene, impact polyvinyl chloride, plasticized polyvinyl chloride, and polyamide-12, were examined for their solubility in the solvents. The solubility (+) and nonsolubility ( - ) are given in Table I. It was expected that the solubility of polystyrene would correlate with the partition coefficients of n-octane and toluene. In Figure 1, the tested solvents are drawn in according to the logarithms of their partition coefficients of octane and toluene. A high K-value for toluene gives a good solubility of polystyrene. For the solubility of the other three polymers, a similar diagram with the logarithms of K m e t h y i ethyl ketone and Krlltr0methane is especially useful. The solvents of the three polymers are placed in small areas close together. Polyamid-12 has been solvated by only two of the tested solvents, rn -cresol and dodecafluoroheptanol. Special Selectivities. The data of Table I were examined for special effects of single solvents with respect to the standard solutes. The solvents with extreme solubility for ethanol have a particular solubility for dioxane (cresol, acetic acid) if K
x 10-3 Nitromethane
Eth ano I solvents
Ethanol
Hexamethyl phosphoric acid triamide Tetramethylguanidine .n-Cresol N, N-Dimethylacetamide Tetramethylurea
38.5
3.6
15.7
19.7 12.3 9.2 7.6
4.0 99.0 4.6 4.1
7.3 2.9 13.2 10.0
Dioxane
K
x
10-3
Ethanol solvents
Ethanol
Dioxane
Nitromethane
N-Methylpyrrolidone Acetic acid Dimethylsulfoxide Dimethylformamide
7.3 7.1 7.1 6.8
4.3 17.8 3.9 4.9
12.6 3.8 14.0 14.0
they are strong proton donors or for nitromethane if they operate as proton acceptors. Thus, hexamethyl phosphoric acid triamide is a selective solvent for acetylene which is working as a proton donor. Nitromethane as a pseudo acid is also able to supply protons for a hydrogen bond. The strongest solvents of methyl ethyl ketone work particularly well with dioxane. K Methyl ethyl ketone solvents
Methyl ethyl ketone
Tetrafluoropropanol Dodecafluoroheptanol m-Cresol Chloroform Ethyl bromide Dichlorethane
28.4 26.3 24.1 9.6 5.8 4.4
x 10-3
Dioxane
Toluene
71.3 162 99.0 35.6 8.9
0.9 0.8 2.3 10.5 11.8 7.4
-
Whereas the first three solvents again give strong hydrogen bonds, the three halogen hydrocarbons are generally good solvents by reason of their small molecular volume. Except for acetonitrile, all good nitromethane solvents are already given in the Table of the good alcohol solvents. Acetonitrile does not dissolve aromatics, alcohols, or ether very well. Its large dipole moment of 3.5 Debye makes it useful for substances with great dipole moments like nitromethane (3.1 Debye). Received for review November 22, 1972. Accepted January 22, 1973.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
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