1267
SOLUBILITY IN WATEROF HYDROCARBONS
rather low effective electron energy in the discharge’ implies that these ions are not formed by dissociation following electron impact. As formation of the ions from the corresponding radicals would require relatively low electron energies, we conclude that if the radicals are present they react rapidly giving a standing concentration below the limits of detection. It is quite certain that reaction occurs in the discharge. Analysis shows the presence of C z and possibly C3 saturated and unsaturated hydrocarbons and of a hydrocarbon deposit. I n view of the dominance of product ions in the electron irradiation of ethylenes we suggest that the ion distribution we observe in the discharge is in part due to the formation of CI, CZ,
and possibly C3 ions by a mechanism similar to that found in the electron-beam experiments, and in part due to other reactions, e.g., ion-molecule reaction with unsaturated hydrocarbons, and ionization of high molecular weight products. This latter suggestion is supported by the observation that the concentration of higher ions increased over the first few minutes when a discharge was struck in a freshly cleaned reaction vessel.
Acknowledgment. We thank Mr. R. A. Mascall for his assistance with the experimental work; this paper is published by permission of the Central Electricity Generating Board.
Solubility in Water of Paraffin, Cycloparaffin, Olefin, Acetylene, Cycloolefin, and Aromatic Hydrocarbons1
by Clayton McAuliffe Chevron Research Company, La Habra, Calqornia (Received November 16, 1966)
The solubilities in water a t room temperature of 65 hydrocarbons have been measured using a gas-liquid partition chromatographic technique. For each homologous series of hydrocarbons, the logarithm of the solubility in water is a linear function of the hydrocarbon molar volume. Branching increases water solubility for paraffin, olefin, and acetylene hydrocarbons. The increased solubilities due to branching apparently are not due to a structural feature of the molecules, but t o the higher vapor pressure of the branched-chain hydrocarbons compared with the corresponding paraffin or olefin hydrocarbon. The “structure” of water is such that, for the same hydrocarbon vapor pressure, approximately the same weight of a Cz through Ce paraffin hydrocarbon dissolves in water. For a given carbon number, ring formation increases water solubility. Increasing unsaturation of the hydrocarbon molecule, chain or ring, increases solubility of the hydrocarbon in water.
Introduction Because of their relatively high solubility in water and ease of measurement by ultraviolet absorption in aaueous so~ution.the solubilities of benzene. toluene.’ and isopropylbenzene have been determined by several investigators.2-10 The solu-
bilities of methane and ethane were determined by Winklerll in 1901 and substantiated by the work of (1) Presented before the Division of Petroleum Chemistry, 148th National Meeting of the American Chemical Society, Chicago, Ill. (2) L. J. Andrews and R. M. Keefer, J . Am. C h m . Sac., 71, 3644 (1949).
Volume 70, Number 4 April 1966
1268
Claussen and Polglase12 and Morrison and Billett.9 Claussen and Polglase also determined the solubilities of propane and n-butane; Morrison and Billett measured propane, n-butane, and ethene. However, the solubilities of only a few additional hydrocarbons have been reported. Fuhner6 in 1924 measured n-pentane, n-hexane, n-heptane, and n-octane. The solubilities of n-hexane, n-heptane, and cyclohexane were determined hy Durand.lo Ethene,13 propene,14 and 2~ e t h y l p r o p e n e l ~solubilities ,~5 in water are reported. With the exceptions cited, solubilities have not been measured for most of the paraffin, cycloparaffin, olefin, acetylene, and cycloolefin hydrocarbons reported here. I n this investigation, hydrocarbon solubilities in water were measured by the direct injection of hydrocarbon-saturated water into a gas chromatograph. The gas chromatographic technique has two principal advantages over previous methods of determining dissolved hydrocarbons in water. (1) The presence of impurities with high water solubility pose no problem unless the relative retention time of the impurity is the same BS the hydrocarbon being measured. (2) The use of a hydrogen-flame ionization detector permits the measurement of relatively low concentrations of hydrocarbons in water. A few preliminary results of the solubility measurements of selected paraffin, cycloparaffin, and aromatic hydrocarbons measured by the gas chromatographic technique were recently published. l6 Experimental Section Materials. The hydrocarbons were obtained either from Phillips Petroleum Co. (99+0/, purity) or from Columbia Chemical Co., Columbia, S. C. The hydrocarbons weye used as received. Equilibration of Hydrocarbon with Water. One atmosphere of the individual gaseous hydrocarbons was maintained over distilled water during equilibration. Gas was added to the equilibration vessel by displacing distilled water. ii rubber balloon in the line from the gas cylinder to the equilibration bottle served as a reservoir. The bottle, three-fourths full of water, was vigorously hand shaken for 5 to 10 min to establish equilibrium between the gaseous hydrocarbon and water. The equilibrated solution was allowed to stand at least 30 min prior to analysis, to permit separation of undissolved gas phase from water. From 10 to 20 ml of individual liquid hydrocarbons was added to 200 ml of distilled water in a 250-ml glass bottle. Saturated solutions of the liquid hydrocarbons in distilled water were prepared by either shaking the bottle vigorously on a reciprocal shaker The Journal of Physical Chemistry
CLAYTON MCAULIFFE
for 1 hr, or stirring at least 1 day with a magnetic stirrer. Speed of stirring was adjusted so that the surface vortex was never greater than one-fourth the solution depth. I n the case of vigorous shaking, the solution was allowed to stand for 2 days to permit separation of small hydrocarbon droplets ; the aqueous phase was examined for the presence of emulsion under a phase-contrast microscope with a magnification of 1700X. No emulsions were found. If droplets were present, they were smaller than 0.2 p. The same solubility values were obtained by stirring the solution. This fact supports the microscopic examination of the absence of emulsified hydrocarbon. Because of greater convenience, most of the hydrocarbons were equilibrated with water by stirring on a magnetic stirrer. No attempt was made to regulate 1.5' maintained by the temperature closer than 25 the air-conditioning system in the laboratory. Samples of the hydrocarbon-saturated water mere obtained from beneath the hydrocarbon phase either by displacement of water through a tube, or by insertion of a hypodermic needle through a silicone rubber septum in a tubulation sealed near the bottom of the glass bottle." A 50-pl sample of the hydrocarbonsaturated water was measured with a Hamilton syringe and injected into the fractionator of the gas chromatograph. Fractionator. The fractionator, a modification of one reported by Zarrella, et a1.,I8 separated the dissolved hydrocarbon from water. The fractionator was a 4.5-in. U-tube made from 6-mm glass tubing. One arm of the U-tube was attached to a modified l / h .
*
(3) L.J. Andrews and R. M. Keefer, J . A m . Chem. Soc., 72, 5034 (1950). (4) R. L.Bohon and W. F. Claussen, ibid., 73, 1571 (1951). (5) W.F. McDevit and F. A. Long, ibid.,74, 1773 (1952). (6) H.FUhner, Chem. Ber., 57, 510 (1924). (7) D. S. Arnold, C. A. Plank, E. E. Erickson, and F. P. Pike, Chem. Eng. Data Ser., 3, 253 (1958). (8) F. Franks, M. Gent, and H. H. Johnson, J . Chem. SOC.,2718 (1963). (9) T.J. Morrison and F. J. Billett, ibid.,3819 (1952). (10) R. Durand, Compt. Rend., 226, 409 (1948). (11) L. W. Winkler, Chem. Be?., 34, 1408 (1901). (12) W. F. Claussen and M. F. Polglase, J . Am. Chem. SOC.,74, 4817 (1952). (13) R.W.Taft, Jr., E. L. Purlee, and P. Riesz, ibid.,77,899 (1955). (14) E. L. Purlee and R. W. Taft, Jr., ibid., 78, 5811 (1956). (15) K. S. Kazanskii, S. G. Entelis, and N. hi. Chirkov, Zh. Fiz. Khim., 33, 1409 (1959). (16) C. McAuliffe, Naiure, 200, 1092 (1963). (17) P. A. Witherspoon, personal communication. (18) W. M. Zarrella, R. J. Mousseau, N. D. Coggeshall, M. S. Norris, and G. J. Schrayer, Preprint of presentation before Division
of Petroleum Chemistry, 144th National Meeting of the American Chemical Society, Los Angeles, Calif., April 1963.
SOLUBILITY IN WATEROF HYDROCARBONS
stainless steel Swagelok T-fitting by means of an 0ring joint. The second arm was attached by an 0ring joint to '/*-in. stainless steel tubing. The O-ring joints were held together with ball-joint clamps. The O-ring joints permitted easy replacement of the glass U-tube. The Swagelok T-fitting was further modified to have a rubber septum as the water injection port. This assembly served as a sample loop of the gas chromatograph. Approximately one-third of the U-tube was packed with 60-80 mesh firebrick, and the remaining twothirds was packed with 8-20 mesh Ascarite. A small plug of glass wool was placed after the Ascarite to prevent solid particles from being carried into the chromatographic column by the helium gas flow. During analysis, helium gas flowed past the injection port and through the U-tube with helium passing first over the firebrick. The U-tube was immersed to just above the O-ring joints in boiling water. A 50-pl sample of mater in the syringe was introduced through the injection port and onto the top of the firebrick. The firebrick permitted partial separation of the dissolved hydrocarbon from water. Ascarite prevented water from entering the chromatographic column. Three successive 50-p1 water samples could be injected before it was necessary to replace the packing in the U-tube. Chromatographic dnalysis. The chromatographic column was 12 ft of 0.25-111. stainless steel tubing, packed with 25% SE 30 gum rubber on 30-60 mesh firebrick. Helium flow through the column was 65 cc/min, with the entire effluent passing into a hydrogen-flame ionization detector (Beckman). Depending upon the relative retention time of the hydrocarbon, the column war operated at approximately 60 (minimum attainable with hydrogen-flame burner in the oven compartment), 100, and 125". The chromatograph was equipped with a 10-port valve (Consolidated Electrodynamic Corp.) which permitted attachment of the U-tube while an analysis was being made, The chromatograph also had a short precut column which permitted backflushing of heavier constituents. Measurement of Hydrocarbon Concentration. The gas chromatographic technique of measuring the concentration of dissolved hydrocarbons in water has a principal advantage in that impurities do not interfere. Only in those instances where the impurity has the same relative retention time on the column as the hydrocarbon being measured will interference occur. Figure 1 demonstrates the ease with which the contributions by impurities can be eliminated. Figure 1 s h o w three superimposed chromatograms. The middle chromatogram was obtained by injecting pure 1-octene
1269
* J i
X 25,600
TIME IN MINUTES
Figure 1. Gas chromatograms of 1-octene compared with chromatogram of water saturated with I-octene.
into the column, with an attenuation setting of 25,600; the top chromatogram was obtained at an attenuation of 256. Note that in the interval analyzed, no impurities are detectable in 1-octene, except for a small peak (256 attenuation) just prior to the emergence of 1octene. However, after 1-octene was equilibrated a number of impurity peaks became apparent, A measuring technique which would have included all organic constituents would have given a solubility for 1-octene very much higher than the true solubility, The impurity peaks are either present in very small amounts in the original 1-octene and have very high water solubility, or the impurity peaks are formed during the equilibration of 1-octene with water either by oxidation, photoreactions, or by bacterial action. However, unless the impurity peaks interfere with the measurement of the hydrocarbon peak, their presence is of no significance. This assumes, of rourse, no interaction between the impurities and the hydrocarbon that would alter the solubility of the latter in water. Although the effect of impurities becomes more pronounced the lower the solubility of the hydrocarbon, impurities can also be present and contribute to solubility measurements, by nonchromatographic techniques, of hydrocarbons having relatively high solubilities in water. Cyclopentane, when analyzed as a pure hydrocarbon on the chromatographic column, showed 0.2Q/, impurity. After equilibration with water, the impurity peak was 25% of the cyclopentane area. Higher molecular weight hydrocarbons might also be present which would not be measured in the time interval used in the analysis. Volume 70, Number Q
April 1966
CLAYTON MCAULIFFE
1270
_____~
~~
Table I:
Solubility in Water a t Room Temperature of Paraffin and Branched-Chain Paraffin Hydrocarbons
Hydrocarbon
hlethane Ethane Propane n-Butane Isobutane n-Pentane Isopentane 2,2-Dimethylpropane n-Hexane 2-Methylpentane 3-Nlethylpentane 2,2-Uimethylbutane n-Heptane 2,4-I)imethylpentane %-Octane 2,2,4-Trimethylpentane 2,2,5-Trimethylhexane
" Standard
deviation from mean.
Solubility, g of hydrooarbon/lOe g of water---This work Lit.
24.4 f 1.0" 60.4 f 1 . 3 62.4 f- 2 . 1 (136)* 61.4 f 2 . 6 (165) 48.9 f 2 . 1 3 8 . 5 f 2.0 47.8 It 1 . 6 (54) 33.2 f 1 . 0 9 . 5 f 1.3 13.8 f 0 . 9 1 2 . 8 f0 . 6 18.4 f1 . 3 2.93 f 0.20 4.06 It 0 . 2 9 0.66 f 0.06 2 . 4 4 & 0.12 1 . 1 5 f.0.08
3608
140,8361O
50,8 1O'O 148
39 55 88.1 100.4 104.3 115.2 116.4 122.: 130.7 131.9 129.7 132.7 146.5 148.9 162.6 165.1 181.3
Calculated solubility of liquid hydrocarbon a t 25'.
A further advantage of the gas chromatographic technique is the unusually high sensitivity obtainable with hydrogen-flame ionization detectors. With the present technique, hydrocarbons in amounts as little as 0.1 ppm can be measured in 50 pl of water. I n practice, because of impurities, it is usually not feasible to measure solubilities that are less than 1ppm. The concentration of hydrocarbon was determined by measuring the area under the chromatograph peak with a planimeter or an electronic integrator (Infotronics Model CRS-1). The hydrogen-flame ionization detector was calibrated by injecting 1.00 pl of a mixture of from eight to twelve carefully weighed pure liquid hydrocarbons. The injection of mixtures of known amounts of several liquid hydrocarbons were also used to determine the relative sensitivity of the detector to all hydrocarbons compared with n-heptane. The detector was found to be 1.16 times more sensitive to benzene than normal heptane and 1.14 times more sensitive to toluene. Detector sensitivity to other hydrocarbons was generally within f-5% of the value for n-heptane.
Results and Discussion The solubilities in water a t room temperature of paraffin and branched-chain paraffin hydrocarbons are shown in Table I. Previously measured solubilities are shown in Table I and the following tables where values are available. The last column in each table gives the molar volume of the individual hydrocarbons obtained from American Petroleum Institute Research Project 44:9 or calculated from data given by Doss.~O T h e Journal of Physical Chemistry
21.7," 22.8,1221.59 56.6," 58.3,1* 51.6O 67.0,12 65.69 72.7.12 67.29
Molar vol., ml/mole at 20°
The solubilities of methane, ethane, propane, and nbutane are close to the values reported by Winkler," Claussen and Polglase,I2 and llorrison and Billett.g However, the solubilities found for n-pentane, nhexane, n-heptane, and n-octane are from 10 to 20 times lower than reported by Fuhner.6 Durand's'O values for n-hexane and n-heptane are much lower than Fuhner's but are still three to four times higher than observed in the present investigation. The solubilities in water of olefin hydrocarbons are shown in Table 11. Values for ethene and propene closely agree with previous work but the value for 2methylpropene is lower. No other olefin solubilities were found in the literature. For a given carbon number, the olefin hydrocarbons have considerably higher solubilities in water than do the paraffin hydrocarbons shown in Table I. Table I11 shows the solubility in water at room temperature of the acetylene hydrocarbons. For a given carbon number, the acetylene hydrocarbons have much higher solubilities than the olefin hydrocarbons. As far as is known, solubilities of acetylene hydrocarbons in water at room temperature have not been measured. The solubilities of cycloparaffin, cycloolefin, and aromatic hydrocarbons are listed in Table IV. Only one comparison value is available for cycloparaffins (19) "Selected Values of Physical and Thymodynamic Properties of Hydrocarbons and Related Compounds, A.P.I. Research Project 44, Carnegie Press, Pittsburgh, Pa., 1953. (20) M. P. Doss, "Physical Constants of the Principal Hydrocarbons," 4th ed, The Texas Go., New York, N. Y., 1943.
SOLUBILITY IN WATEROF HYDROCARBONS
1271
Table 11: Solubility in Water a t Room Temperature of Olefin Hydrocarbons Solubility, g of hydrocarbon/lOB g of water-This work Lit.
Hydrocarbon
Olefins Ethene Propene 1-Butene 2-Methy lpropene 1-Pentene 2-Pentene 3-Methyl-1-butene I-Hexene 2-Methy l-l-pentene 4-Methyl-1-pentene 2-Hepterte 1-Octene Diolefins 1,3-Butadiene 2-Methyl-l,3-butadiene l,&Pentrtdiene 1,5-Hexadiene 1,6-Heptadiene
" Standard
deviation from mean.
131 f 10" (2040)b200 f 27 (615) 222 f 10 (670) 263 f 23 148 f 7 203 f 8 (156) 130 f 14 5 0 f 1.2 7 8 f 3.2 48f 2.6 1 5 f 1.4 2.7 f 0.2
Hydrocarbon
Acetylenes Propyne 1-Butyne 1-Pentyne 1-Hexyne 1-Heptyne 1-Octyne 1-Nonyne Diacetylenes 1,6-Heptadiyne 1,8-Nonadiyne
a Calculated solubility of liquid hydrocarbon a t 25".
Molar vol., ml/mole a t 20°
3640 f 125" (5150)b 2870 f 101 1570 f 33 360 f 17 94 f 3 24 f 0.8 7 . 2 f0 . 5
60 83 98.7 114.8 131.2 147.7 164.1
1650 f 25 125 f 3
112 147
Calculated solubility of
and none for cycloolefins. Durand'O measured the solubility of cyclohexane as 63 ppm, close to the value of 55 ppm measured in this investigation. A number of workers2-10 have measured the solubilities of aromatic hydrocarbons, particularly benzene, toluene, and ethylbenzene. The measured solubilities for benzene and toluene closely agree with those reported by previous workers. However, the solubilities of oxylene, ethylbenzene, and isopropylbenzene are somewhat lower than previously reported.
54.5 81.9 94.3 94.4 109.5 107.0-108.2° 111.8 125.0 123.4 126.7 138.7-140. Oc 157.0 87.1 100.0 103.1 118.7 134.0
*
Solubility, of hydrocarbon/ 106 g of water
" Standard deviation from mean. liquid hydrocarbon a t 25".
289,13 31415
(1980) 735 f 20 642 f 10 558 f 27 169 6 44f 3
Table 111: Solubility in Water a t Room Temperature of Acetylene Hydrocarbons g
134,8 13113 183 a t 30°14
Molar vol., ml/mole a t 20°
Molar volume for cis-trans forms.
The data in Table IV show that as unsaturation of the ring structures increases, solubility in water increases. Lindenberg21 proposed a relationship between the logarithm of the solubility of hydrocarbons in water and the molar volume of the hydrocarbons. Using solubility data from the literature, he calculated constants for various saturated and aromatic hydrocarbons, as well as for chlorinated hydrocarbons. Rather than calculate constants, if the logarithm of the solubilities of the hydrocarbons in water is plotted against the molar volume of the hydrocarbons, a straight line is obtained. Parafin and Branched-Chain Parafin Hydrocarbons. Figure 2 is a plot of the logarithm of the solubility in water of paraffin and branched-chain paraffin hydrocarbons us. the molar volume of the hydrocarbons. A straight line has been drawn through the normal paraffin hydrocarbons having from four through eight carbon atoms. An excellent fit is shown. Two values are shown in Figure 2 for n-butane, isobutane, and 2,2-dimethylpropane (neopentane). The lower value is the solubility measured for these gases at 1 atm pressure and 25'. The value at the point of the arrow represents the calculated solubility the hydrocarbon would possess in the liquid state , namely, the vapor pressure existing in contact with the liquid (21) M. A. Lindenberg, Compt. Rend.,243, 2057 (1956).
Volume 70,Number 4 April 1966
CLAYTON MCAULIFFE
1272
Table IV : Solubility in Water a t Room Temperature of Cycloparaffin, Cycloolefin, and Aromatic Hydrocarbons Molar vol.,
Solubility, g of hydrocarbon/lOB Hydrocarbon
Cycloparaffins Cyt lopentane Cyclohexane Cyt loheptane Cytlooctane Methylc yclopentane Methylcyclohexane
1-czs-2-Dimethylcyclohexane Cycloolefins Cyclopentene Cyclohexene Cytloheptene 1-Methylcyclohexene 1,4-Cyelohexadiene 4-T'inylcyclohexene Cycloheptatriene Aromatics Ber zene Toluene a-X ylene Ethylbenzene 1,2, &Trimethylbenzene Isoprop ylbenzene a
This work
156 f 9" 55 =k 2 . 3 30 i 1 . 0 7 . 9 i1 . 8 42 f 1 . 6 14.0 f 1 . 2 6 . 0 i0 . 8
wate-Lit.
g of
631°
535 i 20 213 i 10 66 f 4 52 i 2 700 i 16 50 f 5 620 i 20 1780 i 45 515 i 17 175 f 8 152 i 8
94.1 108.1 121 135 112.4 127.6 140.9 88.2 101.3 116 118.7 93.6 130 103
1740,21790,* 1775,5 1740,' 1730,8 1720,O 14501" 530,2 627,4 470,b 5369 2042 168,3208,* 140,61659
57 f 4 50 It 5
ml/mole at 20°
733
88.7 106.3 120.6 122.4 137.2 139.5
Standard deviation from mean.
a t 25". The calculated values are shown in parentheses in the tables. The vapor pressures used throughout this paper were obtained from plots of vapor pressure us. temperature for the various hydrocarbons obtained from the literature and compiled by Chevron Research Co. The chief source of vapor pressure data for the compilation was A.P.I. Project 44.lS These data were supplemented with values from the literature. I n general, experimental data were given preference over c:tlculated or generalized data. The solubilities of the branched-chain paraffin isomers are higher in all instances than for the normal paraffin hydrocarbons-the greater the branching, the higher the solubility. This is true also for the corrected solubilities to the liquid state for iso- and nbutane and neopentane. Note the reversal in solubilities of iso- and n-butane from that measured at 1 atm pressure and room temperature. Also, the solubility of neopentane is greater than isopentane; the sequence runs n-C5,iso-C5,neo-Cs. The increased solubilities of the branched-chain paraffins apparently are not due to a structural feature of the molecules but to the higher vapor pressure (fugacity) of the branched-chain hydrocarbons. If the measured solubility of each normal or branchedThe Journal o,f Physical Chemistry
chain paraffin hydrocarbon with a boiling point higher than 25" is divided by its vapor pressure in millimeters at 25" and multiplied by 760 mm, a predicted solubility at room temperature and 1 atm is obtained. The calculated values are shown underlined by the names of the hydrocarbons in Figure 2. With the exception of 2,4-dimethylpentane, the values range from 40 to 59 ppm. The lower values appear to be associated with the heavier hydrocarbons. The solubilities of ethane, propane, iso-, and nbutane range from 49 to 62 ppm. Methane and neopentane, with solubilities of only 24 and 33 ppm at 1 atm pressure, are anomalous. The value of 31 for 2,4dimethylpentane may be anomalous; however, the vapor pressure or the solubility may be in error. With these exceptions, it appears that to a first approximation the weight of a normal or branched-chain hydrocarbon which dissolves in water is proportional to the vapor pressure of the hydrocarbon. The 'Lstructure" of water is such that it will accommodate a given weight of a Ce or C9 paraffin hydrocarbon with almost as much ease as a corresponding weight of ethane or propane. The size and configuration of methane and 2,2dimethylpropane (neopentane) may be such that their
SOLUBILITY IN WATEROF HYDROCARBONS
1273
10000
IO 000
I-
4
J J
8 1000
E
-
y1
a
8
-
o s - I 2
0
P
5
8 Ln
a
8
0.1
M
I
40
I
50
I
M
I
io
I
m
I
w
MOLAR
I
IQO
I IIQ
VOLUME
I
120
IM
I
IUI
I
150
160
1
110
I
IO
I
190
OF HYDROCARBON
( ml/mole 01 2 0 D C . )
1
'50
~
SO
~
70
80
1
90
MOLAR
1
110
100 VOLUME
1
120
~
130
140
'
150
IS0
1 110
OF HYDROCARBON
( ml /mole a! 20%)
Figure 2 . Solubility in water a t 25' of paraffin hydrocarbons as a function of their molar volumes.
Figure 3. Solubility in water a t 25' of olefin hydrocarbons as a function of their molar volumes.
association with water is different than for the other paraffin and branched-chain paraffin hydrocarbons, causing a decrease in observed solubility at 1 atm pressure and 25". The measurement of hydrocarbon solubilities by the gas chromatographic technique reported here is limited to concentrations above 0.1 ppm. If the relationship between the logarithm of the solubility of hydrocarbon in water vs. molar volume of the hydrocarbon is a continuous function, then the solubilities of Cg and heavier hydrocarbons can be predicted by extrapolation of the line shown in Figure 2, or if accurate vapor pressures are available for C9 and heavier hydrocarbons, their solubilities can be estimated from vapor pressures. Olefin Hydrocarbons. The logarithm of the solubility in water of olefin and diolefin hydrocarbons plotted against molar volume of the hydrocarbons is shown in Figure 3. The solubility correction from gas at 1 atm to the liquid state at 2.5' has again been calculated for propene, isobutene, l-butene, 3-methyl-lbutene, l,&butadiene, and 2-methyl-1,3-butadiene. The solubility Correction from gas to liquid state for propene is probably not valid, inasmuch as the molar volume is not accurately known. The liquid density used to make the molar volume calculation was that of propene at its liquefaction temperature. As was shown for the paraffin hydrocarbons, a straight line can be drawn through the straight-chain
olefin and diolefin hydrocarbons. The solubility of 1octene appears to be low, but the other straightchain monoolefins closely fit the relationship postulated. Availability of vapor pressures for the olefin hydrocarbons is more limited than for the paraffin hydrocarbons. Calculated solubilities at 1 atm and room temperature for l-pentene, l-hexene, and l-octene are 186, 224, and 170 ppm. The measured values, Table 11, for propene, l-butene, and isobutene are 200, 222, and 236 ppm. Thus, as for the paraffin hydrocarbons, approximately the same weight of each monoolefin hydrocarbon dissolves in water at a constant vapor pressure. As mas the case for the paraffin hydrocarbons, branching of the olefin hydrocarbons increases water solubility. The same general solubility relationships exist for the olefin hydrocarbons as for the paraffin hydrocarbons, with the exception that the solubility for a given carbon number is higher. Acetylene, Cycloparafin, CyclooleJin, and Aromatic Hydrocarbons. If the solubility values for acetylene, cycloparaffin, cycloolefin, and aromatic hydrocarbons are plotted as for paraffin and olefin hydrocarbons, similar straight-line relationships are shown for each homologous series. l-Nonyne and cycloheptane appear to have solubilities which are a little too high. Cycloheptatriene has a solubility greater than Volume YO, Number 4
A p r i l 1966
1
1
~
CLAYTON MCAULIFFE
1274
toluene. I t s molar volume is less than toluene, and the cycloheptatriene value falls on the aromatic line. Cycloheptatriene, although not normally considered an aromatic hydrocarbon, differs from toluene only in having B seven-membered ring with a methylene carbon in the ring compared to a methyl group external to the six-membered benzene ring. At constant vapor pressure, 760 mm, and 25", benzene, toluene, o-xylene, ethylbenzene, and isopropylbenzene h:tve calculated water solubilities of 1.5, 1.5,2.0, 1.4, and 1.3 X lo4g/106 g of water. With paraffin and olefin hydrocarbons, branching of the chain caused higher water solubility for a given number of carbon atoms in the molecule (Figures 2 and 3). I n the case of cycloparaffins, cycloolefins, and aromatic hydrocarbons, addition of side chains on the ring structures has not increased water solubility for a given carbon number. Water Solubility as Related to Hydrocarbon Type. The straight lines for each homologous series have been plotted in Figure 4 to show the relationship of water solubility vs. molar volume for the various classes of hydrocarbons. Using solubility data from the literature, LindenbergZ1calculated constants from the relationship between $he logarithm of the solubility of hydrocarbons in water and molar volume of the hydrocarbons. He concluded that all hydrocarbons gave a single constant. If plotted, this would mean that all hydrocarbons would fall on a single line. From Figure 4 it is obvious that all classes of hydrocarbons do not fall on a single line. Published data available to Lindenberg were limited. Further, Lindenberg used Fuhner's solubilities for normal CsC8, and, as shown in Table I, they are much too high. I n Figure 4 the solid lines represent the solubilities us. molar volumes for the straight-chain hydrocarbons. The dashed lines represent the solubilities vs. molar volumes for hydrocarbons having ring structures. The solubilities for both chain and ring hydrocarbons increases as unsaturated bonds are added to the molecule. A single double bond increases the solubility of an olefin compared to the normal paraffin. A second double bond increases the solubility still further, and a single triple bond increases the solubility to a greater extent than two double bonds. Ring closure increases water solubility. The addition of unsaturated bonds in the ring progressively increases uater solubility through the sequence cycloolefin, cyclodiolefin, and aromatic hydrocarbons. Figure 4 also demonstrates that although each class of hydrocarbons plot as a straight line when the logarithm of the solubility is plotted against molar The JOUTnd of Phyaical Chemistry
-
-
8
-
VI
-
ce--
0.1
-
I
' 70
So
I
'
R
90
MOLAR
I
IW
110
I
I
I20
110
'
I 140
IS0
160
110
VOLUME OF HYDROCARBON
( mllmole at 2O*C.l
Figure 4. Comparison of the solubility in water of various types of hydrocarbons as B function of their molar volumes.
6
50
60
m
a0
90
MOLAR
IW
110
120
130
110
IS
160
ITO
VOLUME OF HYDROCARBON ( m l / m l e at 20°C.l
Figure 5. Solubility in water of various types of hydrocarbons, each with six carbon atoms in the molecule, as a function of their molar volumes.
volume, the ring structures have a different slope than do hydrocarbons with a straight-chain structure.
SOLUBILITY IN WATEROF HYDROCARBONS
The relaticlnship of solubility us. molar volume of different types of hydrocarbons having six carbon atoms in the molecule is shown in Figure 5. The addition of a single double bond to the molecule changes the molar volume of hexane to a lower value for 1-hexene and increases the water solubility appreciably. Two double bond additions to the six-carbon chain (1,s-hexadiene) further decreases the molar volume arid increases water solubility. The addition of one triple bond further decreases the molar volume and likewise increases water solubility. An additional decrease in molar volume is caused by the addition of a second triple bond to the molecule with a still greater increase in mater solubility. The value for 1,j-hexadiyne is an estimated solubility. From Table I11 the solubility of 1,6-heptadiyne is 17.6 times greater than the solubility of 1-heptyne. The solubility of 1,g-nonadiyne is 17.4 times greater than 1-nonyne. If the solubility of 1,j-hexadiyne is 17.5 times higher than 1-hexyne, the solubility would be calculated to be 6700 ppm. A straight line can be drawn through the four olefin and acetylene hydrocarbons. If ring closure occurs for hexane, a marked decrease in molar volume occurs and the solubility of cyclohexane increases to a greater extent than the addition of a double bond to the hexane molecule. As unsaturation occurs in the ring structure, progressing from cyclohexene to 1,4-cyclohexadiene to benzene, the molar volume of the liquid hydrocarbon decreases and the solubility correspondingly increases. Cyclohexene has a higher vater solubility than does l,5hexadiene. 1,4-Cyclohexadiene has appreciably higher water solubility than 1-hexyne, but 1,5-hexadiyne has greater solubility than benzene. A straight line can, again, be drawn through the four six-carbon ring structures. This line parallels the line drawn through the olefin and acetylene six-carbon hydrocarbons. Similar plots can be made using hydrocarbons having four, five, seven, and eight carbons in the various hydrocarbon structures. The Cg
1275
hydrocarbons were plotted because more information is available.
Conclusions For each homologous series of hydrocarbons, the logarithm of the solubility in water is a linear function of the hydrocarbon molar volume. Branching increases water solubility for paraffin, olefin, and acetylene hydrocarbons, but not for cycloparaffin, cycloolefin, and aromatic hydrocarbons. The increased solubilities due to branching apparently are not due to a structural feature of the molecules, but to the higher vapor pressure of the branched-chain hydrocarbons compared with the corresponding paraffin or olefin hydrocarbon. Methane and neopentane are exceptions; their solubilities are lower than predicted. On an equal hydrocarbon vapor pressure basis, approximately the same weight of paraffin and branched-chain paraffin hydrocarbon dissolves in water. Thus, the “structure” of water is such that it will accommodate a given weight of a Cs or C9 paraffin hydrocarbon with almost as much ease as a corresponding weight of ethane or propane. A similar relationship is shown for olefin and aromatic hydrocarbons. For a given carbon number, ring formation increases m t e r solubility. Double bond addition to the molecule, ring or chain, increases water solubility. The addition of a second and third double bond to a hydrocarbon of given carbon number proportionately increases Tvater solubility. A triple bond in a chain molecule increases water solubility to a greater extent than two double bonds. If the relationship between the logarithm of solubility in water and molar volume of the hydrocarbon for each homologous series is a continuous function, the solubilities of higher molecular weight hydrocarbons can be estimated by extrapolation.
Volume 70,Number 4 April 1966
