Table I.
Approximate Lower Limits of Detection of Some Nitrogenous Compounds on Filter Paper after Chromatography
Puriiiw, Pyrimidines, and Ikrivatives Adenine hdenosine Caffeine Cytosine Guanine C;uanosiiio Hypoxant hiiie
Amount Detected, -//Sq.Cm.
2-~Irth~l-5-ethos3.1iiietli~-l-~-
amiriopyrimidinc
blis cellaneo II s Creatinine Ethanolamine Glucosamine Hippuric acid
1 5 3 7 8 8 1 1 1 4 1 5 1 2
DJ
0 0 0 0 '30 1 0 0 8
Orotic arid Theobromine Thiot hj-mine Thymine Uracil I-ric acid S a n t hine
0 9 0 6
AiminoAcid and Peptide Esters C~lvcineethyl ester Gltcine methyl ester G1ycylglyc:inc methyl ester Serine ethyl ester Tyrosine methyl ester
Amino Acids a-.llaiiine @-Alanine a-.%mino-n-butyric acid r--4miiio-n-butyric acid a-bminoisohutyric acid
0.7 I .4 0.9 1.0 1.1
Proteins" nlasc a-I,actal\)uniiii 8-Lactoglohuliii Iipase (xiieat gci,m) ('at
Urease
11iscellaneous Ace t y1 tryptophan p.4minohenzoic acid Barbituric acid Creatine Creatine phosph;ttc
1
1 I
p-A4ininoisobutyricacid -4rginine iisparagine Aspartic acid Citrulline Cysteine
3.5-4.0 ' 7.1 15.0 0.6 0.8 1.7
0 4 0 7 1 0 T O -spot,
Indole llethylguariidine Phosphoethanolaminc Pyridoxine Sulfadiazine Sulfanilamide Sulfanilic acid Tyramine L-rea
5 4 4 9
6-l fet hJ.1t hiouracil
Amount Detected, r/Sq. Cm.
Cystine
y
7 11 0 0 13 7 2 1 0 3 1 6 9 0 0 2
-4riiount Detected, -//Sq.Cm. 1 0 3 3 35 2 r s o spot,
55 y
1.1 0 5 1 2 2 8 0 4
s o Spot, TO y
s o spot, 50 "/ Cysteic acid 1 0 Cyst athione 1 4 3,4-Dihydroxypheii~l~ila11i1ic 3 .7
Glutamic acid Glutamine
3 5
1 1
Amino Acid Glvciiie Gljxocyaniine Histidine Homocystiiie Hydrox) lysine Hydroxyproline Isoleucine Lanthionine Leucine Lvsine llethionine
Anioun t Detected, yjSq. Cm.
IIethionine sulfone Methionine sulfoxide Methyl histidine Sorleucine Norvaline Ornithine Phenylalanine Phosphoserine Proline Sarcosine Serine Tauiine Thiolhistidine Threonine Tryptophan Tyrosine Valine
1.7 2.0 1.8 0.6 0.5 31.2 13.0 0.4 15.0 0.4 s o spot, 51 -/ 1.5 3.6 1. 0 57.5 51.7 0.6 2.2 7.7 19.2 2.0 0.9 1.2 1.2 2.0 0.3 1. o
0.9
Peptides nL-Alanyl-DL-asparagiiie DL-A~any~-DL-methionille
Carnosine Glutathione Glycylglycirie G1vcylglycylgl~-ciiie Gficyl-L-leucine Glycyl-L-tryptophan 1,eucylglycylglycine
(7) Rydon, H.
5 .0 10.0 0.6 0.8 1.2 1.0 1.5 1 6 1.1
k., dniith, P. \V. G.,
S a t u w 169, 922 (1952).
(8) Teeter, H. A I . , Bell, I:. Syntheses 32, 20 (1952).
\\-., Org.
I~ECEIVEI) for revicn- Julv 5, 1957. cepted Scptemher 28, 1957.
hc-
Gas-Liquid Partition Chromatography of Fluorocarbons T. M.
REED Ill
Department o f Chemical Engineering, University of Florida, Gainesville, Fla.
b Several stationary solvent liquids for the gas-liquid partition chromatography of fluorocarbon mixtures have been compared experimentally. The results are presented for di(2-ethylhexyl) sebacate, n-hexadecane, CI(CF2CFC1)3CF2COOC2H5 (the ethyl ester of Kel-F acid 8 1 14), Kel-F No. 90 grease, perfluorokerosine, and perfluorotributylamine as the stationary solvent media. Fluorocarbon and chlorofluorocarbon media give better resolution of fluorocarbon mixtures
than do hydrocarbon media. A solvent phase of good general application for resolving fluorocarbon mixtures boiling below 150" C. i s CI(CF2CFCI)3CFzCOOC2Hs. Products boiling above 29" C., obtained from the reaction of CFBSF~and C~FS, were subjected to partition chromatography using these stationary solvent media. Compounds obtained as products from this reaction probably include three isomers of CbF12, two isomers of CCF14, two isomers of C7F16, and one isomer of CgF18.
of liytliocarboris a d of hydrocarbon derivativcs have been successfully sryarated by gas-liquid partition chroiiiatography n ith hydrocarbon-typr eulistaiices as the stationary solvent phasc. In addition, Evans and Tatloiv ( 6 ) Iiavc reported that dinonyl phthalate on Celite at 80" C. in a toluiiiii 16 ftct long serves as a satisfactory medium for resolving conipletely a mixture containing dihydroperfluorocycloheuane (boiling point, 78" C.), inonohydroperfluorocyclohexenrs (hoil1x'rcRi.s
VOL. 30, NO. 2, FEBRUARY 1958
221
~~~
~~
~
~
~~
Table 1.
Column No. 1 C
3
Stationary Solvent Phase Di(2-ethylhexyl) sebacate n-Hexadecaned
Solvent 225 (4 mm.) 288
Partitioning Columns
InterGrams of M1. of stitial Solvent Solvent Grams of Column per Gram per Meter Packing Volume, of Celite a t R.T. per Meter Cc.
...
..
..
0.40
2.6
7
23
12
21
9
Cl(CF&FCl)aCF*COOCzHse
ca. 290
1.40
3.7
5
Kel-F No. 90 grease'
...
1.20
3
9
22
170 to 230 176
0.65 1.12
2.5
1.5
7 9
27 24
2 Perfluorokerosineo 8 Perfluorotributylamineh a A t atmospheric pressure and 30" C. b Outlet pressure equals 1 atm. Prepared by Perkin-Elmer Corp.
Operating Conditions Carrier Inlet Column gas flow pressure, ttmg., rate, .cc. lb./sq. Nt/mm.a inch* 18 20.5 30 20.5 80 18 24 20 30 20 24 80 26 72 80 20 25 30 22 24 80 20 80 17 13 18 ~80 lo 125 22 31 30 22 24 80 22 22 100 20 80 37 20 33 30
Eastman Organic Chemicals, P3388. Ethyl ester of Kol-F acid 8114, M. W. Kellogg Co. f From M. W. Kellogg Co. 0 From R Livingston, Oak Ridge National Laboratory. * Fluorochemical N-43 from Minnesota Mining I%Manufacturing Co.
a
ing points, 69" and 71" C.), and perfluorocyclohexadienes (boiling points, 58" and 64" C.). The work reported here is a study of stationary solvent phases which are suitable for the separation of mixtures of fluorocarbon-type materials. The results show that a fluorocarbon stationary phase gives better separations of fluorocarbon mixtures than does a hydrocarbon st,ationary phase. I n the cases of some mixtures, chlorofluorocarbons (Kel-F polymers and derivatives) have a better resolving power than do the fluorocarbon stationary phases studied. A stationary solvent liquid of good general utility for resolving the components of a fluorocarbon mixture boiling below 150" C. is Cl(CF&FCl)&Fy COOC2H6,the ethyl ester of Kel-F acid 8114. APPARATUS
The chromatography apparatus used was a Perkin-Elmer Model 154 vapor fractometer. Samples to be analyzed were placed on the chromatography columns through a rubber disk by hypodermic syringes. Sample sizes for liquids were 0.01 to 0.02 ml., and for gas samples, 1 to 2 cc. a t approximately 30" C. and atmospheric pressure. A Brown Electronik pen recorder was the recording instrument. Tliis instrument had a full-scale (25 mv.) pen speed of 12 seconds. The chart speed was 1 inch in 7.5 minutes. PARTITIONING COLUMNS
The supporting medium for the stationary phase in the partitioning columns was 65- to 100-mesh Celite 545 (Johns-Manville). The Celite was washed first with concentrated hydrochloric acid and then with water to a neutral pH. The substances comprising the stationary solvent phases were 222
ANALYTICAL CHEMISTRY
dissolved in enough suitable volatile solvent to make a slurry when mixed with the Celite. The solvent was then evaporated. The mixtures of Celite and stationary phases were packed in 1-meter lengths of 0.25-inch copper tubing in the form of U-tubes, which were vibrated mechanically during the packing operation. Tests on five columns prepared in this manner are reported here. The total length of each column tested was 2 meters. An amount of stationary solvent phase was used in each packing (except perfluorokerosine) so that there would be approximately 0.6 ml. of solvent phase per gram of Celite. The packing carrying perfluorokerosine had 0.37 ml. of liquid per gram of Celite. A constant volume of solvent per unit amount of supporting solid is desirable for the following two reasons when separating media are compared in gasliquid partition chromatography. The amount of surface of the solvent phase presented to the gas flow in each of the several columns will be the same if there is a constant ratio of solvent volume to Celite and the columns are packed to the same interstitial volume. I n comparing partitioning columns, each should be packed to the same fraction interstitial volume, not only to maintain a constant total solvent-gas interface in a given length, but also to ensure that back diffusion and mixing in the gas phase will be the same in each column. This requirement is met substantially by the experimental columns in Table I, except for column 2 containing the perfluorokerosine. This partitioning medium may perform better than reported if it were packed more compactly and if it contained more solvent phase per gram of Celite. I n one theory of solutions (8) the activity coefficient of a solute is proportional to the square of the volume fraction of the solvent. On this basis
it would be desirable to keep the volume of solvent phase constant from one medium to another. I n other theories of solution-e.g., (!4)-the activity coefficient of solute 1s proportional to the square of the mole fraction of the solvent. On this basis partitioning columns might be better compared with a constant ratio of solvent moles to supporting solid. This latter basis, however, makes the area of solvent phases variable from one column to another. The fluorocarbon and chlorofluorocarbon liquids are two t o three times as dense as hydrocarbon liquids. The weight ratio of stationary solvent phase to supporting solid using fluorocarbon liquids will be two t o three times as great as this same ratio for hydrocarbon solvents when the volume of solvent phase per unit of supporting solid is the same in all partitioning columns. A complete study was not made to determine the optimum ratio of solvent phases to supporting solid for the various media. However, the resolution obtained with the ethyl ester of Kel-F acid 8114 was greatest when each gram of Celite carried 0.6 ml. (1.4 grams) of the ester (7 grams of ester per meter). This column was compared with two other columns 2 meters long containing the same stationary phase. One contained 8.5 grams of ester per meter; the other contained about 2 grams of ester per meter. Both had a much poorer resolution than column 9 (used for the results reported here). The column containing di(2-eth 1hexyl) sebacate was obtained from t e Perkin-Elmer Corp. as the one recommended for fluorocarbon separations. The solvent support was 65- to 100-mesh Celite. Column 8, perfluorotributylamine, was not used a t temperatures above
fl
30" C. because of the low boiling point of the amine. The other columns were run a t temperatures from 30" to 125" C. A temperature of 125' C. m7as too high for column 9; retention volumes were cut in half after 10 hours' operation at, 125" C.
\
SOLVENT PHASE\
1
SEBACATE
14 3 '756
( I N F C 101)
I
Dl(2-ETHbIHEXYL)
Other stationary phases not reported in detail here were studied. Kel-F oils and waxes gave results similar to those of the Kel-F No. 90 grease. A column packed with Teflon partially thermally decomposed was completely ineffective. The potassium salt of
1
30'
1
I'
I
I _
8 O0
4-
/yJ
I
I
125'
I
1
,
L
I
5 K E L - F # 90
~
GREASE
P E R F L UO RO
-
8 on
KEROSINE
\
8 =OR
A L L TRACES
H 10 M I N .
Figure 1.
Chromatograms of standard fluorocarbon compounds
perfluoro-octanoic acid on Celite was also ineffective. STANDARD FLUOROCARBON COMPOUNDS
Three reasonably pure fluorocarbons of known formula weight and physical properties were obtained and used as standard reference points for the chromatograms. Perfluoropentane, CSFIP,was obtained from the electrolysis of pyridine in hydrofluoric acid by the Simons' process. Simons and Dunlap have pfeviously reported (17) the boiling point as 29.2" C. This material was assumed to be essentially n-CaFln(Figure 1). Fluorocarbon 101 (FC 101), manufactured and marketed by Minnesota Mining &: Manufacturing Co., is a mixture of fluorocarbon liquids boiling in the range from 67" to 110" C. About 10 weight yo of this mixture is perfluoroheptane, C7F16. (In Figure 1 peak B on the FC 101 chromatogram is C7FlS.) This C7F16 fraction was separated from the mixture by fractional distillation. The chromatograms of the C7FI6fraction showed only one peak. The boiling point was measured as 82.0" C. a t 760 mm. The refractive index a t 25' C. was 1.2600, and the density a t 25" C. was 1.7274 grams per ml. The molecular weight measured by vapor density vias 387 compared with 388 for the formula C7F16. The boiling point, density, and refractive index of n-C7F16 from the literature (11, I d ) are 82.50" C., 1.71802 grams per ml. a t 25" C.. and 1.25818 ( n a~t 25" C,), respectively. The peak corresponding to CiFIGin the chromatograms of FC 101 n.as used as the standard for CiFi,. A liquid of boiling range 126" to 127 " C., predominantly perfluorononane, C9F120,obtained from h h n e s o t a Mining and Manufacturing Co., was fractionally distilled. A middle cut from this distillation produced a large peak and one or two minor peaks on
Effect of Carrier Gas Flow Rate on Chromatograms of FC 101 in Column 9
Table II.
(Column temperature, 80' C.; volume of charge, 0.01 ml. liquid) tm, Obsd. Nitrogen Peak Time for Flow Rate, Desig- Peak Max., Cc./Min.a nation* Minutes 72 B 3.18 A 6.59 24 B 6.28 A 13.10 13 B 12.5 A 25.8 a I,
e
t s , Time
for 1/2.72 of Peak Max., Minutes 2.87 6.13 5.98 12.27 12.0 24.0
V R ,Total Retention Vol.,
cc. s1=
tm
tL
229 474 151 314 189 391
206 44 1 143 294 182 363
VR'.,CC. NZ s Partition K, Retention Volumed Theo&cal E, tm fa Platese Efficiency' Coefficient0 200 1.16 0.0437 190 171 400 1.10 0.0197 396 368 770 1.10 0.0585 147 140 0.0257 480 1.09 294 289 168 1150 1.09 0.0500 161 0.0222 400 1.08 350 325
Relative Partition Coefficient (C7Fis =
1.00)
1.00
0.45 1.00 0.44 1.oo
0.445
At 1 atm. and 30" C. A = C8Fle0, boiling point, 102' C.; B = C7FIB, boiling point, 82" C. At column temperature. V i = ( V R- V d X (3/2) X [ ( p i / p a )B
V d
'I
= Volume between charge injection and column, plus volume betyeen column exit and sensing element = 3 cc.
Vt: for tm
f E = l + 0 K = Equation 1.
tm
(18).
VOL. 30, NO. 2, FEBRUARY 1958
223
chromatograms (Figure 1). For the present purposes this cut was used without further purification as the standard representing CgFzoin these studies. The boiling point of this cut was 127.5" C. a t 760 mm.; the refractive index a t 25" C. was 1.2737. T'alues of 123" (10) and 127" C. (1) have been reported for the boiling point of CgF20. The refractive indes of CgF20a t 20" C. has been given as 1.281 (10). The vapor pressure-temperature relationship measured in the temperature range from 82" to 128" C. for the CgFjo cut used as standard is represented by log,, Po (mni. Hg) =
- 9072 + 8.0534 T
where T is in degrees Kelvin. The question as to the configuration of these standard compounds is open t o debate. There seenis little doubt that the C,FI2 produced by the electrolysis of pyridine in hydrofluoric acid is in the normal configuration. The structure has not been established by any experiment, however. The CiFls from F C 101 boils 0.5" C. below the CiFIGstudied by Oliver and Grisard (12). n h o used material obtained by the cobalt trifluoride fluorination of n-C7HI6. Glew and Reeves ( 7 ) report the density of n-CiFI6 as 1.72006 grams per ml. at 25" C. and a boiling point of 82.34" C. The CiFIGmaterial from FC 101 has a density of 1.7274 grams per nil. a t 25" C. Obviously, no two of these CiFIGcompounds are identical, although they boil within 0.5" C. of earli other. The configuration of no one of them has been demonstrated experimentally. Similarly, nothing is known about the structure of the CgF20standard. CHROMATOGRAMS OF STANDARD MATERIALS
Some chromatograms typical of the three standard reference fluorocarbon substances obtained by the six partitioning columns described in Table I are shown in Figure 1. On each frame tlie positive time direction is from right t o left. Zero time is the vertical line at the right in each frame. The letters A , B , and C on chroniatograms designate the peaks in order of decreasing peak area (height times half-height width). On the original chromatograms all peaks were complete-that is, none ran off scale on the recorder. The peaks which appear incomplete in the figures are single peaks with no inflections. The tops of these peaks were cut to conserve space. The nitrogen flow rates are soniewhat different among the columns. Some of the effects of nitrogen flow rat? on the chromatograms of FC 101 in column 9 are shown by the data of Table 11. The effects on the two major peaks, A and B , in this material are rompared. Peak A is a compound (or mixture of compounds) corresponding to the formula CsFIGO, \\ hich is a carbon ether (or oxide) (2) boiling a t
224
ANALYTICAL CHEMISTRY
about 102" c. Peak B is C7FI6which boils a t 82" C. The efficiency of this column in terms of theoretical plates is different for each of these components. The number of theoretical plates calculated (18) from the C7FI6peak decreases from 1150 to 200 as the nitrogen flourate is increased from 13 cc. per minute to 72 cc. per niinute.. The nuniber of theoretical p1att.s calculated from the C8FI6Opeak is approximately constant at 400 to 480 over the snme range of nitrogm flow rates. This separation of FC 101 should bc carried out a t a nitrogen floiv rate as low as possible to obtain the maximum st.paration or least overlap of the peaks. Thci greater the number of theoretical plates for rach rompound. tlie narrower will be the pcak obtained for that compound. However, a t too low a flow rate, the complete chromatogram requires an exressive time. =It 7 2 cc. per minute nitrogen flowv.peak A requires 6 minutes to appear, while a t 13 cc. per minute it requires 25 minutes. The linear relationship found by Wicbe ( 1 8 ) between the appcarance time of tlie peak maxima and the reciprocal of the carrier gas flow rate is followed approximately by the values given in Table 11. as shov-n in Figure 2. Honever, this relationship is not a rigorous one that applies over a large range of flox rates. Thc flow ratcls in Figurc 2 and Table I1 vary by :i factor of 5.5 and show a tlcviation from thr exprctrd lincar relationship. Furthermore, if the rrlationship new to hold to infinite carrier gsc: flow rate, the line should pass through zero appcarancp time a t infinite nitrogm flow rate (zero reciprocal nitrogen f l o ~rate). Such is not the case for any reasonably straight line for these data, as 1v(,11 as for IVichc's data (18).
RECIPROCAL Ne
RATE, MIN./CC
Figure 2. Effect of nitrogen flow rate on appearance times of CiF16 and CsFIBOin column 9
There is a minimum in the total retention volume for the peaks of Table I1 at a nitrogen flow rate around 20 cc. per minute. This means that there is a flow rate for operation which requires a minimum of carrier gas to elute the charge. Such a situation may be of some importance in large-scale, gasliquid partition chromatography where large quantities of carrier gas have to be handled. The data of Table I1 show that if a nitrogen flow rate in the range of 13 to 24 cc. per minute is chosen for the operation of column 9 a t 80" C., the carrier gas requirements will be close t o the minimum and the separating efficiency will be as high as possible for reasonable appearance times. Accordingly, column 9 a t 80" C. was operated a t 17 cc. per minute nitrogen flow rate. The appearance time for the C,FI6peak a t 17 cc. per minute is in line with the values of Table 11,as shown in Figure 2. The partition coefficient, K , is defined here as the moles of solute in the mobile gas phase per cubic centimeter of gas phase divided by moles of solute in the stationary solvent phase per cubic centimeter of solvent phase. Assuming infinitely dilute solutions in the solvent, K may be calculated from the retention volume, corrected for pressure drop and dead space, and from the void volunie in the column (V,)and from the \-olume of stationary solvent (V,) in the column (13): K = T78/(T'~o - Vc)
(1)
Values of K obtained in column 9 are given in Table I1 for ciF16 and C8F160in F C 101 as a function of the nitrogen flow rate. The ratio of the partition coefficient of CsF160 to CiF16 is essentially constant a t 0.44 to 0.45. The partition coefficients pass through a maximum value a t an intermediate nitrogen flow rate. It is obvious from Figure 1 that t h r Kel-F acid ester, Cl(CF,CFCl),CF,COOC2H5,produces the best resolution of the F C 101 mixture. There are eight components easily distinguished from the peaks or inflections on this trace. A peak a t 22 minutes \vas cut to conserve space. The n-hexadecane column seems to produce some resolution after the major peak in F C 101 rrhich is not found in the Kel-F acid ester column, The di-(Zethylhexyl) sebacate column is definitely inferior to any of the others for separating the components of FC 101. The perfluorokerosine gives poorer resolution of F C 101 than that obtained on the chlorofluoro compounds. The C5FI2standard s h o w predominantly one peak with one or two minor inflections in the tails of the chromatograms. These niinor peaks are evident in the chromatogranis on n-hexadecane, on the Kel-F acid ester, and on the perfluorotri1)utylnmiiiiIi~.
Table 111. Retention Volume, Vg, of Fluorocarbons (I'p in cc. of nitrogen a t column temperature; corrected for pressure drop)
Materials Partitioned . Fractionsa from Reaction of CFaSFs C3F6 C.Piz C6F14 C7H16 Above C ~ F M B.P., 29-31' C. B.P., 57-59°C. B.P., 82-83 C. B.P. > 83" C. A B A B C A B A B 48.7 ... 71.7 105 . . . 105 95 141 89.6 ... 57.5 60 . . . 68 65 86 70.5 ...
+
Column NO.
3
'9
Stationary Solvent Phase n-Hesadecane Cl(CFzCFC1)aCFzCOOCzHs
Column T ~ ~ Standard ~ . , Compounds ' C. CsFiz C7Fie CsFzo 30 48.7 87 295 80 ... 65 123 30 80 80
117 ...
489 132 143 100 131
~
...
104
...
171 ...
243 89 95.5
281
100 5 Kel-F No. 90 grease ... ... ... , , . 104 100 ... 302 ... .. ... . . . 2 Perfluorokcrosine 80 57 452 ... ... 83 103 . . . 353 8 Perfluorotributylamine 30 319 ... 310 955 1040 a Peak A = peak of greatest area; peak B = peak of next greatest area, etc., for each fraction.
-
Table IV.
336
211 ... 124 157 127 167 . . . . . . 131 144 585 1960
. . . . . 138 230 149 208
. . . . . .
131 1750
219
...
...
138 149
...
131 ...
Partition Coefficients of Fluorocarbons Relative to Partition Coefficient of Standard C:Fls at Various Column Temperatures
Materials Partitioned Fractions5 from Reaction of CRSFS C3Fs C~FI? CEFl4 C7Fl6 Above C 7 F 7 B.P., B.P., B.P., B P., 29-31" C. 57-59" c. 82-83' C. >83" C. B B A A B A B c -4 2.50 ... 1.31 0 . i 8 ... 0.78 0.89 0.54 0.96 ... ... 1.22 1.14 ... 0.93 1.00 0.67 0.88
+
Column
Stationary No. Solvent Phase 3 n-Hexadecane 9 5
CI(CF*CFCl)&F*COOC2Hs Kel-F KO.90 grease
Column TEmz., Standard Compounds C8iz GFis CsFz 30 2.50 1.00 0.24 80 . . . 1.00 0.42 30
80 80
5.10 ... ...
100
1.00 1.00
1.00 1.00 1.00
4.70
2.86
... ... ...
...
0:36
0:37s 0.245
...
...
2.02 1.63 1.65
...
1.74 1.41 1.47
...
Perfluorokerosine 80 3.47 ... ... 1.96 1.37 8 Perfluorotributylamine" 30 5.85 ... ... 5.25 6.05 1.85 1.70 a Peak A = peak of greatest area; peak B = peak of next greatest area, etc., for each fraction. * Peak B in C7F16 fraction is reference partition coefficient for column 8. 2
ABOVE C7fi6
1 DI (2-ETHYLHEXYL) SEEACATE
I FOR ALL TRACES
( 5 '76 - F14 590)
ti
N
N-HEXADECANE
182C7\6 - 830)
( > 83Y
2.3% 1.08 0.82 1.27 0.84 ... ... 1.00 0.89 3.08 0.89
0.95 0.95 ... 1.00
1.00
...
0.53 0.65
0.95 0.95
0.54
1.00
...
...
... ...
The C9FZ0standard is best resolved by the column containing di(2-ethylhexyl) sebacate into one major peak and two minor areas in the tail. The corrected retention volumes, VRO, and the relative partition coefficients of the standard compounds are given in Tables I11 and IV, respectively, for the various partitioning media. The operating conditions at each temperature were those given in Table I. CHROMATOGRAMS OF FLUOROCARBON MIXTURES
l----T PERFLUOROKEROSINE
I n the fractional distillation of the products from the reaction between trifluoroniethyl sulfur pentafluoride, CF3SFZ, and perfluoropropylene, C3F6, Dresdner (3) obtained four fractions of fluorocarbon materials boiling a t and above 20' C. The molecular weights (3) of these fractions showed that each was composed of saturated perfluoroalkanes of one empirical formula. Each of these fractions mas subjected to gasliquid partition chromatography using the several partitioning media described above. Some of the chromatograms
PERFLUOROTRIBUTYLAMINE
Figure 3. Chromatograms of products from reaction of CF3SFs and C3FB VOL. 30, NO. 2, FEBRUARY 1958
225
obtained from these mixtures are shown in Figure 3. For general use with such mixtures the Kel-F acid ester again seems to be the best of those studied. The corrected retention volumes, VRO, and the partition coefficients (with respect to C7F16) for the major peaks obtained on the various partitioning media are given for these fractions in Tables I11 and IV, c5Fl2 Fraction. Dresdner reported (3, 4) that the C5F12 material obtained from the reaction products contained nC5F12 iso-C6F12,and neo-C5F12in the ratio of about 1to 2 to 3. A considerable part of this mixture was the neo-CsFlz. The chromatogram of the C E F Ifraction ~ on perfluorotributylamine shows two peaks close together: a small one a t 11.1 minutes and a large one a t 12.6 minutes. The appearance time of the standard C5F12 was 11.4 minutes under the same conditions (Figure 1). The area of the small peak is much less than one sixth the area of the large peak, however. There is probably no resolution of the isomers of C6F12. The chromatogram of the CsFlz fraction on the Kel-F acid ester (column 9) at 30" C. is somewhat different from that obtained on the perfluorotributylamine. The B peak appears after the major peak and there are fewer minor peaks in the former than in the latter chromatogram. The partition coefficients relative to C7F16 calculated from the A and B peaks on the perfluorotributylamine in the C5F12fraction are plotted in Figure 5. The points are close to that for the standard reference CP12 compound. C6FI4Fraction. Most of the chromatograms of this fraction show two major peaks of about equal size. These compounds were not resolved by the Kel-F grease (column 5 ) or by the perfluorokerosine (column 2) media. The chromatogram showing the greatest number of peaks was obtained on the perfluorotributylamine column. T h e relative positions of the minor peaks on the Kel-F acid ester are again quite different from those on the perfluorotributylamine column. The C peak on the perfluorotributylamine column for this fraction may be a perfluorohexene (6). This is also the C peak on the chromatogram of the fraction a t 30" C. on the KelF acid ester. The partition coefficients (relative to C7Fln) of the peaks in the C814 fraction for each partitioning medium are plotted in Figures 4 and 6 . The A peak corresponds rather closely in each case to the expected value for C814 on the curve for the reference compounds. The B peak has a partition coefficient between the A peak and C7F16. It is probably an isomer of CZF14,as the molecular weight of this entire fraction (3) was that expected for the formula C ~ V . 226
ANALYTICAL CHEMISTRY
The C peaks in the chromatograms of the c814 fraction obtained on the Kel-F acid ester a t 80" C. and on the perfluorokerosine a t 30" and at 80" C. correspond to the C7Fl6 standard, as the relative partition coefficients are about 1.0. C,Fle Fraction. Except on di(2ethylhexyl) sebacate, which seems t o show only one major peak, the chromatograms of the C7FI6 fraction show two major peaks. The n-hexadecane, the Kel-F acid ester, and the Kel-F grease show three very small minor peaks. The low temperature of 30" C. necessary for the perfluorotributylamine column is unsatisfactory for this fraction, which boils a t 82" C. nHexadecane a t 30" C. gives in a shorter time about the same chromatogram as that obtained with the Kel-F acid ester a t 80" C. The two minor peaks preceding the major peaks in the CTF16 fraction correspond to the two major
peaks in the c314 fraction. The partition coefficients (relative to C7FIB) for the peaks in this fraction are given in Table IV and plotted for the respective partition columns in Figures 4 and 5. The B peak is apparently C7F16. The A peak is another C,F,e isomer of lower volatility than the B peak. The molecular weight of the C7Fl6 fraction was 392 (S), which indicates that the major portion of the material was C7F16 (molecular weight, 388). Fraction Boiling above CiFI6. This material showed two major peaks on all the partition columns on which it was run. The more volatile compound (peak B ) had a retention volume and relative partition coefficient equal essentially t o those of the C7F16 standard, as shown in Tables I11 and IV. The larger peak, A , falls practically on the line through the relative volatilities of the standard
C O L U M N NO.
C O L U M N NO. 3 N-HEXADECANE AT 30'C.
\
\
I
I
2
1
I
I
I
9
CI(CF,CFCl13CF2COOC2H5
I
I
I
I
\ r
C O L U M N NO. 2 P E R F L U O R O K E R O S I NE
I t
I
I
I COLUMN AT
NO. 8
3OOC.
'8B\,
\/OB A
i
\ \ \ \
\ \ \ \ \
-2
1
I
I
I
I I
5
6
7
8
9
5
N U M B E R OF
I
I
I
X
6
7
8
9
CARBON
ATOMS P E R MOLECULE
Figure 5. Relative partition coefficients ( c 7 F ~= 1.00) of standard compounds and of reaction products on perfluorokerosine and perfluorotributylamine x---
0-0 0
x
Ideal solution Standard compounds Major peaks in fractions from reaction of CFaSFs and CsFa
compounds a t the expected relative volatility for C8Flsin Figures 4 and 5 for the various partition columns. A better resolution of the two major peaks in this fraction is obtained a t 30" than a t 80" C. on the n-hexadecane niedium a t the same carrier gas flow rate. The identification of structure with a measurable physical property of fluorocarbons has been accomplished only by nuclear magnetic resonance spectra (15). Although spectral differences exist for the saturated fluorocarbons in the infrared, no useful relationship between molecular structure and infrared absorption has been found for these compounds (16). The method (9) of plotting retention volumes obtained on one partitioning column versus the retention volumes for corresponding peaks obtained for the same substance on a different partitioning column, when applied to the data presented here for the fluorocarbon mixtures, reveals no correlation between the various peaks other than that obtained using the standard compounds. The isomers present in the fractions apparently are not in the same homologous series. The small differences in volatility between isomers of perfluoroalkanes are within the limits of the accuracy of these measurements. Thus, it is not possible to state any more definitely than that which has been said above for the standard compounds just what configurations of c6F14and C;F16 are present in the reaction products. It is possible to state, however, that the fluorocarbon material which boils a t 57-59" C. (C6FI4fraction) contains essentially no C7F16 (except peak C which is C7F16), and that the material which boils a t 82-83" C. (C7F16 fraction) contains essentially no C P l 4 and no C8Fts molecules in the major peaks. The presence of two major components in each of these fractions and the absence of sulfur must mean that a t least two isomers of C P l l and a t least two isomers of C7F16 are produced in the reaction of CF3SFsand C3Fe. PARTITION COEFFICIENTS A N D ACTIVITY COEFFICIENTS
If the solutions formed in the stationary solvent phase in the partition columns are ideal, the partition coefficient is given by
Zi
= (Po//,.) (VlIVm)
K,'Kc;F.~=
Po/PQC~F~6
Standard Tfmp., Po, Compounds C. Mm. Hg CPiza 30 782.9 80 3516 C7Fi.t 30 97.4 30
700.9
PO/ P0CiFla
8.03 5.01
1.00 1.00
16.3 0.167 80 149.6 0.214 log,o PQ (mm. Hg) = 22.2092 2108.O/T - 4.9814 loglo T; ( 1 7 ) ; T = 5
K.z/Kb
' K.
loglo Po (mm. Hg) = 6.96493 1196.067/(t 210.36); (11); t = " c. log,, PO (mm. Hg) = 8.0534 - 2072,'T ; T = K.
+
line. The relative partition coefficients of the standard compounds, taken from Table IV, are represented by the solid circles and the solid full lines. I n terms of mole fraction z of solute in the stationary solvent phase and mole fraction y of solute in the mobile gas phase the partition coefficient is K = (Y/s)(Vi/Vm) (4) As y = P/T and P = P Q x y ,where P is the partial pressure of solute in the mobile solvent, T is the total pressure of the mobile phase, and y is the activity coefficient of the solute in the liquid solution, substitution in Equation 4 gives
Use of the ideal gas law equation of state, T V , = RT, gives K = POyVi/RT (6) where T is the temperature of the partition column. Combining Equation 6 with Equation 1 gives the activity coefficient as
(8)
=' Pa'Ya/Pbo7b
The values in Table VI show that y increases as PO decreases, so that the ratio in Equation 8 is closer to unity than the ratio in Equation 3 for ideal solution. That is, ivhcn nonideal solutions are formed, poor separation of homologs can be expected. For the separation of fluorocarbon homologs a solvent should be used which forms ideal solutions with the solute. The partial molal heats of solution, AT,of the C7F16 and CSFZ in the nhexadecane and in the Kel-F acid ester columns may be estimated from the activity coefficients of Table V I a t 30" and 80" C.
Assuming constant composition for a given compound with respect to temperature in the chromatograms
- 1 A H Si R
(5)
A In y / A
'TI
-
(10)
The values obtained are given in Table 1'11. From the data of Table I1 the effect of carrier gas flow rate on the activity
VII. Partial Molal Heats of , Solution of Fluorocarbons
Table
Heat of SoluColumn Stationary tion, Kcal./Mole No. Solvent Phase CTF16 CpFzo 3 n-Hexadecane 5.93 4.6 9 Cl(CFzCFC1)SCFzCOOCzHb 1.5 ...
in terms of the experimental quantities. The activity coefficients of the three standard fluorocarbons are given in Table VI for four of the partitioning mhdia. The activity coefficients de-
Table VI.
Activity Coefficients of Standard Fluorocarbon Compounds as Solutes in Stationary Solvents
Column
Stationary Solvent Phase n-Hexadecane
NO.
3
(3)
9
These ideal ratios and the values of
PO are given in Table V for the standard compounds a t 30" and 80" C. On Figures 4 and 5, the logarithm of these ideal relative partition coefficients versus the number of carbon atoms in the molecule is represented by the dashed
80
C&'zoc
(2)
For an ideal solution the partition coefficient relative to C7F16is
crease as the solvent changes from nhexadecane through the Kel-F acid ester to the perfluorokerosine and perfluorotributylamine. The solutions in the perfluorotributylamine are not far from ideal ( y = 1.0). This medium produced the best separation of the fluorocarbon mixtures. This fact is explained as follows. For nonideal solutions the relative partition coefficient may be obtained from Equation 6 as
Table V. Ratio of Vapor Pressure of Standard Compounds to Vapor Pressure of C7Fla
2
8 a
" c. 30
Cl(CACFCl)&F:COOCzHjb Perfluorokerosinec Perfluorotributylamirie
Equation 7. molar volume Estimated molar volume
* Estimated
Column Temp.,
= =
80
30 80
80
30
Activity Coefficient, CsFiz C7Fl8 16.6 53 ... 13.3 6.7 10.7 ... 7.6 1.69 2.45 1.11 1.52
ya
C eFm 75
26 ... 12.5 2.80
...
270 ml. 370 ml. VOL. 30, NO. 2, FEBRUARY 1958
227
Table VIII. Effect of Carrier Gas Flow Rate on Activity Coefficients of C7Fla and of CBF160 in FC 10 1
Nitrogen Flow Rate. Cc./Min. at 1 Atm. and 30” C.
Activity Coefficient, y Peak BO Peak A a J 13 5.74 3.34 24 6.71 3.86 72 5.03 2.96 a Peak B = C7Hle; peak A = C8F160. * Vapor pressure of C8F16O a t 80’ C. = 535 mm. of mercury. coefficients of C7F16 and of C8FI60may be determined (Table 1‘111). The activity coefficients both shoiv a maximum a t a nitrogen flow rate of 24 cc. per minute, where the retention volumes are minimum. ACKNOWLEDGMENT
The assistance of John F. Walter in the experimental work is gratefully acknowledged. The author also thanks J. H. Simons for the perfluoropentane standard. NOMENCLATURE
E K
efficiency = partition coefficient, moles of =
solute per cc. of mobile gas phase divided by moles of solute per cc. of stationary solvent phase ’ Pi = inlet pressure = outlet pressure P O PO = vapor pressure = number of theoretical plates T = temperature, O K. t = temperature, O C. = time on chromatogram for 1/2.72 t, of maximum peak height = time on chromatogram for maxit, mum peak height VC = void volume in column v d = dead volume between sample injection and detector v1 = molar volume of solvent V , = molar volume of gas phase VR = observed retention volume at column temperature = corrected retention volume = volume of stationary solvent in column = mole fraction of solute in stationX ary solvent Y = mole fraction of solute in mobile gas phase AH = partial molal heat of mixing of solute in stationary solvent = activity coefficient of solute in Y stationary solvent n = total pressure in gas phase
s
LITERATURE CITED
Benning, A. F., Park, J. D., U. S. Patent 2,490,764 (1949).
(2) Brice, T. J., Coon, R. I., J . Ana. C h e m SOC.75, 2921 (1953). (3) Dresdner, R. D., Ihid., 77, 6633 f\ -105.51 Y”V,.
(4) Ibid., 78,876 (1956). (5) Dresdner, R. D., Department of Chemical Engineering, University of Florida, private communication. (6) Evans, D. G. hl., Tatlow, J. C., J . Chern. SOC.(London) 1955,3021. (7) Glen-, D. N., Reeves, L. W., J . Phys. C h e m 60,615 (1956). (8) Hildebrand, J. H , Scott, R. L., “Solubility of Sonelectrolytee,” 3rd ed., Reinhold, New York, 1950. (9) Leais, J. S., Patton, H. IT., Kaye, IT. I., ASAL. CHEM. 28, 1370 (1956). (10) Musgrave, W. K. R., Smith, F., J . Chena. Soc. (London) 1949.3021. (11) Oliver, G. D., Blumkin, S., Cunningham, C. IT’., J . Am. C h e w Soc. 73, 5 i 2 2 (1951). (12) Olivrr, G. D., Grisard, J.
73, 1688 (1951).
W.,Ihid.,
(13) Porter, P. E., Deal, C. H., Stross, F. H., Ibid., 78, 2999 (1956). (14) Prigogine, I., Bellemans, A,, DZScussions Faraday SOC.No. 15, 80 (1953). Radio Frequency Spectroscopy 1, NO.
1, (July 1953). Simons, J. H., ed., “Fluorine Chemistry,” Vol. 11, p. 456j Academic Press, Xew York, 1954. Simons, J. H., Dunlap, R. D., J. Chein. Phys. 18, 335 (1950). Wiebe, -4.I
