766
D. H. DESTYAND W. T. SWANTON
Vol. 65
GAS-LIQUID CHROMATOGRAPHY-SOME SELECTIVE STATIONARY PHASES FOR HYDROCARBON SEPARATION3 BY D. H. DESTYAND W. T. SWANTON The British Petroleum Company Limited, Petroleum Division, BP Research Centre, Chertsey Road, Sunbury-on-Thames Middlesex, England Received S e p t e d e r BI, 1960
7,8-Benzoquinoline and certain related compounds a t temperatures up to 100' have proved very selective for the separation of close boiling aromatic hydrocarbons, particularly m- and p-xylene. More volatile compounds containing condensed aromatic rings employed a t 20' have also been found to be selective for lower boiling paraffin, olefin and naphthene hydrocarbons, especially 2,3-dimethylbutane and 2-methylpentane. Separations achieved are discussed in the light of activity coefficients, derived thermodynamic data and electron polarizabilities of the solutes per unit volume. Selectivities are also compared to the standard liquid, squalane.
Introduction Despite the rapid development of capillary columns and t'he very high efficiencies which they make accessible, selective stationary phases still have an important part to play in gas-liquid chromatography even in t'he resolution of close boiling hydrocarbon mixtures. For example t,he separation of m-xylene and p-xylene has been of considerable int,erest for some time and partial separat,ions of these isomers have been described with various stationary phases in long packed columns'-j and capillary columns.6 Recently, a remarkable separation of t~heseisomers was reported 1vit.h a solid absorbent, mont8morillonit'e, treated with a long chain amine.' Langer, et U Z . , ~ ' ~ have described the use of various tetrahalophthalic esters where the formation of molecular complexes bet'ween the solute and solvent molecules causes m-xylene to he elut'ed first,, the reverse of t'he normal volatility order. In an earlier communication,lo tmheauthors reported that the het,erocyclic amine 7,8-benzoquinoline provided a degree of separat,ion of the xylene isomers not previously att'ained on ot'her stat'ionary phases. As a result of this discovery various condensed ring st'ructures similar to that of 7,B-benzoquinoline were examined and found to separate m- and p-xylene by varying degrees. Though the mechanism of the separation is st,ill obscure, it is possible to relate the degree of separation at'tained to the shape and st'ructure of the st'ationary phase molecule and to t'he nature of the substit,uent' groups. Some of the compounds employed are much too volatile to be of real use above room temperature and several were examined at 20" for possible selectivities amongst t'he lower boiling paraffin, olefin and napht8hene hydrocarbons and some infl) A. Zlatki., 1.. O'Rrien and P. R. Scholly, S a t u r e , 181, 1791 19 i 8 ) . ( 2 ) A. Zlarkis, F. Ling and H. R. Kaufmann, Anal. Chem., 31, [SI 94.5 119%). ( 3 ) R. P. W. Scott, unljublished work. (1)J. Aohenien and ,J. H. Purnell, "Gas Chrumatowapby," e d . 1). H. Dratp, Buttwivortlis, London, 1958, p. 20. ( 5 ) R. 1'. I\-. Scott and .T. D. Cheshire, .Vatwe. 180, 702 (1!137). ( 6 ) >.I. ,Ib;. . Golay, rrf. 4 , 1959, p. 20. (7) M . A . Hughes, D. White and A. L. Roherts, .Vature, 184, 1797 (1939). (8) S. €I. Langer. C. Zrthn and G. Pantasopolos, Chemistry and Ind i L 8 t 7 j i , 1146 (1958). ( 9 ) S. EI. Langer, C. Zalin and 0. Pantazopolos, J . Chromatog., 3, 134 (1960). (10) I). 13. Debt?, .1.Goldup and IT. T. Swanton. Xature, 183, 107 (19SY).
teresting results have been obtained. The present paper discusses these results and compares the selectivity of each stationary phase with the standard compound squalane. Solute-solvent interactions and the separations achieved are discussed in the light of derived thermodynamic data and activity coefficients and calculated electron polarizabilities for the solutes in question. Theoretical The Specific Retention Volume, Activity Coefficient and the Partial Molar Excess Functions.The specific retention volume, V,, defined as the volume of dry gas required to elute a sample, corrected to 0" per g., of stationary phase is given by the equation
where F,
=
gas flow rate measured a t ambient temp., 2',
t
= time between air and sample peak maxima
w j
=
weight of stationary ohase
=
compressibility factor,
L
.
('0
.
OK.
1
p, = inlet pressure p, = outlet pressure, assumed here to be barometric - P H ~ O ) = correction for partial pressure of water vapor Po at T , OK.
The specific retention volume is related t o a partition coefficient, K , defined as the ratio of the weight of solute per unit volume in the liquid phase to that in the gas phase, at infinite dilution, by the equation
v, = 273K TP where p =
density of stationary phase a t the column temp.,
T'K.
Assuming ideal behavior in the gas phase the partition coefficient may be related to the activity coefficient of the solute in the solvent a t infinite dilution, yo, taking the pure solute liquid as the standard state, by the expression" K
=
RT p m
where (11) P. E. Porter, C. H. Deal and F. H. Stross. .I. Am. Cheni. S o r . , 18, 2999 (1956).
GAS-LIQUID CHROMATOGRAPHY
May, 1961
M = molecular weight of the solvent po = vapor pressure of the solute a t the column temp., T'K. V&"P since K = 273 273R
v, = Mr0po
(5)
The activity coefficient is the measure of departure from solution ildeality and may be expressed in terms of the excess partial molar heat, entropy and free energy of solution (AHsE, AssE and x s E , respectively) by the equations
--
AGs" = THs" i&E
= R T In
- TASsE
( 6)
and
70
(7)
T h e excess partial molar heat of solution (sometimes called the "heat of mixing") can therefore be obtained a t infinite dilution from the gradient of a plot of log yoagainst 1/T. Inserting z s E and yo in equation 6, a s E can be obtained readily for any value of T. The excess partial molar heat of solution can also be obtained from plots of log K us. 1/T or log V , vs. l/T.11-13 The latter method is preferable since a knowledge of the density variations of the stationary phase with temperature is not required. From equation 4, taking logarithms and differentiating w.r.t., T
'but d- In - yo dT dIn p 0 dT
-
AHsE RT2
AH, -
RT2
where A H v = part,ial molar latent heat of vaporisation of the solute d In v s = - (AHv - H dT RT2
g E )
(12) A. B. Littlewoos3, C. S. G. Phillips a n d D. T. Price, J . Chem. Soc., 1480 (1955). (13) J. R . Anderson and K. H. Napier, dust. .J. Chom., 10, 250
(1967).
Therefore a plot of log V , vs. 1/T is practically a straight line since temperature variations of AHT> and S s E are fairly small. The slope, 2, is given by Z =
(4)
Thus the activity coefficient may be determined from the specific retention volume without a knowledge of the density of the stationary phase a t the operating temperature. For non-ideal behavior in the gas phase the activity coefficient calculated from equation 3 or 4 represents a hybrid activity coefficient for both liquid and mobile phases. Under these conditions the vapor pressure may be replaced by the fugacity, f, and the true activity coefficient of the solute in the solvent a t infinite dilution, yfois then given by
767
AHv
- THsE 2.3R
A similar procedure applied to equation 3 provides
so that a plot of log K us. 1/T is a curve due t'o the added temperature dependence of t'he terms RT and RT2(d In p/dT). I n some cases the latter t'erm may be small (e.g., for squalane it amount's to ca. 0.3 kcal. over the range 20-100') and can then br neglected. The gradient', 2, then provides the socalled "apparent heat of solution," of Porter, Deal and Stross," Le. Z =
AHv -xHsE 2.3R
- RT
The apparent, heat of solution can however be obtained more easily from t,he slope of t'he plot of log V , us. 1/T, by subtracting the appropriate value of RT. The disadvantage of determining AHsE from such plots, however, is that AHv must be known fairly accurately since =sE is usually small by comparison. Experimental Apparatus and Materials.-Glass columns (120 cm. long and 0.4 cm. internal diameter), packed with Celite 545 (Johns Manville Limited), mesh size 72-100 BSS containing approximately 30% w./w. of the stationary phase were employed in a vapor jacketed gas density meter apparatus similar to that described by Mart'in.I4 High purity grade nitrogen (British Oxygen Company Limited) containing less than 10 p.p.m. of oxygen was employed as carrier gas and the preesure to the columns controlled by means of a mercury pressure regulator. The outlet pressure was barometric, and the gas flow rat'e measured by means of a soap bubble flow meter a t ambienot temperature. For retention volume measurements a t 20 , the columns and gas density meter were thermostated to k0.05' by circulating water through the jacket from a constant temperature bath. Higher temperatures were obtained by refluxing vapor through the jacket a t atmospheric pressure. For a temperature of 56", acetone was used as boiler liquid, 65.5' methanol, 78.5" ethanol, 81' cyclohexane, 101' methylcyclohexane. Variations in column temperature arising from the fluctuations in boiling point with atmoepheric pressure amount to a maximum of 1 0 . 5 " . The squalane used as the standard compound was Embaphase quality (May and Baker Company Limited). Other stationary phases employed were recrystallized products from Light and Company Limited, with the exception of 7,s-benzoquinoline which was obtained from Gesellschaft fur Teerverwertung m.b.H. of Duisberg-Meiderich. The purity of each hydrocarbon examined was above 99 mole
%.
Procedure.-The column packings were prepared by dissolving a known weight of stationary phase in a suitable volatile solvent. The Celite, previously water washed to remove fines, dried a t 300" and rescreened t,hrough a 7 2 mesh sieve to remove lumps, then was added to give 30y0 by weight of the stationary phase. The resulting slurry was well agitated so that the Celite particles were well covered with solution while the solvent was evaporated in a current of warm dry air. The dry and free flowing packing thus obtained was added a little at a time to the glass (14) A . .T. P. Martin and A . T. James, Biochem. J . ( L o n d o n ) . 63,138 (1956).
768
D. H. DESTYAND W. T. SWANTON
Vol. 65
column whilst gently tapping the end to ensure a uniform packing. The columns so prepared had an H E T P varying from 0.4-0.6 mm. For the more volatile stationary hases, aniline and quinoline, which were operated a t 20”, t i e carrier gas was presaturated with the liquid a few degrees above the column temperature in order to minimise the loss from the column. Solutes were added to the top of the column with a calibrated micropipet discharging a maximum of 0.5 pl. For examining the more volatile hydrocarbons, the sample introduction was modified slightly and vapor samples inserted through a serum cap with a syringe. Measurements.-The gas inlet pressure was adjusted to give a flow of about 60 ml. per minute though the analytical column and the retention time for each solute measured from the air peak to sample peak maxima. The results were expressed relative to a standard component (either n-hexane or benzene) for which the specific ret,ention volume was calculated a t various flow rates, from equation 1. Vapor presaures were evaluated from the Antoine equation constants (API Project No. 44) and corrected to the fugacities wherever possible by means of fugacity coefficientslb and critical data.
provide an increase in the relative volatility (or separation fact’or) by preferential retention of the meta isomer. Results for squalane, benzylbiphenyl and diin-propyltetrachlorophthalate are also included in Table I. For all the stationary phases examined (with the exception of the tetrachlorophthalate) p-xylene and m-xylene are eluted in order of their volatilities, a-Kaphthylamine gives the best resolution but p-xylene emerges with ethylbenzene. 7,8-Benzoquinoline with a slightly smaller separation factor for the former pair still retains a separation of p-xylene and ethylbenzene. For phenanthrene, the separation factor is just slightly smaller than that for 7,s-benzoquinoline at the same temperature. This clearly indicates that the selectivity is more dependent on the configuration of the stationary phase molecule than on the presence of the nitrogen atom. A similar selectivity is also observed with various naphResults Table I gives t.he separation factors for m- thalene and quinoline compounds, but of those xylenelp-xylene and p-xylenelethylbenzene for examined 1-nonyl-, 1-nitro- and I-iodonaphthalene various aromatic stationary phases a t different did not separate the isomers. From the data it temperatures. Of these compounds, 7,g-benzo- is apparent that in general the separation of mquinoline, phenanthrene, a-naphthylamine and and p-xylene is only slightly enhanced by rediphenylamine were examined in greater detail placing a carbon atom in the aromatic nucleus and retention volume data for a variety of paraffin, by a nitrogen atom; electron donating groups such olefin, naphthene and aromatic hydrocarbons are as amino and hydroxy slightly increase the sepagiven in Table 11. The vapor pressures of the ration while the electron attracting groups, chloro-, hydrocarbon solutes a t the various operating tem- bromo-, iodo- and nitro-, quickly reduce the selecperatures also are included. Similarly, data for tivity. Also, methyl substitution appears t o a narrower boiling range of hydrocarbons, det,er- decrease the separation which is still further remined a t 20’ only are listed in Table I11 for the duced by increasing the size of the alkyl group. It was interesting to observe that diphenylamine stationary phases aniline, isoquinoline, quinoline, 2-methylquinoline, 4-met8hylquinoline, l-methyl- also separated m- and p-xylene. Perhaps diphenylmethane and diphenyl ether exhibit a naphthalene and 1-chloronaphthalene. similar phenomenon. TABLEI Selectivity of the Substrates in Table 11.-The SEPARATION FACTOR FOR m-XYLENE/p-XYLENE ON VARIOUS specific retention volumes show that the aromatic STATIONARY PHASES hydrocarbons are well retained as the V , for benSeparation factor zene and o-xylene on 7,8-benzoquinoline is approximp-XyleneTemp., X y l e n e ethylRelatiye mat,ely the same as that for squalane. Specific Stationary phase “C. p-xylene benzene volatility retention volumes for phenanthrene are larger Di-n-propyl tetrathan on squalane but those for a-naphthylamine chlorophthalate 110 0.96 .. 1.027 somewhat less. Paraffins, olefins and naphthenes Phenanthrene 101 1.055 1.10 1.029 on the other hand are eluted more rapidly and the 81 1.08 1.06 1.034 7,&Benzoquinoline specificity of the stationary phase toward hydro1-Naphthylamine 81 1.085 1.02 1.034 carbon type separations increases in the order, Squalane 81 1 . 0 3 1.11 1.034 phenanthrene < 7,8-benzoquinoline < diphenylBenzylbiphenyl 78.5 1.04 1 . 0 7 1.035 amine < a-naphthylamine. Thus for a-naphthylI . 12 1.035 1-Konylnaphthalene 78.5 -1.00 amine (and also diphenylamine) benzene emerges I-Nitronaphthalene 78.5 1.03 1.15 1.035 well after n-octane. Cyclopentane is retained .. 1.035 I-Iodonaphthalene 78.5 1.04 after n-hexane on 7,8-benzoquinoline1 a-naphthyl1-Chloronaphthalene 78.5 1.045 1 . 1 3 1.035 amine and diphenylamine and an increase in the 1-Bromonaphthalene i8.5 1.055 1.09 1,035 separation factor for hexene-l/n-hexane from 1-Methylnaphthalene 78.5 1.06 .. 1.035 1.16 with 7,8-benzoquinoline to 1.44 with a%Hydroxyquinoline 78.5 1.07 1.On 1.035 naphthylamine a t 65.5’ is also observed (squalane, Isoquinoline GS.5 1.075 1.055 1.039 0.85). These liquids are also effective for increasing Diphenylamine 56 ~ 1 . 0 5 .. 1.042 separations between normal and branched paraffins, thus with a-naphthylamine the separation Separation of m-Xylene and p-Xylene.-On account of their small differences in volatility m- factor of n-heptanelisooctane is 1.67 (squalane, xylene and p-xylene are quite difficult to resolve 1.08), at 65.5’. Selectivity of the Substrates in Table 111.even on fairly selective stationary phases. A group of condensed aromat’icsubstrates, however, ZlatkisI6 was the first to show that a solution of have proved very successful in this separation and brucine in quinoline facilitated the separation of the (15) J. B. Maxwell, “Data Book on Hydrocarbons, Application to Process Engineering,” D. Van Nostrand Co., New York, N. Y., 1950.
(16) A. Zlatkis, “Advanoes in Gas Chromatography,” A.C.S. Div. Petroleum Chem. Symposium, New York, September, 1957.
May, 1961.
GAS-LIQUIDCHROMATOGAPHY
769
RELATIVE RETEXTIOX T;OLUME
'D.4T.k FOR VARIOUS
TABLE I11 HYDROCARBOSS O S A VARIETY O F
STATIOSARY PH.4SES .4T
30'
(n-Hexane = 1) Hydrocarbon
n-Butane Isopentane n-Pentane 2,2-I>iniethylbutane 2,3-Dimeth\ lbutane 2-Nethylpentane 3-1Iet hvlprntane n-Hexane 2.2-Dimethylpent ane 2,A-Dirnethjlpentane 2.?,3-Ti iniethylhutane 3,3-Dimethylpentane 2,3-l)imethylpentanr 2-LIcthylhexan~ t3-31tlth\ lhexaric S-Eth! lpentane n-Hr)ptane 2,2,4-Triniethj lpentane 2-lIeth\ lpentene-1 Hewirl-1 2-?\lrth\Ipeiitene-2 Hept enc- 1 C?clopentanr Methrlcvclopentane CJ clohexanr Xethylcvclohexane Benzene Sprcific retention volume f x n-heuane, ml./g.
Fugacity,
mm
1513 563 415 259 191 171 153 121 83 78 81 65 54 51 48 45 35 38
157 150 126
.13
IPU-
Squalane
0.088 3 7 5 1 5 4 3 89 16 85 80 24 68 23 41 44 83 7 0 4 96
259 7 110 3 77 30 36 23 75. 19
Aniline
.a1
0.132 ,264
,300
,368
,454
,658 689 ,814 1.000 1.321 1 400 1.514 1.896 2.247 2.177 2.425 2.680 3.236 2.763
0.783 ,799 ,972 2.585 0.625 1.301 1.915 3.969 1.438 520.6
1.000
2.680 1.981 1.614 1.609 1.9$2 4.332 1.266 1.996 3.005 4.751 18.100
quinoline
five isomeric hexanes and in particular 2,3-dimethylhutane and 2-methylpentane. This separation a150 has been achieved usiiig isoquinoline, 2methylqu~nolii~e and 4-methylquinoline1 but this tffert i-. iiot limited t o the use of heterocyclic nniineh biiice 1-methylnaphthalene and l-chloronaphthalene show the Fame characteristicq. Figure 1 i h o m a ieparation of the five isomeric hexanes
1-Methylnaphthalene
1-Chloronaphthalene
0.083 197 294 373 574
0 084 195 "05 365 ,563
. COO
fill . 7416
1.000 1 152
1.000 1 lti7
639 740 1 000 1 140
1.248
1 . 259
1 "21
2 040 1 !I70
2.085 2,076
2 058 2 066
3 132 2 209
:3 . 110
3 X4j 2 310
1 183
1 180
3 . 337 2 392 I . 08 1 I 086 1 286 3.674 0 . 775
:3 ti52 0 785
2.028 4,245 1.221
2 041 4 189 4 232
2-Methylquinoline
&Methylquinoline
0 004 21 1 314 371 567 652 752 1 000 1 076 1 173 1 17.3 1 543 1 955 1 927 2 151 2 307 3 055 2 094 1 205 1 352 1 540 3 850 0 950
0 098 215 318
3 838 0 983
3 753 0 873
3 ,693 0.883
2 254 4 153 8 226
2 351 4 318 8 244
2 215 4 296 6 510
2,222
132 4
60.7
Quina. line
573 640 1 000 1 078
0 214 313 590 646 1 000
0.095 221 ,318 390 599 650
-
3 -
1 104
1 533 1 972 1 918
3 052 2 068 1 202 1 237
127 9
167 1
2,178
4,327 6.763
154.6
,
289,O
1 082
249 1
compounds a11 selectively retain the normal paraffins aiid this is shown quite clearly in the logarithmic plots of relative retention volumes against boiling point, an example of which is shown in Fig. 2, for ivquinoline where the data for the paraffin isomerat each carbon number appear to vary linearly with boiling points. Selectivity to hydrocarbon type separations decrease in the order aniline > quinoline N isoquiiioline, > 4-methylquinoline e 2met hylyuiiioline > 1-chloronaphthalene l-methylnaphthalene, z.e., the order of increasing retention for the standard ?%-hexane. Thus the selectivity is not significantly altered by changing the position of the nitrogen atom from the a to the I 7 6 5 4 3 P-position or by altering the position of the methyl group in the heterocyclic ring. ,igaiii little change in selectivity is observed n-hen the weakly electron donating inethyl group is replaced by the weakly electron attracting chloro group in I-methylnaphthalene. hniliiie 5hows (min) 4 the highest olefin and naphthene ielectivity with a Fig. 1 .--Septtration of the hcsmc. isoniers on l-methyl- vparation factor of 1.61 for hexene-1 '?2-hexan( tiaphthaleiie at, 20": ( 1 ) air: ( 2 ) u-I)utatw: ( 3 ) 2,2-dimoth- aut1 3.44 for cyclopeiitaiiein-pentane, the cyc~loylhutativ; (4)2,:3-clitiicth?.ll)ut:tii(~: ( 5 ) ~ - t i i ~ ~ l h ~ ~ l ~ )peiitanci ( ~ t i t ~ tappeariiig ~~~: well after n-hexane. The ~aluci (ti) : ~ - l l l o t I l ~ ~ l ~ , " l l t n l l(c7~); ra-hes:tnc~. of the> beparation factors for these pairb of hydrowrricd out oii l-met,h?-liiaphthaleile at 20°. rarboiis on isoquiiioline are 1.24 aiid 3.09, resperlhis particular stationary phase is also useful in tively. Isoquiiioline gives the hebt, resolution of resolving hexene-1 from 2-met,hylpeiitene-l, giving 2-methylpentane and 2,3-dimethylbutane (separise t o an increase in the separation factor from 1.02 ration factor, 1.15 squalane, 1.05) but a decreabe for squalane to ahout 1.05. Isoquiiioline, quino- in the separation of 2,3-dimethylpentane and 2line, the methylquinolines and the naphthalene methylhexane is observed compared n-ith squalanc.
4
r 7
May, 1961
GAS-LIQUID CHROMATOGRAPHY
771
TABLE IV A('TI\.ITY (hEFFICYPNTS (*ifo) OF T'ARIOUS
Hydrocarbon
Electron polarizability, per unit volume
x
1023
n-Butane 8,147 Isopentane 8.613 n-Pentane 8.69ti 8.941 2,2-Dirnethylbutnne 2,3-Dirnethylbutane 9.075 2-Rlethylpentane 9,000 3-Methylpentane 9,110 n-Hexane 9.074 2,2-Dirnethylpentane 9.231 2,4-Dirnethylpentane 9.216 2,2,3-Trimethylbutane 9.387 3,3-Dirnethylpentane 9.419 2,3-Diinethylpentane 9.441 2-Metbylhexane 9.289 3-Methylhexane 9,370 9.472 3-Ethylpentarie ?+Heptane 0.349 2,2,4-Trimethylpentane 9.336 9.430 %Met h ylpent me-1 9.354 Hemic-1 2-Meth ylpentene-2 I). 620 Heptene-1 9.608 Cyclopentane 9.719 Methylcyclopentane 9.817 10.164 Coclohexane Met hylcyclohexane lO.O!N Benzene 11.684 Dipole moment (Debye units)
HYDROCARBONS O Y
h 17ARIETY O F
fS0-
Squalane
Aniline
quinoline
0 578 618 617 653 612 652 617 635 695 704 621 617 632 684 658 633 071 721 624 642 627 678 474 537 320 535 712 0
15.11 20.29 19.72
i.01 8.39 7.64 10.36 9.20 8.92 8.65 8.22 11.05 10.88 10.39 9.82 9.41 10.02 9.61 9.16 0.21 12.33 5,.25 5.31 5.12 5.88 4.04
24.8T
31.77 39.43 11.85 12.50 12.29 15.84 9.18 13.70 12.99 17,53 2.22 1.52
The separation factor for le-heptane isooctane on isoquinoline on the other hand is increased to 1.46 (squalane 1.17 and aniline 1.35). Discussion In attempting to account for the separations achieved with these aromatic stationary phases it may be visualised that in addition to the dispersive forces between solvent and solute molecules, the solvent exerts an inductive effect due to its own permanent dipole and to the induced dipole in the wlute molecule. So that whether a hydrocarbon wlectively retained or not will depend on the ease of polarisability as well as solubility in the stationary phast.. Before examining this in detail however. it heems worthwhile to discuss squalane where the inductive effect is absent and the attractive torces het,ween the species are essentially of the disperhi1.e type 1:London). Tablc [\' show:, the activity coefficients (yf[') oi the \miow hydrovnrbonb listed it1 Table 111, for hqiiii1:tiic and the aromatic stationary phase5 :it 20'. 'l'hc solutioii forces in squalane give rist. cweffkieni s which are much less thaii ely due t o the difference in molecular inagnitudc between solvent and solute) and the solutions exhibit negative deviations from Raoult'h law. Probably the hydrocarbon solutes are orientated so as to lie as parallel as possible to the solvent molecules with maximum interaction betv,een the inethyl groups. Small and flexible
5.i2
6.63 1.61 2.54
Quinoline
STkTIONARY PHASES
AT
124Methyl- 1-ChloroMethyl- Methyl- nauhtha- naDhthaiene lene quinoline quinoline
8 3ti
31 83 43 45
3 :30 3 'if 3 40 4 29 3 i8 :3 76 3 63 3 42 4 24
3 3 3 4 3
4 0s
4 21
6 96 7 57
3 67 3 87
:J IT7 3 95
8.35
ti 99 9 16
3 55 4 72
2.67 2 77 1 32 1 59
6 96 8 53 7 81
5 91 5.48
9 43 9 41
6.32 6.43
8 51 11 42
5.88
10 24 9 65 10 42
6 5 7 6 6 6 6 8 8
19 83 61 73 91 57 35 11 26 7 54
55 92 45 56
6.42
4.02
4 35
6 12 4 04
4.32 3.14
4 75 3 36
3 51 4 49 2 55 3 55 2 55 2 57 2 06
5 68 6 60 1 67 2 15
4.16 4.58 1.46 1.86
4 49 4 91 1 52
2 65 2 70 1 31
9 12 5 5
20"
91
3 84
3 71 :3 17
4 40
2 59
2 62 2 07
molecules will be better accommodated within the solvent lattice and thus will be retained longer than larger molecules and will consequently have smaller activity coefficients. These factors will therefore be important in determining the order of elution of fairly close boiling isomers. The relative volatility of 2,3-dimethylbutane and 2methylpentane is 1.12 at 20' and these isomers would appear to be easily separable on a stationary phase providing essentially a boiling point separation. However, the separation factor on lane is reduced to 1.05 and the smaller ac coefficient of 2,3-diniethylbutaiie indicates that the isomer is being selectively retained compared to 2-methylpentane. This fact can be attributed to its smaller molecular volume. 2,2,%Trimethylbutane is also well retained and emerges after 2,4dimethylpentane despite its greater volatility. The molecular volume of the former is smaller by about 4 cc. A%gain2,X-dimethylpentane i$ tlutcd after 3-methylhexane for apparently the *:imp rtlami. Saphthenr hydrocarbons having r:i thrr smaller molecular volumes t hail parafinh of thc hailif carbon iiuriiber, are slightly more rctained :ih indicated hy their lower activity POeficieiits. Benzene, on the other hand, though possessing :til even smaller molecular volume emerges before cyclohexane. This is mainly due to the loss of coohesire energy of the pure solute benzene molecules when in squalane solution. Solute-solvent behavior in the case of the polar
D. H. DESTYAND W. T. SWANTON
772
Vol. 65
0 PARAFFINS
1.2
El OLEFINS
A
NAPHTHENES 0 BENZENE 1.0
Br 0.8
MCH
"p;ld
' 0.6 c1
0
0.4
224 TMP
II
98
4
0.2
0
- 0.2
-0.4
-0.6
-0.8
- 1.0
-1.2
0
10
20
30
40
50
60
70
80
90
100
B.p., "C. Pig. 2.-Log (relative retention volumes) on isoquinoline at 20".
compounds aniline, quinolines and the substituted naphthalenes is complicated by the polarizing effect of the solvent on the solute molecule. The degree of polarity of the solvent and the polarizability of the solute determine for the most part the selectivitv achieved. Thus olefins. nanhthenes and aromatic hydrocarbons which are more readily polarized than paraffins are selectively retained. Selectivities amongst an isomeric group of paraffins will be less spectacular since differences in polarizability are small. The calculated electron polarizabilities for various hydrocarbons included in Table IV refer to unit volume and have been evaluated from the Clausius-Mosotti equation ,
112
ae =:
n2
-
1
3
+ 2 x4--x ?riV
dl
-
p
by dividillg by tile molecular volume 2-1 = n -
3
n2+2'4aN
I
The refractive indices (n) were obtained from AI'I Project No. 44. Owing to the fairly high cohesive energy of the stationary phase molecules and to the non-polar character of the solutes examined, the degree of solute-solvent interaction, solubility and therefore retention volumes are smaller than for squalane. The solutions exhibit positive deviations from
GAS-LIQUID CHROMATOGRAPHY
May, 1961
773
TABLE V ACTIVITYCOEFFIClIENTS
AMINE AT
Hydrocarbon
n-Butane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane 2,2,4-Trimethylperitane Hesene-1 Heptene-1 Octene-1 Cyclopentane Cyclohexane hlethylcyclohexane Ethylcyclohesane Benzene Toluene Ethylbenzene p-Xylene m-X ylene o-Xylene
a-NAPHTHYLAMINE, PHENANTHRENE VARIOUSTEMPERATURES
( y o ) ON SQUALANE, 7,8-BENZOQUINOLINE,
--Squalane65.5'
8l0
1Olo
-7.S-Be~~oq~i~oli~65.5" 81' 101'
0.51 .58 .61 .64 .67 .71
0.59 .61 .65 .69 .7l
0.61 .62 .66 .69 .72
5.19 5.51 5.96 6.41 6.98
5.14 5.35 5.72 6.14 6.57
.68 .61 .64 .G7 .44 .48 .51 .54 .60 .62 .67 .63 .64 .64
.70 .62 .65 .69 .46 .49 .52 .56 .60 .62 .67 .65 .65 .66
.70 .62 .65 .69 .46 .49 .52 .55 .57 .61 .66 .63 .64 .65
8.35 4.03 4.31 4.61 2.72 3.67 4.15 4.38 1.42 1.52 1.66 1.68 1.61 1.50
7.73 3.94 4.20 4.50 2.76 3.50 3.95 4.13 1.43 1.54 1.68 1.70 1.63 1.52
Raoult's law so that activity coefficients are consequently greater than unity and decrease numerically as the polarity of the solvent decreases from isoquinoline to l-methylnaphthalene. Besides providing selectivities among hydrocarbon types these stationary phases also provide an increase in the separation factor between is0 and normal paraffins. Thus the separation factor for n-hexane/2,2-dimethylbutane is 2.69 on isoquinoline (squalane 2.19 and the relative volatility of these isomers is 2.14) and for n-heptane/2,2dimethylpentane, 2.73 (squalane 2.45, relative volatility, 2.37). However, this selectivity cannot be wholly attributed to the differences in polarizability as these are small. Moreover the separation factors between is0 a,nd normal paraffins are found to be about the same with 1-methylnaphthalene as with isoquinoline despite the disparity in the solvent polarities. The somewhat smaller activity coefficients for the normal and less branched isomers provide a further indication that these isomers are being slightly more selectively retained. I n the light of the recent discovery concerning the formation of inclusion complexes of the methylnaphthalenes,l7 j t is possible that hydrocarbons are included within a solvent "cage," the selectivity due to inclusion being perhaps rather more evident in the case of the paraffins than for aromatic and naphthene hydrocarbons where the effect may be masked by the selective retention caused through the greater polarizing effect of the solvent, on the more polarizable molecules. Table v' shows the activity coefficients ( y o ) a t various temperatures for the solutes listed in Table 111. a-Naphthylamine, the most specific of the stationary phases examined gives the largest values of yo which for all the solvents (except (17) J. M k o m , J . Phys. Chsm., 63, 1843 (1959).
5.04 5.34 5.70 6 .OS 6.46 7.10 3.81 4.04 4.28 2.72 3.31 3.73 3.80 1.42 1.53 1.67 1.67 1.63 1.51
a-Naphthylamip 65.5' 81
14.01 15.31 17.81 20.91 24.02 24.67 8.23 9.28 10.75 5.34 7.51 9.26 10.54 1.93 2.27 2.64 2.82 2.68 2.45
12.49 14.13 15.97 18.29 20.83 18.19 7.80 8.82 10.11 7.05 8.51 9.58 1.93 2.26 2.64 2.78 2.65 2.43
AND
DIPHENYL-
Phenanthre2e 101
Diphenylamine 66'
3.93 4.02 4.20 4.36
5.40 6.16 7.00 7.97 8.94
5.27 3.04 3.18 3.30 2.23 2.68 2.90 2.92 1.35 1.40 1.52 1.48 1.44 1.37
8.66 3.96 4.49 5.09 2.84 3.37 4.12 4.66 1.16 1.35
squalane) are greater than unity. For squalane, general tends to increase slightly with temperature but for 7,&benzoquinoline and a-naphthylamine a rapid decrease for paraffins, naphthenes and olefins is observed while the values for the aromatic hydrocarbons remain fairly constant. The comparatively small change in yo with temperature for squalane indicates that the heats of mixing ( B s E ) are small and (since y < 1) negative and departure from ideality can be mainly attributed to a positive change in t,he entropies of
yo in
TABLE VI HEATSAND ENTROPIES OF MIXINGAT INF~NITE DILUTION Squa-
-7,8:Beneoqmnolme
-
AS&
AH$
A&",
lane Hydrocarbon n-Pentane n-Hexane n-Heptane n-Octane n-Nonane 2,2,4-Trimethylpentane Hexene-1 Heptene-1 Octene-1 Cyclopentane Cyclohexane Methylcyclohexane Ethyloyclohexane Benzene Toluene Ethylbenzene p-Xylene m-Xylene o-Xylene
e.u. kcd 4-1.05 t o . 99 + 0 . 6 3 .86 .77 .74 .88 .68 .93 .71 + 1 . 1 6 + 1 . 0 0 4-0.39 $0.86 .46 +0.74 .53 .... 4-1.55 C1.43 .75 $1.31 .76 4-1.16 +l.Ol +l.OZ 4-0.95
....
+ + +
+ + + +
+ + + +
+ + + +
.so .86 .86 .83
.... .... .... .... ...
ASS&,
e.u.
.... -1.58 -1.37 -1.27 -1.15
-0.80 -1.62 -1.57 -1.50
....
a-Naphthylanum
kcal.
....
-1.25 -0.99 -0.12 +6.40 -2.01 f1.01 - 1 . 5 0 f1.29 -0.95
....
4-1.06
-0.71
....
-0.86 -1.03 -1.06 -0.98 -0.85
e.u.
.... ....
f1.43 +1.61 +2.01 +4.32 $0.74
-0.38 .59 .03
+
AXE,
+1.38 f1.56
.... .... .... ....
....
-
.91 .3a .10 -1.31 -1.63 -1.94 -2.04 -1.95 -1.77
mixing For 7,8-benzoquinoline and anaphthylamine on the other hand for hydrocarbons other than aromatics are positive. For the aromatic hydrocarbons, however, where hHeE is small for 7,8-benzoquinoline and ci-
ALEXASDER I. I’OPOV ANI Iioam
774
naphthylamine the magnitudes of the deviations from ideality are determined mainly by the negative changes in n s E . Some values of Z s E and a s E for the three solvents are given in Table TI. The positive values of a s E for squalaiie $how that a greater ease of manoeuverability exists in this solvent, which decreases as the size of the molecule increases. The positive values of H s E for 7,8-benzoquinoline and a-iiaphthylamine occur as a result of the loss in cohesive energy between solvent molecules and the negative values of z s E are due to the solute molecules adopting a less random configuration. The value of AXsE for isooctane on a-naphthylamine is however positive and appears to be particularly large. The slightly larger numerical values of ASsE for p-xylene than for m- and o-xylene with these stationary phases indicates that a slightly more ordered orientation is imposed on the solute molecule. This may arise through steric hindrance caused by the terminal methyl groups of the pxylene molecule or through the manner in which both solute and solvent molecules are mutually polarized. With regard to the second aspect, it is interesting to observe that the order of elution and degree of separation of the CSC9 aromatic hydrocarbons achieved with both 7,8benzoquinoline and a-naphthylamine can be accounted for in the light of their electron polarizabilities. (Table VI1 shows the boiling point and electron polarizabilities of some of these hydrocarbons.) m-Xylene and ethylbeiizene having somewhat larger polarizabilities than p-xylene are selectively retained, the p-xylene now being difficult to separate from ethylbenzene. o-Xylene is eo well retained (ae” = 11.84 X that it
L).
HOLM
1-01, 65
emerges with isopropylbenzene (ae” = 11.50 X despite the difference in boiling point (ca. 8’). On 7,8-benzoquinoline, p-ethyltoluene and m-ethyltoluene are not resolved but severe peak broadening is evident. However, with the more selective a-naphthylamine a t 81’ the two isomers are partially resolved, the para isomer emerging first from the column despite its higher boiling point. This reversed order to that of their volatilities is due to selective retention of the methyltoluene, which may be attributed to its slightly higher polarizability. Again despite the closeness of the boiling points of 1,3,5-trimethylbenzene and o-ethyltoluene the latter is well retained for apparently the same reason (separation factor ca. 1.10 at 81’ on 7,8-benzoquinoline, relative volatility 1.003). TABLE
VI1
ELECTRON POLARIZABILITIES Hydrocarbon
Ethylbenzene p-Xylene m-Xylene 0-Xylene Isopropylbenzene p-Ethyltoluene m-Ethyltoluene o-Ethyltoluene 1,3,5-Trimethylbenzene
B.p., ‘C.
136 138 139 144 152 161 161 165 164
19 35 10 41 39 99 31 15 72
Electron polarizability per u n i t volume
11 58 X 11 54 11 61 11 84 11 50 11 56 11.60 11 75 11 65
Acknowledgments.-The authors wish to thank the Chairman and Directors of the British Petroleum Company for permission to publish the paper and Mrs. D. M. Irving who assisted with the experimental work.
ELECTRIC MOMENT OF 3-ETHYL-3-METHYLGLUTARIMIDE BY ALEXASDER I. POPOV~ AND ROGERD. HOLM Department of Chemistry, Northern Illinois University, DeKalb, Illinois Received September 23?1860
The electric moment of 3-ethyl-3-methylglutarimide has been found to be 2.84 D. in benzene solution. The structure and the direction of the group moments is discussed. It is shown that estimates of bond moments in such compounds are as yet inadequate. The H-h bond moment appears to be smaller than commonly supposed.
Introduction Barbiturates and hydantoins both possess anticonvulsant properties, and both share imide structures. Yet, 3-ethyl-3-methylglutarimide, (“Megimide,” Abbott Laboratories) while possessing a similar structure, is an effective antidote in cases of barbiturate poisoning. It was felt that the structural similarities and physiological activities of these compounds merited further attention, and that an investigation of the electric moments of these drugs may reveal significant differences in the electron distributions of certain bond systems associated with the observed activities. A survey of the literature reveals that exceed-
ingly little information is available on the dipole moments of imides. Accordingly, a study of the dipole moment of Megimide was undertaken as part of a general investigation of dipole moments and physiological activity. and as a means of obtaining information concerning the electron distribution in imide structures. Experimental Part
(1) Department of Chemistry, Northern Illinois University, De-
( 2 ) C. P. S m s t h and W. S. Walls, J. Am. Ckem. Soc., 64, 1854 (1932).
Kalb. Illinoi..
Reagents.-Thiophene-free benzene was purified by shaking it with concentrated sulfuric acid, with water, with sodium carbonate solution, and then again with distilled water. Following the extractions benzene was dried, distilled from PzOj, fractionally crystallized twice, again distilled from P 2 0 6 and then refluxed over sodium ribbon and distilled as needed, n Z 61.4980, ~ lit.2 1.4981.
