July 5, 195G
SOLVENT EFFECTSIN GAS-LIQUIDPARTITION CHROMATOGRAPHY
in the a-functions. The error in C Y Aappears ~ to be about *0.01 unit in the region XcU= 0 to Xcu = 0.4 and to vary approximately linearly thereafter to h0.02 unit a t XcU = 1. The error in acu appears to be hO.01 unit from XcU = 0 to about XcU = 0.7 and to vary approximately linearly thereafter to =tO.O5 unit at XcU = 1, if the nature of the extrapolation has been correctly assumed. The molar enthalpy of mixing is found to be - 2,720 rt 200 cal. a t XcU= 0.5 which is about 800 cal. more negative than Oriani reported and con-
2989
siderably more negative than the calorimetric value of zero which Kawakami12 obtained. The general agreement of our work with that of Oriani leads US to discount the calorimetric value. The partial molar enthalpy of mixing of Cu for the solutions infinitely dilute in Cu is found t o be -10,500 =t 700 cal. The partial molar enthalpy of Au for the solutions infinitely dilute in Xu is found to be -7,290 =k 700cal. (12) M Kawakaml, Sct. Rep Tohokic lmz5 C n i o , 19, 521 (1930)
CHICAGO15, ILLISOIS
[CONTRIBUTION FROM THE SHELL
DEVELOPMENT COMPANY]
Solvent Effects in Gas-Liquid Partition Chromatography BY G. J. PIEROTTI,
c. H. DEAL,E. L. DERRAND P.E. PORTER RECEIVED DECEMBER 9, 1955
For the purpose of considering solvent effects in GLPC, the partition coefficient (HO) is discussed in terms of the vapor pressure of the solute (PO)and the thermodynamically defined activity coefficient of solute in the solvent ( y o ) . This separation of effects leads to a clear understanding of the interrelations of solute and solvent behavior in GLPC operations and provides the basis for exploiting relations which have been found in direct studies of solutions. This in turn leads to a number of observations which are useful in practical GLPC work. The more important of these are: (1) At a given temperature and within an homologous series of solutes the logarithm of the ratio of partition coefficientsin solvents mag be expected to be a linear function of the solute carbon number. The slopes of such plots for different homologous series may be expected t o be the same. ( 2 ) Over the restricted ranges in carbon number of interest in a given GLPC separation and except for the first several members of an homologous series, log HO for the members of an homologous series may be expected to be linear in carbon number. This observation is closely related to ones already appearing in the literature.
James, Martin and other workers in the have discussed solvent effects in gas-liquid partition chromatographic (GLPC) separations in terms of the fundamental nature of the solute-solvent interactions which occur in the liquid phase and in terms of regularities in GLPC behavior observed among the members of homologous series. A companion paper6to the present one has shown that the solvent supports usually used are substantially inert and that GLPC behavior can be largely accounted for in terms of the deviations from' ideal behavior in the bulk liquid phase. It is the purpose of the present paper to discuss GLPC solvent effects in more empirical but more quantitative terms than hitherto has been done, exploiting for this purpose observations drawn from direct studies of the non-ideal behavior of solutions. In the present paper the partition coefficients which characterize GLPC behavior are considered in terms of fundamental solution parameters, Useful and generally applicable relations between solute activity coefficients and structural'groupings of solute and solvent are outlined. These are applied in order to illustrate the interplay of solute and solvent properties in setting GLPC behavior in several cases. Useful methods of treating GLPC data are outlined and illustrated.
where M , is the reciprocal molar volume of the solvent, yo is the activity coefficient of the solute a t infinite dilution in the solvent, Po is the vapor pressure of the solute and RT has its usual meaning. Equation I leads directly to an expression for the relative partition coefficients of two solute materials, 2 and 1, or to the relative volatility of solute 1 to solute 2 , a t infinite dilution in the solvent
T.James and A . J . P . M a r t i n , Biochem. J.. 80, ti79 (1952). ( 2 ) A. T. James and A . J. P. Martin, Bjitish M e d . Bull., 10, 170 (1954). (3) A. T.James, A. J. P. Martin and G. Howard-Smith, Biochcm. J., aa, 23s ( 1 ~ 5 2 ) . (4) A. J. P. Martin and R. L. M. Synge, ibid., 10, 532 (1943). ( 5 ) A. J. P. Martin, Biochem. SOC.Symposia No. 3, 4 (1951). (6) P. E. Porter, C. H. Deal and F. H. Stross, THISJ O U R N A L , 78, 2999 (1956).
This is identical with the separation coefficient normally used to measure the ease of separation of two components by distillation. Although the number of plates in a column of given geometry varies from solvent to solvent as a result of differ-
Experimental Partition coefficients from GLPC measurements have been determined as described in the previous paper for a number of additional solutes in the two solvents, diisodecyl phthalate and squalane, and in a more polar solvent, Triol. Partition coefficients are shown in Table I ; for completeness some of the data presented in the previous paper have been repeated.
Discussion GLPC Partition Coefficients and Fundamental Solute-Solvent Parameters.-As indicated in the previous paper,6 the partition coefficient determined from GLPC data is essentially that operative when the solute is a t infinite dilution in the solvent phase. In terms of fundamental parameters of the solute and solvent, this may be expressed as
(1) A.
2990
c>.J. PIEROTTI, c. H. DE\I.,
E . .L. DERRAXD P. E. PORTER
Vol. 7 s
TABLE I tude of partition coefficients and through its effect For a given set of solute AVERAGEPARTITION COEFFICIENTS FROM GLPC MEASURE-on relative volatilities,
components to be separated, however, relative volatilities are fixed insofar as Po effects are concerned. Diisodecyl Po is generally the most strongly temperature deSolute “Triol”a phthalate Squalaneb pendent of the above quantities, and changes in %-Pentane 16.3 temperature strongly affect the magnitudes of parn-Hexane 27.0 34.8 11 0 tition coefficients through Po Relative volatilities n-Heptane 57.6 72.9 21 0 are not as strongly affected by changes in temper%-Octane 121 30 4 ature insofar as PO effects are concerned because the n-Wonane 240 56 4 heats of vaporization of solutes are frequently 2,2-Dimetliylhu tarie 19 5 roughly comparable. 2,4-Dimethylpentatie 3.;. 2 ( 3 ) yo is the property of solute and solvent i n 2,3-DimethylIieuane 73.3 teims of which solvent effects can be properly ex2,4-Dimethq hexane 89.7 pressed. This quantity affects separation both 2,2,5-Trimethylhexane 93.9 through the magnitude of partition coefficients and Meth ylcyclopentane 38.2 through relative volatilities. lVith a given set of Cyclohexane 52.3 60.1 solutes for separation, i t is only by changes in yc’s hleth ylcyclohexane 97.4 (83.5) brought about by changes in solvent that large Ethylcyclohexane (180) changes in relative volatilities of the solutes can be 1-Hexene 26.9 10.5 made. yo is not as temperature dependent as PI’, 2-Heptene 64.6 18 9 and relative volatilities are usually even less tem1-Octene 119 27 5 perature dependent insofar as yo effects are conToluene 144 cerned. As a result, unfavorable effects on the hlethanol 12.6 magnitudes of partition coefficients brought about Ethanol 20.9 82 5 by the use of a solvent which provides a suitably 1-Propanol 46.2 138 favorable relative volatility can be compensated to 1-Butanol 107 264 a large degree by changes in the temperature of 2-Propanol 81.5 26.1 operation. 2-Butanol 173 77.4 I n order to provide some basis for judging the 2-hlethyl-2-butanol 128 73.5 magnitudes and directions of the solvent effects 3-Methyl-2-butanol 105 which may be exploited in GLPC separations, pri2-Methyl-1-butanol 154 mary attention will here be given to relations be3-Methyl-1-butanol 161 tween T O values and the structures of solutes and 2-Methyl-2-propanol 72.5 30.8 at a given temperature. The magnitudes solvents 2-Methyl-1-pentanol 317 of temperature effects on the quantities considered 2-Methyl-2-pentanol 188 here have already been touched up011 in the coni4-Methyl-2-pentanol 180 panion paper.6 3-Methyl-3-pentanol 142 Some Relations between Activity Coefficients 2-Hexanol 241 and Structural Groupings.-The essential natures 3-Hexanol 239 of such molecular interactions as van der Waals 19.7 32.2 Acetone interactions, dipole-induced dipole, dipole-dipole, 2-Butanone 52.4 42.5 hydrogen bonding, etc., are well understood and 3-Pentanone 91 86 1 have been discussed el~ewhere.~Quantitative esti3-Methyl-2-butanone 69.5 mates of their effects on the non-idealities of solua Triol is a product of Union Carbide and Carbon Co. which is 2,4-dimethy1-2-( 2-liydroxyethoxy)-pentane-l,5-diol. tions can be made on a fundamental basis in only a few special cases; however, qualitative ideas along * Squalane is 2,6,10,15,19,23-hexarnethyltetracosane. these lines are useful when viewed within the frameences in the physical properties of solvents, gener- work of experimental quantitative data and have ally the magnitudes of the partition coefficients, been discussed by Martin and others.‘-j which largely determine plate capacities, and the The Scatchard-Hildebrand relation’ permits a relative volatilities of components being separated, fairly good estimate of yo for the special class of which determine the relative rates of band move- regular solutions from the energies of vaporization ment, are the most important quantities affecting and molar volumes of solute and solvent. This reseparation. lation is not, however, applicable to solutions in Before proceeding to a detailed discussion of sol- which one or both components are polar in nature, vent effects, several general observations regarding and more empirical methods must be resorted to. the above quantities may be noted explicitly. Pierotti, et a1.,8 in their direct solution studies have (1) -Ifsis purely a solvent property. Although determined yo’s for several hundred polar systems its magnitude affects GLPC separation through the so chosen that the variations within homologous magnitudes of the partition coefficients, i t is clear series of solutes and solvents can be systematically from I1 that i t is not this solvent property that reviewed. -4lthough many of their experimental directly affects relative volatilities. The variation data are not concerned with solvents of interest in of Ms with temperature is negligible in comparison (7) J. H. Hildebrand and R. L Scott, “The Solnbility of Nonelecto other temperature effects. trolytes,” A C S Monograph Series, So.17, 3rd E d . , N e w Y o r k . K. Y , ( 2 ) Pu is purely a solute property. It affects 1950. GLPC separation through its effect on the magni(8) Pierotti, et ai., in preparation. MENTS
Solvent
July 5, 1956
SOLVENT EFFECTS IN GAS-LIQUIDPARTITION CHROMATOGRAPHY
2991
c GLPC separations for volatility reasons, the reguR+-----+S larities of behavior noted are broadly applicable and can be exploited to advantage here in setting the magnitudes of solvent effects which can be achieved. Pertinent aspects of this work which is only now being prepared for publication will consequently be outlined here. Pierotti and his co-workers have developed a R ’t--------f X’ semi-empirical model from experimental data which f follows along lines very similar to the treatment of Fig. 1. homologous series of Br#nsted and K o e f ~ e d , ~ Butlerlo and Copp and Everett.” It considers the are constants which are characteristic of the solvent excess partial molal free energy of solution for functional group and apply to an entire homolothe limiting case of solute a t infinite dilution in the gous series of solvents, and A is characteristic of solvent (;.e., R T In y o ) as the sum of individual both the solute and solvent functional group. terms, each of which can be identified with the For the case in which the structure of a solvent is various characteristic groupings of the solute and/or held constant and that of the solute is varied within solvent structure. The essence of the model can be an homologous series, n’ is constant and n is varimost easily described as follom. able. In this case, terms “a” and ‘If” are constant For a mono-functional solute R-X in a mono- and in many homologous series further sirnplificafunctional solvent R’X’, where the R’sare alkyl tions arise. For n-alkanes in polar solvents the groups and X’s are “functional” groups (polar c term does not occur and the expression for log y o groups as -OH or groupings which it is convenient becomes to consider as units as phenyl or naphthyl groups), log yo = K, B ( n / n ’ ) D ( n - %’)* = K , six free energy terms associated with six net interB,n D ( n - n’)* ( I V ) actions of structural groupings can be imagined. where K , includes all constant terms and B , measThese are shown in Fig. 1. The variations of these terms with structure ures the net molecular weight effect of the polar within homologous series are now determined by group per methylene of the alkane, the differences systematic variation of the R’s and X’s, choosing between methyl and methylene groups being negcomponent pairs of increasing complexity and ap- lected. For n-alkylbenzenes the phenyl group is plying the terms of each case to the next most com- considered a functional group and a c term occurs. plex case. Thus, the simplest case is R-H in In this case the expression becomes R’-H,terms a, b, c, e, f are found to be zero or log y o = KAB B,(n - 6) C / ( n - 4) negligible, and an analytical expression for the D(n - x ’ ) ~ (V) term d is derived from a sufficient number of exwhere KAB includes all constant terms, n is the total amples. Having determined an expression for the carbon number including the phenyl group, B , is as “ d ” term, the next most simple case of R-H in R’X’ is considered and so on. From such a sys- before, and n- 6 measures the number of methylene tematic study Pierotti, et al., have shown that limit- plus methyl groups in the molecule. Examples showing the magnitudes of ?“-effects ing log y can be expressed in five terms having the for the above “type” homologous series over a following forms broad range of carbon number with a broad range a:A of solvent polarities are given in Figs. 2 and 3 . h:R nln’ yo’sare here described to within about 5% of their r:C/(n C‘) experimental values for representative solutes in d : D ( n - n’)2 (111) the range up to C ~ and e :. . . . . . . . . . O there is little doubt that f:F/(n‘ F’) interpolations and extrapolations for solutes up to ~ equally accurately described. It is where the upper case letters are constants, where about C S are n and n’ are the carbon numbers of the solute and of particular interest that moderate branching in R solvent, respectively, and where the e term is small or substitution on an aromatic nucleus modifies or can a t least be taken into account in other terms. the -yo’s by only a few per cent. from the value for The form of the “d” term is taken directly from the corresponding straight chain or normal alkyl Bransted and K o e f ~ e d . ~As is apparent from the compound. Coefficients for the above correlations way in which the structural parameters are defined a t a temperature in the temperature region freabove, D is a universal constant which is independ- quently of interest in GLPC separations are shown ent of both solute and solvent structure, C and C’ in Table I. A s already indicated the D coefficient are constants which are expected to be characteris- is structure invarient, the B , coefficient is solvent tic of the solute functional group and apply to an characteristic, the C coefficients are solute charac~ both solute type entire homologous series of solutes,12 F , F‘ and B teristic, and the K , and K A are and solvent characteristic.l 3 (9) J. N. Br0nsted and J. Koefoed. Kgl. Danskc V i d . Selsk. M e d d . , Two factors relating to the relative solvent ef22, 1 (1946). fects for two homologous series or “types” of (10) J. A. V. Butler, J. Chem. Soc., 280 (1935). (11) J. L. Copp and D. H . Everett, Disc. Faraday Soc., 268 (1955). solute in a given solvent will be particularly noted (12) For t h e more complicated case in which t h e solute structure by the comparison shown in Fig. 4. First, except has the form RiRzCHX, the “c” term has t h e related form C ( l / n l +
+
+
+
+
+
+
+
+
+
l / m ) where by one.
ni
and nt are t h e carbon numbers in R I and Rz increased
(13) For more polar solutes the C coefficients are somewhat solvent dependent b u t usually are not critically so.
G. J. PIEROTTI, C. H. DEAL,E. L. DERRAND P. E. PORTER
2992
Vol.
'is
4.
e'
r
0
r
M
M
+
3.
i
2.
1.
0.
-1.0
I
5
l
15 20 25 Carbon Number of Solute.
10
l
30
Fig. 2.--n-Alkanes in various solvents: solvents: 1, n-heptane at 90"; 2, 2-butanone at 90'; 3, phenol at 90"; 4, triethylene glycol a t 90"; 5, water at 25'.
6.0
5.0
4.0
0;
3.0
10 15 20 25 30 Carbon Number of Solute. Fig. 4.-Comparison of activity coefficients in triethylene glycol: solvents: 1, n-alkylbenzene; 2, n-alkanes 5
parallel with slopes which are characterized primarily by the coefficient B,, but also are affected somewhat by the D term when broad carbon number ranges are considered. These two effects have been observed over very broad ranges of solute types and solvents. They may be expected in virtually all solutions except in those in which a strong chemical interaction occurs between the t vvo solution components. For the instance in which the solute is held constant and the solvent is varied within an homologous series, the above formulation gives for the activity coefficient of the solute log yo = [ A
+
F
I/:[
+
72
+
M 3
+'C' + U(?Z -
.~
2.0
It
1.0
0.0
I
5
10
I
I
I
15
20
25
I I
30
Carbon Number of Solute. I'ig. 3 -Alkylbc.iizenes in various solvents: solvents: 1, n-heptane a t 90"; 2, 2-butanone a t 90"; 3 , phenol a t 90"; 4, trietliylcne glycol at 90'; 5 , water a t 25'.
for the first series members (c term important), the two plots are displaced with respect to one another by a n amount which may be associated with the differences in the K , and K . ~ B . Second, except in the first members, the two curves are substantially
12.')'
(Vl)
where the square brackets enclose terms which were held constant in the discussion just preceding. Examples of the relation of these terms to solvent carbon number are shown for the case of alkanones as solutes in primary alkanols as solvent are shown in Fig. 5 . Here the terms have been based on ten systems involving alkanones from propanone to heptanone-3 and alkanols from methanol to 1octanol. It is clear that K and B , coefficients of the preceding discussion become smaller as the carbon number of the solvent is iiicreasetl (over-all polarity decreased by dilution with iriethylene groups). In terms of the plot iii Fig. 4, this results in a decrease iii the displacenient of the two curves and in a lowering of their slopes and has irnportant bearing on the "type" arid molecular weight selectivities of solvents. Some Relations between Solute Vapor Pressure and Structure.-Cseful regularities m,iy also be observed in the Po'sof solute coinponents :it coiistuit
July 5 , 1956
SOLVENT EFFECTS IN GAS-LIQUID PARTITION CHROMATOGRAPHY
2993
3.
2. 8
-
c
.u d
2.6-
4 0 M
ad
a
2.4-
3 E
2. 2 . 2 a
-
M
0 . 2 L
2.0-
1. 8
-
1.
2
1
I/n’.
Fig. 5.-Effects of carbon number of solvent; n-alkanones in alcohol solvents; n’ = carbon number of solvent.
temperature as their structure varies, though the structural changes which are important to Po are in some cases different from those important to yo’s. Plots of log Po as a function of carbon number are shown in Fig. 6.
Carbon Number of Solute. Fig. B.--Vapor pressures of homologous series members: 0, n-alkanes; b , 2-methylalkanes; A , n-alkylbenzenes; V, n-alkylcpclopentanes; E, n-alkylcyclohexanes; 0, 1alkenes; 0, 1-alkanols; -0-,2-alkanols; 0, 2-alkanones.
ities, and in many cases detailed configurational effects become important. Examples illustrating the consistency of contributions for methyl groups and for hydroxyl groups using the same series of TABLE I1 compounds as in the above are shown in Tables CONSTAXTSFOR ACTIVITYCOEFFICIENTCORRELATIONS IV and V. In the case of the methyl group conAlkane and alkylbenzene solutes--Tritributbn to the log Po of paraffins, an average 2-Butaethylene Water SolHeptane Phenol glycol none value of about 0.25 holds with about the same prea t 25’ a t 90° a t 90° vent a t 90” a t 90° cision as in the methylene case. It is of particular KP 0 0.330 0.620 0.680 0.69 interest that moderate branching is much more KAB 0.202 0.239 ,420 ,730 3.55 important to Po than to yo. I n the case of the conBP 0 0.019 ,0581 ,134 0.63 C -0.350 -0.350 -0.360 -0.350 (0.466) tribution to the log Po of alkanols the average C’ -4 -4 -4 -4 -4 value is somewhat lower and cannot be applied D -0.000~ -0.oo06 -0.noo6 -0.oo06 ..... with very good precision, particularly if detailed Within homologous series, particularly over the positional factors are not taken into account. In fairly limited ranges of carbon number which are the case of the contributions of hydroxyl groups to usually of interest in a GLPC separation, the Po’sof log Po,an average value of about 1.45 is obtained successive members of the series differ by a roughly when the alcohol formed is primary, about 1.08 constant factor. This factor, which corresponds when secondary and about 0.95 when tertiary. to the increment log Po (Le., the free energy of These values are, however, not very precisely mainvaporization) per methylene group added to the tained, particularly over the first series members for structure, frequently varies somewhat in the first which the data are available. several members of a given series, but fairly rapidly Some Relations between Partition Coefficients approaches a constant value for a large number of and Structure.-In order to illustrate the interplay homologous series. Examples derived from vapor of solute and solvent properties which can be expressure data of the American Petroleum Institute, ploited in GLPC separations the partition coeffiProject 44, which illustrate this effect are shown in cients for two “types” of materials, n-alkanes and Table 111. Here, the contribution to log Po per n-alkylbenzenes, have been estimated on the above methylene group varies between the extremes of basis for several solvent cases. These cases are about 0.2 and 0.5. If, however, the first one or two those of (1) a hypothetical solvent which forms series members are excluded, the contribution to log ideal solutions with all components, ( 2 ) a non-polar P o is about 0.35 to within about 0.03 in log Po or solvent (C30 paraffin) which gives negative deviaabout 77’ in Po itself, even over the wide range of tions from Raoult’s law (yoI-2-alkanoli 2,3-Diiiiethyl-l-alkallolz a First meiriber of series
3
HOMOLOGOUS S h K I h S , 85'
105 P:
-~ 4
0.46"
- log P i
0.40 .39
0.359 ,381 ,378" .366 .406"
,307"
,328 .336 ,275' .373"
0 364 ,294"
1
6
.47"
0 315
+
J
.354" ,412 ,260" ,406"
7
8
9
0.389 ,374 ,368 ,362 ,347 ,381" ,341 377 ,416 ,358
0.264 ,346 .366 ,362 .36 1 ,339 ,352 ,369 ,339 ,334
0.366
IO
3r; 7 368 3G2
.380 ,372 ,373
I363 ,348 ,299 ,306 Xf;" . "88 ,
. 32,j,a
n - .n .c>Jo
3 72"
TABLE IV CHANGES I S SOLUTE \.'APOR
PRESSURE WITH METSfYL
Carbon number, n
>
1
3
GROUPh D I T I O X , 83' log P: - log P:-+ CHa 0.31"
Alkanes -+ 2-methylalkanes Alkanes -+ 3-methylalkaiies Alkanes -+ 4-metliylalkanes 2-hlethylaikanes -+ 2,2-dimethylalkanes 2-Methylalkanes -+ 2,3-dimethylalkanes 2,4-dimethylalkanes 2-Methylalkanes 1-Alkanols -+ 2-methyl-1-alkanols 1-Alkanols -+ 3-methyl-1-alkanols 1-Alkanols -+ 4-methyl-1-alkanols 2-Methyl-1-alkanols .--L 2,3-dimethyl-l-alkanols 2-Alkanols +. 3-methyl-2-alkanols 2-Alkanols -+ 4-methyl-2-alkanols 2-methyl-3-alkanols 3-Alkanols 2-Alkanols 2-niethyl-2-alkanoIs 3-Alkanols -+ 3-methyl-3-alkanols 2-R.Ietli~-l-l-alknriols 2 , 2 - d i i n e t I i y l - l - a l ~ ~ i i i ~ l ~ First ineniber of series.
3
6
0.30
0.177 ,1301"
4
. 26"
.27 .2G"
-+
-+
-+
-+
TABLE
l-
CHASGEIX SOLUTE VAPORPRESSURE WITH HYDROXYL GROUP AI)I)I.I.IOS,85' loi: P i - log P," + OH-*7
Carbon n u m b e r ,
iz
Alkaiies 1-alkanols 2-Methylalkanes -+ 2-meth~l-l-alkaiioIs :3-llethylalkanes 3-met hyl-1-alkanols 2,2-Dinietliylalkaiie -+ 2,2-dinietli?.l-l-alkanols 3,3-Diniethglalkate -+ 3,3-dinietIiyl-l-alk~iiols Secondary O H Alkane -+ 2-alkauols 3-hlethylalkaiie -+ 3-inetliyl-2-alkaiiols 4-Methylalkane +. 4-methyl-2-alkanols 3,3-Diniethylalkane + 3.3-dimethyl-2-alkaiiols 2-Methylalkane + 2-niethyl-3-alkanol Tertiary OH 2-Methylalkanes +. 2-methyl-2-alkanols 3-Methylalkanes -t 3-rnethyl-3-alkanols 2,3-Diinethylalkane 2,3-diiiietIiylalkaii~~ls First member of series. -+
1
7
3
4
1
1.615 1.56"
1.510 1.47
1.2!)
I .20 (1.17J"
-+
-+
7
--
8
July 5 , 1956
SOLVENT EFFECTS I N
GAS-LIQUIDP.4RTITION
CIIROhI,ZTOGK.\€’l1Y
2‘,\!I
*-,
large positive deviations froin Raoult’s law (7” with ordinary columns and sample sizes, provided >>1). Log Goversas carbon number is shown in the relative volatilities are large enough. The absolute magnitudes may, of course, be increased Fig. 7. by lowering the temperature somewhat without affecting the relative magnitudes for the various components significantly. In the case of the polar solvent yo value effects have become quite significant in relation to Po effects and result in a considerable change in the basis of separations possible, Insofar as the relative volatilities for successive carbon numbered components of a given type are concerned, the idcal relative volatility has been reduced from roughly 2.25 to about 1.7. Insofar as the relative volatilities of types are concerned the ideal factor of about 1.23 has been increased to about 9. As a result of both the decrease in slope and the relative displacement of the log N ocurves for the two “types,” the carbon number spread between the members of the “types” which cannot be separated has been increased from 0.3 to about 5. The solvent may thus be said to be “type” selective. It lnay be noted that the general magnitudes of the H o values are markedly lower than those of the ideal case. This results almost completely from the large y” values involved, AIs in this case being about T as compared to 10 for the ideal case taken. The Ho values, at least in the cases of the lower numbered hydrocarbons, are rather low and depending Carbon Number of Solute, on the details of apparatus may be some%hat low Fig. 7.-Approximate Havalues for several solvent cases: for reasonable resolution with ordinary sample P, alkanes, A, alk5lbenzenes; 1, ideal ( M a = 10) solvent; sizes and charging volumes. As is generally the case, the magnitudes of the H v o alues may be in2 , C30paraffin solvent, 3, triethylene glycol solvent. creased by lowering the temperature. Insofar as In the ideal solvent case, differences in H o y o effects are concerned, this will result in some invalues arise purely from differences in the vapor crease in “type” selectivity and a slight increase i n pressure of the solutes, and separations with such a molecular weight selectivity, corresponding to an solvent might be said to be based purely on solute increase in both “ K , - KAB”and in B),, Po or boiling point. I t may be noted that alkanes cannot be separated from aromatics with carbon numbers about 0.3 unit higher, that the relative volatilities for alkanes and aromatics of the same carbon number are about 1.25 and that the relative volatilities of successive carbon numbered alkanes or aromatics are about 2 . 2 3 . The magnitudes of the He values being greater than 100 for all cases shown, all of the component pairs with relative volatilities greater than about 1.1 or so could be reasonably well resolved in a normal column using normal GLPC charge sizes and charging volumes. I n the case of the paraffinic solvent i t is apparent that, insofar as the Ho values of successive carbon numbered hydrocarbon solutes are concerned, Po effects predominate and separations are substantially according to boiling point. Insofar as the separations between “types” are concerned, however, the Po effects have been substantially overcome by yo value effects with the net result that on the average alkanes and aromatics of the same carbon number can no longer be easily separated. The magnitudes of the N o values are lower than in the ideal case. y o value effects tend to increase the magnitudes ( y o < l ) in this case; however, the M. 3 -0.4 factor is low (-1.9) compared to that assumed for 2 4 6 8 the ideal case (-10) and lowers the magnitudes by Carbon Number of Solute. a constant factor. The magnitudes are still, however, large enough to give reasonable separation Fig. 8.-Alkanes in squalane solvent.
-
2996
G. J. PIEROTTI,C. H. DEAL,E. L. DERRA N D P. E. PORTER
Vol.
is
Plots of log Haveysus carbon number and of log MsRT/HnPaversus carbon number for various homologous series from the three practical GLPC solvents tested in the present work are shown in Fig. S through 11 and further illustrate the effects discussed above. In the first case, that of normal 2. 6,
I I 6 0 Carbon Number of Solute. I
0
2
I 4
I
Fig. 11.-%-Alkanes, 1-alkanols and alkanones in Triol solvent: 0, n-alkanes; A , alkanones; n, I-alkanols
6
8
10
Carbon Number of Solute.
Fig. 9.-Alkanes
in diisodecyl phthalate : 0, straight chain : 0,branched chain.
1.0 0
4 6 8 Carbon Number of Solute.
2
1
Fig. 10 --n-Alkanes, 1-alkanols and alkanones ill Triol solvent; 0, ??-alkanes; C, alkanones; A, 1-alkanols.
alkanes in squalane solvent a t loso, the plots are substantially linear as is to be expected from the yo value and Po value relations, The apparent activity coefficients in squalane are somewhat higher than would be expected for the solutes a t infinite dilution in a normal paraflin of the same molecular weight (for n-pentane 0.36 as compared to an expected value in the region of 0.42). This may result in part from the relatively high concentrations encountered in the liquid phase and in part from a consistent error in the amount of solvent in the column used in the determination. I t is of interest that the temperature coefficients for these solutes are reasonable in order of magnitudeE and that the apparent rate change of lor yo with carbon number (ca. 0.04) is about that expected (-ZD(n -?z') = 0.029). In the case of diisodecyl phthalate, the rz-alkanes show a regular variation of log IIn with carbon number, but the branched chain alkanes haVe lower partition coefficients corresponding to their higher Po's. The log yo s, however, are substantially identical with the corresponding carbon numbered iiormal alkanes as expected from direct solution studies. It is of interest that as a result of the large carbon riuniber of the solvent, the b tern1 would be expected to be small (large n' in Hir n ' ) . This is apparently the case since the apparent rate change of log JI,RT,'IIoPO with carbon iiuniber 15 small. In the ca,e of the Triol solvent, three "types" of substances are shown, n-alkanes, alkanones and alkanols. As is to be expected simply on the ha^ of differences in polarity and hydrogen bonding, the alkanols form the inore ideal solutions, alkanones have moderate positive deviations froin Raoult's law, and the normal alkancs have large positive devid-
July 5 , 1956
SOLVENT EFFECTS IN GAS-LIQUIDPARTITION CHROMATOGRAPHY
299 7
tions. With this solvent, in contrast to the case with diisodecyl phthalate, “type” differences are sufficiently large to stand out over small inconsist, 0.6encies of the data, and the apparent activity coeffic cients have been correlated. Since the range in carbon number is small the d terms have been negv lected, the B, determined by the slope of the n0.4 32 paraffin curve, and in the cases of the alkanones and alkanols c terms and a terms determined from the 3 three data points. In the latter cases the C’S are ; 0.20 determined as the best slopes of a log MaRT/HoPo + -Bn/n‘ versus l / n plots and the K’s as the intercepts. It is clear that the available data are ade0.0quately expressed by the relations derived. It may a be noted that “B,” coefficient for Triol is about the h z same as for triethylene glycol (see Table 11) as P 2 -0.2 might be expected since the “atomic” concentration of the polar hydroxy groups is about the same v as in triethylene glycol (3/12 as compared to ”7). I n addition to the above, several methods of ex% -0.4 M tending GLPC data for homologous series which do not explicitly separate y o and Po effects follow directly from the relations discussed above. The -0.6 first and most broadly applicable of these eliminates Po effects and some of the less convenient terms 1 2 3 4 5 6 7 8 9 10 in the yo relations. Consider a given homologous Carbon Number of Solute. series of solute components in two different solvents, A and B. For such a case, the activity coefficients for the members of any homologous series may be Fig. 12.-Comparison of solvents: 1, 1-alkaIiols; 2, alkanexpressed by ones; 3, n-alkanes; 4, alkenes. log ~ r = k A.4 Ba (n/’n.k) C / ( n C’) experimental points scatter somewhat about the Fa/(nL FA) +D(% - n’)2 four parallel lines drawn in, but the general pattern (VII) 1 l B f BB(n/nh) C/(n c’) of behavior is well founded. log Y i W(nA 4 ) D ( n - n’)Z Other methods of extending data may be based It will be recalled that in such expressions the on more extensive approximations, and on the basis F coefficients depend only on the solvent and the of experience from direct solution behavior studies C coefficients do not depend on the nature of cannot be expected to be as broadly applicable as the solvent critically. I n addition, the D coefi- the foregoing case. Frequently the b terms of the cients are small. As a result, the difference in activity coefficient expression predominate over the the log yo’s of the members of an homologous other terms in characterizing the change in yc with series may be written to a good approximation solute carbon number, particularly if first series members and broad ranges of solute carbon numlog ( Y i / Y i ) = Ki f (BA/nk - BB/nh)n (lrIII) bers are not considered. In this case for two solwhere the difference in the terms involving F’s have vents, A and B, the yo’s of the members of an been included in K ’ , where the terms involving homologous series can be expressed C’s cancel out, and the differences in terms involvlog 71 K.; (BA/n.;)% ing D have been neglected. From VI11 and I, the (XI log ?’\ = Kb (BB/nB)n differences in log HO’s in the two solvents may be written where the K’s include all constant terms. I n this case, plots of log Ho - log Po are expected to be log H:/H: = log ( & / J I ~ ~+ ) k” + linear in carbon number. This of course, differs (BA/ni - BB/%h)n (Ix) from log y o plots discussed above only by the conwhere Po’s have cancelled out exactly. From IX stant log ;Zf,RT. i t may be seen that a plot of the differences in log If first series members are not considered, log Po H oof the corresponding members of an homologous is also frequently linear in carbon number withiu a n series for the two solvents may be expected to be homologous series linear in the carbon number of the homologous selog Po = K pn (SI) ries members. In addition, since IX involves only the solute independent “B” coefficients, the slopes From I, X and X I of such plots for a given pair of solvents may be ex-log H.: = KX (6 B.A/~A)~ pected to be the same for all homologous series. -log H i = K i + ( p + B ~ / n j $ ) n (r;II) This provides a very good basis on which to estimate partition coefficients when only incomplete This is equivalent to the plots shown by Littlewood, data are available. Phillips and Price14 for their V g . For large partiPlots of this type are shown in Fig. 12 for several (14) A. B. Littlewood, C. S G. Phillips and D. T. Prize, J . CIwnz. alkanes, alkenes, alkanols and alkanones. Here the SOL.,1480 (1955). 4
:
.r(
.-(
2 4
s .r(
4
1 1
+
+
+
+ +
+ + + + +
+ +
+
+ +
299s
( > .J .
e.
PIEROTTI, H. DEAL,E.L. DERRA N D
tion coefficients and retention volumes, ;.e., log ( ,’!I HoVs)N constant log (HOVs),it is related to the linear plots of log Vk versus carbon number shown by Ray.lS It should be noted, however, that such plots for different homologous series can be expected to be parallel only when the corresponding Po curves are also parallel. The slopes for different solvents are in general different. Eliminating n from XII, one obtains
+
+
Here the only term on the right that changes from homologous series member to homologous series member is the one involving I€:. Since the coefficient of log J?;is constant within the hornologous series, it is clear that a plot of the logarithms of the partition coefficients of the members of an homologous series for one solvent against those of a second solvent is a linear plot. As long as log PO plots for different homologous series have the same slope (p), such plots for different homologous series are parallel. Examples of such plots are shown in Fig. 13. In these cases, the curves are parallel.
log Hn (Triol).
Fig. 13.-Comparison of solvents: 1, 1-alkanols; 2, secondary alkanols; 3 , tertiary alkanols; 4, alkanones; 5, nalkanes.
The advantage of this plot over the just preceding one lies primarily in that it gives a graphic illustration of the differences in separation which may (1,;)
h’ H Ray, .I Abbl. Chem , 4 , 21 (1954).
P.E. PORTER
1-01. i S
be achieved with the two solvents compared. Thus a vertical line intersects the plots a t carbon numbers which have the same partition coefficients and indicates the materials that cannot be separated with the solvent represented by the abscissa. A horizontal line gives the corresponding information for the solvent represented on the ordinate. It will be noted that in equation XI11 /3 is the order of 0.35 (Table 111) and BA/nh or Bg’nh may take values from 0 to about 0.2 for practical solvents (Table 11). The coefficient of log 11; inay consequently be expected to fall within the limits of unity and about 1.6. For the special case in which the Bin' coefficients are the same for both solvents, the slopes of the plots are simply unity. In this special case equation XI11 may be reduced to I’PtA
= (K;
- I(:)(
Vs%/I.’8B)( I’vYtH
-
I’niH)
f
l’mA
(SI\.)
where the Va‘s are limiting retention volumes for the homologous series members, and the lis's and Vm’s are the volumes of the liquid phase and the gas phase, respectively, in the two columns. I t is clear that equation XIV indicates a linear relation between the limiting retention volumes of the homologous series members determined with the two solvents but that the slopes and intercepts for different homologous series are in general different. This is just the linear correlation plot of V i ’ s vs. ] ’ i f ’ s shown by James and M a r t i n 2 I n addition to the above, it has been useful to correlate partition coefficients of solutes in a given solvent directly with structure very much as reported by Littlewood, Phillips and Price.14 Incremental values of log €Iocorresponding to various structural changes are systematically determined in order to provide a basis for estimating partition coefficients for solutes which have not been determined. Such a procedure is ultimately based on the premise that plots of log Ho uersus carbon number give parallel straight lines for all homologous series. As is clear from the above discussions of y”’s and Po’s, this can be expected to hold to within 5 - 1 0 ~ o only for the higher members of homologous series. It is also clear that, although moderate chain branching of hydrocarbon groupings does not strongly affect yo’s, such structural differences must be taken in to account when P ’ s are correlated. The authors wish to acknowledge the aid and cooperation of their associates in the Emeryville Research Center o f Shell Development Company. EMERYVILLE, CAL.
