Principles of Elution Chromatography as Applied to Separation of Lubricating Oil Components GLENN
E. IRISH
Ethyl Corp.,
AUGUST C. KARBUM Detroit, Mich. and
4 study of the factors which influence the degree of chromatographic separation between lubricating oil components was made so that large scale separations could be made rapidly and quantitatively in order to supply samples for testing in an engine. It was found that precise and accurate separations could easily be niade between classes of components by virtue of the property of aromaticity, providing that the column is not overloaded. Separations within each class, however, become more difficult in that a greatly diminished column load is required. Therefore, although chromatograms obtained in the course of elution work do not generally reflect the true spread of properties of lubricating oil components, quantitative separations into aromatic classes may, however, readily be made.
SSU/2lOo F. viscosity, also from mid-continent crudes, and 2 5 % of a phenol treated neutral of 900 SSU/l0O0 F. viscosity from a coastal crude. Pour point depressant in the range of 0.2% had been added. The lorv viscosity index oil, n-hich is designated naphthenic, was totally distilled from a Gulf Coast crude. It was supplied a t the low additive levels of 0 028% phosphorus and 0.015% zinc. The Chromatographic columns constructed in the cwrse of this work are Iisted in Table I. The stainless steel columns were used mainly for large capacity work. The glass columns were used for preliminary investigations and for cleaning eluents. They lvere all made of borosilicate glass tubing provided with a stopcock. The sectional columns were designed for study of the internal operation of a column whereas the regular columns are only adaptable to study of the column effluent. Most of the experiments were performed on Davison Type 12 silica gel (28-200 mesh). Some background v a s also obtained on Davison Type TS-55 silica gel (60-200 mesh) and type F-20 mesh activated alumina supplied by the Aluminum Co. of America. The hydrorarbon eluents were all Phillips’ commei cia1 grade.
T
HE number of papers concerned with separation of lubricating oil components by chromatographic techniques has increased markedly in the past few years. A few workers have studied the problem from a fundamental standpoint, in that they rmployed pure compounds in their separations (5, 14-17). Displacement chromatography was applied in most of the WOI k published on the lower boiling components of petroleum (9) because it Ras relatively simple, and because it avoided the necessity of using low-boiling eluents which would later he difficult to separate from the petroleum fractions. The displacement technique has been applied to petroleum fractions through the gas oil range (11). Smit ( 1 4 ) investigated the behavior of pure substances on silica gel. and developed the idea of quantitative separation of pure substances by an elution technique. Smit’s technique &as entended by van Nes and van Westen into the gas oil range and applied to a nonaromatic lubricant fraction ( 1 3 ) . During the time which has elapsed since the bibliography was nritten for the van Xes and van Westen book, several articles devoted to this subject have appeared in the literature (2-4, 6-8). The elution technique is useful in the lubricating oil range because the viscosity of oil components is lowered by eluent and the liquid may more readily be passed through the adsorption lied; the rate of attaining adsorption equilibrium is more rapid in the l o x viscosit-b. system n here diffusion is relatively rapid; ruts may conveniently he made between successive classes of oil components because each class can be made to emerge from the column separated from the follon ing class by a layer of solvent containing a negligible quantit? of oil; and eluent ma\’ readily he removed from the oil sample by distillation. In the course of an investigation in this laboratory of the engine performance of lubricating oil components, numerous cliiomatographic separations were made, on both large and small sc&. The small-scale experiments were designed to investigate thr~constitution of lubricants and to determine the factors n-hich influenced the degree of separation between classes of lubricating oil components. Quantities of these components suitable for engine testing xcre obtained in the course of larger scale experiments M4TERIALS
I t was desired to niake a study of a “paraffinic” oil and a “naphthenic” oil. The paraffinic oil chosen in this case vias a 20W, high viscosity index oil, a blend of 60% of a phenol treated neutral of 150-160 SSU/lOO” F. viscosity prepared from midcontinent crudes, 15% of phenol treated bright stock of 150
Table I.
List of Chromatographic Columns
No of Columns Material of Construction 6 5 2 1 1 1 2
Stainless steel Glass Glass Glass Glass Glass Glass sections
Length, Ft. 10 5 8 8
4 4
7
Diameter, Min. 50 60 25 13 ~.
25 13 18
PROCEDURE
Each column was run under a generall?; similar procedure. The column \vas filled n ith adsorbent and packed uniformly by vibration. A mixture of equal volumes of oil and primary eluent was then allowed to flow into the column by gravity. After all of the oil entered the adsorbent, predetermined volumes of the eluents were successively passed through the column. Many small, measured samples were taken from the column if a chromatogram was to be determined. Three to sin samples were taken on large scale separations. Solvents were removed from the samples by distillation under air or nitrogen, evaporation on a hot plate or steam bath, or vacuum distillation, depending upon the size and stabilitp of the sample. Finally, the sample as weighed and one or more of the follon ing properties were measured: refractive index a t one or more wave lengths, density, molecular weight, sulfur content, and fluorescence. Elaborate correlations hetn een physical properties and molecular structure are available in the literature (for example, 6, 12, 13). The tables ( I ) of properties of hydrocarbons of high molecular weight were particularly helpful to\yard an understanding of the chromatographic results. The composition of each chromatogiaphic sample may he estimated in terms of such correlations, but the purpose of this article is to discuss the elution technique rather than the composition of an oil, and references to composition have therefore been held to a minimum Where such references do occur, it should be understood that the information was based on the n-d-m (refractive intie\;. dencity, and molecular weight) method of ring analysis ( I S ) . T H E CHROMATOGRARI
A chromatogram is defined as a graph of a physical property of each small chromatographic sample as a function of the weight of oil vhich has left the column before the sample is taken. I n the following, the use of the term will be restricted to the physical property refractive index (ng). I t will also be instructive to 1445
1446
ANALYTICAL CHEMISTRY
consider the distribution of oil components within a chromatographic column as a function of position, and to distinguish the resultant refractive index function by the term "internal chromatogram." Basic Chromatograms. The basic chromatograms of the naphthenic and paraffinic oils are shown in Figures 1 and 2, respectively. The data plotted in Figure 1 are a composite of two experiments performed under different conditions, and they illustrate several factors affecting the precision of the work.
,
56;
!
fil ,
46,
0
i ,
0
'0
*E G H T i or
Figure 2.
OIL ,PC"I
0 16
u D
1:IGHT
80
oc
IO0
ZAUPLEl
Chromatogram of Paraffinic Oil
Mixture composed of loliters of 2.3-dimethylbutane and 1.5 liters of benzene
53
0
20
60
40
80
I
,
40
60
IO0
WEIGHT X OFOIL
Figure 1. Chromatogram of Naphthenic Oil
-A Expt. 1 Expt. 2.4.
2,3-Dimethylbutane
0 Expt. 2B. Excess n-hexane
Experiment 1, represented by the straight lines, was run in a 10-foot stainless steel column packed with 4500 grams of 28-200 mesh silica gel. The column was charged with 1250 ml. of naphthenic oil mixed with 1250 ml. of n-hexane, and eluted successively with 16 liters of n-hexane, 6 liters of benzene, and 4 liters of acetone. Benzene was then distilled from the entire benzene cut and this cut was rechromatographed with the fullowing successive eluents: 18 liters of carbon tetrachloride, 6 liters of benzene, and 4 liters of acetone. These columns were not shut down during operation. On the other hand, Experiment 2 was run primarily to test the efficiency of three primary eluents: 2,3-dimethylbuLme, nhexane, and cyclohexane. Three 10-foot columns were packed with 28-200 mesh silica gel and charged with a mixture of 0.5 liter of naphthenic oil and 0.5 liter of primary eluent. The circles in Figure 1 represent the n-hexane elution, and the triangles represent the dimethylbutane elution. The cyclohexane results duplicate those obtained with 2,3-dimeth>lbutane. A reproducible chromatogram was obtained in all of these experiments. The following minor sources of error are noted: The largest sample in Experiment 1 has a lower refractive index than the ones preceding and succeeding it. This was most probably caused by incomplete evaporation of solvent, for in all of the large scale runs complete separation of solvent from large samples of oil was found to involve more diffirulty than was experienced with smaller samples. Benzene elution in Figure 1 begins u i t h material having a refractive index greater than 1.61. It can readily be seen that the benzene is adsorbed so strongly that it completely displaces the material of the benzene fraction instead of eluting it. The oil recovery in Experiment 1 (Figure 1 ) was 98 3 % ; that in
I 20
0
WEIGHT
Figure 3.
i
"ELLO*
I
I
I 80
I 100
OF OIL IPOlNT I S H I D - W E I G H T OF SAMPLE1
Chromatogram of Naphthenic Oil after .4niline Extractions
"*GL
4 8CO
200
Figure 4. Weight of Naphthenic Oil Appearing in Effluent of Chromatographic Column (Experiment 1)
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4
1447 Figure 2 was 97.8%. The loss is a reflection of handling loss rather than loss of any material remaining in the column, for in those experiments in which few samples were taken complete recoveries were obtained. A maximum was obtained in the refractive index curve of Kxperiment 2-4 near the end of the 2.3-dimet,hylbutane elution (Le., after about 60% of the oil had passed through the column). This maximum was also obtained in Experiment 2R during the n-hexane elution but it was not obtained in Experiment 1. The discrepancy may be attributed to a slight overloading of the column in Experiment 1. The variable, "column load," is discussed more fully later. The maximum was investigated by the n-d-m method of ring analysis ( I S ) and shown to be a real effect due to the presence of nonaromatic compounds containing four or five naphthene rings. A similar phenomenon may be noted in the mononuclear aromatic range (Le., a t i l % of input oil); it may be attributed to a similar type of constituent containing one aromatic ring.
I63
I61
ri
1.59
I57
As an application of the chromatographic technique, it is interesting to note the effect of aniline extraction on the naphtheriic oil. After five successive aniline extractions, the paraffinic raffinate was diluted a ith 2,3-dimethylbutane, washed with hydrochloric acid, washed with water, dried, and finally chronutographed (Figure 3). With respect to hydrocarbon type, the aniline extracted material appears to be more similar to the parsffinic oil than to the original naphthenic oil. The Internal Chromatogram. I n preliminary static experiments, adsorption equilibrium was established rapidly between silica gel and binary, labile solut'ions. It m s therefore expected that lubricating oil components could be distributed within a column of reasonable length to yield an internal chromatogram which would be a characterist,ic and reproducible property of the oil. An experiment was designed to test this hypothesis. -4n aromatic concentrate had been prepared as follows: Experimtwt 1 (Figures 1 and 4) had been run in multiplicate in six columns; the graphs are the results of one column. The samples from the other five columns were combined into aromatic and nonaromat,ic concentrates on the hasip of fluorescence, and eluent was removed from each by distillation. Samples of the aromatic concentrate (9.7 to 10.3 grams) were eluted with 900 ml. of 2,3-dimethylbutane in each of seven sectional columns (seven sections to each column). The oil in pitch section was then displaced lvith acetone. The 56 samples from the 49 sections and seven column effluents were combined into eight large samples-i.e., all of the samples from the seiwi top sections were combined, all of the second sections Tvere comhined, etc. The eluent was then distilled from each large sample, and
20 C X W 0
5
I55
>
Y
-
0 k-
i'
CC
W LL
1.53
151
I49
I47
I
I
0
20
I
40
60
80
I00
WEIGHT Y. OF INPUT OIL
Figure 5. Chromatogram of Aromatic Constituents of Naphthenic Oil Taken in Sectional Columns
20,
2s
--
-
-
2.3-DIMETHYLBUTINE N.HEXANE
I
c
Figure 6.
33
2: "OL.YE
C'
EFF."ENT
50
'0 YC ,OF
200
Y.
60
Q
I'YFLES,
Comparison of Eluting Power of Three Primary Eluants (Experiment 2)
out the column in the final chromat,ography.
chromatographic column (Figure 6) is a characterist,ic of the oil, and that by proper choice of technique the same distribution Tvill be displayed in the column effluent. The Rate of Emergence Graph. The rate of emergence graph is defined as a graph of the weight
ANALYTICAL CHEMISTRY
1448
of oil in each small sample emerging from the column as a function of the total volume of column effluent before that sample is taken. I t is thus a volume rate rather than a time rate, but it is closely related to a time rate of movement of oil fractions if the time rate of total flow is maintained a t a constant value. One of the advantages of elution chromatography can be seen in Figures 4 and 6, the rate of emergence graphs corresponding to Experiments 1 and 2, respectively. Whereas "what" comes out of the column is described in Figure 1, "how" it comes out is described in Figures 4 and 6. The appearance of the samples is described in Figure 4. The accumulation of samples which appear under one of the peaks is referred to as a "fraction." Fractions A and B were both eluted with n-hexane in Experiment 1 (Figure 4); fraction C was eluted with carbon tetrachloride, fraction D with benzene, and fraction E with acetone. The question immediately arose as to whether fractions A and B could be more completely separated. The answer was established in Experiment 2 (Figure 6). Fractions A and B were effectively separated in two ways. The "weaker" eluents, 2,3-dimethylbutane and cyclohexane, used in experiments 2A and 2C, respectively, eluted fraction A but did not elute fraction B from the column, within the volumes of eluent employed. It was also found that fractions A and B could be separated adequately. eluting with n-hexane (experiment 2B, Figure 6), providing that a lower ratio of oil to silica gel was employed.
IS
O
1.49
SEPARATION BY THERMAL DIFFUSION
/
I
,
--CHROMATOGRAPHIC SEPARATION
I
THERMAL DIFFUSION A 24 HCURS A 4 8 HOURS 0 72 HOURS 120 HOURS
f 1.451
I
I
2!3
40
I
60
WEIGHT X OF WHOLE OIL
Figure 8. Separation of Paraffin-Kaphthene Components of the Paraffinic Oil
WEIGHT OF IDSOPBENT T l l h V E R Y O
IbRAh151
Figure 7. Distribution of Aromatic C Components on Type 12 Silica Gel in Sectional Column. Effect of Elution
The general agreement between the chromatograms illustrated in Figure 1 indicates that the basic chromatogram is a characteristic of the oil, and that by proper choice of such operating variables as the type of eluent employed, this characteristic of the oil may be measured. EFFECT O F OPERATING VARIABLES
A systematic investigation of operating variables has been reported (11) for the technique of displacement chromatography. Some of these have been studied in the course of this work, and some additional variables were encountered which are peculiar to the elution technique. Column Load. Column load is defined simply as the quantity of oil which is chromatographed on a unit weight of silica gel.
A more pertinent quantity would be some measure of the maximum weight of a particular class of components which can be held by a unit weight of adsorbent. The following experiment demonstrates that this quantity is not utilizable conceptually, nor measurable experimentally as a single number. Each of seven sections of a sectional column was packed, stoppered, weighed, and assembled. A weighed quantity of aromatic concentrate, obtained as described above, was then introduced, followed by a measured volume of eluent. The sections were finally septrated; the oil was displaced from each with acetone, and the weight and refractive index of the evaporated samples xvere measured. The data for several columns illustrate the effect of successive quantities of 2,3-dimethylbutane eluant in Figure 7 . Kate that even the most strongly absorbed material was continuoudy eluted through the column. It is evident, therefore, that the concept of maximum weight of aromatics absorbed on silica gel is oversimplified. A more useful concept for chromatographic purposes is the maximum column Ioad for which a quantitative separation can be made between two fractions. I t is shown that this quantity is principally dependent upon the following factors: type of eluant, type of adsorbent, and type of constituents to be separated. A column is said to be overloaded with respect to the variables involved-e.g.. with respect to separation of fraction A from fraction B-if the column load is greater than the maximum column load for the A-B separation. The effect of overloading the column has already been discussed in connection with Figure 1, where fine structure of the chromatogram was inumbrated when the data were taken in an overloaded column. A useful rule of thumb was formulated from the type of result illustrated in Figure I: The maximum weight ratio of aromatics to 28-200 mesh silica gel for uhich the chromatogram may be obtained with sufficient detail and reproducibility is about 0.09'7.
1449
V O L U M E 2 6 , NO. 9, S E P T E M B E R 1 9 5 4
termined and included in the figure to illustrate the eluting effect of n-hexane. The 900 ml. of n-hexane has about the same eluting power as 1750 ml. of dimethylbutane. From this standpoint n-hexane has twice the eluting power of 2,3-dimethylbutane. Some static experiments were run in early work, in which laown mixtures of eluants were contacted with silica gel. Refractive index measurements were made on the initial solution, and a t various times after contact. I n all cases, eydilibrium was attained more rapidly than could be measured (less than 1 minute), and the following series of adsorption strengths Tvere established for an approximately equimolar initial miuture: Benzene = tert-butyl chloride > methyl chloroform > carbon tetrachloride > n-hexane. The ability of several primary eluants to elute oil was compared in experiments similar to those graphed in Figure 6. I n this manner the following relative strengths of primary eluants were established: n-hexane > cyclopentane > a mixture of equal volumes of n-hexane and 2,3-dimethylbutane Philips isohexanes > 2,3-dimethylbutane = cyclohexane 0 nheptane. Commercial isohexanes lyere tried because they contsin a large proportion of methyl pentanes, hydrocarbons which r e p resent an intermediate class of branching between n-hexane and 2,3-dimethylbutane, but the isohexanes were not pure enough to give sufficiently reproducible results for most chromatographic work. Of the above eluents, only 2,3-dimethylbutane, cyclohexane, and n-heptane were sufficiently weak to separate fraction A from fraction B quantitatively a t a column load of 0.20, using the naphthenic oil. Nevertheless, a t lower column load, the separation was quantitative even with n-hexane. B. J. Mair (IO)has compared the relative adsorption of some of the above hydrocarbon pairs by chromatographing a mixture in a displacement procedure. There is complete agreement between the generalizations drawn by Mair and the relative adsorption strength measured in this work. Type of Adsorbent. Type 12 silica gel (28-200 mesh) was employed in the experiments previously described. For purposes of comparison, type TS-55 silica gel (60-200 mesh) and type F-20 alumina were also investigated in the sectional columns. The aromatics separated from the naphthenic oil in a preliminary experiment were chromatographed and the results are plotted in Figure 9. Type TS-55 gel appears to be more selective than Type 12 gel (28-200 mesh), for although the chroniatograms 03 the tvio gels correspond closely, two classes of compounds appear to separate from each other in a more pronounced fashion within the column which is filled with the finer gel. Alumina appears
The accuracy of the chromatogram, in terms of the actual spread of the properties of lubricating oil componente, is, however, limited by other considerations. At lower column load, or by utilization of a weaker eluant, the cuts comprising fraction A could more effectively be separated from those comprising fraction B. However, a column load less than the maximum column load required for the A-B separation does not guarantee that components xithin fraction A will be completely separated, but only that fraction A will be separated from fraction B to the degree specified. For example, a comparison is illustrated in Figure 8 between separations achieved chromatographically and by means of liquid thermal diffusion within fraction A . The chromatographic results were taken a t a column load less than the maximum column load required for the A-B separation, but the components of fraction A were separated more adequately b~ thermal diffusion. Type of Eluant. As mentioned above, the eluaiit type is an important variable in column operation. For example, the solid curves in Figure 7 represent the eluting effect of successive quantities of 2,3-dimethylbutane. An additional curve !vas de-
r x
150
w
*=OH-
1.OSOC-SEYT T P I V C P S F D
Figure 9. Distribution of -4romatic Oil Components in Sectional Column. Effect of Adsorbent Type Elution i n all cases with 900 ml. of 2,3-dimethylbutane
7
E L U T I O N BY I
n - HEXANE I
1
1
n - B U T Y L CHLORIDE
I
I
I
I
I
I
I
I
I
I
BENZENE I I
m
I
ACETONE I I
I 0 26lAi
i5-
I4
PACKING
F - 20 ALUMINA
CHARGE
150 ML AROMATICS FROM NAPHTHENIC OIL 4
!I I \\
E4r I S I I
?I
\
NUMBER OF 200 ML SAMPLES
Figure 10.
Distribution of Oil Components in Effluent of Column
I
1450
to be the most interesting adsorbent, for although it has low capacity for those aromatics which remain adsorbed (Figure 9), it has still lower capacity for one class of aromatics which had been eluted through the column with only 900 ml. of eluent. Two experiments were run on alumina in 10-foot stainless steel coulmns in order to extend these observations. In the first, 100 ml. of the aromatics from a preliminary separation of the naphthenic oil (Le., half the load of the column illustrated in Figure 9 ) Fere eluted with 12 liters of 2,3-dimethylbutane. Less than half of the mononulcear aromatics \\ere eluted, and the second experiment was therefore run with 150 ml. of aromatics and eluted xith 16 liters of n-hexane, 5 liters of n-butyl chloride, 8.75 liters of benzene, and 8 liters of acetone. The distribution of components in the effluent of the column is shown in Figure 10. The samples were only lightly colored through No. 184. The physical properties of fraction B samples indicate that they are mononuclear aromatics. Fractions C and D appear to contain the remaining aromatic hydrocarbons. They are eluted completely by n-butyl chloride and benzene and seem to be eluted rather than displaced, for the chromatogram (Figure 11) is smooth throughout this region instead of showing a “block” effect as in Figure 1. Extremely high-refractive index material was obtained. The material eluted by acetone contained a large percentage of sulfur compounds. I t has not been characterized in detail because it is not very stable toward heat and oxygen and has not been adequately separated from the mesityl oxide which is formed when acetone is passed over alumina. Effect of Prewet Absorbent. TWO advantages were gained in many of these experiments by the procedure of wetting the adsorbent with primary eluent before the oil charge was introduced. Thermal effects due to the large initial heat of adsorption are always encountered. When the columns were prewet with primary eluent, the initial heat of adsorption was dissipated before the oil charge was introduced; hence, thermal effects on the chromatogram were minimized. The prewetting procedure was also found to add to the reproducibility of those experiments in which data such as those plotted in Figure 4 were to be obtained-Le., the weight of oil in each sample of cut A in Figure 4 follows the general outline of a bell-shaped curve. I n this experiment the column XTas prewet. If the column had not been prewet, the bell shape would be skewed. The former type of curve iv, of course, more desirable for making a comparison between the action of tL50 primary eluants. A third advantage has been attributed to prewetting in the literature. Lillard, Jones, and A4nderson( 6 )found that the probability of “smearing” high refractive index material ahead of the initial low refractive index material 15 as lessened by prewetting the column. This conclusion has not been verified in the present work. Figure 2 is a chromatogram of results taken in a prewet column. The initial high refractive index material is not regarded as having been “smeared” ahead of the following material, for it is invariably reproduced under conditions of low column load when the column is prelvet, and it is absent a t high column load. Moreover, it is masked in the large first sample of the column which has not been prewet. Therefore, it is concluded that the initial high-refractive-index material may be expected in some oils, and that it is masked by overloading or not prewetting the column. Some operating variables have not been investigated completely. The effect of flow rate has not been studied, but it is expected to be small, for preliminary static experiments indicated that absorption equilibrium is attained very rapidly. Overnight interruption of column operation was not found to affect the chromatograms significantly. Finally, the effect of column length has not been investigated extensively although the results obtained in 5-fOOt columns mere invariably found to agree with those ohtained in 10-foot columns. The same conclusion may not be as valid for the displacement technique. Mair ( 1 1 )demonstrated that the effect of quadrupling column length was relatively
ANALYTICAL CHEMISTRY small, but significant in the application of the displacement technique to the gas-oil fraction of petroleum. SUMMARY
Elutioii chromatography has been applied to separation of classes of lubricating oil components. The method was also employed to characterize the lubricating oil in terms of a chromatogram. Investigation of the operating variables inherent in elution chromatography of lubricating oils revealed that the most important operating variable is column load. The maximum column load with which an adequate separation can be made betrveeLitwo classes of oil components is in turn principally dependent upon the three variables: classes to be separated, type of eluant, and type of adsorbent. Major classes of components have been separated employing reasonabl! large column loads, and reproducible chromatograms were obtained simultaneously. These chromatograms, however, do not accurately reflect the spread of properties of the components of the oil. Very small column loads would be required to obtain an accurate chromatogram. LITERATURE CITED
(1) Ani. Petroleum Inst., Research Project 42, “Properties of Hydro-
carbons of High Molecular Weight Synthesized by Research Project 42,” Pennsylvania State University, State College, Pa., Sepember 1950. I 68
,
I
I64
I 62
E’’c
160
X W
n
5
cu= W
I58
Y K W
E
I56
I54
I52
I 5 0
I
I
Figure 11. Chroniatograni of .AromaticComponents of Saphthcnic Oil Ilechromatographcd on Alumina (Same Experiment as in Figure 10)
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4
1451
( 2 ) Cady, W. E., and Seelig, H. S., Ind. Eng. Chem., 44, 2636 (1952). (3) Furby, N.W., gaper presented before the Division of Petroleum Chemistry at the 116th Meeting of the A Y E R I C C . ~H~E M I C A L S O C I E T YAtlantic , City, K.J., September, 1949. (4) Hibbard, R. R., Ind. Eng. Chem., 41, 197 (1499). ( 5 ) Kuhn, R., and Winterstein, A , , Helr. Chim.Bcta.. 11, 87, 116, 123, 144 (1928). (6) Lillard, J. G., Jones, W.C., Jr., and .4nderson, .J. .I.,Jr., 2nd. Eng. Chem., 44, 2623 (1952). (7) Lipkin, hl. R., Hoffecker. W. A., Martin, C. C . , and Ledley, R. E., A s . 4 ~ .C H E M . 20, , 130 (1948). (8) Lipkin, 11. R., Martin, C. C., and Hoffecker, W..I.,gaper presented before the Division of Petroleum Chemistry at the 113th hleeting of the A M E R I C ACXH E M I C ASLO C I E T YChicago, , Ill., April 1948. (9) 1Iair, B. J., Ind. Eng. Chem.. 42, 1355 (1950).
(10) Jlair, B. J., Gaboriault. .4.L., and Rossini, F. D., Ibid., 39, 1072
(1947). (11) Mair, B. J., Sweetman, .4. J., and Rossini, F. D., Ibid., 41, 2224 (1949). (12) Mair, B. J., Willingham, C . B., and Streiff, A. J., J . Rrscurch .Vatl. Bur. Standards, 21, 581 (1938). (13) Xes, K. van, and Westen, H. A. r a n , “Aspects of the C‘onstitution of Mineral Oils,” iYew York, Elsevier, 1951. (14) Smit, W. AI., Anal. Chim. Acta., 2, 671 (1948). (15) Willingham, C. B., J . Research 9 a t l . Bur. Standard.?, 22, 321 (1939). (16) Winterstein, A . , and Schon. K., 2.physiol. Chem., 230, 146 (1934). (17) Winterstein, -4.. Schon, K., and Velter, H., I b i d . , p. 158. RECEIVED f o r review August 17, 1953. Accepted June 14, 1954. Presentrd before the Division of Petroleum Chemistry at the 124th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill.. September 1953.
Adsorption Chromatography and liquid Partition of High Polymers Silicones D. W. BANNISTER, C. S. G. PHILLIPS,
and
R. J. P. WILLIAMS
lnorganic Chemistry Laboratory, O x f o r d University, Oxford, England
The molecular w-eightdistribution in high polymers has never been satisfactorily investigated. Two new countercurrent methods were therefore examined as fractionating techniques. The initial experiments have been carried out on silicone polymers which are known to be linear. Experiments using gradient elution, in which carbon was the adsorbent and the eluting solvent was ether in methanol, gave a series of fractions of the silicones, the molecular weight of the fraction increasing with effluent volume. A second method, liquidliquid partition in a vortex column, w-as successfully applied to fractionation of low molecular weight material. The methods allow rapid determination of the molecular weight distribution in silicones. The fractions obtained are sharp and will permit a fresh examination of much of the theory of high polymers.
T
HE problem of the fractionation of high polymers obviously
presents greater difficulties than those usually encountered in the separation of other substances because of the very small differences bet\vern the properties of the components. Present methods available for the fractionation of polymers are almost invariably batch methods, and although some attempts have been made t o utilize continuous multistage processes, none of them have been developed into routine techniques. The only detailed work on the chromatography of high polymers (3) used the frontal analysis method and involved an intricate and uncertain mathematical examinat,ion of the polymer front irhich emerged from the column. This method does not, frnctionate the polymrr. T h e fractionation procedure which is described here can he used either as an analytical method or preparatively. It is a n application of gradient elution ( 1 , 6). I n this technique the zones of the substances, in this case t,he different molecular weight fractions, are adsorbed onto the column material from n poor solvent (or if the substances are liquids they can be put onto the column directly). T h e development of the zones commences with the poor solvent but the elution is carried on Ivith increasingly more powerful eluting solutions by introducing continuously a good solvent for the substances. The good solvent must be so chosen that i t will elute the substances under examination from the column quantitatively and it-ith very small reten-
tion. The apparatus used to obtain the solvent gradient is illustrated in Figure 1 ( I , 6). APPLICATION TO THE SILICONE POLYMERS
Several adsorbent-solvent combinations were tried before a suitable system was found for these polymers. Alumina and silica gel were found to be unsuitable adsorbents as none of the solvents which were examined would elute the higher molecular weight polymer*. However, if the silicones were adsorbed onto charcoal (animal charcoal, Harrington Bros., Ltd., London. England), they could be quantitatively eluted from this adsorbent by diethyl ether. The recovered material was examined by the determinaVESSEL tion oi“ its viscosity. In this way it was proved that none of the material, even of the highest molecular weight,, had been irreversibly adsorbed, as the viscosities obtained in these tests viere always identical x i t h those of the starting materials. F r a c t i o n a t i o n could now be attempted. METHOD O F FRACTIOX.ATIOS
F$gii ducing
zp{iz:
Gradient
T h e carbon column, 35 cm. in length and 1.5 cm. in diameter, \vas prepared in methanol. To the top of the column \vas added 3.5 grams of a silicone fluid-e.g., a Dow Corning 200 (straight-chain polymethylsiloxanes) of bulk viscosity 1000 centistokes of formula:
I
I n
CH,
where n, t h e number of repeating units, is 140 on average. The polymer wa8 then washed onto the column with methanol. The E t o a 200-mI. mixing vessel which was column W ~ connected
