Cesium and barium were adsorbed from 1M HC1, 1M HNO,, 2 M H C l j l M HNO,, 1M HC104, and 1M HzS04. From solutions of mixed-fission and activation products in 8M HNO,, tantalum was found to be adsorbed along with the sodium. The conditions employed in the present investigation were
selected on the basis of these studies, checked, and found suitable. RECEIVED for review May 21, 1970. Accepted October 5, 1970. This work was performed under the auspices of the United States Atomic Energy Commission.
Solvent Effects in Gel Permeation Chromatography J. G . Bergmann and L. J. Duffy Research and Development Department, American Oil Co., Whiting, Ind.
R. B . Stevenson Research and Development Department, Ammo Chemicals Corp., Whiting, Ind. WHEREAS HYDRODYNAMIC VOLUME appears to be the effective parameter for the size separation of high-molecular-weight polymers in Gel Permeation Chromatography ( I ) , molar volume is better for low-molecular-weight compounds such as alkanes and aromatics (2). However, some of these compounds show unusual elution behavior in tetrahydrofuran (THF), a common solvent in GPC. For example, alkanes and aromatics with similar molar volumes show large differences in retention volumes, which have been attributed to a n association between the aromatic and the polystyrene divinylbenzene gel (3). Even within the aromatics class, different elution patterns have been observed, not only in T H F ( 4 ) , but also in methylene chloride ( 5 ) ; with increasing molar volume, retention volumes decrease for the catacondensed series of polynuclear aromatics, but there is an anomalous increase for the peri-condensed series. Thus, a G P C study of petroleum resids-complex mixtures of highmolecular-weight compounds including homologs of alkanes and aromatics-is hampered by these varied elution patterns. To simplify the study of elution behavior, we observed the elution of low-molecular-weight model compounds in various solvents and solvent combinations. These studies were supplemented with infrared, ultraviolet, and solubility measurements to determine whether elution order is affected by adsorption, association, and/or differences in the solubilities of the compounds. Conventional GPC equipment (Waters Associates, Models 100 and 200) was used for this work. The results should be useful with more complex systems such as resids. T o date, we have obtained no clear-cut evidence to explain the anomalous behavior of the peri-condensed molecules. However, we have found that the anomalies disappear when 1,2,4-trichlorobenzene (TCB) is used as the GPC solvent. RESULTS AND DISCUSSION
Figure 1 illustrates the complex elution patterns in T H F compared to TCB. Retention volume data for these compounds are presented in Tables I and 11. In THF, five dis(1) Z . Grubisic, P. Rempp, and H. Benoit, J . Polym. Sci., Part B, 5, 753 (1967). ( 2 ) J. Cazes and D. R. Gaskill, Separ. Sci., 2,421 (1967). (3) G. D. Edwards and Q. Y . Ng, J. Polym. Sci., Part C , 21, 105
(1968). (4) T. Edstrom and B. A. Petro, ibid.,p 171. (5) H. Oelert, presented at the API 60 Summer Meeting, Laramie, Wyo., 1969.
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RETENTION RELATIVE T O BENZENE
Figure 1. Effect of solvent on elution behavior tinct elution patterns are observed for these six series of compounds (the sulfur-heterocyclics align with the catacondensed aromatics). The series of peri-condensed aromatics shows a n unusual reversal in elution behavior-the larger the molecule, the later it elutes, This reversal seems to be related to peri-condensation. Further support for this observation lies in the elution behavior of the nitrogen compounds. Elution is in the expected order through carbazole, but benzcarbazole-a peri-condensed molecule-elutes later than carbazole. The hydrogenated pyrenes show a maximum in relative retention and, although less clear, this would again appear to be a function of structure. By contrast, in TCB these differences between classes of compounds disappear. With that solvent, the retention
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
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Table I. GPC of Model Compounds in THF at 25 "C Molar volume data Retention volume, Data Compound cc/mole source0 cc n-Alkanes Heptane 141 1 147.5 Hexadecane 311 1 127.4 Octacosane 503 1 113.0 Benzene Naphthalene Anthracene Naphthacene Picene
Cata-condensed aromatics 88.9 125.0 160.2 196 232
2 2 2 2 2
164.5 162.3 158.6 155.0 151.5
Pyrene Perylene 1,lZBenzperylene Coronene 0v a1ene
Peri-condensed aromatics 171.5 202 219 228 280
2 2 2 2 2
161.3 162.7 163.8 166.2 179.3
2
161.3 162.5 163.2 156.0
Hydrogenated pyrenes Pyrene 171.5 1,2-Dihydropyrene 175.1 1,2,3,6,7$-Hexahydropyrene 186.O Decahydropyrene 211.3 Pyrrole Indole Carbazole 4H-Benzo(def)car bazole
N-heterocyclics 69.3 102.9 144 149.8
3 3
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149.8 147.0 145.0 147.4
S-heterocyclics Thiophene 79.0 3 165.4 Benzthiophene 115.2 3 162.6 3 Dibenzthiophene 162.7 159.6 a ( l ) J. Cazes and D. R. Gaskill, Seminar Proceedings, Sixth International Seminar on GPC, Miami Beach, Fla., Oct. 7-9, 1968; (2) H. G. Davis and S. Gottlieb, Fuel, 42, 37 (1963); (3) Calculated from molecular weight and density values. Table 11. GPC of Model Compounds in TCB a t 140 "C Molar Retention volume, volume, Compounds cc/mole cc Thiophene 79.0 170.0 Benzene 88.9 166.0 Benzthiophene 115.2 159.6 Naphthalene 125.0 158.4 Anthracene 160.2 148.8 Dibenzthiophene 162.7 149.5 Pyrene 171.5 145.8 1,2-Dihydropyrene 175.1 146.3 1,2,3,6,7&Hexahydropyrene 186.0 143.8 Naphthacene 196 141.O Perylene 202 140.3 Decah ydropyrene 211.3 140.3 1,12-Benzperylene 219 139.1 Coronene 228 136.0 Picene 232 136.3 Ovalene 280 129.7 Hexadecane 311 127.5 Octacosane 112.7 503 Pyrrole Indole Carbazole 4H-Benzo(def)carbazole
N-heterocyclics 69.3 102.9 144 149.8
184.4 173.5 161.1 158.3
Table 111. Effect of Temperature on Elution Behavior in TCB Retention relative to benzene Compound 140 "C 25 "C Pyrene 0.877 0.842 Perylene 0.844 0.812 Coronene 0.818 0.787 Table IV. Solubility of Aromatic Hydrocarbons Solubility, (gramsjiter) at 25 "C Solute Toluene THF TCB Coronene 1.4 3.6 6.4 Perylene 2.9 7.8 11 .o Pyrene v. Sol. v. Sol. v. Sol. Picene 2.6 2.8 2.7
volumes of nearly all the compounds show a simple relationship with the log of molar volume. The only exceptions are the nitrogen compounds, which are slightly displaced, undoubtedly because a small adsorption component remains. The displacement could very likely be eliminated if a small amount of a polar solvent is incorporated in the TCB. The ability of TCB to eliminate unusual elution behaviors is temperature independent; Table 111 shows that at 25" and 140 "C elution orders are identical, although positions are slightly displaced presumably because of differences in swelling characteristics of the gel. The marked contrast in elution behavior between the cataand peri-condensed molecules in T H F and TCB apparently cannot be explained by adsorption, which is a common reason for delayed retention in liquid chromatography. Such adsorption usually causes pronounced tailing whereas GPC peaks for peri-condensed molecules, although wider than expected, do not show such tailing. Furthermore, the adsorption behavior of a large number of polynuclear aromatics on alumina (6) does not reveal any reason for their anomalous GPC behavior. Studies with other solvents also failed to show reasons for the varied elution pattern. I n toluene, coronene is held back even farther with respect t o benzene than in T H F ; the retentions relative to benzene are 1.13 and 1.01. In 10 vol TCB-in-toluene at room temperature and at 50 "C, retention of coronene relative to benzene is the same as in toluene. Increasing the temperature to 75 "C shifted the coronene peak only slightly toward earlier elution. Studies t o determine whether a relationship exists between solubility and retention of the polynuclear hydrocarbons in TCB, THF, and toluene are summarized in Table IV and in Figure 2. For the peri-condensed molecules, there appears t o be a correlation between the degree of solubility in the three solvents and retention. Both coronene and perylene are most soluble in TCB, in which there is apparently no delayed retention; they are least soluble in toluene, in which there is the greatest delay in retention. On the other hand, picene-a cata-condensed molecule-has the same low solubility in all solvents, and it is not delayed. These data suggest that solubility may affect the elution order of the pericondensed but not of the cata-condensed molecules. This suggestion is made cautiously because pyrene shows some delayed retention even though it is highly soluble in all solvents. But the amount of delayed retention is the smallest of the series. (6) L. R. Snyder, J. Pkys. Chem. 67, 234 (1963).
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0 5
0 6
0 7 0 8 0 9 1 0 1 1
1.2
RETENTION RELATIVE T O BENZENE
Figure 2. Relation of solubility to retention volume
1-L-l
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Figure 5. G P C of asphaltene fractions
ELUTION COUNT
Figure 3. Solvent effect on GPC of asphaltene An association between the peri-condensed solutes and the gel phase is possible, and it may be enhanced by poor solubility. However, IR examination of a gel upon which coronene had been deposited revealed no evidence for such an association. A solute-solvent interaction could possibly explain the effectiveness of TCB in eliminating associationcoronene and TCB are good candidates for a charge transfer complex. But examination of the UV spectra of coronene in TCB, THF, and toluene failed to provide solid support for such complexing. I n preliminary experiments, the elution behavior of petroleum asphaltenes has shown a solvent dependence similar to that for peri-condensed model compounds. In T H F , all asphaltenes show a prominent low-molecular weight tail; but in TCB this tail is considerably reduced. These differences are shown in Figure 3. I n addition, a n asphaltene was fractionated in a preparative column in methylene chloride solvent, and the material represented by the tail was collected and then chromatographed in the analytical GPC instrument. As shown in Figure 4, this material is held back
much more in T H F than in TCB; the relative retentions of the peaks compared t o benzene are 0.77 for THF and 0.66 for TCB. Possibly the diminution of the asphaltene tail in TCB and the shift in peak position of this material with solvent may be analogous t o the elution behavior of the simple periand cata-condensed molecules in T H F and TCB. Thus, this behavior may well be caused by naturally occurring pericondensed molecules in asphaltenes. An alternate explanation is that adsorption by sulfur, nitrogen, or oxygen compounds is greater in THF than in TCB. However, this seems unlikely upon examination of the G C P behavior of highmolecular-weight material obtained by fractionation. The chromatogram of such material in THF, together with that of the parent asphaltene, is shown in Figure 5 . On the basis of simple adsorption, a tail should still be present in the derived peak if the polar compounds are assumed to be distributed uniformly across the molecular weight spectrum. These results thus imply that the low-molecular-weight tail of an asphaltene in THF is caused by unique species which show different elution behavior in TCB. But we have not yet established by independent means that the low-molecular-weight asphaltenic material actually is enriched in peri-condensed aromatics.
RECEIVED for review July 6, 1970. Accepted September 28, 1970.
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