Inverse Gas-Liquid Chromatography A New Approach for Studying Petroleum Asphalts T. C. DAVIS, J. C. PETERSEN, and W. E. HAINES laramie Petroleum Research Center, Bureau of Mines, U. S. Department of the Interior, laramie, Wyo.
b A new approach to the characterization of asphalt is described in which the asphalt serves as the liquid substrate in an unusual application of gas liquid chromatography (GLC). The term “inverse GLC” has been applied to differentiate this technique from conventional GLC. In the technique, the asphalt on the column is characterized by measuring the corrected retention volumes of a series of selected test compounds with different functional groups. The retention data are quantified by referencing to the behavior of n-paraffins on the asphalt column. The retention behavior of each test compound depends on interactions with functionality in the asphalt and thus is related to the chemical composition of the asphalt. The new technique was found to be useful in showing differences among asphalts and holds promise as a method of studying asphalt composition and showing chemical changes which occur on oxidation and weathering.
G
as an analytical and research tool has been valuable in solving problems of the organic chemist. The importance of both chemical type and structure on the solute-solvent interactions that determine retention behavior is clearly pointed out by Wehrli and Kovats (6) and further defined by Kovats ( 4 ) . A large number of researchers have discussed the significance of these interactions. Heubner ( 2 ) recognized their importance when he characterized the polarity of surface active agents using methanol as the polar solute. I n the present paper, a new GLC technique is reported by which the chemicd functionality of an asphalt can be studied. Interactions between the volatile solute and the stationary liquid phase form the basis of the application. I n contrast to conventional GLC, however, primary interest is placed on the stationary (aFphalt) phase. The term “inverse GLC” is applied to differentiate the technique from conventional GLC. The volatile solutes (test compounds) , instead of being the subject of analysis, AS-LIQUID CHROMATOGRAPHY
Work done under cooperative agreements between the Bureau of Mines, U. S. Department of the Interior and the University of Wyoming.
serve the purpose of fingerprinting the asphalt phase. I n practice, an asphalt on an inert support is placed in the GLC column and the retention behavior determined for a number of chemically different test, compounds. These test compounds are carefully selected on the basis of differing functional groups. The interactions of each test compound with the asphalt determine how long the test compound remains in the column and so provide information about the chemical makeup of the asphalt. Each test compound produces a different piece of information and, thus, forms the basis for asphalt characterization and classification. To provide a means of quantifying the data, the retention behavior of each test compound on asphalt is referenced to the behavior of a hypothetical n-paraffin of similar molecular weight. The ability of inverse GLC to show differences in asphalts was tested using four petroleum asphalts, a petroleum residuum, a shale-oil residuum, and seven fractions from one of the asphalts. EXPERIMENTAL
Procedure. GLC data were obtained on a Beckman GC-2 gas chromatograph equipped with a 1-mv. Bristol recording potentiometer. The column packing, composed on a weight basis of 1 part asphalt to 10 parts Fluoropak 80, was prepared by adding to the Fluoropak 80 the desired amount of asphalt as a 6: 1 benzeneasphalt solution followed by evaporation of the benzene. Fifty-six grams of the prepared packing was packed into aluminum tubing (‘/*-inch by &foot). The column was placed in the gas chromatograph, and helium a t an inlet gauge pressure of 15 p.s.i. (because of capillary tubing restrictions inherent in the GLC instrument, actual pressure drop across the column averaged 4.0 p s i . ) , was passed through the column while raising the instrument operating temperature to 130’ C.; the column was maintained under these conditions for a minimum conditioning period of 6 hours. Following column conditioning, selected test compounds and a series of a t least three n-paraffins were injected individually in the amount of 0.1 pl,, and their emergence times determined. The emergence time data were calculated as corrected retention volumes (S), VRO, and then expressed as common logarithms.
Calculations. T o quantify the differences in t’est compound retention data and, thus, provide for data comparison, the interaction coefficienh with reference to n-paraffins ( I p ) were calculated using the following equation :
I , = [log VRo (test compound) log V z o (hypothetical n-paraffin)]
x 100
The interaction coefficient is a term devised for expressing inverse GLC data. The hypothetical n-paraffin is one having the same molecular weight as the test compound. The values were multiplied by 100 for the convenience of handling whole numbers. Figure 1 illustrates the method by which the interaction coefficient was calculated. The test compounds and a series of n-paraffins of increasing molecular weight were run on the asphalt column being tested, and the retention data were plotted as a function of molecular weight as indicated. (The molecular weight range of the paraffins should cover t’he molecular weight range of the test compounds.) The interact’ion coefficient then was obtained by subtract’ing the logarithm V R O obtained for the hypothetical n-paraffin (distance BC) from the value obtained for the test compound (dist’ance AC). The I , value also can be derived mathematically. DISCUSSION OF TECHNIQUES
Procedure. Fluoropak 80 was chosen as the solid support because of its apparent neutrality towards the asphalt and the test compounds. The optmimumcolumn loading was 1 part of asphalt to 10 parts Fluoropak 80 by weight. Higher loadings gave packings which were tacky with many asphalts and, thus, difficult to prepare and pack uniformly; lower loadings gave emergence times for many of the test compounds that mere too short for accurate measurement. The 1: 10 packing rat’io produced an asphalt film on the surface of the Fluoropak 80 estimated at an average thickness of 15 microns. The choice of column operating conditions was a compromise, The temperature should be low enough to minimize both irreversible thermal changes in the asphalt column and irreversible interactions with the test compounds during the run. The temperature and flow rate of the carrier gas should be selected to give reasonVOL. 38, NO. 2, FEBRUARY 1966
241
Ttr( Coslpound on Asphalt
Table 1. Comparison of Interaction Coefficients for Homologous Series
Test compound Asphalt no. Methylcyclohexane Ethylcyclohexane Butylcyclohexane 2,3-Dithiabutane 3,CDithiahexane 4,5-Dithiaoctane Butvraldehvde He6taldehide Acetic acid Propionic acid Formarnide Acetamide Phenol 3-Methylphenol
Interaction coefficient (I,) 1 6 24 29 24 29 25 29 32
64
32 31
122
63 61 70 68 76 75 178 171
119
138 140
36
34 76 73 127 118
able emergence times for the different test compounds. On the basis of these considerations, an operating temperature of 130" C. and a column inlet pressure of 15 p.s.i. were selected. Proper selection of test compounds is an important consideration in the inverse GLC technique. Interpretation of retention behavior in terms of asphalt composition will ultimately rest on an understanding of the interactions of functional g r o u p in the test compound with functionality in the asphalt. It would be desirable to select test compounds whose functional groups are as specific as possible for groups present in the asphalt. However, because the purpose of t h b study was to determine if differences in asphalts could be detected by GLC interactions, test compoundq covering a wide variety of functional types were chosen. Among the type3 of compounds included were acids, alcohols, amines, aromatics, cycloparaffins, esters, olefins, sulfur compounds, and certain additional compounds which contained oxygen and nitrogen. I n the initial phase of the study, two or more compounds of the same structural type were used to determine if response of the test compound chosen mas typical for its chemical family. Emergence time data were reproducible within 1%. Within these limits,
Asphalt no.
wt. %
( Ramsbottom) Penetration. 25' c. 100/5,'0.1
mm.
Soctening point, C. R and B C/H, wt./wt. Nitrogen, wt. % Sulfur, wt. % Oils, wt. % Resins, wt. %
Asphaltenes, wt. 73
242
C Moltculor Weight
Figure 1.
Commercial
Commercial
___)
Illustration of interaction coefficient (I,)
AB = Interaction coefficient (Ip) AC = Log VRO test compound on asphalt BC = Log VRO of hypothetical n-paraffin
test compound data were reproducible on asphalt columns prepared repeatedly from the same batch of packing which had been stored under nitrogen. Duplicate GLC test compound data were obtained on each of the six asphalts when separate batches of packing were prepared at intervals of 3 to 12 months. Also, GLC data were reproducible on columns which mere exposed repeatedly three or four times to the same test compounds over a period of several weeks. Calculations. The difference in retention behavior of the test compounds and n-paraffins, as defined in the experimental section and illustrated in Figure l , has been termed the interaction coefficient ( I p ) . The interaction coefficient is thus the logarithm of the ratio of the retention volume of the test compound to the retention volume of a hypothetical n-paraffin of the same molecular weight, both determined on the same asphalt. The use of the molecular weight in establishing the point of reference (Figure 1) was adopted following reasoning similar to that of Littlewood (j),in which he showed that for n-paraffin stationary liquids and any solute the JP where V , is the specific logloV, a
Table II. Asphalt Property and Constituent Analysis Data 5 3 4 I 2
Source Carbon residue,
p
WilmShale oil ington residuum residuum
Commercial
RESULTS AND DISCUSSION
6 Commercial
Asphalts. Six asphalts were selected to test the feasibility of t h e inverse GLC technique for studying asphalts.
t'hetic)
These asphalts include four commercial petroleum asphalts (one a thermally cracked synthetic), a petroleum residuum (Wilmington), and a shaleoil residuum. They were chosen t o cover a wide range in properties. Property and constituent analysis data on the six asphalts are shown in Table 11. Constituent analyses were made by precipitation of asphaltenes (pentane digestion, 40 ml. per gram of asphalt) followed by separation of the resins from the oils on fuller's earth in the conventional manner.
( SYp-
24
29
8
18
19
45
21
33
132
147
73
2
59 8.6 0.55 3.74 13 59
55
28
ANALYTICAL CHEMISTRY
8.5 0.67
4.45 13 61
26
104 16.5 0.14 3.30
65
47 7.8 0.63 0.71 18 67
10
15
>90
34 7.9 2.42 0.75
30 8.4 1.12 2.17
78
10
12
25
retention volume of the solute and 111 is the molecular weight. I n the present work it was assumed that a similar, though inverted, situation exists in which the n-paraffin solutes show interactions similar to Littlewood's stationary phase. The n-paraffins were chosen as reference compounds on the assumption that they would be least affected by polar groups present in the asphalts, and the forces between the asphalt and n-paraffins would be due primarily to electronic dispersion forces. Thus, the interaction coefficient is a measure of the interaction of the functional groups in the test compounds with functionality in the asphalt. The ieteraction coefficients for a homologous series of test compounds is expected to be constant after the first few members of the series. Table I shows I , values for several homologous series determined using two asphalts. (Comparable data also were obtained on the four other asphalts used in the program.) The deviations within groups are minor. Although it may be desirable to choose higher members of a homologous series, volatility considerations at operating temperatures often dictate the use of lower members, amides and acids being examples.
...
...
The ability of inverse GLC to show differences among asphalts is demonstrated by Table 111. Interaction coefficients (I,) were obtained on the six asphalts, using a wide variety of test compound types. Differences in the interaction coefficients are readily seen among the asphalts, thus indicating differences in asphalt chemical composition. When comparing the magnitudes of the interaction coefficients, it should be kept in mind that these values are derived from the logarithms of the retention volumes. For example, the I , of 71 for butanol on asphalt N o . 6 represents more than a fivefold increase in its retention volume over that of the n-paraffin family. Evidence that certain test compounds are selective in indicating differences in functional groups within the asphalt is illustrated by comparing the behavior of 1-methylpyrrolidine and propionic acid on asphalts Sos. 3 and 4. With 1methyl-pyrrolidine there was a net increase in I , of 8 from Nos. 3 and 4, while propionic acid showed a net decrease in I, of 37. The more polar test compounds generally showed the larger Ip’s among the different asphalts. Typical of this response are propionic acid, formamide, and phenol. The more nonpolar test compounds, as typified by methylcyclohexane, 2-thiahexaneJ and toluene, however, showed leqs variation from asphalt to aqphalt. The large differences seen in the interaction coefficients with the more polar test compounds suggest strong interactions with polar or polarizable functions present in the asphalt. Asphalt properties and performance might be studied by establishing correlations with the differences in the interaction coefficients. Interpretation of the interactions should lead to a better knowledge of the chemical composition of asphalt. Asphalt Fractions. The potentiality of inverse GLC as a technique for t h e study of asphalt fractions is indicated by d a t a in Table IV. The seven fractions tested x e r e separated from the Wilmington crude oil residuum (aqphalt S o . 4). Fractions 1 through 6 were obtained by successive elution of the pentane-deasphaltened residuum on a Florex column using solvents of differing polarity and solvent power after the method of Boyd and Illontgomery ( 1 ) . Significant differences are seen among the fractions. Those fractions eluted from the Florex column with the more polar solvents, in general, gave larger interaction coefficients. This generalization should not be surprising, since fractions held more tightly by the Florev column would be expected to contain a greater percentage of the more polar inteiacting groups. However, because many different types of polar groupings exist in both the asphalt
Table 111.
Differentiation of Asphalts by Inverse GLC
Test compound Asphalt no. 1 -2 Butyl acetate 4 1-Decene 2-Methyl-24 pentanethiol 5 Allyl ether 24 Methylcy clohexane 34 Heptaldehyde 36 1-Methylpyrrolidine 40 2-Thiahexane 44 Butanol 47 Toluene 59 2-Methylp yridine 73 Propionic acid 86 Pyrrole 118 Phenol 127 Formamide Table IV.
Interaction coefficient (I,) 2 3 4 -3 5 0 3 2 2 2 3
23 33 37 42 45 47 62 83 89 120 132
6 8 24 42 42 45 60 51 71 136 108 147 164
3 7 24 37 50 43 51 48 69 99 94 125 149
5 -5 2
6 26 8
1
22 30 29 68 61 58 71 68 98 75 124 138 179
2 23 25 36 38 41 43 56 63 80 106 118
Differentiation of Fractions from Wilmington Residuum (Asphalt NO.4 by Inverse GLC
Interaction coefficient (I,) Fraction numbera Test compound 1 2 3 4 5 6 7 3 5 4 15 2 10 - 10 Butyl acetate 4 4 5 7 3 3 4 1-Decene 6 7 7 -3 5 7 2-Meth 12 pentanethiol - 1 Allyl 4 7 10 6 10 9 17 RIethvlcvclohexane 22 22 19 25 22 21 1.5 42 52 39 42 24 40 47 Heptkdehyde 105 114 109 64 101 35 43 1-Methylp yrrolidine 44 42 44 44 52 37 42 2-Thiahexane 54 68 64 70 37 51 58 Butanol 48 48 48 60 48 51 52 Toluene 68 92 79 94 72 49 66 2-Met hylpyridine 67 140 91 236 40 58 126 Propionic acid 89 96 120 74 93 119 118 Pyrrole 104 138 175 81 117 146 184 Phenol 102 139 156 149 206 180 193 Formamide a Fraction No. 1. Eluted with pentane (yield 37.5 wt. yo). No. 2. Eluted with carbon tetrachloride (yield 14.0 a t . 70). No. 3. Eluted with benzene (yield 5.4 wt. 70). No. 4. Eluted with chloroform (yield 14.2 wt. %). No. 5. Eluted with methanol (yield 11.2 wt. 70). No. 6. Eluted with pyridine (yield 7.7 wt. yo). No. 7. Asphaltenes (yield 10.0 wt.
Original asphalt 0
2
3
7 24 37 50 43 51 48 69 99 94 125 149
s).
fractions and the test compounds, many deviations from uniform changes in I , between the fractions are expected. (For example, compare propionic acid and phenol on fractions 5 and 6.) I n fact, these deviations indicate the usefulness of inverse GLC in detecting differences in the functionality present, and the interpretation of these differences offers a means of more fully understanding the chemical composition of the fractions. CONCLUSIONS
Results of the initial work indicate that inverse GLC will be useful in fingerprinting asphalts and studying fractions separated from asphalts. The technique should be useful in studying the effects of oxidation and weathering. With the aid of inverse GLC, the nature of the functional groups in asphalt may eventually be ascertained and quantitatively determined. Thus, inverse GLC
offers a new technique for studying asphalts and possibly other complex mixtures. LITERATURE CITED
(1) Boyd, R L L., Rlontgomery, D. S., Dept. of Mines and Tech. Surveys
(Canada), Fuels and Mining Practice Div., Internal Rept. FMP-61/86-RBS, (May 1961). (2) Heubner, 1‘. R., ANAL.CHEM.34, 488 11962). (3) James, 4. T., Martin, A. J. P., J. Biochem. 50, 679 (1952). (4) Kovats, E., 2. Anal. Chem. 181, 351 (1961). (5) Littlewood, A. B., J. Gas Chromatog. 1, (11) 16 (1963). (6) Wehrli, A., Kovats, E., Helv. Chim. Acta 42, 2709 (1959). RECEIVEDfor review October 15, 1965. Accepted December 6, 1965. Division of Analytical Chemistry, 150th Meeting, ACS, Atlantic City, N . J., September 1965. References to specific commercial materials or models of the equipment in this report are made to facilitate understanding and do not imply endorsement by the Bureau of Mines. VOL. 38, NO. 2, FEBRUARY 1966
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