a technique for utilizing more of the information which has been generated in the laboratory. Thus, a combination of infrared measurements, data treatment and transformation, and discriminant function analysis through computer assistance has resulted in effecting a more precise and accurate method of distinguishing between these heavy petroleum materials. An established statistical technique has hereby been applied successfully to a recently developed infrared procedure for heavier petroleum products to provide a useful and powerful technique for classification and may result in a promising application to further identification.
ACKNOWLEDGMENT It is a special pleasure to acknowledge the support and interest given by D. G. Ballinger, Director of the Analytical Quality Control Laboratory, throughout the investigation. Much credit is due R. C. Kroner, Chief, Physical and Chemical Methods, for his interest and help in this study. The assistance given by Hilda Knueven is greatly appreciated. Received for review January 4, 1973. Accepted August 16, 1973. Mention of the products and manufacturers i s for identification purposes only and does not imply endorsement by the Environmental Protection Agency.
Molecular Interactions of Asphalt: An Infrared Study of the Hydrogen-Bonding Basicity of Asphalt I?.V. Barbour and J. C. Petersen Lararnie Energy Research Center, Bureau of Mines, U.S. Department of the Interior, Laramie, Wyo. 82070
The hydrogen-bonding basicity of asphalts, oxidized asphalts, and asphalt fractions has been investigated by infrared techniques using phenol as the hydrogen-bonding acid. Evidence has been obtained that asphalts exhibit strong hydrogen-bonding basicity. Infrared frequency shift data give enthalpies of formation in the range 6-8 kcal/ mole for the asphalt-phenol hydrogen bond in CCI4 solution. The phenol interaction data suggest a hydrogenbonding base concentration of at least 2 mmoles per gram of asphalt. Air oxidation of asphalts at elevated temperatures increased the measured hydrogen-bonding basicity of the asphalt and suggests the formation of new hydrogen-bonding bases. Sulfur-containing molecules are suggested as important to the hydrogen-bonding basicity of asphalts, and the oxidation of the sulfur in these molecules could account for some of the increased basicity on air oxidation. Data from the methylation of an asphalt and its fractions with diaromethane suggest the occurrence of molecular aggregation via hydrogen bonding.
Intermolecular association forces have long been acknowledged as fundamental to the overall physical properties of materials (1-3). Previous reports ( 4 ) have suggested that intermolecular forces in asphalt, such as dipole, dispersion, electron donor-acceptor, and hydrogenbonding forces, are important to the macroproperties of asphalt; and several studies (4-6) have appeared concerning the possible chemical origin of these attractive forces. These reported studies, and our continued interest in the molecular interactions of asphalt, prompted this in(1) (2)
(3) (4) (5) (6)
D. J. Williams, "Polymer Science and Engineering." Prentice-Hall, Inc., Englewood Cliffs, N.J., 1971, pp 19-22. F. Rodriguez, "Principles of Polymer Systems." McGraw-Hill Book Co.. NewYork. N.Y.. 1970, pp 11-14. M . C. Shen, J. D. Strong, and F. J. Matusik. J. Macrornol. Sci., Parf B, 1, 15, (1967). C. Mack, "Bituminous Materials: Asphalts, Tars, and Pitches, Vol. 1." A. J. Hoiberg. Ed., lnterscience Publishers, New York. N.Y., 1964, pp 31-6. J. C. Petersen, R. V. Barbour. S. M. Dorrence. F. A. Barbour, and R. V. Helm.Ana/. Chern.. 43.1491 (1971). J. C. Petersen, h e / , 46, 295 (1967).
vestigation of hydrogen-bonding in asphalt and, in particular, the existence and nature of hydrogen-bonding bases in asphalt. The formation of intermolecular hydrogen bonds in asphalt requires the specific interaction of two molecular functions-a proton donor, or hydrogen-bonding acid, and a proton acceptor, or hydrogen-bonding base (7).This interaction to form the hydrogen-bonded complex can be depicted in the general case of the donor acid, Asphalt-H, and acceptor base, Asphalt-B, as Asphalt-H
+ Asphalt-B 5 +
[Asphalt-H Asphalt-B] AHf,,, (1) This interaction is a rapid, reversible equilibrium and is characterized by an eqaiilibrium constant, Keq, and a formation enthalpy or hydrogen bond strength, AHcorm. In asphalt the natural occurrence of such hydrogenbonding acids as carboxylic acids, phenols, amides, and pyrroles is generally accepted (8, 9). Previous work has shown that several of these molecular types form hydrogen bonds in asphalt (6). Petersen (5) has tentatively identified a cyclic amide compound type in asphalt and has demonstrated the ability of these amides to associate strongly with carboxylic acids. Another study (6) has pointed to the involvement of OH and NH functionality in asphalt as hydrogen-bonding acids. The hydrogen-bonding bases in asphalt have received much less study. Yen (10) has found that large *-aromatic systems in asphaltenes are capable of electron donor-electron acceptor activity, and he suggests that a-aromatic systems may participate as electron-donor bases. Petersen (5, 11) has shown that carboxylic acids and cyclic amides act as hydrogen-bonding bases in asphalts by self-association to form dimer structures. (7) G. C. Pimental and A. L. McClellan, "The Hydrogen Bond," W. H. Freeman and Co., San Francisco, Calif., 1960. (8) R . N. Traxler. "Asphalt, Its Composition, Properties, and Uses." Reinhold Publishing Corp.. New York, N.Y., 1961, pp 7-32. (9) E. J. Barth. "Asphalt," Gordon and Breach Science Publishers, Inc.. New York. N.Y., 1962, pp 109-81. (10) T. F. Yen, Arner. Chern. SOC..Div. Fuel Chern., Prepr., 15, (1) 93 (1971). (11) J. C. Petersen, J. Phys. Chern., 75, 1129 (1971).
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
273
o'O .5
r--l-A I
0
I
0,012
1
0024
I
0036
I
0.048
I 0060
TOTAL PHENOL CONCENTRATION, M
Figure 1. Absorbance of free phenolic OH (3610 cm-') vs. total phenol concentration for phenol-only and asphalt-phenol mixtures (asphalt concentration, 0.05 gram/rnl)
In this study, we report the spectroscopic observation and measurement of the intermolecular association of the hydrogen-bonding acid, phenol, with asphalts, with oxidized asphalts, and with asphalt fractions. Experimental evidence is presented that asphalts possess considerable hydrogen-bonding basicity, that this basicity increases upon oxidation, and that the hydrogen-bonding bases naturally present are capable of forming energetically strong hydrogen bonds. Additionally, we present evidence for molecular aggregation in asphalts attributable to hydrogen bonding.
EXPERIMENTAL Materials. Five asphalts were used in this study, four of which were used as supplied by the Federal Highway Administration (FHWA) from the California Road Test series (12) and the fifth a Wilmington (Calif.) asphalt previously described (13-16). The oxidized asphalt samples were prepared by air oxidation in an inverse gas-liquid chromatographic column (17)a t 130 "C for 24 hours. Phenol was Eastman reagent grade and was distilled just prior to use. Carbon tetrachloride (CCl,) was Baker and Adamson reagent grade distilled from phosphorus pentoxide prior to use and stored over 4A molecular sieve. Chromatographic-grade basic alumina, 100-200 mesh, Activity 1, from Bio-Rad Laboratories was used for the chromatography without further activation. Diazald, for the preparation of diazomethane. was obtained from .41drich Chemical Company. Tetrahydrofuran was used as received from Eastman Chemical Company. Infrared Spectra. Infrared spectra were recorded on a PerkinElmer Model 521 infrared spectrophotometer. All spectra were obtained using a 0.5-mm pathlength sample cell and a matching reference cell (NaC1 windows). The sample cell was maintained a t h constant temperature of 30 f 0.5 "C using a Barnes Engineering Co. thermoelectric temperature chamber with KBr windows. The sample cell was allowed to thermally equilibrate 20 minutes in the temperature chamber prior to recording each spect.rum. Machine spectral response was monitored and maintained constant throughout each series of spectra by regular recalibration with a 0.060Mphenol solution. Determination of Phenol Interaction Value. Solutions of phenol in cc14 were prepared in 100-ml volumetric flasks a t concentrations of 0.024, 0.050, 0.072, 0.096, and 0.120M. A 100.0 mg/ml solution of the desired asphalt sample in CC14 was prepared in a 10-ml volumetric flask. Spectral samples of the phenol-only solutions were prepared by making a 1 : l quantitative dilution of each phenol solution with CC14 in a 5-ml septum-capped vial, using a hypodermic syringe. Spectra of the phenol-only solutions in the region of 4000-3000 cm--1 a t the concentrations of 0.012, 0.025, 0.036, 0.048, and 0.060M were recorded. In a similar manner, using septum-capped vials, five asphalt-phenol solutions were prepared by mixing quantitatively 1 ml of the asphalt solution (12) F . N. Hveem, E. Zube, and J. Skog, Amer. SOC.Test. Mater., Spec. Tech. Pub/., 227,3 (1959). (13) R . V. Helm. Ana/. Chem., 41, 1342 (1969). (14) R. V. Helm and J . C. Petersen, Anal. Chem., 40,1100 (1968). (15) T. C. Davis, J . C. Petersen, and W. E. Haines, Anal. Chem., 38, 241 (1966). (16) J . W. Ramsey, F. R . McDonald, and J . C. Petersen, lnd. Eng. Chem., Prod. Res. Deveiop. 6,231 (1 967). (17) T. C.Davis and J. C. Petersen, Anal. Chem., 38,1938 (1966). 274
with 1 ml of each of the standard phenol solutions. Spectra, in the region of 4000-3000 cm,-l were recorded for each of these asphalt-phenol mixtures. Absorbance of the free phenolic OH band a t 3610 cm-1 was read directly from the spectrum using the baseline method. Interfering phenolic absorbance a t 3610 cm-1 due to the phenols naturally present in the asphalt as measured by a separate spectrum of asphalt-only (50.0 mg/ml solution), was subtracted from the recorded free phenolic band of the asphaltphenol mixture. A plot (Figure 1) was made of free phenol absorbance us. total added phenol for the phenol-only solutions and the asphalt-pheno1 mixtures. The difference in area under the two lines after extrapolation to the origin, expressed as a percentage of the total area under the phenol-only line, was the phenol interaction value (PIV) for the asphalt sample. PIV's were calculated as follows:
PIV
=
A-B
x loo A
where A = area under phenol-only line and B = area under asphalt-phenol line. PIV's were found to be reproducible to &1PIV unit upon repetitive determi. ations. Fractionation of Wilmington Asphalt. The asphalt, 50 grams, was deasphaltened by digestion overnight with 2.5 1. of n-pentane. The mixture was vacuum-filtered to collect the asphaltenes which were then exhaustively extracted in a Soxhlet extractor with n-pentane and dried under vacuum to give 6.5 grams (13%) asphaltenes. The Soxhlet extract was combined with the initial filtrate and the resulting maltenes were recovered by rotary evaporation of this combined solution. Thirty grams of maltenes was dissolved in 100 ml of n-hexane and charged to a column (1.25 m x 35 mm) of 900 grams of basic alumina wet packed in n-hexane. Successive elutions with 3 1. of n-hexane, 3 1. of benzene, and 3 I. of 4:l benzene/methanol yielded 7.9 grams (26%) saturates, 10.6 grams (35%) aromatics: and 8.8 grams (29%) polar aromatics, respectively. Solvent was removed from each of these fractions by exhaustive rotary evaporation under vacuum. Methylation. Approximately 0.025 mole of diazomethane (explosive and extremely toxic) was generated as previously described (18) using the precursor i"\r-methyl-N-nitroso-p-toluenesulfonamide (Diazald). A high-boiling alcohol, 2-(2-ethoxyethoxy)ethanol, was used in place of ethanol in the diazomethane generator t o avoid contamination of the methylated asphalt with traces of alcohol. The diazomethane-ethyl ether solution was distilled from the generation flask into a stirred, cooled (0 "C) solution of 2.5 grams of asphalt in 100 ml of anhydrous tetrahydrofuran. After addition of the diazomethane, the asphalt solution was stirred for 2 hours at 0 "C and allowed to stand at room temperature for 16 hours. Unreacted diazomethane was removed by bubbling dry nitrogen gas through the solution for 2 hours. Solvent was removed from the asphalt sample by rotary evaporation under vacuum while heating the sample flask with boiling water.
RESULTS AND DISCUSSION The use of phenol as a test hydrogen-bonding acid in both spectroscopic ( 19-23) and calorimetric (24-27) investigations of hydrogen-bonding bases is well documented. In the present study, our choice of phenol as the test hydrogen-bonding acid was based on three factors: the ability to reliably observe and measure spectroscopically the free and the bonded phenolic OH group in the infrared in mixed asphalt-phenol solutions; the ability to work with high (up to 0.06M) test-acid concentrations without seri(18)T. H. J. DeBoer and N. J . Backer, Recl. Trav. Chim. Pays-Bas, 73, 229 (1954). (19) J . H . Neison, L. C. Nathan, and R. 0. Ragsdale, J. Amer. Chem. Soc.. 90,5754 (1968). (20) T . Gramstad, Spectrochim. Acta, 19, 497 (1963). (21) B. 8. Wayland and R . S. Drago, J . Amer. Chem. Soc., 86, 5240 (1964). (22) 2 . Yoshida and E. Osawa, J. Amer. Chem. Soc., 87,1467 (1965). (23) D. P. Eyman and R. S. Drago, J . Amer. Chem. Soc.. 88, 1617 (1966). (24) T. D. Epley and R . S. Drago, J. Amer. Chem. Soc., 89, 5770 (1 967). (25) D . Neerinck, Ann. Chim., 4, 43 (1969). (26) E. M . Arnett, T. S. S. R . M u r t y , P. von R . Schleyer, and L. Joris, J . Amer. Chem. SOC., 89,5955(1967). (27) S. S . Barton, J. P. Kraft, T. R. Owens, and L. J. Skirner, J . Chem. SOC.,3,339 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, F E B R U A R Y 1974
Table I. Phenol Interaction Values for Unoxidized and Oxidized Whole Asphalts PIV Asphalt
Unoxidized
Oxidized4
25 24 13 22 25
31 28 23 32 33
A H J D
Wilmington asphalt a
Air oxidized as 1 5 - p film, 24 hr at 130 "C.
ous complication from test-acid self-association; and the mutual compatibility of phenol and asphalt in CC14 solution. The hydrogen-bonding interaction of direct interest was the association of phenol with asphalt. This interaction can be represented in a simplistic manner as Asphalt B
+ HO
0 ~
[Asphalt B H O o ]
(3)
The determination of a conventional equilibrium constant for the interaction was not possible for lack of a reliable molar concentration term representing the concentration of asphalt bases. Although it was possible to calculate an equilibrium constant based on the osmometric determination of the average molecular weight for the asphalt, the use of average molecular weight as a measure of the molar concentration of hydrogen-bonding base functionality is clearly misleading. The complex chemical composition of asphalt prohibits such use except as a first approximation. An infrared technique was therefore devised to measure the relative hydrogen-bonding basicity of asphalt with respect to phenol. The spectra of a series of asphalt-phenol solutions were obtained holding the asphalt concentration constant while varying the concentration of phenol. The spectra of a corresponding series of phenol-only solutions were also obtained. A plot was then made for each series of spectra, plotting absorbance of the free phenolic OH band a t 3610 cm-' us. total phenol concentration. A representative plot is shown in Figure 1. The difference in area under the lines in the plot was used as a direct measure of the percentage of the added phenol which was involved in a hydrogen-bonding interaction with the asphalt. This percentage of phenol bonded was termed the "phenol interaction value," or PIV. In all samples studied, confirming evidence that the loss of free phenolic OH absorbance resulted from hydrogen bonding between the phenol and a hydrogen-bonding base in the asphalt rather than some unsuspected phenolconsuming reaction was the corresponding appearance in the asphalt-phenol spectra of a broad band centered between 3100-3350 c m - l which is characteristic of' hydrogen-bonded phenol (28). In addition, an increase in temperature of the asphalt-phenol solution in the sample cell caused dissociation of the hydrogen-bonded complex as evidenced by the decrease in bonded OH and increase in free OH absorbances. Phenol Interaction of Asphalts and Oxidized Asphalts. Five asphalts were selected for investigation. Four were asphalts from the California Zaca-Wigmore Road Test series (12), and the fifth was a Wilmington asphalt chosen because of the extensive studies previously reported (13-16) on this particular asphalt. Phenol interac(28) L J Bellamy. "The Infrared Spectra of Complex Molecules," 2nd ed., John Wiley and Sons, Inc , New York, N Y , 1959, pp 95-106
tion values, reported in Table I, were determined for each of these asphalts, unoxidized and oxidized. With all of the un xidized asphalts except asphalt J , 22 to 25% of the added phenol entered into a hydrogen-bonding interaction with the asphalt; with asphalt ,J, only a 13% interaction was observed. Oxidation with air a t 130 "C produced a considerable increase in hydrogen-bonding basicity in every asphalt tested. This increase, ranging from 4 to 10 PIV units for the five asphalts, supports the previous observation ( 6 ) that naturally occurring hydrogen-bonding acids (OH and NH) in asphalt are more highly associated in oxidized asphalts than in unoxidized asphalts. The proposed explanation that oxidation of asphalts leads to the formation of new hydrogen-bonding bases appears to be valid and suggests that a part of the physical hardening observed in asphalts upon oxidative aging is related to an increase in intermolecular association due to hydrogen bonding. With all the asphalts examined, plots of free phenol concentration us. total phenol concentration, a t a fixed asphalt concentration, resulted in a straight line with no noticeable curvature even a t high phenol concentrations. Calculations based on the equilibrium reaction of Equation l indicate that, within the range of phenol concentrations considered in these experiments, straight-line plots would be obtained only under restricted conditions of equilibrium constant and base concentration values. If the equilibrium constant is small (
