Molecular and Unit Sheet Weights of Asphalt Fractions Separated by Gel Permeation Chromatography Graeme A. Haley School of Highway Engineering, University of New South Wales, Box I , Kensington 2033, N . S .W . Australia An unblown Kuwait asphalt and an asphaltene sample separated from an air blown Kuwait asphalt were fractionated by gel permeation chromatography. Molecular weight measurements, using a membrane and a vapor pressure osmometer in four solvents, toluene, chloroform, methylene chloride, and a dioxane-chloroform mixture showed the different abilities of each solvent to break up intermolecular association. In the less powerful solvents toluene and chloroform, incomplete asphaltene-asphaltene dissociation led to high molecular weights. Nuclear magnetic resonance ‘unit sheet’ weights were determined for fractions of both the asphalt and asphaltene. The asphalt fractions’ molecular weights determined in the most powerful solvent, dioxane-chloroform, were consistent with their ‘unit sheet’weights, for a value of the hydrogen to carbon ratio of the nonaromatic group of 1.92. The unit sheet weight of the asphaltenefractions was found to be about 1050 and substantially independent of molecular weight. This suggests that the asphaltene molecules consist of a number of unit sheets linked together rather than one large pericondensed aromatic ring system.
VAPORPRESSURE OSMOMETRY (VPO), because of its use with a wide range of solvents and temperatures, together with its speed and small sample size, has become a convenient method for molecular weight determination of asphalt samples in recent years. Benzene and chloroform have been the main solvents used for the determination of molecular weights by VPO although other solvents have also been tried. Girdler ( I ) found that for two unfractionated asphaltenes, trichloroethylene as a solvent gave a lower molecular weight than benzene. H e believed that less association occurred in the more powerful solvent although some degree of association still remained. Dickie and Yen ( 2 ) in comparing tetrahydrofuran with benzene as VPO solvents for a number of asphaltene and resin samples stated that there appeared to be little difference between the molecular weight obtained for each of the samples. Neumann and Bellstedt ( 3 ) , in determining the molecular weight of an unfractionated petroleum-ether asphaltene, tried a number of VPO solvents ranging in solvent power from carbon tetrachloride t o pyridine at temperatures of 25 to 90 “C. They observed a distinct decrease in molecular weight with increasing temperature and increasing solvent power. Molecular weight differences for different solvents should become more apparent when asphalt separation into more homogeneous fractions is carried out. Determination of the mean molecular weight by either VPO or membrane osmometry gives a number average and, for a material with a wide range of molecular sizes such as the whole asphalt the mean obtained is disproportionately influenced by the size of the lower molecular weight constituents. The molecular weight of a gel permeation chromatography (GPC) fractionated (1) R. B. Girdler, Proc. Ass. Asphalt Paoing Technol., 34, 45 (1965). (2) J. P. Dickie and T. F. Yen, ANAL.CHEM., 39, 1849 (1967). (3) H. J. Neumann and F. Bellstedt, Erdoel Kolile, 22, 19 (1969).
asphaltene has been determined by Altgelt ( 4 ) . Toluene, benzene, chloroform, and xylene were used as VPO solvents for one fraction and the resultant molecular weights appeared quite similar. Aggregation or low solubility was indicated by convex concentration 1‘s. molecular weight curves for some of the lower fractions. Altgelt proposed that the decrease in molecular weight was due t o dissociation of low molecular weight contaminants from a n asphaltene molecule rather than dissociation of a n asphaltene-asphaltene aggregation. Earlier measurements of asphalt molecular weights by VPO, as well as other methods, led Dickie and Yen ( 2 ) to propose that asphalt molecules can be built u p by association of individual asphalt unit sheets. Dickson, Davis, and Wirkkala ( 5 ) attempted to obtain a comparison between the VPO molecular weight of a sample fractionated by G P C with its unit sheet weight, as determined from nuclear magnetic resonance (NMR) data by the method of Ramsey, McDonald, and Petersen (6). The molecular weight diverged from the unit sheet weight in the high molecular weight regicn. This divergence was attributed t o association or polymerization of the asphalt units in solution. Association may lead to larger molecular weights through the combination of unit cells held together by intermolecular forces. These intermolecular forces can be broken down by the use of more powerful solvents. Helm and Petersen (7) have compared the effect of methylene chloride and carbon tetrachloride, as solvents on the infra-red carbonyl absorption of a n asphalt. The stronger absorption with the more powerful solvent, methylene chloride, indicated that some breakdown of intermolecular forces had occurred. EXPERIMENTAL
Preparation of Sample. An asphaltene sample was obtained from an air blown Kuwait asphalt by the n-pentane precipitation method. The asphalt was diluted with 200 times its volume of n-pentane and, after refluxing, the asphaltene precipitate was recovered by filtration. The asphaltene sample and an unblown Kuwait asphalt were fractionated on a 2-cm id x 150-cm long column, packed with two pore size polystyrene gels prepared by the method described by Altgelt (8). The solvent system of a 90% benzene and a 10% methanol mixture was operated at a flow rate of 2 ml/min to give a fraction volume of 5.5 ml. A sample of 1 gram was used to give 25 asphaltene fractions and 35 asphalt fractions suitable for further analysis. Molecular Weight Determinations. Molecular weights of GPC fractions were determined using four different solvents. Toluene was selected as a standardizing solvent and fractions which were thought to be above a molecular weight of 10,000 (4) K . H. Altgelt, Amer. Cliem. SOC.,Dir. Petrol. Chem., Prepr., 13(3), 37 (1968).
(5) F. E. Dickson, B. E. Davis, and R. A. Wirkkala, ANAL.CHEM., 41, 1335 (1969). (6) T. W. Ramsey, F. R. McDonald, and J. C. Petersen, brd. ,Gig. Chem., Prod. Res. Decelop., 6, 231 (1967). (7) R. V. Helm and J. C. Petersen, ANAL.CHEM., 40, 1100 (1968). ( 8 ) K. H. Altgelt, Makromol. Clzem., 88,75 (1965). ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
371
ELUTION V O L U M E mi Figure 1. Weight distribution of fractions separated from the asphalt by GPC
200
400
300 E L U T I O N VOLUME
ml
Figure 3. Molecular weights of selected fractions from the asphalt in different solvents 0 toluene
o chloroform methylene chloride X dioxane-chloroform M.O. membrane osmometer result
+
under vacuum to remove any absorbed benzene. Spectra and five integrals were recorded at a sweep width of 500 Hz and a scan speed of 500 sec. No proton signals were found in the 750-500 Hz range. Solutions of GPC fractions in the N M R solvent of deuterated chloroform were approximately 10% by weight and integrations were from high to low field. 300 ELUTION
VOLUME
400
Figure 2. Weight distribution of fractions separated from the asphaltenes by GPC were run o n a 501 Mechrolab Membrane Osmometer at 37 "C. Four concentrations in the range 0.1-10 g/l. for each fraction were tested and readings were repeated until constant pressures were obtained. Samples below 10,000 were run at 37 "C o n a 301A Mechrolab Thermoelectric Vapor Pressure Osmometer. The other three solvents used were chloroform, methylene chloride, and a 50 :50 vjv dioxane-chloroform mixture. The molecular weights of fractions dissolved in these soIvents were all determined by VPO since the solvents were too volatile for membrane osmometry measurements on the high molecular weight fractions. Chloroform was used to allow a comparison to be made with the results of previous experiments while methylene chloride and the dioxane-chloroform mixture were used to examine the effect of more powerful solvents on the fraction molecular weight. Dioxane-chloroform rather than an acetone mixture was decided upon because of its higher boiling point although both have similar solventpolymer interaction energies. Minimum sample size was 10 mg with a series of four concentrations for each fraction. Resistance readings were taken for all solvents after 2 min, except for the dioxane-chloroform mixture where a 5-min reading time was needed. All solvents were calibrated with benzil. NMR Spectra. The NMR spectra were obtained using a Varian HA 60 spectrometer operating at 60 MHz with tetramethylsilane as an internal standard. All GPC fractions examined were dissolved in carbon tetrachloride and dried 372
RESULTS AND DISCUSSION
mi
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, M A R C H 1971
Molecular Weight Calculations. The membrane osmometry readings were plotted and extrapolated on an osmotic pressure-concentration us. concentration graph. The method of plotting vapor pressure osmometry readings on a resistance/concentration L'S. concentration graph, although suitable for most fractions, was found to be unsatisfactory for some toluene and some chloroform solvent fractions. A method of calculation from the shortened McMillan and Mayer (9) virial expansion equation was used for these fractions. The equation is:
R
= a,
+ alc + a2c2
(1)
and al = KIM
where
a,,
c = sample concentration (gramsjcm3) R = sample resistance at concentration c (ohms) a, and az = virial coefficients K = solvent calibration factor M = molecular weight.
The graphical method assumption that a, is zero, is eliminated using the above equation. Difficulties with the graphical method were also experienced by Adicoff and Murbach (10) and Altgelt ( 4 ) . It was found that for all frac(9) W. G . McMillan and J. E. Mayer, J . Chem. Phys., 13, 276 ( 1945). (10) A. Adicoff and W. J. Murbach, ANAL.CHEM., 39,302 (1967).
=
t
aR C
k4
c3 10 w
3 [L
a
J 3 U
W _I
I
0
10'
200
3 0 0 ml 400 ELUTION VOLUME
Figure 4. Molecular weights of selected fractions from the asphaltenes in different solvents 0 toluene
chloroform methylene chloride X dioxane-chloroform M.O. membrane osmometer result 0
+
(1 1) K . H. Altgelt, J. Appl. Polym. Sci., 9, 3389 (1965). (12) L. R . Snyder, ANAL.CHEM., 41, 1223 (1969). (13) D. Bynurn, R . N. Traxler, H. L. Parker, and J. S. Ham, J. h u t . Petrol., 56, 147 (1970).
L 3
Figure 5. Effect of asphalt fraction concentration in VPO method for measuring molecular weight 0 asphalt A asphaltenes
Table I. Aromatic Hydrogen to Carbon Ratios for a Pericondensed System No. of rings 1
2 3
tions examined a2was between +0.002 and -0.005. Chloroform gave a slightly positive a? while all other solvents had zero or negative a2. Effect of Solvent on Molecular Weight. The GPC chromatograms for the asphalt and the asphaltene samples are shown in Figure 1 and Figure, 2. The extent to which intermolecular bonding can be broken up by the use of more powerful solvents is illustrated by Figure 3 and Figure 4. These curves can also be used to calibrate the chromatograms with respect to molecular weight. Fractions dissolved in toluene and chloroform give molecular weight results for calibration of chromatograms similar t o those obtained by Altgelt (8, I ] ) , Dickson et a / . (9,Snyder (12), and Bynum ef a/. (13). The results for the membrane osmometer appear interchangeable in the high molecular weight region with those of the vapor pressure osmometer. The molecular weights of the dioxanechloroform fractions show a definite difference from those in other solvents, although the methylene chloride fractions were also slightly lower in molecular weight than those in toluene and chloroform. The amount of aggregation in the dioxane-chloroform solvent can be judged from the concentration effect on resistance in Figure 5. The plots t o determine the molecular weights for the dioxane-chloroform fractions d o not have concave slopes in the low molecular weight range, although they d o have a slightly negative slope. This slope tends t o disappear as molecular weight increases with less dependence of molecular weight o n concentration. Altgelt ( 4 ) has plotted apparent molecular weight against concentration, which gives a concave slope if dissociation occurs. H e suggested that the decrease in apparent molecular
30 g/l.
10 20 CONCENTRATION
0
r
4 5
6 7 9 17 22
CP 6
C, 0
CW
6
1.00
8 10
2 4 6 8 10 12 16 32 42
10
0.80
14 16 20 22 24 30
0.71 0.62 0.60 0.54
10 12 12 12 14 18 20
Hor/Car
50
62
0.50
0.47 0.36 0.32
weight, with a decrease in concentration, was caused by dissociation of low molecular weight contaminants from asphaltenes rather than asphaltene-asphaltene dissociation. From Figure 4 and Figure 5 , there appears t o be incomplete asphaltene-asphaltene dissociation with less powerful solvents. A contaminant-asphaltene aggregation, if not broken u p by the G P C solvent of benzene-methanol, with the contaminants separated into the lower GPC fractions, is unlikely t o be broken u p in the less powerful VPO solvent of benzene, where the concentration is higher than the maximum G P C fraction concentration. NMR Calculations. The Ramsey (6) assignments of proton bands in the NMR spectra of asphalts were used t o determine the parameters of Brown and Ladner (14). These were the aromaticity,f,, the degree of substitution, u, and the aromatic hydrogen to carbon ratio, H,,/C,,, of the hypothetical unsubstituted aromatic material. These parameters depend o n the assumptions that more than three rings are pericondensed and that no aromatic rings are joined by single bonds. The theoretical aromatic hydrogen to carbon ratios for a number of hexagonal pericondensed structures are given in Table I. An equation can be obtained by a regression analysis of these data to relate the internal condensed carbon Ct t o the peripheral carbon C,. This equation is: 6Ci
- C,*+ 8C, - B
=
0
(3)
(14) J. K. Brown and W. R . Ladner, Fuel, 39, 87 (1960). ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
373
Table 11. NMR Proton Ratios for Asphalt and Asphaltene Fractions Fraction No. Ha/H Hp/H H-JH HA/H Asphalt 16.3 60.0 19.2 4.5 lop1 17.2 58.5 20.0 4.3 20 11.6 65.2 19.7 3.5 24 14.1 66.3 17.8 1.8 29 61.7 12.5 22.8 3.0 39/40 17.6 55.0 20.8 6.6 Asphaltene 21.6 50.0 20.4 8.0 8 19.8 50.4 22.7 7.1 12 19.5 52.2 21.1 6.2 14 19.7 54.3 20.0 6.0 18 20.8 49.7 21.6 7.9 24 20.8 51.9 21.0 6.3 32/33 23.6 45.9 20.7 9.8 41
I
Table 111. Structural Parameters for Asphalt Fractions Fraction Elution No. volume, ml CN, C/H loill 243.5 24.1 0.826 20 288.8 15.0 0.670 24 307.7 15.9 0.643 29 326.2 14.9 0.636 39/40 378.5 21 .o 0.707 Asphalt ... 15.5 0.649
z
where B is a variable which changes slightly, depending on the values of C i and C,. F o r the pericondensed model being considered, if C,, is the aromatic carbon and H,, is the aromatic hydrogen then: CQ, =
ci
+ c,
(4)
and Substituting Equations 4 and 5 in Equation 3 gives
c u r H=z u -r - T H u r+ d B 6 Results were calculated using a value of 12 for B, since this value gives a n exact fit for one and two rings and a close if not exact fit for the other ring systems. The variation produced by having a number of C i values for the same C , is taken into account by allowing fractional values of C,. Rearranging Equation 6 and dividing both sides by C u r gives Ha,- - 1 -
Cur
+ 1 / 6 C Q r- 11
(7)
Cur
The value of the parameter HQr/CQr is known and its substitution in Equation 7 gives the amount of aromatic carbon. can be obtained by using the aromaticity The total carbon, CT, parameter. CT = Curifu
(8)
The unit sheet weight, USW, is now given by: (9)
where CT72 is the per cent carbon from elemental analysis. A computer program can calculate the unit sheet weight using the above series of equations from the NMR proton 374
e
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
200
I
.
I
I
I
I
1
.
300 mi ELUTION
l
/
40 I
VOLUME
Figure 6. Best X ratio for coincidence of unit sheet weight with molecular weight for the asphalt fractions X 0
VPO molecular weights NMR unit sheet weights
----~
ratios and the relative amounts of carbon, hydrogen, and other elements. Asphalt Structure. The proton ratios in Table I1 for the asphalt GPC fractions show some interesting points. H,/H, hydrogen alpha to aromatic rings, and HA,”, aromatic hydrogen, both produce a minimum near the center of the molecular weight range while H,/H, beta hydrogen and further removed methylene hydrogen, goes to a maximum. The methyl hydrogen gamma or further removed from an aromatic ring, H J H , appears to remain constant. The highest molecular weight fraction is very similar to the lowest molecular weight fraction and it is only the difference carbon to hydrogen ratios, given in Table 111, which causes a difference in unit sheet weight. The carbon to hydrogen ratio also goes to a minimum together with the naphthenic carbon, C N , calculated by Williams’ method ( 1 3 , near the center of the molecular weight range. It appears from the variation of the different functions with molecular weight that the more paraffinic fractions occur in the central molecular weight range while the lower and higher molecular weight fractions contain the more naphthenic and aromatic structures. A comparison between the asphalt molecular weight and its unit sheet weight is shown in Figure 6. Brown and Ladner (14) in studying a number of coals used a value of x = y = 2.0, where x and y are the respective hydrogen to carbon ratios for the alpha carbon groupings and the indirectly or unattached carbon groupings. They found that for this value of x and y , plots of N M R parameters against carbon content gave a linear correlation. Ramsey ( 6 ) and Dickson ( 5 ) adopted x = y = 2.0 for their asphalt N M R experiments. The value of x = I .92 in Figure 6 gives a much better correlation between the unit sheet weight and the dioxane-chloroform molecular weight. All the molecular weights lie between x = 1.89 and 1.95 to give the desired link of molecular weight with unit sheet weight as sought by Dickson ( 5 ) . The development of a condensed naphthenic structure is needed to support a value of x and y lower than 2. Since the (15) R. B. Williams, Amer. Sci. Testhg Muter., STP 224 (1957).
between the asphaltene and asphalt results are shown in comparing their unit sheet and molecular weights. Since the asphaltene unit sheet weight does not change with molecular weight, then it is unlikely that the asphaltenes are formed by an extension of the asphalt pericondensed ring system. The formation of asphaltenes through the polymerization of a number of unit sheet weights is in agreement with conclusions presented by Yen, Erdman, and Pollack (16), from X-ray data o n asphaltenes and also with those of Ferris, Black, and Clelland (17). Ferris found, from X-ray diffraction techniques on a solvent fractionated asphaltene, that the unit sheets were all approximately the same dimensions and postulated a sixteen-ring structure of condensed aromatic sheets linked by saturated bridges. The sixteen-ring system with an aromaticity of 0.45-0.50 would give a unit sheet weight which would agree quite well with the results presented for the GPC fractionated asphaltene. Further work to examine more closely the changes occurring during air blowing are at present being undertaken and this is expected to give a clearer picture of asphaltene formation.
Table IV. Comparison of NMR Unit Sheet Weights with Interpolated VPO Molecular Weights for Asphaltene Fractions Unit MolecFraction Elution sheet ular C/H weight5 weight No. volume C,V % 8 228.3 32.1 0,860 lo00 4200 3260 0.882 1350 247.5 35.7 12 2850 0.845 1080 28.6 14 257.5 2220 0.850 870 29.6 18 276.3 1050 1540 0.856 303.9 32.8 24 0.896 780 910 342.3 28.0 32/33 0.861 880 2900 28.4 Asphaltene = y = 1.92. ,
.
I
aromatic carbon appears to form a condensed ring structure, it is highly likely that the naphthenic carbon is also present in this form. The condensation of the naphthenic carbon will produce internal branching with a simultaneous lowering of the value of x and y . An artificial change in x and y will also be produced b y the number average molecular weight coniparison with the weight average unit sheet weight. This effect would be rather small, however, with fractionated samples. Asphaltene Structure. The asphaltene fractions are all very similar in hydrogen distribution, and there appears to be no indication of increasing or decreasing proton ratios as was the case with the asphalt. The calculated factors, naphthenic carbon, carbon to hydrogen ratio, and the unit sheet weight, given in Table IV, are all similar for the different fractions. The unit sheet weights are contained within the range 1050 300 for x = y = 1.92 whereas the molecular weights increase up to 4200. The most important differences
ACKNOWLEDGMENT
The author thanks Professor D.F. Orchard, Dr. E. J. Dickinson, and Mr. N. W. West for their helpful criticisms and assistance. RECEIVED for review September 21, 1970. Accepted November 16, 1970. Work supported by an Australian Road Research Board Fellowship. (16) T. F. Yen, J. G. Erdman, and S. S. Pollack, ANAL.CHEM., 33, 1587 (1961). (17) S. W. Ferris, E. P. Black, andJ. B. Clelland, Iud. Eng. C/iem., Prod. Res. Decelop., 6 , 127 (1967).
*
Application of Photoelectron Spectrometry to Pesticide Analysis.
II
Photoelectron Spectra of Hydroxy-, and Halo-Alkanes and Halohydrins” A. D. Baker, D. Betteridge,’ N. R . Kemp, and R. E. Kirby2 Department of Chemistry, Unicersity College of Swansea, Singleton Park, Swansea, Glum., U . K .
The photoelectron spectra of ethylene fluoro-, chloro-, bromo- and iodohydrin have been obtained and are discussed in relation to the photoelectron spectra of alkanes, halo alkanes and alcohols, which have also been measured. There i s an interaction between the nonbonding orbitals in dihalo alkanes, ethylene chlorohydrin and ethylene bromohydrin. The extent of interaction depends on the degree of separation of the interacting orbitals and their relative energies. The spectra of the individual compounds are sufficiently different to allow qualitative identification. The analytical significance of the results is briefly discussed and correlation diagrams for 29 compounds are presented.
* Part
I, “Application of Photoelectron Spectrometry to Pesticide Analysis; Photoelectron Spectra of Five-Membered Heterocycles and Related Molecules” appeared in ANAL.CHEM., 42, 1064
(1970). To whom communications concerning this paper should be addressed. On sabbatical leave from Queens College of the City University of New York, Flushing, N.Y. 11367.
ETHYLENE OXIDE AND PROPYLENE OXIDE are widely used as fumigants, but they can react with chloride and bromide in cereals to give toxic chlorohydrins or bromohydrins. This has led to the development of methods for the determination of these toxic residues, most of which are based upon gasliquid chromatography (1-5). Halohydrins are relatively simple molecules containing both an oxygen and a halogen atom and are thus well suited for examination by photoelectron spectrometry (PES) in its present form. This technique, which measures the binding energies of electrons in molecules, and K . A. Scudamore, Cheni. Ifid.(Loitcloii),1967, 1557. (2) Ibid.; p 1054. (3) S. Ben-Yehoshua and P. Krinsky, J . GNSCliromutogr., 6 , 351 (1968). (4) E. P. Ragelis, B. S. Fisher, and B. A. Klimeck, J . Ass. Ofic. A g r . Clieni. 51, 709 (1968). (5) F. Wesley, B. Rourke, and 0. Derbyshire, J . Food Sci., 30, 1037 (1965). (1) S. G. Heuser
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
375
