307
Structural Group Analysis
However, accumulation of other nuclei was still possible because the asphaltene molecules were
considered to contain two or more polynuclear aromatic sheets sigma-bonded by aliphatic carbon
chains. Chains orientation of aliphatic groups, spaced about 5.7Χ apart in the manner of a saturated
carbon chain or loose net of naphthenic rings, was also indicated.
For materials with limited order, such as pyrolytic carbons, the x-ray diffraction (XRD)
technique can be used to determine the repeating sequences of the molecular arrangement. For
a dark-colored, solid material in particular, the vectors of the d spacings between 0.5 and 10 D
can be easily derived from a reduced x-ray spectrum. The crystallite parameters often used
as structural parameters are L a , layer diameter; L c , cluster diameter; d m interlayer distance; d,
interchain distance; and M e , effective layer number. One of the reputed advantages of the x-ray
diffraction technique is that it can also be used for a measurement of the aromaticity, f a , of
the sample. The calculation is based on the relative intensity of the (γ) band and the (002) band
of the pattern.
X-ray diffraction used at very small angles, small angle scattering (SAS), has also been used for
the characterization of heavy fractions of petroleum (Pollack and Yen, 1970). The range of small
angle scattering is in the molecular dimension from 20 to 2000 D. Some of the useful parameters
are f, structure number; O r , surface parameter; l, heterogeneity length; l c , coherence length; and r M ,
radius of gyration. There has been serious criticism of the use of the method for determining absolute structural parameters of petroleum materials (Ebert et al., 1984; Ebert, 1990).
Thus, it is not surprising that x-ray diffraction studies have been used in attempts to elucidate the
structure of the micelle (Wen et al., 1978). Molecular weight data show that asphaltenes associate,
even in dilute solution and under the conditions often prescribed for determination of molecular
weight (Chapters 9, 10, and 12). This has raised the issue of asphaltene–asphaltene interactions as
well as the means by which the asphaltene constituents exist in petroleum.
Small angle x-ray scattering (SANS) has also been used as a means of determining particle size
and asphaltene molecular weight (Espinat et al., 1984; Herzog et al., 1988; Storm et al., 1993). And,
it is interesting to note that the SANS technique gives good agreement with other methods for the
molecular weight of the fractions from gel permeation chromatography for the lower molecular
weight fractions but was unusable for the higher molecular weight fractions.
What is perhaps more important from the findings was that the particle size of the asphaltenes
was found to increase as a result of deasphalting. This raises the issue of the changes caused to
the asphaltene constituents during deasphalting (perhaps analogous to denaturing the protein during the boiling of an egg). Furthermore, it questions the validity of the literal interpretations that
have been made of the physical character of asphaltenes that have been separated from petroleum.
Therefore, deductions about the nature of the micelle, made on the basis of the behavior of separated
asphaltenes, are certainly open to question and reinvestigation.
An x-ray technique (Diamond, 1957, 1958) can be used to estimate the size groups of aromatic
lamellae (sheets) that constitute the molecular structure of synthetic carbons, coals, asphaltenes,
and the like. After correction for polarization and absorption, 31 evenly spaced intensity values
between S = 0.66 and 0.96 (where S = 2 sin θ/λ) are treated with Diamond’s matrix H 5 . Corrections
for Compton scattering due to hetero (e.g., noncarbon) atoms in the layers are also applied, and a
histogram is derived. Each histogram is made up of seven parts showing, from left to right, size
groups consisting of nonaromatic material A and aromatic lamellae of varying diameters, 5.8, 8.4,
10.0, 15.0, 20.0, and 20.0 D.
The essence of this method is to show that complex fractions of petroleum contain a distribution
of ring systems but the method has been largely ignored by the average structure community.
11.2.3 HeteroAtom systems
Although the major focus of spectroscopic techniques appears to have been delineation of
the hydrocarbon skeleton of the nonvolatile constituents of petroleum, efforts have also been
Structural Group Analysis
However, accumulation of other nuclei was still possible because the asphaltene molecules were
considered to contain two or more polynuclear aromatic sheets sigma-bonded by aliphatic carbon
chains. Chains orientation of aliphatic groups, spaced about 5.7Χ apart in the manner of a saturated
carbon chain or loose net of naphthenic rings, was also indicated.
For materials with limited order, such as pyrolytic carbons, the x-ray diffraction (XRD)
technique can be used to determine the repeating sequences of the molecular arrangement. For
a dark-colored, solid material in particular, the vectors of the d spacings between 0.5 and 10 D
can be easily derived from a reduced x-ray spectrum. The crystallite parameters often used
as structural parameters are L a , layer diameter; L c , cluster diameter; d m interlayer distance; d,
interchain distance; and M e , effective layer number. One of the reputed advantages of the x-ray
diffraction technique is that it can also be used for a measurement of the aromaticity, f a , of
the sample. The calculation is based on the relative intensity of the (γ) band and the (002) band
of the pattern.
X-ray diffraction used at very small angles, small angle scattering (SAS), has also been used for
the characterization of heavy fractions of petroleum (Pollack and Yen, 1970). The range of small
angle scattering is in the molecular dimension from 20 to 2000 D. Some of the useful parameters
are f, structure number; O r , surface parameter; l, heterogeneity length; l c , coherence length; and r M ,
radius of gyration. There has been serious criticism of the use of the method for determining absolute structural parameters of petroleum materials (Ebert et al., 1984; Ebert, 1990).
Thus, it is not surprising that x-ray diffraction studies have been used in attempts to elucidate the
structure of the micelle (Wen et al., 1978). Molecular weight data show that asphaltenes associate,
even in dilute solution and under the conditions often prescribed for determination of molecular
weight (Chapters 9, 10, and 12). This has raised the issue of asphaltene–asphaltene interactions as
well as the means by which the asphaltene constituents exist in petroleum.
Small angle x-ray scattering (SANS) has also been used as a means of determining particle size
and asphaltene molecular weight (Espinat et al., 1984; Herzog et al., 1988; Storm et al., 1993). And,
it is interesting to note that the SANS technique gives good agreement with other methods for the
molecular weight of the fractions from gel permeation chromatography for the lower molecular
weight fractions but was unusable for the higher molecular weight fractions.
What is perhaps more important from the findings was that the particle size of the asphaltenes
was found to increase as a result of deasphalting. This raises the issue of the changes caused to
the asphaltene constituents during deasphalting (perhaps analogous to denaturing the protein during the boiling of an egg). Furthermore, it questions the validity of the literal interpretations that
have been made of the physical character of asphaltenes that have been separated from petroleum.
Therefore, deductions about the nature of the micelle, made on the basis of the behavior of separated
asphaltenes, are certainly open to question and reinvestigation.
An x-ray technique (Diamond, 1957, 1958) can be used to estimate the size groups of aromatic
lamellae (sheets) that constitute the molecular structure of synthetic carbons, coals, asphaltenes,
and the like. After correction for polarization and absorption, 31 evenly spaced intensity values
between S = 0.66 and 0.96 (where S = 2 sin θ/λ) are treated with Diamond’s matrix H 5 . Corrections
for Compton scattering due to hetero (e.g., noncarbon) atoms in the layers are also applied, and a
histogram is derived. Each histogram is made up of seven parts showing, from left to right, size
groups consisting of nonaromatic material A and aromatic lamellae of varying diameters, 5.8, 8.4,
10.0, 15.0, 20.0, and 20.0 D.
The essence of this method is to show that complex fractions of petroleum contain a distribution
of ring systems but the method has been largely ignored by the average structure community.
11.2.3 HeteroAtom systems
Although the major focus of spectroscopic techniques appears to have been delineation of
the hydrocarbon skeleton of the nonvolatile constituents of petroleum, efforts have also been