339
Asphaltene Constituents
asphaltenes exist as complex mixtures of components that are separated from petroleum by treatment with a low-boiling liquid hydrocarbon (Mitchell and Speight, 1973; Speight et al., 1984).
A number of molecular models have been proposed for asphaltenes and are described in detail
elsewhere (Speight, 1994), and repetition is not warranted here. It is sufficient to state that any
molecular models derived for asphaltenes must be in keeping with behavioral characteristics.
Efforts were made to describe the asphaltene fraction in terms of a single, representative asphaltene
molecule or molecules (Speight, 1994) incorporating, in the correct proportions, all of the chemical
and elemental constituents known to be present in a given asphaltenic matrix.
Although this approach gives an idea of the structural complexity of the compounds making up
the asphaltene component, it obscures the highly differentiated chemical nature of the molecules
in this petroleum fraction (Long, 1979, 1981). The idealized molecular structures defined by this
method, however, are of some utility for inferring the chemistry and physical/chemical properties
of the asphaltenes (Speight, 1992, 1994).
Briefly, there are essentially three types of models: (1) those models that help to visualize the
three-dimensional architecture and stereochemistry but are not to scale, (2) framework-type models
that indicate correct bond distances and bond angles and can be used to measure distances between
nonbonded atoms in molecules but do not show the atoms as such, and (3) space-filling models that
provide a fairly realistic three-dimensional representation of what the molecule actually looks like.
Attempts to model asphaltenes have usually employed pictographic representations of the first kind
of models. Minor efforts have been made to represent asphaltenes using models of types 2 and 3.
A serious shortcoming, common to all molecular models, is that they have fixed bond angles and
that rotation about single bonds is excessively facile, especially in the first two types of models. In
contrast, in the real situation there are relatively easily definable bond angles and substantial barriers
to rotation about single bonds. Moreover, if it is desired to measure intramolecular and/or intermolecular bond distances, the model must first be fixed in its actual conformation. For molecules having a
number of single bonds, this may be quite inconvenient (even though mechanical devices to stop bond
rotation are available), and it may be difficult to set the actual torsion angles with any kind of precision.
As a result of all these difficulties, modeling of the macromolecules in the asphaltene fraction has
proved to be speculative, although some success has been achieved in attempting to understand the
nature of the constituents and their behavior in thermal processes (Speight, 1994). Success notwithstanding, the chemical dynamics are still speculative because of the speculative nature of the model!
The properties of macromolecules depend on both the chemical structure and physical structure.
The term primary structure describes the chemical sequence of atoms in a macromolecule. The
ordering of the atoms in space relative to each other is referred to as the secondary structure, and
the three-dimensional structure of a molecule is called the tertiary structure.
By analogy, the primary structure of asphaltenes is the two-dimensional structure derived by a variety of analytical techniques and is often presented on paper as an average structure. The secondary and
tertiary structure, and perhaps an often ignored but extremely important aspect of asphaltene chemistry
and physics, is the micelle structure, which represents the means by which asphaltenes exist in crude oil.
A model that may be more in keeping with the amphoteric constituents of the asphaltenes would,
on the basis of the preceding concept, have smaller polynuclear aromatic systems. Such a model
would not only have to be compatible with the other constituents of petroleum but also be able
to represent the thermal chemistry and other process operations. It must also be recognized that
such a model can have the large size dimensions that have been proposed for asphaltenes. It can
also be a molecular chameleon insofar as it can vary in these dimensions and shape depending
upon the freedom of rotation about one or more of the bonds and whether or not it has been isolated from the crude oil. It is not possible to compose a model that represents all the properties of
asphaltenes. The molecular size, as deduced from molecular weight measurements, prohibits this.
It will be necessary to compose models that are representative of the various constituent fractions
of an asphaltene. Then, perhaps, we will truly have representative models.
Asphaltene Constituents
asphaltenes exist as complex mixtures of components that are separated from petroleum by treatment with a low-boiling liquid hydrocarbon (Mitchell and Speight, 1973; Speight et al., 1984).
A number of molecular models have been proposed for asphaltenes and are described in detail
elsewhere (Speight, 1994), and repetition is not warranted here. It is sufficient to state that any
molecular models derived for asphaltenes must be in keeping with behavioral characteristics.
Efforts were made to describe the asphaltene fraction in terms of a single, representative asphaltene
molecule or molecules (Speight, 1994) incorporating, in the correct proportions, all of the chemical
and elemental constituents known to be present in a given asphaltenic matrix.
Although this approach gives an idea of the structural complexity of the compounds making up
the asphaltene component, it obscures the highly differentiated chemical nature of the molecules
in this petroleum fraction (Long, 1979, 1981). The idealized molecular structures defined by this
method, however, are of some utility for inferring the chemistry and physical/chemical properties
of the asphaltenes (Speight, 1992, 1994).
Briefly, there are essentially three types of models: (1) those models that help to visualize the
three-dimensional architecture and stereochemistry but are not to scale, (2) framework-type models
that indicate correct bond distances and bond angles and can be used to measure distances between
nonbonded atoms in molecules but do not show the atoms as such, and (3) space-filling models that
provide a fairly realistic three-dimensional representation of what the molecule actually looks like.
Attempts to model asphaltenes have usually employed pictographic representations of the first kind
of models. Minor efforts have been made to represent asphaltenes using models of types 2 and 3.
A serious shortcoming, common to all molecular models, is that they have fixed bond angles and
that rotation about single bonds is excessively facile, especially in the first two types of models. In
contrast, in the real situation there are relatively easily definable bond angles and substantial barriers
to rotation about single bonds. Moreover, if it is desired to measure intramolecular and/or intermolecular bond distances, the model must first be fixed in its actual conformation. For molecules having a
number of single bonds, this may be quite inconvenient (even though mechanical devices to stop bond
rotation are available), and it may be difficult to set the actual torsion angles with any kind of precision.
As a result of all these difficulties, modeling of the macromolecules in the asphaltene fraction has
proved to be speculative, although some success has been achieved in attempting to understand the
nature of the constituents and their behavior in thermal processes (Speight, 1994). Success notwithstanding, the chemical dynamics are still speculative because of the speculative nature of the model!
The properties of macromolecules depend on both the chemical structure and physical structure.
The term primary structure describes the chemical sequence of atoms in a macromolecule. The
ordering of the atoms in space relative to each other is referred to as the secondary structure, and
the three-dimensional structure of a molecule is called the tertiary structure.
By analogy, the primary structure of asphaltenes is the two-dimensional structure derived by a variety of analytical techniques and is often presented on paper as an average structure. The secondary and
tertiary structure, and perhaps an often ignored but extremely important aspect of asphaltene chemistry
and physics, is the micelle structure, which represents the means by which asphaltenes exist in crude oil.
A model that may be more in keeping with the amphoteric constituents of the asphaltenes would,
on the basis of the preceding concept, have smaller polynuclear aromatic systems. Such a model
would not only have to be compatible with the other constituents of petroleum but also be able
to represent the thermal chemistry and other process operations. It must also be recognized that
such a model can have the large size dimensions that have been proposed for asphaltenes. It can
also be a molecular chameleon insofar as it can vary in these dimensions and shape depending
upon the freedom of rotation about one or more of the bonds and whether or not it has been isolated from the crude oil. It is not possible to compose a model that represents all the properties of
asphaltenes. The molecular size, as deduced from molecular weight measurements, prohibits this.
It will be necessary to compose models that are representative of the various constituent fractions
of an asphaltene. Then, perhaps, we will truly have representative models.