352
The Chemistry and Technology of Petroleum
The means by which asphaltene constituents and resin constituents interact to exist in petroleum remains the subject of speculation, but hydrogen bonding (Moschopedis and Speight 1976a;
Acevedo et al., 1985) and the formation of charge-transfer complexes (Yen, 1974) have been cited
as the causative mechanisms. Indeed, the influence of hydrogen bonding interactions in determining molecular structure is established (Taft et al., 1996) and to ignore the potential for hydrogen
bonding interactions here would be a gross injustice. Finally, there is also evidence that asphaltenes
participate in charge-transfer complexes (Penzes and Speight, 1974; Speight and Penzes, 1978),
but the exact chemical or physical manner in which they would form in petroleum is still open to
discussion. Therefore, it might be more reasonable to consider that the asphaltene constituents exit
as aggregations only when separated and flocculated and that in the crude oil the micelle does not
consist of aggregates of asphaltene molecules (Pelet et al., 1985; Speight, 1994).
The original concept of the asphaltene–resin micelle invoked the concept of asphaltene–asphaltene
association to form a graphite-like stack (Chapter 12; Speight, 1994 and references cited therein) that
acted as the micelle core that, in turn, was stabilized by the resins. Indeed, application of various physical methods to the study of asphaltenes aggregation has shown that the physical dimensions and shape
of the aggregates are functions of the solvent used and the temperature of the investigation (Ravey
et al., 1988; Overfield et al. 1989; Thiyagarajan et al., 1995). In addition, surface tension measurements
have been used to study the self-association of asphaltenes in pyridine and nitrobenzene (Sheu et al.,
1992). A discontinuous transition in the surface tension as a function of asphaltene concentration was
interpreted as the critical asphaltene concentration above which self-association occurs.
When resin constituents and asphaltene constituents are present together in solution, hydrogenbonding studies show resin–asphaltene interactions are preferred over asphaltene–asphaltene interactions (Moschopedis and Speight, 1976a). If the same intermolecular forces can, with any degree
of justification, be projected to petroleum, asphaltene constituents in petroleum as single entities
that are peptized, and effectively dispersed, by resin constituents. However, whatever the means by
which the individual molecular species are included in the micelle, the structure is recognized as
being complex (Bardon et al., 1996).
Therefore, petroleum is a complex system with each fraction dependent upon other systems for
complete mobility and solubility (Koots and Speight, 1975). It is presumed that the resins associate
with the asphaltenes in the manner of an electron donor–acceptor. In addition, there could well be
several points of structural similarity between the asphaltenes and resins that would have an adverse
effect on the ability of the resins to associate with asphaltenes from a different crude oil (Koots and
Speight, 1975; Moschopedis and Speight, 1976a).
It therefore appears that, when resins and asphaltenes are present together, hydrogen bonding
may be one of the mechanisms by which resin–asphaltene interactions are achieved, and resin–
asphaltene interactions are preferred over asphaltene–asphaltene interactions. It has also been
reported that hydrogenation and concurrent desulfurization cause a decrease in asphaltene molecular weights, and the reduction in molecular weight is caused by a decrease in hydrogen-bonding
interactions as a direct result of loss of the hetero element.
If the composition and properties of the precipitated asphaltenes reflect those of the micelles in
solution, the latter should be considered as mixed micelles. Asphaltene constituents vary in character
(Chapter 12). Resins are also complex mixtures of constituents. Therefore, homogeneous micelles
cannot be predicted. This is a reasonable assumption in view of the large quantities of soluble resins
found in the precipitated solid. It should be noted that if mole fractions are used as a composition
measure, the one for resins in the precipitates would be greater than that for asphaltenes.
Considering the micelles to be composed of asphaltene species that are peptized by resins and
asphaltenes is a more reasonable approach. An important corollary of petroleum composition is
that the mole fraction of resins is always larger than that of asphaltenes and hence the micelles are
expected to be richer in resins. The micelle center would be formed from polar asphaltene molecules.
These molecules would be surrounded by other more soluble asphaltene molecules, which would be
placed between the center and the periphery. The inclusion of other asphaltene species is not seen
The Chemistry and Technology of Petroleum
The means by which asphaltene constituents and resin constituents interact to exist in petroleum remains the subject of speculation, but hydrogen bonding (Moschopedis and Speight 1976a;
Acevedo et al., 1985) and the formation of charge-transfer complexes (Yen, 1974) have been cited
as the causative mechanisms. Indeed, the influence of hydrogen bonding interactions in determining molecular structure is established (Taft et al., 1996) and to ignore the potential for hydrogen
bonding interactions here would be a gross injustice. Finally, there is also evidence that asphaltenes
participate in charge-transfer complexes (Penzes and Speight, 1974; Speight and Penzes, 1978),
but the exact chemical or physical manner in which they would form in petroleum is still open to
discussion. Therefore, it might be more reasonable to consider that the asphaltene constituents exit
as aggregations only when separated and flocculated and that in the crude oil the micelle does not
consist of aggregates of asphaltene molecules (Pelet et al., 1985; Speight, 1994).
The original concept of the asphaltene–resin micelle invoked the concept of asphaltene–asphaltene
association to form a graphite-like stack (Chapter 12; Speight, 1994 and references cited therein) that
acted as the micelle core that, in turn, was stabilized by the resins. Indeed, application of various physical methods to the study of asphaltenes aggregation has shown that the physical dimensions and shape
of the aggregates are functions of the solvent used and the temperature of the investigation (Ravey
et al., 1988; Overfield et al. 1989; Thiyagarajan et al., 1995). In addition, surface tension measurements
have been used to study the self-association of asphaltenes in pyridine and nitrobenzene (Sheu et al.,
1992). A discontinuous transition in the surface tension as a function of asphaltene concentration was
interpreted as the critical asphaltene concentration above which self-association occurs.
When resin constituents and asphaltene constituents are present together in solution, hydrogenbonding studies show resin–asphaltene interactions are preferred over asphaltene–asphaltene interactions (Moschopedis and Speight, 1976a). If the same intermolecular forces can, with any degree
of justification, be projected to petroleum, asphaltene constituents in petroleum as single entities
that are peptized, and effectively dispersed, by resin constituents. However, whatever the means by
which the individual molecular species are included in the micelle, the structure is recognized as
being complex (Bardon et al., 1996).
Therefore, petroleum is a complex system with each fraction dependent upon other systems for
complete mobility and solubility (Koots and Speight, 1975). It is presumed that the resins associate
with the asphaltenes in the manner of an electron donor–acceptor. In addition, there could well be
several points of structural similarity between the asphaltenes and resins that would have an adverse
effect on the ability of the resins to associate with asphaltenes from a different crude oil (Koots and
Speight, 1975; Moschopedis and Speight, 1976a).
It therefore appears that, when resins and asphaltenes are present together, hydrogen bonding
may be one of the mechanisms by which resin–asphaltene interactions are achieved, and resin–
asphaltene interactions are preferred over asphaltene–asphaltene interactions. It has also been
reported that hydrogenation and concurrent desulfurization cause a decrease in asphaltene molecular weights, and the reduction in molecular weight is caused by a decrease in hydrogen-bonding
interactions as a direct result of loss of the hetero element.
If the composition and properties of the precipitated asphaltenes reflect those of the micelles in
solution, the latter should be considered as mixed micelles. Asphaltene constituents vary in character
(Chapter 12). Resins are also complex mixtures of constituents. Therefore, homogeneous micelles
cannot be predicted. This is a reasonable assumption in view of the large quantities of soluble resins
found in the precipitated solid. It should be noted that if mole fractions are used as a composition
measure, the one for resins in the precipitates would be greater than that for asphaltenes.
Considering the micelles to be composed of asphaltene species that are peptized by resins and
asphaltenes is a more reasonable approach. An important corollary of petroleum composition is
that the mole fraction of resins is always larger than that of asphaltenes and hence the micelles are
expected to be richer in resins. The micelle center would be formed from polar asphaltene molecules.
These molecules would be surrounded by other more soluble asphaltene molecules, which would be
placed between the center and the periphery. The inclusion of other asphaltene species is not seen