353
Structure of Petroleum
as a graphite-type stack, as has been proposed previously. It is more likely an association of convenience that facilitates the association of the central asphaltene with resin species. In many cases,
more than one asphaltene molecule per micelle is considered unlikely.
However, in such a model, the substances with higher molecular weights and with the most
pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellar (dispersed or oil) phase is gradual
and almost continuous. Since asphaltenes are incompatible with the oil fraction (Swanson, 1942;
Koots and Speight, 1975), asphaltene dispersion is attributable mainly to the resins (polar aromatics).
Empirical observations indicate that the resins play an important role in stabilizing asphaltenes
in crude oil (Koots and Speight, 1975 and references cited therein). Under unfavorable solvent conditions, the asphaltene species are prone to further aggregation into clusters that are unstable and
precipitate from the crude oil (Mitchell and Speight, 1973; Koots and Speight, 1975; Yen, 1990).
Asphaltene constituents ↔ Micelle ↔ Micelle clusters/flocs
This model requires that the asphaltene micelles be composed of an insoluble molecular core that
associates with the resins, thereby providing steric stabilization against flocculation and precipitation. Thus, the phase separation of asphaltenes upon the addition of nonpolar solvents to the crude
oil can be rationalized in terms of reduction of the solubility parameter or polarity of the hydrocarbon medium (Speight, 1992, 1994; Wiehe, 1993). Such a change results in solubilization of the
resins and leads to dissociation of the resin–asphaltene complexes with the result that there is a
destabilization of the unattended asphaltene constituents, followed by flocculation/precipitation/
phase separation.
The vast differences in molecular structure among individual asphaltene molecules prevent
these agglomerates from forming a discotic phase, which requires the monomers to have identical
molecular structure. Instead, the asphaltenes randomly stack to form globular micelles as proposed
in this model. It was also proposed that the elementary micelle units can further cluster into larger
particles similar to flocs.
Acceptance of these models and the concept of a micelle are easier to understand if the constituents of the resin and asphaltene fractions are viewed as two adjacent portions of a single broad compositional continuum that contains the polar, aromatic, and higher molecular weight components of
petroleum. This allows the further acceptance of structural similarities in the asphaltenes and resins
that facilitate formation of the micelle(s) (Speight, 1992, 1994).
Thus, the means by which asphaltenes exist in crude oil is assumed to involve asphaltene–resin
interactions leading to the formation of micelles (Koots and Speight, 1975), but there are still many
unknowns related to the existence of asphaltenes in crude oil.
On this basis, the stability of petroleum can be represented by a three-phase system in which the
asphaltene constituents, the aromatic constituents (including the resin constituents), and the saturate
constituents are delicately balanced (Speight, 1992, 1994, 1996). Various factors, such as oxidation,
can have an adverse effect on the system, leading to instability or incompatibility as a result of
changing the polarity, and bonding arrangements, of the species in crude oil.
An alternative explanation for the stabilizing influence of the resin constituents and for the stability of asphaltene constituents in hydrocarbon media in general is that individual asphaltene species
and micelles are in thermodynamic equilibrium (Cimino et al., 1995). Thus, the degree of association is determined by the relative energies of solvation of the monomers and the micelles and the
entropy and enthalpy changes associated with the self-association of the asphaltene monomers.
According to this model, the stabilization of the asphaltene micelles by the resin fraction derives
from the contribution of the latter to the solvent power (polarity and aromaticity) of the medium.
Destabilization of the asphaltenes, leading to phase separation, occurs when the solvating power of
the medium toward the asphaltene monomers and micelles is reduced to the point at which they are
no longer fully soluble.
Structure of Petroleum
as a graphite-type stack, as has been proposed previously. It is more likely an association of convenience that facilitates the association of the central asphaltene with resin species. In many cases,
more than one asphaltene molecule per micelle is considered unlikely.
However, in such a model, the substances with higher molecular weights and with the most
pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellar (dispersed or oil) phase is gradual
and almost continuous. Since asphaltenes are incompatible with the oil fraction (Swanson, 1942;
Koots and Speight, 1975), asphaltene dispersion is attributable mainly to the resins (polar aromatics).
Empirical observations indicate that the resins play an important role in stabilizing asphaltenes
in crude oil (Koots and Speight, 1975 and references cited therein). Under unfavorable solvent conditions, the asphaltene species are prone to further aggregation into clusters that are unstable and
precipitate from the crude oil (Mitchell and Speight, 1973; Koots and Speight, 1975; Yen, 1990).
Asphaltene constituents ↔ Micelle ↔ Micelle clusters/flocs
This model requires that the asphaltene micelles be composed of an insoluble molecular core that
associates with the resins, thereby providing steric stabilization against flocculation and precipitation. Thus, the phase separation of asphaltenes upon the addition of nonpolar solvents to the crude
oil can be rationalized in terms of reduction of the solubility parameter or polarity of the hydrocarbon medium (Speight, 1992, 1994; Wiehe, 1993). Such a change results in solubilization of the
resins and leads to dissociation of the resin–asphaltene complexes with the result that there is a
destabilization of the unattended asphaltene constituents, followed by flocculation/precipitation/
phase separation.
The vast differences in molecular structure among individual asphaltene molecules prevent
these agglomerates from forming a discotic phase, which requires the monomers to have identical
molecular structure. Instead, the asphaltenes randomly stack to form globular micelles as proposed
in this model. It was also proposed that the elementary micelle units can further cluster into larger
particles similar to flocs.
Acceptance of these models and the concept of a micelle are easier to understand if the constituents of the resin and asphaltene fractions are viewed as two adjacent portions of a single broad compositional continuum that contains the polar, aromatic, and higher molecular weight components of
petroleum. This allows the further acceptance of structural similarities in the asphaltenes and resins
that facilitate formation of the micelle(s) (Speight, 1992, 1994).
Thus, the means by which asphaltenes exist in crude oil is assumed to involve asphaltene–resin
interactions leading to the formation of micelles (Koots and Speight, 1975), but there are still many
unknowns related to the existence of asphaltenes in crude oil.
On this basis, the stability of petroleum can be represented by a three-phase system in which the
asphaltene constituents, the aromatic constituents (including the resin constituents), and the saturate
constituents are delicately balanced (Speight, 1992, 1994, 1996). Various factors, such as oxidation,
can have an adverse effect on the system, leading to instability or incompatibility as a result of
changing the polarity, and bonding arrangements, of the species in crude oil.
An alternative explanation for the stabilizing influence of the resin constituents and for the stability of asphaltene constituents in hydrocarbon media in general is that individual asphaltene species
and micelles are in thermodynamic equilibrium (Cimino et al., 1995). Thus, the degree of association is determined by the relative energies of solvation of the monomers and the micelles and the
entropy and enthalpy changes associated with the self-association of the asphaltene monomers.
According to this model, the stabilization of the asphaltene micelles by the resin fraction derives
from the contribution of the latter to the solvent power (polarity and aromaticity) of the medium.
Destabilization of the asphaltenes, leading to phase separation, occurs when the solvating power of
the medium toward the asphaltene monomers and micelles is reduced to the point at which they are
no longer fully soluble.