325
Asphaltene Constituents
For any one method, the observed molecular weights suggest that asphaltenes form molecular
aggregates, even in dilute solution, and this association is influenced by solvent polarity, asphaltene
concentration, and the temperature at which the determination is made. The precise mechanism
of the association has not been conclusively established, but hydrogen bonding and the formation of charge-transfer complexes have been cited as responsible for intermolecular association
(Yen, 1974). In fact, intermolecular hydrogen bonding could be involved in asphaltene association
and may have a significant effect on observed molecular weights (Moschopedis and Speight, 1976c;
Speight et al., 1985).
It is also interesting to note that use of a solvent of low dielectric constant (benzene) does not
cause any variation in the molecular weight when the concentration of the asphaltenes is varied
over the range 2%–7% by weight. However, use of a solvent of higher dielectric constant (pyridine)
caused significant variation in the observed molecular weights over this particular range. Molecular
weight determinations at lower (<2%) and higher (>8%) concentrations are subject to instrument
sensitivity and solubility interference, respectively. Extrapolation of the pyridine data to infinite
dilution, when asphaltene–asphaltene interaction may be assumed negligible, affords molecular
weights of the same order (∼1800) as those recorded using nitrobenzene as the solvent. On the other
hand, extrapolation of the pyridine data to higher concentrations suggests that molecular weights of
the order of those recorded in a solvent of lower dielectric constant (e.g., benzene) may be obtained.
The higher molecular weights recorded when solvents of low polarity are employed are undoubtedly the result of intermolecular association between the asphaltene nuclei, but solvents of high
dielectric constant are able to bring about dissociation of these asphaltene agglomerations to what,
in fact, appear to be single asphaltene particles. Furthermore, this observation precludes asphaltene
structures that invoke the concept of a polymer molecule to account for the high molecular weights
observed in nonpolar solvents.
Determination of asphaltene molecular weight is subject to the presence of occluded resin
constituents (Ali et al., 1985) and removal of the resin constituents gives rise to higher observed
molecular weights of the purified asphaltenes. In addition, and as noted previously for the whole
asphaltenes (Moschopedis et al., 1976), the molecular weights of the purified asphaltenes also varied with the solvent used for the determination; that is, solvents of high dielectric constant decrease
the observed molecular weights. Furthermore, extraction of freshly precipitated asphaltene fractions using a Soxhlet extractor and different solvents followed by molecular weight determinations
of the insoluble material show a decrease in the asphaltene molecular weight with the dielectric
constant of the solvent.
Thus, the tendency of asphaltenes to undergo association and/or dissociation depending upon the
nature of the solvent also appears true for the series of higher molecular weight fractions. However,
it should be noted here that although the results with asphaltenes available from several crude oils
(Moschopedis et al., 1976) suggest that molecular weight varies with the dielectric constant of the
solvent, there may be other factors that may in part also be responsible for this phenomenon. The
final phenomenon that influences the molecular weight of the asphaltene is the relative polarity of
the solvent used in the precipitation technique.
It is of interest to speculate at this point about the means by which the resin material is retained
by the asphaltenes. For example, asphaltenes may be analogous to coal insofar as it appears that
asphaltenes have adsorption characteristics and may even exhibit a distinct physical structure or
may even participate in clathrate systems. If this is the case, swelling by a solvent such as pyridine
would undoubtedly free resin material from the asphaltenes. On the other hand, if the resin material
is retained purely by a surface adsorption phenomenon, it may be expected to be removed from the
asphaltenes by the repetitive precipitation. Both effects could play a part in the retention of resin
(i.e., precipitant-soluble) material by the asphaltenes.
Obviously, many other facets of asphaltene precipitation need to be investigated, but it is obvious from these data that it is extremely difficult to obtain clean (resin-free) asphaltenes without a
multiple precipitation technique. The amount of resin may have some influence on the asphaltene
Asphaltene Constituents
For any one method, the observed molecular weights suggest that asphaltenes form molecular
aggregates, even in dilute solution, and this association is influenced by solvent polarity, asphaltene
concentration, and the temperature at which the determination is made. The precise mechanism
of the association has not been conclusively established, but hydrogen bonding and the formation of charge-transfer complexes have been cited as responsible for intermolecular association
(Yen, 1974). In fact, intermolecular hydrogen bonding could be involved in asphaltene association
and may have a significant effect on observed molecular weights (Moschopedis and Speight, 1976c;
Speight et al., 1985).
It is also interesting to note that use of a solvent of low dielectric constant (benzene) does not
cause any variation in the molecular weight when the concentration of the asphaltenes is varied
over the range 2%–7% by weight. However, use of a solvent of higher dielectric constant (pyridine)
caused significant variation in the observed molecular weights over this particular range. Molecular
weight determinations at lower (<2%) and higher (>8%) concentrations are subject to instrument
sensitivity and solubility interference, respectively. Extrapolation of the pyridine data to infinite
dilution, when asphaltene–asphaltene interaction may be assumed negligible, affords molecular
weights of the same order (∼1800) as those recorded using nitrobenzene as the solvent. On the other
hand, extrapolation of the pyridine data to higher concentrations suggests that molecular weights of
the order of those recorded in a solvent of lower dielectric constant (e.g., benzene) may be obtained.
The higher molecular weights recorded when solvents of low polarity are employed are undoubtedly the result of intermolecular association between the asphaltene nuclei, but solvents of high
dielectric constant are able to bring about dissociation of these asphaltene agglomerations to what,
in fact, appear to be single asphaltene particles. Furthermore, this observation precludes asphaltene
structures that invoke the concept of a polymer molecule to account for the high molecular weights
observed in nonpolar solvents.
Determination of asphaltene molecular weight is subject to the presence of occluded resin
constituents (Ali et al., 1985) and removal of the resin constituents gives rise to higher observed
molecular weights of the purified asphaltenes. In addition, and as noted previously for the whole
asphaltenes (Moschopedis et al., 1976), the molecular weights of the purified asphaltenes also varied with the solvent used for the determination; that is, solvents of high dielectric constant decrease
the observed molecular weights. Furthermore, extraction of freshly precipitated asphaltene fractions using a Soxhlet extractor and different solvents followed by molecular weight determinations
of the insoluble material show a decrease in the asphaltene molecular weight with the dielectric
constant of the solvent.
Thus, the tendency of asphaltenes to undergo association and/or dissociation depending upon the
nature of the solvent also appears true for the series of higher molecular weight fractions. However,
it should be noted here that although the results with asphaltenes available from several crude oils
(Moschopedis et al., 1976) suggest that molecular weight varies with the dielectric constant of the
solvent, there may be other factors that may in part also be responsible for this phenomenon. The
final phenomenon that influences the molecular weight of the asphaltene is the relative polarity of
the solvent used in the precipitation technique.
It is of interest to speculate at this point about the means by which the resin material is retained
by the asphaltenes. For example, asphaltenes may be analogous to coal insofar as it appears that
asphaltenes have adsorption characteristics and may even exhibit a distinct physical structure or
may even participate in clathrate systems. If this is the case, swelling by a solvent such as pyridine
would undoubtedly free resin material from the asphaltenes. On the other hand, if the resin material
is retained purely by a surface adsorption phenomenon, it may be expected to be removed from the
asphaltenes by the repetitive precipitation. Both effects could play a part in the retention of resin
(i.e., precipitant-soluble) material by the asphaltenes.
Obviously, many other facets of asphaltene precipitation need to be investigated, but it is obvious from these data that it is extremely difficult to obtain clean (resin-free) asphaltenes without a
multiple precipitation technique. The amount of resin may have some influence on the asphaltene