356
The Chemistry and Technology of Petroleum
As a note, and to avoid unnecessary confusion, there is another scale by which the solubility
parameter can be measured. The numbers are precisely half of those used earlier. Either scale is
adequate to the task but must be defined to avoid any misinterpretation of the data.
The material precipitated from n-heptane to toluene insoluble constituents may have a
δ-distribution between 19 and 22 MPa 1/2 . This observation is in disagreement with the data from the
titration experiments that give an upper limit of 20.7 MPa 1/2 , and hence were not able to predict the
presence of toluene insoluble material. One titration experiment using 2,4-dimethylpyridine showed
a flocculation threshold of 0.42 (18.3 MPa 1/2 ) indicating an upper limit of 22.3 MPa 1/2 .
This is to be anticipated since, on the basis of molecular weight data, asphaltene constituents
associate less in pyridine (Moschopedis et al., 1976; Speight et al., 1985). Hence, the interactions
leading to dispersion of the least soluble part are not present, and the compounds behave as single
molecular entities. The latter is possible as the asphaltene concentration of this sample is below
the critical micelle concentration (CMC) (4.32 g/L) at the beginning of the titration (Andersen and
Birdi, 1991a,b). Hence, if solubility parameters are determined from titration experiments, the data
are relative to the conditions such as solvent system and equilibrium time.
According to the group contribution approach for calculation of the solubility parameter (δ), an
increase in molecular weight does not strictly imply an increase in the magnitude of the solubility
since long chain paraffin systems have low δ values (Barton, 1983). Hence, the precipitation may be
governed by molecular weight differences between the solvent and the solute.
HPLC coupled with selective elution chromatography has shown that a high concentration of
toluene in the precipitant leads to shorter peak retention time, and changes take place on the low
molecular weight side of the eluted profile. The largest change in profiles was seen when going
from pure heptane to a mixture containing 10% toluene. The n-heptane asphaltenes had a close to
bimodal profile whereas for 10% (or higher amounts) of toluene, the profile is a single tailing peak
indicating that the incremental material, soluble in toluene/heptane but insoluble in heptane, is
composed of lower molecular weight species. This suggests that asphaltenes may be composed of
different molecular weight types as suggested from fractionation studies (Bestougeff and Darmois,
1947, 1948; Speight, 1971, 1975).
Hydrogen bonding is involved, to some extent, in asphaltene association and dispersion
(Moschopedis and Speight, 1976a). Indeed, an examination of the Fourier transform infrared spectra,
particularly in the range 3100–3700 cm −1 , showed that the main change in the nature of the precipitates from that obtained with pure heptane is the occurrence of less hydrogen-bonded structures.
In the spectrum of the precipitate from the 40% toluene mixture, distinct, relatively large, bands
for free hydroxyl and carboxylic hydroxyl groups are found. In the presence of the coprecipitated
resin-type material, these bands are extremely weak—indicating that dispersion of any toluene
insoluble material may take place through association with the coprecipitate. A similar concept has
been proposed for resin–asphaltene interaction (Koots and Speight, 1975). Coprecipitated material
was in all samples of lower molecular weight.
Discrepancies between methods (equilibrium precipitation and flocculation titration) employed
to determine the incipient precipitation of asphaltenes have been found and may be related to slow
kinetics of precipitation (Speight et al., 1982, 1984).
Solubility parameters of asphaltenes can be measured by the titration method but relative to
the experimental parameters such as time, solvent, and solute (asphaltene or oil) concentration. The
recovery of a toluene insoluble fraction from a toluene soluble fraction shows the complexity of the
asphaltene aggregation during precipitation. Evidence of rearrangements of molecular interactions
is suggested.
The self-association of asphaltenes has led to the concept of the critical micelle concentration
(CMC), present in feedstocks and due to the polarity of the asphaltenes. Indeed, polarity is believed
to play a role in the behavior of crude oils and also offers a means of predictability of crude oil behavior. In addition, combined with solubility parameter studies, knowledge of the CMC may also be an
aid to a more precise definition of crude oil behavior during refining operations. It may also be of
The Chemistry and Technology of Petroleum
As a note, and to avoid unnecessary confusion, there is another scale by which the solubility
parameter can be measured. The numbers are precisely half of those used earlier. Either scale is
adequate to the task but must be defined to avoid any misinterpretation of the data.
The material precipitated from n-heptane to toluene insoluble constituents may have a
δ-distribution between 19 and 22 MPa 1/2 . This observation is in disagreement with the data from the
titration experiments that give an upper limit of 20.7 MPa 1/2 , and hence were not able to predict the
presence of toluene insoluble material. One titration experiment using 2,4-dimethylpyridine showed
a flocculation threshold of 0.42 (18.3 MPa 1/2 ) indicating an upper limit of 22.3 MPa 1/2 .
This is to be anticipated since, on the basis of molecular weight data, asphaltene constituents
associate less in pyridine (Moschopedis et al., 1976; Speight et al., 1985). Hence, the interactions
leading to dispersion of the least soluble part are not present, and the compounds behave as single
molecular entities. The latter is possible as the asphaltene concentration of this sample is below
the critical micelle concentration (CMC) (4.32 g/L) at the beginning of the titration (Andersen and
Birdi, 1991a,b). Hence, if solubility parameters are determined from titration experiments, the data
are relative to the conditions such as solvent system and equilibrium time.
According to the group contribution approach for calculation of the solubility parameter (δ), an
increase in molecular weight does not strictly imply an increase in the magnitude of the solubility
since long chain paraffin systems have low δ values (Barton, 1983). Hence, the precipitation may be
governed by molecular weight differences between the solvent and the solute.
HPLC coupled with selective elution chromatography has shown that a high concentration of
toluene in the precipitant leads to shorter peak retention time, and changes take place on the low
molecular weight side of the eluted profile. The largest change in profiles was seen when going
from pure heptane to a mixture containing 10% toluene. The n-heptane asphaltenes had a close to
bimodal profile whereas for 10% (or higher amounts) of toluene, the profile is a single tailing peak
indicating that the incremental material, soluble in toluene/heptane but insoluble in heptane, is
composed of lower molecular weight species. This suggests that asphaltenes may be composed of
different molecular weight types as suggested from fractionation studies (Bestougeff and Darmois,
1947, 1948; Speight, 1971, 1975).
Hydrogen bonding is involved, to some extent, in asphaltene association and dispersion
(Moschopedis and Speight, 1976a). Indeed, an examination of the Fourier transform infrared spectra,
particularly in the range 3100–3700 cm −1 , showed that the main change in the nature of the precipitates from that obtained with pure heptane is the occurrence of less hydrogen-bonded structures.
In the spectrum of the precipitate from the 40% toluene mixture, distinct, relatively large, bands
for free hydroxyl and carboxylic hydroxyl groups are found. In the presence of the coprecipitated
resin-type material, these bands are extremely weak—indicating that dispersion of any toluene
insoluble material may take place through association with the coprecipitate. A similar concept has
been proposed for resin–asphaltene interaction (Koots and Speight, 1975). Coprecipitated material
was in all samples of lower molecular weight.
Discrepancies between methods (equilibrium precipitation and flocculation titration) employed
to determine the incipient precipitation of asphaltenes have been found and may be related to slow
kinetics of precipitation (Speight et al., 1982, 1984).
Solubility parameters of asphaltenes can be measured by the titration method but relative to
the experimental parameters such as time, solvent, and solute (asphaltene or oil) concentration. The
recovery of a toluene insoluble fraction from a toluene soluble fraction shows the complexity of the
asphaltene aggregation during precipitation. Evidence of rearrangements of molecular interactions
is suggested.
The self-association of asphaltenes has led to the concept of the critical micelle concentration
(CMC), present in feedstocks and due to the polarity of the asphaltenes. Indeed, polarity is believed
to play a role in the behavior of crude oils and also offers a means of predictability of crude oil behavior. In addition, combined with solubility parameter studies, knowledge of the CMC may also be an
aid to a more precise definition of crude oil behavior during refining operations. It may also be of