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Acid–base chemistry enables reversible colloid-to-solution transition ofasphaltenes in non-polar systems†
Sara M. Hashmi,* Kathy X. Zhong and Abbas Firoozabadi*
Received 29th April 2012, Accepted 21st June 2012
DOI: 10.1039/c2sm26003d
The conjugated p-bonding in asphaltenes, a naturally occurring member of the polyaromatic
hydrocarbon family, provides a unique platform for investigating electrostatics and electronics in non-
polar systems, but at the same time causes asphaltenes to be insoluble in all except aromatic liquids.
Asphaltenes precipitate from petroleum fluids under a variety of conditions, including depressurization
and compositional changes, plaguing both recovery operations and remediation in the case of
equipment failure. Aromatic solvents like toluene dissolve asphaltenes, but only at very high
concentrations, nearly 50% by weight. Polymeric dispersants can stabilize asphaltene colloids, and in
some cases can inhibit asphaltene precipitation entirely. Strong organic acids such as dodecyl benzene
sulfonic acid (DBSA) can dissolve precipitated asphaltenes when introduced in concentrations as little
as 1 percent by weight. Here we demonstrate for the first time that DBSA enables a reversible transition
from unstable to stable colloidal-scale asphaltene suspensions to molecularly stable solutions. A
continuum of acid–base reactions explains the apparent dual-action of DBSA. The suspension–
solution transition occurs through the protonation of heteroatomic asphaltene components and
subsequent strong ion pairing with DBSA sulfonate ions, effectively forming DBSA-doped asphaltene
complexes with a solvation shell.
Introduction
While asphaltene precipitation from petroleum fluids may hinder
oil production, asphaltenes also constitute a unique p-conju-gated system for studying electrostatics and self-assembly in non-
polar systems. Assemblies of p-conjugated systems show muchpromise for incorporation into supramolecular electronics
devices.1 Being polycyclic aromatic hydrocarbons, asphaltenes
are chemically related to compounds like graphene and hex-
abenzocoronene.2 Graphene is highly exploited for its unique
electronic characteristics.3–5 However, heteroatoms, metals, and
aliphatic defects must be introduced to improve solubility across
a variety of solvents and prevent against self-association of
graphene sheets and platelets.6–8 Hexabenzocoronene (HBC),
containing 13 fused aromatic rings, can self-assemble into
columnar structures, conveniently arranging its electron donor
and acceptor sites for incorporation into photovoltaic devices.9,10
Various substituted relatives of HBC can exhibit high charge-
carrier mobilities, can self-assemble into graphitic nanotubes,
and can form stable high-performance photosensitive field effect
transistors.11–13 However, being more difficult to characterize
than graphene, HBC is nearly insoluble in almost every solvent,
and requires functionalization by aliphatic chains.14 By contrast,
atomic-scale electronic defects including heteroatoms, metals,
and aliphatic chains are found naturally in asphaltenic materials,
without the need of synthesis. Here we exploit the heteroatomic
content of asphaltenes to tune colloidal stability, dissolution
characteristics, and conductivity of asphaltenes in a non-polar
medium, heptane. We do so by assembling asphaltenes via
protonation by a strong organic acid, dodecyl benzene sulfonic
acid (DBSA).
DBSA can dissolve asphaltenes, which are soluble in aromatics
but insoluble in light and medium alkanes.15–17 Asphaltenes dis-
solved by DBSA have molecular sizes less than 5 nm, as
measured by SAXS, comparable to that of asphaltenes in toluene
alone.18,19 Kinetics of asphaltene dissolution by DBSA are often
assessed by fixed wavelength UV-visible spectroscopy or
turbidity measurements.16,20–23 Because it dissolves asphaltenes to
the molecular scale, DBSA also changes the thermodynamic
dependence of asphaltene precipitation on composition or pres-
sure.17,24–26 Molecular simulations suggest that the acidic DBSA
headgroup interacts strongly with asphaltenes, forming layers of
individual DBSA molecules and DBSA hemimicelles around
asphaltenes.27,28 However, simulations rely on van der Waals
interactions or transfer and interaction free energies between
aromatic and aliphatic components, and do not include chemical
reactions: they neither determine the importance of nor
Yale University, Department of Chemical and Environmental Engineering,New Haven, CT, USA. E-mail: [email protected]; [email protected]† Electronic supplementary information (ESI) available: UV-vismeasurements on DBSA in heptane, measurements of the criticalmicelle concentration in heptane, and the dissolution screening done bylight scattering. See DOI: 10.1039/c2sm26003d
8778 | Soft Matter, 2012, 8, 8778–8785 This journal is ª The Royal Society of Chemistry 2012
Dynamic Article LinksC
differentiate between mechanisms such as Brønsted acid/base
reactions or hydrolytic cleavage.29,30
In compositional environments promoting the phase separa-
tion of asphaltenes out of molecular solutions, asphaltene
precipitation to the micron scale and larger can be eased by
colloidal stabilization through the use of non-ionic surfactants.
However, the ability of the ionic DBSA to stabilize asphaltene
colloids has not yet been explored.31,32 In the colloidal domain,
asphaltene stabilization occurs through electrostatic stabilization
in non-polar media.33,34 The origin of charge in non-polar
suspensions remains somewhat mysterious given the high ener-
gies, compared to thermal fluctuations, required to separate
charges in low dielectric media. Ionic surfactants at concentra-
tions above the critical micelle concentration can provide elec-
trostatic stabilization to colloids in non-polar media, suggesting
that charge disproportionation between pairs of neutral micelles
can lead to transient micellar charging.35–37 Other proposals for
the origin of charge in non-polar suspensions involve acid–base
interactions and other types of interfacial chemistry or ion-
exchanges.38
In this study, we find that intermediate DBSA concentrations
can electrostatically stabilize asphaltene colloids in non-polar
systems, as evidenced through light scattering measurements.
Additional DBSA achieves a complete molecular dissolution of
asphaltenes, but still at an order of magnitude lower in concen-
tration than that required by aromatic solvents. We explain this
apparent duality by appealing to the acid–base chemistry
observed through UV-vis spectroscopy. To develop a molecular-
scale understanding, we explore similarities with polyaniline
(PANI), a conducting polymer with aromatic and heteroatomic
contents showing promise for a wide variety of applications.39,40
While the utility of PANI is limited by its low solubility in non-
polar media, doping with DBSA both dissolves PANI and
enhances its electronic characteristics.41–43 We find the transition
from colloidal asphaltene suspension to molecular solution to be
mediated by a similar doping process which also enhances
solution conductivity. Adding a base completely reverses the
transition, suggesting a high degree of tunability in the assembly
of asphaltene–DBSA complexes.
Materials and methods
Materials
We obtain three petroleum fluids, which we call SB, QAB, and
CV, and measure the asphaltene and metal content, as well as
their densities and the densities of the asphaltenes.
The asphaltene content is measured as reported previously, by
mixing 1 g of petroleum fluid with 40 mL of heptane (Fisher).31
The mixtures are sonicated for 1 minute and filtered through
0.2 mm pore-size cellulose nitrate membrane filters (Whatman).The filtrate is collected, dried and weighed to give f, the weight
fraction of asphaltenes in the petroleum fluid. The petroleum
fluids’ densities ro are measured using a densitometer (Anton
Paar). The density ra of the asphaltenes is measured by preparing
a solution of 0.005 g asphaltenes per g toluene and measuring the
density of the mixture. Results are based on 10–12 measurements
each, and the results are within the error bars of typical values
presented in the literature.32,44,45 Quantitative elemental analysis
is conducted using ICP-AES (inductively Coupled Plasma-
Atomic Emission Spectroscopy) at the facilities of Lubrizol in the
UK, following ASTM D5185 to identify V, Fe, Ni, and Zn. The
total metal content cm of CV is 413 ppm, compared to 6 ppm in
SB and 23 ppm in QAB. Table 1 summarizes f, ro, ra and cm for
the three petroleum fluids.
We obtain dodecyl benzene sulfonic acid (DBSA), with
molecular weight 348 (Acros Organics), containing isomers with
chain lengths between C10 and C13. We prepare stock solutions of
DBSA in heptane at various concentrations c in ppm by weight.
We obtain triethylamine (TEA), with molecular weight 101 (J.T.
Baker).
Sample preparation
To investigate the effect of DBSA, we first prepare a model oil by
dissolving the filtered asphaltenes in toluene (J.T. Baker) in a
ratio of f ¼ 0.005 g g"1. We precipitate asphaltenes by addingheptane: all asphaltene suspensions are made by combining 1 g of
the model oil with 20 mL heptane. The total amount of asphal-
tenes in each suspension is thus #340 ppm, corresponding to theasphaltenic volume fraction, f # 0.0002. Given an estimate ofthe asphaltenic molecular weight at 750, the molarity of
asphaltenes in each suspension is #0.3 mM.2 We preparesuspensions at volumes of 3 mL and sonicate for 1 minute before
performing measurements. To study the effect of DBSA, we
combine heptane with the stock dispersant solutions at various
ratios to obtain DBSA concentrations between 10 < c < 50 000
ppm with respect to heptane, corresponding to a range of
molarities between 0.02 and 100 mM.Given the constant amount
of asphaltenes in the suspensions, the estimated stoichiometric,
or molar ratio of DBSA to asphaltene ranges from less than 0.1
to more than 300.
To test the reversibility of the DBSA–asphaltene doping, we
titrate drops of TEA into an asphaltene suspension prepared
with 50 000 ppm DBSA. We titrate TEA at concentrations up to
100 mM, to match the molar quantity of DBSA.
UV-visible spectroscopy
UV-visible spectroscopy is performed on DBSA solutions in
heptane and supernatants of the asphaltene suspensions (Agilent
8453). All measurements are carried out in UV-transparent
quartz cuvettes (Cole Parmer). Using several stock solutions of
DBSA in heptane, we measure dispersant solutions after centri-
fugation to confirm that DBSA micelles do not sediment under
the given amount of gravitational forcing. The spectral signature
of DBSA falls at wavelengths less than #280 nm (ESI†).To assess the effect of DBSA, suspensions of asphaltenes are
prepared as noted above, then centrifuged for 1000 minutes at 16.1
Table 1 Material properties of the petroleum fluids: density ro,asphaltene content f and asphaltene density ra, and total content cm ofmetallic components V, Fe, Ni, and Zn
Petroleumfluid ro (g mL
"1) f (g g"1) ra (g mL"1) cm (ppm)
SB 0.844 0.0069 1.10 $ 0.09 6QAB 0.865 0.0125 1.11 $ 0.01 23CV 0.905 0.1180 1.23 $ 0.08 413
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rcf (Eppendorf 5415 D). Given ra, this centrifugation time guar-
antees that asphaltene colloids larger than approximately 20 nm are
driven to the bottom of the centrifuge tubes. The resultant super-
natants are isolated and measured. When DBSA is added at
concentrations above #25 000 ppm, no sediment appears aftercentrifugation, indicating complete asphaltene dissolution.
Dynamic light scattering
We assess the effect of DBSA on colloidal stability using phase-
analysis light scattering (PALS) to measure the electrophoretic
mobility m of the colloidal asphaltene (ZetaPALS, Brookhaven
Instruments). To characterize particle size a, we use dynamic
light scattering (DLS) at wave vector q ¼ 0.01872 nm"1 (Zeta-PALS, Brookhaven Instruments). We monitor I/I0 in model
asphaltene suspensions with various concentrations of DBSA, to
assess asphaltene solubility (ESI†). Also by DLS, the measured
cmc of DBSA is 100 ppm with a micellar size of 33 $ 4 nm(ESI†).
Bulk conductivity measurements
We measure the impedance U (Solartron Impedance/Gain Phase
Analyzer) of the CV asphaltene suspensions as a function of
DBSA concentration over the range 500–500 000 ppm. We
report conductivity s ¼ k/U, where the cell constant k ¼ 0.1 forthe 0.1 mL electrode cells (Biorad Gene Pulser cuvettes). The
impedance measurement ceiling of the instrument is nearly 1010
ohms, giving a floor of s # 5 % 10"11 S m"1. We prepare samplesas indicated above, using the CV model oil with f ¼ 0.005 g g"1asphaltenes in toluene, and a more concentrated CV model oil
with f ¼ 0.025 g g"1 in toluene. We also measure s for DBSAdissolved in heptane, and in heptane with #3% by weighttoluene, to mimic the solvent conditions of the asphaltene
solutions.
Results and discussion
Asphaltene dissolution
To initiate asphaltene precipitation, we combine model asphal-
tene solutions in toluene with heptane and various concentra-
tions c of DBSA. A roughly constant amount of precipitate
appears for c below #1000 ppm, and decreases at higherconcentrations until no precipitation occurs at c > #25 000 ppm,indicating full dissolution. The simultaneous color change with
dissolution indicates the possibility of a chemical reaction, as
seen with the SB asphaltenes in Fig. 1(a). After centrifuging out
the asphaltene precipitate, the color gradation persists in the
resulting supernatants, as seen in Fig. 1(b). These observations
match other studies indicating decreased asphaltene precipitate
with increased DBSA content.25 At c < #1000 ppm, the super-natants are lighter in color than the original suspensions, due to
the centrifugation of the precipitated asphaltenes. A small
amount of color remains even at c ¼ 0 ppm, indicating thepresence of#50 ppm asphaltene remaining in the supernatant, asseen in previous studies with SB asphaltenes.33
An aromatic solvent like toluene can dissolve asphaltenes
when present at approximately 50% by weight, as seen on the left
in Fig. 1(c), by altering the bulk solution to achieve favorable
asphaltene–solvent interactions. However, DBSA can dissolve
asphaltenes at concentrations ten times less than required by
toluene, resulting in a color change, as seen in the middle of
Fig. 1(c). Given that DBSA is a strong acid, with aqueous pKa¼ "1.8, the dissolution of asphaltenes by DBSA might signifyirreversible hydrolytic activity, whereby a strong acid breaks
down large molecules into smaller pieces. However, if DBSA
dissolves asphaltenes simply through acid–base interactions,
then the process should be reversible by the addition of a base to
the DBSA–asphaltene system. Basic sites on asphaltene mole-
cules could include heteroatomic nitrogen, oxygen or sulfur, or
amine groups on alkyl chains or adjacent to an aromatic ring. We
choose triethylamine (TEA), a base with pKa close to 11, similar
to that of a wide range of alkyl amines. We start with a fully
dissolved asphaltene solution with 50 000 ppm DBSA, approxi-
mately 100 mM, as shown in the middle of Fig. 1(c). Upon
titration of five drops of TEA, an approximately equal molar
volume of TEA (#100 mM), the asphaltene solution completelydestabilizes, immediately generating the asphaltene precipitate,
as shown on the right in Fig. 1(c). The reversibility of dissolution
indicates that the process occurs entirely due to acid–base
interactions, and signifies that no hydrolytic activity occurs.
Both the color change of the asphaltene suspensions with
DBSA and the reversibility of dissolution with TEA indicate that
acid–base chemistry is responsible for dissolution. We can
confirm these chemical changes with UV-visible spectroscopy of
the asphaltene suspension supernatants. Optical spectroscopy in
this energy region reveals electronic signatures and charge
transfer processes of the molecules and assembled complexes in
solution. At c¼ 0 ppm, the UV-vis spectrum nearly saturates at l< 250 nm. Strong absorption A in the range below 300 nm
indicates p–p* transitions in the asphaltenic fused aromaticrings. This signature decays rapidly into the visible region,
typical of asphaltene compounds.46 Small amounts of DBSA in
suspension do not significantly alter the UV-visible spectra of the
asphaltene supernatants. However, above a few hundred ppm
Fig. 1 Dissolution by DBSA. (a) shows color gradation in SB asphal-
tene suspensions and (b) in the suspension supernatants. The DBSA
content ranges from c ¼ 50 ppm on the left to c ¼ 10 000 ppm on theright, as indicated by the labels in between (a) and (b). (c) shows CV
asphaltenes dissolved in 46% by weight toluene on the left, dissolved in
50 000 ppm DBSA in the middle, and, on the right, destabilized by the
addition of 100 mM TEA to the 50 000 ppm DBSA solution.
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DBSA, a shoulder develops in A between 300 and 350 nm. For
instance, this shoulder appears clearly at c > 750 ppm in
suspension supernatants of SB asphaltenes, and at 250 < c < 1000
ppm in suspension supernatants of CV asphaltenes, as seen in
Fig. 2(a) and (b), respectively. At very high DBSA concentra-
tions in the SB asphaltenes, c¼ 50 000 ppm, the absorption bandextends beyond 400 nm. In the CV suspension supernatants, an
additional shoulder appears between 400 and 450 nm above c ¼2500 ppm. Investigating A(c) at fixed l, we observe equivalence
point plateaus, indicating the complete protonation of different
heteroatomic groups by the acid. For the SB asphaltenes, A(c)
suggests the presence of multiple equivalence points, shown in
the inset of Fig. 2(a). For CV there is one main equivalence
plateau, and the gradual increase in A(c) below 1000 ppm may
mask several smaller plateaus, as shown in the inset of Fig. 2(b).
The features seen in the UV-vis spectra of asphaltene–DBSA
systems suggest a caveat for using fixed wavelength spectroscopy
for assessing dissolution kinetics, and furthermore may indicate
important chemical changes with the addition of DBSA.16,20–22
The features between 300 and 450 nm are similar to those seen in
the conducting polymer polyaniline (PANI), which contains
alternating benzene rings and amine groups. DBSA dopes PANI
by protonating its heteroatomic nitrogens, generating the con-
ducting emeraldine salt of PANI and leaving signatures in both
UV-visible and FTIR spectra.43,47–49 Full doping of PANI is ach-
ieved with a 1 : 1 ratio of DBSA to the aniline monomer, with
DBSA doping every other basic nitrogen in the PANI back-
bone.48,49 UV-vis spectral signatures in PANI–DBSA near 350 nm
indicate n–p* and p–p* electron and polaron transitions.41,43,49 Atvery high levels of doping, a band forms at 420 nm in PANI–DBSA
systems, which may even overlap with the one at 350 nm to form a
single flat peak.49,50
We propose that asphaltene dissolution by DBSA occurs
through a protonation process similar to that in DBSA-doped
PANI systems. Asphaltenes contain heteroatoms N, S, and O, each
of which having one or two lone pairs of electrons susceptible to
protonation. An asphaltene molecule characteristically constitutes
between seven and ten fused aromatic rings.2 Given an estimated
asphaltene molecular weight of 750 Da, the appearance of the
strong band at 450 nm in the CV asphaltenes beyond c¼ 2500 ppmmay correspond to more than 10 DBSAmolecules per asphaltene.2
Just as in PANI–DBSA, the long non-polar tail of DBSA solvates
the aromatic asphaltenes in non-polar solvents.47 Fig. 3 shows a
schematic of this protonation process in asphaltenes. A model
asphaltene compound with molecular weight #750, consisting of10 fused benzene rings, side chains and heteroatoms, can be
protonated by DBSA even at low DBSA concentrations, as in
Fig. 3(a). At higher DBSA concentrations, the degree of doping
increases, and several DBSA molecules can protonate the asphal-
tene molecules simultaneously. Eventually, at high enough DBSA
concentrations, the asphaltene molecule is effectively surrounded
by DBSA and solvated by its long-chain tails, as suggested in
Fig. 3(b). An increased amount of doping, possibly combined with
p-p interactions between the aromatic rings of the DBSA andasphaltene, may account for the flattening of the band between 350
and 450 nm, seen in the CV asphaltenes. Interestingly, acid–base
interactions themselves can be considered weak in comparison to
hydrogen bonds, for instance, and are therefore not considered in
asphaltene–DBSA simulations.30
The observation of acid–base interaction results suggests an
additional interaction: ion-pairing through electrostatic interac-
tions. Due to the non-polar medium, heptane, the ion pairing
between the protonated asphaltene ion and the DBSA sulfonate
ion is very strong, facilitating immediate complexation of the
solvated DBSA–asphaltene. Electrostatic binding energy E for
ions separated by r # 1 nm is on the order of E ¼ e2/(4p30Dr) #30 kBT, where e is the elementary charge, 30 the permittivity of
free space, D the dielectric constant, kB the Boltzmann constant,
and T the temperature. Thermal energy alone cannot maintain
separation for ions which are closer together than the Bjerrum
length lB ¼ 4p30DkBT/e2; in a low-dielectric constant mediumsuch as heptane, with D ¼ 2, lB ¼ 27 nm.51 In this way DBSAdissolves asphaltenes at ten times lower concentrations than
required by aromatic solvents: rather than altering the bulk
solution, DBSA alters asphaltene molecules themselves. Elec-
trostatic interactions facilitate the assembly of complexes with
more favorable interactions with the non-polar background
solution.
Fig. 2 UV-visible spectroscopy on model asphaltene suspension super-
natants. (a) and (b) show selected spectra for SB and CV model asphal-
tene suspensions respectively, at DBSA concentrations in ppm as listed in
the legend. In both (a) and (b), the arrow denotes increasing DBSA
concentration. In each, the inset plot shows A(c) at three wavelengths, as
listed in the legends, from within the shoulder region of the spectra in the
main plots.
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The structures shown in Fig. 3 may also suggest a reason for
the difference in the SB and CV spectra seen in Fig. 2: if CV
asphaltenes have more basic heteroatomic sites than SB
asphaltenes, the degree of protonation by DBSA may be
greater, leading to the prominence of the shoulder above
400 nm in the CV asphaltene system. The importance of the
number of ‘‘active sites’’ for interactions between asphaltenes
and DBSA has been described by free energy thermodynamic
models.30 Depending on the heteroatomic content, a high degree
of protonation by DBSA could effectively lead to the encap-
sulation of an asphaltene molecule within an inverse micelle of
DBSA. Interestingly, literature studies suggest that other
compounds in the benzene sulfonic acid family with shorter
aliphatic tails are less effective in asphaltene dissolution.16,52
Furthermore, the DBSA self-assembly is known to stabilize
guest molecules: for instance, DBSA micelles can promote
emulsion polymerization of PANI in aqueous systems.49,53
These observations match well with the molecular-scale model
we propose for the cascading action of DBSA in protonating
and eventually solvating asphaltenes.
Colloidal asphaltene characteristics
The acid–base interactions revealed by UV-vis may have inter-
mediate effects at DBSA concentrations lower than that which
fully dissolves the asphaltenes. The presence of the centrifuged
precipitate indicates a colloidal asphaltene phase which may or
may not aggregate before settling. While acid–base interactions
have been proposed as a mechanism for electrostatic stabilization
in non-polar suspensions, the effect of DBSA on colloidal
asphaltene stability has not been fully explored. At low concen-
trations, DBSA does not stabilize asphaltene sedimentation
dynamics.23 We investigate the effect of moderate concentrations
of DBSA on the stabilization of the colloidal asphaltenes using
dynamic light scattering (DLS) to assess aggregation dynamics,
particle size, and electrophoretic mobility. We measure suspen-
sions with c < 1000 and 2500 ppm DBSA, before the onset of
asphaltene dissolution. Asphaltene dissolution increases back-
ground absorption in the suspensions, thereby decreasing the
effectiveness of DLS as a colloidal characterization tool, as dis-
cussed in the ESI.†
Asphaltenes are overall charge-neutral materials, exhibiting
both positive and negative charges on the colloids suspended in
heptane.33,34 The positive charges may arise from the asphaltene
metal content.21,34,54 Doping of the basic sites by DBSA
suppresses their contribution to the negative charges, promoting
net positive charging on asphaltene colloids. The average elec-
trophoretic mobility hmi of the colloidal asphaltenes increaseswith DBSA, indicating an increase in colloidal stability before
the onset of dissolution. In the CV suspensions, hmi increasesfrom nearly 0 below c ¼ 100 ppm to #0.2 % 10"8 m2 V"1 s"1above 100 ppm. The more gradual increase in hmi in the SB andQAB suspensions also occurs as c surpasses 100 ppm, as seen in
Fig. 4(a). The acid–base chemistry revealed by UV-vis clearly
plays an important role in colloidal asphaltene stability, as in the
electrostatic stabilization of other colloidal systems.38 Unlike in
more traditional colloidal systems, however, DBSA disintegrates
asphaltene colloids at higher concentrations, before completely
dissolving them.
The stabilizing activity of DBSA also manifests in colloidal
aggregation dynamics and particle size. As seen with other
dispersants, without sufficient amounts of DBSA in suspension,
asphaltenes form micron-scale colloids which persist for a few
minutes at a roughly constant size and then abruptly aggregate to
a much larger scale, ten microns and larger.31–33 We define the
aggregation onset time tagg as the time of abrupt increase from
the initial particle size. Aggregation slows and tagg increases with
c from approximately 5 minutes at 0 ppm to nearly 20 or more at
c up to 100 ppm, as seen in Fig. 4(b). Aggregation ceases in SB
model suspensions beyond c ¼ 50 ppm, beyond c ¼ 25 ppm forQAB, and beyond c¼ 100 ppm for CV. Closer inspection revealsthat DBSA also changes the pre-aggregate particle size a0, which
has proven to be a useful measure of colloidal asphaltene
stability.33 As with hmi, c ¼ 100 ppm seems to act as a switchfor all three suspension types, as seen in Fig. 4(c). Below c ¼ 100ppm, a0 > 1 mm, while above 100 ppm, a0 falls to 500 nm or less.This switching behavior is seen most easily in the CV suspen-
sions. By contrast, the behavior in the QAB suspensions is more
gradual, nearly following a power law decrease over the range
investigated.
Fig. 3 Proposed chemical mechanism for molecular assembly. (a) shows
a schematic asphaltene molecule with a heteroatomic nitrogen proton-
ated at low DBSA concentrations. At sufficiently high DBSA concen-
trations, as in (b), the heteroatomic nitrogen, oxygen, and sulfur groups
are all protonated. In both (a) and (b), blue denotes asphaltene and red,
DBSA.
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DBSA forms micelles at c ¼ 100 ppm (Fig. S2†), the sameconcentration facilitating changes in a0 and hmi. Most ionicsurfactants stabilize colloids when added above the critical
micelle concentration (cmc), while non-ionic dispersants can
maximize the stability at the cmc.33,38,55,56 While micellization
plays an important role in non-polar colloidal stabilization, its
role in the asphaltene–DBSA systemmust be considered together
with its acidity. Approaching the cmc enhances acidity, helping
DBSA halt aggregation and alter both a0 and hmi for c # 100ppm.57 As suggested in Fig. 3, a cooperative self-assembly
process involving DBSA–asphaltene complexes enables disinte-
gration and eventual dissolution of colloidal asphaltenes.
Conductivity measurements
Just as doping in PANI–DBSA films and nanodispersions can
increase electrical conductivity by an order of magnitude or
more, so too does conductivity s increase in asphaltene–DBSA
systems.42,49,50,58 Without asphaltenes, s of DBSA solutions rises
nearly linearly with concentration, as shown in Fig. 5. We
measure DBSA in heptane and DBSA in heptane with 3%
toluene by weight, mimicking the asphaltene solution solvent
conditions. Toluene does not strongly affect the result, displayed
with blue circles in Fig. 5. The dashed-line is a power law fit with
exponent 0.65; the measurements increase to #2 % 10"10 S m"1 atc ¼ 25 000 ppm. At concentrations above 25 000 ppm, deviationfrom the fit indicates the possibility of electrochemistry occurring
at the aluminum electrodes. Although the solutions are non-
polar, charges could arise from electrochemistry or through ion-
exchange interactions between DBSA micelles.36
Adding asphaltenes to the system increases s by an order of
magnitude, further indicating asphaltenes as an important source
of charge. In asphaltene–DBSA systems, s increases nearly
linearly over a few orders of magnitude in c, obeying a power-law
with a greater exponent than in DBSA–heptane solutions. In
suspensions with the same composition as those used in UV-vis
and DLS measurements, the asphaltene volume fraction f # 3 %10"4, the power law has an exponent 0.77, and s rises from 10"10
to 8 % 10"9 S cm"1 between c ¼ 500 and 100 000 ppm. Whenasphaltenes are present at #3 times higher concentration at f #10"3, again the rise in s is nearly linear, with a power law expo-
nent 0.85. Furthermore, s similarly increases by a factor of 2–3
Fig. 4 Colloidal particle characteristics in asphaltene suspensions. (a)
shows the average electrophoretic mobility hmi as a function of DBSAconcentration in the three types of asphaltene suspensions. (b) shows the
aggregation onset time tagg as a function of c for the concentrations at
which aggregation occurs, and (c) shows the pre-aggregate colloidal
particle size a0, all as a function of c.
Fig. 5 Conductivity of CV asphaltene solutions. The plot indicates s in
heptane with DBSA as a function of c in ppm. The three datasets
represent different volume fractions of asphaltenes as indicated in the
legend, and the dashed lines represent fits to the data.
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with the corresponding increase in f, as seen in Fig. 5. Above
100 000 ppm, 10% DBSA by weight, the data deviate from the fit
in both asphaltene systems, again suggesting the possibility of
electrochemistry at the electrode. However, asphaltenes effec-
tively incorporate the acid molecules into DBSA–asphaltene
complexes such that electrochemistry occurs at a higher
concentration than in the DBSA solutions.
Conclusions
When protonated by a strong organic acid, the heteroatomic
defects naturally present in asphaltenes facilitate a tunable
transition from colloidal suspension to molecular solution.
DBSA protonates heteroatomic nitrogen, oxygen, and sulfur
groups in asphaltenes, while its long chain tail provides efficient
solvation. The strength of electrostatics in non-polar media
allows for a very strong ion-pairing between the protonated
asphaltene and sulfonate ion, effectively altering asphaltene
molecules without the need of covalent synthesis. Low degrees of
protonation suffice to stabilize asphaltene colloids, while higher
degrees can disintegrate them entirely. In this way, cooperative
asphaltene self-assembly within DBSA micelles enables full
dissolution in non-polar media. The cascading behavior of
increasing protonation leads to an apparent dual-nature of
DBSA, with colloidal stability at intermediate concentrations
and molecular dissolution at high concentrations. The effec-
tiveness of DBSA in solubilizing polyaromatic hydrocarbons like
asphaltenes bodes well for its wider use in solution-processing of
substituted p-conjugated organic semiconductors.Furthermore, the enhanced solubility of asphaltene–DBSA in
non-polar media suggests the possibility of applications using
asphaltene-derived materials. Indeed, asphaltenes are related in
chemistry to graphenic materials and other polycyclic aromatic
hydrocarbons already in development for a wide variety of
applications. The molecular defects in asphaltenes, including
heteroatomic content and other basic sites, allow for acid–base
chemistry to reversibly stabilize the material in non-polar
solvents. Conductivity measurements indicate that asphaltenes
are an important source of charges even in low-dielectric
constant media, and suggest that s increases with asphaltene
content. While our current measurements are in systems at low
asphaltene concentrations, up to f # 10"3, scaling to higherconcentrations may allow conductivities in asphaltene thin films
to be on the same order as conducting polymers films such as
those made with DBSA–PANI.
Acknowledgements
We gratefully acknowledge the support of RERI member institu-
tions, and experimental assistance from Salvatore DeLucia and
Anjali Khetan. SMH thanks Ulrich Hintermair for helpful
conversations.We appreciate the use of theUV-visible spectroscopy
equipment belonging to Menachem Elimelech, and acknowledge
Lubrizol Corporation for performing the ICP-AES analysis.
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