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 much

promise 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: sara.hashmi@yale.edu; abbas.firoozabadi@yale.edu† 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

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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 ! 0.005 g g"1. We precipitate asphaltenes by adding

heptane: 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 the

asphaltenic volume fraction, f # 0.0002. Given an estimate of

the asphaltenic molecular weight at 750, the molarity of

asphaltenes in each suspension is #0.3 mM.2 We prepare

suspensions 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 after

centrifugation, 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 ! 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 ! k/U, where the cell constant k ! 0.1 for

the 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 samples

as indicated above, using the CV model oil with f ! 0.005 g g"1

asphaltenes in toluene, and a more concentrated CV model oil

with f ! 0.025 g g"1 in toluene. We also measure s for DBSA

dissolved in heptane, and in heptane with #3% by weight

toluene, 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 higher

concentrations 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 ! 0 ppm, indicating the

presence of#50 ppm asphaltene remaining in the supernatant, as

seen 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 pKa

! "1.8, the dissolution of asphaltenes by DBSA might signify

irreversible 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 completely

destabilizes, 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 c! 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 aromatic

rings. 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 ! 50 ppm on the left to c ! 10 000 ppm on the

right, 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, c! 50 000 ppm, the absorption band

extends beyond 400 nm. In the CV suspension supernatants, an

additional shoulder appears between 400 and 450 nm above c !2500 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 At

very 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 c! 2500 ppm

may 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 of

10 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 and

asphaltene, 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 ! 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 ! 4p30DkBT/e2; in a low-dielectric constant medium

such as heptane, with D ! 2, lB ! 27 nm.51 In this way DBSA

dissolves 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 increases

with DBSA, indicating an increase in colloidal stability before

the onset of dissolution. In the CV suspensions, hmi increases

from nearly 0 below c ! 100 ppm to #0.2 % 10"8 m2 V"1 s"1

above 100 ppm. The more gradual increase in hmi in the SB and

QAB 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 ! 50 ppm, beyond c ! 25 ppm for

QAB, and beyond c! 100 ppm for CV. Closer inspection reveals

that 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 ! 100 ppm seems to act as a switch

for all three suspension types, as seen in Fig. 4(c). Below c ! 100

ppm, 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 ! 100 ppm (Fig. S2†), the same

concentration facilitating changes in a0 and hmi. Most ionic

surfactants 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 # 100

ppm.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 at

c ! 25 000 ppm. At concentrations above 25 000 ppm, deviation

from 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 ! 500 and 100 000 ppm. When

asphaltenes 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 DBSA

concentration 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 higher

concentrations 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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