Structure and Dynamic Properties of Colloidal Asphaltene Aggregates

Article pubs.acs.org/Langmuir

Structure and Dynamic Properties of Colloidal Asphaltene Aggregates Joel̈ le Eyssautier,†,‡ Didier Frot,† and Loïc Barré*,† †

IFP Energies nouvelles, 1-4 Avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France Physique de la Matière Condensée, CNRS−École Polytechnique, UMR 7643 CNRS, 91128 Palaiseau Cedex, France

‡

ABSTRACT: The abundant literature involving asphaltene often contrasts dynamic measurements of asphaltene solutions, highlighting the presence of small particle sizes between 1 and 3 nm, with static scattering measurements, revealing larger aggregates with a radius of gyration around 7 nm. This work demonstrates the complementary use of the two techniques: a homemade dynamic light scattering setup adapted to dark and fluorescent solutions, and small-angle X-ray and neutron scattering. Asphaltene solutions in toluene are prepared by a centrifugation separation to investigate asphaltene polydispersity. These experiments demonstrate that asphaltene solutions are made of Brownian colloidal aggregates. The hydrodynamic radii of asphaltene aggregates are between 5 and 10 nm, while their radii of gyration are roughly comparable, between 3.7 and 7.7 nm. A small fraction of asphaltenes with hydrodynamic and gyration radii around 40 nm is found in the pellet of the centrifugation tube. The fractal character of the largest clusters is observed from small angle scattering nearly on a decade length scale. Previous results on aggregation mechanisms are confirmed (Eyssautier, J., et al. J. Phys. Chem. B 2011, 115, 6827): nanoaggregates of 3 nm radius, and with hydrodynamic properties also frequently illustrated in the literature, aggregate to form fractal clusters with a dispersity of aggregation number. aggregates are discs of 3.2 nm radius and 0.7 nm height.9 At larger length scale, small angle scattering techniques show a further aggregated structure of radius of gyration RG = 7.0 nm, organized in mass fractals clusters. However, previous measurements of hydrodynamic properties did not yield large sizes. Small angle scattering is the only technique that shows large aggregates for asphaltenes in solution. Recently, Goual et al.10 presented DC-conductivity measurements on a wide concentration range and highlighted two breaks at 0.2 g/L and 2 g/L, attributed to nanoaggregation and clusterization, respectively. In opposition to the kinetic approach commonly applied to asphaltenes, which describes colloidal-stabilized aggregates detailed in the previous paragraph, asphaltenes in a good solvent are sometimes described as dispersed molecules near phase transitions,11 in which short lifetime concentration fluctuations occur. The small angle scattering signal is then associated to the correlation length of temporary fluctuations. However, Dechaine and Gray12 proved with membrane selfdiffusion experiments that the asphaltene solution is mostly made of aggregated molecules not passing through the membrane after 7 days. They concluded that the rate of exchange between molecules and aggregates is extremely slow. The dynamic light scattering (DLS) technique is another way to examine aggregate lifetimes.

1. INTRODUCTION In the study of complex systems in soft matter, the confrontation of their static and dynamic properties is of great interest for elucidating structural organization and characteristic length and time scales. Asphaltenes belong to the complex systems category. They are operationally defined as forming precipitates in alkanes and being soluble in toluene. They represent the heaviest and highly aromatic components of heavy oils and bitumen, which remain a significant energy resource nowadays. Their aggregation properties at various length scales impact the oil industry from extraction, to transport, to refining. They are blamed for the increase in viscosity of fluids, contributing to solid deposits when depletion occurs in the well and to plugging the pores of catalytic networks during refinery operations.1,2 Asphaltenes have been investigated through numerous characterization techniques. High resolution mass spectrometry recently enabled determination of an average molecular weight between 500 and 1000 g/mol.3 Property changes of asphaltene in toluene solutions were observed upon increasing concentration and were ascribed to the formation of nanoaggregates.4 Further studies showed that, at asphaltene concentrations larger than 0.1−0.2 g/L, nanoaggregates are the prevalent species. Measurements of hydrodynamic properties of asphaltene solutions such as the diffusion coefficient gave hydrodynamic radii between 1.0 and 3.6 nm.5−8 Recently, nanoaggregates were characterized from a structural study combining SAXS and SANS measurements, and the results showed that nano© 2012 American Chemical Society

Received: April 26, 2012 Revised: July 19, 2012 Published: July 24, 2012 11997

dx.doi.org/10.1021/la301707h | Langmuir 2012, 28, 11997−12004

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Figure 1. Schematic representation of the dynamic light scattering setups. (a) Initial Vasco geometry (side view). (b) In-house setup with a bandpass filter (top view). preparation is not considered in this study. Centrifugation B: the initial asphaltene solution F0 is centrifuged for 11 h at 24 000 rpm in a Beckman Coulter ultracentrifuge fitted with a JS 24.38 rotor, leading to an average acceleration of 104 000g. The asphaltene fraction adhering to the centrifugation tube is recovered and dissolved in toluene (fraction F7). We note that this fraction is hardly soluble in toluene. Each of the seven fractions is characterized as follows: (1) Asphaltene concentration by weighing an aliquot before and after solvent evaporation until a constant weight is reached. (2) Chemical composition (CHONS). C, H, and N are determined using the ASTM D5291 method. O and S are measured by infrared detection of CO and SO2, respectively, generated by pyrolysis at 1100 °C and 1150 °C, respectively. (3) Asphaltene mass density d at 20 °C as described elsewhere19 has been determined by measuring specific gravity of asphaltene solutions at 25 °C using an Anton Paar DMA5000 densitometer. Indeed, the specific volume of the solutions versus mass fraction of asphaltene showed a linear behavior, in the range 1−5 wt %, which permits an extrapolation at null dilution. (4) SAXS, SANS, and DLS measurements: It is known from SAXS and SANS measurements20 that no major modification occurs in the Guinier region upon dilution from 50 to 0.5 g/L. On the one hand, to perform SAXS experiments on a reasonable acquisition time, fractions F1−F6 are measured at 5 g/L in toluene. The accessible q range is extended for the F7 fraction by measuring a 20 g/L solution, filtered at 0.45 μm, in deuterated toluene using SANS. On the other hand, dynamic light scattering is performed on solutions of F1 through F7 filtered at 0.45 μm and diluted at c = 0.5 g/L. This low concentration is chosen to limit strong absorption effects of the solutions in the visible range, possibly leading to local temperature elevation and solvent viscosity modification. All scattering measurements are performed at 23 °C. The fluorescence spectra are obtained with a Varian Cary Eclipse fluorescence spectrophotometer equipped with an extended range PMT (Photomultiplier tube) detector. 2.2. Static and Dynamic Scattering. In static and dynamic scattering, the scattered intensity is collected at 2θ angle and the magnitude of the scattering wave vector q is given by q = 4πn sin (θ)/ λ0, where n is the refractive index of the medium and λ0 is the radiation wavelength in vacuum. For static X-ray and neutron scattering (SAXS and SANS), the intensity is analyzed as a function of q. For dynamic light scattering (DLS), the time-dependent intensity fluctuations are collected at a single q value, where q−1 is related to the characteristic length of the observation box where fluctuations occur. a. Small Angle X-ray and Neutron Scattering (SAXS and SANS). Equipment. SAXS measurements of asphaltene fractions are processed at 23 °C on an in-house experimental setup as described elsewhere21 in a flow-cell quartz capillary. A copper rotating anode is used (λ = 0.154 nm) and a total q range of 10−1 to 3 nm−1 is reached. SANS measurements are carried out at the Laboratoire Leon Brillouin (LLB,

Dynamic light scattering is presumably a relevant complementary technique to small angle scattering because the size window overlaps the window observed by SAXS and SANS. Although not a priori adapted for dark and opaque systems such as asphaltene solutions, several DLS measurements on asphaltene solutions are reported in the literature, using a special backscattering setup.13 However, all results involve precipitated asphaltenes in heptane/toluene (Heptol) solution. Yudin et al.131415 measured the aggregation onset, defined as the amount of heptane added to the toluene for asphaltene precipitation to occur. The sizes reported are roughly micronic. A kinetic study showed that depending on the asphaltene content, low concentrations led to diffusion-limited aggregation while high concentrations led to reaction-limited aggregation.15 Hashmi et al.1617 studied the effect of a dispersant on asphaltene sedimentation dynamics in Heptol mixtures. Until now, stabilized colloidal asphaltene aggregates (e.g., in toluene) were never studied by DLS. We propose in the present study to develop and adapt this technique for asphaltene solutions in toluene. It will clarify the dynamic status of asphaltene: Brownian particles or short lifetime concentration fluctuations. We determine static and dynamic properties of asphaltenes in toluene, for centrifuged fractions of reduced polydispersity. The radius of gyration and the molecular weight of aggregates are measured by small-angle X-ray and neutron scattering, in parallel to the hydrodynamic radius by DLS. As a matter of fact, similar sizes are observed by both techniques. The relationship between RG and RH is investigated for various aggregate sizes. The knowledge of the characteristic ratio between the radius of gyration and the hydrodynamic radius is a way to investigate aggregation mechanisms.18

2. EXPERIMENTAL SECTION 2.1. Sample Preparation and Characterization. Samples are made of asphaltenes in toluene. Asphaltenes are obtained from a vacuum residue (Safaniya) by n-heptane precipitation followed by a Soxhlet-type purification, as described elsewhere.9 A 3 wt % solution of asphaltenes in toluene is prepared, and two separations by centrifugation are performed at 20 °C. Centrifugation A: the initial asphaltene solution F0 was centrifuged for 7 h at 32 000 rpm in a Beckman Coulter ultracentrifuge fitted with a SW 32 Ti rotor, leading to an average acceleration of 175 000g. Six fractions of the supernatant are recovered: F1 (top) down to F6 (bottom). The pellet of this 11998


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G2(q , t ) = α + βg12(q , t )

Saclay, France) on the PAXE instrument, and the procedure is described elsewhere.9 The wavelength is set at 1.7 nm, and the sampleto-detector distance is 5 m, covering a total q range of 3 × 10−2 to 3 × 10−1 nm−1. Data Treatment. The scattered intensity for a two-phase system of particles, at volume fraction ϕ in a solvent, is governed by the general equation:22 2

I(q) = ϕ(1 − ϕ)Δρ F(q)S(q)

where α is a baseline and β is the coherence factor. The time dependence of g1(q,t) is related to the dynamic properties of the solution. For Brownian, monodisperse, and noninteracting particles, g1(q,t) is an exponential decaying function:

g1(q , t ) = exp(−Γt )

g1(q , t ) =

(6)

where Ai represents the scattered intensity contribution of particles of class i and Γi its decay rate. The method for correlogram decomposition, which is a complex but well-known problem26,27 is based upon the combined use of the Laplace transform and of Padé approximants, which does not require an a priori hypothesis as to the number of discrete components Γi. For monomodal systems, the correlogram is well described by a single exponential decay, and the Pade approximant method yields a decay time that is also the mean decay time provided by cumulant method. For polydisperse systems, the Z-Pade/SVD algorithm operates a SVD (singular value decomposition) rank determination to choose the number of characteristic decay times.28 From this sum of exponential decaying functions, a standard Levenberg−Marquardt method is used to adjust the weight of each exponential decaying function. Good agreement between the data and the fit is reached when residual minimizing the χ-square criterion are statistically distributed around zero without variations larger than a third of the noise envelope. When the diffusion process is Brownian, the self-diffusion coefficient of the particles Di is related to Γi by Γi = Diq2. In the high dilution limit, the self-diffusion coefficient Di enables calculation of the hydrodynamic radius RHi of the hard sphere-equivalent noninteracting particles using the Stokes−Einstein equation:

(2)

To estimate the scattering cross section at zero angle I(0), an approximation of I(q) at small q values is needed. Zimm formalism,23 defined by eq 3, has been found more robust than the classical Guinier approximation because validity of eq 3 can be extended for swollen particles24 and asphaltene aggregates19 up to qRG < 2 .

⎞ q2R G2 1 1 ⎛ ⎜1 + = + ...⎟ I(q) I(0) ⎝ 3 ⎠

∑ Ai exp(−Γit ) i

I(0)dNa ϕ(1 − ϕ)Δρ2

(5)

where Γ is the decay rate. For a polydisperse system, g1 (q,t) can be decomposed into a sum of exponential decaying functions:

(1)

In the dilute regime, the structure factor S(q) is close to 1. The scattering length density difference between particle and solvent, Δρ, is determined from density and elemental composition of solvent and particles.22 F(q) is the form factor normalized by the volume v of one particle (i.e., F(0) = v). By combining the previous equations, the scattering intensity at zero q value is directly related to the molar mass MW of scatterers:

MW =

(4)

(3)

Scattering data are processed through the model-independent Zimm approximation, leading to the determination of the radius of gyration RG (nm) and the molecular mass of the aggregates MW (g/mol). For a polydisperse system, a mean radius of gyration is obtained by a zaverage, overweighing the large sizes. The radius of gyration represents the radius of a hollow sphere having the same mass moment of inertia as the aggregate. b. Dynamic Light Scattering (DLS). Equipment. DLS measurements are first processed on a particle size analyzer in backscattering light detection mode (Vasco, Cordouan Tech.), illustrated in Figure 1a. The wavelength of the incident beam is 656 nm, the beam goes through a prism and reaches a thin film of sample, and the backscattered light is recorded at 135° by an APD detector. This setup is suitable for opaque and concentrated systems.13 These first measurements highlight the strong fluorescence properties of asphaltenes, characteristic of their aromatic chemical structure.25 But in DLS experiments, this fluorescence signal gives rise to an incoherent background, impoverishing the relaxation signal of the autocorrelated scattered intensity of the particles. A second setup is built up, introducing a band-pass filter to reduce the fluorescence effect on the recorded signal. This geometry is sketched in Figure 1b. The incident beam (λ = 656 nm) passes through the sample in a 10 mm Hellma quartz cell. The scattered light at 90° is filtered by a removable band-pass filter centered at 660 nm with a 10-nm fwhm (Fb 660-10, Thorlabs) and recorded by an APD detector. The intensity of the scattered signal is optimized by positioning the laser and collecting the scattered signal in the vicinity of the wall with great care to avoid the heterodyne mixing mode. This is achieved with a micrometric turntable of displacement and the monitoring of the scattered intensity while we optimize the relative position of the laser beam with the quartz cell wall. As a highly selective spatial filter, we use a single mode fiber in both setups to collect the scattered signal to enhance the coherence factor β (see eq 4). The time-averaged scattered intensity, monitored all along the experiment duration, remains constant, which means that no sedimentation or creaming occurs. Data Treatment. From time-dependent intensity fluctuations, the normalized autocorrelation function of the scattered intensity G2(q,t) (e.g., a correlogram) is built. G2(q,t) is related to the modulus of the normalized field autocorrelation function g1(q,t) by the Siegert relationship:

Di =

kBT 6πηRHi

(7)

where kB and η are, respectively, the Boltzmann constant and the solvent viscosity. When dealing with low polydisperse solutions, average hydrodynamic radii are calculated. A volume average consists of the following calculation:

RHv

⎛ ∑ B R 3 ⎞1/3 i Hi ⎟ = ⎜⎜ i ∑ B ⎟ ⎝ i i ⎠

(8)

where Bi represents the number contribution of particles of class i. To compare RH to the radius of gyration given by SAXS and SANS, a z-average is calculated according to:

RHz

⎛ ∑ B R 8 ⎞1/2 i Hi ⎟ = ⎜⎜ i 6 ⎟ ⎝ ∑i Bi RHi ⎠

(9)

3. RESULTS 3.1. Static Small Angle Scattering. SAXS and SANS spectra were obtained for fractions F1−F6 and F7, respectively, diluted in toluene at 5 g/L. At such low concentration, it has been shown for asphaltene fractions in toluene,19 that S(q) is close to 1 and consequently the approach described in section 2.2.a to obtain MW and RG remains relevant. As shown in Figure 2 and Table 1, scattering spectra of the seven asphaltene fractions and their calculated radii of gyration and molecular masses show an evident separation obtained by centrifugation. Besides, asphaltene mass densities are similar for fractions F2− F6 (see Table 1): separation is obtained by size for these fractions. The deeper the sampling in the tube, the bigger and 11999


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aggregation of primary particles of fractal dimension 1.8. The asphaltene aggregate can be defined by its aggregation number N, expressed as N = MW /MNA where MNA is the mass of the primary particle, called nanoaggregate in reference to Eyssautier et al.9 The aggregation number is illustrated by the right axis in Figure 3. The fractions of the study are made of asphaltenes of aggregation numbers between 4 and 200.

Figure 3. (circles) Molar mass versus the radius of gyration of the asphaltene aggregates in toluene separated by centrifugation. The linear trend yields MW ∝ RG1.8. (Star) Data for the asphaltene nanoaggregate from Eyssautier et al.9 Right axis: aggregation number (see text).

Figure 2. (Circles) SAXS spectra of asphaltene fractions of centrifugation A (F1−F6), (squares) SANS spectra of asphaltene fraction of centrifugation B (F7). All data are normalized by the concentration and the contrast term according to eq 2. The lines are the corresponding Zimm approximation (eq 3). Extrapolation to q → 0 gives the molecular mass of the aggregates, MW. The slope indicates the fractal dimension of the aggregates, 1.8.

Moreover, as shown in Figure 3, the mass MW and the radius of gyration RG are related through the relation MW ∝ RdGf, characteristic of a fractal aggregation with fractal dimension df. This is in line with previous results from Barré et al.19 and Eyssautier et al.9 The fractal dimension obtained here is 1.8, in agreement with the master slope of the scattering spectra shown in Figure 2. 3.2. Dynamic Light Scattering. a. Dynamic Properties of Asphaltene Solutions. Figure 4 shows the autocorrelation functions of an asphaltene solution (F6) measured at two scattering angles (2θ = 90° and 135°). From eqs 4 and 6, g1(q,t) can be fitted by the sum of two exponential decaying functions, giving satisfactory residua. The resulting fits are shown in Figure 4. This primary observation is consistent with the Brownian properties of the asphaltene solution. We then plot the decay rates Γi, obtained from the inversion of g1(q,t), versus q2 for the two scattering vectors. The results are shown

heavier the particles. Radii of gyration of aggregates are between 3.7 and 37.5 nm, while masses are between 3.88 × 104 and 2.10 × 106 g/mol. Note that for fraction F6, the Guinier plateau is not reached at the lowest q value scanned. Neither molecular weight nor gyration radius can be estimated. The same comment could be made for fraction F7, for which a large error bar has been set. Spectra in Figure 2 show that the fractions differ in molecular weight and size, but the large q values behave the same, which means that the structure on a small length scale is independent of the size of the aggregates. At intermediate q values, a power law (I(q) ∝ q−1.8) is found. The range of scattering vectors over which this behavior is observed is more extended for the heavier fractions (between q ≈ 0.2 and 1 nm−1 for F7) and is characteristic of a fractal

Table 1. z-Average Radius of Gyration RG,a Weight-Average Molecular Mass MW,a and Hydrodynamic Radii RHi,b Volume, and z-average hydrodynamic radius RHc NAd F1 F2 F3 F4 F5 F6 F7 F0

d

RG (nm)

MW (g/mol)

RH1 (nm)

RH2 (nm)

R̅ Hv (nm)

R̅ Hz (nm)

1.117 1.182 1.169 1.189 1.198 1.186 n.a. 1.195

2.00 3.70 4.14 5.55 6.87 7.71 n.a. 37 ± 3 n.a.

9.58 × 103 3.88 × 104 4.42 × 104 8.96 × 104 1.14 × 105 1.49 × 105 n.a. 2.1 ± 0.8 × 106 n.a.

n.a. 4.8 2.4 5.2 6.2 6.7 8.1 10.0 13.0

n.a. 7.4 5.6 10.4 11.6 11.7 35.7 40.9 40.4

2.57 4.8 5.4 5.5 6.6 7.2 n.a. n.a. n.a.

2.57 5.2 5.6 8.9 9.8 10.0 n.a. n.a. n.a.

a

Calculated from the Zimm approximation on the SAXS and SANS spectra. bCalculated from the Stokes−Einstein equation (eq 7) on the DLS correlogram fits. cFrom eqs 8 and 9. dData for the nanoaggregate from Eyssautier et al.9 12000


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Figure 4. Autocorrelation functions of F6 asphaltene fraction in toluene at two scattering angles: 2θ = 90° and 135°, respectively, with and without band-pass filter. The lines represent the multiexponentiallike fits.

Figure 6. Spectrogram of toluene and of asphaltenes in toluene (F3) excited at λ = 656 nm.

does not fluoresce (flat spectrum), the asphaltene solution shows a broad emission band after the excitation wavelength (656 nm). The pertinence of the 90° setup with the use of a band-pass filter, centered on the laser beam wavelength, is unambiguous and shown in Figure 7. Although the fluorescence signal is not entirely absorbed, the relaxation to baseline ratio is enhanced by a factor 10.

in Figure 5. The relation Γi = Diq2 expected for Brownian particles is observed. The dynamic properties of the asphaltene

Figure 7. Autocorrelation function for F3 asphaltene solution in toluene with the 2θ = 90° setup, with and without band-pass filter. Figure 5. Decay rates obtained from the fits of Figure 4 (eq 6) at two scattering wave vectors q. The relations Γi ∝ q2 indicate the diffusive properties of the solution through diffusion coefficients Di. D1 = 4.5 × 10−11m2/s and D2 = 0.9 × 10−11m2/s.

c. Hydrodynamic Dimensions. Using the band-pass filter setup, the scattered intensity autocorrelation functions for the seven asphaltene fractions are shown in Figure 8 together with the Padé−Laplace fits. A high signal-to-noise ratio is reached, which leads to good quality fits, as observed from the residua in Figure 9. Note in the bar diagrams of Figure 9 that the correlograms of F1 to F5 fractions are well described by a size domain from 5 to 15 nm whereas for F6, F7, and F0 correlograms, a secondary larger size domain needs to be introduced near 40 nm to get good residua except for fraction F7 that is hardly soluble in toluene. However, particles between 5 and 15 nm are still found in the heavy fractions, corresponding to the particles initially present in the fraction before centrifugation. Note in Figure 8 that the F0 correlogram baseline remains flat from 7 × 102 to 3 × 103 μs, meaning that no size larger than 40 nm is found in the asphaltene solution. Calculated volume and z-average hydrodynamic radii for fractions of relatively low polydispersity (F1 to F5) are reported in Table 1. Aggregate size clearly increases with sampling depth. Hydrodynamic radii are spread from 5 to 10 nm. When the two radii domain are significantly different (F6, F7, and F0), averaging is not adequate.

particles can be estimated through the self-diffusion coefficients calculated from the slope of the lines passing through zero. We find D1 = (4.5 ± 0.3) × 10−11 m2/s and D2 = (0.9 ± 0.1) × 10−11 m2/s corresponding to sphere-equivalent hydrodynamic radii of 8.0 ± 0.5 nm and 40 ± 5 nm. b. Fluorescence of Asphaltenes. Preliminary measurements on a Vasco granulometer (2θ = 135°), not equipped with a band-pass filter, have highlighted a strong fluorescence signal from asphaltenes, particularly for the light fractions (F1−F3). Their scattering intensity is low, according to their small sizes and masses recorded by SAXS, compared to the roughly constant fluorescence signal. The fluorescence intensity will significantly contribute to the background α (eq 4), impoverishing the correlation events (eq 5) that are of interest. This phenomenon generates error bars (Figure 5, q2 = 7 × 10−4 nm−2) larger than those observed with the in-house setup (Figure 5, q2 = 4 × 10−4 nm−2). The fluorescence effect is illustrated by the spectrogram in Figure 6. While the toluene 12001


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Figure 8. Autocorrelation functions of the asphaltene samples in toluene (F0 to F7), measured in the 90° configuration with the 660− 10 nm band-pass filter. Lines: Padé−Laplace fits. The relative positions of the fractions are represented on the sketch of the centrifuge tube.

4. DISCUSSION 4.1. Dynamics of Asphaltene Solutions. The analysis of the dynamic properties of asphaltene solutions by DLS enabled the validation of the presence of Brownian clusters in solution. The exponential decay of the autocorrelation function and the linear q2 to time dependence of relaxation rates are characteristics of long-lifetime aggregates with finite sizes. With the help of the decay rates of Figure 5, the aggregates observed in the largest box (90°) are followed during a minimum time of 55 to 250 μs, corresponding to Γ−1, which is related to long lifetimes. This result is in agreement with the slow exchange rates observed by Dechaine and Gray.12 Thus, the data obtained from small angle scattering can be interpreted as a scattering signal from particles of finite size dispersed in a solvent and not as a correlation length resulting from fast density fluctuations. It validates the colloidal kinetic approach commonly applied for asphaltene solutions, and on the contrary, it does not confirm the thermodynamic approach considering the asphaltene solution as an equilibrium near phase-transition. 4.2. Structure of Asphaltene Solutions. The fractal character of the largest asphaltene clusters is confirmed in this study by SAXS and SANS. The power law obtained for mass versus radius of gyration is found on more than one decade of size, and the same power law on the scattering spectra of largest clusters is observed on almost a q-decade. The fractal dimension (df = 1.8) is relative to low density clusters made of primary units. In Eyssautier et al.,9 the geometry, dimensions, and composition of this primary unit, called nanoaggregate, were optimized based on a multiple fitting procedure of SAXS and SANS spectra recorded under various solvent conditions. The optimization resulted in a disk made of a dense aromatic core surrounded by an alkylic shell. The dimensions of the nanoaggregate (total radius of 3.2 nm) are in line with the present mass-to-radius of gyration relationship (Figure 3). Asphaltenes are clusters of nanoaggregates. These clusters, roughly 100 nanoaggregates for the largest one, are structured ina mass fractal organization. The mean aggregation number of clusters, around 10 nanoaggregates, is in good agreement with various oil field studies regrouping numerous measurement techniques,29 which strongly reinforces the present results. 4.3. Polydispersity and Size of Asphaltene Clusters. By centrifugation of the initial F0 solution, one can sample, from the top to the bottom, different fractions from F1 to F7;

Figure 9. (Left) Hydrodynamic radii obtained from Padé−Laplace fits of DLS correlograms in Figure 8 for each asphaltene solution, with corresponding relative amplitudes (eq 6). Black dotted bars: volumeaverage hydrodynamic radius (eq 8). Gray dash bars: z-average hydrodynamic radius (eq 9). (Right) Residua from Padé−Laplace fits. Acquisition time for F1 was much longer (3 days) than for F2−F7 (2 h) to reach a stationary signal-to-noise ratio setting the reasonable time limit.

fractions F1−F5 contain particle sizes from 5 to 15 nm while F0, F6, and F7 fractions remain more polydisperse. They contain both small particles, initially present in the solution before centrifugation, and large particles with a hydrodynamic radius around 40 nm. From the DLS analysis (Figure 9), F0, F6, and F7 fractions are roughly size comparable, except for the 12002


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5. CONCLUSION The hydrodynamic properties of asphaltene solutions obtained by a centrifugation process are investigated by dynamic light scattering along with the static properties of asphaltene aggregates obtained from small angle scattering techniques. The dynamic properties of the solutions show that clusters of asphaltenes are made of long-lifetime aggregates, in agreement with membrane diffusion experiments illustrated in the literature. Their hydrodynamic radii, between 5 and 10 nm, are in total agreement with radii of gyration obtained from small angle scattering, measured between 3.7 and 7.7 nm. A small fraction of asphaltenes is made of larger clusters with a maximum radius of around 40 nm, observed by both dynamic and static scattering. While previous studies opposed dynamic and static measurements on asphaltene solutions, the present study showed that when adapting length scales and time scales, both techniques provide evidence for the same characteristic length scale corresponding to asphaltene clusters.

relative amplitude of the largest size that is enhanced by the centrifugation process. Because static scattering gives z-average radii of gyration, the sizes resulting from DLS analyses have to be averaged to compare the hydrodynamic and gyration radii. For moderately polydisperse systems (F1−F5), the hydrodynamic average has been performed using eq 7 whereas for F0, F6, and F7 fractions, averaging is not relevant. The z-average hydrodynamic radii, between 5 and 10 nm, correspond to one of the fractal clusters observed and defined by SAXS and SANS. In this study, cluster dynamic properties are experimentally determined and are in good agreement with the size of asphaltene structures determined by Dechaine and Gray by membrane diffusion,12 between 5 and 9 nm. Previous analyses of asphaltene solutions by high Q ultrasonic measurements,4ususing a quartz crystal microbalance with dissipation,7 and by nuclear magnetic resonance68 report high diffusion coefficients, relative to particle radii between 1 and 3 nm, which corresponds to the dimensions of the nanoaggregate described in Eyssautier et al.9 Higher dimensions are not reported, with the exception of DC-conductivity measurements recently presented by Goual et al.,10 and the present innovative measurements, which point out the sensitivity of dynamic light scattering, adapted to the cluster length scale. 4.4. RG to RH Dependence. The RG to RH dependence is reported in Figure 10. Despite a polydispersity reduction of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank C. Michel from Cordouan Technologies for his help on DLS data treatment, J. Jestin (LLB Saclay) for SANS measurements, Y. Benoit (IFP Energies nouvelles (IFPEN)) for fluorescence spectra, P. Paul, L. Jonchier, and S. Gautier (IFPEN) for centrifugation experiments, and D. Espinat (IFPEN) and P. Levitz (CNRS) for helpful discussions.

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REFERENCES

(1) Sheu, E. Y. Petroleum Asphaltene: Properties, Characterization, and Issues. Energy Fuels. 2002, 16, 74−82. (2) Mullins, O. C. Petroleomics and Structure−Function Relations of Crude Oils and Asphaltenes . In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Vol. 1, pp 11−16. (3) Rogers, R. P.; Marshall, A. G.; Petroleomics: Advanced Characterization of Petroleum-Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Vol. 3, pp 63−93. (4) Andreatta, G.; et al. Nanoaggregates and Structure−function Relations in Asphaltenes. Energy Fuels 2005, 19 (4), 1282−1289. (5) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Asphaltene Molecular Size by Fluorescence Correlation Spectroscopy. Energy Fuels 2007, 25 (5), 2875−2882. (6) Lisitza, N. V.; et al. Study of Asphaltene Nanoaggregation by Nuclear Magnetic Resonance (NMR). Energy Fuels 2009, 23, 1189− 1193. (7) Abudu, A.; Goual, L. Adsorption of Crude Oil on Surfaces Using Quartz Crystal Microbalance with Dissipation (QCM-D) under Flow Conditions. Energy Fuels 2009, 23, 1237−1248. (8) Kawashima, H.; et al. Determining Asphaltene Aggregation in Solution from Diffusion Coefficients As Determined by Pulsed-Field Gradient Spin−Echo 1H NMR. Energy Fuels 2008, 22 (6), 3989−3993. (9) Eyssautier, J.; et al. Insight into Asphaltene Nanoaggregate Structure Inferred by Small Angle Neutron and X-ray Scattering. J. Phys. Chem. B 2011, 115 (21), 6827−6837.

Figure 10. Hydrodynamic radius versus the radius of gyration (data from Table 1) for asphaltene fractions from F1 to F5 (●, centrifugation A), for F7 (■, centrifugation B), and for the nanoaggregate (★9). When not indicated, the experimental error is relative to the size of the symbol. Empty circles: volume-average for RH calculation (eq 8). Full circles: z-average for RH calculation (eq 9). Dashed line: RH = RG. Arrows indicate the upper Guinier regime available with both SANS and SAXS techniques.

asphaltene solution by centrifugation, we observe that the fractions somehow still remain polydisperse. The result is dependent on the averaging employed (z- or volume averaging in this case) and on the inversion algorithm and model inherent to each technique. However, we see that RG roughly equals RH from the nanoaggregates (2.5 nm) up to the largest clusters (40 nm) present in the solution. Note that this larger size class represents only a few percent of the asphaltene volume fraction. The wider the RG and RH range is, the better the knowledge of the aggregate structure should be.18 Nevertheless, asphaltene solutions exhibit only a 2.5−40 nm range. 12003


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