On the Size Distribution of Self-Associated Asphaltenes - Energy

Article pubs.acs.org/EF

On the Size Distribution of Self-Associated Asphaltenes H. W. Yarranton,*,† D. P. Ortiz,† D. M. Barrera,† E. N. Baydak,† L. Barré,‡ D. Frot,‡ J. Eyssautier,‡ H. Zeng,§ Z. Xu,§ G. Dechaine,§ M. Becerra,§ J. M. Shaw,§ A. M. McKenna,∥ M. M. Mapolelo,∥ C. Bohne,⊥ Z. Yang,⊥ and J. Oake⊥ †

Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr. NW., Calgary, Alberta, Canada T2N 1N4 ‡ IFP Energies Nouvelles, 1 & 4, Avenue de Bois-Préau, 92852 Rueil-Malmaison, Cedex, France § Department of Chemical and Materials Engineering, University of Alberta, 114 Street 89 Avenue NW, Edmonton, Alberta, Canada, P6G 2M7 ∥ Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, 1800 E. Paul Dirac Dr., Tallahassee, FL, 32310-3706, United States ⊥ Department of Chemistry, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, Canada, V8P 5C2 S Supporting Information *

ABSTRACT: A variety of experimental techniques were applied to a single source asphaltene sample at the same experimental conditions in order to reveal the possible size distributions of asphaltene monomers and aggregates. The asphaltene sample was divided into solubility cuts by selective precipitation in solutions of heptane and toluene. Asphaltene self-association was assessed through a combination of density, vapor pressure osmometry (VPO), elemental analysis, Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry, and time-resolved fluorescence emission spectra measurements performed on each cut. The physical dimensions of the asphaltenes were assessed using SAXS, DLS, membrane diffusion, Rayleigh scattering, and nanofiltration measurements. Molecular and nanoaggregate dimensions were also investigated through a combination of interfacial tension, interfacial adsorption, and surface force measurements. All of the measurements indicated that approximately 90 wt % of the asphaltenes self-associated. Ultrahigh resolution spectrometry suggests that the nonassociated asphaltenes are smaller and more aromatic than bulk asphaltenes indicating that the associating species are larger and less aromatic. On the basis of VPO, the average monomer molecular weight was approximately 850 g/mol, while the molecular weight of the nanoaggregates spanned a range of at least 30000 g/mol with an average on the order of 10000 to 20000 g/mol. SAXS and DLS gave molecular weights 10 times larger. The physical dimensions of the nanoaggregates were less than 20 nm based on nanofiltration and with average diameters of 5 to 9 nm based on diffusion and Rayleigh scattering. SAXS and DLS gave average diameters of 14 nm and indicated that the nanoaggregates had loose structures. Film studies were consistent with the lower molecular weights and dimensions and also demonstrated that asphaltene monolayers swell by a factor of 4 in the presence of a solvent. The most consistent interpretation of the data is that asphaltenes form a highly polydisperse distribution of loosely structured (porous or low fractal dimension) nanoaggregates. However, the discrepancy between VPO and SAXS molecular weights remains unresolved.

1. INTRODUCTION After almost a century of research, the structure and molar mass distributions of asphaltene molecules and self-associated aggregates continue to be debated. The subject is challenging because asphaltenes are a mixture of hundreds of thousands of different chemical species and a fraction of the asphaltene molecules self-associate. This brief introduction reflects the span of current debates on pertinent topics but does not comprise an exhaustive review. Considerable progress has been made in establishing the average molecular weight of asphaltene monomers. Vapor pressure osmometry measurements extrapolated to zero concentration suggest average monomer molecular weights on the order of 1000 g/mol.1,2 Significantly lower average molecular weights are inconsistent with the wide product distributions observed after hydrotreating or thermal cracking.3 However, the size and structural distributions of the monomers © 2013 American Chemical Society

are unknown. Few data are available for the size distribution of asphaltene monomers because they tend to self-associate at very low concentrations. High-resolution mass spectrometry provides qualitative results for heavy oil fractions and indicates a wide distribution of molecular weights ranging from a few hundred to approximately 1500 g/mol.4 Structural distributions have been inferred from a variety of measurements5−7 but with conflicting interpretations. The traditional view of asphaltene molecular structure is a “continent”, a highly condensed aromatic center containing some heteroatoms and some nalkyl side chains.8 A more recently proposed alternative structure is an “archipelago”, smaller condensed aromatic groups with some heteroatoms connected by alkyl bridges.5 Received: April 22, 2013 Revised: July 21, 2013 Published: August 5, 2013 5083

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Figure 1. Molecular, nanoaggregate, and precipitate length scales.

dimensions is used to constrain the possibilities for the dominant type of asphaltene aggregate structure.

Similarly, while it is well-established that asphaltenes selfassociate, the size and shape distributions of the asphaltene nanoaggregates remain uncertain (Figure 1). The traditional view is of a colloid,9,10 often considered to be a π−π-bonded stack of continent-like asphaltene monomers.11 A two-step aggregation has been proposed where the colloids flocculate into larger structures after the initial molecular aggregation.7,11,12 An alternative view is of an oligomer, an analogy to a polymer except that the aggregate is held together by intermolecular forces (π−π bonding, acid−base interactions, and hydrogen bonding) rather than covalent bonds.13,14 Note, the oligomers are not constructed of identical molecules, are not held together by identical forces, and could also be referred to as supramolecules. The oligomer or supramolecular model13,18 is more consistent with the archipelago structure. It is also unclear how self-association relates to asphaltene precipitation. The traditional mechanism is that asphaltenes are a colloidal dispersion with resins as a stabilizing agent and that precipitation is the result of colloid aggregation.15 An alternative mechanism is that asphaltenes are in solution with the oil and precipitation is a liquid or solid phase separation.16 Identifying the correct mechanism is necessary to construct accurate phase behavior models in order to predict the conditions where asphaltene precipitation occurs as well as to develop effective mitigation strategies. Establishing the size and structure distribution of asphaltene monomers and aggregates is a necessary step to validate and improve these models. It seems likely that asphaltenes are a continuum of monomers and a continuum of aggregates exhibiting a wide variety of sizes and structures.17,18 Different experimental techniques illuminate different parts of the asphaltene continuum, leading to different interpretations. Reconciliation of the results from different methods can potentially set limits on the interpreted size and structure distribution. The goal of this study is to apply diverse experimental techniques to the same asphaltene sample at the same experimental conditions in order to reveal the possible asphaltene monomer and molecular aggregate size and structure distributions and to set constraints on the interpretation of self-association. Asphaltenes are divided into solubility fractions and the molecular weights and densities of these fractions are measured to estimate the property distributions. Elemental analysis, high-resolution mass spectrometry, and time-resolved fluorescence spectroscopy data are used to supplement the interpretation of the distribution data. The physical dimensions of asphaltene aggregates are assessed from interfacial film properties, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), membrane filtration, Rayleigh scattering, and nanofiltration data. A comparison between the molecular weight and apparent physical

2. EXPERIMENTAL METHODS 2.1. Materials. A bitumen from the Peace River area (WC_B1) was supplied by Shell Canada. The bitumen was obtained from a steam-assisted gravity drainage process and had been heated to 50 °C and centrifuged to remove water and solids. The residual water content was less than 1 wt %. Some comparisons are provided in the Supporting Information for a second bitumen from the Athabasca area (WC_A1) which was supplied by Syncrude. The bitumen is the product from the naphtha recovery unit after froth treatment and therefore is a topped bitumen. The residual water content was less than 1 wt %. Toluene, n-heptane, n-pentane, and acetone for the University of Calgary (UofC) experiments were obtained from Aldrich Chemical and were 99% pure. HPLC-grade toluene from Fisher Scientific with a purity 99% was used in experiments at the University of Alberta (UofA). Spectrograde toluene from Caledon Laboratory Chemicals was used at the University of Victoria (UVic). Rectapur-grade toluene from VWR International was used at IFP Energy Nouvelles (IFPEN). All solvents used for FT-ICR mass spectral characterization were highperformance liquid chromatography (HPLC)-grade (Sigma-Aldrich Chemical Company, St. Louis, MO). 2.2. Asphaltene Precipitation and Fractionation (University of Calgary). Asphaltenes were extracted from bitumen using npentane and n-heptane following the procedure of Sztukowski et al.19 To precipitate asphaltenes, n-pentane or n-heptane was added to the bitumen at a 40:1 (cm3/g) solvent-to-bitumen ratio. The mixture was sonicated for 45 min and left to equilibrate for 24 h. The supernatant was decanted and filtered through a Whatman #2 filter paper. Additional solvent was added to the remaining solution, which was equilibrated for another 12 h. The mixture was filtered through the same filter paper. The filter cake was washed with approximately 30 cm3 of solvent three times a day for 5 days. After all the washing, the filter cake was dried and then redissolved in toluene. The insoluble material was centrifuged out from the toluene−asphaltene solution. The supernatant was decanted, and the toluene evaporated off to recover “solid-free” asphaltenes. n-Pentane and n-heptane extracted asphaltenes are designated C5- and C7-asphaltenes, respectively. The C7-asphaltenes were fractionated into solubility cuts by first dissolving the asphaltenes in toluene and then precipitating a portion of the asphaltenes by adding a specified mass of n-heptane. To dissolve asphaltenes in toluene, the solution was sonicated for 45 min. After adding heptane, the solutions were equilibrated for 24 h. The concentration of asphaltenes after adding heptane was 10 g/L in all cases. The solution was then centrifuged, and the supernatant was decanted. The precipitate was dried to recover the insoluble heavy fraction of the asphaltenes. The solvent in the supernatant was evaporated in a fume hood to recover the soluble light fraction of the asphaltenes. Each cut is designated as H##H or H##L, where H## indicates the mass percent n-heptane in the solution used to produce the cut and H or L indicates the heavy phase (precipitate) or light phase (dissolved) cut. All cuts were produced at the University of 5084

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2.6. Elemental Analysis and Mass Spectrometry (FSU). Elemental Analysis. The bulk CHNS/O weight percents of the samples were determined using a flash elemental analyzer (C.E. Elantech, Inc.) model 1112. Quadruplicate samples (∼1.5 mg) were weighed for the CHNS analysis (combustion) and O analysis (pyrolysis). Calibration values were developed by the use of sulfanilamide and 2,5-bis-(5-tert-butylbenzoxazol-2-yl)-thiophene (BBOT) standards. The H:C ratio of the samples was determined based on the elemental analysis results. 9.4 T Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Bitumen and fractions were analyzed with a custom-built FT-ICR mass spectrometer21 equipped with a 9.4 T horizontal 220 mm bore diameter superconducting solenoid magnet operated at room temperature (Oxford Corp., Oxney Mead, U.K.) and a modular ICR data station (PREDATOR) facilitated instrument control, data acquisition, and data analysis.22 Ions generated at atmospheric pressure were accumulated in an external linear octopole ion trap.23 ICR time-domain transients were collected from a 7 segment open cylindrical cell with capacitively coupled excitation electrodes based on the Tolmachev configuration.24,25 The 100−200 individual transients of 5.6−6.1 s collected for bitumen fractions were averaged, apodized with a full-Hanning (magnitude spectrum) or halfHanning (absorption mode) weight function,26 and zero-filled once prior to fast Fourier transformation. For all mass spectra, the achieved spectral resolving power approaches the theoretical limit27 over the entire mass range (e.g., average resolving power at m/z 500 was approximately 1 × 106 in magnitude-mode and 1.3 × 106 in the absorption mode for all ionization modes). Mass Calibration and Data Analysis. ICR frequencies were converted to ion masses based on the quadrupolar trapping potential approximation.28 Each m/z spectrum was internally calibrated with respect to an abundant homologous alkylation series differing in mass by integer multiples of 14.01565 Da (mass of a CH2 unit), confirmed by isotopic fine structure based on the “walking” calibration equation.29 Experimentally measured masses were converted from the International Union of Pure and Applied Chemistry (IUPAC) mass scale to the Kendrick mass scale to identify homologous series for each heteroatom class (i.e., species with the same CcHhNnOoSs content, differing only by their degree of alkylation). Peak assignments were performed by Kendrick mass defect analysis, as previously described.30 For each elemental composition, CcHhNnOoSs, the heteroatom class, type (double-bond equivalents, DBE = number of rings plus double bonds involving carbon), and carbon number, c, were tabulated for subsequent generation of heteroatom class relative abundance distributions and graphical DBE versus carbon number or H:C ratio versus carbon number images. Electrospray Ionization (ESI). ESI Source. Sample solutions were infused via a microelectrospray source ENREF_2231 (50 μm i.d. fused silica emitter) at 400 nL/min by a syringe pump. Typical conditions for negative ion formation were emitter voltage, −2.5 kV; tube lens, −350 V; and heated metal capillary current, 4 A. Positive-mode ion formation occurred at the same conditions as the negative mode but with positive values. Electrospray samples were diluted to a final concentration of 500 μg/mL in 50:50 (v/v) toluene/methanol and further modified with 1% (by volume) formic acid to aid protonation of basic species, whereas ammonium hydroxide was used to deprotonate acidic species. 2.7. Time-Resolved Fluorescence (TRF) Experiments (University of Victoria). Dilute samples (10 mg/L asphaltene in toluene) were measured using a 90-degree arrangement between the excitation and emission optics because the absorbance at the excitation wavelength (310 nm) was ca. 0.4−0.5. Samples with 100 mg/L or higher concentrations were measured using a front-face sample holder because the absorbance at the excitation wavelengths is above 3, and no light would penetrate to the detection volume if the 90-degree arrangement was used. The latter sample holder eliminates any selfabsorption of the asphaltene emission. Quartz cells (10 × 10 mm) were used for all measurements. Steady-state fluorescence spectra were obtained with a PTI QM-2 fluorimeter, where samples were excited at 310 nm with a Xe-arc lamp. The bandwidths for the emission and

Calgary and distributed to the participants to ensure consistent samples. 2.3. SARA Fractionation to Recover Resins (University of Calgary). Asphaltenes were precipitated with n-pentane using the above procedure. Saturates, aromatics, and resins were extracted from the WC_B1 and WC_A1 bitumens using a modified ASTM D2007 M method where the asphaltenes are washed with sonication.20 Only the resin fractions were used in this study. 2.4. Density Measurement (University of Calgary). The densities of selected cuts were determined as follows. The asphaltene cut was dissolved in toluene at different concentrations and the density of each solution measured at 23 °C and atmospheric pressure with an Anton Paar density meter. The density of the asphaltene cut was calculated from a best fit of the specific volumes (reciprocal of density) with an ideal mixing rule (Figure 2). Note, while the data are linear

Figure 2. Specific volume of solutions of WC_B1 asphaltenes and toluene at 23 °C and 1 atm fitted with the ideal mixing rule (vid = w1v1 + w2v2) and an example of a nonideal mixing rule (vmix = vid − w1w2(v1 + v2)β12). Insert shows that although both mixing models fit the data equally well, they extrapolate to different asphaltene densities. within the error of the measurements, measurements are only possible at relatively low concentrations where asphaltenes can be readily dissolved in the toluene. Small deviations from ideal mixing may not be detected (Figure 2), and therefore, there is some uncertainty in the calculated densities. The repeatability of the density determinations was ±0.0008 g/cm3. 2.5. Vapor Pressure Osmometry (University of Calgary). The principle and application of vapor pressure osmometry (VPO) are described in detail elsewhere.2 The molecular weight of asphaltenes is related to a measured voltage difference as follows:

MA =

K 0CA ΔV

(1)

where ΔV is the voltage difference, CA is the asphaltene concentration, and K0 is a calibration constant. Note that to obtain eq 1, higher-order concentration dependent terms were assumed to be negligible. The molecular weight of resins calculated using eq 1 is constant versus concentration,13 indicating that the higher-order terms can be neglected for resins. Since asphaltenes are chemically similar to the resins, it was assumed that the higher-order terms are also negligible for asphaltenes. Note that for polydisperse materials such as asphaltenes, VPO gives a number average molecular weight. The molecular weights of asphaltenes in toluene at 50 °C were measured with a model 833 VPO from Jupiter Instrument Company. This osmometer has a detection limit of 5 × 10−5 mol/L when used with toluene. Sucrose octaacetate (679 g/mol) was used to calibrate the instrument, and octacosane (395 g/mol) was used to check the calibration. The measured molecular weight of octacosane was found to be within 2% of the correct value. The repeatability of asphaltene molecular weight measurements is ±15%, and therefore, several repeats were performed. 5085

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excitation monochromators were set to 2 nm. An Edinburgh Instruments OB920 single photon counter was employed to measure fluorescence decays. Samples were excited at 310 nm (EPLED 310, Edinburgh Instruments Ltd.), and the bandwidth for the emission monochromator was 16 nm. The number of counts in the channel of maximum intensity was 10000, and a stop rate of 1−2% was employed. The instrument response function (IRF) was measured using a Ludox solution, where the scattered light was detected at 310 nm. The emission decay was collected at 450 or 520 nm, and the decays were analyzed as a sum of three exponentials (eq 2, i = 3):

ΓA =

1

(2)

where Ai is the pre-exponential factor for each emissive species, i, with a different lifetime (τi = 1/ki). The sum of Ai is unity. At least 5 decays were collected and were analyzed individually. The individual lifetimes and A values were averaged. The IRF was reconvoluted in the F900 software from Edinburgh Instruments with the calculated fit and the reconvoluted calculated fit compared to the experimental data. Fits of the experimental decays were considered to be acceptable when the χ2 value was between 0.9 and 1.2 and when the residuals between the calculated and experimental data and the autocorrelation were random.32 Average lifetimes were calculated from the individual lifetimes and pre-exponential factors as follows: i

⟨τ ⟩ =

∑ Aiτi 1

(4)

where mt is the total mass of asphaltenes in the emulsion, d32 the Sauter mean diameter of the emulsion droplets, Vw the total volume of 0 the water phase, Ceq A the asphaltene equilibrium concentration, and CA the initial asphaltene concentration. The initial asphaltene concentration and the total volume of water were known. The Sauter mean diameter was found from drop-size distributions of samples taken from a settled emulsion. The drop-size distributions were measured with a Carl Zeiss Axiovert S100 inverted microscope equipped with a video camera and image analysis software. Approximately 400−500 drops were used, giving an expected error of 5−10%. Details on the drop size distributions of water-in-asphaltene/ Heptol emulsions are provided elsewhere.33 The equilibrium concentration of the asphaltenes in the continuous phase of the emulsion was determined from a gravimetric analysis of the separated continuous phase. The continuous phase was decanted from the top of the settled emulsion, and its volume measured. The solvent was allowed to evaporate until only dry asphaltenes remained, and their mass was calculated gravimetrically. The equilibrium concentration of asphaltenes is the measured mass divided by the volume of evaporated solution. 2.9. Surface Forces Apparatus (SFA) and Atomic Force Microscope (AFM), (University of Alberta). A surface forces apparatus (SFA) was used for monitoring the swelling of asphaltene film in toluene vapor and measuring the normal force between an asphaltene surface and a model clay surface (i.e., mica) in toluene. Detailed setup for SFA experiments has been reported elsewhere.34,35 Briefly, a thin mica sheet (1−5 μm thick) was first glued onto a cylindrical silica disk [radius (R) = 2 cm]. Two prepared silica disks with or without asphaltene films coated mica were then mounted into the SFA chamber in a cross-cylinder geometry which was locally equivalent to a sphere of radius R interacting with a flat surface or to two spheres of radius 2R when the separation of the two surfaces was much smaller than R.34 To prepare an asphaltene film, 0.5 wt % asphaltene solution was prepared by dissolving the asphaltenes in toluene and filtering through a 0.2 μm PTFE filter before use. The asphaltene films were prepared by an adsorption method. Briefly, several drops of asphaltene solution were placed on a thin mica sheet glued on a cylindrical silica disk. The asphaltenes were allowed to adsorb for 10 min in a sealed chamber saturated with toluene vapor. The surfaces were then washed with pure toluene before loading into the SFA chamber. During the force measurements, surface separation D was obtained by an optical technique called multiple beam interferometry (MBI) in the SFA, using interference fringes of equal chromatic order (FECO). The reference distance (D = 0) was determined at the adhesive contact of two bare mica surfaces in air before coating the asphaltene films. The force between the two cylindrical surfaces, F, was determined as a function of the separation D based on the deflection of the supporting spring. The measured force F(D) can be converted to interaction energy per unit area between two flat surfaces W(D) through the Derjaguin approximation when D ≪ R.34 Asphaltene films were also deposited on mica sheets at the toluene− water interface using a Langmuir−Blodgett interfacial trough (KSV Instruments, Finland) for topographic imaging. The trough and barriers were first thoroughly cleaned with toluene and acetone followed by ultrapure water of 18.2 MΩ cm resistivity. The asphaltene films were prepared by first spreading 50 μL of 2 g/L asphaltene in toluene solution on an ultrapure water subphase followed by careful pouring of 100 cm3 optima toluene (top phase) on the aqueous phase. After 30 min of equilibrium, compression and expansion of interfacial films were then carried out at a barrier speed of 5 mm/min. The asphaltene films were deposited on mica sheets at an interfacial pressure of 5 mN/m with a pulling speed of ∼1 mm/min during the first cycle of compression. An Agilent 5500 molecular imaging atomic force microscope (AFM) was used to characterize and image the deposited asphaltene surfaces.

i

I(t ) = I0 ∑ Ai e−kit

mtd32 ⎛ C eq ⎞ ⎜1 − A0 ⎟ 6Vw ⎝ CA ⎠

(3)

Time-resolved emission spectra (TRES) are emission spectra collected for a specific time window after the excitation pulse. The steady-state fluorescence emission spectra are related to the emission of all chromophores in solution, while TRES are related to the species that emit within a particular time window. TRES were constructed from time-resolved fluorescence decays collected for the same amount of time, where the intensity was integrated for a particular time window. 2.8. Interfacial Property Measurements (University of Calgary). Interfacial Tension. Interfacial tension was measured using an IT Concept Drop Shape Analyzer. A known mass of asphaltenes was dissolved in a solution of 25 vol % heptane and 75 vol % toluene (heptol25). The mixture was injected through a U-shaped needle into an optical glass cuvette containing water. The profile of the drop was captured using a CCD camera and analyzed using a video image profile digitizer board connected to a personal computer. The temperature of the cuvette was controlled by a water jacket. The interfacial tension was measured for at least 1 h. The interfacial tension was approximately constant after 10 min, and the average reading after 10 min was taken to be the equilibrium interfacial tension. Emulsion Preparation. Emulsions were required for the mass surface coverage experiments. To prepare the emulsions, a known mass of asphaltene was dissolved in heptol25 (concentrations from 5 to 25 g/L). This heptane/toluene ratio was chosen to obtain stable emulsions and to ensure that the asphaltenes remained solubilized. The mixture was sonicated prior to and after the addition of heptane to ensure complete asphaltene dissolution and mixture homogeneity. Water in a 40 vol % ratio was added dropwise to the hydrocarbon phase, while the mixture was homogenized at 18000 rpm with a CAT520D homogenizer for five minutes. The emulsion was allowed to settle for 1.5 h, after which a continuous phase consisting solely of asphaltenes dissolved in heptol25 and a concentrated emulsion phase had separated. The drop size distribution did not change during the settling period, nor did any water evolve from the emulsion. Asphaltene Mass Surface Coverage. Adsorption isotherms for asphaltenes on the hydrocarbon/water interface were constructed from data from the emulsions described above. A brief description of the method follows but details are provided elsewhere.33 The asphaltene mass surface coverage, ΓA, is given by: 5086

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2.10. SAXS Measurements (IFPEN). Samples for SAXS and DLS experiments were prepared by mixing approximately 50 mg of asphaltene powder with 1 g of toluene. Aliquots of this stock sample were diluted further on a mass basis to prepare asphaltene solutions in the 0.5−50 g/L concentration range (volumes determined using UofC density data). SAXS measurements were performed at IFPEN, Rueil-Malmaison (France) on an in-house experimental setup described elsewhere.36 The position of each pixel, relative to the position of the direct beam, is converted into the magnitude of the scattering vector q (defined as q = 4π sin(θ)/λ for a scattering angle 2θ) based on measurements of an external standard (Ag behenate). The scattering intensity, which was found isotropic for each measurement, was azimuthally averaged. Two sample-to-detector distances, 60 and 150 cm, were used, allowing a total q range from 7 × 10−3 to 7 × 10−1 Å−1 to be covered. After normalization with respect to thickness, transmission, and measuring time, the toluene signal was subtracted from the sample signal and the raw intensities were converted to the scattering cross section I(q) in absolute scale (cm−1) by using a multiplication factor. This conversion factor was determined by comparing the measured toluene intensity to a calculated value, using the known isothermal compressibility of toluene. The toluene and then the asphaltene solutions were measured at increasing asphaltene concentrations. For each measurement, the solution was loaded in a brass cell (optical path 1.86 mm, closed by two mica windows) using a syringe and the temperature was raised by 5 °C/min up to 50 °C and stabilized for 5 min prior to the 500 s measurement. Before unloading, the liquid level at room temperature before and after measurements was checked to ensure that no toluene evaporation occurred. The mica windows were changed for each concentration series. The scattering curves are used to extract the weight average molecular weight Mw and the z average radii of gyration RG of asphaltene aggregates using the Zimm approximation.37 Briefly, in the so-called Guinier region (at scales larger than the typical size of particles), the measured I(q) is approximated by

⎞ q2R G2 1 1 ⎛ ⎜1 + = + ...⎟ or qR G < 2 I(q) I(0) ⎝ 3 ⎠

introduced in a 10 mm Hellma quartz cell and measured at room temperature (21.8 °C). Dynamic light scattering (DLS) measurements were performed at IFPEN, Rueil-Malmaison (France) on an in-house experimental setup described elsewhere.37 Asphaltene solutions emit a strong fluorescent signal compared to the small amount of scattered light emitted by particles, particularly very small particles (or molecular aggregates). The fluorescence signal was screened out using a band-pass filter centered on the laser line (λ = 656 nm). Apart from this small modification, the DLS set up is an up-to-date 90° light scattering instrument equipped with a high quantum efficiency APD solid detector. Classically, the light-scattering intensity autocorrelation function G2(q,t), is inverted to the field autocorrelation function g1(q,t) to extract the scattered intensity contribution, Ai, and the characteristic decay rate, Γi, of particles of class i. G2(q,t) is related to the modulus of the normalized field autocorrelation function g1(q,t) by the Siegert relationship:

G2(q , t ) = α + βg12(q , t )

where α is a baseline and β is the coherence factor. For dilute and polydispersed solutions where Brownian particles do not interact, g1(q,t) can be decomposed into a sum of exponential decaying functions: g1(q , t ) =

(5)

Di =

kBT 6πηRHi

(10)

where kB and η are the Boltzmann constant and the solvent viscosity, respectively. When dealing with low polydisperse solutions, the volume average hydrodynamic radii were calculated using the Rayleigh formalism:

(6)

where Δρ, c, d, and A2 represent, respectively, the scattering length density difference between asphaltene and toluene (cm−2), the asphaltene concentration (g/cm3), the mass density (g/cm3) of asphaltene, and the second virial coefficient (mole cm3 g−2). Note that, in the Zimm formalism, the apparent molecular weight, Mapp, to concentration dependence is ascribed to interactions and not to aggregation with concentration. The scattering length density ρSLD for toluene and the asphaltene fractions was determined from a summation over the N different atoms as follows:

RHv

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

(11)

where Bi represents the number contribution of particles of class i. 2.12. Membrane Permeation and Rayleigh Scattering Experiments (University of Alberta). For detailed descriptions of the apparatus and data analysis techniques, refer to Dechaine and Gray.39 All diffusion measurements were done using a 30 kDa Millipore Ultracel YM membrane (YM30) with an average pore size of 5 nm. For information on the Rayleigh scattering measurements and analysis procedures, refer to Derakshesh et al.40 An SI-Photonics (Tucson, AZ) model 440 spectrophotometer, using a combination of a deuterium light source for wavelengths of 200−460 nm and a tungsten source for wavelengths from 460 to 950 nm, was used for all measurements. Both 10.00 ± 0.01 mm and 1.00 ± 0.01 mm path length quartz cuvettes (Hellma Canada limited, Concord, Ontario) were used along with an SI-Photonics fiber optic cuvette holder. All spectra were collected at ambient conditions. 2.13. Nanofiltration Experiments (University of Alberta). A 162 cm3 aliquot of a 1 wt % mixture of WC_A1 heptane asphaltenes in toluene was prepared and then sonicated for one hour at room

N

∑1 niZile V

(9)

For polydisperse systems, the Z-Pade/SVD algorithm employs a SVD (singular value decomposition) rank determination to choose the number of characteristic decay times, Γi.38 From this sum of exponential decaying functions, a standard Levenberg−Marquardt method is used to adjust the weight Ai 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 with variations no larger than one-third of the noise envelope. When the diffusion process is Brownian, the selfdiffusion coefficient of the particles Di is related to Γi by Γi = Diq2. In the high dilution limit, the self-diffusion coefficient (Di) and the hydrodynamic radius (RHi) of the hard sphere-equivalent noninteracting particles are related through the Stokes−Einstein equation:

(Δρ)2 c 1 1 = (1 + 2A 2 M w c) = qR G < 2; Mw Mapp(c) Nad 2 I(0)

ρSLD =

∑ Ai exp(−Γit ) i

and

A 2 M w c < 0.25

(8)

(7)

where n is the number of atoms, Z is the atomic number of each atom, le is the scattering length of an electron (0.281 × 10−12 cm), and V is the volume considered in the chemical composition. The validity of this expression has been checked experimentally for asphaltenes using contrast matching small-angle neutron scattering.12 2.11. Dynamic Light Scattering Measurements (IFPEN). For the DLS technique, asphaltene solutions were prepared a few months later from the initial (SAXS) solution by dilution to 0.5 g/L. Then, without either micronics filtration or centrifugation, they were 5087

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Table 1. Samples Used for Each Experiment (X Indicates Experiment Performed) experiment density molecular weight elemental analysis mass spectroscopy TRF interfacial tension mass on interface atomic force surface force SAXS DLS membrane permeation rayleigh scattering nano filtration a

WC_B1 resins

WC_B1 C5-asphaltene

WC_B1 C7-asphaltene

WC_B1 C7-asphaltene cuts

WC_A1 C7-asphaltenea

WC_A1 C7-asphaltenea

X X X X X

X X X

X X X X X

X X

X X

X

X

X

X X X X X X X X X X

X X

X X

X X X

See the Supporting Information.

temperature. The mixture was then passed through a zirconia filter with a nominal pore diameter of 50 nm. The permeate was recovered. The asphaltenes were dried first in an oven at 100 °C to evaporate the bulk of the solvent and then in a vacuum oven at 60 °C overnight. The dried asphaltenes were then weighed, remixed in toluene to obtain a 1 wt % mixture, and sonicated. This mixture was then fed through a 20 nm zirconia filter. The permeate recovery and remixing procedures were then repeated sequentially for a 10 and a 5 nm alumina filter. The filtration experiments were carried out at room temperature and approximately 1.5 bar. A detailed description of the apparatus is presented elsewhere.41 The fraction of asphaltenes passing through a filter is assessed in two ways: (1) from the ratio of the wt % asphaltenes in the effluent stream to the wt % asphaltene in the feed, and (2) from the ratio of the mass of asphaltenes passing through the filter during filtration + flushing the apparatus (feed reservoir + transfer lines to the filter and the lines down stream of the filter) to the mass of asphaltenes in the feed. As the filtrations were performed sequentially from higher to lower pore sizes, the mass fraction of asphaltenes smaller than a nominal size is the product of the mass fractions passing through all of the filters in the sequence leading to the last filtration in the series.

Table 2. Number Average Molecular Weight (VPO at 50°C in Toluene) and Density (at 23°C) of WC B1 Resins and Asphaltene Solubility Cuts at 23°C and Atmospheric Pressure

cut resins C5-asphaltene C7-asphaltene H60L H77L H92L H60H H77H H92H recombined H60b recombined H77b recombined H92b

3. RESULTS Table 1 summarizes all the experiments performed in this study. Only results for WC_B1 are reported here. The data for the WC_A1 C7-asphaltenes and their cuts simply confirmed the trends observed for the WC_B1 asphaltenes and are provided for comparison in the Supporting Information. The asphaltene and resin contents of the WC_B1 bitumen were as follows: resins (wt %), 23.1; C5-asphaltenes (wt %), 19.4; C7asphaltenes (wt %), 17.0. A preliminary set of experiments was performed on the WC_B1 resins and pentane-extracted asphaltenes (C5asphaltenes). The WC_B1 heptane extracted asphaltenes (C7-asphaltenes) were the main samples and were used in all but one experiment. Unless otherwise indicated, all results are reported for the C7-asphaltenes. 3.1. Asphaltene Property Distributions. We begin with the distribution of molecular weight, density, and elemental ratios within the asphaltenes. Table 2 lists the mass fraction, average density, and apparent molecular weight at 10 and 40 g/ L of the asphaltenes and asphaltene fractions. The apparent molecular weight (a number average of the nonassociated monomer and the aggregate molecular weights) increases from the most soluble cut (H92L) to the least soluble cut (H60H).

order of most to lease soluble cut

mass fraction of C7asphaltene

MWa at 10 g/L (g/mol)

MWa at 40 g/L (g/mol)

density (g/cm3)

− − − 3 2 1 6 5 4 −

− − 1.00 0.82 0.46 0.21 0.18 0.54 0.79 1

900 2500 2900 1600 1400 800 6900 6000 5500 1900

950 3100 3800 1700 1500 900 12300 10300 9500 2200

1.075 1.173 1.173 1.159 1.136 1.078 1.184 1.183 1.184 1.166

−

1

2400

2900

1.163

−

1

2500

3200

1.167

MW from curve fit of data (lines in Figure 3). bThe recombined values represent the whole C7-asphaltenes and are a number average of the corresponding light and heavy phase cuts; for example, H60L and H60H. a

There is a notable increase in density from Cut H92L to H77L; that is, from the most soluble 20 wt % of the asphaltenes to the next cut. The density is nearly uniform for the least soluble 80 wt % of the asphaltenes (cuts H60H, H77H, and H92H). The factors that control the differences in asphaltene solubility in these solutions are the molar volume (molecular weight/density) and the solubility parameter of the asphaltene species. Since the asphaltene densities fall within a narrow range, the higher apparent molecular weight in the less soluble cuts suggests that a major factor determining differences in asphaltene solubility is the size of the aggregates. Some differences between the solubility of the cuts may also arise from nonassociated species, which would distribute into the different solubility cuts according to their distribution of solubility parameters and molar volumes. 5088

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Figure 3. The apparent number average molecular weight of WC_B1 asphaltenes in toluene at 50 °C: (a) light-phase solubility cuts and (b) heavyphase solubility cuts. Lines are exponential decay function curve fits provided as a visual aid. Note that the y axis is scaled differently on each plot.

Asphaltene Molecular Weight Distribution from VPO Data. Figure 3 shows the apparent number average molecular weight of the light and heavy phases solubility cuts from the WC_B1 asphaltenes at different asphaltene concentrations. For most of the cuts, the molecular weight increases monotonically with concentration, as expected with self-associating components. However, the molecular weight of the least soluble cut (H92L), which represents 21 wt % of the asphaltenes, only increases from approximately 750 to 900 g/mol as the asphaltene concentration increases from 1 to 60 g/L. This variation is within the scatter of VPO measurements, and the average value is approximately 800 g/mol slightly less than the 900 g/mol measured for the resins. This cut evidently contains relatively few associated species and therefore the average value of 800 g/mol is the approximate molecular weight of the asphaltene monomers. Note that the more self-associated cuts extrapolate to higher y-intercept molecular weights. Given the high curvature in the molecular weight trend at low concentrations, the extrapolations are questionable. It is also possible that self-association persists to very low concentrations. If the extrapolated monomer molecular weights are valid, then the value of 800 g/mol may be the lower end of a distribution of monomer molecular weights. Ideally, the molecular weights of the cuts could be used to reconstruct the molecular weight distribution of the whole asphaltenes. However, since the asphaltenes self-associate, the distributions of the cuts can change when they are redispersed in toluene for the VPO measurement. To test how significant the self-association effect is, the average molecular weight of the whole asphaltenes was calculated as a number average of the molecular weights of the corresponding light and heavy phase fractions (e.g., H92L and H92H or H60L and H60H). Table 2 shows that all of the calculated average molecular weights fall below the measured values, strongly suggesting that some redistribution did occur. Despite this potential source of error, a molecular weight distribution was estimated that approximately fit the molecular weight data for the cuts at 40 g/L. Note, the same approach could be applied at any concentration with similar results. The asphaltenes were assumed to consist of two types of material: nonassociating and associating components. The nonassociating components were assigned a fixed molecular weight of 850

g/mol, and the associating components were represented with a Gamma function given by: f (M ) =

( M − M m ) βΓ − 1 Γ(βΓ ) ⎡ ⎤β ⎡ βΓ M − Mm ⎤ ⎢ ⎥ exp⎢β ⎥ ⎢⎣ Γ Mavg − M m ⎥⎦ ⎣⎢ Mavg − M m ⎥⎦

(12)

where f(M) is the molar distribution function, Mm and Mavg are the monomer molecular weight and number average “associated” molecular weight of asphaltenes, and βΓ is a parameter which determines the shape of the distribution. To fit the distribution, the mass percent of nonassociated asphaltenes, the number average molecular weight of the aggregates, and the shape parameter were adjusted. After each adjustment, the distribution was recalculated and then numerically integrated to calculate the number average molecular weight of any given cut, assuming that the lowest molecular weight material always reported to the most soluble cut and the highest molecular weight material to the least soluble cut. The calculated molecular weights were then compared with the measured molecular weights. A narrow distribution (high value of βΓ) provided the best fit but was not sensitive to βΓ at values above 25; therefore, βΓ was set to 25. Then, the mass percent of nonassociated asphaltenes and the average aggregate molecular weight were adjusted to minimize the error. The parameters for the fitted distribution are provided in Table 3, and the Gamma distribution (without the Table 3. Parameters for Molecular Weight Distributions of WC_B1 C7-Asphaltenes (40 g/L in Toluene at 50°C) parameter nonassociating components wt % of asphaltenes M (g/mol) associating components wt % of asphaltenes monomer M (g/mol) average M (g/mol) shape Factor 5089

WC_B1 15 850 85 850 9800 25

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Figure 4. (a) Estimated molecular distribution of associating components of WC_B1 asphaltenes at 40 g/L in toluene at 50 °C. (b) The average apparent molecular weight of WC_B1 solubility cuts calculated from the distribution.

increase in density (negative excess volume) when asphaltene monomers aggregate is also plausible. When aggregation occurs, the associating monomer moves from a solvent-rich surrounding to an asphaltene-rich surrounding. Also, the forces that drive association would likely draw parts of the asphaltene structures closer together. Therefore, the average distance between the molecules is likely to decrease. Note, the asphaltene densities determined from solutions of asphaltenes in toluene will be most strongly influenced by the densities measured at high asphaltene concentration and therefore include the contribution of the maximum self-association in the given asphaltene fraction (molecular weight plateau). In ideal solutions, the density distribution within the whole asphaltenes can be reconstructed from the density of the solubility cuts. For now, the possible effects of the apparent small nonideality of solutions of asphaltenes and toluene were neglected. The simplest distribution to fit the data was to assume a uniform density for the monomers and a different uniform density for the aggregates:

nonassociating components) is shown in Figure 4a. Interestingly, it appears that the aggregates form a normal distribution on a mass basis. Note that the calculated mass fraction of nonassociating material is less than the mass of the least soluble fraction; the least soluble fraction (H92L) is estimated to consist of approximately 25% associating components. The calculated distribution is compared with the data for the cuts in Figure 4b. While the agreement is qualitative, the calculated distributions suggest that most of the aggregates are less than 30000 g/mol or 40 monomers per aggregate. No definitive conclusion can be drawn about the upper limit of the molecular weight because small amounts of high molecular weight aggregates could not be distinguished from an analysis of number average molecular weights. Nonetheless, McKenna et al.42 also report molecular weight distributions for asphaltene aggregates between 6000 and 20000 g/mol detected by electrospray ionization time-of-flight mass spectrometry (TOF-MS), the first direct measurement of the asphaltene aggregate molecular weight. They also confirm the presence of the majority of asphaltenes as stable aggregates even at subparts per billion concentrations. Asphaltene Density Distribution. Table 2 lists the densities of the solubility cuts from the WC_B1 asphaltenes. The density increases monotonically from the most soluble fraction to the least soluble fractions. The density of the most soluble fraction (H92L) approaches the resin density. Interestingly, the density of the least soluble 80 wt % of the asphaltenes (H60H, H77H, H92H cuts) is nearly uniform. The similar density of the heavier asphaltene cuts is consistent with self-associated asphaltenes. Asphaltene monomers likely have a distribution of properties, including density. However, if the asphaltenes that form aggregates are randomly drawn from the monomers, the property variations are averaged in the aggregates. There is a small but significant change in density between the least soluble nonassociated asphaltenes (H92L) and the associated asphaltenes (all other cuts). Possible explanations are (1) the associating molecules are denser than the nonassociating molecules, (2) there is a density change when aggregates form. It is possible that self-association involves denser structures within the asphaltenes (e.g., π−π bonding of aromatic sheets8,11 or heteroatomic structures17), and that associating asphaltenes contain more of these structures and therefore are denser than nonassociating asphaltenes. A small

for W < WNAρ = ρNA forW > WNAρ =

(13)

W WNAρNA + (W − WNA )ρAA

(14)

where W is the cumulative mass fraction, ρ is density, and subscripts NA and AA indicate nonassociated and associated asphaltenes. The parameters used to fit the density of the WC_B1 cuts are provided in Table 4. Note that the mass fraction of nonassociating components was set equal to the values used to fit the molecular weight distributions. The density distributions were integrated to determine the density of any given cut, and the calculated cut densities are compared with the data in Table 4. Parameters for Density Distributions of WC_B1 Asphaltenes at 23°C

5090

parameter

WC_B1

Wna ρNA (g/cm3) ρAA (g/cm3)

0.15 1.08 1.189

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Figure 5. The fit to the data is reasonable (within 1.3%), considering the uncertainties in the extrapolated density data.

ratios of the least soluble 20 wt % of the asphaltenes approach the atomic ratios of the resins. These observations are consistent with the interpretation that approximately 15 wt % of the WC_B1 asphaltenes are nonassociating. Note, the trends in elemental ratios are consistent with the trends observed for Mexican asphaltene fractions.44 Also note, a possible inference from this interpretation is that there is no selectivity for the types of monomers that aggregate. A lack of selectivity implies that there is not one dominant intermolecular interaction for aggregate formation. The inferred lack of selectivity is supported by the mass spectrometry data presented later. 3.2. Evidence from Time-Resolved Fluorescence, TRF. Asphaltenes show intrinsic fluorescence due to the presence of different fluorophores,45−48 and for this reason the steady-state fluorescence spectra are broad with none of the distinguishing features observed for isolated polyaromatic hydrocarbons. Increase in the asphaltene concentration led to a slight red shift of the emission maxima (Figure S.5 of the Supporting Information). Time-resolved experiments differentiate between fluorophores with different lifetimes. The spectra for the fluorophores with long lifetimes can be separated from the spectra for all fluorophores when comparing the TRES obtained for different time windows after the excitation pulse; for example, see Figures S.6 and S.7 of the Supporting Information for H60L and H60H, respectively. It is important to note that at short delays, the TRES corresponds to the spectra from all fluorophores, while for long delays, the spectra correspond only to the emission of fluorophores with long lifetimes. In general terms, the same features were observed for each sample where a red shift occurs at longer delays, suggesting that the excited fluorophores with lower excitedstate energy have longer lifetimes. This shift was more pronounced at the higher concentrations because the excited state fluorophores with higher energies, which emit at short wavelengths, are quenched more effectively than fluorophores with lower excited state energies. Quenching leads to a shortening of the excited state lifetimes either because of the formation of aggregates or because at high asphaltene concentrations bimolecular quenching reactions with other asphaltene molecules occur. Both these mechanisms have been previously proposed.48−51 Comparison of the TRES for the various samples for the same integration windows provides information on how different fluorophores were distributed between the different cuts. The TRES for asphaltene at a concentration of 100 mg/L for the short and long time windows showed a maximum at shorter wavelengths for H92L followed by H60L, while the maxima for H92H and H60H were the same and were red shifted compared to the spectra for the other two cuts (Figure 7). This trend suggests that in the low-cut fractions, molecules were solubilized that have higher excited-state energies, which in general are related to smaller polyaromatic hydrocarbons. The same trends at an asphaltene concentration of 10 g/L (Figure S.8 of the Supporting Information) are less prominent because the lifetimes for the asphaltene emission are shorter, leading to less discrimination between the spectra of the fluorophores with different lifetimes. However, it is important to note that lifetimes for the resin sample and H92L are significantly different, suggesting that the fluorophores in these two samples are different. The decays for the emission of asphaltene were fit adequately to the sum of three exponentials. This approach groups together fluorophores with similar lifetimes, and it should not

Figure 5. Measured and modeled densities at 23 °C of WC_B1 asphaltene solubility cuts.

When considered together, the density and molecular weight data indicate that approximately 15 wt% of n-heptane precipitated asphaltenes are nonassociating components and the remainder are self-associated. This observation is consistent with previous results for asphaltenes from other sources, such as Boscan asphaltenes.43 The properties of the nonassociated monomers (850 g/mol and 1.078 g/cm3) are consistent with the properties of WC_B1 resins (950 g/mol and 1.075 g/cm3). Note that the apparent molecular weight of the resins did not increase significantly with resin concentration, indicating the resins are primarily, if not entirely, made up of nonassociating components. Asphaltene Elemental Distributions. Figure 6 shows the H/ C, S/C, N/C, and O/C atomic ratios from elemental analyses

Figure 6. Atomic ratios of WC_B1 resins and asphaltene cuts (diamonds, hydrogen/carbon ratio; squares, sulfur/carbon; circles, oxygen/carbon; triangles, nitrogen/carbon).

performed on the WC_B1 asphaltene fractions and resins. The ratios are relatively uniform for the least soluble 60 to 80 wt % of the asphaltenes (cumulative mass from 40 wt % up). This relative uniformity is consistent with aggregation since aggregates will likely contain a random selection of associating monomers. Aggregates formed from random groups of monomers will have an elemental analysis approaching the average elemental analysis of all the monomers. The atomic 5091

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Figure 7. Normalized TRES integrated between 0 and 5 ns (left) and 15 and 20 ns (right) after the excitation pulse for 100 mg/L of H92L (a, black), H60L (b, blue), H92H (c, green), and H60H (d, red).

Figure 8. Average lifetimes for samples excited at 310 nm for the emission at 450 nm (left) or 520 nm (right) of C5-Asph (green, solid line, closed circle), C7-Asph (blue, solid line, closed circle), H60L (red, solid line, open square), H60H (black, solid line, open square), H92L (red, dashed line, open triangle), H92H (black, dashed line, open triangle) and resin (green, dashed line, solid diamond). The lines were added to guide the eye.

Figure 9. Distribution of N1 from positive ion FT-ICR-MW for WC_B1 fractions including WC_B1 bitumen, resins, the most soluble 21% of the C7-asphaltenes (H92L), the most soluble 46 wt % of the asphaltenes (H77L), the most soluble 82 wt % of the asphaltenes (H60L), and whole C7asphaltenes.

be interpreted that asphaltenes contain only three fluorophores. The objective of the fluorescence experiments in this work was not to distinguish between the two mechanisms of quenching but to establish differences between the fluorophores present in the different cuts of asphaltene. Lifetime measurements were performed for concentrations between 10 mg/L and 30 g/L,

and the increments were large since within the experimental errors the average lifetimes did not vary significantly, up to concentrations of 1 g/L. Smaller increments were used above 10 g/L. The asphaltene emission decays were measured at 450 and 520 nm. The former wavelength was chosen because it is close to the maximum in the TRES, and the wavelength of 520 5092

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nm was chosen because it is representative of the shoulder that developed in the TRES. For all samples, the average asphaltene lifetimes were shown to decrease at some asphaltene concentrations (Figure 8 and Table S.5 in the Supporting Information). The exception is the emission for the resin sample, for which the average lifetime increased at intermediate concentrations. The average lifetime for the resin sample was always longer than for all asphaltene samples. This result is consistent with resins containing relatively small polyaromatic compounds that do not interact and are therefore not quenched. The H92L sample was the only cut for which different average lifetimes were observed when compared to the other asphaltene samples. The average lifetimes were longer, making the fluorescence behavior of the H92L intermediate similar to that observed for resins and the other asphaltene cuts. This result is in line with the H92L cut, containing a large portion of the nonassociated components of asphaltene, and with the fact that the TRES emission from this cut is blue shifted when compared to the other asphaltene samples. 3.3. Evidence from Mass Spectrometry. Figure 9 shows double-bond equivalents (DBE), number of rings plus double bonds to carbon, and DBE = C − h/2 + n/2 + 152 calculated from molecular formulas, CcHhNnOoSs, for members of the N1 class determined by positive-ion electrospray ionization FTICR MS. Note, FT-ICR MS characterization of H92H, H77H, and H60H fractions was not possible due to a very low signalto-noise ratio. Since positive-ion electrospray produces ions through protonation of basic species, the most basic component of petroleum (pyridinic nitrogen) is highlighted through this technique. Here, Peace River whole bitumen molecules within the N1 heteroatom class contain an average of C38 per molecule and 12 DBE but contain molecules with carbon number distributions between C15−C60. Resins within the N1 class contain molecules with similar carbon number and DBE distributions and cover the same compositional space as the parent bitumen. The relative abundance-weighted average N1 molecules report DBE values between 12 and 13 across a wide carbon number distribution that correspond to pyridinic nitrogen molecules (i.e., benz[a,c]acridine and naphtoquinoline) known to exist in heavy crude oil53 with varying degrees of alkylation. However, a shift to lower carbon number (C32) and higher DBE (20) is observed for solubility fractions (H92L, H77L, and H60L) and whole C7-asphaltenes. Importantly, asphaltenes are known to form stable nanoaggregates at low concentration (263000 175000

− 7.1 4.0 2.3 >100 7.2

− − − 0.53 16. 12.1

RH1 (nm)

A1

0.53 12.1 11.7

− − − 0.28 0.12 0.07

RH2 (nm)

A2

Rspc (nm)

6.4 65. 61

− − − 0.72 0.88 0.92

− 3.8 2.4 1.6 − 3.9

a

Mw from Zimm approximation (eq 6). bEstimated on the most diluted sample (eq 5). cRadius of compact sphere estimated from Mw and density measurement.

5096

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Figure 16. SAXS measurements at ≈10 to 50 g/L of extreme fractions in toluene at 50 °C: (a) most soluble cut, H92L; (b) least soluble cut, H60H. Lines are the Zimm approximation (eq 5). Note that for the H60H no Guinier region appears at small q values; instead, an increase of I(q) is observed, represented by the continuous line of slope −1.

Figure 17. Reverse apparent molecular weight as a function of concentration of WC_B1 asphaltene in toluene at 50 °C: (a) light-phase solubility cuts and (b) heavy-phase solubility cuts. Lines are the Zimm equation for q = 0 and dotted line for qmin = 7 × 10−3 Å−1. Note, the y axis is scaled differently on each plot.

some cases and can likely be ascribed to toluene subtraction issues. In the following discussion, only intensities larger than 10−2 cm−1 will be considered, and we will focus mainly on the Guinier region where intensities are always larger than 5 × 10−2 cm−1. Duplicate measurements (H92L, ϕ = 0.021; H60H, ϕ = 0.015; H60H, ϕ = 0.043) indicate that the repeatability of intensities is approximately ±10%. Similar variations in intensity are observed when measurements from two different q ranges are compared. For the most soluble cut (H92L) (Figure 16a), a Guinier region with a plateau at small q values is observed. The q dependence in this region is strong evidence of the presence of aggregates in the solutions. A much higher intensity is observed at small q values for the least soluble cut (H60H) (Figure 16b), but no Guinier region appears. Instead, a q−1 behavior is observed, indicating a radius of gyration larger than 1/qmin ≈

compact colloidal structure. Note that this conclusion may seem to contradict the previous observation that an asphaltene molecule in an aggregate is denser than when nonassociated in solution. However, while association may pull asphaltene molecules closer together (affecting their specific volume or density), solvent occlusion can still occur in a loose nanoaggregate structure (affecting the aggregate density). 3.5. SAXS/DLS Results. The calculation of scattering length density, eq 7, are summarized in Table 5. Their values are nearly constant (10.8 ± 0.1 × 1010 cm−2), except for the lightest solubility cuts (10.1 × 1010 cm−2). Nevertheless, they all differ clearly from the toluene value (7.96 × 1010 cm−2), indicating that SAXS will be sensitive to all the species regarding contrast. The SAXS measurements of extreme fractions at approximately 10 to 50 g/L in toluene at 50 °C are shown in Figure 16. A flattening of the intensities at large q values is observed in 5097

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range of 16 nm, whereas the z-averaged gyration radii are larger than 100 nm. The marked discrepancy between hydrodynamic and gyration radii could be ascribed to aging effects. Indeed, the DLS observation of the H60H solution aged for few months found aggregates up to one micrometer in radius. Beside the rather good RG and RH agreement, Table 5 highlights a large size polydispersity when fractions are compared: the most soluble cut contains aggregates in the 2 nm range, whereas the least soluble cut has aggregates up to 65 nm. One of the interesting features of static scattering measurement methods are their ability to estimate in the same experiment both the molecular weight (MW) and size (RG) of aggregates. From the MW and the mass density, the radius of the corresponding dense sphere Rsp can easily be estimated and compared with the RG values (Table 5). For each fraction, at least two different decay rates (or hydrodynamic radii) are necessary to fit the experimental autocorrelation function. These differences are a direct proof of a large polydispersity, even in fractions. The gyration radii are found to be larger, by a factor of 2, than the corresponding Rsp, suggesting that aggregates are not dense. Moreover, the q−1 behavior at small q value is a strong indication of the fluffiness of the largest aggregates. This conclusion is consistent with previous studies by SAXS and SANS.12,36,37 While the SAXS and DLS results confirm a continuous polydispersity and loose structure of the asphaltene aggregates, the molecular weight and dimensions of the aggregates are at least an order of magnitude larger than indicated by VPO and the film studies. This discrepancy will be discussed later. 3.6. Asphaltene Aggregate Size Based on Permeation and Nanofiltration Experiments. Diffusion Measurements. The results of the diffusion measurements with the WC_B1 asphaltenes are summarized below in Table 6. The effective diffusion coefficient of the WC_B1 C7−asphaltene sample in toluene at 25 °C and 1 g/L is within the range obtained for asphaltene samples of various other origins. This similarity implies that, at least at these conditions, the sizes of the

100 nm. The intensity at q = 0 will also be larger than I(qmin), which means a Mw larger than I(qmin)/cK (see eq 6). The SAXS measurements of the other solubility cuts (not shown here) have intermediate behaviors: a clear plateau is obvious for H60L, whereas the Guinier region is barely observed for H92H and the whole WC_B1 asphaltene. However, the Zimm formalism (eqs 5 and 6) has been applied to these cuts and the extracted values are likely underestimated. The full treatment of the SAXS measurements using Zimm formalism is reported in Figure 17, where the reverse apparent molecular weight is plotted as a function of asphaltene concentration. Positive slopes, indicating a positive second virial coefficient (repulsive interactions or stable systems), are observed, except for the least soluble cut (H60H) where I(qmin) is not concentration dependent. The extracted values of Mw and RG are reported in Table 5. The DLS measurements of WC_B1 asphaltene solubility cuts are shown in Figure 18. For the most soluble fraction,

Figure 18. Intensity autocorrelation function of WC_B1 asphaltene solubility cuts. The lines represent the multiexponential-like fits according to eq 9. Dotted and dashed lines represent the baseline α in eq 8.

Table 6. Summary of Effective Diffusion Coefficients for WC_B1 Asphaltenes in Toluenea De (× 10−6 cm2/s)

H92L, the fluorescence intensity contributes significantly to the background α (eq 8) leading to a small amplitude signal of relaxation associated with short decay times (corresponding to small sizes ranging from 0.5 to 6 nm). Note, in this case, that the volume average will strongly highlight the small sizes. For the less soluble fractions, H92H and H60H, the intensity correlation function reaches a stable baseline value with a better balance between fluorescence and scattering intensity; the corresponding sizes range from 12 to 60 nm and from 12 to 65 nm, respectively. The different sizes from the SAXS and DLS interpretations, RG and RH, respectively, are compared in Table 5. The order of magnitude is the same for the lightest solubility cut, H92L. The larger RG value (2.3 nm), compared to RH (0.53 nm), could be ascribed to the different averages determined with each method (z and volume averages, respectively) and indicates a large size polydispersity even for an asphaltene fraction. The intermediate H92H cut has similar hydrodynamic and gyration radii (7.2 and 12.1 nm, respectively). The small difference is likely due to the limited q range probed in the SAXS experiment. The least soluble cut, H60H has similar hydrodynamic size values in the

experimentb WC_B1 C5asphaltenes WC_B1 C7asphaltenes AA C739 AA C739 SA C739 VA C739 APDA C739

375.8 and 384.7 nm

500.8 nm

(A) 1 g/L, 25 ± 0.2 °C, 5 nm (YM30) 0.77 ± 0.03 0.40 ± 0.05 0.67 ± 0.03

0.32 ± 0.05

0.51 ± 0.07 0.26 ± 0.04 0.62 ± 0.04 0.32 ± 0.04 0.74 ± 0.04 0.36 ± 0.04 0.74 ± 0.04 0.32 ± 0.03 0.41 ± 0.03 0.28 ± 0.03 (B) 1 g/L, 50 ± 0.2 °C, 5 nm (YM30) WC_B1 C5 Asph. 1.20 ± 0.04 0.62 ± 0.05 (D) 0.1 g/L, 50 ± 0.2 °C, 5 nm (YM30) WC_B1 C5 Asph. 1.58 ± 0.09 1.0 ± 0.4

600.1 nm 0.27 ± 0.08 0.17 ± 0.06 0.18 0.20 0.18 0.26 0.13

± ± ± ± ±

0.05 0.06 0.05 0.06 0.04

0.36 ± 0.08 0.7 ± 0.6

a

Some of the data of Dechaine and Gray39 are included for reference and comparison. bThe new abbreviations used are as follows: AA = Athabasca asphaltenes, SA = Safaniya asphaltenes, VA = Venezuelan asphaltenes, and APDA = Athabasca partially demetalated asphaltenes. 5098

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concentration. The measurements were taken above 600 nm because 600 nm coincided with the lack of absorbance observed in the permeate of the diffusion experiments.39 Although there may still be some molecular absorption in this region, it is anticipated that its effects will be small compared to the scattering effects and should not affect the analysis. Equation 20 indicates that if Rayleigh scattering is responsible for the measured extinction in the visible region (>600 nm), then the measured absorbance should vary linearly with the inverse fourth power of the wavelength (1/λ4). In contrast, if the observed extinction were due to absorption, then the measured absorbance would vary with 1/λ.63 Eq 20 can therefore be used to estimate the size of the scatterers for each solution. This is done by first plotting the raw spectra for each asphaltene solution as A versus λ−4 (Figure 19). A linear

aggregated structures for the WC_B1 asphaltenes is similar to those from other origins (between 5 and 9 nm1). The effective diffusion coefficient of the WC_B1 C5asphaltenes is 15% higher than that of the WC_B1 C7asphaltenes, implying that the diffusing species on average are slightly smaller for C5 asphaltenes. The magnitude of this increase is marginal, and it is difficult to draw any concrete conclusions. However, the VPO molecular weight of the WC_B1 C5-asphaltenes at 10 g/L was 2500 g/mol, slightly smaller than the 2900 g/mol WC_B1 C7-asphaltenes. Since the C5-asphaltenes contain all of the C7-asphaltenes, the additional components in the C5-asphaltenes must be smaller and are likely nonassociated material. It is also possible that the asphaltenes self-associate into a different size distribution in the presence of these additional components. Since the diffusion coefficient for the C5 asphaltenes is comparable to that of the C7-asphaltenes, the size of the aggregated structures is likely still larger than 5 nm. And although measurements were not done using a membrane with 9 nm pores, based on the similarity of the results of the current measurements for both the WC_B1 C5- and C7-asphaltenes with the results for Athabasca asphaltenes,39 it is likely that the aggregated structures have a primary dimension that is less than 9 nm. The diffusion coefficient of the WC_B1 C5-asphaltenes was also measured at 50 °C and 1 g/L. For a temperature increase from 25 to 50 °C, the diffusion coefficient of the C5asphaltenes increased by a factor of 1.56. In accordance with the Stokes−Einstein equation, if the size of the diffusing species does not change over this temperature range, then the ratio Dη/T will remain constant.39 For toluene viscosities of 0.5542 mPa s and 0.4208 mPa s at 25 and 50 °C, respectively,62 the ratio of diffusion coefficients would then be 1.43. This value is close to the measured ratio of 1.56, indicating that the majority of the increase in the measured effective diffusion coefficient is due to thermal motion and viscosity effects rather than a decrease in the size of the aggregated structures. Note that the apparent increase in size with temperature contradicts results from vapor pressure osmometry2 and interfacial property measurements (this study and Yarranton et al.2). It is possible that the aggregates become more swollen or porous with an increase in temperature. However, the effect of temperature on asphaltene association is an unresolved issue arising from this study. Rayleigh Scattering Analysis. UV−vis spectra of both WC_B1 C5- and C7-asphaltenes were collected and analyzed using a Rayleigh scattering formalism.40 Very briefly, if it is assumed that the observed extinction in the visible region (>400 nm) is a function of Rayleigh scattering only, then Beer’s law can be written as63 A = bCξ

r6 λ4

Figure 19. Rayleigh scattering curves for WC_B1 C7-asphaltenes in toluene (T = 23 °C, 10 mm cuvette).

regression with a forced zero intercept is then performed on each of these transformed spectra. All spectra were highly linear for λ > 600 nm when plotted in this form (R2 > 0.99 in all cases). The slope of these lines can then be combined with eq 20 to estimate the average radius of the scatterers in the solution. First, it is necessary to estimate the average molecular weight of the scatterers in solution at each concentration. The molecular weights were estimated using the stepwise-polymerization model of Agrawala and Yarranton.13 Then, the constant ξ in eq 1 is calculated using refractive indices of 1.496 and 1.7 for toluene and asphaltenes, respectively.64,65 Finally, the value of the average radius of the scatterer, r, can be calculated. The results of these calculations are summarized in Figure 20. Note, the stepwise method was used to calculate molecular weights to be consistent with the comparison data40 presented in Figure 20. The calculated molecular weight distributions are similar to the Gamma distributions presented earlier for the WC_B1 asphaltenes, and the derived sizes presented in Figure 20 will be within 0.5 nm of the sizes that would be determined from the Gamma distribution. The results shown in Figure 20 indicate that the WC_B1 C7asphaltene aggregates in toluene are approximately 0.2 nm smaller than C7-asphaltenes from other origins. In the case of the WC_B1 C5-asphaltenes, the Rayleigh scattering analysis indicates that they are smaller than the C7-asphaltenes by

(20)

where 2 128π 5nS4NA ⎡ (nP /nS)2 − 1 ⎤ ⎥ ⎢ ξ = cons tan t = 3 × 2.303 ⎣ (nP /nS)2 + 2 ⎦

(21)

and A = absorbance = −log10(I/I0), I = intensity of light transmitted through the sample, I0 = baseline intensity of light through solvent, b = path length, r = radius of scattering particle, NA = Avogadro’s number, nS = refractive index of the solvent, nP = refractive index of the particle, and C = analyte 5099

dx.doi.org/10.1021/ef400729w | Energy Fuels 2013, 27, 5083−5106

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Figure 21. Cumulative size distribution for asphaltenes arising from sequential nanofiltration measurements with 50, 20, 10, and 5 nm nominal filter pore sizes. Full triangles: mass based; open circles: composition based.

Figure 20. Calculated diameter of asphaltene aggregates in toluene at 23 °C based on a Rayleigh scattering analysis. The data for Athabasca, Safaniya, and Venezuelan C7-asphaltenes are from ref 40. Closed and open symbols are for 10 and 1 mm pathlengths, respectively.

approximately 0.3 nm. This small difference agrees with the diffusion coefficient measurements since the diffusion coefficient for the C5-asphaltenes was only slightly higher (15%) than the value obtained for C7-asphaltenes. These trends are in qualitative agreement with the molecular weights measured in 10 g/L asphaltenes in toluene at 50 °C (4900 g/mol for Athabasca C7-asphaltenes, 2900 g/mol for WC_B1 C7asphaltenes, and 2500 g/mol for WC_B1 C5-asphaltenes). Nanofiltration Results. The results of the nanofiltering measurements with 1% WC_A1 C7-asphaltenes in toluene are summarized in Table 7 and Figures 21 to 23. The mass balance Table 7. Nanofiltration Results for 1 wt % WC_A1 C7Asphaltenes + Toluene Mixtures

Figure 22. Size distribution for the 1% WC_A1 C7-asphaltenes in toluene. Dark bars: mass based; light bars: composition based.

asphaltene mass fraction passing through a filter (%)

asphaltene fraction below nominal filter size (%)

filter pore size (nm)

asphaltene mass balance closure (%)

composition ratio

mass ratio

composition ratio

mass ratio

50 20 10 5

99.8 98.8 92.4 89.5

≤1.0 0.5 ≤1.0 0.2

0.99 0.51 0.96 0.28

≤100 ≤50 ≤50 ≤10

99 51 49 14

filter. A superficial interpretation of the results would indicate that the asphaltenes comprise two nominal size ranges with approximately equal weight fractions: 20−50 nm and less than 10 nm. However, in prior work, the radii of gyration of asphaltene-rich species in permeates were found to be significantly smaller than the nominal pore size of the filters. For example, the radii of gyration of objects present in Athabasca Bitumen and Safaniya vacuum residue permeates that passed through a 20 nm zirconia filter were approximately 2 nm66 and 3 nm,67 respectively. Thus, object sizes obtained from nanofiltration are overestimates. This arises in part from the build up of deposits on the upstream sides of the filters or within filters that prevent smaller objects from passing through them. Sorption of asphaltenes on acidic and basic surfaces68 is another effect that can inhibit passage of asphaltenes. This would appear to be of secondary importance as sorption was not detected on the 50 nm filters. The visual appearance of the 20 nm permeate 0.5 wt % asphaltenes (opaque) and the 5 nm permeate 0.2 wt % asphaltenes (transparent) (Figure 22) is also suggestive. The 5 nm permeate would appear to be “aggregate free” as the mixture does not appear to scatter visible light significantly in comparison with the 20 nm permeate, even though the wt % asphaltenes in both cases are comparable. Dechaine also qualitatively observed in his Rayleigh scattering measurements that a permeate filtered to 5 nm (all material >5 nm removed) did not scatter light.

closures for the filtration measurements and the wt % of the asphaltenes passing through individual filters and sequences of filters, based on composition and mass ratios described in Experimental Methods, are reported in Table 7. The mass balance closures are excellent for the 50 and 20 nm filtration experiments and deteriorate to ∼90% as the mass of asphaltenes decreased during the filtration sequence. The two measures of filter performance, based either on the mixture composition or on the mass of asphaltenes recovered, agree to within the anticipated level of experimental errors. From the cumulative distribution (Figure 21), essentially all of the asphaltenes pass through the 50 nm filter. The binned results are shown in Figure 22. Fifty percent of the asphaltenes are trapped by the 20 nm filter. Little is trapped by the 10 nm filter, and an additional 30−35 wt % is trapped by the 5 nm filter. 10−14 wt % of the asphaltenes pass through the 5 nm 5100

dx.doi.org/10.1021/ef400729w | Energy Fuels 2013, 27, 5083−5106

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Figure 23. Permeates obtained from the 20 and 5 nm filtration experiments for 1 wt % WC_A1 C7-asphaltenes in toluene. The compositions are 0.5 and 0.2 wt % WC_A1 C7-asphaltenes, respectively.

4. DISCUSSION The VPO, density, elemental analysis, and TRF data all indicate that asphaltenes are a mixture of nonassociating and associating species. The nonassociating species appear to make up approximately 15 wt% of the C7-asphaltenes. The molecular weight and diffusion data also show that the C5-asphaltenes are, on average, smaller and have lower molecular weight than the C7-asphaltenes. Since the C5-asphaltenes include all of the C7asphaltenes, their lower molecular weight indicates that many of the additional components in the C5-asphaltenes are likely nonassociated as well. Similarly, the VPO molecular weight of resins shows no evidence of association. Hence, it appears that 80 to 85 wt % of the bitumen consists of nonassociating species. This nonassociated material has a slightly higher density and molecular weight and lower H/C ratio than the resins fraction. It is likely that this material is part of the continuum of aromatic and resin monomers, which have a trend of increasing density, molecular weight, polarity, and heteroatom content with increasing solubility parameter values. It appears that the upper end of this monomer distribution is insoluble in npentane. The nanoaggregates have more uniform properties than the nonassociating species, suggesting that self-association brings together a random selection of associating monomers in each aggregate. High-resolution mass spectrometry data, while lacking a material balance, suggest that the nonassociating species are the smaller, more aromatic asphaltenes and therefore that the associating species are larger and less aromatic. Smaller aromatic molecules emit at shorter wavelengths in agreement of the TRES for the cut with a higher content of soluble fractions. Hence, although the nanoaggregates are more aromatic than the rest of the oil, the self-associating components appear to be relatively more aromatic than the nonassociated components in the same solubility cut. 4.1. Asphaltene Molecule and Aggregate Mass. A comparison of the molecular weight data from different methods is provided in Table 8. The VPO data indicate that the number average molecular weight of asphaltene monomers is approximately 850 g/mol with nanoaggregate molecular

Table 8. Comparison of Asphaltene and Solubility Cut Molecular Weights Determined from VPO, Interfacial Film Measurements, and SAXS cut

VPO MW at 10 g/L (g/mol)

VPO MW at 40 g/L (g/mol)

film data MW at 10 g/L (g/mol)

SAXS MW dilute extrapolation (g/mol)

2900

3800

∼3500

165000

1600 1400 800 6900 6000 5500

1700 1500 900 12300 10300 9500

− − − − − −

42000 − 12000 >263000 − 175000

C7Asphaltene H60L H77L H92L H60H H77H H92H

weights ranging up to 30000 g/mol based on the previous distribution fitting to the VPO data reproduced in Table 9. The Table 9. Effect on MWN and MWW of adding high molecular weight material to a molecular weight distribution with original MWN = 3800 g/mol and MWW = 10100 g/mol high MW lump (g/mol)

wt %

new MWN (g/mol)

new MWW (g/mol)

MWW/MWN

200000 500000 500000 1 × 106 1 × 106

50 20 10 10 5

7500 4700 4200 4200 4000

105000 108000 59000 109000 60000

14 23 14 26 15

average molecular weights obtained from VPO data are supported by asphaltene film dimensions. The weight average SAXS molecular weights have similar trends to the VPO molecular weights but the values are approximately 20 times greater. Some possible explanations for the apparent discrepancy are discussed below. Irrespective of these differences, both VPO and SAXS data confirm the broad distribution of nanoaggregate sizes in asphaltenes with a minimum molecular weight range of 800 to 40000 g/mol. Non-Ideal Solution Behavior. The molecular weights calculated from VPO assume ideal solution behavior, which may not be the case for asphaltenes. VPO data from aromatics 5101

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and resins (not reported here) indicate that these effects are relatively small (less than approximately 10% deviation) for aromatic components in toluene at concentrations up to 20 g/ L. Also, these deviations would be expected to increase with asphaltene concentration. The discrepancies between VPO and SAXS molecular weights are far greater and are observed at all concentrations. Different Measures of Polydispersed Size Distributions. SAXS measures a weight average molecular weight (MWW), while VPO measures a number average molecular weight (MWN). For a polydispersed size distribution, the mass average molecular weight can be significantly greater than the number average because one large object contains the same mass as a large number of smaller objects. Further, the number average is sensitive to errors in the amount of low molecular weight material in a sample, while the mass average is sensitive to errors in the amount of high molecular weight material. Interconversion of weight and number averages is nontrivial. Consider the distribution determined in this work: 15 wt % nonassociated asphaltenes with a molecular weight of 850 g/ mol plus the Gamma distribution of associated asphaltenes with a number average molecular weight of 9800 g/mol. The MWN of the combined distribution is 3800 g/mol and the MWW is 10100 g/mol. Therefore, the difference in the type of averaging, based on an assumed gamma distribution in this case, accounts for a factor of 2−3 difference between the measured MWN and calculated MWW values. However, if a small mass fraction of much larger molecular weight material is present but was not included in the originally calculated distribution, then the difference between the calculated MWW and measured MWN values grows dramatically, as illustrated in Table 9. Conversely, the measured MWW values obtained from SAXS measurements, while sensitive to high molecular weight material, are insensitive to small species irrespective of whether or not they are present or distinguished from the background. For the example above, if species smaller than 5000 g/mol are ignored, the MWW only increases by 0.7 g/mol. Hence, to reconcile the large observed differences between the VPO and SAXS measurements, some very high molecular weight material must be present in the samples. In fact, Table 8 shows that the largest differences between MWN and MWW are in the insoluble cuts where high molecular weight material is most likely to be collected. Table 9 illustrates the impact of large molecular weight material on the ratio of MWW to MWN and how much high molecular weight material is required to reconcile the difference between MWW and MWN values for C7-asphaltenes. In the absence of other considerations, the high molecular weight material must extend above 0.5−1 million g/ mol. Partial Solubility of Asphaltenes. While asphaltenes disperse in toluene, not all of them may be soluble. VPO only detects soluble material, while SAXS detects all species with a large enough contrast whether dissolved or dispersed in toluene. A problem with this hypothesis is that large diameter particles were not detected in the membrane permeation experiments or on the asphaltene films. Flocculation of Nano-Aggregates. The nanoaggregates may flocculate like colloids. The flocculation would likely be invisible to VPO, the film measurements, and the permeation experiments but would likely be detected by SAXS. 4.2. Reconciliation of Asphaltene Molecular Weight and Size. Table 10 provides a comparison of the dimensions

Table 10. Comparison of C7-Asphaltene Dimensions Determined from Different Methods method

dimension

magnitude

SAXS diffusion rayleigh scattering nanofiltration IFT surface mass (collapsed film) AF (dry film) SFA (collapsed film) SFA (expanded film)

diameter diameter diameter diameter surface area thickness thickness thickness thickness

14 nm