Characterization of Asphaltenes by Nonaqueous Capillary

Jan 5, 2011 - Nonaqueous capillary electrophoresis was used for the separation and characterization of asphaltene samples from different sources. For ...
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Energy Fuels 2011, 25, 208–214 Published on Web 01/05/2011

: DOI:10.1021/ef100921d

Characterization of Asphaltenes by Nonaqueous Capillary Electrophoresis Wim Th. Kok,*,† Anna J. T€ ud€ os,‡ Mark Grutters,‡ and Andrew G. Shepherd§ †

Analytical Chemistry Group, van’t Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, 1090 GD Amsterdam, The Netherlands, ‡Shell Projects and Technology, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands, and § Nederlandse Aardolie Maatschappij B.V., Schepersmaat 2, 9405 TA Assen, The Netherlands Received July 20, 2010. Revised Manuscript Received December 16, 2010

Nonaqueous capillary electrophoresis was used for the separation and characterization of asphaltene samples from different sources. For the separation medium (background electrolyte), mixtures of tetrahydrofuran and a high-permittivity organic solvent could be used. The best results were obtained with an 80:20 mixture of tetrahydrofuran and acetonitrile, containing 1-10 mM of lithium perchlorate. In this separation medium, asphaltene samples were found to be composed of two fractions that could be clearly separated: one fraction of neutral species and a fraction that carries a positive charge in the solvent mixture employed. Between samples of different origin, differences were found in the relative amounts of the neutral and the charged fractions and in the average electrophoretic mobility of the charged components. Taylor dispersion analysis was applied to estimate the average diffusion coefficient of the asphaltene species in the solvent mixture used. From the results, it is concluded that the asphaltenes are present as nanoaggregate clusters of 3000-4000 Da and that the charged aggregates carry a net charge of approximately þ1. The possible correlation between the electrophoretic properties of asphaltenes in crudes of different origin and their field behavior is discussed.

particles on surfaces, and the consequences for precipitation and deposition under field conditions. Most of the surveillance work on asphaltene problematic fluids was obtained with dead oil samples. However, advances in downhole fluid analysis for actual wells have also provided critical insights on “in situ” asphaltene behavior.7 Despite the major research effort devoted to the elucidation of the molecular characteristics of asphaltenes in the past 50 years, the area is still the subject of a lively debate.8-11 Major advances in the confirmation and verification of asphaltene molecular weight and architecture have been achieved by time-resolved fluorescence depolarization (TRFD),12 smallangle X-ray scattering (SAXS),13 small angle neutron scattering (SANS),14 and mass spectrometry.15 It has been shown

Introduction In the petroleum industry, asphaltenes are defined as a solubility class of compounds in crude oil not soluble in n-alkanes such as heptane but soluble in aromatic solvents such as toluene.1-3 Methods to separate asphaltenes from hydrocarbon systems are well documented in the literature. The procedures used (e.g., the solvent type, contact time, or filtration method) usually dictate the overall separation yields as well as the characteristics of the fractions themselves.4-6 Within the asphaltene fraction, a wide range of molecules can be found, from relatively small polar compounds to larger structures. The presence and structure of asphaltenes in crude oil is linked to the kerogen type in source rocks and thermal maturity of the expelled oil. The exact molecular characterization of asphaltenes is a continuous challenge for the petroleum industry. Understanding the asphaltene structure is important as it may help to better predict the mechanism of aggregation, adhesion of

(7) Mullins, O. C.; Ventura, G. T.; Nelson, R. K.; Betancourt, S. S.; Raghuraman, B.; Reddy, C. M. Visible-near-infrared spectroscopy by downhole fluid analysis coupled with comprehensive two-dimensional gas chromatography to address oil reservoir complexity. Energy Fuels 2008, 22 (1), 496–503. (8) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Characterization of heavy hydrocarbons by chromatographic and mass spectrometric methods: An overview. Energy Fuels 2007, 21 (4), 2176–2203. (9) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Contrasting perspective on asphaltene molecular weight. This comment vs the overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti. Energy Fuels 2008, 22 (3), 1765–1773. (10) Strausz, O. P.; Safarik, I.; Lown, E. M.; Morales-Izquierdo, A. A critique of asphaltene fluorescence decay and depolarization-based claims about molecular weight and molecular architecture. Energy Fuels 2008, 22 (2), 1156–1166. (11) Mullins, O. C. Rebuttal to Strausz et al. Regarding TimeResolved Fluorescence Depolarization of Asphaltenes. Energy Fuels 2009, 23, 2845–2854. (12) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Asphaltene molecular size by fluorescence correlation Spectroscopy. Energy Fuels 2007, 21 (5), 2875–2882. (13) Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. On the distribution of chemical properties and aggregation of solubility fractions in asphaltenes. Energy Fuels 2006, 20 (2), 705–714.

*To whom correspondence should be addressed. E-mail: W.Th. [email protected]. (1) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. The MolecularStructure of Asphaltene - an Unfolding Story. Fuel 1992, 71 (12), 1355– 1363. (2) Speight, J. G. Petroleum asphaltenes - Part 1 - Asphaltenes, resins and the structure of petroleum. Oil Gas Sci. Technol. 2004, 59 (5), 467– 477. (3) Mullins, O. C., Sheu, E., Hammami, A., Marshal, A. G., Eds. Asphaltenes, Heavy Oils and Petroleomics; Springer: New York, 2007. (4) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. The role of asphaltene solubility and chemical composition on asphaltene aggregation. Pet. Sci. Technol. 2003, 21 (3-4), 461–489. (5) Kharrat, A. M.; Zacharia, J.; Cherian, V. J.; Anyatonwu, A. Issues with comparing SARA methodologies. Energy Fuels 2007, 21 (6), 3618– 3621. (6) Schabron, J. F.; Rovani, J. F. On-column precipitation and redissolution of asphaltenes in petroleum residua. Fuel 2008, 87 (2), 165– 176. r 2011 American Chemical Society

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positive charge. The authors indicated that the charge of deposited asphaltenes on electrodes could be reversed by adding resins to the system or by changing the solvent types. Further arguments were put forward that electrodeposition of asphaltenes required charge transfer via dissociation on surface groups and adsorption of ionized solutes. Examination of the dc conductivity of asphaltene solutions in toluene clearly showed nanoaggregate formation above certain concentration levels.23,24 From these studies it appeared that the more polar asphaltenes were also the more conductive. Electrokinetic measurements are a good way to study the charge properties of asphaltenes and its contribution to precipitation and adhesion. In the studies conducted so far, measurements have been performed on suspensions in nonsolvents. The diverse results of these experiments show that the electrophoretic properties of the suspended particles are mostly dictated by the interaction with the medium. Kokal et al. measured the electrokinetic properties of asphaltenes separated from crude oils in aqueous suspensions.25 The results of these measurements were dependent on pH, ionic strength, composition, and degree of hardness of the electrolyte solution. The authors reported that asphaltene particles were negatively charged at neutral pH. A charge reversal was observed for higher pH values in multivalent cation solutions. Experiments conducted in nitromethane indicated that asphaltene particles are positively charged in this medium. Similar results were obtained by Gonzalez et al.26 In aqueous suspensions, the asphaltene particles showed a negative charge and resins were shown to bind to the asphaltene particles. This interaction, however, did not significantly change the electrophoretic mobility of the asphaltene particles. Leon et al. performed electrophoretic measurements on asphaltenes in n-heptane, showing again that asphaltene particles carry a positive charge, and also in nonpolar lowconductivity solvents.27 Charge speciation on the molecular level or on the level of nanoaggregates as they exist in crude oils, rather than for suspended particles, may be a valuable tool for predicting deposition tendencies directly from problematic and nonproblematic crudes. Capillary electrophoresis (CE) is a now widely accepted technique for the separation and characterization of charged species in solution.28 In CE, ionic compounds are separated in capillaries filled with a background electrolyte (BGE). A separation is obtained on basis of differences in charge and/or size, resulting in differences in electrophoretic mobility. The vast majority of the applications of CE involves the separation of water-soluble compounds in aqueous BGE solutions. However, nonaqueous capillary

that, in contrast to what was believed earlier, asphaltenes are not high-molecular weight species, and consensus appears to be now that asphaltene molecules contain on average a single heteroatomic aromatic chromophore with short aliphatic side chains and that the covalent molecules have masses on the order of 500-1000 Da. Peri-condensed as well as archipelago models have been used to explain the hierarchical structure of asphaltenes.3,16 Since often only a fraction of the precipitated asphaltenes form deposits on the walls of tubing and process equipment,17 it is important to find reliable methods to identify the fractions responsible for this behavior. This may help to improve existing thermodynamic models and predictive screens for precipitation and deposition. Experimental studies with model fluid systems have suggested it is the most polar fraction of asphaltenes in crude oil that has the tendency to precipitate.4 Wattana et al. reported that the asphaltenes that were extracted from unstable crude oils and from solid deposits contained substantially larger portions of the higher polar fractions and had a higher polarity, compared to the asphaltenes obtained from crude oils with no asphaltene stability problems in the field.18 Since asphaltene polarity and charge are believed to be associated with the deposition phenomena, most of the early research into deposition was carried out using electrodeposition measurements, forcing crude oil components to deposit under the influence of an electric field. Katz and Beu performed electrical deposition experiments with crude oils in the laboratory.19 These authors were among the first to suggest that streaming potentials could also lead to the precipitation of asphaltenes in the reservoir porous media. Their results indicated that asphaltenes that deposited on electrodes in crude oil were negatively charged. However, in more comprehensive electrodeposition tests Lichaa and Herrera showed that asphaltenes from problematic Venezuelan fluids were positively charged,20 using crude oil samples diluted with mineral oil. Khvostichenko and Andersen looked at the effect of electric fields on asphaltene/resin model systems and deposition on electrodes in different solvents.21,22 Most of the deposited asphaltenes in these experiments had a (14) Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Effects of temperature and pressure on asphaltenes agglomeration in toluene. A light, X-ray, and neutron scattering investigation. Energy Fuels 2004, 18 (5), 1243–1249. (15) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (5), 1965–1972. (16) Sabbah, H.; Morrow, A. L.; Pomerantz, A. E.; Mullins, O. C.; Tan, X. L.; Gray, M. R.; Azyal, K.; Tykwinski, R. R.; Zare, R. N. Comparing Laser Desorption/Laser Ionization Mass Spectra of Asphaltenes and Model Compounds. Energy Fuels 2010, 24, 3589–3594. (17) Dubey, S. T.; Waxman, M. H. Asphaltene adsorption and desorption from mineral surfaces. SPE Reservoir Eng. 1991, 6 (3), 389–390. (18) Wattana, P.; Fogler, H. S.; Yen, A.; Garcia, M. D.; Carbognani, L. Characterization of polarity-based asphaltene subfractions. Energy Fuels 2005, 19 (1), 101–110. (19) Katz, D. L.; Beu, K. E. Nature of asphaltic substances. Ind. Eng. Chem. 1945, 37 (2), 195–200. (20) Lichaa, P. M.; Herrera, L. SPE 5304, presented at the International Symposium of Oilfield Chemistry, Dallas , TX, January 16-17, 1975. (21) Khvostichenko, D. S.; Andersen, S. I. Electrodeposition of Asphaltenes. 1. Preliminary Studies on Electrodeposition from OilHeptane Mixtures. Energy Fuels 2009, 23 (1), 811–819. (22) Khvostichenko, D. S.; Andersen, S. I. Electrodeposition of Asphaltenes. 2. Effects of resins and additives. Energy Fuels 2010, 24 (4), 2327–2336.

(23) Zeng, H.; Song, Y. Q.; Johnson, D. L.; Mullins, O. C. Critical Nanoaggregate Concentration of Asphaltenes by Direct-Current (DC) Electrical Conductivity. Energy Fuels 2009, 23, 1201–1208. (24) Sedghi, M.; Goual, L. Role of Resins on Asphaltene Stability. Energy Fuels 2010, 24, 2275–2280. (25) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and Adsorption Properties of Asphaltenes. Colloids Surf., A 1995, 94 (2-3), 253–265. (26) Gonzalez, G.; Neves, G. B. M.; Saraiva, S. M.; Lucas, E. F.; de Sousa, M. D. Electrokinetic characterization of asphaltenes and the asphaltenes-resins interaction. Energy Fuels 2003, 17 (4), 879–886. (27) Leon, O.; Rogel, E.; Torres, G.; Lucas, A. Electrophoretic mobility and stabilization of asphaltenes in low conductivity media. Pet. Sci. Technol. 2000, 18 (7-8), 913–927. (28) Kok, W. T. Capillary electrophoresis: Instrumentation and operation. Chromatographia 2000, 51, S5–S89.

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Table 1. Asphaltene Sample Properties, Including Weight Percent of Asphaltenes in the Crude Oil (wt % A) and Standarized Elemental Composition sample

description

location

wt % A

Ni/V

O/N

O/S

stability in field conditions

1 2 3 4

downhole deposit IP 143 asphaltene IP 143 asphaltene IP 143 asphaltene

Gulf of Mexico (Field 1) Gulf of Mexico (Field 1) South America (Field 2) South America (Field 3)

3.2 3.2 7.2 2.5

2.2 2.1 2.8 4.2

3.4 2.2 1.3 1.2

1.3 2.1 0.5 2.7

unstable unstable unstable stable

electrophoresis (NACE) has been applied increasingly in the past years for solutes that do not dissolve easily in water.29,30 A prerequisite for a (meaningful) separation by CE is that the BGE is sufficiently conductive. Salts to be used for the BGE should be soluble in the solvent used and when dissolved ionized to a reasonable extent. Therefore, solvents with high dielectric constants are usually selected for NACE, and methanol and acetonitrile (ACN) are most frequently employed.30 However, mixtures of less polar solvents such as THF and chloroform with methanol or ACN can also be used.31 In the current work, NACE was applied for the study of the charge properties of asphaltenes. First, the method was investigated using solutions of a downhole solid identified as an asphaltene field deposit. Different solvent mixtures were tested for their suitability, with respect to the solubility of the asphaltenes on one hand and the electrical conductivity of the solution on the other. The optimized NACE method was also applied to SARA fractions of crude oils from the Gulf of Mexico (GoM) and Venezuela (see Table 1). Two of the three crude oils are unstable with respect to asphaltene precipitation during normal production and form asphaltene deposits in the well. While the charge characteristics of asphaltene samples were determined by NACE, the same instrumentation was used to estimate the molecular size of the asphaltene species by Taylor dispersion analysis (TDA). The long-term objective of this work is to explore the possibilities of these techniques for characterization and fingerprinting of asphaltenes as a means to understand and predict their phase behavior and the precipitation and deposition mechanisms. Information obtained from NACE and TDA measurements may be correlated with deposition behavior and could possibly be used in future deposition studies. Separation of asphaltenes according to their charge properties may also serve to simplify molecular characterization, as a preliminary step before other analytical techniques. Furthermore, simple empirical correlations from NACE may in the future aid the selection of fit-for-purpose, production chemicals to mitigate or prevent asphaltene precipitation and deposition.

Samples. The asphaltene sample used in most of the experiments described in this work was a downhole deposit obtained from a field in the Gulf of Mexico. A 2 wt % solution was prepared from this sample using toluene (Merck P.A.). For comparison, asphaltene samples collected from SARA fractionation were also analyzed. Data on the samples tested are given in Table 1. The SARA method employed here is a modified proprietary IP 143 procedure. Topped samples were used with excess hot n-hexane as the precipitation solvent. Separation of the asphaltene from the maltene fraction was carried out after reflux followed by filtration and washing with dichloromethane. The asphaltene weight percent data of the crude, the normalized elemental composition (wt %) of the asphaltene samples, and the nickel/vanadium concentration ratio of the crude oils studied are included in Table 1. Elemental analysis of the asphaltenes was conducted using a proprietary procedure developed for a LECO instrument. The measurement of nickel and vanadium of the parent crudes was carried out using a proprietary inductively coupled plasma mass spectrometry method (ICPMS) applied directly on the samples. The asphaltene solutions in toluene were diluted in the appropriate BGE solution to final concentrations of 0.1 to 2.0 mg/mL for NACE analysis. Apparatus. The instrument used was an HP 3D CE from Agilent (Waldbronn, Germany) with a diode array detector. Detection was performed at 260 and 350 nm. Except when stated otherwise, the signal at 350 nm is shown in the figures in this paper. Fused silica capillaries with an internal diameter of 75 μm were obtained from Polymicro Technologies (Phoenix, AZ). The total length of the capillaries was typically 0.425 m, with the detection window at 0.32 m. From the ends of the capillary, in contact with the solutions, the protective coating on the outside was removed. New capillaries were etched with a 0.1 M NaOH solution and subsequently flushed thoroughly with water and THF. Between experiments, the capillary was flushed with THF and with the BGE for at least 2 min each. Samples were injected hydrodynamically (20 mbar  5 s). Voltages were applied from -30 kV to þ30 kV. To speed up the elution of the peaks of interest, a pressure of 10 or 20 mbar was applied on the inlet vial during the separations. The same instrumentation was used for the estimation of diffusion coefficients by TDA measurements. In these experiments a pressure in the range of 5-30 mbar was applied on the inlet vial, without applied voltage.

Experimental Section

Results

Chemicals. The solvents used for the preparation of the BGE were tetrahydrofuran (THF), acetonitrile (ACN), dimethyl formamide (DMF), and dimethyl acetamide (DMAC). They were of analytical grade quality, obtained from standard suppliers, and were used without further purification. Salts (lithium perchlorate, tetramethylammonium hexafluorophosphate, tetramethylammonium tetrafluoroborate) were obtained from SigmaAldrich (Zwijndrecht, The Netherlands.

BGE Selection. Two criteria for the selection of a nonaqueous solvent were applied: the solubility of asphaltenes in the particular organic phase and the associated ionization properties. The asphaltene solution in toluene could not be used directly in the NACE experiments; because of the low dielectric constant of this solvent, any (sparingly) soluble salt is not ionized in toluene. To test for the first solvent criterion, the 2% (w/v) asphaltene solution in toluene (sample 1) was diluted at 1:20 (v/v) in candidate solvents, yielding a final concentration of 1 mg/mL asphaltenes. The mixture of solvents was checked for precipitation or cloudiness. The asphaltene samples were found to be well soluble in THF. However, the conductivity of a solution of salt (LiClO4) in pure THF is virtually zero. Therefore, mixtures of THF with

(29) Riekkola, M. L. Recent advances in nonaqueous capillary electrophoresis. Electrophoresis 2002, 23 (22-23), 3865–3883. (30) Geiser, L.; Veuthey, J. L. Non-aqueous capillary electrophoresis 2005-2008. Electrophoresis 2009, 30 (1), 36–49. (31) Cottet, H.; Struijk, M. P.; Van Dongen, J. L. J.; Claessens, H. A.; Cramers, C. A. Non-aqueous capillary electrophoresis using nondissociating solvents - Application to the separation of highly hydrophobic oligomers. J. Chromatogr., A 2001, 915 (1-2), 241–251.

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Table 2. Relative Conductivities of 10 mM LiClO4 in Solvent Mixtures for NACE solvent mixture

composition

relative conductivity

THF-ACN THF-ACN THF-ACN THF-DMF THF-DMAC ACNa

95:5 90:10 80:20 80:20 50:50 100

0.03 0.06 0.26 0.47 0.32 1.00

a

100% ACN used as reference only; asphaltenes precipitated from ACN.

solvents with a higher permittivity were tested. No precipitation or flocculation of asphaltenes was observed for THF mixtures with up to 20% (v/v) of ACN or DMF and with up to 50% (v/v) of DMAC. Solutions of asphaltene in these mixtures were stable for at least several days. The ionization properties of the solvent mixtures were tested by measuring the currents obtained at voltages of 10-30 kV with a 10 mM solution of an appropriately soluble salt (LiClO4) in the capillary. The currents were compared with that obtained with a LiClO4 solution in (pure) ACN. Results are shown in Table 2. Although the conductivities measured were lower than in 100% ACN, at least a substantial ionization of the salt was observed. The ionization increases with the volume fraction of the high-permittivity solvent. The relative conductivities cannot be directly translated into a degree of ionization because of differences in viscosity. Electrophoretic Separation. Detection was performed at two wavelengths simultaneously. The signal at 350 nm can be regarded as selective for the asphaltenes. Toluene, present in the samples for NACE as the solvent from the original asphaltene solution, does not absorb at this wavelength. The signal at 260 nm can be used to monitor the velocity of the toluene peak that was used as a neutral marker (indicating the velocity of the BGE by the pressure induced flow and the electroosmotic flow together). Asphaltenes absorb at 260 nm, but the signal is virtually that of toluene alone due to its much higher concentration in the samples injected. In Figure 1A, electropherograms are shown of the asphaltene sample in a BGE of THF-ACN (80:20) containing 1 mM LiClO4, with an applied voltage of þ30 kV and -30 kV, respectively. In both electropherograms, a clear fractionation of the asphaltene sample into two distinct peaks is observed. With a positive polarity (i.e., with the negative electrode at the detector side of the capillary) one asphaltene peak elutes before the neutral marker and one at the same time. With a negative polarity, part of the asphaltenes migrate with the velocity of the neutral marker and the other part elutes later. These results clearly show that the asphaltene samples tested here contain a neutral fraction and a fraction with a positive charge under the conditions examined. Using an Excel routine developed in-house, the electropherograms (absorbance vs time traces) were converted into mobility abundance plots, showing the relative abundance of asphaltenes as a function of their mobility (Figure 1B). The curves obtained from the electropherograms at positive and negative applied voltage are almost identical. This shows clearly that the fractionation obtained with NACE is related to differences in charge and is not caused by, e.g., selective adsorption on the capillary wall. It should be noted that the abundance data are based on the absorption values

Figure 1. (A) Electropherograms of an asphaltene deposit sample obtained with an applied voltage of þ30 kV (dark line) and -30 kV (gray line). During the separation, a pressure of 15 mbar was applied on the capillary inlet. For other experimental details see the text. (b) Mobility abundancy plots constructed from the electropherograms in part A.

Figure 2. Mobility abundance plots obtained with sample concentrations of 0.2, 0.4, 1.0, and 2.0 mg/mL. BGE, THF-ACN (80:20) containing 10 mM of LiClO4; applied voltage, þ30 kV. For other experimental details, see the text.

measured at 350 nm. The data cannot be translated directly into mass or mole fractions, since the extinction may vary over the different fractions. In Figure 2, similar abundance plots are shown obtained with different concentrations of asphaltenes. Varying the sample concentration from 0.1 to 1.0 mg/mL does not have a significant effect on the (concentration) ratio of the neutral and the charged fraction nor on the mobility of the charged fraction. BGE Effects. NACE experiments were performed with different solvent mixtures for the BGE. Figure 3A shows the effect of the ACN content of THF-ACN mixtures. With an increasing volume fraction of ACN, a more pronounced 211

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those obtained earlier. Also the addition of an acidic (HCl) or a basic compound (triethanolamine) to the BGE in concentrations of 1-5 mM did not affect the electropherograms obtained. Although the general appearance of the electropherograms with the asphaltene samples was consistent, the repeatability of quantitative data was not completely satisfactory. In a series of repeated experiments, often a trend was seen in the average mobility and in the relative abundance of the charged fraction. Both parameters tended to increase over time after preparation of the diluted sample. The chemical background of these trends is still not clear. It could be related to the aging of the diluted solution, accompanied with a change in the aggregation state. Redistribution of asphaltenes between different aggregation states is known to be extremely slow.1 On the other hand, it cannot be precluded that the changes are related to changes in the composition of the sample solution or the BGE by, e.g., selective evaporation of the solvents. Estimation of the Molecular Size and Charge. For the determination of the diffusion coefficient of the asphaltenes, Taylor dispersion analysis (TDA) was applied.32-34 TDA is based on the broadening of zones of dissolved compounds in narrow tubes or capillaries when these zones are transported by a laminar flow of liquid. The broadening of a zone by the Poisseuille profile of the flow is counteracted by molecular diffusion in the radial direction. Thereby, the broadening observed for a compound is related to its diffusion coefficient. For TDA measurements, narrow plugs of the sample solution were injected and flushed through the capillary by applying a pressure on the inlet vial. By simultaneous detection at two different wavelengths, the peaks of asphaltene and of toluene could be detected independently, even when the compounds were not separated. Figure 4 shows some examples of the peaks observed. From the variance of a peak, σt2 (calculated from the observed width at half height) and the peak top time tp, the diffusion coefficient can be estimated as

Figure 3. (A) Mobility abundance plots obtained with a THFACN solvent mixture as the BGE containing (a) 5%, (b) 10%, and (c) 20% (v/v) of ACN. (B) Mobility plots obtained with a BGE solvent mixture of (a) THF-DMF (80:20), (b) THF-DMAC (50:50), and (c) THF-ACN (80:20).

separation of the neutral and (positively) charged asphaltene fractions is obtained and the relative amount of charged asphaltenes appears to increase. In Figure 3B, separations with different high-permittivity solvents in the mixtures were compared. ACN clearly gives the best separation. Still, the overall picture with all BGE compositions does not vary: in these solvents asphaltenes are partly present as neutral species and partly as positively charged species. Differences in the average mobility of the charged species are relatively limited and could be related to differences in ionization properties and/or viscosity of the solvents tested. Different salts, known to be relatively well soluble in organic solvents, were tested for the BGE: apart from lithium perchlorate these were lithium chloride, tetramethylammonium hexafluorophosphate, and tetramethylammonium tetrafluoroborate. No significant changes in the electropherograms were observed. The concentration of the salt (LiClO4) in the BGE was varied between 1 and 10 mM. With increasing salt concentration, a decrease of the average mobility of the charged asphaltene fraction was observed. This is in accordance with the ionic-strength effect in aqueous CE.28 No special precautions had been taken to keep the samples free of water. To test if the effective charge on a fraction of the asphaltenes was related to the presence of traces of water in the solvents, water was added to the BGE and the sample solutions at a concentration of 1% (v/v). The addition of water increased the conductivity of the BGE considerably and affected the long-term stability of the asphaltene solutions. In some water containing samples, precipitation of asphaltenes was observed after a few hours. Still, the electropherograms obtained after addition of water to the BGE and to the sample solutions were not significantly different from

D ¼

dc 2 tp 96 σt 2

ð1Þ

where dc is the diameter of the capillary. Equation 1 is valid only within certain boundary conditions, i.e., the time of the experiment should be long enough for adequate radial diffusion and longitudinal diffusion should be negligible.35 Calculations showed that in our experiments both conditions were met. Experiments were performed using a range of applied pressures from 5 to 30 mbar, and values for D were calculated. For toluene, the average value obtained for D was 1.50  10-9 m2 s-1. For the asphaltenes, an average value of 0.24  10-9 m2 s-1 was found in the THF-ACN mixture. Assuming that the diffusion coefficient scales with MW-0.5, the average molecular weight of the species as they exist in (32) Bello, M. S.; Rezzonico, R.; Righetti, P. G. Use of Taylo-Aris dispersion for measurement of a solute diffusion-coefficient in thin capillaries. Science 1994, 266 (5186), 773–776. (33) Sharma, U.; Gleason, N. J.; Carbeck, J. D. Diffusivity of solutes measured in glass capillaries using Taylor’s analysis of dispersion and a commercial CE instrument. Anal. Chem. 2005, 77 (3), 806–813. (34) Le Saux, T.; Cottet, H. Size-based characterization by the coupling of capillary electrophoresis to Taylor dispersion analysis. Anal. Chem. 2008, 80 (5), 1829–1832. (35) d’Orlye, F.; Varenne, A.; Gareil, P. Determination of nanoparticle diffusion coefficients by Taylor dispersion analysis using a capillary electrophoresis instrument. J. Chromatogr., A 2008, 1204 (2), 226–232.

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Figure 4. Peaks obtained in TDA analysis with an applied pressure of (A) 20 mbar and (B)10 mbar for asphaltenes (350 nm, black traces) and toluene (260 nm, gray traces).

Figure 5. Mobility abundance plots obtained with different asphaltene samples. Deposit (sample 1), SARA fractions of parent crude from deposit (sample 2), unstable crude (sample 3), and stable crude (sample 4). BGE: THF-ACN (80:20) containing 10 mM of LiClO4; applied voltage, þ30 kV.

the solvent mixture used in this study, can be estimated in the order of 3000-4000 Da. For charged species in solution, the mobility μ, charge number z, and diffusion coefficient D are related by μ ¼

ze D kT

heteroatom rings in the main structure of the (covalent) molecules, either present in the original samples or induced by traces of dissolved oxygen in the solvents after dilution. However, titration measurements by Barth et al. have indicated that the majority of asphaltenes species are nonbasic.37 Enrichment of acidic heteroatom species (acidic oxygen and SXOX) has been reported in asphaltene deposits generated in the laboratory with more advanced techniques.36 Analysis of the heteroatom ratios from the samples in Table 1 shows it is difficult to draw strong conclusions about differences between samples from stable crude oils versus those from unstable crude oils (in regards to asphaltene precipitation and deposition) in this small set of samples. No clear trends in O/N or O/S are visible, although the stable sample from field 3 has a clearly higher O/S than the samples from the unstable crudes. Alternatively, complexation with trace metals (nickel and or vanadium) could play a role. McKenna et al. have identified various vanadyl porphyrins in asphaltenes.38 Also, a recent review by Dechaine and Gray pointed out that metal species may associate with the heteroatom constituents of asphaltenes.39 From Table 1 it can be seen that the nickel/ vanadium ratio of unstable crudes (samples 2 and 3) is lower than the nickel/vanadium of a stable crude (sample 4). If it is assumed that most of the metal species in the crude oil are found in the asphaltene SARA fraction, then further speciation work is necessary for stronger conclusions in regards to the role of metal species on the NACE result. The NACE method as developed was applied to asphaltene samples of different origin and behavior as outlined in Table 1. Figure 5 shows abundance plots for four different asphaltene samples. The figure shows that asphaltenes from the unstable crude oils (2 and 3) contain one positively charged fraction, whereas the asphaltenes from the stable crude oil (4) contain an additional positively charged peak. The much higher mobility of this peak may indicate that the nanoaggregate

ð2Þ

where e is the electron charge and k the Boltzmann constant. For the charged fraction of the asphaltene deposit sample, an average mobility value of 8.3  10-9 m2 V-1 s-1 was found in a BGE of THF-ACN 80:20 with 1 mM salt. This would mean that the average effective charge of the charged species is þ0.89. Discussion The NACE studies of a field asphaltene deposit as reported here have clearly shown that the asphaltene field deposit consisted of two major fractions in dilute solutions: a smaller neutral fraction and a larger fraction which is positively charged. The optimum solvent for the NACE separations consisted of 80:20 mixtures of THF-ACN with LiClO4. However, similar results were obtained with other solvent mixtures and BGE salts, and addition of small amounts of water or acidic or basic compounds to the samples or the BGE did not change the results significantly. TDA experiments have shown that the species existing in the solvent mixture used have an average molecular weight on the order of 3000-4000 Da. In view of the latest insights in the molecular structure of asphaltenes, this implies that the asphaltene molecules are present as nanoaggregate clusters. Thus the molecular weights obtained here are not of individual asphaltene species but of the clusters formed in the solvent used. The charged fraction of these aggregates has a positive charge of approximately þ1. In one of the samples from a crude oil, also a fraction with a charge number of approximately þ2 was found (see Figure 5). An issue that still has to be addressed is the molecular nature of the difference between the charged and the neutral fractions. Several explanations are possible. The charge difference could for instance be related to the oxidation state of

(37) Barth, T.; Hoiland, S.; Fotland, P.; Askvik, K. M.; Myklebust, R.; Erstad, K. Relationship between the content of asphaltenes and bases in some crude oils. Energy Fuels 2005, 19 (4), 1624–1630. (38) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Identification of Vanadyl Porphyrins, in a Heavy Crude Oil and Raw Asphaltene by Atmospheric Pressure Photoionization Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry. Energy Fuels 2009, 23, 2122–2128. (39) Dechaine, G. P.; Gray, M. R. Chemistry and Association of Vanadium Compounds in Heavy Oil and Bitumen, and Implications for Their Selective Removal. Energy Fuels 2010, 24, 2795–2808.

(36) Juyal, P.; Yen, A. T.; Rodgers, R. P.; Allenson, S.; Wang, J. X.; Creek, J. Compositional Variations between Precipitated and Organic Solid Deposition Control (OSDC) Asphaltenes and the Effect of Inhibitors on Deposition by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry. Energy Fuels 2010, 24, 2320–2326.

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: DOI:10.1021/ef100921d

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clusters of this fraction are much smaller or are doubly charged. Although speculative at this moment it may point at the presence of a fraction in the asphaltenes that help to solubilize the overall asphaltenes in the stable crude oil, which is absent in the unstable crude oils. Comparing the field deposit (sample 1) to the asphaltenes from its parent crude oil (sample 2) shows that the field deposit is somewhat depleted in the neutral fraction and relatively enriched in the positively charged fraction. This first NACE study shows that asphaltenes are composed of neutral and positively charged fractions, and the results suggest that charge may play an important role in precipitation/aggregation and deposition. The differences in

asphaltene stability between the investigated crude oils and their affinity to form wellbore deposits may be explained by the relative distribution and abundance of the charged fractions in the asphaltenes. There were surprising similarities between samples from unstable crude oils in regards to charge distribution. The sample from a stable crude oil showed a different charge distribution. If such trends hold for a wider set of samples from different backgrounds, then potentially the NACE procedure described here could be used as an additional surveillance tool. Key to this development is the applicability of the technique directly on crude oils. This will be the focus of future work in our laboratories.

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