The Properties of Asphaltenes and Their Interaction with Amphiphiles

Energy & Fuels 2009, 23, 3625–3631

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The Properties of Asphaltenes and Their Interaction with Amphiphiles Jiqian Wang,*,†,‡ Chuan Li,† Longli Zhang,† Guohe Que,† and Zhaomin Li‡ State Key Laboratory of HeaVy Oil Processing, College of Chemistry and Chemical Engineering, China UniVersity of Petroleum, Qingdao 266555, China and College of Petroleum Engineering, China UniVersity of Petroleum, Qingdao 266555, China ReceiVed December 31, 2008. ReVised Manuscript ReceiVed May 1, 2009

The functional groups on asphaltene surfaces of two kinds of Chinese residue oil were analyzed by X-ray photoelectron spectroscopy (XPS). The ζ potential and electrophoretic mobility of asphaltene solutions and residue solutions were measured through phase analysis light scattering (PALS) technique. The ability to stabilize asphaltenes of two typical ionic amphiphiles, dodecyl benzene sulfonic acid (DBSA) and dodecyl trimethyl ammonium bromide (DTAB), were investigated. Karamay asphaltenes contain large amount of carboxyl and calcium and are negatively charged; whereas Lungu asphaltenes are rich in nickel, vanadium, and pyrrolic structures and are positively charged. DBSA has good ability to stabilize Lungu asphaltenes but has no effect on Karamay asphaltenes. Differently, DTAB has good ability to disperse Karamay asphaltenes but has no obvious effect on Lungu asphaltenes. It is concluded from these results that the charges might derive from the dissociation of metal ions and the deprotonation of acid groups (such as COOH, OH, and SH) or basic groups (such as pyridinic groups) on asphaltene surface. The electric property of asphaltenes plays an important role in the interaction between asphaltenes and amphiphiles. The negatively charged asphaltenes tend to be dispersed by cationic amphiphiles, whereas the positively charged asphaltenes tend to be dispersed by anionic amphiphiles.

1. Introduction The deposition of asphaltenes gives rise to many problems in oil recovery, transportation, and refining. For example, the deposition of asphaltene plugs the porous formation and changes the wettability of oil reservoir during oil production; the aggregation and deposition causes fouling, and thus plugging of pipelines and heat exchangers during transportation and refining. Researchers have studied dispersion and stability of asphaltenes for several decades. As early as the 1990s, Gonza´lez had investigated the dispersing of asphaltenes by various oilsoluble amphiphiles, including long alkyl chain benzenes, aliphatic alcohols, alkyl phenols, and primary aliphatic amine.1 Alkyl phenol shows good peptizing properties, and it prevents the precipitation of asphaltenes induced by light alkanes, such as heptane. Primary aliphatic amines show some ability to disperse asphaltenes, but long-chain benzenes and aliphatic alcohols show no asphaltene-dispersing ability. Lian studied the partial precipitation of asphaltenes in a fixed amount of aromatic hydrocarbon system (such as toluene), with gradual addition of paraffinic hydrocarbon (such as pentane) in the presence of various surfactants.2 These surfactants affect the asphaltene precipitation, either by acceleration (cetyl amide) or by retardation (nonyl phenol), depending on the structural types and quantities of the surfactants. The natural resin separated from the same oil serves as a good peptizing agent since the polar fractions of resin also contain surfactants (amphiphiles). Further study by Al-Shhaf shows that nonyl phenol (NP), DBSA, and * To whom correspondence should be addressed. Phone: +86-53286981562; fax: +86-532-86981569; e-mail: [email protected]. † College of Chemistry and Chemical Engineering. ‡ College of Petroleum Engineering. (1) Gonza´lez, G.; Middea, A. Colloids Surf. 1991, 52, 207–217. (2) Lian, H.; Lin, J.-R.; Yen, T. F. Fuel 1994, 73, 423–428.

dodecyl resorcinol (DR) are better than resin to retard asphaltene precipitation.3 Ramos chose some polymers as additives as well as sulfonic and carboxylic acids, phenols, and amides and tested their stabilization effect on asphaltenes. The polymer additives were block copolymers (styrene/butadiene, oxyethylene/oxypropylene), block copolymers with ionic groups (styrenesulfonic acid), and other polymers (hydroxyalkyl celluloses, siloxanes). Among the various chemical compounds tested for the inhibition of asphaltene precipitation in crude oils, nonionic surfactants, such as ethoxylated alcohols/phenols, showed the best performance. The results revealed that effective inhibitors would need to display a significant interaction with both oil and asphaltenes. That is to say, the solubility of additives in crude oil is important.4 Because vegetable oils, such as sweet almond, andiroba, coconut essential oil, and sandalwood essential oil, contain aliphatic acid, such as palmytic, linoleic, and caprylic acid, they can also be used as inhibitors of asphaltene precipitation, and they show fairly good effect and low economical cost. Further fraction of vegetable oils may improve their performance.5,6 Permsukarome and Fogler also studied the kinetics of asphaltene dissolution in amphiphile/alkane solution. The amphiphiles were DBSA and nonylphenol, and the five alkane solvents ranged from hexane and hexadecane. The dissolution rate increased steadily at low amphiphile concentrations and (3) Al-Shhaf, T A.; Fahim, M. A.; Elkilani, A. S. Fluid Phase Equil. 2002, 194-197, 1045–1057. (4) da Silva Ramos, A. C.; Haraguchi, L.; Notrispe, F. R.; Loh, W.; Mohamed, R. S. J. Pet. Sc. Eng. 2001, 32, 201–216. (5) Rocha Junior, L. C.; Ferreira, M. S.; Carlos da Silva Ramos, A. J. Pet. Sc. Eng. 2006, 51, 26–36. (6) Moreira, L. F. B.; Lucas, E. F.; Gonza´lez, G. J. Appl. Polym. Sci. 1999, 73, 29–34.

10.1021/ef801148y CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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Figure 1. Lungu asphaltenes XPS spectra and curve fitting results, A: carbon 1s, B: oxygen 1s, C: nitrogen 1s, and D: sulfur 2p.

reached a plateau at higher amphiphile concentrations when asphaltenes were completely dissolved. The dissolution rate follows the Arrhenius temperature dependence. On the basis of this result, it was deduced that one of the rate-controlled paces of asphaltene dissolution was the reactions involving the transition from asphaltene-asphaltene associations to asphaltene-amphiphiles associations through the redistribution of intermolecular hydrogen bonding and charger transfer interactions.7 Auflem studied the dispersive effects of amphiphiles and naphthenic acids on asphaltenes in heptane-toluene solution through near-IR spectroscopy. The method is based on the scattering from preferentially large aggregates. They deduced the same conclusion as former researchers: additives that are efficient in replacing hydrogen bonds possess dispersive power and can serve as inhibitors.8 According to Gonza´lez’s study, the most obvious mechanism of the peptizing process is the interaction between polar functional groups on asphaltene surface and polar head of amphiphiles to reduce asphatene polarity. The interaction may not be restricted to the polar groups, but may also include an association in which π-electron of aromatic structure of asphaltene may act as H-donors to form hydrogen bonds with hydroxyl groups of amphiphiles.1 Lian also suggested that nonyl phenol absorbed on the asphaltene surface through hydrogen bonds.2 Chang used a series of well-defined compounds to study the asphaltene-amphiphile interaction as well as the ability to disperse asphaltenes. The amphiphiles were alkylbenzenederived compounds with a long side-chain alkylbenzenes; and the structural differences included different polar head groups, different lengths of alkyl tails, and the existence of extra groups, including the side group or modified tail. Their results show that asphaltenes from crude oil can be stabilized by oil-soluble amphiphiles. Increasing the polarity of amphiphile’s headgroup (7) Permsukarome, P.; Chang, C.-L.; Fogler, H. S. Ind. Eng. Chem. Res. 1997, 36, 3960–3967. (8) Auflem, I. H.; Havre, T. E.; Sjo˜blom, J. Colloid Polymer Sci. 2002, 280, 695–700.

strengthens the attraction of amphiphiles to asphaltenes through acid-base interaction. Thus, the asphaltene dispersing ability is increased. Increasing the length of alkyl tail of amphiphile can improve its dispersing ability. But, if the alkyl tail is too long, its effectiveness to dispersing asphaltene is slightly decreased because of its affinity to asphaltenes is decreased. Amphiphiles with short tails cannot peptize asphaltenes by forming a steric stabilization layer but will codeposit with asphaltenes. Addition of another polar group to the amphiphile will increase its ability to stabilize asphaltenes; however, a polar group on the tail will decrease the stability of the amphiphile itself in the solution. Chang also used Fourier transform infrared (FTIR) spectroscopy and small-angle X-ray scattering (SAXS) to unveil the interaction between asphaltenes and two amphiphiles, p-alkylphenol and p-alkylbenzenesulfonic acid. The FTIR results verified the hydrogen bonds formed between asphaltenes and the two amphiphiles. The size change given by SAXS also proved the association of asphaltenes and amphiphiles.9,10 The matching of crude oil and amphiphiles is important to improve the colloidal stability of asphaltenes when the heptane flood experiments are conducted in oil instead of asphaltene solution. DBSA disperses asphaltenes effectively in low aromatic oil in terms of kinetics, whereas it is effective in aromatic oil in terms of equilibrium. NP exhibits its high inhibition efficiency (10%), in terms of the onset point on the stable oil.11 Asphaltenes from crude oil were fractionated into components of different polarities by Fogler. The fractions displayed significantly different rates of dissolution in a differential reactor using an amphiphile/alkane solution of 10 wt % dodecylbenzenesulfonic acid in heptane.12,13 The adsorption of amphiphilic (9) Chang, C.-L.; Fogler, H. S. Langmuir 1994, 10, 1749–1757. (10) Chang, C.-L.; Fogler, H. S. Langmuir 1994, 10, 1758–1766. (11) Ibrahim, H. H.; Idem, R. O. Energy Fuels 2004, 18, 1038–1048. (12) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, S. Ind. Eng. Chem. Res. 1999, 38, 964–972. (13) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14, 25–30.

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Figure 2. Karamy asphaltenes XPS spectra and curve fitting results, A: carbon 1s, B: oxygen 1s, and C: nitrogen 1s.

molecules is dependent not only on their functional groups, but also on the chemical structure (e.g., aromaticity and acidic groups) of the asphaltenes, that is to say, the acid-base interaction, or other kind of interaction, between amphiphiles and functional groups plays an important role. The differences of asphaltenes can be analyzed through element contents of carbon, hydrogen, sulfur, nitrogen, and metals, while the functional groups of asphaltenes surface can be characterized by X-ray photoelectron spectroscopy (XPS).14-17 Although electric charge and potential may represent important factors in the colloidal stability of crude oil and asphaltene dispersions, the nature (negative or positive), magnitude, and/ or the mechanism of formation of the charge on the surface of the asphaltene particles is still not well understood. Furthermore, the contribution of surface charge to asphaltene dispersions stability has not been experimentally substantiated so far. In aqueous solution, the nature of ζ potential of asphaltenes depends on the pH value. Asphaltenes in aqueous suspensions exhibit an isoelectric point (IEP) at a certain pH value. Dissociation of acid functional groups (most likely carboxylic, hydroxyl, and phenols groups, or SH groups associated to mercaptans or organic sulfides) is responsible for the negative ζ potential of asphaltenes above the pHIEP, whereas protonation of nitrogen-containing basic surface groups results in positive ζ potentials below pHIEP. Both a cationic and an anionic surfactant are found to adsorb specifically onto asphaltenes through electrostatic and/or hydrophobic interactions and to reverse the sign of the ζ potential under appropriate conditions. Because the electrical charge of asphaltenes can be modified (14) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227–1234. ¨ stlund, J. A.; Nyda´n, M.; Fogler, H. S.; Holmberg, K. Colloids (15) O Surf., A 2004, 234, 95–102. (16) Zhao, S.; Xu, Z.; Xu, C.; Chung, K. H.; Wang, R. Fuel 2005, 84, 635–645. (17) Larachi, F.; Dehkissia, S.; Adnot, A; Chornet, E. Energy Fuels 2004, 18, 1744–1756.

with these surfactants, it might be potential candidates to control the stability of crude oil dispersions.18-26 The charges of asphaltenes in organic solvent are more complex. Most of the researchers have found that the sign of asphaltene charge is positive in organic solvent such like heptane, toluene, ethanol, and nitromethane.27-31 The origin of the electric charges is a consequence of an electron transfer between the organic solid particles and the liquid organic phase. By hydroviscoreduction, both the electric charges and the resistance against flocculation were increased.32 The sign of the charges may also affect the adsorption of various amphiphiles on asphaltenes in organic solutions. The addition of DBSA decreased the electrophoretic mobility of asphaltene particles in ethanol.31,36 (18) Abraham, T.; Christendat, D.; Karan, K.; Xu, Z.; Masliyah, J. Ind. Eng. Chem. Res. 2002, 41, 2170–2177. (19) Parra-Barraza, H.; Herna´ndez-Montiel, D.; Lizardi, J.; Herna´ndez, J.; Urbina, R. H.; Valdez, M. A. Fuel 2003, 82, 869–874. (20) Jada, A.; Salou, M. J. Pet. Sci. Eng. 2002, 33, 185–193. (21) Jada, A.; Salou, M.; Siffert, B. Pet. Sci. Technol. 2001, 19, 119– 127. (22) Jada, A.; Ait Chaou, A.; Bertrand, Y.; Moreau, O. Fuel 2002, 81, 1227–1232. (23) Jada, A.; Ait Chaou, A. J. Pet. Sc. Eng. 2003, 39, 287–296. (24) Jada, A.; Ait Chaou, A. Fuel 2002, 81, 1669–1678. (25) Gonza´lez, G.; Neves, G. B. M.; Saraiva, S. M.; Lucas, E. F.; de Sousa, M. A. Energy Fuels 2003, 17, 879–886. (26) Salou, M.; Siffert, B.; Jada, A. Fuel 1998, 77, 343–346. (27) Wright, J. R.; Minesinger, R. R. J. Colloid Sci. 1963, 18, 223– 236. (28) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Colloids Surf., A 1995, 94, 253–265. (29) Morrison, I. D. Colloids Surf., A 1993, 71, 1–37. (30) Siffert, B.; Rageul, P.; Papirer, E. Fuel 1996, 75, 1625–1628. (31) Leo´n, O.; Rogel, E.; Torres, G.; Lucas, A. Pet. Sci. Technol. 2000, 18, 913–927. (32) Siffert, B.; Kuczinski, J.; Papirer, E. J. Colloid Interface Sci. 1990, 135, 107–117. (33) Alboudwarej, H.; Jakher, R. K.; Svrcek, W. Y.; Yarranton, H. W. Pet. Sci. Technol. 2004, 22, 647–664. (34) Dubey, S. T.; Waxman, M. H. SPE ReserVoir Eng. 1991, 6, 389– 395. (35) Grijalva-Monteverde, H.; Arellano-Ta´nori, O. V.; Valdez, M. A. Energy Fuels 2005, 19, 2416–2422.

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In this article, we studied two kind of heptane asphaltenes separated from Chinese crude oil. One is Karamay oil, and the other is Lungu crude oil. Both of them originated from Xinjiang oilfield, but their properties are totally different. The element contents of the asphaltenes were analyzed. XPS was employed to investigate if there were any distinct deviations in the content of functional groups between the two kinds of asphaltenes. The ζ potential and electrophoretic mobility of asphaltenes in organic solvents were measured through a Zeta PALS instrument (Brookhaven Inc.). Moreover, two amphiphilic molecules, DBSA and DTAB, were studied with respect to their asphaltene stabilizing ability. Few researchers have studied the interaction between asphaltenes and cationic amphiphiles, such as alkyl quaternary ammonium salt, in organic solution. The aim was to try to unveil the interaction mechanism between asphaltenes and amphiphiles. 2. Experimental Section 2.1. Materials. Karamay and Lungu residue were from the distillation unit of refinery plants. The maximum temperature of the distillation tower is lower than 350 °C. Asphaltenes in the residues have not cracked under this temperature. DBSA and DTAB were purchased from Tokyo Chemical Industry Co. Ltd. (TCI), and their purity was >99%.The amphiphiles were used as received without further purification. All solvents used in this study were from Sinopharm Chemical Reagent Co. Ltd. (SCRC). They were in analytic pure grade and were used directly, except for those in the ζ potential measurement. The solvents used for ζ potential measurement were dehydrated by zeolites. 2.2. Preparation of Asphaltenes and Asphaltene Stock Solution. Asphaltenes were precipitated from residue oil by gently mixing oil and n-heptane at the solvent-to-oil ratio of 30 cm3/g. Then the mixture was stirred at 70 °C for 1 h to ensure sufficient mixing, and it was kept at room temperature for 4 h in dark. The mixture was then filtrated through a microfilter (pore diameter 0.22 µm) to separate the asphaltenes from the diluted residue oil. The filter cake was extracted with n-heptane by a Soxhlet extractor to remove the soluble components completely and dried to constant weight. The dried asphaltenes were dissolved in toluene with the concentration of 1 wt % and filtered through 0.22 µm microfilter. The stock solution was allowed to equilibrate with constant stirring for 48 h before using. 2.3. XPS Analyses. XPS technique provides information about elemental composition (types of atoms and how they are bonded to each other) in the outer layer of a material. The technique was used in this study for investigating functional groups on asphaltene surface. All XPS spectra were recorded on a Perkin-Elmer Physics Electronics PHI5300 Multi Technique System by the use of Mg KR (1253.6 eV) radiation. The pressure in the vacuum chamber was 1.0 × 10-8 Pa during the experiments. The resolution was 0.8 eV, the sensitivity was 80 Kcps, and the angle resolution was 5-90°. The data were processed through XPS Peaks 4.1 software supplied by Hong Kong University. Binding energy was calibrated according to carbon 1s binding energy at 284.6 eV to eliminated charging effect. 2.4. ζ Potential Measurement. The oil was diluted by toluene or 1:1 (V:V) toluene/heptane at the ratio of 500:1 (mL/g) to ensure the solution was transparent to the laser. The amphiphile content in residue oil was 0.1 wt %. The concentration of asphaltene solution was 200 µg/g. ζ potential and electrophoretic mobility were measured using a Brookhaven Zeta PALS apparatus. It utilizes phase analysis light scattering (PALS) to determine the electrophoretic mobility of charged particles. The advantage of the PALS technique is that it does not require the application of high potential electric fields that may result in thermal problems since the particle (36) Neves, G. B. M.; de Sousa, M. A.; Travalloni-Louvisse, A. M.; Lucas, E. F.; Gonz´alez, G. Pet. Sci. Technol. 2001, 19, 35–43. (37) Goual, L.; Firoozabadi, A. AIChE J. 2004, 50, 470–479.

Wang et al. Table 1. The Elemental Analysis of Asphaltenesa

a

element content

KM asphaltenes

LG asphaltenes

carbon/wt % hydrogen/wt % nitrogen/wt % sulfur/wt % nickel/µg/g vanadium/µg/g calcium/µg/g oxygen/wt %

84.44 8.49 1.75 0.82 36 1 4377 4.06

83.99 7.31 1.41 4.38 162 1301 220 2.74

LG: Lungu asphaltenes, KM: Karamay asphaltenes.

needs to move only a fraction of its own diameter to yield good results in the measurement of phase change. The velocity of the charged particle is measured and the electrophoretic mobility is determined by dividing the measured velocity by the electric field strength. At least six consecutive measurements were taken for each sample and averaged. The laser used in the experiment had the wavelength of 660 nm. The voltage between to electrodes was 200 V with a frequency of 2.00 Hz, and the grad of the electrical field was 540 V/cm. Particle size analysis was conducted using the same instrument as ζ potential. Particles suspended in a liquid are subject to Brownian motion. Small particles diffuse “faster”, whereas large particles diffuse “slower”. The time variation of the scattered intensity was analyzed by examining their autocorrelation. From this, a diffusion coefficient can be derived. The particle size and polydispersity number were calculated from the determined diffusion coefficient. All the samples were measured at 25 °C. 2.5. Preparation of Amphiphiles Solution. DBSA was dissolved in heptane, and DTAB was dissolved in ethanol to make amphiphile stock solutions with the concentration of 5 g/L and 20 g/L. 2.6. Stabilization Effects of Amphiphiles to Asphaltene. Samples were prepared by taking 5 mL asphaltene stock solution and adding 5 mL of n-heptane (DBSA), or ethanol (DTAB) solutions that contained known concentrations of amphiphilic molecules and diluting the mixture with 40 mL of n-heptane. The solution then consisted of 0.1 wt % asphaltenes in a mixture of n-heptane and toluene with a ratio of 90/10 (v/v) (DBSA), or n-heptane, toluene, and ethanol with a ratio of 80/10/10 (v/v/v) (DTAB). The sample was stirred constantly for 4 h and then filtrated with a microfilter (pore diameter 0.22 µm). The filtrate was withdrawn and analyzed by the use of a Cary 50 UV-vis spectrophotometer at the wavelength of 450 nm, where the concentration of asphaltene was usually measured.33,34 To determine asphaltene concentrarion from the UV-vis absorption spectra, the absorbance was calibrated to a series of asphaltene solutions with known concentration. The concentrations ranged from 25 to 300 mg/L with the interval of 25 mg/L. To avoid error, we used the same kind of asphaltene in the original calibration to determine the unknown asphaltene concentration. The increased percentage of asphaltene concentration with the addition of amphiphile was defined as dispersing index (DI), that is, DI ) (Ca - C0)/Ca. In the formula, Ca means asphaltene concentration with amphiphile, and C0 means asphaltene concentration without amphiphile.

3. Results and Discussion 3.1. Surface Functional Groups of Asphaltenes. The result of asphaltene elemental analysis is shown in Table 1. Both of the two kinds of asphaltenes have heteroatoms, such as nitrogen, sulfur, oxygen, nickel, and vanadium, besides carbon and hydrogen. Lungu asphaltenes are rich in sulfur and vanadium, whereas Karamay asphaltenes are abundant in calcium and oxygen. XPS analyses were performed to study surface functional groups of Lungu asphaltenes and Karamay asphaltenes. The spectra of carbon and heteroatoms of two asphaltenes are shown in Figures 1 and 2, respectively. XPS technique provides

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Table 2. Lungu Asphaltenes XPS Peak Curve Resolution Results peak

bind energy (ev)

functional group

fwhm (eV)

area

area %

carbon1 carbon2 oxygen1 oxygen2 nitrogen1 sulfur1 sulfur2

284.5 286.2 532.6 531.3 400.2 163.3 164.3

C-C,C-H C-O,CdO CdO C-O pyrrolic aliphatic thiophenic

1.47 1.27 2.00 1.62 5.08 1.512 1.610

9480 703 2404 994 664 182.2 144.8

93.10 6.90 70.74 29.26 100 55.72 44.28

information about the elemental composition on the surface of asphaltenes. The fitted subpeaks represent atoms in different functional groups. In Figure 1A, the carbon 1s (C 1s) spectrum of Lungu asphaltenes is divided into two subpeaks. Peak 1 at 284.5 eV corresponds to carbons bonded with carbon and hydrogen, and peak 2 at 286.2 eV corresponds to carbon bonded with oxygen. The oxygen 1s (O 1s) spectrum is also divided into two subpeaks (Figure 1B). Peak 1 at 532.6 eV corresponds to oxygen atoms in the type of -CdO, such as carbonyl and lactone, and peak 2 at 531.3 eV is attributed to hydroxyl and etheric oxygens. The relative contents of each kind of functional groups could be calculated through the area of subpeaks, and the results are shown in Table 2. More than two-thirds of oxygen atoms are in the types of carbonyl and lactone. The result of nitrogen 1s (N 1s) of Lungu asphaltenes (Figure 1C) shows that most of nitrogen compounds are of pyrrolic type. As we know, vanadium and nickel in asphaltenes are associated with porphyrin rings with the basic structure of four pyrrolic rings connected by methane. Because Lungu asphaltenes contain vanadium and nickel of 1301 and 162 µg/g respectively, most nitrogen compounds are in the pyrrolic rings of porphyrin structure. Figure 1D shows the spectrum of sulfuric species. The resolved curves show that there are thiophenic and aliphatic sulfuric compounds in Lungu asphaltenes. Figure 2 shows the XPS spectra of Karamay asphaltenes. Similar to the case of Lungu asphaltenes, the C 1s spectrum of Karamay asphaltenes can also be resolved as -CH2 and -C-C (284.6 eV) and -CdO (286.2 eV). However, the O 1s spectrum (Figure 2B) shows three subpeaks, corresponding to carboxylic oxygen (533.3 eV), carbonyl or lactone (532.2 eV), and hydroxyl or etheric oxygen (531.3 eV), respectively. The N 1s spectrum is divided into two peaks, pyridinic nitrogen (398.5 eV) and pyrrolic nitrogen (400.2 eV). All the curve-resolution results are listed in Table 3. Because most of calcium compounds in residue oil are calcium naphthenates, and Karamay asphaltenes contain calcium of more than 4000 µg/g, more than 1/3 of

Figure 3. Dispersing ability of CTAB and DBSA at different concentrations. LG Asp: Lungu asphaltenes, KM Asp: Karamay asphaltenes.

oxygen atoms in Karamay asphaltenes are carboxylic. It is the main difference between Lungu asphaltenes and Karamay asphaltenes. 3.2. Electrophoretic Mobility and ζ Potential of Asphaltenes. The ζ potential and electrophoretic mobility of Karamay residue, Lungu residue, and their asphaltenes were measued in toluene/heptane (1:1); the results are listed in Table 4. The charges and electophoretic mobility of Karamay asphaltenes are negative, whereas those of Lungu asphaltenes are positive. The difference of charge sign between two asphaltenes was attributed to their chemical composition. The origin of the charges on surface of asphaltene is still not well understood in nonaqueous solution. The main opinion accepted by most researchers is an ion adsorption and ion dissociation mechanism. The electrical charges are stabilized against neutralization by being held in large structures such as micelles or complex macroions.25,31 The sign and magnitude of charges on asphaltene could be affected by three factors, including the dissociated ions, the acid functional groups, and the basic functional groups. The dissociation of charged ions caused opposite surface charges on asphaltenes. The basic functional groups, such as pyridinic groups, produce negative charges in heptane/toluene solution. Hydroxyl associated to carboxylic acids or alcohols and phenols and mercapto groups associated to mercaptans or organic sulfides may be considered as the acid groups and produce positive charges in organic solution.25 Because Karamay asphaltenes are rich in calcium naphthenates, the sign of charges on the asphaltene surface is negative after the dissociation of Ca2+. The basic pyridinic groups also produce negative charges. Although the OH and SH groups produce positive charges, the main factor is the dissociation of Ca2+. The negative charges of particles of Karamay asphaltenes indicate that negatively charged surface groups predominate over the positive groups. Lungu asphaltenes are rich in nickel and vanadium. Ni2+ and VdO2+ in asphaltenes exist in porphyrin compounds, which are rather stable, and the positively charged metal ions, Ni2+ and VdO2+, do not tend to dissociation like that of Ca2+ in naphthenates. According to the XPS results, Lungu asphaltenes have many hydroxyl and mercapto groups, which also produce positive charges in nonaqueous solution. Therefore, the particles of Lungu asphaltenes show positive charges in heptane/toluene solution. The results of residue oil solution show that the adsorption of resin does not change the sign of charges, but increases ζ potential value and electrophoretic mobility velocity. The mean radius of asphaltene particles in residue solution is much smaller than that of asphaltene solution. Also, the polydispersity of asphaltene particles in residue solution is larger than that of an asphaltene solution (Table 4). The difference of radius and polydispersity between residue solution and asphaltene solution proved that resin worked as dispersing agents. With the existence of resin, the Karamay asphaltene particles are more polydispersed (polydispersity parameter is 4.954) with the mean radius of 39.8 nm, whereas in the Karamay asphaltene solution without resin, the asphaltene particles are less polydispersed (polydispersity parameter was 1.198) with the mean radius of 608 nm. Lungu asphaltenes have the same tendency with Karamay asphaltenes. They have the mean radius of 180.6 nm compared to 750.2 nm in asphaltene solution, and the polydispersity parameter is 0.005-1.584. 3.3. Interaction between Asphaltenes and Amphiphiles. The eletrophoretic mobility and ζ potential of residue oil solution with amphiphiles were measured, and the results are shown in Table 4. Both DBSA and DTAB did not change the sign of

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Table 3. Data of Karamay Asphaltenes XPS Carbon 1s Peak Curve Resolution peak

bind energy (eV)

functional group

fwhm (eV)

area

area %

carbon1 carbon2 oxygen1 oxygen2 oxygen3 nitrogen1 nitrogen2

284.6 286.2 533.2 532.2 531.3 400.2 398.5

C-C, C-H C-O, CdO, carboxylic carboxylic CdO C-O pyrrolic pyridinic

1.44 1.01 1.55 1.38 1.79 2.98 3.33

10916 603 1381 1149 1296 518 432

94.76 5.23 36.10 30.03 33.87 54.53 45.47

Table 4. Electric Characteristics of Colloidal Particles of KM Residue and LG Residue with Amphiphiles sample Karamay asphaltenes Karamay residue Karamay residue + DBSA Karamay residue + DTAB Lungu asphaltenes diluted Lungu residue diluted Lungu residue + DBSA diluted Lungu residue + DTAB a

u/(µ/s)/(V/cm)a -2

-2.48 × 10 -4.50 × 10-2 -1.18 × 10-2 -9.06 × 10-3 1.17 × 10-2 1.20 × 10-2 -1.17 × 10-3 -4.10 × 10-3

ξ/mVb

R/nmc

polydispersity

-9.83 -17.89 -4.68 -3.6 4.65 4.78 -0.47 -1.63

608 39.8 41.6 20.1 750.2 180.6 86.3 206.0

1.198 4.954 5.586 5.600 0.005 1.584 2.956 1.675

u: electrophoretic mobility. b ξ: zeta potential. c R: mean radii.

particle charge in Karamay residue solution, but decreased the value of electrophoretic mobility and ζ potential. In Lungu residue solution, DBSA and DTAB decreased the value of electrophoretic mobility and ζ potential, and the sign of asphaltene surface charge has been reversed to negative. Because the electrical behavior of amphiphilic molecules has not been clearly understood in nonaqueous solution, the interaction between amphiphiles and asphaltene particles could not be elucidated only through electrophoretic mobility and ζ potential. The dispersing ability and changes of mean radius and polydispersity would be helpful to understand the interaction between amphiphiles and asphaltene particles. The mean radius of Karamay asphaltene particles changed from 39.8 to 20.1 nm after the addition of DTAB, but from 39.8 to 41.6 nm after the addition of DBSA. Differently, the mean radius of Lungu asphaltene particles changed from 180.6 to 86.3 nm after the addition of DBSA, but from 180.6 to 206.0 nm after the addition of DTAB. The increase of polydispersity parameter and the decrease of mean radius indicate that original asphaltene particles have been separated into small particles with different radii. The DI of DBSA and DTAB also proved the point (Figure 3). DATB improves the dispersion of Karamay asphaltenes, whereas it has no obvious effect on the dispersion of Lungu asphaltnenes. Differently, DBSA improves the dispersion of Lungu asphaltenes but decreases the dispersion of Karamay asphaltene. In Leon’s study, the adsorption of DBSA decreased the electrophoretic mobility, which suggested that the surface charges were neutralized by DBSA.31 The eletrophoretic mobility and ζ potential also decreased in Karamay residue solution with the addition of DTAB and in Lungu residue solution with DBSA. As we know from the DI, mean radius, and polydispersity, DTAB increases the dispersion of Karamay asphaltenes, and DBSA increases the dispersion of Lungu asphaltenes. It could be deduced that DTAB and DBSA must have adsorbed on corresponding asphaltenes through acid-base interaction or electrostatic interaction. The surface charges on asphaltenes were partially neutralized by absorbed amphiphiles. Figure 3 shows that DTAB has no effect on Lungu asphaltenes dispersion, and DBSA even decreases the dispersion of Karamay asphaltenes to a certain extent. It indicates that DTAB and DBSA do not absorb on asphaltenes or the amount of absorbed amphiphiles is not large enough to disperse asphaltenes. The unabsorbed amphiphilic molecules do change electric

charges of the residue solution although their performance is not clearly understood. DBSA is a good dispersing agent to Lungu asphaltenes in our experiments; however, it would be effective only if beyond a certain concentration. When the concentration is too low, it shows a retrograde effect and the asphaltenes aggregate. Chang9,10 and Goual37 also reported this phenomenon. Chang gave his explanation: DBSA molecules donate their protons to the asphaltenes’ basic groups and/or CdC bonds, therefore, irreversibly attaching to asphaltene surfaces. The strong binding of DBSA to asphaltenes makes DBSA effective enough to stabilize asphaltenes sterically. On the other hand, DBSA amphiphiles have a strong tendency to self-associate in nonpolar medium. This association takes place between the DBSA molecules attached on different asphaltene particles, asphaltenes then flocculate and precipitate out if the amount of DBSA is too low to provide with sufficient steric stabilization.

Figure 4. Possible mechanism of the dispersion of asphaltene particles by amphiphiles. Red indicates that asphaltenes (Karamay) and amphiphile (DBSA) are negatively charged, and blue indicates that asphaltenes (Lungu) and amphiphile (DTAB) are positively charged.

Interactions Betweem Asphaltenes and Amphiphiles

Although the interaction between asphaltenes and amphiphiles has not been exactly understood at the molecular level, the electric property of asphaltenes plays an important role in the process. The negatively charged asphaltenes tend to be dispersed by traditional cationic amphiphiles, whereas the positively charged asphaltenes tend to be dispersed by anionic amphiphiles.

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the dissociation of acid groups (OH and SH) or basic groups (such as pyridinic groups) on asphaltene surface. The electric property of asphaltenes plays an important role in the interaction between asphaltenes and amphiphiles. The negatively charged asphaltenes tend to be dispersed by traditional cationic amphiphiles, while the positively charged asphaltenes tend to be dispersed by anionic amphiphiles.

4. Conclusion By elemental analysis and XPS analysis, difference is detected between Karamay asphaltenes and Lungu asphaltenes; Karamay asphaltenes are rich in carboxyl and calcium, whereas Lungu asphaltenes are rich in nickel and vanadium and pyrrolic structures. The ζ potential and electrophoretic mobility show that Karamay asphaltenes are negatively charged whereas Lungu asphaltenes are positively charged in toluene/heptane solution. The charges might derive from the metal ion dissociation and

Acknowledgment. The authors thank for the funding of National Natural Science Foundation of China (No. 20506017 and No. 20776160) and China Postdoctoral Science Foundation (No. 20070421098). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. EF801148Y