Energy Fuels 2009, 23, 4584–4592 Published on Web 08/19/2009
: DOI:10.1021/ef9004175
Aggregation Behavior of Heavy Crude Oil-Ionic Liquids Solutions by Fluorescence Spectroscopy )
J. Alberto Murillo-Hern andez,† Isidoro Garcı´ a-Cruz,‡ Sim on L opez-Ramı´ rez,‡ C. Duran-Valencia,§ ‡ J. Manuel Domı´ nguez, and Jorge Aburto*, Programa Acad emico de Posgrado, ‡Programa de Ingenierı´a Molecular, §Programa de Recuperaci on de Hidrocarburos, and Programa de Procesos de Transformaci on, Instituto Mexicano del Petr oleo, Eje Central L azaro C ardenas Norte 152, Col. San Bartolo Atepehuacan, M exico D. F. 07730, M exico )
†
Received May 6, 2009. Revised Manuscript Received July 27, 2009
Asphaltene aggregation is a two-step process concerning phase separation and asphaltene particle growth which provoke crude oil destabilization and significant problems during the production, transport, and refining of heavy and extra heavy crude oils. A recent and innovative approach to overcome this problem is the use of ionic liquids (ILs) as inhibitors or stabilizers of asphaltene aggregation. Since the information concerning the properties of the studied ILs is scarce, we characterized some of their electronic properties and critical aggregation concentration (CAC) by quantum chemistry and spectrofluorometry, respectively. We found that the presence of a complex anion such as [AlCl4]-, [BF4]-, and [PF6]- incremented the HOMO-LUMO gap (ΔH-L), electronegativity (χ), absolute hardness (η), and dipole moment (μ) when compared to [Br]--containing ILs. Moreover, the ILs’ CAC values showed a linear correlation with the dipole moment. Afterward, we studied the effect of various commercial ILs on the aggregation point (AP) of a heavy crude oil (HCO) due to the increment of (a) its concentration in toluene solutions or (b) the n-heptane volume by means of fluorescence spectroscopy. We have found that the aggregation of HCO occurs at larger crude oil concentration or n-heptane volume in the presence of some ILs. Here, ILs set a polar microenvironment around HCO asphaltenes, which stabilized them against further aggregation and precipitation. The better performance of ILs as inhibitors or stabilizers of asphaltene aggregation was found with those comporting a complex anion, a pyridinium ring, or a shorter alkyl substitution on the cation. Such ILs present the higher values of the calculated electronic properties. them, already present in the crude oil.10-12 Most recently, Goual and Firoozabadi13 suggested that both asphaltene and resins molecules are polar and associated as micelles. In fact, asphaltenes and resins coexist in a petroleum fluid and may be found in the form of monomers or associated as micelles. In the latter form, the micellar core is formed by the selfassociation of asphaltene molecules with adsorbed resins at the surface to form a shell that also contains an oil fraction. Indeed, resins are essential in asphaltene aggregation because they attach to asphaltene micelles through their polar heads and hence stretch their aliphatic groups outward to form a steric-stabilization layer surrounding asphaltene molecules. The formation and properties of such micelles is governed by the relative concentration of asphaltenes and resins. When the resins are desorbed from the micellar core surface, they give rise to the asphaltene phase.14,15 Hence, resins are the natural solvent of asphaltenes. In addition, asphaltenes present in crude oil are polycyclic, rigid molecules with π bonds which emit light as fluorescence when they are exposed to ultraviolet or X-ray radiation.16 Pietraru and Cramb17 carried out a research about the
Introduction Asphaltene aggregation represents a very serious and constant problem in the oil industry with an enormous economic impact. This is because asphaltene aggregation occurs spontaneously in oil wells, provoking the formation of an extremely dense phase that prevents oil extraction and in many cases completely stops the oil production.1-8 Asphaltene molecular structure is largely unknown, but a proposed model includes polyaromatic condensed rings with short aliphatic chains and polar heteroatoms such as nitrogen, oxygen, and sulfur.9 The stability of asphaltenes in crude oil is due to the presence of some neutral polar substances, resins among *Corresponding author. Tel.: þ52 55 9175 8204. Fax: þ52 55 9175 8429. E-mail address:
[email protected]. (1) Speight, J. G. In The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker Inc.: New York, 1999; Chapter 11. (2) Murgich, J.; Rogel, E.; Le on, O.; Isea, R. Pet. Sci. Technol. 2001, 19, 436. (3) Murgich, J.; Abanero, J. A. Energy Fuels 1998, 12, 239. (4) Mansoori, G. J. Pet. Sci. Technol. Eng. 1997, 17, 101. (5) Buerrostro-Gonzalez, E.; Espinoza-Pe~ na, M.; Andersen, S. I.; Lira-Galeana, C. Pet. Sci. Technol. 2001, 19, 299. (6) Carbognani, L.; Orea, M.; Fonseca, F. Energy Fuels 1999, 13, 351. (7) Oh, K.; Deo, M. D. Energy Fuels 2002, 16, 694. (8) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (9) Nalwaya, V.; Tangtayakom, V.; Piumsomboon, P.; Fogler, H. S. Ind. Eng. Chem. Res. 1999, 38, 964. (10) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 17749. (11) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 17758. (12) Scotti, R.; Montanari, L. In Asphaltenes, Fundamentals and Applications, 1st ed.; Plenum Press: New York, 2001. r 2009 American Chemical Society
(13) Goual, L.; Firoozabadi, A. AIChE 2002, 48, 2646. (14) Garcı´ a-Cruz, I.; Martı´ nez-Magadan, J. M.; Salcedo, R.; Illas, F. Energy Fuels 2005, 19, 998. (15) Pacheco-Sanchez, J. H.; Alvarez-Ramı´ rez, F.; Martı´ nezMagadan, J. M. Energy Fuels 2004, 18, 1676. (16) Rouessac, F.; Rouessac, A. In An alisis Quı´micos. M etodos y T ecnicas Instrumentales Modernas, 1st ed.; McGraw-Hill: Madrid, 2003; Chapter 11. (17) Pietraru, G. M.; Cramb, D. T. Langmuir 2003, 19, 1026.
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aggregation and precipitation of asphaltenes in a toluene and o-chlorinebenzene solution through fluorescence. They observed that the polarity increased with asphaltene concentration. They found a lineal correlation for low asphaltenes concentration (0.2-1.5 g L-1) and a bathochromic shift. Goncalves et al.18 analyzed the asphaltene aggregation process of two types of Venezuelan crude oil by means of fluorescence. The first oil presenting precipitation problems and the second one without. They identified two distinctive bands in the spectra (530 and 564 nm), which were attributed to the different structures of fluorescent compounds in asphaltenes. They realized that a bathochromic shift takes place when increasing asphaltene concentration and that the crude oil with asphaltene precipitation problems starts the aggregation process with a lower concentration than the sample without problems. Ghosh et al.19 studied the effect of the asphaltene concentration over the aggregation of an asphaltene sample (Barari, India) in different solvents, such as benzene, toluene, and carbon tetrachloride through fluorescence. They also noticed a bathochromic shift of the spectra when increasing the asphaltene concentration. In addition, it was concluded that the aggregation process was gradual and it depended on the asphaltene concentration in the solution, regardless of the solvent used in their study. On measurements of depolarization fluorescence, Groenzin and Mulins20,21 found a strong correlation between the size of an individual fused ring system in an asphaltene molecule and the overall size of the corresponding molecule, showing that asphaltene molecules have one or two fused ring systems per molecule. In this case, the asphaltene fluorescence emission is significant in the range of 400-600 nm. Here, the HOMO-LUMO gap increased with number of rings on the aromatic core. They also found that asphaltene micelles with smaller molecular weights possess a lower energy barrier to break apart, which may also have important operational implications. The term ionic liquids (ILs) have been coined in recent years to describe a class of organic salts that are liquid in their pure state at or near room temperature. Some of the more widely studied ILs are heterocyclic cations, based on a substituted pyridine or imidazole ring plus an inorganic anion.22 When an ionic liquid is used to replace classical organic solvents, it offers a new environmentally benign approach toward modern chemical processes. Additionally, the implementation of task-specific ILs further enhances their versatility for the cases where both reagent and medium are coupled. The increased interest in ILs by chemists and technologists is clearly due to the utility of ionic liquids as solvents for different chemical reactions, including catalytic reactions and the characterization of solvation interactions.23,24 On the subject of ILs application to the asphaltene problems, Hu et al.25 studied for the first time the dissolution of asphaltenes in ILs. They mixed asphaltene samples of Shengli crude oil with ILs and heated them up to different temperatures (50, 80, 135, and 150 °C) in order to determine their
Table 1. Properties and Composition of a Heavy Mexican Crude Oil Physical Properties at 25 °C molecular mass (g/mol) °API density (g/cm) viscosity (cP) interfacial tension (dyn/cm) water content (%)
486 11.60 0.9859 53028 18.24 0.05
chemical composition
(% w/w)
carbon hydrogen nitrogen oxygen sulfur
84.28 10.28 0.41 0.01 5.02
SARA composition
(% w/w)
saturates aromatics resins asphaltenes
7.94 5.28 70.93 15.85
solubility. It was observed that the ILs based on pyridinium cations can dissolve asphaltenes better than imidazolium cations. Also, the ILs capacity to dissolve asphaltenes decreases with the length of the alkyl chain. The effect of the anion over the capacity to dissolve asphaltenes was found to be proportional to its size and charge density. Furthermore, Hu and Guo26 studied the effect of ILs in the inhibition of asphaltene precipitation. They used a high-pressure cell where the crude oil and the ILs were mixed and CO2 was injected as an asphaltene precipitation agent. Here, the pyridinium-based ILs capacity to inhibit asphaltene precipitation increased with the alkyl chain length and was attributed to the charge density delocalization between the cation and anion. The aim of this work is to ascertain the effect of ionic liquids on the aggregation point (AP) of a heavy mexican crude oil (HCO), by varying the concentration of crude oil or n-heptane as asphaltene precipitant by means of fluorescence spectroscopy of ILs-HCO solutions. Moreover, we obtained the electronic structure properties and critical aggregation concentration (CAC) of the ILs by quantum chemical techniques and spectrofluorometry, respectively. Experimental Method Materials. We used a heavy crude oil (HCO), from an oil field located in Southern Mexico, and its properties are summarized in Table 1. Imidazolium- and pyridinium-based ionic liquids (ILs) over 99% purity were provided by Sigma-Aldrich, Mexico, and are listed in Table 2. Prior to use, ILs were dissolved in acetonitrile to ensure a homogeneous dispersion in toluene-rich solutions containing HCO. HPLC-grade solvents such as acetonitrile, n-heptane, and toluene were acquired from Techrom and Sigma-Aldrich, Mexico. Computational Details. An accurate structural study of different ILs has been carried out using density functional theory (DFT) by quantum chemical techniques. In this part of the study, we want to obtain actual values of different indices from reactivity, such as the HOMO-LUMO gap (ΔH-L), electronegativity (χ), absolute hardness (η), and dipole moment (μ), with the clear objective to understand how the ionic liquids can dissolve asphaltenes present in heavy crude oil. The minimum-energy geometries of the ILs used in the present work were determined by ab initio geometry optimizations at the
(18) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Fuel 2004, 83, 1823. (19) Ghosh, A. K.; Srivastava, S. K.; Bagchi, S. Fuel 2007, 86, 2528. (20) Groenzin, H.; Mulins, O. C. Energy Fuels 2000, 14, 667. (21) Groenzin, H.; Mulins, O. C. J. Phys. Chem. A 1999, 103, 11237. (22) Holbrey, J. D.; Seddon, K. R. Clean Prod. Process. 1999, 1, 223. (23) Xing, H.; Wang, T.; Zhou, Z.; Dai, Y. J. Mol. Catal. A: Chem. 2007, 264, 53. (24) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (25) Hu, Y. F.; Liu, Y.; Wang, H.; Xu, Ch.; Ji, D.; Sun, Y.; Guo, T. M. Chin. J. Chem. Eng. 2005, 13, 564.
(26) Hu, Y. F.; Guo, T. M. Langmuir 2005, 21, 8168.
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Table 2. Structures of ILs Used in This Work
B3LYP/6-31þþG** level27 using the Gaussian 03 package;28 then, a vibration analysis was performed to ensure the absence of imaginary frequencies and verify the existence of true minima. A schematic representation of ILs is given in Figure 1 with the purpose to identify the bond distance and the bond angle of each studied IL. In this scheme, X represents the [Br]- anion or any halogen atom from other anions. Details of electronic properties as well as geometrical parameters are described in the Supporting Information. Fluorescence Measurements. Steady-state fluorescence measurements were performed on a RF-5301PC Shimadzu Spectrofluorometer equipped with a 150 W Xe lamp and a cell temperature controller. The emission spectra of HCO and assays with ILs were recorded between 300 and 600 nm using a λexc of 288 nm at 25 °C. Such emission spectra serve to identify the less and more aggregated fractions of the crude oil. There is considerable evidence that asphaltenes self-associate into molecular aggregates of colloidal size but the nature and extent is still widely debated.29 Such aggregates are held together with π-π, acid-base and/or hydrogen bonding. Several complex molecules like polyaromatic hydrocarbons (PAHs), carbazole, and dimethyl dibenzothiophene present in crude oil and distillates form actually excited-ground state dimer complexes under light incidence. Such complexes present different fluorometric properties that permit identification of them from the less
aggregated or dispersed crude oil species.30,31 In this work, we determined the spectral center of mass (SCM, eq 1) as well as the relative polarity (RP, eq 2) from the emission spectra of each HCO assay. This permits to study the microenvironment around HCO molecules and how it varies by changes on HCO concentration, solvents, additives, etc. P λIðλÞ SCM ¼ P ð1Þ IðλÞ
(27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (28) Frisch, M. J.; et al. et al. Gaussian 03, revision D.02; Gaussian Inc.; Wallingford CT, 2004. (29) Yarranton, H. Y. J. Dispers. Sci. Technol. 2005, 26, 5.
(30) Aburto, J.; Correa-Basurto, J.; Torres, E. Arch. Biochem. & Biophys. 2008, 480, 33. (31) Correa-Basurto, J.; Aburto, J.; Trujillo-Ferrara, J.; Torres, E. Mol. Simul. 2007, 33, 649.
Figure 1. Schematic representation of ILs used in this work. (a) IL with imidazolium ring. (b) IL with pyridinium ring.
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Figure 2. Effect of the increasing concentration of IL-1 over (a) the emission spectra of HCO (180 ppm) and (b) the SCM value of the HCO for the determination of the CAC value.
where λ is the wavelength and I(λ) represents the fluorescence intensity at every λ. SCM RP ¼ SCM°
Results and Discussion Critical Aggregation Concentration of the ILs. The critical aggregation concentration (CAC) of every single IL was experimentally determined by the consecutive addition of an IL to HCO, as the fluorescence probe, in order to obtain the emission spectra between 300 and 600 nm. We selected the fluorescent probe in order to evaluate the overall aggregation of every IL with a complex mixture, HCO, comporting many types of molecules. The emission spectra of HCO in the absence and presence of ILs represent then the average of all present HCO molecules and their interactions among them and with ILs. Therefore, the evaluated CAC value represents the mean of all possible interactions favoring IL aggregation around HCO molecules. For instance, the emission spectra obtained for IL-1 in the presence of a fixed HCO concentration are shown in Figure 2a. Here, the molecules of IL-1 surround and clog the fluorophore molecules of HCO, which enhances fluorescence emission at increasing IL concentration. The aggregation phenomena of IL molecules around HCO molecules may be then asserted from the change of the emission spectra of a fluorophore probe, as the HCO, at constant concentration. Indeed, the fluorometric determination of surfactant’s CMC using pyrene as a fluorophore probe is well-known and correlates well with other methods such as interfacial tension, conductivity, etc.32 The surfactant CMC determination is indeed done in an aqueous system where the surfactant forms micelles with polar heads oriented toward the aqueous medium and clogs pyrene molecules in an apolar environment. Here, the SCM value tipically diminishes with surfactant concentration indicating an increase of the nonpolar environment sensed by pyrene. In our system, the continuous media is toluene where HCO molecules are very soluble.
ð2Þ
where SCM° and SCM correspond to the SCM value of HCO in the absence and presence of the corresponding IL, respectively. The critical aggregation concentration (CAC) of the ILs was determined by mixing it with HCO (180 ppm) and toluene in order to obtain a 2000 μL solution with an IL concentration ranging from 10 to 150 ppm in acetonitrile. The CAC value is measured at the IL concentration where the emission fluorescence is augmented due to the surrounding or inclusion of HCO by IL molecules. Since the CAC value indicates the onset of IL aggregation, this term was preferred instead of the critical micellar concentration (CMC). Then, the emission spectra were obtained, and the SCM values calculated, normalized for acetonitrile red-shifting, and graphed versus the IL concentration for each assay. The CAC value was estimated from the second derivative of the latter curve for every IL. The aggregation point (AP) of HCO in the absence and presence of ILs (45 ppm) was calculated by incrementing the HCO concentration from 0 to 200 ppm, and the respective emission spectra were recorded. Afterward, the SCM and the RP value of each assay were graphed as a function of the crude oil concentration. Here, the SMC° value is 418.49 nm and corresponds to the mixture of HCO (20 ppm) in toluene. Finally, the second derivative of the SCM curve was estimated in order to obtain the AP from the graphic inflection point. Also, we studied the effect of ILs over the AP of HCO by varying the n-heptane concentration from 0 to 90% volumen. In a typical experiment, we prepared a 2000 μL solution containing 100 ppm of crude oil, 45 ppm of an IL dissolved in acetonitrile, and the respective volume (% v/v) of toluene and n-heptane. The respective emission spectra and the SCM and RP values were obtained as stated above. Here, the SMC° value is 444.47 nm which corresponds to the solution of HCO (100 ppm) in neat toluene. Finally, the AP of HCO in n-heptane was calculated from the SCM curve as stated above.
(32) Domı´ nguez, A.; Fernandez, A.; Gonzalez, N.; Iglesias, E.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227.
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Scheme 1. Schematic Aggregation of HCO Molecules by (a) Increasing Its Concentration in Toluene and (b) Increasing the n-Heptane Content
The addition of ILs molecules provokes also the paulatine diminution of the SCM value of HCO, for all ILs, until a minimum is reached and where we identified the CAC value. Here, the HCO molecules sense also a surrounding more apolar microenvironment at the CAC value, suggesting that they are clogged into a direct micelle as well (see below). The CAC values of every IL were then obtained experimentally by using HCO (180 ppm) as a fluorescence probe (Table S1 of the Supporting Information). For instance, the increment of IL-1 concentration resulted in a decrease of the SCM value of HCO until a plateau was reached. Such blue-shifted behavior of the SCM denotes a more hydrophobic microenvironment around HCO molecules with IL-1 (Figure 2b). The plotting of the second derivative of the SCM curve helped us to identify the corresponding CAC value for every tested IL (Figure 2b, dotted line). If we considered further that ILs interact through their nonpolar tail with resin-stabilized asphaltene aggregates, we should then observe the clogging of HCO molecules into a more apolar environment as the diminution of the SCM value suggests. Indeed, it has been proposed that both asphaltene and resin molecules are polar and associated as micelles and those resins solvate asphaltene micelles through their polar heads and hence stretch their aliphatic groups outward to form a steric-stabilization layer surrounding asphaltene molecules.13-15 We observed that the CAC value increases from IL-1 to IL-2 as a result of a larger alkyl chain of the ILs containing an imidazolium ring and bromide anion (Table S1). When we maintained a constant the alkyl chain length (n-butyl group), we observed that IL-6 with a pyridinium cation possesses a lower CAC value when compared to IL-2 with an imidazolium cation. Here, we can say that a lower polarity of the IL, observed by the μ value, results in an increment of the CAC value. Nevertheless, the opposite phenomenon occurs for ILs with the same imidazolium cation but different counterion. Indeed, the CAC value increased with polarity expressed by the μ value (IL-2 to IL-5): [BF4]- < [Br]- = [PF6]- < [AlCl4]-. Here, we see that there is some kind of correlation between the μ and CAC values for some tested ILs. Effect of Crude Oil Concentration and n-Heptane Content on the Aggregation Behavior of HCO. First, we study the
aggregation behavior of HCO through fluorometry by increasing (a) the HCO concentration in a toluene solution and (b) the n-heptane content. Since asphaltene aggregation is stabilized by resins13-15 and mediated by polar interactions,17-19 we expect first an augmentation of the emission spectra due to HCO solubilization in toluene and followed by the formation of higher HCO aggregates which provokes an important quenching of emission signal (Scheme 1a). We can notice here the important resin/asphaltene ratio (4.48) of the HCO that should contribute to the low °API and high viscosity (Table 1). On the other side, the increasing n-heptane content causes a progressive asphaltene aggregation with an important quenching of the emission signal until the critical aggregation point occurred with an augmentation of the emission signal due to the remaining soluble HCO molecules (Scheme 1b). Here, the stability of asphaltenes should be a function of the concentration of the ILs in solution, the fraction of asphaltene surface sites covered by ILs and the equilibrium conditions between ILs in solution and on the surface of asphaltenes as proposed in an earlier work.33 For instance, we observed first that the emission spectrum of HCO in toluene shows two characteristic signals, one large and resolved peak at 386 nm and a second broader peak at 435 nm (Figure 3a). It is well-known that molecules present in HCO like polyaromatic hydrocarbons (PAHs) are fluorophores, which emit energy under the incidence of a light source.34 It is also well-known that we can identify two groups of molecules by spectrofluorometry: the first corresponds to a lower state of aggregation and/or high solvated group of molecules at low wavelength or high emission energy, i.e. in the blue region of the spectrum. The second group, known as excited-ground state dimer complexes or excimers, corresponds to a higher state of aggregation and/or less solvated group of molecules at higher wavelength or lower emission energy.30,31 In our study, the low and high aggregated molecules correspond to the identified bands at 386 and 435 nm, respectively. Such technique identifies then (33) Leontaritis, K. J.; Mansoori, G. A. SPE J. 1987, 16258. (34) Lakowickz, J. R. In Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: New York, 1999; Chapter 1.
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Figure 3. Effect of the concentration of HCO on (a) the emission spectra and (b) the change of the SCM value for determination of the AP value. Second derivative of the SCM curve (dashed line).
the changes on the aggregation state of HCO molecules by the intensity ratio (I435/I386) but rather on the overall spectrum through the calculation of the SCM value (see below). The latter is more accurate in determining the AP value than the intensity ratio (I435/I386) or by UV-vis techniques since it measures the spectrum changes on emission intensity and wavelength shifting due to changes on the self-interaction of HCO molecules. On contrast, the UV-vis techniques determine the flocculation onset of crude oil or fractions by measuring the light absorption throughout the complete spectrum or at specific wavelengths (ca. 280 nm or at the Soret band, ca. 400 nm). When crude oil molecules start to flocculate due to concentration or presence of a flocculant agent, i.e. n-pentane or n-heptane; the increment of solution’s absorbance is not linear and it is observed a change on the slope of the absorbance vs concentration curve. Both UV-vis and fluorometry techniques permit estimation of the flocculation onset or AP value, respectively; that should be situated around the same value. But, we prefer the last one because it is more precise and makes the difference between low and high aggregated states of crude oil or fraction molecules. We observed then that the increase of the HCO concentration, between 20 and 60 ppm, resulted in a high emission fluorescence of both peaks (Figure 3a, black lines). Then, the emission fluorescence of the first peak associated with the low aggregated HCO molecules quenches from 60 to 400 ppm (Figure 3a, gray lines). The second band at 435 nm, associated with the high-aggregated HCO molecules, suffered a slighter emission quenching and broadening of the peak (Figure 3a, gray lines). Furthermore, the vibronic structure of the emission spectra of HCO changed by increasing its concentration as easily determined by the intensity ratio (I435/I386) of the mentioned wavelength bands. The increment in the I435/I386 ratio suggests that more high aggregated molecules are formed at higher concentrations of
HCO in toluene solution. For example, the I435/I386 augmented from 0.81 to 1.24 at 20 and 120 ppm of HCO, respectively. We proceeded then to evaluate the change of the SCM values of HCO in a toluene solution by spectrofluorometry. We easily observed that the SCM value increment with the HCO concentration until a plateau is reached (Figure 3b). The change on the slope of the curve indicates the aggregation point (AP) of the crude oil. Since the SCM value shifts to the red spectrum region where the high aggregation molecules appear, we assumed that such change on the curve’s slope indicates the AP of HCO as confirmed above by the I435/I386 ratio. The AP value (60 ppm) of HCO was easily determined from the second derivative of the SCM curve (Figure 3b, dashed line). Such an HCO concentration indicates the critical aggregation of the crude oil in toluene. With respect to the effect of the n-heptane volume on HCO aggregation, we observed here the quenching of HCO emission fluorescence between 0 and 30% (Figure 4a, black lines), followed by a higher emission between 50 and 90% v/v of nheptane (Figure 4a, gray lines). Here, the asphaltenes molecules, among others species in HCO, sense an incrementing nonpolar microenvironment that favors their aggregation through polar interactions. This results in quenching of the emission fluorescence until 30% v/v of n-heptane. Beyond this point, asphaltenes molecules precipitate since they are expulsed from the n-heptane phase with an increment of the emission fluorescence coming from the remaining soluble HCO molecules. Here, the vibronic structures of the emission spectra changed slightly when compared to those obtained by increasing HCO concentration. Nevertheless, we could observed that the I435/I386 ratio incremented from 0.96 to 1.12 between 0 and 50% v/v of n-heptane. Afterward, the I435/I386 ratio followed a light decrement until 1.03 at 90% v/ v n-heptane. This was confirmed by the estimation of an AP value of 30% v/v n-heptane using the second derivative of the 4589
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Figure 4. Effect of n-heptane volume on (a) the emission spectra of HCO (180 ppm) and (b) the change of the SCM value for determination of the AP value. The second derivative of the SCM curve (dashed line).
SCM curve (Figure 4b). Indeed, the SCM value red-shifted due to an increasing polar microenvironment around HCO molecules until 30% v/v n-heptane was reached. Afterward, the SCM value blue-shifted due to the dominating nonpolar microenvironment associated to n-heptane. Effect of Imidazolium- and Pyridinium-Based ILs on the Aggregation Behavior of HCO. For cations with imidazolium or pyridinium rings, it is well-known that the methyl and butyl groups (electron donor) activate aromatics ring and act as ortho/para directors, while the anions [Br]-, [AlCl4]-, [BF4]-, and [PF6]- could act as weak electron acceptors, because they weakly deactivate the aromatic ring. Since we described how HCO aggregates in function of its concentration and n-heptane content, we pursue the study to the effect of imidazoliumand pyridinium-based ionic liquids on HCO aggregation by following the RP and AP values. HCO aggregates at 60 ppm and 30% v/v n-heptane as discussed above. For HCO aggregation as a function of its concentration, we observed first that the RP value, i.e. the relative polarity of HCO at the AP value in absence and presence of an IL, increased with all ILs excepting IL-4 (Figure 5). The increment in relative polarity can be explained in terms of the hydrophilic nature of ILs that sets a polar microenvironment around HCO molecules, especially on asphaltenes, which allows further HCO-HCO and/or an IL-stabilized HCO micelle interactions until the critical aggregation is reached at a higher AP value. Since asphaltenes are polar molecules, their aggregation is ruled by polar and π-π interactions.17-19 The decrement in the RP value produced by IL-4 may be attributed to a slighter apolar microenvironment around HCO molecules. In the case of 1-butyl-3-methylimidazolium-based ILs, we tested four ILs with different anions. We observed that the aggregation was shifted to higher HCO concentration with [PF6]- < [Br]- < [AlCl4]- < [BF4]- (Figure 5). Some studies have been done to correlate ILs properties with
Figure 5. Relationship of the relative polarity (RP) set by imidazolium- and pyridinium-based ILs and the aggregation point of HCO.
asphaltene aggregation. For example, a linear free-energy relationship was proposed to characterize some ionic liquids in the basis of solvation interactions. The latter study takes in account the ability of ILs to interact with π- and n-electrons of coexisting compounds, the ILs’ dipolarity/polarizability, the hydrogen-bond acidity and basicity, and dispersion forces of ILs.24 The reported hydrogen-bond basicity (a parameter) at 40 °C for [Bmim]þ[PF6]- (IL-4) and [Bmim]þ[BF4]- (IL-5) are 1.887 and 2.219, respectively. This means that hydrogen-bonding or electron donor interactions are more feasible between acidic HCO molecules and [BF4]than with [PF6]- with permits to shift the aggregation at higher HCO concentration. Moreover, the presence of a pyridinium cation (IL-6) allowed a higher stability of HCO against aggregation when compared to IL-2 with an imidazolium ring (Figure 5). 4590
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Scheme 2. Schematic Representation of HCO Aggregation in the Presence of ILs by Incrementing (a) HCO Concentration and (b) n-Heptane Content
density, such as that established by Hu et al.25,26 Here, coverage of HCO molecules by ILs should be homogeneous (mono or multilayer) or a more specific IL-stabilized HCO interaction should exist because the AP is delayed at higher HCO concentrations (Scheme 2a). We assumed that asphaltene aggregates are stabilized through their interaction with resins to form a nonpolar outer layer. ILs may then attach to the outer resin layer, which increments the apolar microenvironment around more polar HCO molecules, i.e. asphaltenes; as sensed by fluorometry. Then, it seems that ILs aggregate around asphaltenes/resin micelles with their polar groups oriented toward the organic phase, i.e. a direct micelle should then be formed. The presence of ILs polar groups on the outer phase of stabilized HCO micelles is suggested also by the increment of the relative polarity in presence of ILs when compared to the HCO blank without ILs (Figure 5). ILs with lower RP and AP values are not capable to avoid critical aggregation, which can be explained in terms of a heterogeneous or incomplete coverage of HCO molecules by ILs or to more specific HCO-HCO interactions that displace HCO-IL interactions. Respecting the aggregation of HCO as a function of the n-heptane content (% v/v), we observed that HCO reaches its critical aggregation at a RP value of 1.01 in absence of ILs. But, the RP value diminishes to 0.97 for all ILs. The fact that all assays showed the same RP value, at different HCO aggregation points, indicates that the microenvironment around HCO molecules, at the AP, is set now for n-heptane molecules that displace the ILs molecules (Scheme 2b). IL-HCO interaction should favor a major dilution of HCO molecules in n-heptane until such interaction is broken or diminished. At this point, HCO molecules aggregate and precipitate. Indeed, the presence of aliphatic chains on aromatic sheets, like in asphaltenes, introduces disruptions on the aromatic-sheet stacking and favors dissolution.35 Moreover, the adsorption of p-alkylphenols and p-alkylbenzenesulfonic acids on asphaltenes have served to stabilize them in apolar alkane solvents.10,11,36,37
This can be understood by the higher basicity of the pyridinium cation that acts as an electron-pair donor with HCO molecules. On the contrary, the imidazolium cation is a weaker base because of a major delocalization of the nonbonding electron-pair. Nevertheless, IL-5 with [BF4]- and an imidazolium cation provided a better stabilization against HCO aggregation than IL-6 and IL-2 with [Br]- anion. Here, the major capacity of [BF4]- to interact with HCO acidic molecules should be at the origin of a higher stabilization against aggregation. Besides the presence of an ethyl group (IL-1) in an imidazolium bromide-based IL permitted a higher stabilization of HCO against aggregation with respect to IL-2 with a butyl group (Figure 5). Such a trend has also been observed by Hu and Guo working with [Cnpy]þ[Cl]- ILs in the inhibition of asphaltene precipitation from CO2-injected reservoir oils.26 A plausible explanation is that the concentration and activity of [Br]- increases as the alkyl chain length of the cationic head ring decreases because the molar volumes/polarities of the [Cnim]þ cations are reduced/promoted from [Bmim]þ to [Emim]þ. It is important to notice that a linear correlation exists between RP and AP values (Figure 5). Since both parameters are estimated from the same series of fluorometric experiments, such linear correlation just indicates that HCO molecules covered by ILs reached a higher degree of polarity when compared to the blank which contains no ILs. Then, ILs set a polar environment around HCO molecules, which allows HCO-ILs-HCO interactions, limits HCO aggregation, and shifts the AP value to a larger HCO concentration. The higher AP values were then found for ILs presenting an intermediate μ value between 13.11 and 13.62 D (see Table S1 of the Supporting Information). Such ILs should interact and cover HCO molecules in such a manner that permits HCO-HCO or IL-stabilized HCO micelle interactions, as seen by a higher RP value, but limits its critical aggregation as mentioned above. Due to the high values of the negative charge in ILs, the X14 halogen ion can be easily attracted by H11 with a positive charge, favoring a hydrogen bond between the cation and the anion (Figure 1). This important charge distribution on cations and anions results in a significant dipole moment of ILs that could explain their ability to shift the aggregation point of HCO. Furthermore, the effect of the anion is proportional to its size and charge
(35) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martı´ nez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732. (36) Gonzalez, G.; Middea, A. Colloids Surf. 1991, 52, 207. (37) Hernandez-Trujillo, J.; Martı´ nez-Magadan, J. M.; Garcı´ a-Cruz, I. Energy Fuels 2007, 21, 1127.
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: DOI:10.1021/ef9004175
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the AP value from 30 to 50% v/v n-heptane content as obtained for 1-butyl- (IL-2) and 1-ethyl-3-methylimidazolium bromide (IL-1), respectively (Figure 6, right). Here again, the IL with a higher dipolar moment allowed the displacement of the aggregation point of HCO to higher values. As mentioned earlier, the reduction of the molar volume and increase of polarity should increase the stabilization of HCO by the shorter alkyl chain of [Emim]þ[Br]-. Conclusions We have determined for the first time the CAC value and electronic properties of some commercial imidazolium- and pyridinium-based ILs by means of spectrofluorometry and theoretical quantum chemistry, respectively. The quantum chemistry studies revealed that the presence of a complex anion, such as [AlCl4]-, [BF4]-, or [PF6]-, substantially incremented the electronegativity (χ) and dipole moment (μ) of the studied ILs. This may be at the origin of the better performance of such ILs to inhibit asphaltene aggregation in an HCO. The CAC values of studied ILs were equal or below 50 ppm, which indicates the existence of strong self- and IL-resin interactions through an outer stabilized layer that protect asphaltenes against agglomeration and precipitation. HCO self-interaction to form larger aggregates seems to be of polar nature as suggested by the increment in the spectral center of mass versus HCO concentration. This may be attributed to the aggregation of asphaltenes; one of the more polar molecules in crude oils. On the other hand, HCO aggregation in n-heptane resulted in the increment of the spectral center of mass until the aggregation point is reached. Afterward, the SCM diminishes because asphaltenes and other HCO molecules encounter an apolar microenvironment due to surrounding n-heptane molecules. The presence of ILs in HCO solution shifted the AP value to higher HCO concentration or n-heptane volume. In the first case, the ILs modified the relative polarity of the microenvironment around HCO molecules and shifted the HCO aggregation to higher values. Here, ILs with a larger dipole moment inhibit HCO aggregation until a specific relative polarity is reached. HCO molecules aggregate in n-heptane as seen by the spectrofluorometric studies and probably due to the insolubility of polar asphaltenes in an apolar medium. The presence of ILs with a higher dipole moment such as [AlCl4]-, [BF4]-, and [PF6]-, a pyridinium cation, or a shorter alkyl chain, shifted the AP value (% v/v n-heptane) of HCO to higher values. This may be attributed to a certain polarization by ILs of HCO molecules that protect them from self-aggregation.
Figure 6. Effect of the anion (left), cation (middle), and alkyl chain (right) of imidazolium- and pyridinium-based ILs on the aggregation point of HCO in n-heptane.
Nevertheless, HCO coverage by some ILs allowed shifting the AP value at higher n-heptane contents as seen in Figure 6. For instance, the use of anions like [BF4]- = [AlCl4]- < [PF6]- in 1-butyl-3-methyl imidazolium based ILs enhanced the efficiency to displace the AP value to higher n-heptane contents when compared to the bromide anion (IL-2) which has no effect on the AP value (Figure 6, left). We noticed here that an IL with a dipolar moment higher than 13.0 D allowed the displacement of the AP to higher n-heptane contents, but the relationship was not linear. It seems to us that other IL properties or specificity of the HCO-IL interaction should rule the aggregation of HCO as a function of n-heptane content. For example, IL-4 is able to interact with compounds containing π- and n-electrons in water as shown by its r-coefficient of 1.29. But, such capacity enhances 2.5 times in n-heptane with a correlation of r = 3.28.38 Moreover, an interesting work has recently shown that polarity (expressed as the dipolarity/polarizability factor, π*, and the dielectric constant, ε) varies inversely with the molar volume of an IL. The authors also proposed that an IL with high dipolarity/ polarizability could be attained by incorporating very small ions or incorporating ionic structures that frustrates the formation of nanoscale domains but that the insight is still unclear.39 In our case, IL-4 has a higher molar volume (345 versus 311 A˚3) than IL-5, but the latter is more polar (π* = 1.05 and ε = 11.7) when compared to IL-4 (π* = 0.95 and ε = 11.4).39,40 Further studies may provide additional insight in the effect of ILs on the aggregation of crude oil. The use of [Bpyr]þ[Br]- (IL-6) with a higher dipolar moment (13.62 D) instead of [Bmim]þ[Br]- (IL-2, 12.89 D) incremented the AP value from 30 to 70% v/v n-heptane content (Figure 6, middle). This behavior could be also attributed to the higher basicity of pyridinium cation that allows its interaction with HCO acidic molecules. Finally, the use of a shorter alkyl chain allowed the displacement of
Acknowledgment. The authors thank the Programa Academico de Posgrado of IMP for the economic support of this work. I.G.-C. and J.A.M.-H. are thankful to the Centro de Superc omputo de Catalunia (CESCA) in Spain and to the Direcci on General de Superc omputo Academico (DGSCA) of the UNAM for the support for carrying out the calculations of electronic structure. J.A.M.-H. thanks CONACYT and the IMP for the economic support granted during his Ph.D. studies. Supporting Information Available: More specialized description of theoretical electronic properties and optimized geometrical parameters of different ILs at the B3LYP/6-31þþG** level of theory. This material is available free of charge via the Internet at http://pubs.acs.org/.
(38) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003, 375, 191. (39) Kobrak, M. N. Green Chem. 2008, 10, 80. (40) Wikiro, Ch.; Oleinikova, A.; Ott, M.; Weingaertner, H. J. Phys. Chem. B 2005, 109, 17028.
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