Aggregation Behavior of Heavy Crude Oil−Ionic Liquids Solutions by Fluorescence Spectroscopy

4584r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4584–4592 : DOI:10.1021/ef9004175Published on Web 08/19/2009

Aggregation Behavior of Heavy Crude Oil-Ionic Liquids Solutions by Fluorescence

Spectroscopy

J. Alberto Murillo-Hern�andez,† Isidoro Garcıa-Cruz,‡ Sim�on L�opez-Ramırez,‡ C. Duran-Valencia,§

J. Manuel Domınguez,‡ and Jorge Aburto*, )

†Programa Acad�emico de Posgrado, ‡Programa de Ingenierıa 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 growthwhich provoke crude oil destabilization and significant problems during the production, transport, andrefining of heavy and extra heavy crude oils. A recent and innovative approach to overcome this problem isthe use of ionic liquids (ILs) as inhibitors or stabilizers of asphaltene aggregation. Since the informationconcerning the properties of the studied ILs is scarce, we characterized some of their electronic propertiesand critical aggregation concentration (CAC) by quantum chemistry and spectrofluorometry, respec-tively.We found that the presence of a complex anion such as [AlCl4]

-, [BF4]-, and [PF6]

- incremented theHOMO-LUMO gap (ΔH-L), electronegativity (χ), absolute hardness (η), and dipole moment (μ) whencompared to [Br]--containing ILs. Moreover, the ILs’ CAC values showed a linear correlation with thedipole 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) then-heptane volume by means of fluorescence spectroscopy. We have found that the aggregation of HCOoccurs at larger crude oil concentration or n-heptane volume in the presence of some ILs. Here, ILs set apolar microenvironment aroundHCO asphaltenes, which stabilized them against further aggregation andprecipitation. The better performance of ILs as inhibitors or stabilizers of asphaltene aggregation wasfound with those comporting a complex anion, a pyridinium ring, or a shorter alkyl substitution on thecation. Such ILs present the higher values of the calculated electronic properties.

Introduction

Asphaltene aggregation represents a very serious and con-stant problem in the oil industry with an enormous economicimpact. This is because asphaltene aggregation occurs spon-taneously in oil wells, provoking the formation of an extre-mely dense phase that prevents oil extraction and in manycases completely stops the oil production.1-8 Asphaltenemolecular structure is largely unknown, but a proposedmodelincludes polyaromatic condensed rings with short aliphaticchains and polar heteroatoms such as nitrogen, oxygen, andsulfur.9 The stability of asphaltenes in crude oil is due to thepresence of some neutral polar substances, resins among

them, already present in the crude oil.10-12 Most recently,Goual and Firoozabadi13 suggested that both asphaltene andresins molecules are polar and associated as micelles. In fact,asphaltenes and resins coexist in a petroleum fluid andmay befound in the form of monomers or associated as micelles. Inthe latter form, the micellar core is formed by the self-association of asphaltene molecules with adsorbed resins atthe surface to form a shell that also contains an oil fraction.Indeed, resins are essential in asphaltene aggregation becausethey attach to asphaltene micelles through their polar headsand hence stretch their aliphatic groups outward to form asteric-stabilization layer surrounding asphaltene molecules.The formation and properties of such micelles is governed bythe relative concentration of asphaltenes and resins.When theresins are desorbed from the micellar core surface, they giverise to the asphaltene phase.14,15 Hence, resins are the naturalsolvent of asphaltenes.

In addition, asphaltenes present in crude oil are polycyclic,rigid molecules with π bonds which emit light as fluorescencewhen they are exposed to ultraviolet or X-ray radiation.16

Pietraru and Cramb17 carried out a research about the

*Corresponding author. Tel.: þ52 55 9175 8204. Fax: þ52 55 91758429. 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.

(13) Goual, L.; Firoozabadi, A. AIChE 2002, 48, 2646.(14) Garcıa-Cruz, I.; Martınez-Magad�an, J. M.; Salcedo, R.; Illas, F.

Energy Fuels 2005, 19, 998.(15) Pacheco-S�anchez, J. H.; �Alvarez-Ramırez, F.; Martınez-

Magad�an, J. M. Energy Fuels 2004, 18, 1676.(16) Rouessac, F.; Rouessac, A. In An�alisis Quımicos. 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 ando-chlorinebenzene solution through fluorescence. They ob-served that the polarity increased with asphaltene concentra-tion. They found a lineal correlation for low asphaltenesconcentration (0.2-1.5 g L-1) and a bathochromic shift.Goncalves et al.18 analyzed the asphaltene aggregation pro-cess of two types of Venezuelan crude oil by means offluorescence. The first oil presenting precipitation problemsand the second one without. They identified two distinctivebands in the spectra (530 and 564 nm), which were attributedto the different structures of fluorescent compounds in as-phaltenes. They realized that a bathochromic shift takes placewhen increasing asphaltene concentration and that the crudeoil with asphaltene precipitation problems starts the aggrega-tion process with a lower concentration than the samplewithout problems. Ghosh et al.19 studied the effect of theasphaltene concentration over the aggregation of an asphal-tene sample (Barari, India) in different solvents, such asbenzene, toluene, and carbon tetrachloride through fluores-cence. They also noticed a bathochromic shift of the spectrawhen increasing the asphaltene concentration. In addition, itwas concluded that the aggregationprocesswas gradual and itdepended on the asphaltene concentration in the solution,regardless of the solvent used in their study.Onmeasurementsof depolarization fluorescence, Groenzin and Mulins20,21

found a strong correlation between the size of an individualfused ring system in an asphaltene molecule and the overallsize of the corresponding molecule, showing that asphaltenemolecules have one or two fused ring systems permolecule. Inthis case, the asphaltene fluorescence emission is significant inthe range of 400-600 nm. Here, the HOMO-LUMO gapincreased with number of rings on the aromatic core. Theyalso found that asphaltene micelles with smaller molecularweights possess a lower energy barrier to break apart, whichmay also have important operational implications.

The term ionic liquids (ILs) have been coined in recent yearsto describe a class of organic salts that are liquid in their purestate at or near room temperature. Some of the more widelystudied ILs are heterocyclic cations, based on a substitutedpyridine or imidazole ring plus an inorganic anion.22When anionic liquid is used to replace classical organic solvents, itoffers a new environmentally benign approach toward mod-ern chemical processes. Additionally, the implementation oftask-specific ILs further enhances their versatility for the caseswhere both reagent and medium are coupled. The increasedinterest in ILs by chemists and technologists is clearly due tothe utility of ionic liquids as solvents for different chemicalreactions, including catalytic reactions and the characteriza-tion of solvation interactions.23,24

On the subject of ILs application to the asphaltene pro-blems, Hu et al.25 studied for the first time the dissolution ofasphaltenes in ILs. They mixed asphaltene samples of Shenglicrude oil with ILs and heated them up to different tempera-tures (50, 80, 135, and 150 �C) in order to determine their

solubility. It was observed that the ILs based on pyridi-nium cations can dissolve asphaltenes better than imidazo-lium cations. Also, the ILs capacity to dissolve asphaltenesdecreases with the length of the alkyl chain. The effect ofthe anion over the capacity to dissolve asphaltenes was foundto be proportional to its size and charge density. Further-more, Hu and Guo26 studied the effect of ILs in the inhibi-tion of asphaltene precipitation. They used a high-pressurecell where the crude oil and the ILs were mixed and CO2 wasinjected as an asphaltene precipitation agent. Here, thepyridinium-based ILs capacity to inhibit asphaltene precipi-tation increased with the alkyl chain length and was attri-buted to the charge density delocalization between the cationand anion.

The aimof thiswork is to ascertain the effect of ionic liquidson the aggregation point (AP) of a heavy mexican crude oil(HCO), by varying the concentrationof crude oil or n-heptaneas asphaltene precipitant by means of fluorescence spectros-copy of ILs-HCO solutions. Moreover, we obtained theelectronic structure properties and critical aggregation con-centration (CAC) of the ILs by quantum chemical techniquesand spectrofluorometry, respectively.

Experimental Method

Materials.We used a heavy crude oil (HCO), from an oil fieldlocated in Southern Mexico, and its properties are summarizedin Table 1. Imidazolium- and pyridinium-based ionic liquids(ILs) over 99%puritywere provided by Sigma-Aldrich,Mexico,and are listed in Table 2. Prior to use, ILs were dissolved inacetonitrile to ensure a homogeneous dispersion in toluene-richsolutions containing HCO. HPLC-grade solvents such as acet-onitrile, n-heptane, and toluene were acquired from Techromand Sigma-Aldrich, Mexico.

Computational Details. An accurate structural study of dif-ferent ILs has been carried out using density functional theory(DFT) by quantum chemical techniques. In this part of thestudy, we want to obtain actual values of different indices fromreactivity, such as the HOMO-LUMO gap (ΔH-L), electro-negativity (χ), absolute hardness (η), and dipole moment (μ),with the clear objective to understand how the ionic liquids candissolve asphaltenes present in heavy crude oil. The mini-mum-energy geometries of the ILs used in the present workwere determined by ab initio geometry optimizations at the

Table 1. Properties and Composition of a Heavy Mexican Crude Oil

Physical Properties at 25 �Cmolecular mass (g/mol) 486�API 11.60density (g/cm) 0.9859viscosity (cP) 53028interfacial tension (dyn/cm) 18.24water content (%) 0.05

chemical composition (% w/w)

carbon 84.28hydrogen 10.28nitrogen 0.41oxygen 0.01sulfur 5.02

SARA composition (% w/w)

saturates 7.94aromatics 5.28resins 70.93asphaltenes 15.85

(18) Goncalves, S.; Castillo, J.; Fern�andez, 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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B3LYP/6-31þþG** level27 using the Gaussian 03 package;28

then, a vibration analysis was performed to ensure the absenceof imaginary frequencies and verify the existence of trueminima.A schematic representation of ILs is given in Figure 1 with thepurpose to identify the bond distance and the bond angle of eachstudied IL. In this scheme, X represents the [Br]- anion or anyhalogen atom fromother anions.Details of electronic propertiesas well as geometrical parameters are described in the Support-ing Information.

Fluorescence Measurements. Steady-state fluorescence mea-surements were performed on a RF-5301PC Shimadzu Spectro-fluorometer equipped with a 150 W Xe lamp and a celltemperature controller. The emission spectra of HCO andassays with ILs were recorded between 300 and 600 nm usinga λexc of 288 nm at 25 �C. Such emission spectra serve to identifythe less and more aggregated fractions of the crude oil. There isconsiderable evidence that asphaltenes self-associate into mo-lecular aggregates of colloidal size but the nature and extent isstill widely debated.29 Such aggregates are held together withπ-π, acid-base and/or hydrogen bonding. Several complexmolecules like polyaromatic hydrocarbons (PAHs), carbazole,and dimethyl dibenzothiophene present in crude oil and dis-tillates form actually excited-ground state dimer complexesunder light incidence. Such complexes present different fluoro-metric properties that permit identification of them from the less

aggregated or dispersed crude oil species.30,31 In this work, wedetermined the spectral center ofmass (SCM, eq 1) as well as therelative polarity (RP, eq 2) from the emission spectra of eachHCO assay. This permits to study the microenvironmentaround HCO molecules and how it varies by changes on HCOconcentration, solvents, additives, etc.

SCM ¼P

λIðλÞP

IðλÞ ð1Þ

Table 2. Structures of ILs Used in This Work

Figure 1. Schematic representation of ILs used in this work. (a) ILwith imidazolium ring. (b) IL with pyridinium ring.

(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.

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where λ is the wavelength and I(λ) represents the fluorescenceintensity at every λ.

RP ¼ SCM

SCM�ð2Þ

where SCM� and SCM correspond to the SCMvalue of HCO inthe absence and presence of the corresponding IL, respectively.

The critical aggregation concentration (CAC) of the ILs wasdetermined by mixing it with HCO (180 ppm) and toluene inorder to obtain a 2000 μL solution with an IL concentrationranging from 10 to 150 ppm in acetonitrile. The CAC value ismeasured at the IL concentration where the emission fluores-cence is augmented due to the surrounding or inclusion of HCOby IL molecules. Since the CAC value indicates the onset of ILaggregation, this term was preferred instead of the criticalmicellar concentration (CMC). Then, the emission spectra wereobtained, and the SCM values calculated, normalized for acet-onitrile red-shifting, and graphed versus the IL concentrationfor each assay. The CAC value was estimated from the secondderivative of the latter curve for every IL.

The aggregation point (AP) of HCO in the absence andpresence of ILs (45 ppm) was calculated by incrementing theHCO concentration from 0 to 200 ppm, and the respectiveemission spectra were recorded. Afterward, the SCM and theRP value of each assay were graphed as a function of the crudeoil concentration. Here, the SMC� value is 418.49 nm andcorresponds to themixture ofHCO (20 ppm) in toluene. Finally,the secondderivative of the SCMcurvewas estimated in order toobtain the AP from the graphic inflection point.

Also, we studied the effect of ILs over the AP of HCO byvarying the n-heptane concentration from 0 to 90%volumen. Ina typical experiment, we prepared a 2000 μL solution containing100 ppm of crude oil, 45 ppm of an IL dissolved in acetonitrile,and the respective volume (%v/v) of toluene and n-heptane. Therespective emission spectra and the SCM and RP values wereobtained as stated above. Here, the SMC� value is 444.47 nmwhich corresponds to the solution of HCO (100 ppm) in neattoluene. Finally, the AP of HCO in n-heptane was calculatedfrom the SCM curve as stated above.

Results and Discussion

Critical Aggregation Concentration of the ILs. The criticalaggregation concentration (CAC) of every single IL wasexperimentally determined by the consecutive addition ofan IL to HCO, as the fluorescence probe, in order to obtainthe emission spectra between 300 and 600 nm. We selectedthe fluorescent probe in order to evaluate the overall aggre-gation of every IL with a complex mixture, HCO, comport-ingmany types ofmolecules. The emission spectra ofHCO inthe absence and presence of ILs represent then the average ofall present HCO molecules and their interactions amongthem and with ILs. Therefore, the evaluated CAC valuerepresents the mean of all possible interactions favoring ILaggregation around HCO molecules.

For instance, the emission spectra obtained for IL-1 in thepresence of a fixed HCO concentration are shown inFigure 2a. Here, the molecules of IL-1 surround and clogthe fluorophore molecules of HCO, which enhances fluor-escence emission at increasing IL concentration. The aggre-gation phenomena of IL molecules around HCO moleculesmay be then asserted from the change of the emission spectraof a fluorophore probe, as the HCO, at constant concentra-tion. Indeed, the fluorometric determination of surfactant’sCMCusing pyrene as a fluorophore probe is well-known andcorrelates well with other methods such as interfacial ten-sion, conductivity, etc.32 The surfactant CMCdeterminationis indeed done in an aqueous system where the surfactantformsmicelles with polar heads oriented toward the aqueousmedium and clogs pyrene molecules in an apolar environ-ment. Here, the SCM value tipically diminishes with surfac-tant concentration indicating an increase of the nonpolarenvironment sensed by pyrene. In our system, the continuousmedia is toluene where HCO molecules are very soluble.

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 forthe determination of the CAC value.

(32) Domınguez, A.; Fern�andez, A.; Gonz�alez, N.; Iglesias, E.;Montenegro, L. J. Chem. Educ. 1997, 74, 1227.

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The addition of ILs molecules provokes also the paulatinediminution of the SCM value of HCO, for all ILs, until aminimum is reached and where we identified the CAC value.Here, the HCO molecules sense also a surrounding moreapolar microenvironment at the CAC value, suggesting thatthey are clogged into a direct micelle as well (see below).

The CAC values of every IL were then obtained experimen-tally by using HCO (180 ppm) as a fluorescence probe (TableS1 of the Supporting Information). For instance, the incre-ment of IL-1 concentration resulted in a decrease of the SCMvalue of HCO until a plateau was reached. Such blue-shiftedbehavior of the SCM denotes a more hydrophobic microen-vironment around HCOmolecules with IL-1 (Figure 2b). Theplotting of the secondderivative of the SCMcurve helped us toidentify the corresponding CAC value for every tested IL(Figure 2b, dotted line). If we considered further that ILsinteract through their nonpolar tail with resin-stabilized as-phaltene aggregates, we should then observe the clogging ofHCO molecules into a more apolar environment as thediminution of the SCM value suggests. Indeed, it has beenproposed that both asphaltene and resin molecules are polarand associated as micelles and those resins solvate asphaltenemicelles through their polar heads and hence stretch theiraliphatic groups outward to form a steric-stabilization layersurrounding asphaltene molecules.13-15

We observed that the CAC value increases from IL-1 toIL-2 as a result of a larger alkyl chain of the ILs containing animidazolium ring and bromide anion (Table S1). When wemaintained a constant the alkyl chain length (n-butyl group),we observed that IL-6 with a pyridinium cation possesses alower CAC value when compared to IL-2 with an imidazo-lium cation. Here, we can say that a lower polarity of the IL,observed by the μ value, results in an increment of the CACvalue. Nevertheless, the opposite phenomenon occurs forILs with the same imidazolium cation but different counter-ion. Indeed, the CACvalue increasedwith polarity expressedby 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 in-creasing (a) the HCO concentration in a toluene solutionand (b) the n-heptane content. Since asphaltene aggrega-tion is stabilized by resins13-15 and mediated by polar inter-actions,17-19 we expect first an augmentation of the emissionspectra due to HCO solubilization in toluene and followed bythe formation of higher HCO aggregates which provokes animportant quenching of emission signal (Scheme 1a). We cannotice here the important resin/asphaltene ratio (4.48) of theHCO that should contribute to the low �API and highviscosity (Table 1). On the other side, the increasing n-heptanecontent causes a progressive asphaltene aggregation with animportant quenching of the emission signal until the criticalaggregation point occurred with an augmentation of theemission signal due to the remaining soluble HCO molecules(Scheme 1b). Here, the stability of asphaltenes should be afunction of the concentration of the ILs in solution, thefraction of asphaltene surface sites covered by ILs and theequilibrium conditions between ILs in solution and on thesurface of asphaltenes as proposed in an earlier work.33

For instance, we observed first that the emission spectrumofHCO in toluene shows two characteristic signals, one largeand resolved peak at 386 nm and a second broader peak at435 nm (Figure 3a). It is well-known that molecules presentin HCO like polyaromatic hydrocarbons (PAHs) are fluor-ophores, which emit energy under the incidence of a lightsource.34 It is also well-known that we can identify twogroups of molecules by spectrofluorometry: the first corre-sponds to a lower state of aggregation and/or high solvatedgroup of molecules at low wavelength or high emissionenergy, i.e. in the blue region of the spectrum. The secondgroup, known as excited-ground state dimer complexesor excimers, corresponds to a higher state of aggregationand/or less solvated group ofmolecules at higher wavelengthor lower emission energy.30,31 In our study, the low and highaggregated molecules correspond to the identified bands at386 and 435 nm, respectively. Such technique identifies then

Scheme 1. Schematic Aggregation of HCOMolecules by (a) Increasing Its Concentration in Toluene and (b) Increasing the n-Heptane Content

(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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the changes on the aggregation state of HCO molecules bythe intensity ratio (I435/I386) but rather on the overall spec-trum through the calculation of the SCM value (see below).The latter is more accurate in determining the AP value thanthe intensity ratio (I435/I386) or byUV-vis techniques since itmeasures the spectrum changes on emission intensity andwavelength shifting due to changes on the self-interaction ofHCO molecules. On contrast, the UV-vis techniques deter-mine the flocculation onset of crude oil or fractions bymeasuring the light absorption throughout the completespectrum or at specific wavelengths (ca. 280 nm or at theSoret band, ca. 400 nm). When crude oil molecules start toflocculate due to concentration or presence of a flocculantagent, i.e. n-pentane or n-heptane; the increment of solu-tion’s absorbance is not linear and it is observed a change onthe slope of the absorbance vs concentration curve. BothUV-vis and fluorometry techniques permit estimation ofthe flocculation onset or AP value, respectively; that shouldbe situated around the same value. But, we prefer the last onebecause it is more precise and makes the difference betweenlow and high aggregated states of crude oil or fractionmolecules.

We observed then that the increase of the HCO concen-tration, between 20 and 60 ppm, resulted in a high emissionfluorescence of both peaks (Figure 3a, black lines). Then, theemission fluorescence of the first peak associated withthe low aggregated HCO molecules quenches from 60 to400 ppm (Figure 3a, gray lines). The second band at 435 nm,associated with the high-aggregated HCO molecules, suf-fered a slighter emission quenching and broadening of thepeak (Figure 3a, gray lines). Furthermore, the vibronicstructure of the emission spectra of HCO changed by in-creasing its concentration as easily determined by the in-tensity ratio (I435/I386) of the mentioned wavelength bands.The increment in the I435/I386 ratio suggests that more highaggregatedmolecules are formed at higher concentrations of

HCO in toluene solution. For example, the I435/I386augmented from 0.81 to 1.24 at 20 and 120 ppm of HCO,respectively.

We proceeded then to evaluate the change of the SCMvalues of HCO in a toluene solution by spectrofluorometry.We easily observed that the SCM value increment with theHCO concentration until a plateau is reached (Figure 3b).The change on the slope of the curve indicates the aggrega-tion point (AP) of the crude oil. Since the SCMvalue shifts tothe red spectrum region where the high aggregation mole-cules appear, we assumed that such change on the curve’sslope indicates the AP of HCO as confirmed above by theI435/I386 ratio. The AP value (60 ppm) of HCO was easilydetermined from the second derivative of the SCM curve(Figure 3b, dashed line). Such an HCO concentration in-dicates the critical aggregation of the crude oil in toluene.

With respect to the effect of the n-heptane volume onHCOaggregation, we observed here the quenching of HCO emis-sion fluorescence between 0 and 30%(Figure 4a, black lines),followed by a higher emission between 50 and 90% v/v of n-heptane (Figure 4a, gray lines). Here, the asphaltenes mole-cules, among others species in HCO, sense an incrementingnonpolar microenvironment that favors their aggregationthrough polar interactions. This results in quenching of theemission fluorescence until 30% v/v of n-heptane. Beyondthis point, asphaltenes molecules precipitate since they areexpulsed from the n-heptane phase with an increment of theemission fluorescence coming from the remaining solubleHCO molecules. Here, the vibronic structures of the emis-sion spectra changed slightly when compared to those ob-tained by increasing HCO concentration. Nevertheless, wecould observed that the I435/I386 ratio incremented from 0.96to 1.12 between 0 and 50% v/v of n-heptane. Afterward, theI435/I386 ratio followed a light decrement until 1.03 at 90% v/v n-heptane. This was confirmed by the estimation of an APvalue of 30%v/v n-heptane using the second derivative of the

Figure 3.Effect of the concentration ofHCOon (a) the emission spectra and (b) the change of the SCMvalue for determination of theAPvalue.Second derivative of the SCM curve (dashed line).

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SCM curve (Figure 4b). Indeed, the SCM value red-shifteddue to an increasing polar microenvironment around HCOmolecules until 30% v/v n-heptane was reached. Afterward,the SCM value blue-shifted due to the dominating nonpolarmicroenvironment associated to n-heptane.

Effect of Imidazolium- and Pyridinium-Based ILs on the

Aggregation Behavior of HCO. For cations with imidazoliumor pyridinium rings, it is well-known that the methyl and butylgroups (electron donor) activate aromatics ring and act asortho/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 howHCOaggregates in functionof its concentrationandn-heptanecontent, we pursue the study to the effect of imidazolium-and pyridinium-based ionic liquids on HCO aggregation byfollowing the RP and AP values. HCO aggregates at 60 ppmand 30% v/v n-heptane as discussed above.

ForHCO aggregation as a function of its concentration, weobserved first that the RP value, i.e. the relative polarity ofHCO at the AP value in absence and presence of an IL,increasedwith all ILs excepting IL-4 (Figure 5). The incrementin relative polarity can be explained in terms of the hydrophilicnature of ILs that sets a polarmicroenvironment aroundHCOmolecules, especially on asphaltenes, which allows furtherHCO-HCOand/or an IL-stabilizedHCOmicelle interactionsuntil the critical aggregation is reached at a higher AP value.Since asphaltenes are polar molecules, their aggregation isruled by polar and π-π interactions.17-19 The decrement inthe RP value produced by IL-4 may be attributed to a slighterapolar microenvironment around HCOmolecules.

In the case of 1-butyl-3-methylimidazolium-based ILs, wetested four ILs with different anions. We observed thatthe aggregation was shifted to higher HCO concentrationwith [PF6]

- < [Br]- < [AlCl4]- < [BF4]

- (Figure 5). Somestudies have been done to correlate ILs properties with

asphaltene aggregation. For example, a linear free-energyrelationship was proposed to characterize some ionic liquidsin the basis of solvation interactions. The latter study takes inaccount the ability of ILs to interact with π- and n-electronsof coexisting compounds, the ILs’ dipolarity/polariza-bility, the hydrogen-bond acidity and basicity, and disper-sion 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. Thismeans that hydrogen-bonding or electron donor interactionsare more feasible between acidic HCOmolecules 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 aggregationwhen compared to IL-2 with an imidazolium ring (Figure 5).

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 ofthe AP value. The second derivative of the SCM curve (dashed line).

Figure 5. Relationship of the relative polarity (RP) set by imidazo-lium- and pyridinium-based ILs and the aggregation point of HCO.D

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This can be understood by the higher basicity of the pyr-idinium cation that acts as an electron-pair donor with HCOmolecules. On the contrary, the imidazolium cation is aweaker base because of a major delocalization of the non-bonding electron-pair. Nevertheless, IL-5 with [BF4]

- andan imidazolium cation provided a better stabilization againstHCOaggregation than IL-6 and IL-2with [Br]- anion.Here,the major capacity of [BF4]

- to interact with HCO acidicmolecules should be at the origin of a higher stabilizationagainst aggregation.

Besides the presence of an ethyl group (IL-1) in animidazolium bromide-based IL permitted a higher stabiliza-tion of HCO against aggregation with respect to IL-2 with abutyl group (Figure 5). Such a trend has also been observedby Hu and Guo working with [Cnpy]

þ[Cl]- ILs in theinhibition of asphaltene precipitation from CO2-injectedreservoir oils.26 A plausible explanation is that the concen-tration and activity of [Br]- increases as the alkyl chainlength of the cationic head ring decreases because the molarvolumes/polarities of the [Cnim]þ cations are reduced/pro-moted from [Bmim]þ to [Emim]þ.

It is important to notice that a linear correlation existsbetweenRP andAP values (Figure 5). Since both parametersare estimated from the same series of fluorometric experi-ments, such linear correlation just indicates that HCOmolecules covered by ILs reached a higher degree of polaritywhen compared to the blank which contains no ILs. Then,ILs set a polar environment around HCO molecules, whichallows HCO-ILs-HCO interactions, limits HCO aggrega-tion, and shifts the AP value to a larger HCO concentration.

The higher AP values were then found for ILs presentingan intermediateμ value between 13.11 and 13.62D (seeTableS1 of the Supporting Information). Such ILs should interactand cover HCO molecules in such a manner that permitsHCO-HCO or IL-stabilized HCO micelle interactions, asseen by a higher RP value, but limits its critical aggregationas mentioned above. Due to the high values of the negativecharge in ILs, the X14 halogen ion can be easily attracted byH11 with a positive charge, favoring a hydrogen bondbetween the cation and the anion (Figure 1). This importantcharge distribution on cations and anions results in a sig-nificant dipole moment of ILs that could explain their abilityto shift the aggregation point of HCO. Furthermore, theeffect of the anion is proportional to its size and charge

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 HCOinteraction should exist because the AP is delayed at higherHCO concentrations (Scheme 2a). We assumed that asphal-tene aggregates are stabilized through their interaction withresins to form a nonpolar outer layer. ILs may then attach tothe outer resin layer, which increments the apolar micro-environment aroundmore polarHCOmolecules, i.e. asphal-tenes; as sensed by fluorometry. Then, it seems that ILsaggregate around asphaltenes/resin micelles with their polargroups oriented toward the organic phase, i.e. a directmicelleshould then be formed. The presence of ILs polar groups onthe outer phase of stabilized HCO micelles is suggested alsoby the increment of the relative polarity in presence of ILswhen compared to the HCO blank without ILs (Figure 5).ILs with lower RP and AP values are not capable to avoidcritical aggregation, which can be explained in terms of aheterogeneous or incomplete coverage of HCOmolecules byILs or to more specific HCO-HCO interactions that dis-place HCO-IL interactions.

Respecting the aggregation of HCO as a function ofthe n-heptane content (% v/v), we observed that HCOreaches its critical aggregation at a RP value of 1.01 inabsence of ILs. But, the RP value diminishes to 0.97 for allILs. The fact that all assays showed the same RP value,at different HCO aggregation points, indicates that themicroenvironment around HCO molecules, at the AP,is set now for n-heptane molecules that displace the ILsmolecules (Scheme 2b). IL-HCO interaction should favora major dilution of HCO molecules in n-heptane untilsuch interaction is broken or diminished. At this point,HCO molecules aggregate and precipitate. Indeed, thepresence of aliphatic chains on aromatic sheets, like inasphaltenes, introduces disruptions on the aromatic-sheetstacking and favors dissolution.35 Moreover, the adsorp-tion of p-alkylphenols and p-alkylbenzenesulfonic acids onasphaltenes have served to stabilize them in apolar alkanesolvents.10,11,36,37

Scheme 2. Schematic Representation of HCO Aggregation in the Presence of ILs by Incrementing (a) HCO Concentration and (b) n-Heptane

Content

(35) Buenrostro-Gonz�alez, E.; Andersen, S. I.; Garcia-Martınez,J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732.

(36) Gonzalez, G.; Middea, A. Colloids Surf. 1991, 52, 207.(37) Hern�andez-Trujillo, J.; Martınez-Magad�an, J. M.; Garcıa-Cruz,

I. Energy Fuels 2007, 21, 1127.

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Nevertheless, HCO coverage by some ILs allowed shiftingtheAP 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-heptanecontents when compared to the bromide anion (IL-2) whichhas no effect on theAP value (Figure 6, left).We noticed herethat an ILwith a dipolarmoment higher than 13.0D allowedthe displacement of the AP to higher n-heptane contents, butthe relationship was not linear. It seems to us that other ILproperties or specificity of the HCO-IL interaction shouldrule the aggregation of HCO as a function of n-heptanecontent. For example, IL-4 is able to interact with com-pounds containing π- and n-electrons in water as shown byits r-coefficient of 1.29. But, such capacity enhances 2.5 timesin n-heptane with a correlation of r = 3.28.38 Moreover, aninteresting work has recently shown that polarity (expressedas the dipolarity/polarizability factor, π*, and the dielectricconstant, 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 smallions or incorporating ionic structures that frustrates theformation of nanoscale domains but that the insight is stillunclear.39 In our case, IL-4 has a higher molar volume (345versus 311 A3) 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 insightin the effect of ILs on the aggregation of crude oil.

The use of [Bpyr]þ[Br]- (IL-6) with a higher dipolarmoment (13.62 D) instead of [Bmim]þ[Br]- (IL-2, 12.89 D)incremented the AP value from 30 to 70% v/v n-heptanecontent (Figure 6, middle). This behavior could be alsoattributed to the higher basicity of pyridinium cation thatallows its interaction with HCO acidic molecules. Finally,the use of a shorter alkyl chain allowed the displacement of

the AP value from 30 to 50% v/v n-heptane content asobtained for 1-butyl- (IL-2) and 1-ethyl-3-methylimidazo-lium bromide (IL-1), respectively (Figure 6, right). Hereagain, the IL with a higher dipolar moment allowed thedisplacement of the aggregation point of HCO to highervalues. As mentioned earlier, the reduction of the molarvolume and increase of polarity should increase the stabili-zation of HCO by the shorter alkyl chain of [Emim]þ[Br]-.

Conclusions

We have determined for the first time the CAC value andelectronic properties of some commercial imidazolium- andpyridinium-based ILs by means of spectrofluorometry andtheoretical quantum chemistry, respectively. The quantumchemistry studies revealed that the presence of a complexanion, such as [AlCl4]

-, [BF4]-, or [PF6]

-, substantiallyincremented the electronegativity (χ) and dipole moment (μ)of the studied ILs. This may be at the origin of the betterperformance of such ILs to inhibit asphaltene aggrega-tion in an HCO. The CAC values of studied ILs were equalor below 50 ppm, which indicates the existence of strongself- and IL-resin interactions through an outer stabilizedlayer that protect asphaltenes against agglomeration andprecipitation.

HCO self-interaction to form larger aggregates seems to beof polar nature as suggested by the increment in the spectralcenter of mass versus HCO concentration. This may beattributed to the aggregation of asphaltenes; one of the morepolar molecules in crude oils. On the other hand, HCOaggregation in n-heptane resulted in the increment of thespectral center of mass until the aggregation point is reached.Afterward, the SCM diminishes because asphaltenes andotherHCOmolecules encounter an apolarmicroenvironmentdue to surrounding n-heptane molecules.

The presence of ILs inHCOsolution shifted theAPvalue tohigher HCO concentration or n-heptane volume. In the firstcase, the ILs modified the relative polarity of the microenvir-onment around HCO molecules and shifted the HCO aggre-gation to higher values.Here, ILswith a larger dipolemomentinhibit HCO aggregation until a specific relative polarity isreached.HCOmolecules aggregate in n-heptane as seen by thespectrofluorometric studies and probably due to the insolu-bility of polar asphaltenes in an apolar medium. The presenceof 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 highervalues. Thismay be attributed to a certain polarization by ILsof HCO molecules that protect them from self-aggregation.

Acknowledgment. The authors thank the ProgramaAcad�emico de Posgrado of IMP for the economic support of thiswork. I.G.-C. and J.A.M.-H. are thankful to the Centro deSuperc�omputo de Catalunia (CESCA) in Spain and to theDirecci�on General de Superc�omputo Acad�emico (DGSCA) ofthe UNAM for the support for carrying out the calculations ofelectronic structure. J.A.M.-H. thanks CONACYT and the IMPfor the economic support granted during his Ph.D. studies.

Supporting Information Available: More specialized descrip-tion of theoretical electronic properties and optimized geome-trical parameters of different ILs at the B3LYP/6-31þþG**level of theory. This material is available free of charge via theInternet at http://pubs.acs.org/.

Figure 6. Effect of the anion (left), cation (middle), and alkyl chain(right) of imidazolium- and pyridinium-based ILs on the aggrega-tion point of HCO in n-heptane.

(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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