Molecular Mechanics and Microcalorimetric Investigationsof the Effects of Molecular Water on the Aggregation of

Asphaltenes in Solutions

Juan Murgich*

Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas,Apartado 21827, Caracas 1020A, Venezuela

Daniel Merino-Garcia and Simon Ivar Andersen

Department of Chemical Engineering, Building 229, Technical University of Denmark,DK-2800 Lyngby, Denmark

Jose Manuel del Rıo and Carlos Lira Galeana

Programa de Ingenierıa Molecular, Instituto Mexicano del Petroleo,Avda. Lazaro Cardenas No. 152, Colonia San Bartolo Atepehuacan,

CP 07730, Mexico DF, Mexico

Received April 30, 2002. In Final Form: July 2, 2002

The interaction of two model asphaltene molecules from the Athabasca sand oil with a water moleculein a toluene solution was studied by means of molecular mechanics calculations. It was found that waterforms bridging H bonds between the heteroatoms of asphaltenes with a considerable span in energies. Thestronger H bond found has energies higher than those corresponding to the stacking of the aromatic areasof the same asphaltene molecules. This shows that the water molecule may generate additional mechanismsof aggregation of asphaltenes in toluene solution, as found experimentally. The H bond mechanism dependson the heteroatoms involved, the extension of the aromatic regions, and the steric interference presentin the asphaltene molecules. The simulation results have been compared with experimental values ofenthalpy of association of two different petroleum asphaltenes obtained by titration calorimetry. A simpledimer dissociation model was used to derive the information about the heat and the constant of dissociationfrom asphaltenes of Mexico and Alaska obtained from the calorimetric data. The association enthalpiescalculated were found to be in excellent agreement with those measured, although the simulation onlyemployed the interaction between averaged molecular structures.

Introduction

The effects of traces of water on the aggregation ofasphaltenes of several crude oils in solutions of toluenehave been recently studied by means of calorimetrictitration and other techniques.1 It was found that thepresence of even minute amounts of water in the solvent(<0.015%) had significant effects on the aggregationbehavior of the asphaltenes studied.1 As small amountsof water are present in most if not all crude oils,2 it is ofinterest to study the effects of molecular water in theaggregation process of asphaltenes occurring in solutionsof toluene. In this work, the study of the interaction of twomodel asphaltene molecules from the Athabasca sand oil3

with a water molecule in a toluene solution was under-taken using molecular mechanics calculations. It wasfound that water molecules form bridging H bonds betweensome of the heteroatoms of the asphaltenes that haveenergies lower or higher than those corresponding to thestacking of the asphaltene molecules.4 The existence ofrather strong H bonds shows that water molecules mayprovide an additional aggregation mechanism for similar

asphaltenes and aggregate asphaltenes in solution. Tocheck the values of the heats of interaction calculatedwith molecular mechanics, a number of calorimetricmeasurements have been carried out. The experimentaltechnique chosen was isothermal titration calorimetry5,6

(ITC), which allows the measurement of the heat developedin the titration of a solution of known concentration ofasphaltenes when dissolved in a solvent with known waterconcentration contained in the calorimeter. The experi-mental data were interpreted by means of a model that,for simplicity, only contemplated the dimer-monomerdissociation process. It was found that the heats ofdissociation derived from ITC measurements using thismodel were in agreement with the lowest values found bythe molecular mechanics calculations. Therefore, thecalculatedheatsofdissociation fromthemodelasphaltenesfrom Athabasca seem to also be representative of theinteraction between other types of asphaltenes andbetween these asphaltenes and water molecules. However,this result should be used with caution, as the simpledimer model contains approximations, plus the fact thatthe number of model asphaltene molecules used in thecalculation is rather limited and that these models maynot be the best representation of the molecules of thespecific fraction under study.

* E-mail: jmurgich@qumica.ivic.ve and dmg@kt.dtu.dk.(1) Andersen, S. I.; del Rio, J. M.; Khvostitchenko, D.; Shakir, S.;

Lira Galeana, C. Langmuir, 2001, 17, 307 and references therein.(2) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd

ed.; Marcel Dekker: New York, 1999.(3) Strausz, O. P. Private communication, 1999.(4) Murgich, J. Pet. Sci. Technol. 2002, 20, 1029.

(5) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142,497-502.

(6) Andersen, S. I.; Christensen. Energy Fuels 2000, 14, 38.

9080 Langmuir 2002, 18, 9080-9086

10.1021/la025882p CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 10/19/2002

Molecular Mechanics Methodology

Molecular mechanics determines the most stable con-formation of molecules and their aggregates both in avacuum and in solution.7 This method makes use ofanalytical functions to represent bond stretching, bending,and torsional as well as nonbonded (electrostatic interac-tions, dispersion attraction, and exchange repulsion)energies of molecules. In molecular mechanics, an initialconfiguration is specified and the interatomic distancesand bond angles are adjusted, using an iterative compu-tational method, until the minimum energy configurationis obtained.7 The algorithm used in this work was part ofthe InsightII and Discover set of programs.8 Molecularmechanics can only guarantee to locate the nearest localminimum of the energy surface to the starting point ofthe calculation. For this reason, several different startingconfigurations were used in each case in order to see if thesame minima were obtained for each of them. If this is thecase, then one can assume that the minima reached arerepresentative of the conformations of interest.7

A box containing two different model asphaltenemolecules (T4 and T5 shown in Figures 1 and 2), 100molecules of toluene, and a molecule of water was builtwith the Amorphous Cell program.8 The average densitywas fixed at 0.871 g/cm3, and the temperature during theruns was kept at 298 K. The calorimetric experimentshave been carried out at 303 K. This small temperaturedifference is expected to have only a very small or negligible

influence on the values of the heats of interaction. Thedielectric constant used in the calculations was equal to3.09. Periodic boundary conditions were employed in thecalculations so that an unbound liquid medium could bemodeled around the asphaltenes and water molecule.7

The diffusion of large molecules such as the modelasphaltenes in a liquid is a very slow process in terms ofthe usual molecular times (10-12 s). So, instead of followingtheir diffusion, which will require the use of prohibitivelylong computer times, four different boxes were employedto simulate the different molecular arrangements ofinterest. Each of these configurations is expected to occurduring the diffusion process present in solutions ofasphaltenes in toluene with traces of water. One of theboxes contained the asphaltene and water moleculesseparated by relatively large distances (see Figure 3). Thisbox represents the conformation of a set of free andindependent asphaltene and water molecules in a toluenesolution. Two more boxes were built where a watermolecule bridged the asphaltene molecules with H bondsbetween different heteroatoms (see Figure 4). A fourthboxwasbuiltwhere theasphaltenemoleculeswereallowedto form a dimer while the water molecule was away fromthem (Figure 5). Five initial configurations for each box

(7) Leach, A. R. Molecular Modelling, 2nd ed.; Prentice Hall: London,2001.

(8) Accelrys Inc., San Diego, CA, 2001.

Figure 1. Model asphaltene T4 molecule from the Athabascasand oil.3

Figure 2. Model asphaltene T5 molecule from the Athabascasand oil.3

Figure 3. Distances in angstroms between the water moleculeand the asphaltene molecules corresponding to the set of freemolecules.

Figure 4. Simulation box containing two asphaltenes bridgedby a water molecule. In this box, the water and asphaltenemolecules are shown as space filling objects while the toluenemolecules are drawn with lines only for simplicity.

Aggregation of Asphaltenes in Solutions Langmuir, Vol. 18, No. 23, 2002 9081

were chosen with noticeably different starting points, andeach of them was minimized until the gradient was lessthan 0.1 kcal/mol‚Å. The resulting energy values in eachset agreed within 4%, and only the lowest ones arereported.

The cell multipole summation method was used in thecalculation together with a summation cutoff distance of14 Å. The well-proven CVFF interatomic force field8 hasbeen employed in this work to describe the intra- andintermolecular interactions. The box containing the freeasphaltene and water molecules was taken as our refer-ence conformation (see Figure 3) and was set as the zeroof energy. The first H bond involved one of the O atomsof the asphaltene T4 and an N atom (N1) in T5, as seenin Figures 1 and 2. The second one involved the N fromthe asphaltene T4 and the N1 atom from T5.

Molecular Mechanics Results

The concentration dependence of several properties ofthe asphaltenes in toluene has been used to determine anapparent critical micellar concentration (cmc) of thisfraction.1,5,6 However, the cmc is obviously more opera-tional than indicative of real monomer-molecular ag-gregate equilibrium. Experimental results showed bothherein and in other papers1,5,6 indicate that there is nocritical concentration at which micelles are formed withinthe concentration range examined (0.05-9 g/L toluene).The molecular characteristics of the asphaltenes are verydifferent from those of the typical surfactants that formmicelles,9 as seen in Figures 1 and 2. Even if only theaverage molecular structure of the asphaltenes is known,none of the available models contain the well-defined polarheads and the long hydrocarbon tails that characterizethe surfactant molecules. Instead, the average modelasphaltenes have saturated rings and rather short alkanechains attached to their aromatic cores (see Figures 1 and2). Some of the chains act as bridges between the aromaticregions, so that the molecules have quite complex spatialconformations. In general, several heteroatoms (S, N, O)are also present in these molecules. The anisotropy of themolecular polarity of the model asphaltenes is markedlydifferent from that of the typical surfactants.4 In the caseof the model asphaltene molecules, the presence of a small

amount of heteroatoms and of asymmetric C-C bondsgenerates a charge distribution with a finite dipolemoment. This polarity is, in general, rather weak becausethe amount of highly electronegative atoms such as Oand N is low (<2-3%) in most asphaltenes. Moreover, theintrinsic polarity of model asphaltenes is also low becausethe molecular volume is, in general, large and theestimated dipole moment is small.4 Consequently, thedriving mechanism responsible for the formation of theasphaltene aggregates is likely to involve a differentbalance of the intermolecular forces that are present inthe micelle formation.4 The existence of aromatic regionsprovides the anisotropy that drives the formation of themany asphaltene aggregates. Molecular mechanics cal-culations showed that this contribution together with thatgenerated by the molecular net charges is responsible forthe formation of the asphaltene aggregates when no waterwas present.10 The existence of atomic groups containingbasic N and O atoms implies not only that the asphaltenemolecules will be polar but also that they may interactwith molecules capable of H bonding such as water andacidic asphaltenes and resins. The H bond is a stronginteraction that contains contributions from all theintermolecular forces.11 It involves the interaction of a Hatom, attached to a highly electronegative one (i.e., O, N),with an electronically rich atom of a neighboring moleculeor molecular fragment. This type of interaction betweenfragments and molecules with closed electronic shells ismostly determined by the local atomic charges of both ofthem(electrostatic interaction).Littleornocharge transferin the H bond is found, although noticeable chargerearrangements may occur in both molecules, throughpolarization and induction effects.4 H bonding maycontribute to the aggregate generation through its forma-tion in different sites of the asphaltene molecules. Thesesites must be free of steric hindrances in such a way thatproper interatomic contact is possible between the ap-proaching water (or other acidic) molecule and theaccepting groups. Moreover, the strength of H bonding isorientation dependent, so the approach of the moleculesinvolved should be at proper angles to generate a stronginteraction.12 Otherwise, the H bond will be weak andcontribute little to the aggregation process. As theasphaltene molecules may contain several kinds of basicsites, it is expected that H bonding will contribute to theaggregate formation if they are available for interaction.As in other compounds, the extension of the H bondingdepends on the chemical composition of the asphaltenemolecules and on their shapes. The great chemicalcomplexity of the heavy fractions2 suggests that a widedistribution of H bonds is likely to exist in the molecularaggregates formed by the different asphaltenes. Theexistence of H bonding does not mean that the other partsof the asphaltene molecules are not important in theformation of the aggregates.4 Depending on the strengthof the interactions present in the aggregates, in some ofthem the interaction between the aromatic regions willbe the governing factor while in others the H bonds willbe the dominant ones. A mixed aggregate may also beenvisioned. The complex 3D shapes of most of the modelasphaltene molecules are such that their aggregatesformed by, for example, stacking may leave some emptyintermolecular spaces, thus reducing the number of thepossible favorable atomic interactions. Small moleculessuch as water or methane could fill these empty spaces

(9) Evans, D. F.; Wennestrom, H. The Colloidal Domain, 2nd ed.;Wiley-VCH: New York, 1999.

(10) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 68.(11) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.;

Academic: New York, 1991.(12) Muller-Dethlefs, K.; Hobza, P. Chem. Rev. 2000, 100, 143.

Figure 5. Simulation box containing an asphaltene dimer anda free water molecule. The water molecule can be seen in theupper right corner of the box. All the molecules were drawn asin Figure 4.

9082 Langmuir, Vol. 18, No. 23, 2002 Murgich et al.

and act as an adhesive by contributing with furtherattractive interactions to the system.13 This type ofbehavior is found in many crystals of complex moleculessuch as proteins and other complex macromolecules wherethe inclusion of water (or other similar solvent) moleculesstrongly stabilizes the resulting structures.13 In thisregard, it was recently shown14 that water plays animportant role in stabilizing the entire system of ligandand duplex DNA by means of H bonds and that the watermolecule bridges play an important role in the recognitionof DNA by a bonding protein15 (trp-repressor). If the smallmolecules have strong intrinsic polarity, as water does,then H bonding will compete with the van der Waalsinteraction when the empty inter-asphaltene spaces arefilled with them and receptor sites are available forbonding. If the asphaltene molecular 3D shapes are suchthat the basic O and N atoms are available for H bonding,then a water molecule may act as a bridge between twosuch asphaltenes. At high concentrations, these asphalt-enes may also form trimers, tetramers, and so forth, ifadditional water molecules are available for H bonding.The dissociation model used in the fit of the experimentalvalues only considers dimers and monomers. Some of thesemolecular arrangements have been studied in this workusing two molecular models of asphaltenes from Atha-basca. In Table 1 are shown the changes in enthalpyobtained for the different configurations of the model boxcalculated in this work.

The H bonds between different heteroatoms of theasphaltene molecules may have quite different energies,as seen in Table 1. This reflects the fact that these bondsare quite sensitive to the heteroatoms intervening in them,the intermolecular orientation, and the interatomic dis-tances.11 The last point can be seen in Table 1, where thestrongest H bonds are those with the shortest interatomicdistances.

It is reasonable to assume that the molecular complexityof the asphaltene fraction will generate a distribution inthe H bond strength with water molecules. If the molecularstructure of the asphaltenes is such that it is able toproduce strong H bonds, as in the T4-O- - -HOH- - -N1-Tcase or similar cases, this interaction could compete withor even easily overcome the stacking process and generateaggregates linked by water molecules acting as H bondbridges. This mechanism will be particularly importantin asphaltene molecules with relatively small aromaticregions and strong basic sites. Then, it is not surprisingthat the presence of traces of water in toluene has an

important role in the formation of the aggregates of theasphaltenes in solution. The large values of the changesin enthalpy involved in some of the conformations mayexplain the generation of asphaltene aggregates observedwhen the amount of water is increased in the toluenesolution. At low water content, the asphaltene stacks aremore likely to occur than the formation of aggregates withwater bridges. At higher water concentration, the bridgesbecome more likely to occur and will then generate anadditional mechanism for the formation of the aggregatesof asphaltenes. It is also possible to visualize the formationof water bridges between different aggregates if theconcentrations of both of them are high enough.

The water molecules are not the only ones that maycontribute to the formation of aggregates of asphaltenesthrough the formation of intermolecular H bonds. In theasphaltenes there are generally a basic, an acidic, anamphoteric, and a neutral fraction.2 The water moleculeswill then have to compete with the acidic asphaltenes inorder to form the intermolecular H bonds. Thus, asphalt-enes with a large basic fraction but with a small acidicfraction will be more prone to be affected by the H bondsformed by the water molecules than the others where theacidic one is the larger fraction. Further work is under-way in order to study the influence of the different frac-tions on the aggregation behavior of the asphaltene insolution.

Calorimetric Measurements

Asphaltenes have been obtained following a modifiedversion of the IP143 standard.16 The solutions have beenprepared at least 24 h before the experiments and storedin sealed vials in a dark place. To remove dissolved gases,the samples have been placed in an ultrasonic bath for 1h. Toluene has been dried with molecular sieves, and thesolutions with the different water contents have beenprepared by mixing dried and saturated toluene. Experi-ments have been performed in a VP-ITC Microcal calo-rimeter17 with asphaltenes from two different sources:Alaska 95 and KU (Mexico). The experiments consist ofthe injection of known volumes (10 µL) of asphaltenesolutions into a cell that contains toluene with the samewater content as that of the asphaltene solution. Thecalorimeter was placed in a glovebox that had silica gelin its interior to ensure low humidity. The syringecontained concentrated solutions of asphaltenes (5, 30, or50 g/L) to ensure that they are mainly in the associatedstate. When these solutions were injected into the toluene,the degree of association decreases until the equilibriumis reached and the heat developed is counteracted by thecontrol system of the calorimeter, to keep the isothermalconditions (303 K). This heat is recorded and displayed inthe computer, and the integration of the peaks gives theheat developed in each injection (see Figure 6). The firstinjection point is not used in the treatment of the data,because the tip of the syringe has been in contact with thesolution in the cell during the equilibration time, prior tothe injections, and it is possible that some asphalteneshave diffused into the cell.

The optimization of the calorimeter has allowed us towork at very low concentrations and heats measured (<5µcal/injection), yet keep a good noise level and peakdefinition, as seen in Figure 6.

(13) Kitaigorodsky, A. I. Molecular Crystals and Molecules; Aca-demic: New York, 1973.

(14) Yang, L.; Feng, X.; Lee, I.; Bai, C. J. Mol. Struct. 1998, 444, 13.(15) Suenaga, A.; Yatsu, C.; Komeiji, Y.; Uebayasi, M.; Meguro, T.;

Yamato, I. J. Mol. Struct. 2000 526, 209.

(16) IP 143/90 (or ASTM D3279-90), Institute of Petroleum, London,U.K., 1985.

(17) Microcal Inc., Northampton, MA 01060-2327, 1995.

Table 1. Changes in Enthalpy of the DifferentConfigurations of the Model System

configurationa

changes inenthalpy(kcal/mol) distance (Å)

(T4-O- - -HOH- - -N1-T5) -21.5 T4-O- - -H-O 2.89T5-N1- - -H-O 3.09

(T4-N2- - -HOH- - -N1-T5) -2.1 T4-N2- - -H-O 4.22T5-N1- - -H-O 2.92

T4- - -T5 dimer andfree water

-6.3

a T4-O- - -HOH- - -N1-T5 corresponds to the bridging of theasphaltene T4-O and T5-N atoms by water while T4-N2- - -HOH- - -N1-T5 indicates the same between the N atoms. The right columnshows the distance between the N or O atoms of the asphaltenesand the O atom of the water molecule.

Aggregation of Asphaltenes in Solutions Langmuir, Vol. 18, No. 23, 2002 9083

Dissociation ModelSome investigators5,18,19 have suggested that asphalt-

enes in solution have, as the surfactants, a well-definedcmc. If there was a cmc in the concentration range studied(0.2-9 g/L), then the first injections (when Ccell < cmc)should give a constant heat signal, as all the micelles wouldbreak and asphaltenes would exist in the cell only in themonomeric state (Figure 6). However, in the experimentspresented here, the heat developed decreased as theconcentration was increased, which means that theaggregates break less and less as the solution gets moreand more concentrated. This rules out the possibility ofhaving a cmc in this range of concentrations1,6 and givessupport to the idea that the asphaltenes studied formsimple molecular aggregates even at very low concentra-tions. It is, however, possible to argue that the cmc is ateven lower concentrations than the values usually foundin the literature. Nevertheless, according to the calori-metric experiments, the association goes on at higherconcentrations than the “cmc”, following a stepwisemechanism contrary to the behavior found for micelles.9In this particular type of molecular aggregates formedonly by surfactants, an increase in concentration abovethe cmc increases the number rather than the size of themicelles. These results lead to the development of adifferent model than that of micelles to fit the experimentaldata for the dissociation of the aggregates of asphaltenes.As a first approach, a simple dimer-monomer dissociationmodel has been chosen. It has to be taken into accountthat asphaltenes are a very complex fraction with a widedistribution of molecular weights, aromaticity, and hetero-atoms content,2 so the application of a more complex modelthat will include the formation of higher aggregates is notwarranted at this stage. The simple model considers that

asphaltenes present only equilibrium between the mono-meric species A and the dimers A2, so that

where Kd is the dissociation constant. In this model, allthe asphaltenes interact with the same equilibriumconstant Kd. The starting solution is one of high concen-tration, so the asphaltene molecules will mainly be in theaggregated state. When this solution is injected into thecell, it is no longer in equilibrium, and therefore theaggregates dissociate until the equilibrium conditions arereached again. The heat developed is then calculated ashcalc ) moles of aggregates that dissociate × ∆Hd. Then,∆Hd is the heat of dissociation in kilocalories per mole.Consequently, the heat of interaction between asphaltenemolecules will be -∆Hd. The parameters of the dissociationmodel are ∆Hd and Kd, and their values are optimized tofit the model to the experimental data. In Figure 7 areshown some examples of the fit of the simple dissociationmodel. Table 2 contains the results for the three concen-trations and the two water contents used in the experi-ments. The measurements have been performed at threedifferent water concentrations: dried toluene, saturatedtoluene, and 75% water saturated toluene (31SD Toluene).The fitting of the simple dissociation model shown inFigure 7 is surprisingly good considering the approxima-tions employed in its derivation. Figure 8 shows the

(18) Rogacheva, O. V.; Rimaev, R. N.; Gubaidullin, V. Z.; Khakimov,D. K. Colloid J. USSR 1980, 42, 586 (Translated version).

(19) Sheu, E. Y.; De Tar, M. M.; Strom, D. A.; Decanio, S. J. Fuel1992, 71, 99.

Figure 6. Titration of a solution of KU asphaltenes in driedtoluene into dried toluene. The asphaltene concentration inthe measurement shown was 30 g/L.

Figure 7.

A2 T 2A w Kd ) [A]2/[A2] (1)

9084 Langmuir, Vol. 18, No. 23, 2002 Murgich et al.

comparison of the experimental heats of interaction withthe ones obtained in the molecular mechanics calculations.There is excellent agreement among the experimentalresults obtained for the different concentrations, whichsuggests that the mechanism of interaction that prevailsis the same in the whole concentration interval studied.The KU asphaltenes appear to have slightly lower heatsof interaction than those of the Alaska 95 crude oil.

The heats of dissociation measured in the experimentsfor both types of asphaltenes are similar to the lower oneobtained in the molecular mechanics calculations for waterbridges between N atoms. This seems to suggest that theH bonds present in these asphaltene aggregates are mostlyof medium strength. It is interesting to mention that theheat of interaction obtained in the simulations shown inFigure 4 (O- - -HOH- - -N type) seems to be much higherthan the experimental values of the calorimetric experi-ments, so the occurrence of this type of strong H bond inthese asphaltenes is expected to be negligible in the KUand Alaska 95. Also, the water content, in the case of KUasphaltenes, does not seem to have any influence on theheat of dissociation. In the case of asphaltenes from theAlaska 95 crude oil, the decrease in water content leadsto an increase in the heat of interaction. This can beexplained as follows: when the concentration of waterdecreases, the capacity of the system to form H bondsdecreases and therefore the heat of stacking becomes moreand more predominant. As seen in Table 1, the calculatedheat of stacking may be higher than the heat of interactionby H bonds between N atoms of the model asphaltenes.This result may explain the small rise in heat of dis-

sociation observed in the experiments. It is important,however, to take into consideration that the drying processof the toluene is not complete and it has not been possibleto dry it below 10% saturation. The solubility of water intoluene at 25 °C is approximately 0.055%, and the driersolutions contain an average of 0.007% water, measuredby Karl Fischer titration. In the most unfavorable case(the highest concentration of asphaltenes in the cell is 8g/L), the ratio water/asphaltene in moles in the cell is 0.4(one molecule of water for approximately every two ofasphaltene). This experimental problem must be takeninto account in the analysis of the influence of the watercontent, as it is present in all tests. With this in mind, wemay say that the results obtained by ITC showed thenthat a decrease in the water content reduces the degreeof H bonding and increases the contribution of the heatof stacking for the asphaltenes studied. This is inagreement with the conclusions drawn from the molecularmechanics calculation, where one can see that the stackingaggregation mechanism may have a larger energy, at leastfor the model asphaltenes of Athabasca. The dissociationconstants obtained from the simple model are observedto increase with asphaltene concentration and to decreasewith decreasing water content at fixed concentration. Botheffects can be explained by the same mechanism: as theconcentration of asphaltenes increases, the relative watercontent on a molecular level decreases, hence favoringthe dissociation. When more water is present at fixedasphaltene concentration, we observe that the dissociationconstant decreases as the water bridges keep asphaltenemolecules together. No transition from one mechanism toanother can be determined, as the heat of dissociation isalmost independent of concentration.

It is important to mention that all these results mustbe used with some caution because of the approximationsemployed in the dissociation model and in the molecularmechanics calculations. First, the molecules of the Atha-basca asphaltenes (Figures 1 and 2) do not necessarilycontain the heteroatoms in the same positions as those inthe asphaltenes of the KU and Alaska 95 crude oils usedin the experiments. Molecular variations are expected tooccur in the asphaltene fractions,2 so differences in thenumber and character of the H bond acceptor sites areexpected to be present in the asphaltenes.

Second, the average molecular weight for these otherasphaltenes might be quite different than the averageone assumed (1200 g/mol‚g) for the asphaltenes from theAthabasca sand oil. Moreover, the use of just a singlevalue to characterize a complex MW distribution such asthat found in asphaltene fractions is still open to discus-sion. Third, the heats of dissociation measured by ITCcontain contributions of the dissolution of aggregatescontaining all (or most of) the molecules that form theasphaltene fraction. Each measured value is actually aweighted average over the many different types ofaggregates formed by the great variety of molecules of theasphaltene fraction. It is known that, in asphaltenes, theMW distribution is complex, sometimes is multimodal,and covers a wide range of values.20,21 Similar distributionsare expected to occur in other asphaltenes,2 thus showingthat this fraction encompasses a large variety of differentmolecules. Therefore, the values of the heats of dissociationmeasured contain contributions from different types ofmolecular aggregates. On the other hand, the valuescalculated by molecular mechanics only reflect the in-

(20) Selucky, M.; Kim, S. S.; Skinner, F.; Strausz, O. P. In Chemistryof Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in ChemistrySeries 195; American Chemical Society: Washington, DC, 1981; p 83.

(21) Andersen, S. I. J. Liq. Chromatogr. 1994, 17, 4065.

Figure 8. Squares represent KU asphaltenes, and circlesrepresent those of Alaska 95. The solid markers represent testswith 75% water saturated toluene, and the open ones representtests with dried toluene. The dashed line represents thecalculated heat of interaction with water as a bridge betweenthe N atoms (N- - -HOH- - -N) (see Table 1).

Table 2. Dissociation Constants and Heats ofDissociation Obtained from the ITC Measurements

dried toluene 75% water saturated toluene

conc(g/L)

∆Hd(kJ/mol)

103Kd(mol/L)

∆Hd(kJ/mol)

103Kd(mol/L)

KUAsphaltene5 0.9 0.83 1.0 0.69

30 1.3 3.25 1.2 1.80

Alaska 95 Asphaltene5 1.7 0.12 1.4 0.79

30 2.1 5.04 1.8 3.8650 1.6 9.11 1.4 5.19

Aggregation of Asphaltenes in Solutions Langmuir, Vol. 18, No. 23, 2002 9085

teraction between a specific pair of asphaltene moleculesthat may or may not be the most representative of thefraction of the crude oil under study. Therefore, the heatsof interaction displayed in Table 1 only provide an estimateof its order of magnitude and should then be used withcare. The results obtained are then more of a qualitativethan quantitative nature, and further studies are requiredto evaluate their general validity.

ConclusionsMolecular simulation of association of petroleum as-

phaltenes including association with water has beenperformed and compared with experimental results ofheats of association obtained from titration calorimetry.

The results fall within the same order of magnitude,verifying that H bonding indeed may be an importantfactor in the association of asphaltenes. In this, watermay act as a promoter of the association because of itssmall size and high intrinsic polarity.

Acknowledgment. The authors appreciate the valu-able technical help provided by Jose A. Abanero, and theythank Prof. O. P. Strausz for providing the molecularstructure of the asphaltenes from Athabasca prior topublication. D.M.-G. and S.I.A. thank the Danish Techni-cal Research Council for financial support under the Talentprogram.

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