Energy & Fuels 2005, 19, 477-484
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Dispersant Adsorption during Asphaltene Aggregation Studied by Fluorescence Resonance Energy Transfer (FRET) Fabricio Arteaga-Larios,† Ana Cosultchi,‡ and Elı´as Pe´rez*,‡ CIEP Facultad de Ciencias Quı´micas and Instituto de Fı´sica, Universidad Auto´ noma de San Luis Potosı´, Alvaro Obrego´ n 64, 78000 SLP, Mexico and Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´ leo, 152 Eje Central L. Ca´ rdenas, 07720, Mexico Received March 19, 2004. Revised Manuscript Received October 23, 2004
Fluorescence resonance energy transfer (FRET) is used to study the adsorption of a commercial dispersant on asphaltenes in a toluene-heptane solution. We induce the asphaltene aggregation process by changing the heptane volumetric fraction, where heptane plays the role of a precipitant agent. The dispersant and asphaltenes are naturally fluorescent, and no special labeling was performed to use this fluorescent technique. Low concentrations of asphaltene and dispersant were used in this works0.007 and 0.055 g/L, respectivelysto avoid self-association of the dispersant and to induce asphaltene aggregation only by the presence of heptane in the solution. The results show different behavior of the energy transfer below and above the precipitation onset of the asphaltenes, corresponding to a heptane volumetric fraction of 0.6. The dispersant adsorption was followed during the induced aggregation by measuring the resonance energy transfer from the dispersant (energy donor) to the asphaltenes (energy acceptor) above a heptane volumetric fraction of 0.7, where aggregates are formed and stabilized by the dispersant. The fluorescent experiments presented here also give information about the asphaltene stability in the solvent below the asphaltene precipitation onset.
Introduction Asphaltene deposition in petroleum pipes has produced a negative impact on the petroleum economy for many decades, and methods to resolve the problem are continuously being investigated.1 The asphaltene precipitation may occur as a consequence of thermodynamic changes during the crude oil production, which favors the precipitation and aggregation of asphaltenes.2 The most common methods used for asphaltene removal are mechanical, solvent, and chemical methods. In the mechanical method, the attached asphaltene is removed by scraping. In the solvent method, hot oil, water, or steam is used to melt paraffinic deposits, and diesel or aromatic solvents are used to dissolve the asphaltene particles. In the chemical method, dispersants are used alone or in association with solvents to prevent asphaltene associations. In the latter case, dispersants are amphiphilic molecules that play the role of resins, which are amphiphilic molecules that stabilize the asphaltenes in the crude oil.3 * Author to whom correspondence should be addressed. Telephone: (52) 444 826 23 63, ext. 134. Fax: 444 826 13 38. E-mail address:
[email protected]. Instituto de Fisica. † CIEP Facultad de Ciencias Quı´micas. ‡ Programa de Ingenierı´a Molecular. (1) Sharma, M. K., Yen, T. F., Eds. Asphaltene Particles in Fossil Fuel Exploration, Recovery, Refining and Production Processes; Plenum Press: New York, 1994. (2) Pfeiffer, J. P.; Saal, R. N. J. Phys. Chem. 1940, 44, 139. (3) Leontaritis, J. K. Asphaltene Deposition: A ThermodynamicColloidal Model, Ph.D. Dissertation, University of Illinois at Chicago, Chicago, IL, 1988.
The dissolution of precipitated asphaltenes involves an asphaltene solvation followed by dissociation of the asphaltenes.4 Compared to the solvent method, dispersants do not dissolve asphaltene aggregates but they do disaggregate and generate new small aggregates that are stabilized by the dispersants. The capacity of dispersants to interact and reduce the asphaltene aggregate sizes is dependent on the polarity headgroup and the saturated hydrocarbon tail size of the dispersants.5 Therefore, the asphaltenes remain in the solution only when the dispersant-asphaltene or solvent-asphaltene interactions are stronger than asphalteneasphaltene interactions.6 There are two models related to the stability of asphaltenes in a solution. In the first model, resins stabilize the asphaltenes and precipitation occurs as a consequence of the decrease of the resin amount adsorbed at the asphaltene surface.2 This is referenced as the steric-stabilization model. In the second model, which is referenced as the solubilization (lyophilic) model, asphaltenes are assumed to be nonassociated molecules that are dissolved in a hydrocarbon medium.7 Apparently, the steric-stabilization model describes well (4) Zhang, Y.; Takanohashi, T.; Sato, S.; Kondo, T.; Salto, I. Energy Fuels 2003, 17, 101. (5) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749. (6) Sheu, E. Y., Mullins, O. C., Eds. Asphaltenes, Fundamentals and Applications; Plenum Press: New York, 1995. (7) Cimino, R.; Correa, S.; Del Bianco, A.; Lockhart, T. P. Solubility and Phase Behavior of Asphaltenes in Hydrocarbon Media. In Asphaltenes, Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter 3.
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a fresh asphaltene solution, whereas the solubilization model was determined to be adequate to illustrate an aging asphaltene solution.8,9 Asphaltenes are organic solids mainly formed by polar and polyaromatic compounds, which are highly fluorescent systems.10 Dispersants used in the oil industry generally contain fluorescent groups, according to their chemical compositions, that mimic the resins; therefore, a fluorescent technique is appropriate to evaluate the molecular adsorption of dispersants onto asphaltenes. Fluorescence spectroscopy has already been used to study oil systems.10 For example, fluorescence quenching has been used to show that different crude oils contain the same chromophores, but in different concentrations, depending on their origin and their API gravity.11,12 Fluorescence spectra are related to electronic levels, which are dependent on the molecular conformation and their environment where the fluorescent molecule is placed.10 In the present work, we propose that the fluorescence resonance energy transfer (FRET) technique can be used in the study of the dispersant adsorption onto asphaltenes. This technique can be incorporated in the so-called optical spectroscopy, which uses the excitation of the valence electrons as probes.10 The FRET phenomenon occurs when two fluorescent molecules are close to each other. In this process, part of the energy of one molecule is transferred to the other fluorescent molecule by a dipolar resonance process.13 These are called donor and acceptor molecules, respectively. Energy transfer is assured only if the emission spectrum of the donor overlaps the excitation spectrum of the acceptor, and its efficiency is dependent on the distance between the fluorescent groups. Typically, the distance is on the order of several nanometers. Several studies concerning the average distance between two labeled molecules have been performed using this technique14 (for example, monolayer assembly formation,15 end-to-end chain distance determination,16 latex particle structure,17 and also changes in asphaltene microenvironments18). The experiments in the present work were designed to study the behavior of asphaltene dissolved into a toluene-heptane mixture in the presence of a dispersant. Asphaltenes were dissolved into toluene and precipitated with increasing amounts of heptane with or without the dispersant. The entire process was followed by the FRET technique. FRET is presented here to be a useful and powerful method to evaluate dispersant (8) Mannistu, K. D.; Yarranton, H. W.; Masliyah, J. H. Energy Fuels 1997, 11, 615. (9) Cosultchi, A.; Bosch, P.; Lara, V. H. Colloid Polym. Sci. 2003, 281, 325. (10) Mullins, O. C., Sheu, E. Y., Eds. Structure and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (11) Ralston, C. Y.; Wu, X.; Mullins, O. C. Appl. Spectrosc. 1996, 50, 1563. (12) Downare, T. D.; Mullins, O. C. Appl. Spectrosc. 1995, 49, 754. (13) Fo¨rster, Th. Discuss. Faraday Soc. 1959, 27, 7. (14) Van der Meer, B. W., Coker, G., Simon-Chen, S. Y., Eds. Resonance Energy Transfer: Theory and Data; Wiley-VCH: New York, 1991. (15) Kuhn, H.; Mobius, D.; Bucher, H. Spectroscopy of Monolayer Assemblies. In Physical Methods of Chemistry; Weissberger, A., Rossiter, P., Eds.; Wiley: New York, 1973. (16) Lakowicz, J. R.; Wiczk, W.; Gryczynski, I.; Fishman, M.; Johnson, M. L. Macromolecules 1993, 26, 349. (17) Pe´rez, E.; Lang, J. Langmuir 1996, 13, 3180. (18) Pietraru, G.-M.; Cramb, D. T. Langmuir 2003, 19, 1026-1035.
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performance and asphaltene stability in this complex system. Materials and Methods Materials. Heptane (analytical grade, Fermont, Mexico) and toluene (technical grade, Caledon, Mexico) were used as received. Asphaltenes were precipitated from a Maya crude oil with an excess of 40 volumetric parts of heptane; the suspension was sonicated, filtrated, and dried. The solid fraction was dissolved in 10 wt % of CH2Cl2. The solvent was evaporated and the asphaltenes were separated by repeated heptane reprecipitations, eliminating the supernatant phase in each step. An appropriate dispersion of asphaltene in heptane was reached in each step, following ultrasonic shaking and centrifugation cycles. The number-averaged and weightaveraged molecular weights (Mn and Mw, respectively) of the precipitated asphaltene with different toluene/heptane ratios were determined by size exclusion chromatography (SEC) at 5 wt % in tetrahydrofuran (THF), using polystyrene standards, and their values are Mn ≈ 1000 g/mol and Mw ) 3000-4000 g/mol.19 The commercial dispersant evaluated in this work (InSol AW) was used as received from Kosta Oil Field Tech Company. The Mn value of the dispersant was 765 g/mol; this value was obtained by the vapor pressure osmometry (VPO) technique, using a Corona Wescan model 232A VPO device. The dispersant is dissolved in benzene, which is also used as a reference. The infrared analysis shows that the most probable chemical composition of this dispersant corresponds to an ethoxylate compound; thus the head is non-ionic. In the same way, the tail may be a large alkyl chain with ether groups integrated inside it. These groups allow the dispersant to dissolve in polar or weakly polar liquids, such as toluene. Sample Preparations. Fluorescent spectra were recorded from three different sets of samples: dispersant-tolueneheptane, asphaltene-toluene-heptane, and asphaltene-dispersant-toluene-heptane. The asphaltene-dispersant-heptane-toluene system and the asphaltene-heptane-toluene system were composed by 13 samples, each with a volume of 5.0 mL, which were prepared with different volumetric fractions of heptane (0.0-0.96). The measurements were performed in toluene-heptane solutions, whereas the asphaltenes are well dispersed. The selected asphaltene concentration used for the dispersant adsorption experiments was 0.007 g/L, whereas the dispersant concentration was 0.055 g/L. At these concentrations, the dispersant/asphaltene molar ratio is 40:1, which corresponds to an excess of dispersant that we assume is adequate for the complete adsorption of dispersant onto asphaltenes. All systems were analyzed after 1 h of preparation under ambient conditions. The results are reported as a function of the heptane volumetric fraction. Fluorescence Spectra. The emission and excitation spectra are a function of the radiation wavelength and are measured in fluorescent experiments. Emission (excitation) spectrum is obtained by fixing wavelength of excitation (emission). These spectra are very sensitive to the molecular structure of fluorescent molecules; as a general trend, large structures correspond to spectra at higher wavelengths.20,21 Fluorescent spectra are due to delocalized electrons; in aromatic molecules, these correspond to π-electrons.22 Large (19) Buenrostro-Gonzalez, E.; Andersen, S. I.; Garcia-Martinez, J. A.; Lira-Galeana, C. Energy Fuels 2002, 16, 732. (20) Ralston, C. Y.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 1996, 10, 623. (21) Albuquerque, F. C.; Nicodem, D. E.; Rajagopal, K. Appl. Spectrosc. 2003, 57 (7), 805. Goncalves, A. S.; Castillo, J. A.; Fernandez, A.; Acevedo, S. Proc. SPIE-Int. Soc. Opt. Eng. 2003, 4829, 829. Goncalves, A. S.; Hung, J.; Gutierrez, H.; Fernandez, A.; Castillo, J. A. Rough Surface Scattering and Contamination. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3784, 393.
Dispersant Adsorption during Asphaltene Aggregation structures are associated with smaller energy electronic transitions and, therefore, higher fluorescent wavelengths. Emission and excitation spectra were obtained using a steady-state spectrofluorometer (Photon Technology International, model PTI/C-700). The chromatic light was produced by a 75-W xenon arc lamp, two monochromators select the excitation and emission wavelength, and a photon-counting photomultiplier detects the fluorescence intensity. A circular quartz cell (1 cm in diameter) is used to measure each sample, and detection is practiced at 90°. Emission and excitation spectra are obtained as functions of the wavelength. A step size of 1 nm was used in all fluorescence spectra with a time interval of 1 s. The fluorescent light intensity is controlled by the lamp intensity and excitation and emission slits. The spectra reported in each figure were obtained under similar experimental conditions; the fluorescent intensity was normalized to 100 a.u. in all cases. Fluorescence Radiative Energy Transfer (FRET). The FRET process is based on the energy transfer of electronic excitation from donor to acceptor molecules, which must be in close proximity. The donor molecule is excited and the acceptor emission is registered. The emission from the acceptor molecule is produced by dipolar resonance, as a consequence of energy transferred from the donor. The efficiency of energy transfer between two molecules is given by 1/[1 + (r/Ro)],6 where r is the distance between the two molecules and Ro is the called the Fo¨rster radius, which typically varies from 10 Å to 100 Å.23 The conditions to use the FRET technique are as follows: (C1) The excitation spectrum of the acceptor must overlap the emission spectrum of the donor; and (C2) Donor and acceptor molecules must be in close proximity, at a distance of ∼Ro. Condition C2 dictates that the donor and acceptor molecules be close to each other, at a distance of ∼Ro, which generally is on the order of the molecular length. The study of molecular adsorption is feasible only when these conditions are satisfied. Dynamic Light Scattering (DLS). A dynamic light scattering (DLS) apparatus is formed by a multiple tau digital real time correlator (ALV GmbH model ALV-6010/160), a single photon detector (ALV GmbH model ALV/SO-SIPD), a goniometer (Brookhaven model BI-200SM), and a helium-neon laser of 40 mW, emitting at a wavelength of λs ) 633 nm. The detection angle was fixed at 30°. Because the light scattered by the dispersant-asphaltene systems was low, the maximum laser intensity was used. DLS is sensitive to large molecules and aggregates that scatter the light;24 thus, we have observed a good resolution for particles with a diameter of >λs/20. Therefore, the hydrodynamic size is measured by this technique, and it corresponds to asphaltene aggregates rather than to single or associated molecules. The DLS determinations were performed after the fluorescent experiments were conducted. In addition, the refraction index and the viscosity of the solutions were required to calculate the size of the aggregates. Thus, the refraction index for each of the heptanetoluene mixtures were measured in a Digital Abbe refractometer apparatus. The viscosity was calculated based on the viscosity of pure compounds, using a linear mixing rule for each solution and the solvent molar fractions as factors. Linear behavior is expected for this mixture, according to previously obtained experimental results for toluene-alkane mixtures.25 The hydrodynamic sizes of aggregated formed in the dispers(22) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic-Plenum Publishers: New York, 1999. (23) Berlman, B. I., Ed. Energy Transfer Parameters of Aromatic Compounds; Academic Press: New York, 1973. (24) Berne, B. J., Pecora, R., Eds. Dynamics Light Scattering; Krieger Publishing Company: Melbourne, FL, 1990. (25) Vavanellos, T. D.; Asfour, A.-F.; Slddlque, M. H. J. Chem. Eng. Data 1991, 36, 201. Asfour, A.-F.; Slddlque, M. H.; Vavanellos, T. D. J. Chem. Eng. Data 1990, 35, 199.
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Figure 1. Excitation and emission spectra of the dispersant and asphaltene in toluene at a heptane volumetric fraction of 0.2. The shaded area represents the overlap area between the dispersant emission and the asphaltene excitation. ant-asphaltene systems were calculated using a cumulant method, assuming a spherical form.
Results FRET First Condition for the Dispersant. Spectra of the asphaltene and dispersant in a solution of 0.8 toluene/0.2 heptane are shown in Figure 1. The first FRET condition (C1) is satisfied here, because the dispersant emission spectrum overlaps the asphaltene excitation spectrum. Therefore, we would excite the dispersant and detect the asphaltene emission due to FRET from the dispersant to the asphaltene. Thus, in this system, the dispersant may play the role of donor and the asphaltene the role of acceptor. Spectra at higher wavelengths have been associated with larger molecular fluorescent structures.20,21 Thus, the fluorescent structure in asphaltene molecules is larger than that of the dispersant, according also to their respective molecular weights. Selection of Asphaltene and Dispersant Concentrations. Any possible changes in dispersant or asphaltene structures, which can occur during the experiments independent of the adsorption process, may distort our interpretation. Normally, the solvent type and high solute concentrations could induce such changes. Structural variations that are due to any of the components of the solution could be tested directly from the fluorescence measurements. Thus, the dispersant emission and asphaltene excitation spectra were separately registered for different concentrations of the dispersant and asphaltene. Critical micelle concentrations (CMCs) of the dispersant in toluene and heptane were first determined from the fluorescence spectra. The emission intensity was measured at different dispersant concentrations, and they are shown in Figure 2A for toluene and Figure 3A for heptane. Integrated fluorescence intensity is defined as the area under the curve of the fluorescence intensity. The integrated fluorescence intensity, as a function of the concentration, is shown in Figure 2B for toluene and Figure 3B for heptane. The slope discontinuities give us the CMC values: 0.65 g/L for toluene and 0.5 g/L for heptane. As expected, the dispersant emission spectra
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Figure 2. (A) Emission spectra from the dispersant in toluene at different concentrations. Arrow indicates the direction of increasing concentration. (B) Critical micelle concentration (CMC), as determined from the slope discontinuity of the integrated fluorescence intensity.
at different concentrations below the CMC in toluene and heptane were the same, as shown in Figures 2A and 3A. In the following experiments, we used a dispersant concentration of 0.055 g/L, which is approximately one tenth below the CMC value in heptane. The dispersant emission spectra at different heptane volumetric fractions are shown in Figure 4 and correspond to spectra with a maximum peak at ∼325 nm. The wavelength of excitation used here (290 nm) is able to excite both the dispersant and the solvent; therefore, the change observed in the emission spectra is only apparent and it contains the solvent contribution. The emission spectra due to the solvent are also presented in Figure 4, and they have the maximum peak at ∼358 nm. The dispersant emission spectra presented in Figure 5 were obtained after the solvent contributions were subtracted. Thus, independent of the heptane volumetric fraction, the dispersant emission spectrum is practically unaltered. In the case of asphaltene dissolved in toluene, the variations observed for the excitation spectrum at different concentrations are shown in Figure 6, indicating a strong dependence on the asphaltene concentration. From 0.009 g/L to 0.018 g/L, only intensity changes are observed, whereas at higher concentrations (0.036, 0.054, and 0.090 g/L), there is a systematic increase of the red shift, which suggests the association of asphalt-
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Figure 3. (A) Emission spectra from the dispersant in heptane at different concentrations. Arrow indicates the direction of increasing concentration. (B) CMC, as determined from the slope discontinuity of the integrated fluorescence intensity.
Figure 4. Emission spectra of the dispersant and solvent excited at 290 nm at different heptane volumetric fractions. Maximum peaks at ∼325 and ∼358 nm correspond to the dispersant and the solvent, respectively.
enes.21 To induce only associations by the presence of heptane, the experiments were performed at an asphaltene concentration of 0.007 g/L, which is indeed below the reported CMC in toluene.26 The corresponding excitation spectrum at this concentration is shown in Figure 7.
Dispersant Adsorption during Asphaltene Aggregation
Figure 5. Dispersant emission spectra after subtraction of the solvent signal.
Figure 6. Asphaltene excitation spectra in toluene at different asphaltene concentrations. The main peak moves to a longer wavelength (red-shift) as the asphaltene concentration increases.
Figure 7. Asphaltene excitation spectra as a function of the heptane volumetric fraction. The continuous bold line corresponds to the average dispersant emission spectrum obtained from Figure 5.
Fo1 rster Distance for the Dispersant-Asphaltene System. Increasing amounts of heptane induces asphaltene association21 and modulates the dispersant adsorption onto the asphaltenes. The changes in the asphaltene excitation spectra, as functions of the heptane volumetric fraction, must be taken into consider(26) Roux, J. N.; Broseta, D.; Dem, B. Langmuir 2001, 17, 5085.
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Figure 8. (b, left axis) Overlap area between dispersant emission and asphaltene excitation and ([, right axis) calculated Forster radii (Ro), each as a function of the heptane volumetric fraction. The line corresponds to the average Ro value.
Figure 9. Spectra registered by fluorescent resonance energy transfer (FRET) in the precipitation experiments. The twopeak band from 325 nm to 375 nm corresponds to dispersant emission, whereas the wide band from 400 nm to 510 nm corresponds to asphaltene emission due to energy transfer.
ation in the FRET technique, as shown in Figure 7. The excitation intensity decreases as the heptane volumetric fraction increases. The average emission spectrum of dispersant (from Figure 5) is included in Figure 7, where we can observe the changes of the overlapping areas between the dispersant emission and the asphaltene excitation spectra; their corresponding values are shown in Figure 8, as a function of the heptane volumetric fraction. The Fo¨rster radius (Ro) is calculated from these spectra, based on the formulas given in the Appendix. The Ro value, which has been calculated for the different heptane volumetric fractions, is shown in Figure 8. It is practically constant for any heptane volumetric fraction; the average value is 20.4 ( 1.7 Å. Therefore, it is an appropriate distance to use the FRET phenomenon in the study of dispersant adsorption onto asphaltenes. FRET from Dispersant and Solvent. Emission spectra registered for the asphaltene-dispersanttoluene-heptane system are presented in Figure 9. The dispersant was excited at 290 nm and the fluorescence emission were registered in the range of 300-510 nm. This range could be divided into two regions:
