Characterization of the Charge Carriers in Bitumen - ACS Publications

Energy & Fuels 2006, 20, 2099-2108

2099

Characterization of the Charge Carriers in Bitumen Lamia Goual,† Ge´za Horva´th-Szabo´,* Jacob H. Masliyah, and Zhenghe Xu Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed April 6, 2006. ReVised Manuscript ReceiVed July 10, 2006

This work aims to identify the nature of the charge carriers in Athabasca bitumen to better understand the mechanisms of emulsion stabilization and electrically induced separations. A quartz crystal microbalance (QCM) with nanogram sensitivity is used to determine the electrodeposited mass in toluene-based systems. The use of low voltages for the first time allows a small amount of deposited material to be determined accurately. The method consists of applying a DC voltage between two gold-coated QCM crystals immersed in toluene solutions of asphaltenes, maltenes, or solid-free bitumen. Application of a 250 V/cm electric field strength produces an electrocollection of cationic and anionic species at the negatively and positively charged QCM crystals, respectively. A small fraction of the electrocollected material eventually electrodeposits and forms a transparent colorless film at the electrode surfaces. The specific mass (i.e., mass over charge) is found to be higher for the positive species than it is for the negative species. With this method, the characteristic time of relaxation of the bituminous solutions is estimated during the early stage of voltage application. This time could be used to establish a minimum frequency criterion for the critical field methods of water electrocoalescence in oil. Solids separated from bitumen are mainly negatively charged, and their specific mass is relatively high. This study shows that electrodeposition can efficiently separate solids and solidstabilized water droplets from bitumen. Thus, an electrostatic treatment processes could be successful in removing solids and water from diluted bitumen systems.

Introduction Polar components found in petroleum fluids have a tendency to accumulate at interfaces enhancing the formation of disperse systems that lead to operational problems during fluid processing and transportation. This work aims to provide a better insight into the behavior of one class of polar components, notably the ion pairs. These components are in association and dissociation equilibrium and can thus be detected in the form of charge carriers. The motivation behind understanding the nature of electric charges in petroleum fluids is 2-fold: First, it is essential because of its direct effect on several issues encountered in the oilsands industry such as the stabilization of water-in-oil emulsions. Second, electrically induced separations can take advantage of the ionic nature of species and reduce the use of chemical additives during bitumen processing. The stabilization mechanisms of charge carriers in nonpolar media such as petroleum fluids are not clear. Since the Born energy of ions is higher in low-permittivity media than in highpermittivity media, petroleum fluids usually contain a negligible amount of ions compared to water. This is the reason they usually have a small electrical conductivity. However, the fact that the Born energy decreases with increasing ion size implies that large ions can be stable in organic media. Indeed, ionic liquids with sizable anions and cations are good electric conductors. Small ions can also be stabilized in organic media if they are incorporated in large structures such as micelles. This was the mechanism that Morrison,1 as well as Dukhin and Goetz,2 suggested to interpret the high electrical conductivity * To whom correspondence should be addressed. E-mail: [email protected]. Now at Schlumberger Reservoir Fluids Center, Oilphase - DBR, 9450 17 Ave., Edmonton AB Canada T6N 1M9, Phone: 780-577-1308. † Now at Enhanced Oil Recovery Institute, University of Wyoming, Laramie, WY 82071. (1) Morrison, I. D. Colloids Surf. A 1993, 71, 1.

of certain organic solvents. Morrison1 interpreted the conductivity of organic systems, using the original expression of Fuoss,3 assuming a low ionic strength and neglecting the degree of dissociation of ion pairs when compared to unity. Under these constraints, the degree of dissociation, R, is dependent on the relative dielectric permittivity of the medium, , and the centerto-center distance of the ion pair, d, according to the following formula

(

x

3

R ) d-2

3 -z2e2 exp 4πn0 2d0kBT

)

(1)

where n0 is the number of ions per unit volume, z is the charge number of a symmetrical electrolyte, and the other notations have their usual meanings. In chemical terms, the degree of dissociation in our case equals twice the number of free ions divided by the number of all ion pairs (before dissociation) in the system. This formula suggests the existence of a finite number of ions even in a low-permittivity media. Moreover, the number of charge carriers in these media can be considerable provided that the ions have a reasonably large size. This feature is the basis of a more simplified approach stating that the only ion pairs that can be broken by the thermal energy are those in which the ionic radius of a 1-1 electrolyte exceeds the Bjerrum radius, governed by4

rB )

e2 8π0kBT

(2)

The physics behind this classical formula is based on the fact that the Coulombic attraction between point charges is inversely proportional to the square of the distance between the charges. (2) Dukhin, A. S.; Goetz, P. J. Electroanal. Chem. 2006, 588 (1), 44. (3) Fuoss, R. M. J. Am. Chem. Soc. 1958, 80, 5059. (4) Onsager, L. J. Chem. Phys. 1934, 2, 599.

10.1021/ef0601521 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006

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Therefore, ions must have a large enough size to exist as independent kinetic units in a liquid; otherwise, the attraction would overpower the thermal motion. Because the Coulombic attraction is also inversely proportional to the permittivity of the medium, the separation distance of kinetically independent cations and anions with unit charge should be at least 28 nm in toluene as opposed to 0.7 nm in water.2 Therefore the minimum radius of free ions with identical size, expressed by the Bjerrum radius, is about 14 nm in toluene. If this is the case, then kinetically free charge carriers are likely to exist in asphaltenecontaining systems because of the strong tendency of asphaltenes to associate into large structures. Despite the lack of detailed experimental evidence, the mechanism of surface charging in petroleum fluids without additional electrolytes is believed to be similar to donoracceptor or acid-base interactions, in which acidic functional groups undergo dissociation and protonation occurs in basic functional groups.5 The change of particle charge with the donor or acceptor numbers of the media6 is analogous to the change in the point of zero charge (isoelectric point) of the oxides in water as a function of pH.7 At oil-water interfaces, the adsorption of surface-active organic species (asphaltenes, resins, and toluene-insoluble organic matter, such as, carbenes, carboids, etc.) often associated with mineral solids leads to the creation of stable interfacial films. To describe the resistance of these films against coalescence, most electrocoalescence investigations under AC electric fields use the field at the point of coalescence as a critical parameter to characterize emulsion stability.8-10 However, as will be shown in this work, application of an electric field may induce interfacial polarization, as well as electrodeposition, of charge carriers at oil-water interfaces leading to structurally modified films. Although the impact of the field-induced structural alterations on the film stability could be negligible at high AC frequencies, the frequency increase may introduce other problems such as heating. Hence the electrocoalescence investigations must have an optimal frequency range where polarization, electrodeposition, and heating effects are negligible. In this work, we estimate the characteristic time of relaxation of bituminous systems in an electric field, which can be used to establish a minimum frequency criterion for the electrocoalescence methods. The DC electrical conductivity of hydrocarbon systems shows a short-time decay before reaching a stationary value,11,12 attributed to (1) ion depletion or discharge through charge transfer reactions at the electrodes, (2) nonuniform charge distribution, and (3) electrode polarization. In hydrocarbon media with low dielectric permittivity, the extent and nature of ionization are less readily defined, although conductivity measurements have clearly shown that ionization does occur.13,14 (5) Lyklema, J. AdV.Colloid Interface Sci. 1968, 2, 64. (6) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978; p 19. (7) Siffert, B.; Kuczinski, J.; Papirer, E. J. Colloid Interface Sci. 1990, 135 (1), 107. (8) Sæther, O.; Sjo¨blom, J. Colloid Polymer Sci. 1999, 277 (6), 541. (9) Spiecker, P. M.; Sullivan, A. P.; Zaki, N. N.; Kilpatrick, P. K. Presented at the 219th ACS National Meeting, San Francisco, CA, March 26-30, 2000; GEOC-098. (10) Beetge, J. H. Presented at the Oilsands 2006 Conference, Edmonton, Alberta, Canada, February 22-24, 2006. (11) Gemant, A. Phys. ReV. 1940, 58, 904. (12) Douwes, C.; Van der warden, M. J. Inst. Pet. 1967, 53, 523, 237. (13) Fotland, P.; Anfindsen, H. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. (14) Hasnaoui, N.; Achard, C.; Rogalski, M.; Behar, E. ReV. Inst. Fr. Pet. 1998, 53 (1), 41.

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According to Gemant,11 two kinds of ions may exist in homogeneous media: (1) fully dissociated ones, these are usually large species that are quickly depleted or persist as charged ions, thus creating space charge at the electrodes, and (2) partially dissociated ones, usually of a molecular size, these are continuously regenerated, in permanent ionic equilibrium, and hardly depleted. Douwes and Van der warden12 stated that the degree of ion depletion depends on the dissociation constant of species and that residues of petroleum distillations have an intermediate tendency for depletion. AC electrical conductivity measurements proved to be a powerful tool for understanding the association-dissociation properties of asphaltenes as it became clear from the work of Fotland and Anfindsen.13 These authors gave a detailed assessment of the previous literature, which will not be repeated here. To interpret their AC conductivity data, they assumed the existence of ion pairs, which is partly in line with the approach suggested here. However, we will also consider the contribution of large charge carriers (about 30 nm) to the conductivity and electrodeposition phenomena. Previous studies on the nature of charge carriers in petroleum fluids are usually based on electrophoresis. In organic media, which is most of interest to us, Wright and Minesinger15 and Kokal et al.16 measured positive surface potentials for asphaltene dispersions in nitromethane. The authors suggested possible adsorption of protons dissociated from nitromethane on asphaltene surfaces. More recent work by Leon et al.17 and Gonzalez et al.18 in heptane or heptane/toluene mixtures suggests that the origin of the positive charges is not solely from proton adsorption. Electroacoustic spectroscopy studies showed that the continuous adsorption of bituminous components at the interface of freshly formed water-in-oil emulsions governs the surface charge of water droplets.19 The assessment of previous studies indicates that electrically charged interfacial layers can develop in organic systems, the electrical properties of which cannot be ignored in the study of the stabilization mechanisms of emulsions and suspensions. Electrodeposition methods attempt to study the adhesion of charged particle deposits on oppositely charged electrodes. The adhesion, often irreversible, is governed by electrochemical reactions and charge-dipole or dipole-dipole interactions at the electrode. The sign of charges is found, with this approach, to vary depending on the colloidal state of asphaltenes in the medium. Asphaltene particles in crude oils carry a negative charge,20-22 while asphaltene precipitated by n-heptane or acids carry a positive charge in organic phases.22-25 In an attempt to rationalize these observations, Taylor26 proposed a model for aggregates in which positively charged asphaltenes are stabilized (15) Wright, J. R.; Minesinger, R. J. Colloid Sci. 1963, 18 (3), 223. (16) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Colloids Surf. A 1995, 94, 253. (17) Leon, O.; Rogel, E.; Torres, G.; Lucas, A. Petr. Sci. Technol. 2000, 18 (7, 8), 913. (18) Gonzalez, G.; Neves, G. B. M.; Saraiva, S.; Lucas, E. F.; dos Anjos de Sousa, M. Energy Fuels 2003, 17, 879. (19) Magual, A.; Horva´th-Szabo´, G.; Masliyah, J. H. Langmuir 2005, 21, 19, 8649. (20) Presckshot, G. W.; DeLisle, N. G.; Cottrell, C. E.; Katz, D. L. Am. Inst. Met., Trans. 1943, 151, 188. (21) Katz, D. L.; Beu, K. E. Ind. Eng. Chem. 1945, 37 (2), 195. (22) Moore, E. W.; Crowe, C. W.; Hendrickson, A. R. J. Pet. Technol. 1965, SPE 1143. (23) Lichaa, P. M.; Herrera, L. J. Pet. Technol. 1975, SPE 5304. (24) Henry, J. D.; Jacques, M. T. AIChE J. 1977, 23 (4), 607. (25) Khvostichenko, D.; Andersen, S. I. Proceedings of the 5th International Conference on Petroleum Phase Behavior and Fouling, Banff, AB, Canada, June 13-17th, 2004. (26) Taylor, S. E. Fuel 1998, 77, 8, 821.

Charge Carriers in Bitumen

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by negatively charged resins. Thus, the resin-to-asphaltene ratio defines the net charge of the aggregate. There are limitations associated with the electrophoretic methods in nonpolar media. The estimation of the ionic strength of the medium is, for example, challenging. The electrodeposition methods often require the application of high electric fields, which may trigger field-induced dissociation,4 charge reversal,27 and other related effects. The primary objective of this work is to identify the nature of the charge carriers present in bitumen to better understand the mechanisms of emulsion stabilization and electrically induced separations. Our approach is similar to the earlier electrodeposition methods of crude oil components in organic media; however, we use a quartz crystal microbalance (QCM) as a sensor for deposition. With this approach, a very small amount of deposited material can be measured under relatively low potential differences. The method is simple and consists of applying a DC voltage between QCM crystals suspended in toluene-based solutions. Model compounds with different chemical groups (alcohol, acid, and amine) are first considered to investigate their electrodeposition in toluene. Then, bituminous compounds (asphaltenes, maltenes, and solids) are analyzed in toluene. At this stage of investigation, the preliminary results determined from a simplified mass-frequency relationship are interpreted qualitatively. Asphaltene deposits (with and without solids) are further characterized by X-ray photoelectron spectroscopy to gain insights into their chemical composition. We also investigate the electrodeposition of bitumen in toluenebased solutions and water-in-toluene diluted bitumen emulsions to highlight the impact of solids on deposition. Materials and Methods Materials. An Athabasca bitumen sample was taken from the feed to a vacuum distillation unit at Syncrude Canada Ltd. The certified ACS grade toluene (99%) and acetone (99%) and HPLC grade n-heptane (99.6%) were purchased from Fisher Scientific. Hexadecyl acid, 1-hexadecylamine, and hexadecanol, all 99% purity, were purchased from Aldrich. Methyltrioctylammonium bis(trifluoromethylsulfonyl)-imide (98%) was purchased from Solvent Innovation. The water used in this study was prepared by an Elix 5 followed by an Ultra Millipore Q/UV purification system. Separation Procedure. Bitumen is fractionated into asphaltenes and maltenes by a procedure that involves the mixing of the sample with 40 volumes of n-heptane at room temperature. After it is stirred for 1 h, the solution is left to settle overnight, and then it is filtered with a 0.2 µm pore-size Whatman filter paper. The filter cake is repeatedly washed with n-heptane until the washing liquid is colorless. The filtrate is transferred into a rotary evaporator to evaporate n-heptane from the maltenes. The solids remaining in the asphaltenes are removed by centrifuging 5 wt % solutions of filter cake in toluene at 30 000 g for 3 h, followed by filtration of the supernatant with a 0.2 µm pore-size Whatman filter paper. Asphaltenes (solid-free) are recovered from the filtrate after toluene evaporation. The same procedure is applied to remove solids directly from bitumen. Quartz Crystal Microbalance (QCM). The specifications and principle of operation of the QCM (Maxtek Inc.) are presented elsewhere.28 The setup is modified to include a 6487 picoammeter/ voltage source (Keithley Instruments) parallel to the QCM (see Figure 1a). This modification allows voltages ranging from 0 to 250 V to be applied between the two QCM crystals (5 MHz Cr/Au polished, 25 °C) spaced 1 cm apart and suspended in the liquid. The electric current range is set to 20 µA with a 2.5 mA limit, and (27) Eldib, I. A. Am. Chem. Soc., DiV. Pet. Chem. 1962, 7 (1), 31. (28) Goual, L.; Horva´th-Szabo´, G.; Masliyah, J.; Xu, Z. Langmuir 2005, 21 (18), 8278.

Figure 1. (a) Experimental setup of the QCM with DC voltage source. (b) Effect of the electrostatic field inhomogeneity on electrodeposition in a QCM cell.

the current data are collected at a slow sampling rate with no digital filtering. To collect more material on the crystals for further characterization, higher voltages (up to 20 kV) are applied between the crystals by directly connecting an SL 10 high-voltage source (Spellman) to the crystal holders. The simplified approach of the mass-frequency shift relationship,29 adopted in almost all QCM literature, is used to calculate the mass of deposited material. This approach is justified in diluted systems where the mass corrections are usually small.28 During the QCM experiments and DC conductivity measurements, the temperature was kept constant at 25.0 °C ( 0.01 °C. Procedure. The experiment is initiated by injecting 200 g of solvent (toluene or toluene-heptane mixtures) into the cell to establish the baseline for the frequency and resistance shifts. After crystal capacitance cancellation, the solvent is subjected to the same electric field as the solution for five minutes to remove any impurities. It is important to note that the applied electric field did not change the frequency or resistance of the solvents. After a certain amount of bituminous material dissolved in toluene is introduced into the cell, the progress of adsorption is recorded in the absence of electric field by detecting the frequency and resistance shifts. When adsorption reaches saturation, indicated by a constant frequency, a 250 V potential difference is applied (29) Sauerbrey, G. Z. Phys. 1959, 155, 206.

2102 Energy & Fuels, Vol. 20, No. 5, 2006 between the crystal faces, and further shifts in the resonance frequency and resistance are recorded. In the case of model compounds, the experiment is initiated by directly injecting 200 g of solution (model compound in toluene solvent) into the cell to establish the baseline for the frequency and resistance shifts. This is because of the solubility limits of the chemicals in toluene. A 250 V potential difference is then applied between the crystals, and the shifts in the resonance frequency and resistance are recorded. In this setup, one crystal functions as an electron donor (negative electrode or cathode) and the other as an electron acceptor (positive electrode or anode). Fourier Transform Infrared (FT-IR) Spectrometry. Infrared spectra of the deposited film on the QCM electrodes are obtained using an FTS 6000 (Bio-Rad Laboratories) spectrometer. The QCM crystals are directly mounted on a 30Spec 30° specular reflectance accessory (Pike Technologies). Measurements are performed in reflectance mode and diffuse reflectance spectra are given in Kubelka-Munk units. For each run, 128 scans are recorded in the 4000-450 cm-1 spectral range with a wavenumber resolution of 8 cm-1. The single-beam background spectrum is determined with a blank crystal. X-ray Photoelectron Spectroscopy (XPS). XPS experiments are carried out using an AXIS 165 (Kratos Analytical) spectrometer. Soft X-ray photons (anode mono Al KR radiation, 210 W) are used to excite the electrons of analyzed surfaces with thicknesses higher than 8 nm. The excitation of the inner-shell electrons induces direct emission of photoelectrons whose energy is characteristic of the target chemical element. A survey spectrum within a binding energy range from 0 to 1100 eV is collected at an analyzer pass energy of 160 eV. Several characteristic peaks are also measured with high resolution (at pass energy of 20 eV) to evaluate their chemical bonding from their core electron-binding energies. Optical Microscope. Microscope imaging is performed with an Axioskop 40 (Carl Zeiss) optical microscope attached to a Qicam Fast 1394 (Q imaging) digital camera and mounted on a TCM 65500 antivibration table (Technical Manufacturing Corporation). Images are taken under bright-field reflectance mode using homogeneous illumination from a 35 W HAL-100 halogen lamp. The QCM crystals are placed with extreme care on a glass support and directly viewed under the objectives with a magnification of 2.5 and 10. When water is present in the deposited film, the crystals are immediately covered with a slide cover to avoid water loss.

Results and Discussion Charge versus Dipole Deposition at the QCM Electrodes. The details of the geometry of the QCM cell are presented in Figure 1b. The quartz crystals are sandwiched between front and rear gold electrodes. The front electrodes are immersed in the solution, facing each other. The 5 MHz oscillating electrostatic field forces the parts of the quartz crystals, between the overlapping areas of the front and rear electrodes, to oscillate in a shear mode. The areas of the rear gold electrodes are 1/4 of those of the front electrodes. Thus, it is only the corresponding 1/ areas of the front electrodes that are sensitive to the change 4 in the deposited mass, although deposition occurs on the whole area of the front electrodes because of the DC field applied between them. The electrostatic field is homogeneous in the center of the cell and has an effect on the charge carriers only but not on the dipoles. The field is inhomogeneous at the edges of the front electrodes and has an effect on both the charge carriers and the dipoles. Consequently, the presence of (permanent or induced) dipoles introduces no artifact into the deposited mass of charge carriers because the latter is measured in the central part of the crystals, while dipole deposition would occur only at the edges of the front electrodes where the sensitivity of the crystal to the deposited mass is zero. To support the above explanation, we observed no deposited mass from a toluene solution of hexadecane (although one could

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Figure 2. Mass density, Γ, and resistance, R, of QCM electrodes vs time under a 250 V potential difference for hexadecanol (10 wt % in toluene) at 25.0 °C. The electrodeposition manifests in a continuous, nearly linear, increase of deposited mass density on the anode with a simultaneous increase in the resistance. Table 1. Model Compounds Studied name

formula

hexadecanol hexadecyl acid hexadecylamine methyltrioctylammonium bis(trifluoromethylsulfonyl)-imide

CH3-(CH2)15-OH CH3-(CH2)14-COOH CH3-(CH2)15-NH2 [(C8H17)3-N+-CH3] [N(CF3SO2)2]-

electrodeposition at anode anode cathode anode/cathode

expect the formation of induced hexadecane dipoles). Similarly, the zero cathodic deposited mass from hexadecanol permanent dipoles (See Figure 2) confirms that the presented cell geometry has zero sensitivity to dipole deposition, which would produce a symmetrical cathodic and anodic response. Here, Γ is the deposited mass per unit area, and R is the mechanical resistance of the crystal to the oscillation expressed in Ω. Model Compounds. The chemicals listed in Table 1 are selected with various functional groups to investigate their electrodeposition in toluene. In the QCM cell containing 200 g of solution in toluene, a 250 V potential difference is applied between the crystals, and the shifts in resonance frequency and resistance are recorded. Using this procedure, we find that alcohols and carboxylic acids undergo an anodic oxidation under a 250 V potential difference, while amines and nitrogen-based compounds undergo a cathodic reduction resulting in an electrodeposited mass at the corresponding electrode (see the last column of Table 1). From all the investigated model compounds, we choose to analyze in detail only the hexadecanol. The electrodeposition of hexadecanol manifests in Figure 2 as a continuous, nearly linear increase of deposited mass density, Γ, (mass per unit area) on the anode with a simultaneous increase in the resistance, R. When the anode is viewed under the optical microscope (see Figure 3), the deposition product is seen to consist of networks, probably, containing alkyl chains with more than 16 carbons that form toluene-insoluble waxlike crystals. This altered chemical structure may explain the irreversible deposition observed. The electric current, I, recorded during the first 15 min (see Figure 4) is small and decays rapidly to an asymptotic value. This expected behavior could be attributed to ion depletion, electrode polarization, or both, as explained



Figure 5. FT-IR spectra of deposited films on QCM electrodes from hexadecanol (10 wt % in toluene) under a 250 V potential difference. Comparison between the cathodic and anodic deposits reveals the presence of a band at 1696 cm-1 on the anode characteristic of the CdO stretching of aldehyde groups as a result of the anodic oxidation of hexadecanol.

Figure 3. Optical micrograph of deposited films on QCM electrodes from hexadecanol (10 wt % in toluene) under a 250 V potential difference. The deposition product on the anode consists of networks probably containing alkyl chains with more than 16 carbons that form waxlike crystals which are insoluble in toluene.

Figure 6. Mass density and resistance of QCM electrodes vs time under a 250 V potential difference for asphaltenes (500 ppm in toluene) at 25.0 °C. The potential difference produces a sharp increase in deposition (higher on the cathode than the anode) possibly caused by the formation of a space charge from large species. The small variation of the electrode resistance against oscillation indicates a film with a good mechanical elasticity. On the cathode, a fraction of the electrocollected species electrodeposit, as can be seen from the mass increase when the potential is off. On the anode, a secondary adsorption develops when the potential is off, possibly because of the polarization of the adsorbed layers.

Figure 4. Electric current vs time under a 250 V potential difference for hexadecanol (10 wt % in toluene.) at 25.0 °C. The small electric current decays rapidly and tends toward an asymptotic value. This expected behavior could be attributed to ion depletion or electrode polarization.

earlier. The FT-IR spectra of the material deposited on the QCM electrodes are shown in Figure 5. A comparison of the spectra reveals the presence of a band at 1696 cm-1 on the anode but

not on the cathode. This band is characteristic of the CdO stretching of aldehyde groups as a result of the anodic oxidation of hexadecanol. Asphaltenes, Maltenes, and Solid-Free Bitumen. Figure 6 shows the mass density, Γ (mass per unit area), and mechanical resistance, R (expressed in ohms), of the material collected over time at the surface of the QCM electrodes. After the baseline of QCM in toluene was established, an asphaltene-toluene mixture is introduced into the cell so that the final concentration of asphaltenes in the solution is 500 ppm (0.05 wt %). A primary adsorption layer develops at the gold/liquid interface reaching


saturation after about 50 min. At this time, a 250 V potential difference is applied between the QCM electrodes, thereby producing a rapid increase in deposition, followed by a steady decrease to a constant value. The abrupt increase in the mass density during the first few seconds of voltage application is in stark contrast to the smooth change observed in model compound systems. Furthermore, when the electrical potential difference is released, a sudden drop in the mass density is observed in the case of bituminous components, in contrast to the constant mass density observed for the model compounds (see Figures 2 and 6). In all cases, the deposition results in thin transparent films on both electrodes. We believe that the colloidal, not the chemical, structure causes the difference observed between model compounds and petroleum fluids. Indeed, the various functional groups of the model compounds result in only an alteration of the cationic or anionic nature of deposition, but they are unable to simulate the transient mass change. The large structures of associated bituminous components appear to energetically stabilize electrical charges and sterically prevent their neutralization at the electrode surface. Thus, the sharp increase in the mass density of bituminous systems during the first few seconds of voltage application could be attributed to a space charge (i.e., electrical double layer) developing at the electrode as a result of the electrocollection of charged species. The mass density of the electrocollected material is higher on the cathode than on the anode. Furthermore, the small variation of the electrode resistance against oscillation throughout the process indicates a film with a good mechanical elasticity. Unlike previous investigations, where only one type of charge deposition was observed on one of the electrodes from a petroleum fluid, our results show that both positive and negative charge carriers can be electrically collected or deposited from asphaltene in toluene solutions. When the electric potential is switched off, the majority of the electrocollected species are released from the cathode. However, the increase in the adsorption saturation value at the cathode following the application of the potential (compare the mass densities of the cathode at 40 and 280 min) clearly indicates that a fraction of the electrocollected material was in fact electrodeposited. One could argue that the voltage interruption might provoke a simultaneous release of electrocollected species and a secondary adsorption resulting from the polarization of the primary adsorption layer, as seen on the anode. However if that were the case, then an additional layer would also develop on the cathode after the second voltage application because of the polarization of the second adsorbed layer. The fact that there is no electrodeposition during the second voltage application is probably because of the full coverage of the electrode surface after the initial voltage application. A similar behavior with maltenes (5 wt % in toluene) and solid-free bitumen (1 wt % in toluene) is observed in Figure 7. In these cases, relatively small amounts of material are electrocollected, and there is no noticeable electrodeposition on the cathode. The nature of the charge carriers in maltenes and resins appears to be similar to that in asphaltenes. This observation contradicts Taylor’s model,27 where asphaltene and resins are believed to carry opposite charges. Considering the electroneutrality of the solution and the fact that the specific mass (i.e., mass per unit charge) of the electrocollected particles is higher for positive than negative species imply that (1) the positive species have a higher mass than the negative species if both carry the same number of charges or (2) the negative species carry more charges than positive species if both have the same mass (see Figure 8).

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Figure 7. Mass density vs time under a 250 V potential difference for maltenes (5 wt % in toluene) and solid-free bitumen (1 wt % in toluene) at 25.0 °C. The behavior is similar to that of asphaltenes but with relatively lower amounts of electrically collected material and no noticeable electrodeposition on the cathode.

Figure 8. Schematic of electrically collected bitumen charge carriers on QCM electrodes. (1) Positive species have a higher mass than negative ones if both carry the same number of charges (m+/q > m-/q), and (2) negative species carry more charges than positive ones if both have the same mass (q-/m > q+/m).

Figure 9 shows the electric current recorded during voltage application in asphaltene and maltene solutions. A sharp increase during the first 2 min is observed and was not expected. In fact, electrodeposition currents should decrease with time because of ion depletion, as seen with model compounds (see Figure 4). When the approximately 0.2 pF capacitance of the cell is considered, the observed increase in electric current cannot be attributed to a charging current; however, it could be the result of an increase in the rate of charge transfer at the electrode interface. The current would subsequently stabilize at a constant value or slightly decay as a result of increasing electrode coverage with time. The decay is more significant under high voltages because of the additional ion-depletion process in the bulk. All these observations suggest the following:


Figure 9. Electric current vs time under a 250V DC potential difference for asphaltenes (500 ppm in toluene) and maltenes (1 wt % in toluene) at 25.0 °C. The unexpected current increase during the first 2 min may be the result of an increase in the rate of charge transfer of small-mass ions at the electrode interface. The current subsequently stabilizes at a constant value and is probably maintained by a constant rate of charge depletion of small-mass ions that are continuously produced by partial dissociation of small ion pairs with a low dissociation constant.

(1) The relatively constant electric current after the initial sharp increase is probably caused by a constant rate of charge depletion of small-mass ions that either cannot be detected at the electrode interface under the mass sensitivity constraints of our device or the electrochemical processes produce soluble species from the ions as presented in Figure 10a. The concentration of these ions decreaces slowly, indicating that they are continuously produced by partial dissociation of small ion pairs with a low dissociation constant. These ions, which are in a permanent ionic equilibrium, are continuously regenerated and hardly depleted, as was suggested by Douwes and Van der warden.12 (2) The abrupt increase in the deposited mass over the first minutes of the electrolysis (see Figure 6) is attributed to the development of a space charge from the large and fully dissociated ions. It was Gemant11 who first hypothesized the formation of space charge from large non-neutralized ions. On the basis of our results, these ions have a considerable mass density at the electrode compared to the adsorption saturation value in the absence of potential. However, to our best knowledge, this is the first time that the formation of space charge is experimentally detected using the mass of these species. (3) The subsequent increase followed by a decrease in the mass at the early stages of voltage application could be the result of a molecular reorganization of the large ions collected at the electrodes. This reorganization somehow enhances the depletion rate of small-mass ions and accounts for the initial increase in the electric current, which is higher than the current decay resulting from charge depletion. (4) From the experimental results presented in this paper, as well as our current understanding of the association processes of asphaltenes, we recommend the following model. Lewis acids and bases are present in petroleum fluids. In an organic solvent, these acids and bases form ion-pairs and permanent dipoles, the dissociation constant of which is very low. Nevertheless, the continuous dissociation of these species (through electronpair donors in bases and acceptors in acids) produces smallsize charge carriers that are involved in charge transfer reactions at the electrodes and are responsible for the DC conductivity


Figure 10. (a) Schematic of the contribution of charge carriers to the DC conductivity in petroleum fluids. Lewis acids and bases form ion pairs and permanent dipoles in petroleum fluids. The dissociation of these species produces small-size charge carriers that are involved in charge-transfer reactions at the electrodes and are responsible for the DC conductivity. These acids and bases are also involved in asphaltene association processes leading to high aggregate mass. In these aggregates, the numbers of Lewis acids and bases are not balanced exactly. Electron exchange between the electrodes and these associated acids and bases is hindered (because of steric constrains) so they can only be electrocollected and not electrodeposited at the electrodes; they are quickly released from the electrode surface when the potential is turned off. These species do not contribute to the DC conductivity. (b) Schematic of the contribution of charge carriers to the AC conductivity in petroleum fluids. The charge carriers with a total ionic size near the Bjerrum radius are in association/dissociation equilibrium. The charge carriers with a total ionic size larger than the Bjerrum radius are fully dissociated. Both the small and large charge carriers contribute to the AC conductivity; however, the contribution from the small carriers is probably negligible because of their small concentration, compared to the large carriers.

of petroleum liquids (see Figures 9 and 10a). These small charge carriers, as part of asphaltenes, are also involved in asphaltene association processes leading to aggregate species with high molar mass. In these aggregates, the numbers of Lewis acids and bases are not balanced exactly, and the high molar mass aggregates have an overall Lewis base character, while the Lewis acids have an excess in the smaller molar mass aggregates (see Figure 8). This is the reason positive species have a higher deposited mass than negative species in Figures 6 and 7. The electron exchange between the electrodes and these associated acids and basis are hindered (because of steric constrains), so they can only be electrocollected and not electrodeposited (see Figure 10a). Therefore, these species do not contribute to the DC conductivity. Nevertheless they do contribute to the AC conductivity19 since these species with a radius higher than the Bjerrum radius exist as fully dissociated charge carriers as suggested in Figure 10b. To gain chemical information about the structure of Lewis acids and bases in petroleum liquids, one should isolate and analyze them separately. Unfortunately, because of association processes, we can only deposit mixed species with a slight excess of either the acids or bases. In the electrocollected deposits with a Lewis acid character, the oxygen to nitrogen atomic ratio is higher than 1, while it is lower than


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Figure 11. Mass density and resistance of QCM electrodes vs time under a 250 V potential difference for solids (150 ppm in toluene) at 25.0 °C. The potential difference produces a large irreversible deposition on the anode, almost six times larger than that on the cathode. The specific mass is high compared to asphaltenes and is accompanied by a substantial increase in the resistance of the anode.

1 in the deposits with a Lewis base character. Thus, one might visualize the Lewis acids of Athabasca bitumen as containing -COOH units, while the bases as containing :Nt units. This will be confirmed later in the XPS analysis of asphaltenes, presented in the next sections. Solids. Figure 11 shows the change of the mass density and resistance of QCM electrodes over time during the electrodeposition of bituminous solids (i.e., toluene-insoluble organic matter and mineral clays) from a 150 ppm (0.015 wt %) solid-toluene solution under a 250 V potential difference. The application of an electrical potential produces a large irreversible deposition of solids on the anode, almost six times larger than that on the cathode. The specific mass of deposited solids is high compared to asphaltenes. The deposition is accompanied by a substantial increase in the resistance of the anode. The deposited material in this case forms a rigid and compact layer. The deposition kinetics on the anode displays a short peak before increasing to a plateau value. This behavior is different from the previous cases. The inability of the anode to collect more materials after the first voltage application is probably the result of the large electrode coverage that reaches saturation after 30 min. Asphaltenes with Solids. When the solids in asphaltenes are not removed by centrifugation, the electrical behavior of asphaltenes with solids (1000 ppm in toluene) under a 250 V potential difference is similar to the one presented in Figure 6. However, at higher field strengths (such as 20 kV/cm), large amounts of solids deposit on the anode and produce a thick, black, coarse layer, as observed visually and under the optical microscope (see Figure 12). On the other hand, the thin and transparent layer on the cathode looks identical to that deposited on both electrodes from solids-free solutions under a 20 kV/ cm field strength. The circular patterns shown in Figure 12 represent different layer thicknesses, which developed around the electrodeposited impurities visible at the center of the patterns. Because the amounts of asphaltene deposits are small, it was not possible to characterize them by FT-IR. Instead, we used an XPS survey to determine their chemical composition. The results shown in Figure 13 indicate that the anodic film has slightly more oxygen, chlorine, fluorine, and sulfur but less

Figure 12. Optical micrograph of deposited films from asphaltenes with solids (1000 ppm in toluene) on QCM electrodes under a 20 kV potential difference. Large amounts of solids deposit on the anode and produce a thick, black, coarse layer. A thin, transparent layer forms on the cathode; the circular patterns represent different layer thicknesses, which developed around the electrodeposited impurities visible at the center of the patterns.

nitrogen-based groups than the cathodic films, which is in agreement with deposition results of the model compounds previously investigated (see Table 1). The small differences in elemental composition suggest that species on both electrodes contain similar anionic and cationic functional groups, and there is only a difference in the net charge of these groups. However, these differences become larger when solids are present in asphaltenes, possibly resulting from the presence of silica and alumina. High-resolution XPS analysis reveals that oxygen is in the form of hydroxide and sulfate, sulfur is in the form of thiophene, and there are more unsaturated nitrogen bonds (such as cyanide and azide) on the anode than on the cathode. Bitumen. Figure 14 shows the change of mass density as a function of time during the application of a 250 V potential difference in a cell containing 1 wt % bitumen (with original solids) in toluene and in Heptol (20/80 wt mixture of toluene/ heptane). The electrical behavior of bitumen in toluene under the electric potential difference is similar to that of solid-free bitumen in toluene (see Figure 7). The absence of a noticeable electrodeposition on the anode, despite the presence of solids, suggests that bitumen may be neutralizing the surface charges of solids by coating their surfaces. When the voltage is switched off, a second adsorption accompanied by a large increase in the resistance (not shown here) is observed at the cathode, probably because of the polarization of the primary adsorption


Figure 13. XPS analysis of asphaltenes (1000 ppm in toluene) deposited on QCM electrodes under a 20 kV potential difference. The anode has slightly more oxygen, chlorine, fluorine, and sulfur and less nitrogen-based groups than the cathode does. The small differences in elemental composition suggest that species on both electrodes contain similar anionic and cationic functional groups, and there is only a difference in the net charge of these groups. However, these differences become larger when solids are present in asphaltenes, possibly because of the presence of silica and alumina.

Figure 14. Mass density vs time under a 250 V potential difference for bitumen (1 wt % in toluene and 1 wt % in Heptol, i.e., 80/20 wt mixture of heptane/toluene) at 25.0 °C. The behavior of bitumen is similar to that of solid-free bitumen; however, a secondary adsorption occurs at the cathode when the voltage is off probably because of a wettability alteration of the polarized adsorption layer. The absence of electrodeposition on the anode suggests that bitumen is neutralizing the surface charge of solids by coating their surface. However, in Heptol, the continuous electrodeposition at the anode is attributed to the presence of solids whose surface charge is not fully neutralized by bitumen after asphaltene precipitation.

layer. This polarization may lead to a wettability alteration that could be the driving force for the further deposition of solids. The characteristic time of the relaxation process during the electrocollection is estimated by fitting the function Γ ) A + B exp ((t - t0)/tR) to the experimental points of the cathodic process after the potential is applied. In this expression, A and B are arbitrary fitting parameters, while t0 is the start of the perturbation on the time axis. The relaxation time, tR, is 1.60 s


Figure 15. Estimation of the characteristic time of the cathodic relaxation process for 1 wt % bitumen in toluene. The function, Γ ) A + B exp ((t - t0)/tR), is fitted to the mass vs time experimental data points at the start of voltage application. The relaxation time, tR, in the increasing region (voltage on) is 0.81 s, corresponding to a 1.2 Hz frequency, while it is 14.8 s in the decreasing region (voltage off), corresponding to a 0.07 Hz frequency.

for 1 wt % solid-free bitumen and 0.81 s for 1 wt % bitumen, corresponding to 0.6 and 1.2 Hz frequencies, respectively. Figure 15 illustrates a fitting example for 1 wt % bitumen in toluene. The relaxation time in the increasing region (voltage on) is 0.81 s, corresponding to 1.2 Hz, while the relaxation time in the decreasing region (voltage off) is 14.8 s, corresponding to 0.07 Hz. Higher bitumen concentrations in toluene will exhibit larger frequencies. Although most of the critical field methods8-10 of water electrocoalescence in oil use higher AC frequencies (in the kilo- or megahertz range), our study demonstrates that they can operate at a considerably reduced frequency to avoid heating effects. In Heptol, where the conditions for asphaltene precipitation are favorable, a continuous electrodeposition is observed on the anode during voltage application. This behavior is attributed to the presence of solids, as such phenomenon was not observed with solid-free bitumen in Heptol. It is most likely that the surface charge of solids is not fully neutralized by bitumen after asphaltene precipitation. These findings reveal that solids can be electrodeposited from diluted bitumen under field strengths of 250 V/cm or even lower. Water-in-Oil Emulsions (W/O). W/O emulsions are prepared from 1 wt % Millipore Q water and 10 wt % bitumen in toluene by sonicating for about 20 min. The QCM crystals are placed inside the emulsion and a 250 V potential difference is applied for 10 min. The crystals are then immediately removed from the solution after voltage interruption. A sludge of solids and water is observed on the anode but not on the cathode. This observation indicates that solid-stabilized water droplets can be electrodeposited and hence carry an excess charge in toluene-diluted bitumen. Under the optical microscope (see Figure 16), the anodic deposit is seen to contain several water droplets dispersed by solids. When the same test is performed with solids-free bitumen, no electrodeposition of water is observed on either electrode. The applied potential may not be sufficiently high to cause the electrocoagulation of asphaltene/ resin-stabilized water drops in the emulsion. It would appear that solids have a beneficial effect on dewatering using an electrodeposition method at a relatively low electric-field strength.


Figure 16. Optical micrograph of deposited W/O emulsions (1 wt % Millipore Q water and 10 wt % bitumen in toluene) on QCM electrodes under a 250 V potential difference. The anodic deposit contains several water droplets dispersed by solids while the cathode remains clear.

Conclusions The main conclusions drawn from this study are as follows: (1) In toluene-diluted bitumen, the application of a DC electric field between gold-coated QCM crystals results in a sudden increase in the mass because of the electrocollection of large and fully dissociated positive and negative species, leading to a space charge buildup. The subsequent decrease in the mass of this layer may be the result of a molecular reorganization of the large species, which enhances the charge transfer rate of small-mass species that are partially dissociated. This explains the initial increase in the electric current, which is higher than the decrease in the current caused by charge depletion. After an extended period, the electric current stays constant with time because of the continuous regeneration of the small-mass species in the bulk. (2) The specific mass (i.e., mass over charge) of the QCM crystals after application of electric potential is higher for

Goual et al.

positive than negative species suggesting that (i) positive species have a higher mass than negative species if both carry the same number of charges or (ii) negative species carry more charges than positive species do if both have the same mass. (3) The electrodeposited layers of bituminous components including asphaltenes are transparent in the absence of tolueneinsoluble solids, indicating that the electrodeposition methods based on visual observations may not be always reliable. This also indicates that a fraction of Athabasca asphaltenes is colorless and cannot be described by a condensed aromatic core. (4) The species deposited on the anode contain more oxygen, chlorine, fluorine, and sulfur but less nitrogen-based functions than those deposited on the cathode. (5) The electrodeposition and electrocollection phenomena observed in diluted Athabasca bitumen compounds can be explained by the presence of Lewis acids and bases which are involved in the formation of large aggregates. (6) The characteristic time of relaxation under electric potentials is estimated for bituminous systems and can be used to establish a minimum frequency criterion for petroleum dewatering methods under electric fields (7) Solids are mainly negatively charged in toluene, and their specific mass is high compared to asphaltenes. (8) Solids and solid-stabilized water droplets in bitumen can be readily deposited under an electric field. Thus, an electrostatic treatment process could be contemplated in removing solids and water from diluted bitumen systems. The very observation of a colorless deposit separated from asphaltene/toluene systems on both the anode and the cathode is intriguing and could give some insight into the molecular structure of asphaltene unimers. The black color of asphaltenes is interpreted by the electron structure of their polycyclic aromatic hydrocarbon ring system (i.e., by the “palm” model).30 According to this approach, the condensed core is sterically stabilized by side chains (or fingers). There are, however, a group of researchers who argue that Athabasca asphaltenes should be described by a multicentered “archipelago” model which would predict a colorless appearance.31 Our observations show that at least a fraction of Athabasca asphaltenes is colorless. Acknowledgment. The financial support through NSERC Oil Sands Research Chair in Oil Sands Engineering (Jacob Masliyah) is gratefully acknowledged. The authors would like to thank Emiliy Zholkovskiy for fruitful discussions, as well as Dimitre Karpuzov, Anquang He, and Shihong Xu from the Alberta Centre for Surface Engineering and Science for the XPS measurements. EF0601521 (30) Ruiz-Morales, Y.; Mullins, O. C. Proceedings of the 7th International Conference on Phase Behavior and Fouling, Asheville, NC, June 25-29, 2006. (31) Dettman, H. C.; Salmon, S. L.; Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Proceedings of the 7th International Conference on Phase Behavior and Fouling, Asheville, NC, June 25-29, 2006.

Characterization of the Charge Carriers in Bitumen - ACS Publications

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