Water Interface - ACS Publications

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Energy & Fuels 2007, 21, 274-285

Asphaltene Films at a Toluene/Water Interface Li Yan Zhang,*,† Patrick Breen,† Zhenghe Xu,‡ and Jacob H. Masliyah*,‡ Fluids Separation Technology, Baker Petrolite, 2323 91 AVenue, Edmonton, Alberta T6P 1L1, Canada, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed July 6, 2006. ReVised Manuscript ReceiVed October 31, 2006

A multilayer asphaltene film of about ∼3-4 layers thick was prepared by spreading an excess amount of asphaltene on a water subphase. Once toluene was placed on top of the multilayer film, some of the asphaltenes migrated to the bulk toluene topphase from the interface, leading to the formation of an asphaltene monolayer at the toluene/water interface. The presence of a monolayer film at the toluene/water interface was confirmed by Langmuir trough experiments, UV spectroscopic measurements of the collected topphase, and atomic force microscopy (AFM) imaging of the transferred Langmuir-Blodgett (LB) asphaltene films on silicon wafers. This study shows that the Langmuir trough technique can be used directly to observe interfacial characteristics, which otherwise cannot be easily accessed by other techniques.

1. Introduction Adsorption of asphaltene at an oil/water interface plays a key role in many processes of the oil-related industry.1 For example, stable water-in-oil emulsions form because of the adsorption of asphaltene at the oil/water interface during extraction of bitumen from oil sands using a variation of the Clark Hot Water Extraction process. The adsorbed asphaltene forms a protective interfacial film that prevents the dispersed water droplets from coalescence, thereby stabilizing water-in-oil emulsions.2,3 Asphaltenes by definition are a solubility class of material that is insoluble in n-heptane but soluble in toluene. Asphaltene molecules are believed to consist of a flat sheet of condensed polycyclic aromatic rings that may be interconnected by sulfide, ether, and alkyl side chains.4,5 The physical and chemical properties of asphaltenes can vary, depending upon how they are obtained. Asphaltenes obtained by precipitation methods normally contain a small amount of trapped maltene (n-heptanesoluble fraction) from a crude oil or bitumen.6 Recently, the importance of obtaining a “pure’’ asphaltene fraction has been emphasized in several papers.7-11 These authors indicated that * To whom correspondence should be addressed. Telephone: 1-780416-6440. Fax: 1-780-416-1824. E-mail: [email protected] (L.Y.Z.); Telephone: 1-780-492-4673. Fax: 1-780-492-2881. E-mail: [email protected] (J.H.M.). † Baker Petrolite. ‡ University of Alberta. (1) Horvath-Szabo, G.; Masliyah, J. H.; Elliot, J. A. W.; Yarranton, H. W.; Czarnecki, J. Adsorption isotherms of associating asphaltenes at oil/ water interfaces based on the dependence of interfacial tension on solvent activity. J. Colloid Interface Sci. 2005, 283, 5-17. (2) Bouriat, P.; El Kerri, N.; Graciaa, A.; Lachaise, J. Properties of a two-dimensional asphaltene network at the water-cyclohexane interface deduced from dynamic tensiometry. Langmuir 2004, 20, 7459-7464. (3) Sztukowski, D. M.; Jafari, M.; Alboudwarej, H.; Yarranton, H. W. Asphaltene self-association and water-in-hydrocarbon emulsions. J. Colloid Interface Sci. 2003, 265, 179-186. (4) Yen, T. F. Asphaltene: Types and sources. In Structure and Dynamics of Asphaltenes; Plenum Press: New York, 1998; pp 1-20. (5) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980. (6) Strausz, O. P.; Peng, P.; Murgich, J. About the colloidal nature of asphaltenes and the MW of covalent monomeric units. Energy Fuels 2002, 16, 809-822.

a thorough washing of the precipitated asphaltene with the alkane used for asphaltene precipitation is needed to remove as much coprecipitated/entrained maltenes as possible. Freer et al.,12 for example, showed that maltene in the extracted asphaltene desorbs from a toluene/water interface, leaving an asphaltene film at the toluene/water interface. The irreversible nature of the asphaltene monolayer at a toluene/water interface13 would indicate the absence of smaller maltene molecules in the extracted asphaltene fraction. In the early work of Bartell and Neiderhauser,14 the presence of asphaltene interfacial films has been noted using a pendent drop apparatus. Reisberg and Doscher15 and Strassner16 showed that asphaltenes form rigid films at an oil/water interface. Jeribi et al.17 reported from visual observation that a rigid skin forms at the water-asphaltene-toluene interface if the concentration (7) Gurgey, K. Geochemical effects of asphaltene separation procedures: Changes in sterane, terpane, and methylalkane distributions in maltenes and asphaltene co-precipitates. Org. Geochem. 1998, 29, 11391147. (8) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Investigation of asphaltene association with vapor pressure osmometry and interfacial tension measurements. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (9) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadeh, K. Sensitivity of asphaltene properties to separation techniques. Energy Fuels 2002, 16, 462-469. (10) Zhang, L. Y.; Lawrence, S.; Xu, Z.; Masliyah, J. H. Studies of Athabasca asphaltene Langmuir films at air-water interface. J. Colloid Interface Sci. 2003, 264, 128-140. (11) Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Langmuir and LangmuirBlodgett films of mixed asphaltene and a demulsifier. Langmuir 2003, 19, 9730-9741. (12) Freer, E. M.; Svitova, T.; Radke, C. J. The role of interfacial rheology in reservoir mixed wettability. J. Pet. Sci. Eng. 2003, 39, 137158. (13) Zhang, L. Y.; Lopetinsky, R.; Xu, Z.; Masliyah, J. H. Asphaltene monolayers at a toluene-water interface. Energy Fuels 2005, 19, 13301336. (14) Bartell, F. E.; Neiderhauser, D. O. Film forming constituents of crude petroleum oils. In Fundamental Research on Occurrence and RecoVery of Petroleum; Americal Petroleum Institute: New York, 1949; p 57. (15) Reisberg, J.; Doscher, T. M. Interfacial phenomena in crude-oilwater systems. Prod. Mon. 1956, 21, 43-50. (16) Strassner, J. E. Effect of pH on interfacial films and stability of crude oil-water emulsions. J. Pet. Technol. 1968, 20, 303-312.

10.1021/ef0603129 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006

Asphaltene Films at a Toluene/Water Interface

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of asphaltene in toluene is greater than 10% by weight. At lower asphaltene concentrations (less than 10% by mass), no skin can be observed visually. However, after a drop of an asphaltenein-toluene solution immersed in water was compressed, the drop was distorted. They suggested that the irreversibility of asphaltene adsorption at the toluene/water interface is the cause of liquid drop distortion. Recently, Freer et al.12 showed that the adsorbed interfacial species at an oil/water interface form rigid skins during retraction of an oil drop. They assumed that the adsorbed species are asphaltenes and the adsorption process is irreversible at the oil/water interface. They also showed that the adsorbed species at the oil/water interface do not migrate to the aqueous phase by replacing the aged aqueous phase with a fresh one. Freer and Radke18 showed that adsorption of asphaltenes at a toluene/water interface is irreversible. Asphaltene interfacial films at various oil/water interfaces have been studied using rheological measurements. Acevedo et al.19 studied an asphaltene film at a xylene/water interface and showed that the interfacial asphaltene film behaved like a viscoelastic and elastic film. They speculated that the accumulation of asphaltenes at the xylene/water interface could lead to the formation of a multilayer asphaltene film. Through viscosity measurements, Li et al.20 indicated that an asphaltene interfacial film at an oil/water interface is two-dimensional in nature at low asphaltene concentrations in the oil but the film changes to three-dimensional at high asphaltene concentrations. Asphaltene interfacial film at an oil/water interface can alter the wettability of reservoirs, which is an important factor in oil recovery by waterflooding or enhanced oil recovery.21 Wettability alteration is caused by the rupture of the water film separating the oil and solid surface, leading to the coating of the adsorbed asphaltene interfacial film on solid surfaces.12,22 The asphaltene interfacial film, initially confined to the oil/water interface, can also deposit directly onto the rock surface,12,15 thereby changing its wettability. Numerous studies of asphaltene adsorption lead several authors to conclude that the adsorption film is a multilayer in nature at an oil/water interface. For example, Cairns et al.23 studied the viscosity of interfacial films at a crude oil/water interface. They attributed the sudden increase in initial viscosity to the formation of a multilayer interfacial asphaltene film. Jeribi et al.17 suggested that the formation of a multilayer asphaltene film is due to the adsorption of asphaltene from the bulk toluene phase to the toluene/water interface at asphaltene concentrations higher than 10 wt %. Asphaltene stacking has been reported in a number of papers in the literature. Using X-ray diffraction (XRD), Dickie and

Yen24 demonstrated that solid asphaltenes are organized into stacks of 3-5 unit sheets through interactions between aromatic rings and hydrogen bonding. Recent XRD measurements on solid asphaltenes25 suggested the presence of a stacked cluster of eight aromatic sheets. Brandt et al.26 studied asphaltene stacking in a given solvent using statistical thermodynamics and computer-aided molecular modeling. Asphaltene molecules were modeled as flat hard discs (unit sheets). It was found that asphaltene unit sheets can stack to any arbitrary degree in a solvent. They indicated that asphaltene stacking occurs above an asphaltene volume fraction of 0.2 in a good solvent such as benzene. Yen4 stated that asphaltene stacking is the precursor of asphaltene self-association and aggregation. Asphaltene aggregation in organic solvents was observed by Pfeiffer and Saal.27 It has been postulated in the literature that asphaltenes can form micelle-like aggregates in aromatic solvents above a certain concentration, which is known as the critical micelle concentration (cmc). From surface tension measurements, cmc values for an asphaltenes-in-toluene solution have been reported to be 6.5 mg/mL for a Brazilian asphaltene sample28 and 10.5-22.9 mg/mL for four Venezuelan asphaltene samples. However, for Athabasca asphaltenes, Yarranton and co-workers3,8 did not observe any cmc in their surface or interfacial tension measurements. Sheu and co-workers29,30 noted that asphaltenes in a good solvent below the cmc are in a single molecular state, whereas above the cmc, asphaltene micelle formation occurs in a manner similar to that in surfactant systems but with less uniformity in the micelle structure. Sheu29 further pointed out that the structure and shape of asphaltene micelles/aggregates appear to be much more complicated than those of classic surfactant micelles. This is not unexpected if one considers asphaltenes as a mixture of an infinite number of molecular species of similar structures (polynuclear aromaticity). These studies indicate that asphaltene aggregation or association is controlled by multiple interactions:31 aggregates of 2-5 nm in size are either present in a molecular form or formed by strong intermolecular attractive forces; larger aggregates are self-associates, formed from small aggregates by weaker interaction forces. Recently, Porte et al.32 indicated that asphaltene aggregation in solvents is dominated by dispersion or van der Waals forces. They suggested that the strong attractive van der Waals forces drive the association of two-dimensional asphaltene unit sheets, eventually leading to the formation of a multilayer asphaltene structure. Using a Langmuir trough, many studies showed that asphalt-

(17) Jeribi, M.; Almir-Assad, B.; Langevin, D.; Henaut, I.; Argillier, J. F. Adsorption kinetics of asphaltenes at liquid interfaces. J. Colloid Interface Sci. 2002, 256, 268-272. (18) Freer, E. M.; Radke, C. J. Relaxation of asphaltenes at the toluene/ water inteface: Diffusion exchange and surface rearrangement. J. Adhes. 2004, 80, 481-496. (19) Acevedo, S.; Escobar, G.; Gutie´rrez, L. B.; Rivas, H.; Gutie´rrez, X. Interfacial rheological studies of extra-heavy crude oils and asphaltenes: Role of the dispersion effect of resins in the adsorption of asphaltenes at the interface of water-in-crude oil emulsions. Colloid Surf., A 1993, 71, 65-71. (20) Li, M.; Xu, M.; Ma, Y.; Wu, Z.; Christy, A. A. Interfacial film properties of asphaltenes and resins. Fuel 2002, 81, 1847-1853. (21) Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and adsorption properties of asphaltenes. Colloid Surf., A 1995, 94, 253-265. (22) Ward, A. D.; Ottewill, R. H.; Hazlett, R. D. An investigation into the stability of aqueous films separating hydrocarbon drops from quartz surfaces. J. Pet. Sci. Eng. 1999, 24, 213-220. (23) Cairns, R. J. R.; Grist, D. M.; Neustadter, E. L. The effect of crude oil-water interfacial properties on water-crude oil emulsion stability. In Theory and Practice of Emulsion Technology; Academic Press: New York, 1976; p 135.

(24) Dickie, J. P.; Yen, T. F. Macrostructures of asphaltic fractions by various instrumental methods. Anal. Chem. 1967, 39, 1847-1852. (25) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Characterization of asphaltene aggregates using X-ray diffraction and small-angle X-ray scattering. Energy Fuels 2004, 18, 11181125. (26) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. Thermodynamic modeling of asphaltene stacking. J. Phys. Chem. 1995, 99, 10430-10432. (27) Pfeiffer, J. P.; Saal, R. N. J. Asphaltic bitumen as colloid system. J. Phys. Chem. 1940, 44, 139-149. (28) Mohamed, R. S.; Ramos, A. C. S.; Loh, W. Aggregation behavior of two asphaltenic fractions in aromatic solvents. Energy Fuels 1999, 13, 323-327. (29) Sheu, E. Y. Physics of asphaltene micelles and microemulsionss Theory and experiment. J. Phys.: Condens. Matter 1996, 8, A125-A141. (30) Storm, D. A.; Sheu, E. Y. Characterization of colloidal asphaltenic particles in heavy oil. Fuel 1995, 74, 1140-1145. (31) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Fractal structure of asphaltene aggregates. J. Colloid Interface Sci. 2005, 285, 599-608. (32) Porte, G.; Zhou, H.; Lazzeri, V. Reversible description of asphaltene colloidal association and precipitation. Langmuir 2003, 19, 40-47.

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Zhang et al.

enes can form a monolayer at an air/water interface10,11,33-39 or various oil/water interfaces,11,13,36,38-41 suggesting that asphaltenes act as molecular surfactants. In a previous study,13 we showed that asphaltene forms a monolayer at a toluene/water interface. Repeated washing of the same asphaltene monolayer up to 3-4 times using fresh toluene showed that the interfacial pressure-area (π-A) isotherms obtained among different washings are nearly identical, indicating the absence of asphaltene loss from the interfacial film to the washing toluene. These results clearly show that asphaltenes in the monolayer do not migrate from the toluene/ water interface to the water subphase or the toluene topphase, thereby showing irreversible adsorption at the toluene/water interface. The focus of this study is to investigate whether asphaltenes can form a stable multilayer at a toluene/water interface. In this regard, a multilayer asphaltene film is first artificially created at the air/water interface by spreading an excess amount of asphaltene on the surface of a water subphase. After complete evaporation of the spreading toluene solvent, an asphaltene multilayer, presumably in the form of stacked monolayers, is prepared at the air/water interface. A bulk toluene phase is added as the topphase, in an attempt to disrupt the stacked asphaltene multilayer by dissolving the excess asphaltenes into the toluene topphase, leading to the formation of an asphaltene monolayer at the toluene/water interface. We will show that the multilayer asphaltene film is only weakly held together at the air/water interface and readily reorganizes to become a stable monolayer at the toluene/water interface upon the addition of a bulk toluene topphase.

bitumen was dissolved in toluene at a toluene/bitumen volume ratio of 5:1. Solids were removed from the toluene-diluted bitumen by centrifugation at 20 000 rpm (35000g) for 30 min. Toluene in the diluted bitumen was then removed by natural evaporation under ambient conditions in a fume hood for 1 week. The bitumen prepared as such was added to technical-grade n-heptane at a heptane/bitumen volume ratio of 40:1. The heptane-diluted bitumen was left overnight for asphaltene precipitation and subsequent settling. The supernatant was then removed carefully by decantation, and the precipitates were washed with an excess amount (1 L) of technical-grade heptane for 17 washings to remove any coprecipitated maltenes. After such a large number of washings, the prepared asphaltene can be considered as maltene-free. The density of the prepared asphaltene was determined using an Anton Paar DMA 45 density meter10 to be 1.2 g/cm3 at 20 °C. The molecular mass was determined to be 7070 g/mol.11 2.2. Langmuir Trough Experiments. Asphaltene interfacial films were investigated using a KSV Langmuir interfacial balance (KSV Instruments, Finland) at a toluene/water interface. The interfacial trough, having an area of 17 010 mm2, and the two symmetric barriers are all made of Delrin. The trough has two compartments: a lower compartment for a heavier phase such as water and an upper compartment for a lighter phase such as oil. The holes in the two barriers allow the oil phase to flow freely while compressing an oil/water interface. The trough was placed in an enclosure on an antivibration table. The trough was cleaned by rinsing with heptane and wiping its surface with acetone-soaked Texwipe wipers. The temperature of the trough was controlled by a circulation water bath to 20 ( 0.1 °C. A strip of filter paper (Whatman 1 CHR), attached to an electronic balance, was used as the Wilhelmy plate to measure the interfacial pressure. The interfacial pressure is defined as the difference of interfacial tension42

2. Materials and Methods

π ) γ0 - γ

2.1. Materials. Vacuum distillation feed bitumen was supplied by Syncrude Canada. Optima-grade toluene (99.8%), HPLC-grade acetone, isopropanol, n-heptane, and technical-grade n-heptane were all purchased from Fisher Scientific Canada. The asphaltene fraction used in this study was the same as the whole asphaltene used in our previous studies.11,13 Details of the extraction procedures, molecular weight, and chemical composition of the asphaltene can be found in refs 11 and 13. A brief description of the extraction procedures is given here. Vacuum distillation feed (33) Leblanc, R. M.; Thyrion, F. C. A study of monolayers of asphalts at the air/water interface. Fuel 1989, 68, 260-262. (34) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Dewatering of crude oil emulsions. 2. Interfacial properties of the asphaltic constituents of crude oil. Colloid Surf., A 1993, 80, 237-242. (35) Ese, M. H. Film forming properties of asphaltenes and resins. A comparative Langmuir-Blodgett study of crude oils from North Sea, European continent and Venezuela. Colloid Polym. Sci. 1998, 276, 800809. (36) Ese, M. H.; Galet, L.; Clausse, D.; Sjoblom, J. Properties of Langmuir surface and interfacial films built up by asphaltenes and resins: Influence of chemical demulsifiers. J. Colloid Interface Sci. 1999, 220, 293301. (37) Ese, M. H.; Sjoblom, J.; Djuve, J.; Pugh, R. An atomic force microscopy study of asphaltenes on mica surfaces. Influence of added resins and demulsifiers. Colloid Polym. Sci. 2000, 278, 532-538. (38) Gundersen, S. A.; Ese, M. H.; Sjoblom, J. Langmuir surface and interface films of lignosulfonates and Kraft lignins in the presence of electrolyte and asphaltenes: Correlation to emulsion stability. Colloid Surf., A 2000, 182, 199-218. (39) Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Characterization of adsorbed Athabasca asphaltene films at solvent-water interfaces using a Langmuir interfacial trough. Ind. Eng. Chem. Res. 2005, 44, 1160-1174. (40) Kimbler, O. K.; Reed, R. L.; Silsberberg, I. H. Physical characteristics of natural films formed at crude oil-water interfaces. Soc. Pet. Eng. J. 1966, 6, 153-165. (41) Lawrence, S.; Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Langmuir and Langmuir-Blodgett asphaltene films at heptane-water interface. Can. J. Chem. Eng. 2004, 82, 821-828.

(1)

where γ0 and γ are the interfacial tensions of an oil/water interface in the absence and presence of surface-active material, respectively. Ultrapure water of a resistivity of 18.2 MΩ cm, prepared with a Millipore system, was used as the subphase. The subphase water was cleaned several times by closing the barriers to a small area and removing its surface layers with a pipet connected to a vacuum line equipped with a waste trap, until the surface pressure reading became smaller than 0.10 mN/m at the air/water interface. Asphaltene was dissolved in toluene at a concentration of 2 mg/ mL, a value that is much lower than the cmc values for various asphaltenes discussed earlier in the Introduction. The prepared asphaltene-in-toluene stock solution was centrifuged and filtered according to the procedures described by Zhang et al.10 to remove any solid particles prior to its use. In this study, we adopt the conventional technique to prepare Langmuir films of asphaltenes as described by Schwartz.43 In this method, ultrapure water (130 mL) was first poured into the lower compartment of the trough. A volume of 15-100 µL of the prepared solids-free asphaltene-in-toluene solution was spread dropwise on the surface of the water subphase with a Hamilton precision microsyringe. It should be mentioned that the films prepared using the spreading volume of 15-30 µL are monolayer films and those using a volume of 100 µL are multilayer films, as will be discussed in section 3.1. The spreading toluene solvent was allowed to evaporate for a period of 10-30 min, and the π-A isotherms obtained at different toluene evaporation periods were nearly identical. In this study, at least a 10 min period was allowed for evaporation of the spreading solvent. The bulk toluene topphase was poured into the upper compartment of the trough with the aid (42) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (43) Schwartz, D. K. Langmuir-Blodgett film structure. Surf. Sci. Rep. 1997, 27, 241-334.

Asphaltene Films at a Toluene/Water Interface of a glass funnel. The asphaltene films prepared as such are referred to as “spread films”. The prepared asphaltene interfacial films were compressed after 30 min of equilibration. The interfacial pressure reading was zeroed prior to compression of the prepared asphaltene films at the oil/ water interface. Another method to prepare an interfacial film is by adsorption of asphaltene from a dilute asphaltene-in-toluene solution to a toluene/water interface upon contacting the solution with water to form an adsorbed monolayer. The monolayers prepared using this method are referred to as “Gibbs films” or “adsorbed films”. The adsorbed asphaltene films can also be prepared by placing a small volume of the prepared asphaltene-in-toluene stock solution (2 mg/ mL) on the surface of the water subphase first and then covering the water surface immediately with a toluene topphase. π-A isotherms were obtained by compressing a prepared oil/ water interface at a specified compression speed, typically at 1 cm/ min per barrier. At least three runs were repeated for each of the isotherms to ensure reproducibility, and the averaged values are reported in this paper. To check whether asphaltenes migrate from a prepared oil/water interface, dilution experiments were performed using the following procedure. A prepared asphaltene film at a toluene/water interface was compressed first to an interfacial pressure of ∼30 mN/m, followed by immediate expansion of the two symmetric barriers to a full expansion with a zero pressure reading. The topphase was removed, collected as much as possible with a pipet, and replaced immediately with a fresh toluene as a new topphase. After an equilibration period of 30 min, the interfacial asphaltene film was compressed. This dilution procedure was repeated 2 more times to investigate if successive dilution would affect the behavior of the asphaltene interfacial film. The removal of asphaltenes from the interfacial film was determined by placing the collected washing toluene back on a fresh water subphase and compressing the newly formed toluene/water interface after an equilibration period of 30 min. 2.3. Langmuir-Blodgett (LB) Deposition. Silicon wafers used for LB deposition were cleaned using the cleaning procedures described by Zhang and Srinivasan.44 Briefly, the wafers were soaked overnight in a Nochromix (Fisher Scientific Canada) containing sulfuric acid (96%) solution. The soaked wafers were flushed with excess tap water and rinsed several times with ultrapure water. The cleaned hydrophilic silicon wafers were stored in ultrapure water and were ready for use for depositing LB films. Asphaltene films at a toluene/water interface were transferred onto the cleaned hydrophilic silicon wafers by vertical LB deposition.42 A hydrophilic silicon wafer was first immersed in the water subphase parallel to the barriers before it was covered with a spread asphaltene film and a bulk toluene topphase. An asphaltene film was compressed and held at a constant interfacial pressure while pulling the substrate up (upstroke) through the toluene/water interface, transferring the asphaltene film onto the substrate. Multilayer asphaltene films were deposited onto hydrophobic silicon wafers by the horizontal Langmuir-Schaeffer (LS) deposition technique45 at the air/water interface. Hydrophilic silicon wafers were rendered hydrophobic by soaking them overnight in a solution of 10% by volume of dichlorodimethylsilane (99%, Aldrich) in toluene.11 The wafers were rinsed several times with toluene and dried in a fume hood by natural evaporation. They were stored in a sealed vial before being used for LS deposition. The hydrophobic silicon wafers have a water contact angle of ∼100°. 2.4. AFM Imaging. AFM images of deposited LB asphaltene films were obtained with a Nano Scope IIIa (Digital Instruments) atomic force microscope (AFM) operated in tapping mode in air at room temperature. A MultiMode-Scanning Probe Microscope (44) Zhang, L. Y.; Srinivasan, M. P. Hydrodynamics of subphase entrainment during Langmuir-Blodgett film deposition. Colloid Surf., A 2001, 193, 15-33. (45) Ulman, A. An Introduction to Ultra-thin Organic FilmssFrom Langmuir-Blodgett Films to Self-Assembly; Academic Press: New York, 1991.

Energy & Fuels, Vol. 21, No. 1, 2007 277 (MM-SPM) head and a J-scanner were used for AFM imaging. Tapping mode imaging was performed at a scan rate of 1-2 Hz using a silicon tip (Digital Instruments) with a resonance frequency of 300-400 kHz. Integral and proportional gains of the feedback loop were set to 0.1 and 1, respectively. Phase imaging was performed simultaneously along with topography imaging. At least three separated spots and two samples prepared independently under the same conditions were imaged by tapping mode AFM. 2.5. UV Spectroscopy. To quantify how much asphaltene was migrated from a prepared asphaltene multilayer film to the toluene topphase, a Cary 50 UV-vis spectrophotometer (Varian, Palo Alto, CA) was used to determine the concentration of asphaltene in the collected toluene. To this end, a series of dilute asphaltene-intoluene solutions with known concentrations were prepared by dilution of the prepared asphaltene-in-toluene stock solution and used to obtain a calibration curve. All of the asphaltene-in-toluene solutions were stored in glass vials capped with Teflon caps. UV spectra were obtained by dipping a Fiber Optics probe (Varian) into a prepared asphaltene-in-toluene solution. The probe was washed and rinsed with fresh toluene after each measurement. Pure toluene was used as the background. Each individual spectrum was obtained by zeroing the instrument background reading using clean toluene. UV spectra were collected at a UV-vis scan rate of 60 nm/min and a spectral interval of 5 nm. A wavelength of 370 nm was selected for concentration calibration because the absorbance of asphaltene at this wavelength is the strongest. 2.6. Washing of Water-in-Toluene Emulsion. To investigate the reversibility of asphaltene adsorption at a toluene/water interface, asphaltene-stabilized water-in-toluene emulsions were prepared. Ultrapure water (20 mL) was added to a glass bottle, followed by the addition of 4 mL of a prepared asphaltene-in-toluene stock solution at a concentration of 2 mg/mL. The toluene phase was then topped up to a final volume of 80 mL. The volume fraction of water was 0.2, and the asphaltene concentration in the whole emulsion system was at 0.009 wt %. Emulsification was accomplished by homogenizing the water/toluene/asphaltene mixture at 30 000 rpm for 3 min using a homogenizer (Power Gen 125, Fisher Scientific Canada). The prepared water-in-toluene emulsion was washed by removing and replacing the organic phase with 80 mL of fresh toluene, followed by homogenization for 3 min. This washing process was repeated 3 times until the organic phase became colorless.

3. Results and Discussion 3.1. Monolayer Asphaltene Films. Before preparing asphaltene multilayer films, it is important to determine the amount of asphaltene needed to form a monolayer film. One approach is to determine the molecular area a priori. The area occupied by an asphaltene “molecule” at the solid state of the monolayer at the air/water interface, extrapolated to a zero surface pressure, is about 3.3 nm2.11 The total number of asphaltene molecules required to occupy the entire available trough area of 170.1 cm2 to form a close-packed monolayer is therefore 5.15 × 1015 molecules. Because 1 µL of an asphaltenein-toluene solution at a concentration of 2 mg/mL contains 2 × 10-6 g asphaltene, the number of asphaltene molecules in 1 µL of such an asphaltene-in-toluene solution is estimated to be 1.70 × 1014 molecules using a molecular weight of asphaltene of 7070 g/mol. The total volume of the prepared asphaltenein-toluene solution required to form a close-packed asphaltene monolayer over the entire trough area is therefore 30 µL. When the density of asphaltene (1.2 g/cm3) and the total available trough area (170.1 cm2) are known, the thickness of a spread asphaltene film at the air/water interface can be estimated. For example, for a spreading volume of 30 µL of an asphaltene-in-toluene solution at a concentration of 2 mg/mL, the thickness of the spread film will be the total volume of asphaltene divided by the trough area, i.e., ∼2.9 nm.


Figure 1. Comparison of π-A isotherms at a toluene/water interface in the presence and absence of an asphaltene monolayer. (a) π-A isotherm of a toluene/water interface only. (b) π-A isotherm of the toluene/water interface in the presence of an asphaltene monolayer using 30 µL of an asphaltene-in-toluene stock solution. (c) π-A isotherm of the recovered toluene/fresh water interface. The recovered toluene was collected from the Langmuir trough after one compression of an asphaltene monolayer at the toluene/water interface to an interfacial pressure of ∼30 mN/m.

The above estimate indicates that an asphaltene monolayer can be prepared using a spreading volume of less than 30 µL. When the spreading volume is ,30 µL, a sparsely packed asphaltene monolayer can be prepared. In other words, the trough area is not fully covered by asphaltenes in this case. However, when the spreading volume is 30 µL, the entire trough area is nearly saturated with asphaltenes and a closely packed asphaltene monolayer is expected to form. In this case, the trough area is fully covered by asphaltenes. Figure 1 shows π-A isotherms at a toluene/water interface in the presence and absence of an asphaltene monolayer. The dotted line (a) is the π-A isotherm of a pure toluene/water interface without any added asphaltene at the interface. The interfacial pressure reading is close to zero even when the toluene/water interface is compressed to a small area, indicating the absence of interfacial-active material at the toluene/water interface. This in turn suggests the absence of interfacial-active species in toluene. The solid line (b) of Figure 1 is the π-A isotherm in the presence of an asphaltene monolayer prepared using 30 µL of the prepared asphaltene-in-toluene stock solution at the toluene/water interface. To identify whether asphaltene is transferred from the interface to the toluene topphase upon compression, the toluene topphase after completing one compression of an asphaltene monolayer was recovered and placed on top of a fresh water subphase. The π-A isotherm obtained with the collected toluene phase on a fresh water surface is shown in Figure 1 by the dashed line (c). In this case, the π-A isotherm is nearly identical to that of the pure toluene/water interface, indicating the absence of any interfacial-active material in the collected toluene phase. This finding in turn indicates that the spread asphaltene monolayer remains at the toluene/ water interface and the asphaltenes do not migrate into the bulk toluene topphase from the interface. In other words, the asphaltene monolayer is irreversibly adsorbed at the toluene/ water interface. 3.2. Multilayer Asphaltene Films. As discussed in section 2.2, a multilayer asphaltene film can be prepared by spreading excess asphaltenes at an air/water interface. In this study, we spread 100 µL of the prepared asphaltene-in-toluene stock solution at a concentration of 2 mg/mL to prepare a multilayer asphaltene film. As mentioned in the Introduction, the cmc of an asphaltene-in-toluene solution is >6.5 mg/mL as reported

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in the literature. In our case, we used a concentration of 2 mg/ mL of an asphaltene-in-toluene solution, which is well below the cmc concentration. As a result, the solution is in a single molecule state. The reason for a higher value of molecular weight of 7070 g/mol is because of the way the preparation of our asphaltene sample is different from others as reported in the literature. Our asphaltene sample was washed 17 times with n-heptane as described in ref 11. The prepared asphaltene sample is a “pure” asphaltene sample in which the entrained maltenes were removed by washing with heptane. Therefore, this molecular-weight value is valid for the nature of our asphaltene sample at the toluene/water interface. As a result, an asphaltene monolayer is formed below a full coverage of the total available trough area. On the other hand, an asphaltene multilayer film is a three-dimensional film of several layers with a full coverage of the available trough area. On the basis of the discussions in section 3.1, the film formed as such corresponds to a multilayer film composed of about 3.5 monolayers stacked together. After a period of 30 min for toluene evaporation, the prepared asphaltene film is visible on the water surface, showing large islands surrounded by small brownish fingers, as shown in Figure 2a. Cracks in the asphaltene multilayer film can be observed from the photograph. Such features were absent in asphaltene monolayer films prepared at the air/water interface. Using the LS deposition technique, the multilayer asphaltene film was transferred onto a hydrophobic silicon wafer and the deposited film was examined under an optical microscope and AFM. Figure 2b shows a typical optical micrograph of deposited multilayer asphaltene films transferred from the air/water interface. The image clearly shows that the deposited film is a multilayer as evidenced by a strong variation in color at different locations. Figure 2c shows an AFM image of a transferred multilayer asphaltene film. Clearly, the AFM image is different from that observed for asphaltene monolayers observed at the air/water interface.10,11 Moreover, the z-scale in Figure 2c of 300 nm is about 60 times higher than that of a single-layer asphaltene film,11 indicating that the prepared asphaltene film as shown in Figure 2 is much thicker than a single-layer film. A section analysis of the AFM image in Figure 2c between the bright band and the dark areas gives a thickness value of ∼22 nm of the bright band. It seems that the multilayer asphaltene film has become folded by one multilayer film superimposing on another to form the bright band-like structure, suggesting that the bright band area is composed of ∼7 asphaltene monolayers. Nevertheless, Figure 2 shows that a multilayer asphaltene film can be prepared at the air/water interface. It should be mentioned that asphaltene monolayers transferred onto solid substrates cannot be imaged with an optical microscope because the deposited monolayer films were too thin to be visible under an optical microscope. Washing experiments similar to those of asphaltene monolayers used to obtain Figure 1 were carried out to investigate asphaltene migration from the prepared multilayer film to the added bulk toluene topphase. After 100 mL of toluene was added as the topphase to cover the multilayer asphaltene film and a period of 30 min of equilibration, the toluene/water interface obtained was compressed to an interfacial pressure of ∼30 mN/m. The two barriers were immediately expanded to a zero interfacial pressure. The topphase was collected and replaced with fresh toluene. After another 30 min of equilibration, the newly prepared toluene/water interface was compressed. This expansion/collection/replacement and compression was repeated 1 more time.



Figure 2. Multilayer asphaltene film at the air/water interface. (a) Photo taken after 10 min of spreading 100 µL of an asphaltene-in-toluene solution at a concentration of 2 mg/mL. (b) Image obtained with an optical microscope of a deposited multilayer asphaltene film from the air/water interface. (c) AFM image of a deposited multilayer asphaltene film from the air/water interface.

The collected toluene topphase was investigated for interfacial activity by placing it on the surface of a fresh water subphase. After the recovered toluene topphase was contacted with the fresh water subphase for 30 min, the newly prepared toluene/ water interface was compressed. π-A isotherms using the procedures described above are shown in Figure 3. The collected toluene refers to the toluene topphase recovered from the Langmuir trough in the presence of a “multilayer” asphaltene film at a toluene/water interface. For a pure toluene/water interface, the π-A isotherm of curve a shows an interfacial pressure close to zero. For the prepared “multilayer” asphaltene film, the π-A isotherm shown by curve b, is nearly identical to the π-A isotherm of an asphaltene monolayer as shown in Figure 1. For the collected toluene topphase on a fresh water subphase, the π-A isotherm of curve c shows adsorption of

asphaltene molecules at the toluene/water interface. In this case, the interface can be compressed to an interfacial pressure of ∼20 mN/m. This interfacial pressure is lower than that obtained from an asphaltene monolayer film at the same trough area, indicating that there is less asphaltene present at the interface. In other words, the trough surface has less coverage in the case of using collected toluene from the topphase of an asphaltene “multilayer”. Clearly, the collected toluene topphase contains interfacial-active asphaltene material, which can adsorb at the toluene/water interface and form an adsorbed monolayer. This in turn indicates that part of the asphaltenes in the original multilayer film migrated from the interface to the toluene topphase, suggesting a weak binding between asphaltenes in different adjacent layers. The π-A isotherm of curve d in Figure 3 shows close-to-zero interfacial pressures, indicating that a


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Figure 3. π-A isotherms at the toluene/water interface. (a) Toluene/ water interface as the background. (b) “Multilayer” asphaltene film. (c) Toluene collected from the topphase after completing experiment b and put back on a fresh water surface. (d) Original toluene topphase removed and replaced with a fresh toluene topphase; after completing the experiment of curve c, the topphase was collected and put back on a fresh water surface.

Figure 5. Comparison of π-A isotherms of an asphaltene monolayer with an asphaltene “multilayer”. (a) Original and (b) dilution 2.

Figure 4. π-A isotherms at a toluene/water interface for an asphaltene “multilayer” after multiple dilutions or washings. In dilution 1, the original organic phase was removed and replaced immediately with fresh toluene after completing the experiment with the original and expanding the two barriers to an interfacial pressure reading of 0. The same dilution procedure as used in dilution 1 was repeated for dilution 2.

negligible amount of asphaltene is present in the collected toluene topphase, which was used for a second washing of the asphaltene interfacial film formed from the first washed solution. The π-A isotherms from consecutive dilution experiments by washing the same asphaltene “multilayer” film 3 times with fresh toluene are shown in Figure 4. The three π-A isotherms, which correspond to two consecutive dilutions or washings, show a small but consistent expansion of the same asphaltene “multilayer” film. The π-A isotherms obtained after multiple dilutions of the same “multilayer” asphaltene film are nearly identical, indicating that there is no material loss from the asphaltene interfacial film. The slight difference in π-A isotherms as observed in Figure 4 among the three isotherms can be attributed to the rearrangement of asphaltene at the toluene/water interface. To determine whether the prepared asphaltene multilayer remains to be a multilayer at the toluene/water interface before and after consecutive washings with a fresh toluene, π-A isotherms obtained from an asphaltene “multilayer” were compared to that of an asphaltene monolayer. Figure 5a shows that the π-A isotherms obtained from the original asphaltene “multilayer” are nearly identical to that of an original asphaltene monolayer before any washings. As shown in Figure 5b, after

two dilutions or washings, the π-A isotherms obtained from washed asphaltene “multilayers’’ and an asphaltene monolayer are nearly identical. After one dilution with fresh toluene, the π-A isotherms obtained are also nearly identical for an asphaltene “multilayer” and an asphaltene monolayer. These results suggest that the prepared “multilayer” asphaltene film behaves like a monolayer asphaltene film in the presence of a toluene topphase. Any excess asphaltene material would migrate to the added bulk toluene phase from the interface. It appears that only an asphaltene monolayer is critical to form a protective interfacial film, which is responsible for the stability of the toluene/water interface and therefore the stability of waterin-toluene emulsions. The similarity of the π-A isotherms as shown in Figure 5 between an asphaltene monolayer and an asphaltene “multilayer” demonstrates that dilution by replacing the existing topphase with a fresh toluene topphase did not affect the behavior of the asphaltene interfacial film at the toluene/ water interface. A practical application for bitumen froth treatment during commercial oil sands processing is that dilution with fresh solvent such as toluene will not improve demulsification of stable water-in-bitumen emulsions. Experiments were conducted to study the effect of evaporation of the spreading solvent on π-A isotherms. Immediately after spreading 100 µL of the prepared asphaltene-in-toluene stock solution on the surface of the water subphase without evaporation of the spreading solvent, 100 mL of fresh toluene were poured into the Langmuir trough. After 30 min of equilibration, the toluene/water interface was compressed, and the π-A isotherm is shown in Figure 6 as a dashed line (a). It is apparent that the π-A isotherm obtained without toluene evaporation has shifted toward a smaller trough area at a given interfacial pressure in comparison with that of a spread asphaltene



Figure 6. Comparison of adsorbed asphaltene films with a spread asphaltene monolayer. (a) Adsorbed asphaltene film prepared by placing 100 µL of an asphaltene-in-toluene solution at a concentration of 2 mg/mL. The toluene solvent was not allowed to evaporate in this case. The air/water interface was immediately covered with a bulk toluene topphase. (b) Spread asphaltene monolayer prepared by spreading 30 µL of an asphaltene-in-toluene solution at a concentration of 2 mg/ mL. The spreading solvent toluene was completely evaporated in this case. (c) Adsorbed asphaltene film from the adsorption of asphaltene from an asphaltene-in-toluene solution at a concentration of 0.002 mg/ mL. (d) Adsorbed asphaltene film from the adsorption of asphaltene in collected toluene topphase. The collected toluene topphase was recovered after one compression of an asphaltene “multilayer”.

monolayer as shown by a solid curve (b) in Figure 6. This shift indicates that there is less asphaltene material present at the toluene/water interface without evaporation of the spreading solvent than for the case of a spread asphaltene film of curve b with complete evaporation of the spreading solvent. It is clear that the toluene/water interface is not fully covered with asphaltenes to reach the saturation coverage at the interface. In adsorption experiments, a volume of 100 µL of the prepared asphaltene-in-toluene stock solution (2 mg/mL) was mixed with 100 mL of fresh toluene to obtain a dilute asphaltene-in-toluene solution at a concentration of 0.002 mg/ mL. Then, 100 mL of the prepared dilute asphaltene-in-toluene solution was poured on the surface of a fresh water subphase. After 30 min of adsorption, the prepared toluene/water interface was compressed, and the resultant π-A isotherm is shown as a dotted line (c) in Figure 6. It is clear that the π-A isotherm of the adsorbed asphaltene film is nearly identical to that obtained from spreading 100 µL of the prepared asphaltene-in-toluene solution without evaporation of the spreading solvent. Judging from the similarity in shape between the two π-A isotherms of a and b, Figure 6 suggests that an adsorbed asphaltene monolayer from a dilute asphaltene-in-toluene solution in curve a behaves similarly to that of a spread asphaltene monolayer in curve b at the toluene/water interface. Moreover, the π-A isotherms for curves a and c of adsorbed monolayers were shifted to the left to that of the spread asphaltene monolayer of curve b, indicating that there is less asphaltene material present at the interface for the adsorbed monolayers than that for the spread asphaltene monolayer. For comparison purposes, the π-A isotherm obtained from adsorption of a collected toluene from the topphase of a “multilayer” asphaltene film is also included in Figure 6 as a dash-dotted curve (d). It is interesting to note that the π-A isotherm obtained from the collected toluene topphase shifts to a smaller trough area at a given π value in comparison to the π-A isotherm of the adsorbed asphaltene monolayer from a dilute asphaltene-in-toluene solution at a concentration of 0.002 mg/mL. This shift indicates that the asphaltene concentration

Figure 7. Calibration of an asphaltene-in-toluene solution and UV spectra. (a) UV spectra of an asphaltene-in-toluene solution measured from 200 to 700 nm. (b) Concentration calibration of an asphaltenein-toluene solution.

in the collected toluene is less than 0.002 mg/mL, a result that will be confirmed by UV measurements as is discussed later. The results of π-A isotherms shown in Figure 6 indicate that an adsorbed asphaltene monolayer behaves similarly to a spread asphaltene monolayer. AFM images obtained for LB films prepared from adsorbed asphaltene monolayers at a toluene/water interface showed nearly identical topographical features as observed in the AFM image of the LB asphaltene film prepared from a spread asphaltene monolayer as shown later. The mass of the adsorbed asphaltene monolayers, however, is less than that of the corresponding spread asphaltene monolayer as shown in Figure 6. This is judged on the basis of the fact that the π-A isotherm of the adsorbed asphaltene monolayers shifts to a smaller trough area at a given interfacial pressure than that of a spread asphaltene monolayer. Although the original mass of 0.2 mg used to obtain the adsorbed asphaltene monolayer (curve b in Figure 6) is more than that of 0.06 mg for the spread asphaltene monolayer (curve a in Figure 6), the actual asphaltene mass present at the toluene/ water interface of the adsorbed monolayer is less than that of the spread monolayer. This can be expected because, during preparation of the adsorbed asphaltene monolayer in curve b of Figure 6, only part of the total asphaltene mass was adsorbed to the toluene/water interface. Figure 6 clearly shows that the area coverage is higher for the spread asphaltene monolayer than adsorbed asphaltene monolayers. As mentioned earlier, the whole trough area is covered with asphaltenes in the case of a prepared asphaltene multilayer film. 3.3. Asphaltene Concentration Determination by UV. UV-vis spectroscopy was used to quantify the asphaltene concentration in the collected topphase for the case of a


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Figure 8. Comparison of AFM images of an asphaltene monolayer and an asphaltene “multilayer” film transferred onto silicon wafers from the toluene/water interface at π ) 10 mN/m and 20 °C. (a) AFM image of an LB asphaltene monolayer film. (b) AFM image of an LB asphaltene “multilayer” film.

“multilayer’’ asphaltene film. The corresponding UV spectra over a wavelength range of 200-700 nm are shown in Figure 7a. Figure 7a shows that all of the asphaltene UV spectra are similar in shape. The UV spectrum obtained from the collected topphase is nearly identical to the spectra obtained from asphaltene-in-toluene solutions at concentrations of 0.002, 0.001, and 0.0008, indicating that asphaltenes that migrated from the prepared multilayer film to the toluene topphase do not seem to be different from the original asphaltenes present in the prepared asphaltene-in-toluene solutions, suggesting that asphaltenes that migrated to the toluene topphase are not different from that present at the toluene/water interface. A calibration curve of known asphaltene concentrations at a wavelength of 370 nm is shown in Figure 7b, which demonstrates that the UV absorbance of asphaltene is a linear function of the asphaltene concentration at a wavelength of 370 nm.

The exact asphaltene concentration in the collected toluene topphase can be readily determined. For the collected topphase of spread multilayer films, the asphaltene concentration is determined to be 0.0014 mg/mL. This value is smaller than the value of 0.002 mg/mL obtained from dilution of an asphaltenein-toluene stock solution. This finding is consistent with the trend observed in the π-A isotherms of adsorbed asphaltene monolayers as shown in Figure 6. Because the mass of asphaltene in the originally prepared “multilayer” film is known to be 0.2 mg, the asphaltene mass remaining at the toluene/water interface is therefore 0.06 mg. The remaining asphaltene at the toluene/water interface corresponds to the maximum mass of 0.06 mg needed to prepare an asphaltene monolayer. This is a clear indication that the prepared asphaltene multilayer at the air/water interface became a monolayer at the toluene/water interface once toluene was added as the topphase. The asphaltene mass present at the toluene/


water interface as determined from the asphaltene concentration measurement by UV spectroscopy in the collected toluene topphase confirms the results of π-A isotherms that only an asphaltene monolayer is present at the toluene/water interface after the addition of a toluene topphase. 3.4. AFM Images of LB Asphaltene Films. AFM was performed in tapping mode to image the topography of the transferred LB asphaltene films from the toluene/water interface. Figure 8 shows a comparison of AFM topography images 5 × 5 µm in size of LB asphaltene monolayer (Figure 8a) and “multilayer” (Figure 8b) films. These LB asphaltene films were transferred onto hydrophilic silicon wafers at 10 mN/m and 20 °C from the toluene/water interface. It is apparent that the two AFM images are nearly identical in topographical features, with both images showing the presence of asphaltene nanoaggregates. For the case of the deposited LB asphaltene monolayer in Figure 8a, the nanoaggregates are about 20 nm as determined by section analysis of the AFM image. However, the asphaltene nanoaggregates in the asphaltene “multilayer” in Figure 8b are about ∼50 nm in size. The difference in size as observed in Figure 8a for an asphaltene monolayer and Figure 8b can be attributed to the mass difference of asphaltene spread at the air/water interface in preparing these films at the toluene/water interface. In the “multilayer” case, more asphaltene material was spread at the air/water interface first, therefore creating a saturated bottom layer at the interface, leading to a full coverage of the total available trough area. As a result, the toluene/water interface was still saturated with asphaltenes when toluene was added as the topphase, forming larger nanoaggregates in comparison to the case of an asphaltene monolayer. Nevertheless, the two AFM images in Figure 8 are very similar, suggesting that the “multilayer” asphaltene film is a monolayer asphaltene film. This supports the π-A isotherm results, as well as the UV concentration results as presented earlier, that a single-layer asphaltene film is present at the toluene/water interface, even though the asphaltene film was prepared as a multilayer. As discussed earlier, once a toluene topphase was placed on the prepared asphaltene multilayer film, the multilayer film became a monolayer at the toluene/water interface. 3.5. Molecular Size of Asphaltene at the Toluene/Water Interface. As shown in Figure 9, the molecular size of asphaltene as extrapolated to zero interfacial pressure in the solid state from the π-A isotherm at the toluene/water interface is A0 ) 1.5 nm2. It is smaller than A0 ) 3.3 nm2 as extrapolated at the air/water interface.11 Because the π-A isotherms in Figure 5 are nearly identical for both an asphaltene monolayer and an asphaltene “multilayer”, the mass of the asphaltene material present at the toluene/water interface for the case of a “multilayer” asphaltene film will be the same as that for the spread asphaltene monolayer. In fact, the mass of asphaltene present at the toluene/water interface as determined by UV spectroscopy is nearly the same as the maximum mass allowed to prepare a spread asphaltene monolayer. Therefore, we can plot the isotherm for the case of a “multilayer” asphaltene film in Figure 9 as a dashed curve in terms of area per asphaltene molecule. The extrapolated area per molecule for an asphaltene “multilayer” is therefore the same for an asphaltene monolayer, i.e., 1.5 nm2. The thickness of a closely packed asphaltene film in the solid state at the toluene/water interface can be calculated as the


Figure 9. Molecular size of asphaltene at the toluene/water interface at 20 °C.

diameter of a spherical asphaltene molecule: 2 × x1.5/3.14 ) 1.4 nm. The calculated film thickness of 1.4 nm, as determined from π-A per molecule isotherms for an asphaltene monolayer and a “multilayer”, is smaller than the measured thickness values of 3.0 nm through section analysis using AFM for LB asphaltene films from the spread asphaltene monolayer. However, all of the thickness values are indicative that the asphaltene interfacial film at the toluene/water interface is a monolayer in nature. 3.6. Washing of a Water-in-Toluene Emulsion. To further support the irreversible behavior of asphaltene adsorption as observed by Langmuir trough experiments, asphaltene-stabilized water-in-toluene emulsions were prepared and washed or diluted with fresh toluene. Figure 10 shows a comparison of two bottles of asphaltene-stabilized emulsions before (left) and after (right) washing with toluene. The bottle on the right-hand side was washed 3 times with fresh toluene. The organic phase on the left-hand side is brown, indicating the presence of some unadsorbed asphaltene molecules. However, the organic phase of the right-hand-side bottle is colorless, similar to the color of pure toluene. The colorless nature of the organic phase of the right-hand-side bottle clearly shows that the bulk toluene phase in the washed emulsion system is free of asphaltenes. Consequently, the initially unadsorbed or free asphaltenes in the bulk organic phase can be washed away by repeatedly removing and replacing the organic phase with fresh toluene. However, the adsorbed asphaltenes, which form interfacial films at the toluene/ water interface and stabilize water-in-toluene emulsion, cannot be washed away by adding fresh toluene. It is surprising to note that the toluene-washed asphaltenestabilized emulsion is not broken. In fact, both of these two emulsions as shown in Figure 10 before and after washing with fresh toluene are extremely stable even after 1 month of aging. Moreover, these two emulsions were still stable when they were subjected to heat treatment at 60 °C for 1 h in a water bath. These findings clearly show that asphaltene adsorption at the toluene/water interface in a water-in-toluene emulsion system is irreversible, consistent with the results obtained from Langmuir trough experiments. 3.7. Discussion: Asphaltene Irreversible Adsorption. The results discussed above demonstrate that an asphaltene monolayer is present at the toluene/water interface even though an asphaltene multilayer was prepared at the air/water interface. The fact is that the added toluene topphase dissolved the excess asphaltene material in the prepared multilayer film. Furthermore, the results show that an asphaltene monolayer is irreversibly


Figure 10. Digital photograph of asphaltene-stabilized water-in-toluene emulsions before (left) and after (right) washing with toluene. The righthand-side emulsion was washed 3 times by removing and replacing the top organic phase with fresh toluene.

adsorbed at the toluene/water interface. In this section, we attempt to explain why asphaltenes are irreversibly adsorbed at the oil/water interface. It is possible that the first asphaltene layer at a toluene/water interface is strongly bonded with water, such that this layer cannot be removed from the interface by the solvency power of toluene. In other words, the interactions between this asphaltene layer and water are very strong. On the other hand, the results presented above suggest that the interactions between the first layer of the asphaltene film and the immediate second layer of the asphaltene film in a prepared multilayer asphaltene film are not strong enough to maintain its multilayer film structure at the toluene/water interface. However, the multilayer film structure can be easily maintained at the air/water interface, as demonstrated in the images of Figure 2.

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becomes difficult if not impossible. This type of mechanism has been attributed to the irreversible adsorption observed for proteins.48 The lateral interactions including van der Waals forces between asphaltene molecules at the toluene/water interface are important in keeping the asphaltene film irreversibly adsorbed at the interface. Judged from the two-dimensional aggregated asphaltene structures as observed in the AFM images in parts a and b of Figure 8, it seems that the interactions between hydrocarbon side chains of asphaltene molecules at the toluene/ water interface are strong. The asphaltene aggregates cannot be washed off from the solid substrate when pulling the solid substrate through the bulk toluene topphase during deposition of LB asphaltene films at the toluene/water interface. Moreover, our experiments showed that once asphaltene films were deposited on silicon wafers, they cannot be washed off by toluene. AFM images obtained for LB asphaltene films after soaking them in toluene for 30 min showed that the structures of asphaltene aggregates remain unchanged in comparison with images obtained for these films before soaking, thereby indicating that asphaltenes in the films are not soluble in toluene anymore. It should be mentioned that asphaltenes were completely soluble in toluene in preparing the stock asphaltene-intoluene solution (2 mg/mL). These findings indicate that the association of asphaltene aggregates in the LB films is strong and irreversible in nature, which is consistent with the results from the molecular dynamics simulation that van der Waals interactions between aliphatic chains are responsible for stabilization of asphaltene aggregates.49 In other words, once two-dimensional asphaltene aggregates are formed in an asphaltene monolayer at the toluene/water interface, they become very stable and are not soluble in toluene anymore, giving rise to the observed irreversibility. 4. Summary and Conclusions

On the basis of the hypothesized asphaltene structure of Murgich et al.,47 we can assume that Athabasca asphaltene molecules contain both hydrophobic and hydrophilic patches. We can propose that the origin of asphaltene irreversible adsorption is due to numerous contacts between the hydrophilic patches and water at the toluene/water interface, such that desorption of asphaltene molecules from the interface

Results from Langmuir trough experiments indicated that an asphaltene monolayer is present at the toluene/water interface even though a multilayer film is initially prepared at the air/ water interface first by placing an excess amount of asphaltene on a water surface. However, once a toluene topphase is added, the adjacent upper second and third layers can be completely solubilized in the bulk toluene phase and only the first layer contacting water stays at the toluene/water interface. UV spectroscopic measurements showed that the mass of asphaltenes remaining at the toluene/water interface corresponds to the maximum mass needed in preparing an asphaltene monolayer. Results from washing experiments of multilayer asphaltene films in Langmuir trough experiments and of asphaltene-stabilized water-in-toluene emulsions indicate that asphaltene monolayers are actually responsible for the stabilization of water-in-toluene emulsions. Moreover, our results indicate that dilution with a fresh solvent would not improve demulsification of stable water-in-bitumen emulsions as encountered in the commercial bitumen froth treatment process during bitumen extraction from oil sands. Results of AFM imaging of transferred LB asphaltene films further support the results of π-A isotherms that an asphaltene

(46) Khvostichenko, D. S.; Andersen, S. I.; Viktorov, A. I. Solubility and binding of water in toluene solutions of asphaltenes. Russ. J. Appl. Chem. 2004, 77, 1013-1018. (47) Murgich, J.; Abanero, J. A.; Strausz, O. P. Molecular recognition in aggregates formed by asphaltene and resin molecules from the athabasca oil sand. Energy Fuels 1999, 13, 278-286.

(48) Lu, J. R.; Su, T. J.; Thirtle, P. N.; Thomas, R. K.; Rennie, A. R.; Cubitt, R. The denaturation of lysozyme layers adsorbed at the hydrophobic solid/liquid surface studied by neutron reflection. J. Colloid Interface Sci. 1998, 206, 212-223. (49) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Molecular dynamics simulation of the heat-induced relaxation of asphaltene aggregates. Energy Fuels 2003, 17, 135-139.

We have shown previously that asphaltene monolayer films are bonded with water by Fourier transform infrared spectroscopy (FTIR) measurements of the transferred LB asphaltene films.13,39 It seems that the chemical composition of asphaltenes at the toluene/water interface has been changed through bonding or reacting with water, because the FTIR spectra of asphaltene films are different from the originally precipitated asphaltene powders. Khvostichenko et al.46 also observed using FTIR that asphaltenes are bonded with water for a mixture of asphaltene, toluene, and water.


monolayer is present at the toluene/water interface. Once asphaltene monolayer films are present at a toluene/water interface, they will not leave the interface because of irreversible adsorption of asphaltene to the interface. Washing of asphaltene-stabilized water-in-toluene emulsions further support the irreversible adsorption nature of asphaltene interfacial films at the toluene/water interface. Moreover, these washing results indicated that washing with fresh toluene will not help demulsification. One practical implication for the oil


sands industry is that dilution with a good solvent such as toluene will not help demulsification of stable water-in-bitumen emulsions during the froth treatment process in bitumen recovery from oil sands. Acknowledgment. We thank the NSERC Industrial Research Chair Program in Oil Sands Engineering (held by J.H.M.) for financial support of this work. EF0603129

Water Interface - ACS Publications

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