Langmuir and Langmuir−Blodgett Films of Mixed Asphaltene and a

Oct 18, 2003 - The extracted maltene-free asphaltene was characterized for physicochemical properties through molecular weight, chemical composition, ...
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Langmuir and Langmuir-Blodgett Films of Mixed Asphaltene and a Demulsifier Li Yan Zhang, Zhenghe Xu, and Jacob H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada T6G 2G5 Received May 23, 2003. In Final Form: July 16, 2003 Asphaltene was extracted from Athabasca oil sands bitumen with n-heptane. The extracted maltenefree asphaltene was characterized for physicochemical properties through molecular weight, chemical composition, and functional group analysis. Monolayer characteristics of mixtures of asphaltene and a polymeric demulsifier were studied using a Langmuir interfacial trough at air-water and heptol-water interfaces through measurements of pressure-area and relaxation isotherms (“heptol” refers to a solvent mixture of heptane and toluene). Single-layer Langmuir-Blodgett (LB) films from monolayers of mixed asphaltene and demulsifier were deposited on silicon wafers at air-water and heptol-water interfaces. The deposited LB films were characterized by atomic force microscopy (AFM) imaging and contact angle measurements. Asphaltene forms nanoaggregates at both air-water and heptol-water interfaces. Asphaltene monolayers at a heptol-water interface are more compressible than those at an air-water interface and show little hysteresis. The presence of a demulsifier in a mixed monolayer renders the monolayer more compressible at both air-water and heptol-water interfaces. The asphaltene and demulsifier aggregates follow ideal mixing behavior at both air-water and heptol-water interfaces. AFM images of deposited monolayers reveal the presence of nanosize aggregates. The presence of demulsifier in an asphaltene monolayer can reduce the mechanical strength of the asphaltene monolayer as indicated by reduced rigidity with increasing demulsifier content in the compressibility plot.

1. Introduction A major challenge in a variety of industrial operations, such as the recovery, transportation, and upgrading processes of crude oil and oil sands bitumen, is the formation of stable water-in-oil (W/O) emulsions. Emulsions in the form of fine water droplets dispersed in a continuous oil phase form when crude oil is pumped through pipes, valves, chokes, and so forth under high pressure/temperature.1 At the refinery, some of the washing water introduced to wash off salts and fine solids in the crude oil can be further emulsified. Crude oil spillage on the sea also results in the formation of a water-in-oil emulsion containing 80-90% water which must be demulsified.1 Formation of water-in-oil emulsions during the Clark Hot Water Extraction (CHWE) process in recovering bitumen from oil sands is well documented.2-4 After a two-stage centrifugation, a reduction of water content from 60% to 2-3 wt % is reported.4 Surface-active asphaltenes are known to adsorb at a water-oil interface and form a rigid and protective interfacial film surrounding the water droplets.5-7 The formation of a rigid and mechanically strong interfacial film prevents coalescence of the dispersed water droplets.8 * To whom correspondence should be addressed. Phone: 1-780-492-4673. Fax: 1-780-492-2881. E-mail: Jacob.Masliyah@ ualberta.ca. (1) Angle, C. W. In Encyclopedic handbook of emulsion technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001; pp 541-594. (2) Clark, K. A.; Pasternak, D. S. Ind. Eng. Chem. 1932, 24, 1410. (3) Zhou, Z. A.; Xu, Z.; Masliyah, J. H.; Czarnecki, J. Colloids Surf., A 1999, 148, 199. (4) Gu, G.; Xu, Z.; Nandakumar, K.; Masliyah, J. H. Fuel 2002, 81, 1859. (5) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. J. Colloid Interface. Sci. 2000, 228, 52. (6) Lee, R. F. Spill Sci. Technol. Bull. 1999, 5, 117. (7) Khristov, K.; Taylor, S. D.; Czarnecki, J.; Masliyah, J. Colloids Surf., A 2000, 174, 183. (8) Sullivan, A. P.; Kilpatrick, P. K. Ind. Eng. Chem. Res. 2002, 41, 3389.

In the case where an interfacial film becomes a continuous rigid membrane, the oil and water phases are completely separated by the membrane.9 Stable water-in-oil emulsions are undesirable in the oil industry due to high costs incurred in transportation, corrosion, heat and mechanical energy demands, and other problems encountered in the upgrading and refining of crude oil and bitumen.1 The emulsions must be treated to remove the dispersed water droplets and associated inorganic fine solids to meet the standards for transportation and storage and to reduce corrosion and catalyst poisoning in the downstream upgrading processes.10 The crude oil market requires that the water content be reduced to a level of less than 0.5% of solids and water.1 To separate oil and water from emulsions, several demulsification methods, including chemical treatment and aggressive techniques such as centrifugation, sonic action, and usage of high-voltage electric fields, are commonly used.11 Among these techniques, chemical demulsification is an effective, convenient, and inexpensive method.12 However, the mechanism of chemical demulsification remains to be understood.13 Although it is generally accepted that the role of a demulsifier is to change the interfacial rheological properties and to destabilize the interfacial films,14-16 a quantitative description is rare in the literature. Monolayer behaviors of natural surfactants and extracted asphaltenes from various crude oils have been (9) Kimbler, O. K.; Reed, R. L.; Silsberberg, I. H. Soc. Pet. Eng. J. 1966, 6, 153. (10) Kokal, S.; Al-Juraid, J. J. Pet. Technol. 2000, 52, 41. (11) Sams, G. W.; Zaouk, M. Energy Fuels 2000, 14, 31. (12) Tambe, D.; Paulis, J.; Sharma, M. M. J. Colloid Interface Sci. 1995, 171, 463. (13) Goldszal, A.; Bourrel, M. Ind. Eng. Chem. Res. 2000, 39, 2746. (14) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Colloids Surf., A 1993, 80, 237. (15) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Colloids Surf., A 1994, 83, 261. (16) Kim, Y. H.; Wasan, D. T. Ind. Eng. Chem. Res. 1996, 35, 1141.

10.1021/la034894n CCC: $25.00 © 2003 American Chemical Society Published on Web 10/18/2003

Films of Mixed Asphaltene and a Demulsifier

studied at both air-water and oil-water interfaces using a Langmuir trough. Kimbler et al.,9 for example, measured interfacial pressure-area isotherms of natural surfactants from several crude oils at a benzene-diluted crude oilwater interface. It was found that addition of Triton X-100 (0.1 vol %) into the aqueous subphase considerably lowers the interfacial pressure of the adsorbed monolayer of surfactants from a crude oil. This reduction was attributed to the penetration of Triton X-100 molecules into the adsorbed natural surfactant monolayer. Jones et al.17 showed that a mixture of alkyoxylated phenol with alkyoxylated ester was a good demulsifier which can displace and inhibit the formation of interfacial films from the natural surfactants at an Iranian heavy crude oilwater interface, while a mixture of carboxylic acids with alkyoxylated ester was a poor demulsifier which can only inhibit the formation of the natural surfactant interfacial film. Singh18,19 studied the effect of addition of a commercial demulsifier on monolayer behavior of an Indian crude oil at the air-water and diluted crude oil-water interfaces. It was found that adding a demulsifier into the crude oil slightly reduced the surface or interfacial pressure. Leblanc and Thyrion20 studied monolayer behavior of asphaltenes from a Venezuela crude oil and the deasphaltened oil at an air-water interface. The shapes of the pressure-area isotherms of the pentane- and heptaneprecipitated asphaltenes were found to be similar. Upon compression, the maximum surface pressure attainable is about 60 mN/m for asphaltene monolayers, while for a monolayer of deasphaltened oil, a maximum surface pressure of ∼20 mN/m can be obtained. Mohammed et al.14 showed that a monolayer of Buchan asphaltene at an air-water interface can be compressed to a surface pressure of 45 mN/m at a temperature of 25 °C and 70 mN/m at temperatures higher than 30 °C. Gundersen et al.21 reported that lignosulfonates and Kraft lignins dissolved in an aqueous subphase can drastically decrease the interfacial pressure of the adsorbed asphaltene film at an oil-water interface. Ese and co-workers22-25 studied monolayers of asphaltenes from the North Sea, Venezuela, and France at the air-water interface. They showed that addition of a demulsifier to the oil phase increased the compressibility and reduced the rigidity of the interfacial asphaltene films. The addition of a complex copolymer demulsifier in the oil phase was shown to lower the interfacial pressure of the asphaltene films considerably. However, the reported surface pressure-area isotherms for asphaltenes and for several demulsifiers are hard to justify as the highest attainable surface pressure value reported is well above the corresponding surface tension value of water. In a previous study at the air-water interface,26 we showed that monolayers of the fractionated and unfractionated whole Athabasca asphaltenes behave similarly by a close resemblance of the pressure-area, relaxation, (17) Jones, T. J.; Neustadter, E. L.; Whittingham, K. P. J. Can. Pet. Technol. 1978, 17, 100. (18) Singh, B. P. Energy Sources 1994, 16, 377. (19) Singh, B. P. Energy Sources 1997, 19, 783. (20) Leblanc, R. M.; Thyrion, F. C. Fuel 1989, 68, 260. (21) Gundersen, S. A.; Ese, M. H.; Sjoblom, J. Colloids Surf., A 2000, 182, 199. (22) Ese, M. H. Colloid Polym. Sci. 1998, 276, 800. (23) Ese, M. H. J. Dispersion Sci. Technol. 1998, 20, 1849. (24) Ese, M. H.; Galet, L.; Clausse, D.; Sjoblom, J. J. Colloid Interface Sci. 1999, 220, 293. (25) Ese, M. H.; Sjoblom, J.; Djuve, J.; Pugh, R. Colloid Polym. Sci. 2000, 278, 532. (26) Zhang, L. Y.; Lawrence, S.; Xu, Z. H.; Masliyah, J. H. J. Colloid Interface Sci. 2003, 264, 128.

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and hysteresis isotherms. We also showed that the asphaltene monolayers at 20 °C can be compressed to a surface pressure of 70 mN/m without abrupt collapses. The objective of this study is to quantitatively investigate the effect of addition of a demulsifier on the stability and mechanical strength of an asphaltene monolayer at both air-water and oil (heptol)-water interfaces. The word “heptol” refers to a solvent mixture of heptane and toluene. The heptol is used in the current study because the aromaticity of the solvent can be changed readily by changing the heptane to toluene volume ratio. The use of heptol allows us to understand the issues of commercial processes in which solvents of different aromaticity are used to control the precipitation of asphaltene and the stability of emulsions. The physicochemical properties of the n-heptane-extracted asphaltene from Athabasca oil sands bitumen are characterized with techniques such as molecular weight, elemental analysis, and Fourier transform infrared (FTIR) spectroscopy. Monolayer characteristics of mixed asphaltene and a polymeric demulsifier are studied using a Langmuir interfacial trough at both air-water and heptol-water interfaces. The morphology of transferred Langmuir-Blodgett (LB) films on solid substrates is examined using atomic force microscopy (AFM) and contact angle measurements. 2. Experimental Section 2.1. Materials. Coker feed bitumen was supplied by Syncrude Canada Ltd. HPLC grade toluene, acetone, and n-heptane and technical grade n-heptane (96%) were all purchased from Fisher Scientific. A polymeric demulsifier, known as emulsion breaker BOH2-32B, dissolved in toluene (10 wt %), was supplied by Champion Technologies Inc. The demulsifier is a mixture of oxyalkylated alkylphenol formaldehyde resin having an average molecular weight of 3000 Da. This family of resin-based demulsifiers is synthesized using a mixture of nonylphenol, tertbutylphenol, and formaldehyde under base-catalyzed conditions. The active aryl hydroxyls of the resultant calixarene resin are then oxyalkylated, using oxiranes such as ethylene oxide or propylene oxide. This type of demulsifier has been in use in the demulsification of crude oil emulsions at a dosage of ∼100 ppm since 1950.1,27 Ultrapure water with a resistivity greater than 18.2 MΩ cm produced by a Millipore system was used as the aqueous subphase. 2.2. Asphaltene Extraction. Figure 1 shows the procedure used for extracting asphaltene from coker feed bitumen and washing of the extracted asphaltene using technical grade n-heptane. Coker feed bitumen was dissolved in toluene at a toluene/bitumen volume ratio of 5:1. The dissolved bitumen was shaken on a laboratory shaker for 2 h. Solids were removed by centrifugation of the toluene-diluted bitumen at 20 000 rpm (35 000g) for 30 min. The mass of the recovered solids is 0.75% of that of the original bitumen. Toluene in the diluted bitumen was removed by natural evaporation under ambient conditions in a fume hood for 1 week. The toluene-free bitumen was then added to technical grade n-heptane at a heptane/bitumen volume ratio of 40:1. The heptane-diluted bitumen was shaken on a laboratory shaker for 2 h and left overnight for asphaltene precipitation. The supernatant was carefully decanted, and the precipitated asphaltene was washed with an excess amount (1 L) of technical grade heptane to remove any coprecipitated maltene fraction. The washing was accomplished by shaking the mixture of asphaltene in heptane on the laboratory shaker for 2 h and leaving the mixture overnight for the asphaltene particles to settle down. The supernatant was carefully decanted, and the washing was repeated 15 times. HPLC grade heptane was used for the final 2 washes. After 17 washes, the supernatant becomes colorless. The asphaltene precipitates were then left in a fume hood for 3 days to remove n-heptane by natural evaporation. The mass of the dried asphaltene is 11.92% of that of the original (27) Staiss, F.; Bohm, R.; Kupfer, R. SPE Prod. Eng. 1991, 6, 334.

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Figure 1. Procedure of asphaltene extraction from bitumen and washing of the extracted asphaltene. The ratio numbers show a volume ratio of toluene to bitumen or heptane to bitumen. Table 1. Molecular Weight (M, g/mol), Chemical Composition (C, H, N, O, S, and Total Heteroatoms E ) O + S + N, % by Weight), Hydrogen-to-Carbon Ratio (H/C), and Aromaticity (fa) of Asphaltenesa asphaltene

M

C

H

N

O

S

E

H/C

fa

asphaltene 6868 79.06 7.93 1.14 1.37 7.62 10.13 1.20 0.53 (this work) asphaltene 7072 79.71 8.25 1.20 1.33 7.81 10.34 1.23 0.50 (ref 26) a Asphaltene in the first row was extracted with technical grade n-heptane used in this work; asphaltene in the second row was extracted with HPLC grade n-heptane used in our previous work.

bitumen. The total time needed for extracting and washing of the asphaltene was about 1 month. 2.3. Characterization of the Extracted Asphaltene. Asphaltene obtained according to the extraction and washing procedures described in the previous section was characterized. Molecular weight was determined with vapor pressure osmometry (VPO), while functional groups were analyzed by FTIR spectroscopy. Details of VPO and FTIR analyses can be found elsewhere.26 Table 1 shows a comparison of the technical grade n-heptane extracted asphaltene used in this study and the HPLC grade n-heptane extracted asphaltene in ref 26 in molecular weight, elemental composition, hydrogen-to-carbon ratio, and aromaticity using the correlation given in refs 28 and 29. In Table 1, E is the total weight content of heteroatoms (E ) O + S + N). The molecular weight of the asphaltene sample used in this study by VPO is 7070 g/mol, which is used to obtain the pressurearea isotherms in terms of the area occupied by an asphaltene molecule. The elemental compositions, hydrogen-to-carbon ratios, and aromaticities of the two asphaltenes are very similar, indicating that the quality of the n-heptane used in asphaltene extraction does not affect the quality of the extracted asphaltene samples. The FTIR spectrum of the asphaltene powders used in this paper, obtained according to the extraction procedure shown in Figure 1, was identical to the FTIR spectrum of the whole asphaltene powders reported in ref 26. (28) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1998, 77, 783. (29) Mazumdar, B. K. Fuel 1999, 78, 1239.

We emphasize the importance of thorough washing, which is necessary in obtaining a maltene-free asphaltene material as has been shown by Zhang et al.26 and Yarranton and coworkers.30,31 2.4. Monolayer Preparation. Monolayers composed of a mixture of asphaltene and demulsifier were characterized at the air-water and heptol-water interfaces using a KSV Langmuir interfacial minitrough (KSV Instruments, Finland). The interfacial trough has an area of 17 010 mm2 and is made of Delrin. The two symmetric barriers are also 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 an oil. Several holes in the two barriers enable the lighter phase to flow freely while compressing an interfacial film. The trough is placed in an enclosure and on a vibration isolation table as described previously.26 The trough is cleaned by rinsing with HPLC grade n-heptane and wiping its surface with acetonesoaked Texwipe wipers prior to each run. The aqueous subphase is further cleaned by removing its surface layer with a pipet connected to a vacuum system while repeatedly closing the barriers to a small area until the surface pressure reading becomes smaller than 0.10 mN/m. Asphaltene in toluene solution at a concentration of 2 mg/mL was prepared. The solution was centrifuged and filtered following the procedures described elsewhere26 to remove any solids insoluble in toluene. The demulsifier in toluene solution (10 wt %) was further diluted with toluene to a concentration of 1.91 mg/mL. The prepared asphaltene in toluene solution was premixed with the demulsifier solution to obtain mixtures with various demulsifier mole fractions. To study the monolayer characteristics of the pure components, 15 µL of the asphaltene in toluene solution (2 mg/mL) or 50 µL of demulsifier in toluene solution (1.91 mg/mL) was spread dropwise at the air-water interface by a Hamilton precision microsyringe. Similar procedures were used for composite monolayers of asphaltene and demulsifier at the airwater interface. The volume of the mixture solution was varied from 17 to 65 µL, depending on the mole fraction of demulsifier used. A waiting period of 10 min was allowed for toluene evaporation. The technique of spreading the prepared mixture solution on an aqueous subphase described is the classical spreading method.32,33 A composite monolayer of asphaltene and demulsifier can also be obtained by spreading dropwise the asphaltene solution in toluene first, followed immediately by spreading dropwise the demulsifier solution in toluene. This method is called the sequential spreading method. We found that the monolayers prepared by both techniques exhibited identical pressure-area and relaxation isotherms, topographies of AFM images, and contact angle measurements on transferred LB films. Since both spreading methods yielded indistinguishable results, the classical spreading method was used in this study. To prepare a monolayer at the heptol-water interface, the classical spreading method was used. After spreading a mixture of asphaltene and demulsifier solution and waiting 10 min for toluene evaporation, heptol, a mixture of heptane and toluene, was carefully poured along a glass rod to cover the interface. Heptol with a heptane volume fraction (φ) of 0.4, 0.5, 0.6, and 0.9 was used as the oil phase. A waiting period of 10 min was allowed before compressing the prepared monolayer at the heptol-water interface. Pressure-area isotherms were obtained by compressing the prepared monolayer at the air-water and heptol-water interfaces at a specified compression rate of 20 mm/min (10.8 cm2/ min trough area). In relaxation tests, compression was performed at 20 mm/min to a specified surface or interfacial pressure at which the barriers were stopped. The surface or interfacial pressure changes as a function of time at a given trough area were monitored. At least two runs were repeated for each of the isotherms to ensure reproducibility. (30) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916. (31) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadeh, K. Energy Fuels 2002, 16, 462. (32) Gaines, G. L., Jr. Insoluble monolayers at liquid-gas interfaces; Wiley-Interscience: New York, 1966. (33) Minones, J., Jr.; Minones, J.; Conde, O.; Seoane, R.; DynarowiczLatka, P. Langmuir 2000, 16, 5743.

Films of Mixed Asphaltene and a Demulsifier All the experiments were performed at a temperature of 20 ( 0.1 °C controlled by a circulating water bath (Fisher Scientific, Isotemp 3006) through a built-in jacket in the trough. 2.5. Langmuir-Blodgett and Langmuir-Schaefer Films. Langmuir films of asphaltene, demulsifier, and mixed asphaltene and demulsifier at the air-water and heptol-water interfaces were deposited onto hydrophilic silicon wafers by LB deposition. The silicon wafers were placed parallel to the barriers, and LB films were deposited during the first upstroke of the wafer through the air-water and heptol-water interfaces at a substrate withdrawal speed of 5 mm/min. The silicon wafers were cleaned according to the cleaning procedure of Zhang and Srinivasan.34 Basically, the wafers were soaked overnight in sulfuric acid (96%) with added Nochromix (Fisher), flushed with an excess amount of tap water, and rinsed several times with ultrapure water. They were stored in ultrapure water before being used in deposition experiments. Single-layer Langmuir-Schaefer (LS) films were deposited by placing a hydrophobic silicon wafer horizontally on a compressed monolayer at a very high surface pressure of ∼65 mN/m at the air-water interface. In this study, silicon wafers were rendered hydrophobic by soaking them overnight in a solution of 10 vol % of dichlorodimethylsilane (99%, Aldrich) in toluene. 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 single-layer LS deposition. The hydrophobic silicon wafers have a water contact angle of ∼100°. 2.6. AFM Imaging and Contact Angle Measurement. AFM imaging was performed in air under ambient conditions using a Nanoscope IIIa multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA) using a J scanner. We performed tapping mode AFM using a silicon tip (Digital Instruments) with a resonance frequency of 300-400 kHz. A scan rate of 1 Hz was used for imaging. Integral and proportional gains of the feedback loop were set to 0.1 and 1, respectively. The resonance frequencies of a silicon cantilever and the piezo drive were obtained by performing an “auto tune” in the AFM imaging software. Phase imaging was performed simultaneously along with topography imaging. Phase imaging measures the phase lag of the cantilever oscillation relative to the piezo drive. Spatial variations in sample properties cause shifts in the cantilever phase which are mapped to produce a phase image. Phase imaging is a powerful tool for mapping variations in composition, friction, viscoelasticity, and adhesion of a sample surface.35 Contact angle measurements of the deposited LB films were carried out as described in our previous paper.26 The contact angle was taken through the water phase at intervals of 1 s for 30 s in air at room temperature (22 °C).

3. Results and Discussion 3.1. Surface Pressure-Area (π-A) Isotherms at the Air-Water Interface. Molecular behaviors of monolayers of mixed asphaltene and a polymeric demulsifier at the air-water interface were characterized with the Langmuir interfacial trough, through measurements of surface pressure-area (π-A) and relaxation isotherms. Pressure-area isotherms will be referred as “surface pressure-area” isotherms at the air-water interface, while “interfacial pressure-area” isotherms will be used at the heptol-water interface. 3.1.1. Surface Pressure-Area (π-A) Isotherms. Tests were conducted to confirm that asphaltene and the demulsifier are insoluble in water. An amount of 5 µL of asphaltene in toluene solution (2 mg/mL) was spread on a surface of 400 mL of ultrapure water contained in a beaker with a surface area of 6000 mm2. After waiting for 30 min, a funnel was inserted into the beaker. Water was then taken from the lower section of the beaker through the inserted funnel and transferred to the Langmuir trough using a pipet. After waiting for 30 min, the aqueous (34) Zhang, L. Y.; Srinivasan, M. P. Colloids Surf., A 2001, 193, 15. (35) Garcı´a, R.; Pe´rez, R. Surf. Sci. Rep. 2002, 47, 197.

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Figure 2. Air-water interface: pressure-area (π-A) isotherms of mixed monolayers of asphaltene and demulsifier at various molar ratios of asphaltene to demulsifier. The curves of mixed monolayers are plotted against area per molecule of the mixture.

subphase surface was compressed. We found that the recorded surface pressure remained at zero during compression of the aqueous subphase surface, clearly indicating that the surface of the aqueous subphase was clean and free of asphaltene molecules. From this observation, we can conclude that the asphaltene sample used is insoluble in water. Similar results were obtained for the demulsifier BOH2-32B and a mixture of the demulsifier and the asphaltene sample. These results show that asphaltene and the demulsifier, when spread at the airwater interface, remain at the interface and do not migrate into the aqueous subphase. Surface pressure-area isotherms of mixed asphaltene and demulsifier monolayers at the air-water interface are shown in Figure 2. Here, xd is the mole fraction of demulsifier in the initial mixture, which can be regarded as the mole fraction of demulsifier in the mixed monolayer. Surface pressure is plotted in Figure 2 as a function of area per molecule for monolayers of demulsifier and asphaltene alone and for a mixture of demulsifier and asphaltene. Although asphaltene in the prepared toluene solution exists as individual molecules,30 the exact state of asphaltene molecules at the air-water or oil-water interface is not well established. For the purpose of comparison, area per molecule is used in this paper as a normalized scale, as conventionally practiced when presenting Langmuir film results. The surface pressure-area isotherm of the asphaltene monolayer without any demulsifier added (xd ) 0) in this work is the same as that of the whole asphaltene reported by Zhang et al.26 The surface or interfacial pressure is defined as32

π ) γ0 - γ where γ0 and γ are the surface or interfacial tension in the absence and presence of surface-active molecules, respectively. The origin of the surface or interfacial pressure is due to the interactions (bombardment)36 of surface-active asphaltene or demulsifier components residing at airwater or oil-water interfaces. The monolayers of asphaltene and mixed asphaltene and demulsifier are found to have gas (G), liquid (L), and solid (S) phases. However, no clear transition points from gas to liquid or from liquid to solid phase are observed for monolayers of asphaltene and mixed asphaltene and demulsifier in Figure 2. The absence of clear transition (36) Adamson, A. W.; Gast, A. P. Physical chemistry of surfaces; Wiley: New York, 1997.

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Figure 3. Air-water interface: compressibility Cs of monolayers of asphaltene (xd ) 0) and mixed asphaltene and demulsifier (xd ) 0.88) plotted as a function of surface pressure. The inset shows the corresponding trough area-surface pressure curves.

points in these isotherms is a manifestation of asphaltene being composed of many components, possibly each with a different transition point.26 The extrapolated area per molecule for the asphaltene monolayer from the solid phase is Aa0 ) 3.4 nm2. Likewise, Ad0 ) 0.2 nm2 for the demulsifier monolayer. The superscripts a, d, and m denote asphaltene, demulsifier, and a mixture of asphaltene and demulsifier, respectively. For a demulsifier monolayer, a transition at ∼24 mN/m from liquid to solid phase is observed (see also Figure 7b which will be discussed later). The collapse pressure, defined as the highest pressure to which a monolayer can be compressed without detectable expulsion of molecules to form a new phase,32 is πdc ∼ 47 mN/m for the demulsifier monolayer. For monolayers of asphaltene and a mixture of asphaltene and demulsifier, no abrupt collapses were observed. However, visible streaks of brown color were observed near the two barriers above a critical surface pressure πc. According to Gaines,32 appearance of streaks is an indication of monolayer collapse. An AFM image (see Figure 11b) of an LB film of asphaltene deposited at a surface pressure of 65 mN/m discussed later demonstrates that some asphaltene molecules were expelled from the monolayer to form a buckled three-dimensional bulk phase. The critical surface pressure of a monolayer of mixed asphaltene and demulsifier (πm c ) was found to be composition dependent. However, no specific trend of πm c as a function of demulsifier mole fraction xd was observed, as shown by the curve drawn at the critical surface pressures. The critical pressure decreases to a minimum when xd decreases from 0 to 0.43. The monolayer behavior is dominated by asphaltene for xd e 0.43. On the other hand, the critical surface pressure is dominated by demulsifier for xd g 0.60. Similar behavior for mixed monolayers of arachidic acid and copper tetra-tert-butyl phthalocyanine has been reported by Emelianov and Khatko.37 When xd ) 0.79 and 0.88, the critical pressure is larger than that of the demulsifier or asphaltene monolayer only. This could be due to a more homogeneous distribution of the hydrophilic headgroups and hydrophobic side chains of the asphaltene molecules through incorporation of demulsifier molecules. Beyond the critical surface pressure, monolayers of asphaltene and mixed asphaltene and demulsifier are still compressible, suggesting that these monolayers are elastic. The surface pressure, however, increases slowly as buckling of the monolayer continues. Figure 3 shows a comparison of monolayer compressibility Cs calculated from the inserted trough area-surface (37) Emelianov, I. L.; Khatko, V. V. Thin Solid Films 1999, 354, 237.

Zhang et al.

Figure 4. Air-water interface: area per molecule (A) as a function of mole fraction (xd) of demulsifier in the mixed monolayers of asphaltene and demulsifier at a surface pressure of 5 and 30 mN/m.

pressure curves using eq 1, for monolayers of asphaltene (xd ) 0) and mixed asphaltene and demulsifier (xd ) 0.88), as a function of surface pressure. Monolayer compressibility Cs is defined as32

Cs ) -

1 dAt At dπ

(1)

where At is the trough area and π is the surface pressure. Above the critical or collapse surface pressure (πc), Cs increases slightly with increasing demulsifier mole fraction (xd), indicating that mixed asphaltene and demulsifier monolayers become more compressible and less rigid. Figure 3 also demonstrates that the compressibility of monolayers of asphaltene and mixed asphaltene and demulsifier decreases from the gas phase to the liquid phase to a lowest constant value at the solid phase. Figure 4 shows a plot of the area per molecule in the mixed monolayers of asphaltene and demulsifier as a function of mole fraction of demulsifier, obtained from Figure 2, at a surface pressure of 5 and 30 mN/m, respectively. The straight lines drawn in Figure 4 express a compositional dependence of the averaged molecular area. It is evident that the experimental data points with increasing demulsifier mole fraction agree with the straight lines. Clearly, the additive rule32 is satisfied,

Amixture ) xaAa + xdAd

(2)

where Amixture is the area per molecule of the mixture; xa and xd are the mole fractions of asphaltene and demulsifier in the mixture, respectively; Aa and Ad are the molecular areas of the pure component of asphaltene and demulsifier at the same surface pressure, respectively. If an ideal mixed monolayer is formed or the two components are completely immiscible in a binary mixed monolayer, a plot of Amixture as a function of xa or xd at a given surface pressure will be a straight line. In our case, a compositiondependent collapse surface pressure (πc) as shown in Figure 2 indicates miscibility of the two components32 of asphaltene and demulsifier. These findings indicate that asphaltene and demulsifier show ideal mixing behavior at the air-water interface. 3.1.2. Relaxation Isotherms. In relaxation experiments, the prepared monolayers were compressed at 20 mm/min to a preset surface or interfacial pressure at which the barriers were stopped. Change of surface or interfacial pressure was monitored as a function of time at a given trough area. The surface

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Figure 5. Air-water interface: relaxation isotherms of asphaltene and mixed asphaltene and demulsifier monolayers, compressed at 20 mm/min, to a surface pressure of π0 ≈ 20 mN/m.

Figure 6. Heptol-water interface: comparison of pressurearea curves at multiple compressions for monolayers of asphaltene (xd ) 0) and mixed asphaltene and demulsifier (xd ) 0.43) at the heptol-water interface for φ ) 0.5.

or interfacial pressure decrease may be a result of molecular reorganization, loss of material due to dissolution into the aqueous subphase, evaporation, collapse, and so forth.38 Collectively, the rate of pressure change can be an indication of monolayer stability at an interface. In this paper, we refer to the pressure decrease as a measure of monolayer stability. The stability of a monolayer can be examined by studying monolayer relaxation characteristics.32,39 For comparison, the surface pressure π(t) at time t was normalized by the compressed surface pressure π(0) at time zero to give a nondimensional surface pressure π*(t) ) π(t)/π(0). Figure 5 shows the relaxation (π*(t) vs t) isotherms at the air-water interface for various mixed monolayers compressed to a surface pressure of ≈20 mN/m. At shorter times (t < 5 min), the normalized surface pressure π*(t) decreased rapidly soon after stopping the barriers as a result of a fast reorientation of the asphaltene and demulsifier molecules. At longer times (t > 5 min), π*(t) reached a plateau region due to a slower reorientation of the asphaltene and demulsifier molecules. The relaxation phenomena observed in the mixed asphaltene and demulsifier monolayers are consistent with the relaxation characteristics observed for several asphaltene monolayers at the air-water interface.26 The stability of a mixed monolayer decreases with increasing demulsifier mole fraction xd, as evidenced by a faster rate of relaxation in the relaxation isotherms. It is clear that addition of a demulsifier into an asphaltene monolayer destabilizes the asphaltene monolayer. 3.2. Interfacial Pressure-Area (π-A) Isotherms at the Heptol-Water Interface. Initial tests were conducted to confirm that the asphaltene and demulsifier molecules spread at the heptol-water interface remain at the interface and do not migrate into the bulk phases. In one set of tests, multiple monolayer compressions were performed at different mole fractions of demulsifier. A monolayer was first compressed to a predetermined pressure and relaxed for 45 min. Then the barriers were fully expanded, and the monolayer was compressed for a second time. For monolayers of asphaltene (xd ) 0) and a mixture of asphaltene and demulsifier (xd ) 0.43) at a heptol-water interface for φ ) 0.5, Figure 6 shows that in both cases there is little difference between the first and second compression, suggesting that there is little, if any, migration of the demulsifier/asphaltene into the bulk

phases within the time frame of the tests. At other heptolwater interfaces for φ ) 0.4, 0.6, and 0.9, similar results were observed. In another set of experiments, tests similar to those described in section 3.1.1 were performed. An amount of 5 µL of asphaltene in toluene solution (2 mg/mL) was spread on a surface of 200 mL of ultrapure water contained in a beaker with a surface area of 6000 mm2. After waiting 10 min for toluene evaporation, 200 mL of heptol at a heptane volume fraction of φ ) 0.5 was poured into the beaker to cover the water surface. After waiting for 30 min, 100 mL of heptol was taken from the upper part of the beaker and poured onto a clean water surface in the Langmuir interfacial trough. After waiting for 30 min, the heptol-water interface was compressed. We found that the recorded pressure remained at zero during compression of the heptol-water interface. This observation clearly indicates that the migration of asphaltene molecules into the bulk heptol phase was negligible. Similar results were obtained for the demulsifier BOH232B and a mixture of the demulsifier and the asphaltene sample. Similar results ware observed for other heptolwater interfaces for φ ) 0.4, 0.6, and 0.9. Results from the two sets of tests clearly indicate that molecules of asphaltene, demulsifier, and a mixture of asphaltene and demulsifier once spread at a heptol-water interface remain at the interface and do not migrate into either of the bulk phases within the time frame of our experiments. Therefore, the composition of the initial mixture can be assumed to be the same as in the monolayer. 3.2.1. Interfacial Pressure-Area (π-A) Isotherms. Figure 7 shows a comparison of π-A isotherms of asphaltene and demulsifier monolayers at the heptolwater interfaces with that obtained at the air-water interface. For an asphaltene monolayer at the heptol-water interfaces as shown in Figure 7a, the states of the monolayers are much more difficult to identify at heptolwater interfaces. Nevertheless, the buckling at a volume ratio of heptane to toluene of 0.9 does suggest the presence of the solid state of the asphaltene monolayer. However, the highest attainable interfacial pressure decreases with decreasing heptane volume fraction φ in heptol. Accompanied with this decrease in the highest attainable interfacial pressure is the disappearance of monolayer buckling and the various phases become almost indistinguishable, suggesting that the asphaltene monolayer is more fluidlike at the interface. It is clear that the asphaltene monolayer at the air-water interface behaves

(38) Rodrı´guez Patino, J. M.; Rodrı´guez Nin˜o, M. R.; Carrera Sanchez, C. Langmuir 2002, 18, 8455. (39) Lee, Y. L. Langmuir 1999, 15, 1796.

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Figure 7. Comparison of pressure-area (π-A) isotherms of asphaltene (xd ) 0) and demulsifier (xd ) 1) monolayers, at the heptol-water and air-water interfaces: (a) asphaltene monolayers at heptol-water interfaces for φ ) 0.5, 0.6, and 0.9 and at the air-water interface; (b) demulsifier monolayers at the heptol-water interface for φ ) 0.5 and at the air-water interface.

differently from that at the oil-water interface. It appears that heptol at a high toluene volume fraction provides a more favorable environment for asphaltene backbones to reside; that is, they become more flexible and compressible. The interfacial pressure-area isotherm at the heptolwater interface is more expanded than that at the airwater interface when the interfacial pressure is below a critical value. The critical interfacial pressure value increases with increasing heptane volume fraction in heptol. At a pressure of π < 20 mN/m, the pressure-area isotherm shown in Figure 7b for a demulsifier monolayer at a heptol-water interface for φ ) 0.5 is also more expanded than that at the air-water interface. This result is similar to that for the asphaltene monolayers observed in Figure 7a. A transition at ∼24 mN/m from liquid (L) to solid (S) phase can be observed for the demulsifier monolayer at the air-water interface, as indicated by the arrow in Figure 7b. However, no clear phase transition was observed for the demulsifier monolayer at the heptolwater interface for φ ) 0.5. Figure 8 shows pressure-area isotherms of monolayers of asphaltene and demulsifier with a demulsifier mole fraction of xd ) 0.43 at heptol-water interfaces with a heptane volume fraction of φ ) 0.5, 0.6, and 0.9. Figure 8a demonstrates that the highest attainable interfacial pressure upon compression of the mixed monolayers increases with increasing heptane volume fraction φ, a result similar to the trend observed for asphaltene

Zhang et al.

Figure 8. Heptol-water interfaces: comparison of interfacial pressure-area (π-A) isotherms of monolayers of asphaltene and demulsifier: (a) asphaltene and demulsifier monolayers with a demulsifier mole fraction of xd ) 0.43 at heptol-water interfaces for φ ) 0.5, 0.6, and 0.9 and at the air-water interface; (b) asphaltene and demulsifier monolayers at various demulsifier mole fractions xd at the heptol-water interface for φ ) 0.5.

monolayers as shown in Figure 7a. However, the molecular area of a mixed monolayer is smaller than that of an asphaltene monolayer. For comparison, the π-A isotherm for a mixed monolayer of asphaltene and demulsifier at xd ) 0.43 at the air-water interface is also included in Figure 8a. At xd ) 0.43, a monolayer of mixed asphaltene and demulsifier at a heptol-water interface is more expanded than that at the air-water interface at low pressures, similar to the results obtained for asphaltene monolayers as shown in Figure 7a. Figure 8b shows interfacial pressure-area isotherms for monolayers of asphaltene and demulsifier at the heptol-water interface for a heptane volume fraction of φ ) 0.5 with various demulsifier mole fractions in the monolayers. With increasing demulsifier mole fraction (xd) in the monolayer, the area per molecule occupied at the heptol-water interface for φ ) 0.5 decreases. This trend is similar to the trend observed for asphaltene and demulsifier monolayers at the air-water interface. However, no buckling was observed for asphaltene and demulsifier monolayers at the heptol-water interface for φ ) 0.5. Figure 9 shows a plot of the average area per molecule at the heptol-water interface for φ ) 0.5 as a function of mole fraction of demulsifier (xd) in the mixed monolayer. At a low pressure of π ) 5 mN/m, the results shown in Figure 9 at the heptol-water interface for φ ) 0.5 are similar to those shown in Figure 4 at the air-water interface. At higher pressures, the area per molecule at the heptol-water interface is much smaller than that at the air-water interface. Nevertheless, the averaged area

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Langmuir, Vol. 19, No. 23, 2003 9737 Table 2. Transfer Ratio TR of Single-Layer LB Films of Mixed Asphaltene and Demulsifier Deposited at a Surface Pressure of 0, 5, 30, and 40 mN/m at a Molar Ratio of xd ) 0, 0.60, 0.79, and 0.88 at 20 °C on Oxidized Silicon Wafers at the Air-Water Interfacea π, mN/m xd

0

5

30

40

0 0.60 0.79 0.88

2.32 1.90 2.72 1.76

1.34 1.13 1.12 1.16

1.01 0.76 1.06 1.26

0.95b 1.01 1.22 1.00

a The withdrawal speed used was 5 mm/min. b Deposited at 50 mN/m.

Figure 9. Heptol-water interface for φ ) 0.5: area per molecule (A) as a function of mole fraction (xd) of demulsifier in monolayers of asphaltene and demulsifier at an interfacial pressure of 5, 10, and 15 mN/m.

Figure 10. Heptol-water interface: comparison of relaxation isotherms. Relaxation isotherms of monolayers of asphaltene (xd ) 0) and asphaltene and demulsifier (xd ) 0.43) at heptolwater interfaces for φ ) 0.5 and 0.9.

per molecule decreases linearly with increasing demulsifier mole fraction, suggesting ideal mixing of asphaltene and demulsifier molecules. Ideal mixing behavior is further confirmed by AFM images discussed later for LB films of mixed asphaltene and demulsifier deposited at the heptol-water interface for φ ) 0.5. 3.2.2. Relaxation Isotherms. Figure 10 shows the effect of adding a demulsifier on asphaltene monolayer relaxation characteristics. The monolayers were compressed to an interfacial pressure of π(0) ≈ 10 mN/m. For comparison, interfacial pressure π(t) was normalized by π(0) as described previously in section 3.1.2. In Figure 10, an asphaltene monolayer is more stable at the heptolwater interface for φ ) 0.9 than at the heptol-water interface for φ ) 0.5. A similar trend can be observed for a mixed asphaltene and demulsifier monolayer at a demulsifier mole fraction of xd ) 0.43. However, the relaxation rate is much faster for φ ) 0.5 than for φ ) 0.9. The relaxation isotherm at φ e 0.5 without demulsifier addition shows some degree of fluctuation. This fluctuation is highly reproducible, and no such fluctuation was observed at φ g 0.6. Although the exact reason for the fluctuation is not clear, it indicates that asphaltene monolayers at the heptol-water interface for φ e 0.5 become less stable. The presence of demulsifier in an asphaltene monolayer has a significant effect on the stability of the mixed monolayer. The normalized interfacial pressure (π*(t)) of a monolayer of mixed asphaltene and demulsifier relaxes much faster than that of a monolayer of asphaltene only. Figure 10 shows that the stability of monolayers of

Figure 11. AFM height images of single-layer LB films of asphaltene (xd ) 0) deposited at a surface pressure of 30 mN/m by the Langmuir-Blodgett method and at a surface pressure of 65 mN/m by the Langmuir-Schaefer method, at the airwater interface: (a) π ) 30 mN/m, TR ) 1.01; (b) π ) 65 mN/m, TR ≈ 1.

asphaltene and mixed asphaltene and demulsifier decreases with decreasing heptane content in the heptol phase, indicating that the heptol phase is important in determining monolayer stability. Earlier results of Figure 6 showed that there was no material loss either into the aqueous subphase or into the heptol phase. The asphaltene and demulsifier molecules relaxed back to their original conformational state at a heptol-water interface. Therefore, results of Figure 10 indicate that reorientation of the asphaltene and demulsifier molecules at the heptol-water interfaces is responsible for the observed relaxation characteristics. The relaxation characteristics caused by the presence of demulsifier in the mixed monolayer at the heptol-water

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Figure 12. AFM height image of a single-layer demulsifier LB film deposited on a hydrophilic silicon wafer at a surface pressure of π ) 30 mN/m at the air-water interface.

interface are similar to those observed at the air-water interface. However, the destabilization effect is more pronounced at the heptol-water interface than at the airwater interface. Since the transfer of asphaltene and demulsifier from the interface into either bulk phase is negligible as discussed earlier, the observed relaxation characteristics cannot be attributed to the loss of asphaltene or demulsifier from the interface. It appears that the demulsifier breaks the association of asphaltene molecules/aggregates at the interface, which allows them to rearrange more easily, resulting in a faster relaxation. 3.3. Deposition of LB Films of Mixed Asphaltene and Demulsifier. Single-layer LB films were deposited on hydrophilic silicon wafers. Table 2 lists the transfer ratio (TR) of single-layer LB films of mixed asphaltene and demulsifier on silicon wafers at 20 °C deposited from the air-water interface at surface pressures from 0 to 40 mN/m. A substrate withdrawal speed of 5 mm/min was used for LB deposition. Entrainment of the subphase water between the depositing monolayer and the solid substrate was observed for all of the deposition cases listed in Table 2, indicating that the deposition is nonreactive.32 In other words, the interactions between the hydrophilic headgroups of the asphaltene molecules/aggregates and the hydrophilic silicon wafer are weak. Table 2 shows that a good transfer ratio close to unity can be obtained at a surface pressure of π g 5 mN/m, indicating that molecules of monolayers of asphaltene and mixed asphaltene and demulsifier are packed in an orderly manner at the air-water interface above this pressure. 3.4. AFM Images and Contact Angle of LB Films of Mixed Asphaltene and Demulsifier: Air-Water Interface. 3.4.1. AFM Images. In all of the AFM images of the LB films discussed hereafter, the dipping direction is downward from the top to the bottom of the page. Inserted close-up images of size 1 × 1 µm were taken from

the lower right corner of the 10 × 10 µm images in Figures 11, 13, and 14. Figure 11a shows an AFM height image of a singlelayer LB film of asphaltene deposited from the air-water interface at a surface pressure of π ) 30 mN/m. At this surface pressure, discotic aggregates and disks connected to rodlike structures are visible in the image. The absence of clear patterns suggests that the asphaltene aggregates are randomly packed at the air-water interface, independent of the compression direction of the barriers. However, upon further compression to a higher surface pressure of π ) 65 mN/m, buckling of the asphaltene monolayer perpendicular to the compression direction was observed, as shown in Figure 11b. Note the difference in the height scale between Figure 11a (5 nm) and Figure 11b (200 nm). It is clear that the deposited LB film becomes thicker at π ) 65 mN/m than at π ) 30 mN/m, indicating that a new three-dimensional domain was formed. Individual asphaltene aggregates can be seen from the image. The inserted magnified image indicates that the asphaltene aggregates become elongated and are more compressed. Figure 12 shows an AFM height image of an LB film of demulsifier. The LB film was deposited at a surface pressure of π ) 30 mN/m at the air-water interface. The size of the demulsifier aggregates is ∼5 nm, which is much smaller than the size of asphaltene aggregates of ∼100 nm. The height scale in the height image of the demulsifier LB film in Figure 12 is 2 nm, which is smaller than that of the height image of the asphaltene LB film in Figure 11a (5 nm), indicating that the demulsifier LB film is smoother than the asphaltene LB film. Figure 13 shows an AFM height image of an LB film of mixed asphaltene and demulsifier with xd ) 0.60. The film was deposited at a surface pressure of 30 mN/m as compared with the absence of demulsifier as shown in

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Figure 13. AFM height images of a single-layer LB film of mixed asphaltene and demulsifier (xd ) 0.60) deposited at a surface pressure of 30 mN/m by the Langmuir-Blodgett method with TR ) 0.76, on a hydrophilic silicon wafer at the air-water interface.

Figure 14. AFM height images of a single-layer LB film of mixed asphaltene and demulsifier (xd ) 0.88) deposited at a surface pressure of 30 mN/m by the Langmuir-Blodgett method with TR ) 1.26, on a hydrophilic silicon wafer at the air-water interface.

Figure 11a. Addition of demulsifier at a mole fraction of xd ) 0.60 does not change the topography of the mixed film. Individual and interconnected discotic structures can still be observed in the presence of demulsifier in the

asphaltene monolayer. Those discotic and rod structures are the asphaltene aggregates, while the structures of the demulsifier molecules are probably not visible in the darker areas in the AFM image. At a surface pressure of

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Figure 15. Static contact angle θ against water as a function of deposition surface pressure π for LB films deposited from binary mixtures of asphaltene and demulsifier at the air-water interface.

π ) 65 mN/m, the folding of the mixed asphaltene and demulsifier monolayer at xd ) 0.60 is similar to that of the asphaltene monolayer as shown in Figure 11b. Further addition of demulsifier significantly changes the topography of the deposited LB film. Figure 14 shows that with a demulsifier mole fraction of xd ) 0.88, large dark areas are observed at a surface pressure of 30 mN/ m. Judging from the AFM images of LB films of asphaltene and demulsifier alone (see Figures 11a and 12), one can assume that the discotic and long rod structures are the asphaltene aggregates, while the dark areas are most likely occupied by demulsifier. To some extent, the continuity of the asphaltene film formed by asphaltene is disrupted by the demulsifier. It is evident that the packing of the asphaltene aggregates is looser with a higher demulsifier mole fraction of xd ) 0.88. Individual long rod and randomly orientated discotic structures are also observed. Many of the small discotic domains, which are similar to those observed in the asphaltene LB film shown in Figure 11a, remain unassociated. A close-up image (Figure 14 inset) shows a resemblance to the large asphaltene aggregates shown in the inset in Figure 11a. This resemblance indicates that the aggregated structures in the deposited monolayer are asphaltene aggregates. Folding behavior at a surface pressure of 65 mN/m was also observed in the AFM image of a deposited LB film of mixed asphaltene and demulsifier at a demulsifier mole fraction of xd ) 0.88. 3.4.2. Contact Angle. Figure 15 shows that the contact angle (θ) is about 42° for an LB film of demulsifier alone; it is fairly insensitive to the surface pressure at which the LB film was prepared. For an LB film of asphaltene alone, there is a slight dependence of the contact angle on the surface pressure, particularly in the low surface pressure region. The contact angle values varied from 72 to 88°. The contact angles of a mixed monolayer are also shown for different demulsifier mole fractions. For xd ) 0.88, that is, a mole fraction of asphaltene of 0.12, the value of the contact angle is very close to the case of asphaltene alone. 3.5. AFM Images and Contact Angle of LB Films of Mixed Asphaltene and Demulsifier: HeptolWater Interfaces. 3.5.1. AFM Images. Single-layer LB films were deposited from the heptol-water interface at various φ values and a pressure of π ) 10 mN/m. This interfacial pressure was chosen as it represents the same degree of monolayer compression, defined by π/γ0, as that at the surface pressure of 30 mN/m for the air-water interface. This allows us to have a fair comparison between the two cases.

Figure 16. AFM height images of single-layer mixed asphaltene and demulsifier LB films deposited on hydrophilic silicon wafers at an interfacial pressure of π ) 10 mN/m at the heptolwater interface for φ ) 0.5: (a) xd ) 0, TR ) 2.03; (b) xd ) 0.43, TR ) 1.96.

For the case of asphaltene alone at the heptol-water interface for φ ) 0.5, Figure 16a shows an AFM height image of the deposited LB film. A comparison between Figure 11a of a deposited asphaltene LB film at the airwater interface and Figure 16a of a deposited asphaltene LB film at the heptol-water interface for φ ) 0.5 indicates that asphaltene aggregates at the heptol-water interface are smaller and more diffused. For the case of demulsifier alone at the heptol-water interface for φ ) 0.5, the AFM image is very similar to that for the deposited LB demulsifier film from the airwater interface as shown in Figure 12. For the case of a mixed monolayer at xd ) 0.43 at the heptol-water interface for φ ) 0.5, the AFM image of Figure 16b is similar to that of the asphaltene LB film shown in Figure 16a. The similarity of AFM images in panels a and b of Figure 16 suggests that the aggregates shown in these images are mainly asphaltene aggregates and the presence of demulsifier does not break the asphaltene aggregates. Although miscibility was suggested from Figure 9, Figure 16 shows that such a miscibility is not at a molecular scale. 3.5.2. Contact Angle. The static contact angle (θ) of water on LB films deposited at an interfacial pressure of 10 mN/m from mixed monolayers of asphaltene and demulsifier at the heptol-water interfaces was found to be ∼90°, independent of the heptane volume fraction (φ) or the mole fraction of demulsifier (xd). These contact angle

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values are almost the same as those obtained from LB films deposited at the air-water interface, confirming that the presence of demulsifier does not affect the hydrophobicity of the deposited films. 4. Summary Asphaltene forms nanoaggregates at both the air-water and heptol-water interfaces. The asphaltene monolayers at a heptol-water interface are more compressible than those at an air-water interface and show little hysteresis; that is, reorientation of asphaltene and demulsifier molecules/aggregates is responsible for the observed relaxation characteristics at the heptol-water interfaces. The presence of the demulsifier in the mixed monolayer renders the monolayer more compressible at both the airwater and heptol-water interfaces. The stability of a

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monolayer decreases with increasing content of demulsifier at both the air-water and heptol-water interfaces. The asphaltene and demulsifier aggregates appear to follow ideal mixing behavior at both the air-water and heptol-water interfaces. AFM images of deposited monolayers reveal the presence of nanosize aggregates. The presence of demulsifier in an asphaltene monolayer can reduce the mechanical strength of the asphaltene monolayer as shown by reduced rigidity with increasing demulsifier content in the compressibility plot. Acknowledgment. We are grateful to Champion Technologies Inc. for providing the demulsifier. This project was supported by the NSERC Industrial Research Chair in Oil Sands Engineering. LA034894N