Characterization of Adsorbed Athabasca Asphaltene Films at Solvent

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Characterization of Adsorbed Athabasca Asphaltene Films at Solvent-Water Interfaces Using a Langmuir Interfacial Trough Li Yan Zhang, Zhenghe Xu, and Jacob H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

Characteristics of adsorbed monolayers of fractionated high molecular weight, low molecular weight, and unfractionated whole asphaltenes at a heptol (a mixture of heptane and toluene)water interface have been studied with a Langmuir interfacial trough. The deposited monolayer Langmuir-Blodgett (LB) asphaltene films were characterized with atomic force microscopy (AFM), contact angle measurements, and Fourier transform infrared spectroscopy (FTIR). Monolayers of the three asphaltene samples behave similarly at a heptol-water interface, as characterized by close resemblance of the corresponding pressure-area, hysteresis, and relaxation isotherms. For pressure-area isotherms, a monolayer of the low molecular weight asphaltene is the most expanded, while a monolayer of the high molecular weight asphaltene is the most condensed. In terms of relaxation characteristics, a monolayer of the high molecular weight asphaltene relaxes most quickly. Similarity among the three asphaltene samples is further evidenced by close resemblance of topographies of AFM images, contact angle values, and FTIR spectra of the deposited LB asphaltene films. In general, an asphaltene monolayer is more flexible at a heptol-water interface than at an air-water interface. 1. Introduction Oil sands, also known as tar sands and bituminous sands, are unconsolidated sand deposits which are impregnated with viscous petroleum, normally referred to as bitumen. The largest deposit in the world discovered so far is in the Athabasca region of Alberta, Canada, with a total estimate of 830 billion barrels of oil in reserve.1 With a continual decline of conventional oil reserves, oil sand deposits will continue to become increasingly significant as an energy source in North America. Water-in-bitumen emulsions form during the Clark Hot Water Extraction (CHWE) process to recover bitumen from oil sands.2 In the CHWE process, mined oil sands are digested in rotating drums or hydrotransport pipelines with addition of hot water and a small quantity of sodium hydroxide to facilitate bitumen liberation from sand grains. The detached bitumen droplets are recovered by flotation in gravity separation vessels. After dilution with a solvent, most of the solids and water entrained in the recovered bitumen froth are removed through a centrifugation process. After centrifugation, however, the diluted bitumen (bitumen/ solvent mixture) still contains 2-3 wt % water in the form of a water-in-oil emulsion, with water being in droplets of 1-5 µm in size. The water-in-oil emulsion is extremely stable such that the dispersed water is extremely difficult to remove, if not impossible. Dewatering of stable water-in-oil emulsion is a continuous challenge to the oil sands industry. Asphaltenes (or other materials associated with asphaltenes) are proposed to be responsible for stabilizing water-in-bitumen or water-in-oil emulsions.2-9 It is commonly considered that surface active asphaltenes are the actual components responsible for promoting * To whom correspondence should be addressed. Phone: (780) 492-4673. Fax: (780) 492-2881. E-mail: Jacob.Masliyah@ ualberta.ca.

formation and subsequent stabilization of water-in-oil emulsions,3,10,11 due to adsorption and formation of a rigid and protective asphaltene film surrounding the dispersed water droplets. To understand the mechanism of stabilization of water-in-oil emulsions, a number of studies have been carried out by Dr. Wasan’s research group to measure the interfacial tension and rheological properties12-14 in the presence of asphaltene films at an oil-water interface. Asphaltenes can be present as colloidal particles, precipitates, and/or surface active molecules in a mixture of hydrocarbons3 such as a mixture of n-heptane and toluene. Leblanc and Thyrion,15 Mohammed et al.,16 and Zhang et al.17 observed that asphaltenes can form a monolayer at an air-water interface, indicating that asphaltenes act as surface active molecules. Eley et al.18 found that water-in-oil emulsions are most stable at the condition when asphaltene starts to precipitate in a mixture of heptane and xylene. McLean and Kilpatrick19,20 also found that water-in-crude oil emulsions were most stable near the incipient point of asphaltene precipitation in a mixture of heptane and toluene. Yang and Czarnecki21 observed that a water-in-diluted bitumen emulsion attains the highest stability at the incipient point of asphaltene precipitation in a mixture of naphtha and bitumen at a naphtha-to-bitumen volume ratio of 4. These results indicate that the adsorbed asphaltene molecules and colloidal particles are collectively responsible for stabilization of waterin-oil emulsions. Asphaltenes can be present as surface active molecules or molecules and colloidal particles at a heptol-water interface, depending on the volume ratio of heptane to toluene in heptol. In this paper, we show that precipitated asphaltene colloidal particles as well as asphaltene molecules are capable of forming an interfacial film at a heptol-water interface. Asphaltene is highly soluble in pure toluene and in heptol (a mixture of heptane and toluene) containing low concentrations of heptane. It is noticed that asphaltene

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starts to precipitate in heptol when the heptane concentration exceeds a volume fraction φ of 0.55. When asphaltene is added to a heptol phase with φ < 0.55, and contacted with a heptol-water interface, any asphaltene presence at the heptol-water interface will arise from asphaltene molecular adsorption from the bulk heptol phase. However, for φ g 0.55, in addition to adsorption of asphaltene molecules from the bulk heptol phase, precipitated asphaltene particles will also be present at the heptol-water interface. Monolayers at various oil-water interfaces have been studied to some extent using a Langmuir trough. However, most of the studies have been focused on monolayers of phospholipids at an oil-water interface to model biological membranes.22 Yue and Jackson,23 for example, studied monolayer characteristics of distearoylphosphatidylcholine (DSPC) at a heptanewater interface through pressure-area isotherm measurements and found that a DSPC monolayer exhibits a temperature-dependent phase transition. Thoma and Mo¨hwald24,25 studied monolayers of dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidylcholine (DPPC) at a number of oil-water interfaces. They used hexadecane, dodecane, and bicyclohexyl as the oil phases and observed that the pressure-area isotherms for both DPPE and DPPC monolayers depend on the type of oil used and are sensitive to temperature. They further showed that the oil can partition in the liquid condensed monolayer phase (LC) for a DPPE monolayer only if there is a close match of chain length between the lipid and hydrocarbon, while for a DPPC monolayer oil partitioning occurs even if the chain lengths of the lipid and hydrocarbon differ significantly. Grandell and Murtoma¨ki26 studied monolayers of DPPC at a dichloroethane-water interface. Phase transitions were not observed in the pressure-area isotherm of a DPPC monolayer at a dichloroethane-water interface, contrary to the phase transition observed at an air-water interface. Other monolayers studied at an oil-water interface include block copolymers,27 cyclam derivatives,28 sorbitan monoesters,29 latex particles,30 and poly(n-hexyl isocyanate).31 Monolayer characteristics of asphaltenes at an oilwater interface have been studied to a lesser extent. At an oil-water interface, Ese et al.32 studied asphaltenes extracted from crude oils from France and the North Sea. Gundersen et al.33 studied asphaltene extracted from a Venezuelan crude oil. Recently, Zhang et al.34 studied mixed monolayers of asphaltene and a demulsifier at solvent-water interfaces. In a previous study17 at an air-water interface, we characterized the behavior of high molecular weight and low molecular weight subfractions, along with whole asphaltene, from Athabasca oil sands. We showed that Athabasca asphaltenes act as amphiphilic molecules and can spread and form a Langmuir monolayer at an air-water interface. In this study, we use a newly developed Langmuir interfacial trough to study adsorbed asphaltene monolayers at a solvent-water interface. We choose a mixture of heptane and toluene at various volume ratios in the current study because the aromaticity of the solvent can be changed readily by changing the heptaneto-toluene ratio. The use of heptol allows us to understand the issues of commercial bitumen extraction processes in which solvents of differing aromaticity are

used to control the precipitation of asphaltenes and the stability of emulsions. As asphaltenes are soluble in toluene but insoluble in heptane, this choice enables us to control the state of an asphaltene monolayer, i.e., as molecules or a combination of molecules and colloidal particles, at a solvent-water interface. Our objective is to obtain information on the state of an asphaltene interfacial film formed at a heptol-water interface and to study the behavior of such interfacial films. The knowledge obtained will help us to understand the mechanism of emulsion stabilization. Measurements of pressure-area, hysteresis, and relaxation isotherms were performed at a heptol-water interface with two fractionated subfractions and unfractionated whole Athabasca asphaltenes. Asphaltene monolayers at a heptol-water interface were transferred onto a solid substrate using the Langmuir-Blodgett (LB) deposition technique. Atomic force microscopic (AFM) images of the deposited LB films revealed that asphaltene forms a molecular monolayer at a lower heptane volume fraction of φ e 0.5 in heptol, while at a higher heptane volume fraction of φ g 0.9 asphaltene presents as a monolayer of molecules and asphaltene particles. 2. Experimental Methods 2.1. Materials. Vacuum distillation feed bitumen was supplied by Syncrude Canada Ltd. HPLC grade toluene, acetone, and n-heptane were all purchased from Fisher Scientific Canada. Asphaltene samples used in this study were the same as used in our previous study.17 Details of the fractionation and extraction procedures and characterization of physicochemical properties of the asphaltene samples can be found in ref 17. Briefly, bitumen was diluted with toluene at a toluene-tobitumen volume ratio of 5:1. The toluene-diluted bitumen was centrifuged at 35000g to remove fine solids. Toluene was then allowed to evaporate from the diluted bitumen in a fume hood for 1 week to obtain solids-free bitumen. The whole asphaltene sample was extracted from the solids-free bitumen by addition of n-heptane at an n-heptane-to-bitumen volume ratio of 40:1. Asphaltene fractionation was performed by dissolving solids-free bitumen in a mixture of n-heptane and toluene at varying n-heptane to toluene to bitumen (H: T:B) volume ratios. The high molecular weight asphaltene subfraction was obtained at an H:T:B volume ratio of 5:5:1. The low molecular weight asphaltene subfraction was obtained at an H:T:B volume ratio of 60:5:1. Prior to their use, the prepared asphaltenes were dissolved in toluene at a concentration of 2 mg/mL. All the asphaltene solutions were centrifuged and filtered to remove solid particles, as described by Zhang et al.17 2.2. Langmuir Interfacial Trough. Interfacial asphaltene films at a heptol-water interface were characterized with a Langmuir interfacial trough (KSV Instruments, Finland). A schematic of the interfacial trough is shown in Figure 1. The trough (having an area of 17010 mm2) and two symmetric barriers are made of Delrin. The trough has two compartments: a lower compartment hosting a heavier phase such as water and an upper compartment hosting a lighter phase such as oil. The holes in the two barriers allow the oil phase to flow freely while changing the area of an oil-water interface. The trough was placed on an antivibration table and in an enclosure. Before each test, the trough was throughly cleaned by rinsing with heptane and wiping its surfaces with

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Figure 1. Schematic of Langmuir interfacial trough. All units are in mm.

acetone-soaked Texwipe wipers. The temperature of the trough was controlled by a circulation water bath to 20 ( 0.1 °C. Either a platinum plate or a strip of filter paper (Whatman 1 CHR) was used as the Wilhelmy plate to record interfacial pressure-area (π-A) isotherms. However, only filter paper strips were used to obtain adsorption, hysteresis, and relaxation isotherms and to control the interfacial pressure during LB deposition. We found that filter paper is more accurate than a platinum plate at an oil-water interface in monitoring the interfacial pressure, since the filter paper ensures a complete wetting by the water subphase with a zero contact angle. Ultrapure water with a resistivity of 18.2 MΩ cm prepared with a Millipore system was used as the water subphase. A volume of 130 mL of the ultrapure water was poured into the lower compartment of the trough as the subphase. The subphase was cleaned by removing its surface layers with a pipet connected to an aspirator by repeatedly closing the barriers to a small area until the surface pressure reading became smaller than 0.10 mN/m. The oil phase was poured from a glass beaker carefully along a glass stirring rod, which was inserted in the water subphase. The glass rod was inclined at an angle so that the oil phase flowed down along it and then spread smoothly on the water surface without disturbing the water subphase. A volume of 100 mL of a mixture of heptane and toluene at a heptane volume fraction (φ) of 0.1, 0.5, 0.9, and 1.0 was used as the oil phase. A volume of 0.025 mL (0.05 mg of asphaltene) of

the prepared solids-free asphaltene-in-toluene solution, having a concentration of 2 mg/mL, was spread dropwise on the heptol surface by a Hamilton precision microsyringe. A period of 10 min was allowed before compressing the heptol-water interface. The monolayers formed by adsorption of asphaltene material to an oilwater interface will be called adsorbed monolayers. In this paper, we focus on adsorbed asphaltene monolayers. Interfacial asphaltene monolayers can also be obtained by spreading a known volume of asphaltene-intoluene solution on the water subphase first, allowing for toluene evaporation, and then covering the water subphase with heptol to obtain an oil-water interface. The asphaltene monolayers formed as such are spread monolayers. Details of preparing spread asphaltene monolayers can be found in our previous study.34 Interfacial pressure-area isotherms were obtained by compressing the adsorbed or spread asphaltene material at the heptol-water interface at a specified compression speed of 10.8 cm2 of trough area/min. The interfacial pressure reading was zeroed before compression of the heptol-water interface was initiated. In compressionexpansion tests, compression was performed at a specified speed to a designated interfacial pressure and followed by an immediate expansion to a specified low interfacial pressure (normally 0 mN/m). A waiting period of 5 min was allowed between two consecutive compression-expansion cycles. In relaxation tests, the prepared interfacial film was compressed to a specified interfacial pressure at which the barriers were stopped and the interfacial pressure was monitored as a function


of time. At least three runs were repeated for each of the isotherms to ensure reproducibility. 2.3. LB and Langmuir-Schaefer (LS) Deposition. Hydrophilic silicon wafers used for LB deposition were cleaned using the cleaning procedures described by Zhang and Srinivasan.35 Briefly, the wafers were soaked overnight in Nochromix containing 96 wt % sulfuric acid. The wafers were then rinsed with an excess amount of tap water, followed by ultrapure water for several times, and stored in ultrapure water until use in deposition experiments. Hydrophilic silicon wafers were rendered hydrophobic by soaking them overnight in a 10 vol % solution of dichlorodimethylsilane (99%; Aldrich) in toluene. Adsorbed asphaltene films at heptol-water interfaces were transferred onto hydrophilic silicon wafers by a LB deposition method. In this case, a hydrophilic silicon wafer was first immersed in the water subphase and placed parallel to the barriers. An adsorbed asphaltene monolayer was compressed to a predetermined interfacial pressure. The interfacial pressure was then held constant by a feedback control system while pulling the substrate up (upstroke) through a heptol-water interface, transferring the asphaltene material onto the substrate. An alternative method of building a LB film is by horizontal lifting of a substrate in contact with a compressed monolayer, also known as LS deposition, which is useful for deposition of rigid films at the solid region of a surface pressure-area isotherm.36 In this method a flat hydrophobic substrate is placed horizontally on the compressed monolayer film. When the substrate is lifted and separated from the subphase surface, the monolayer is transferred onto the solid substrate, maintaining the same molecular direction.36 The LS method was used to deposit an asphaltene film at a high interfacial pressure where LB deposition became difficult or impossible, because the area between the two barriers under such a high interfacial pressure became extremely small. In this case, after an asphaltene film was compressed to a desired interfacial pressure, the substrate was completely immersed in the oil phase first horizontally and then was contacted with the interfacial asphaltene film. The substrate was pulled out of the oil phase slowly and left in a well-ventilated particle-free fume hood for 2 h for the entrained oil to evaporate. The substrate with deposited LS film was kept in a desiccator until use for AFM imaging. 2.4. AFM Imaging. Images of the deposited LB and LS asphaltene films were obtained with a NanoScope IIIa (Digital Instruments) atomic force microscope (AFM) operated in both contact and tapping modes in air at room temperature (22 °C). AFM imaging was performed with a multimode-scanning probe microscope (MM-SPM) head and a J scanner. A silicon nitride tip was used for contact mode imaging, and a silicon tip was used for tapping mode imaging. Images were obtained on at least three separate spots on each sample. Two samples prepared independently under the same deposition conditions were used for AFM imaging. We found that both AFM imaging modes give nearly identical AFM topographic images of the deposited asphaltene films. 2.5. Contact Angle. Static contact angles of the deposited monolayer LB asphaltene films were measured with a Kruss drop shape analysis system (DSA 10-MK2) at room temperature (22 °C) using the proce-

dures described in our previous study.17 The contact angle was measured through the water phase in air and at the two opposite three-phase contact points (left and right) immediately after a water drop was placed on the surface of an asphaltene film. The contact angle was taken at 1-s intervals for 1 min. The drop shape analysis software reported an average value of the contact angle. Contact angle values were obtained at a minimum of four separate locations for each sample, using at least two independently deposited samples for each set of conditions. The averaged contact angle values are reported in this paper. 2.6. FTIR Spectroscopy. The infrared spectrum of the deposited LB films was obtained using a Bio-Rad FTS 6000 Fourier transform infrared (FTIR) spectrometer. FTIR spectroscopic measurement was performed with a specular reflectance accessory (Pike Technologies) with a fixed incidence angle of 30°. The scan speed was set at 5 kHz with a spectral resolution of 2 cm-1. The sensitivity parameter was set to 4. A total of 128 scans was used to obtain a spectrum. 3. Results 3.1. Interfacial Isotherms. The monolayer behavior of the three asphaltene samples at heptol-water interfaces was characterized with a Langmuir interfacial trough. The interfacial pressure was monitored as a function of time upon spreading asphaltene on a heptol surface. The measured interfacial pressure versus time results in the adsorption kinetic (π-t) isotherm. The interfacial asphaltene film was compressed while interfacial pressure was monitored, resulting in a pressure-area (π-A) isotherm. To further study the interfacial behavior of asphaltene films, we investigated hysteresis and relaxation characteristics. A comparison between a spread asphaltene monolayer and an adsorbed asphaltene monolayer from the three asphaltene samples was made in order to estimate the mass of the adsorbed asphaltene material at a heptane-water interface. The volume fraction of heptane φ was varied, which allowed us to change the aromaticity of the organic phase and to study the impact of aromaticity change on asphaltene adsorption characteristics and interfacial behavior. The Langmuir interfacial trough was calibrated by reproducing pressure-area isotherms of a distearoylphosphatidylcholine (DSPC) monolayer at a heptane-water interface23 and a dipalmitoylphosphatidylcholine (DPPC) monolayer at a hexadecane-water interface.25 3.1.1. Adsorption (π-t) Isotherms. Adsorption of asphaltenes at a heptol-water interface was monitored by measuring interfacial pressure (π) as a function of time (t) with a filter paper Wilhelmy plate. The trough was initially filled with ultrapure water as the subphase. Heptol with a given heptane volume fraction was poured carefully on the surface of the water subphase. A volume of 0.025 mL of asphaltene-in-toluene solution (2 mg/mL) was spread dropwise on the heptol surface while the interfacial pressure change as a function of time was recorded. Interfacial pressure (π) is defined as the difference between the interfacial tensions of an oil-water interface in the absence and presence of a surface active material:37

π ) σ0 - σ

(1)

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Figure 3. Interfacial pressure-area (π-A) isotherms for high molecular weight, low molecular weight, and whole asphaltenes, compressed at 10.8 cm2/min at heptane-water interface at 20 °C.

Figure 2. Adsorption kinetics for high molecular weight, low molecular weight, and whole asphaltenes, monitored with a filter paper Wilhelmy plate at 20 °C at (a) heptane-water and (b) heptol-water (φ ) 0.5) interfaces.

where σ0 and σ are the interfacial tensions of an oilwater interface in the absence and presence of a surface active material, respectively. Figure 2 shows typical adsorption isotherms for adsorption of high molecular weight, low molecular weight, and whole asphaltenes at (a) a heptane-water interface and (b) a heptol-water interface for φ ) 0.5. The starting time t ) 0 is defined as the time when the first drop of asphaltene-in-toluene solution touches the heptol surface. The sharp increase of the interfacial pressure from t ) 0 s to t ≈ 100 s as observed in Figure 2 reflects the adsorption of asphaltene as molecules and precipitates (a) and as molecules alone (b). These adsorbed states of asphaltene molecules at the heptane-water interface were derived from AFM observations. As will be discussed later in the AFM image section, the adsorbed asphaltene at a heptane-water interface is present as both asphaltene molecules and particulate precipitates. However, at a heptol-water interface for φ e 0.5, the adsorbed asphaltene is present mainly as molecular aggregates. At the heptane-water interface at t ≈ 100 s, the interfacial pressure for high molecular weight asphaltene decreases sharply and then more gradually with time. The sharp decrease of the interfacial pressure is an indication of redistribution of asphaltene molecules/or precipitates at the heptanewater interface, while the gradual change reflects relaxation and rearrangement of the molecular constituents. It is conceivable that high molecular weight asphaltene molecules become precipitates when they are in contact with heptane. These precipitates reach the interface quickly before they have time to redistribute across the interface, resulting in the observed spike at t ≈ 90 s. For whole asphaltene, the interfacial pressure π at the heptane-water interface reaches a maximum at t ≈ 100 s and then decreases gradually with time,

presumably as a result of molecular reorganization and relaxation. For the low molecular weight asphaltene subfraction, π at the heptane-water interface increases monotonically with time, first rapidly to 100 s and then gradually. The slow increase of π at t > 100 s for low molecular weight asphaltene subfraction indicates a continuous adsorption of asphaltene molecules at the heptane-water interface. This contrasting behavior to that of whole and high molecular weight asphaltenes illustrates the significant effect of molecular weight on surface activity at a heptane-water interface. In general, at a given time, the interfacial pressure value for the three asphaltene samples follows the trend πlow > πwhole > πhigh. A similar trend was observed at the heptol-water interface for φ ) 0.5, as shown in Figure 2b. However, it should be mentioned again that the adsorbed materials at the heptol-water interface for φ ) 0.5 are present as asphaltene molecules only. This was derived from AFM observations as will be discussed later. 3.1.2. Interfacial Pressure-Area (π-A) Isotherms. Our tests showed that, within certain limits, the mass of asphaltene spread on the heptane surface does not affect the shape of the interfacial pressure-area isotherms for the three asphaltene samples studied, indicating that the asphaltenes indeed form a monolayer at the heptane-water interface. For example, when we spread a volume of 0.015 or 0.025 mL of the high molecular weight asphaltene-in-toluene solution (2 mg/ mL) on the heptane surface, the pressure-area isotherms were identical in terms of area per mass of asphaltene added. The asphaltene monolayers were compressed after 10 min of spreading the asphaltenein-toluene solution on the heptane surface. Since the mass of adsorbed asphaltene monolayer at the heptanewater interface is unknown, one cannot normalize the trough area in terms of area per molecule. Instead, the trough area occupied by an asphaltene monolayer is used hereafter. For the purpose of this study, use of trough area to compare molecular behavior at the oilwater interface is sufficient. Figure 3 shows interfacial pressure-area (π-A) isotherms for adsorbed monolayers of the three asphaltene samples at the heptanewater interface. As shown in Figure 3, the asphaltene monolayers at the heptane-water interface are more expanded at lower pressures than those at the airwater interface17 when the pressure is normalized by interfacial tension (σ0) and plotted versus trough area.


This is caused by the presence of hydrocarbon (heptane) molecules in the asphaltene monolayers. The presence of heptane molecules in an asphaltene monolayer provides a more favorable environment for hydrocarbon chains and reduces the interactions between the asphaltene molecules/precipitates. No abrupt phase transition points are visible on the π-A isotherms. The absence of any clear phase transition is an indication that asphaltene is composed of a mixture of many chemical species with varying phase transition points. Although no sharp transition points are observed, distinctive gas phase (G), liquid phase (L), and solid phase (S) regions of asphaltene monolayers are evident at the heptane-water interface. At an air-water interface, asphaltene monolayers were shown to form molecular aggregates by asphaltene molecules.17 At the heptane-water interface, however, the asphaltene monolayers are formed by asphaltene molecules and precipitates, as will be shown later in the AFM images of deposited LB asphaltene films. In the gas phase, the asphaltene molecules and precipitates are far apart and do not interact with each other. When compressed by moving the barriers close to each other to reduce the interfacial area, the asphaltene molecules and precipitates become closer and start to interact with each other, thereby creating a liquid phase (L). In this state, molecules of the surrounding hydrocarbon solvent phase partition into the monolayer.38 In the solid state (S), the asphaltene molecules and particles are closely packed, their alkyl side chains are assumed to be oriented vertically, and the molecules of the surrounding solvent are squeezed out from the asphaltene monolayer. Above an interfacial pressure of ∼45 mN/m, the asphaltene monolayers begin to fold and buckle. Brown streaks can be visually observed near the edges of the two compressing barriers at the heptane-water interface. The shapes of the π-A isotherms of monolayers of the three asphaltene samples are similar with nearly identical slopes in the solid state region, indicating a close similarity of interfacial asphaltene films among the three asphaltene samples under compression. The critical interfacial pressures (πc) at which the interfacial asphaltene monolayers begin to fold are also similar, occurring at πc ≈ 45 mN/m. However, the high molecular weight asphaltene monolayer is the most condensed, while the low molecular weight asphaltene monolayer is the most expanded. To quantify the mass of adsorbed asphaltene at the heptane-water interface, a series of experiments was carried out. The experiments were performed with spread asphaltene monolayers at the heptane-water interface. A known mass of an asphaltene sample, ranging from 0.01 to 0.05 mg (0.005 to 0.025 mL), was spread on the subphase, followed by a period of 10 min evaporation of the spreading solvent (toluene). The water surface was then covered with 100 mL of heptane. After 10 min, the spread asphaltene monolayer was compressed as described previously. Figure 4 shows a comparison of the pressure-area isotherms of the low molecular weight sample of the adsorbed monolayer (see also Figure 3) with that of the spread monolayers prepared with 0.025, 0.015, 0.010, and 0.008 mL (0.05, 0.03, 0.02, and 0.016 mg) of low molecular weight asphaltene-in-toluene solution. In the case of spread monolayers, the shape of the pressurearea isotherms depends on the mass of asphaltene

Figure 4. Comparison of interfacial pressure-area (π-A) isotherms for low molecular weight asphaltene, for adsorbed monolayer (bold curve) and spread monolayers at varying concentrations, compressed at 10.8 cm2/min at heptane-water interface at 20 °C.

Figure 5. Comparison of interfacial pressure-area (π-A) isotherms for high molecular weight asphaltene, for adsorbed monolayer (bold curve) and spread monolayers at varying concentrations, compressed at 10.8 cm2/min at heptane-water interface at 20 °C.

spread on water subphase. However, the concentrationdependent shape change does not affect our comparison. In Figure 4, the curve of spread monolayer of 0.010 mL is identical to that of the adsorbed asphaltene monolayer up to an interfacial pressure of 35 mN/m. This finding would suggest that the mass of the adsorbed low molecular weight asphaltene at the heptane-water interface is nearly the same as that of the known mass of spread asphaltene (0.010 mL × 2 mg/mL ) 0.02 mg). In other words, about 0.02 mg of the original 0.05 mg of low molecular weight asphaltene that was placed on the heptane surface adsorbed to the heptane-water interface. Figure 5 shows a comparison of the pressure-area isotherm of an adsorbed monolayer (see also Figure 3) with those obtained from spread monolayers prepared from high molecular weight asphaltene-in-toluene solution (2 mg/mL) with 0.025, 0.010, 0.008, 0.006, and 0.005 mL. The pressure-area isotherm of the adsorbed monolayer is located between the spread monolayers prepared from 0.005 and 0.006 mL of asphaltene-in-toluene solution, indicating that the mass of the high molecular weight asphaltene present at the heptane-water interface is about 0.010-0.012 mg. Similar results, as shown in Figures 4 and 5, were observed for the whole asphaltene sample. It is apparent from the results shown above that the mass of adsorbed material at the heptane-water interface for the three asphaltene samples in Figure 3 follows the order of low molecular weight asphaltene > whole

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Figure 6. Interfacial pressure-area (π-A) isotherms for (a) whole asphaltene and (b) low molecular weight asphaltene, compressed at 10.8 cm2/min at 20 °C at heptol-water interfaces with a heptane volume fraction of φ ) 0.1, 0.5, 0.9, and 1.0.

asphaltene > high molecular weight asphaltene. This trend is consistent with the results shown in Figures 2 and 3. Figure 6 shows the interfacial pressure-area isotherms for monolayers of the whole asphaltene and monolayers of the low molecular weight asphaltene at the heptol-water interface with varying heptane volume fraction (φ) in heptol. The composition of the hydrocarbon phase shows a dramatic effect on the interfacial pressure-area isotherms; i.e., the highest interfacial pressure attainable increases with increasing heptane content in heptol. As expected, the compressibility of the asphaltene monolayers decreases as the heptane content increases since the rigidity of an asphaltene monolayer increases with increasing heptane content. The critical interfacial pressure at which monolayers begin to buckle at φ ) 0.9 is ∼40 mN/m, lower than the value of ∼45 mN/m at the heptanewater interface (see also Figure 3). No buckling was observed for asphaltene monolayers for φ ) 0.1 and 0.5, indicating that at heptol-water interfaces asphaltene monolayers are not as rigid at lower heptane content (φ e 0.5) as those at higher heptane volume fractions of φ ) 0.9 and 1.0. Clearly, heptol with a higher content of toluene is favorable for asphaltene molecules to reside. In other words, as φ decreases, asphaltene molecules, as opposed to colloidal asphaltene particles, preferentially adsorb at the heptol-water interface. Similar results for monolayers of the high molecular weight asphaltene were observed. 3.1.3. Hysteresis Isotherms. Figure 7 shows hysteresis isotherms for monolayers of the high molecular weight asphaltene at the heptane-water interface. The asphaltene monolayers were compressed and expanded

Figure 7. Hysteresis isotherms for monolayers prepared from the high molecular weight asphaltene sample at the heptane-water interface at 20 °C. The asphaltene monolayers were compressed at a speed of 10.8 cm2/min to a preset interfacial pressure of (a) 20 and (b) 45 mN/m.

at a rate of 10.8 cm2/min to a preset interfacial pressure of 20 or 45 mN/m for three cycles of compressionexpansion. It is clear that the decompression branch deviates significantly from the compression branch, thereby showing a hysteresis. A much larger hysteresis is observed when the monolayer is compressed to a higher interfacial pressure of π ) 45 mN/m than when the monolayer is compressed to a relatively low interfacial pressure of π ) 20 mN/m. The compression leg of the hysteresis for both cases (20 and 45 mN/m) shifts to the left between cycles 1 and 3, indicating that smaller molecules or precipitates are being squeezed out of the monolayer during the previous compression. A slightly reduced hysteresis is observed with increasing compression-expansion cycles, due to some irreversible transfer of small molecules and precipitates during previous compressions. Tighter binding of the asphaltene aggregates may also contribute to the reduced hysteresis, especially in the later cycles. The asphaltene aggregates do not relax to their original state after full expansion of the barriers. For monolayers prepared from the low molecular weight and the whole asphaltene samples at the heptane-water interface, similar hysteresis isotherms as shown in Figure 7 were observed. We also performed hysteresis experiments for the three asphaltene samples as a function of φ and found that hysteresis becomes smaller with decreasing heptane volume fraction for an adsorbed asphaltene monolayer at a heptol-water interface. This finding suggests that, at a heptol-water interface, asphaltene molecules for the case of φ e 0.5, as opposed to asphaltene precipitates for the case of φ g 0.9, induce a smaller hysteresis. Asphaltene molecules

Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 1167 Table 1. Transfer Ratio, TR, of Monolayer LB Films of High and Low Molecular Weight Asphaltenes and Whole Asphaltene Deposited at Interfacial Pressure of π ) 0, 5, 10, 20, 30, and 40 mN/m at 20 °C on Oxidized Silicon Wafers at Heptane-Water Interfacea π, mN/m TR

0

5

10

20

30

40

high molecular weight low molecular weight whole

3.65 1.90 2.59

1.99 1.39 1.61

1.81 1.21 1.74

1.57 1.27 1.35

1.02 1.14 1.25

1.15 0.99b 1.07

b

Figure 8. Relaxation isotherms for monolayers prepared from the high molecular weight, low molecular weight, and whole asphaltene samples at the heptane-water and heptol-water interfaces for φ ) 0.5 at 20 °C. The asphaltene monolayers were compressed and expanded at a speed of 10.8 cm2/min.

in an asphaltene monolayer may relax to their original configuration when the barriers are expanded after a previous compression of the monolayer at a heptolwater interface. However, asphaltene precipitates may not relax to their original configuration when the barriers are expanded after a previous compression, due to irreversible aggregation of asphaltene precipitates, thereby leading to a larger hysteresis between two consecutive compressions. Furthermore, formation of a network of asphaltene precipitates as observed in AFM images may also contribute to the observed larger hysteresis for a higher heptane content of φ g 0.9. 3.1.4. Relaxation Isotherms. Stability of asphaltene monolayers at the heptane-water and heptol-water interfaces was investigated by monolayer relaxation characteristics. The monolayers were compressed to a preset interfacial pressure at which the movement of the barriers was stopped and the trough area was maintained constant. Interfacial pressure decrease was monitored and recorded as a function of time. The decrease of interfacial pressure with time is an indication of how “stable” a monolayer is, when it is compressed to a given interfacial pressure. Figure 8a shows a comparison of the relaxation isotherms for monolayers of the high molecular weight asphaltenes, low molecular weight asphaltenes, and whole asphaltenes at a heptane-water interface. For comparison purposes, the interfacial pressure was normalized with the initial interfacial pressure (π0 ≈ 20 mN/m) to which a monolayer was compressed. At the

a A withdrawal speed of 5 min/min was used for deposition. Deposition at 38 mN/m.

heptane-water interface, relaxation characteristics of asphaltene monolayers prepared from the three asphaltene samples are similar in shape. In comparison with small molecules, large molecules have less possibility to rearrange themselves into a more favorable configuration during compression. As a result, they have a high potential to rearrange themselves during relaxation, thereby exhibiting a quicker relaxation characteristic. Monolayers of the low molecular weight asphaltene relax at the slowest rate. Figure 8b shows a comparison of the relaxation isotherms for monolayers of the high molecular weight asphaltenes, low molecular weight asphaltenes, and whole asphaltenes at a heptol-water interface for φ ) 0.5. The relaxation isotherms of the asphaltene monolayers are nearly identical with a slight difference in the relaxation rates following the trend: high molecular weight asphaltene > whole asphaltene > low molecular weight asphaltene. Similar relaxation characteristics were observed at the heptane-water interface. However, asphaltene monolayers relax at a faster rate at a heptol-water interface than that at a heptane-water interface. This difference is attributed to the fact that an asphaltene monolayer is more flexible at the heptolwater interface than at the heptane-water interface (Figure 6). With increasing toluene content (1 - φ) in heptol, an asphaltene monolayer becomes less rigid, as clearly shown in Figure 6, independent of the asphaltene fractions studied. The heptol composition plays an important role in controlling the rigidity of an asphaltene monolayer, although the exact role of the heptol phase in this regard remains to be established. 3.2. LB Asphaltene Films at Heptane-Water and Heptol-Water Interfaces. Monolayer LB films of asphaltenes were deposited onto hydrophilic silicon wafers. The transfer ratio is defined as

TR )

decrease in area of adsorbed monolayer area of transferred film on solid substrate

The transfer ratio (TR) was found to depend on the interfacial pressure at which LB deposition was performed. Table 1 lists the transfer ratio for depositing a monolayer LB asphaltene film on an oxidized silicon wafer from monolayers of the three asphaltene samples at 20 °C. The deposition was performed from a heptane-water interface at interfacial pressures of 0, 5, 10, 20, 30, and 40 mN/m. A substrate withdrawal speed of 5 mm/min was used for deposition. It can be seen from Table 1 that deposition of LB films from asphaltene monolayers can be achieved at an interfacial pressure of π g 30 mN/m with a good transfer ratio (TR) close to unity. At lower interfacial pressures, the transfer ratio was found to be much larger than unity. A large transfer ratio, as high as 2, has been reported in the litera-

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Figure 9. Tapping mode AFM topographic images of deposited monolayer films from the high molecular weight asphaltene sample deposited at π ) 5, 20, and 50 mN/m at 20 °C on silicon wafers at heptane-water interface: (a and b) LB film; (c) LS film.

ture39,40 for surfactant bearing systems. Such a high transfer ratio is normally attributed to restructuring of surfactant molecules. In our case, the observed large transfer ratio is probably caused by condensation of the depositing monolayer during deposition at the solventwater interface. Such condensation appears to change the packing density of the monolayer at the solventwater interface, which has been previously observed at the air-water interface by Spratte and Riegler.41 This type of condensation can also be viewed as molecular aggregation on the solid substrate surface as reported in ref 42 during deposition of monolayers of fatty acids at the air-water interface. At π ) 5 mN/m, high transfer ratios of 2-3 were also observed with increasing toluene content in heptol during deposition of LB asphaltene films at heptolwater interfaces.

3.3. AFM Images of Deposited Asphaltene Films. Atomic force microscopy (AFM) was used to image the topography of the transferred LB and LS asphaltene films deposited from the heptane-water and heptolwater interfaces. Figure 9 shows tapping mode AFM topographic images of transferred adsorbed asphaltene monolayers prepared from the high molecular weight asphaltene sample deposited from a heptane-water interface on silicon wafers at π ) 5, 20, and 50 mN/m. The deposition temperature was maintained at 20 °C. A substrate withdrawal speed of 5 mm/min was used for deposition. The withdrawal direction of the substrate was upward on all of the AFM images. The arrows in Figure 9a show the direction of compression. As mentioned in the experimental procedure, when 0.025 mL of asphaltene-in-toluene solution (2 mg/mL)


Figure 10. Contact mode AFM topographic images of monolayer LB films from the high molecular weight asphaltene sample deposited at a volume fraction of φ ) 0.1, 0.5, 0.9, and 1.0 on silicon wafers at π ) 10 mN/m at 20 °C at heptol-water interfaces.

was spread dropwise onto the bulk heptane (100 mL) surface, asphaltene precipitated out as fine solid particles. The precipitates settle through the heptane layer (∼3 mm) and adsorb at the heptane-water interface. The bright domains of interconnected and individual aggregates formed from precipitated solid asphaltene particles (∼0.5 µm) are visible in Figure 9a. Darker areas in the image were identified to be continuous and composed of asphaltene molecular aggregates, as revealed by closeup imaging of those dark areas. Phase imaging clearly revealed the material differences between the bright and dark areas in the AFM topographic images. The aggregated solid asphaltene particles are incorporated into or embedded in the asphaltene molecular layer, probably through strong attractive intermolecular interactions between the solid asphaltene particles and the asphaltene molecules. Figure 9 shows that the aggregates of the asphaltene particles become larger and the packing of the asphaltene particles becomes denser with increasing deposition pressure. At an intermediate interfacial pressure of π ) 20 mN/m (Figure 9b), solid asphaltene aggregates form networked structures. At a very high interfacial pressure of π ) 50 mN/m (note that the z-scale in Figure 9c is 5 times larger than that in Figure 9a,b), the high molecular weight asphaltene monolayer becomes buckled (Figure 9c) in the direction perpendicular to the compressing barriers. Brown streaks can be seen near the inner edges of the two compressing barriers. Results similar to those shown in Figure 9 were observed for LB films from low molecular weight and whole asphaltene samples. Figure 10 shows topographic variations of monolayer LB asphaltene films deposited from heptol-water in-

terfaces at varying heptane content (φ). The LB asphaltene films were deposited from the high molecular weight asphaltene sample at π ) 10 mN/m. The AFM images were obtained by contact mode imaging in air. Parts a and b of Figure 10 show that the LB asphaltene films obtained at values of φ ) 0.1 and 0.5 are composed of asphaltene molecular aggregates. A comparison between the aggregated structures observed in Figure 10a,b and those deposited from the air-water interface17 indicates that the packing of the asphaltene molecules is closer at the heptol-water interface than at the air-water interface. Also, the size of the asphaltene molecular aggregates at the heptol-water interface for φ ) 0.1 and 0.5 is smaller than that at the air-water interface. It should be noted that the packing of the asphaltene molecular aggregates is random and uniform, independent of the dipping direction. AFM images in Figure 10 show that, with increasing φ, the composition of the adsorbed asphaltene monolayer changes from molecules for φ e 0.5 to a combination of molecules and colloidal particles for φ g 0.9. This is a clear indication that an asphaltene monolayer becomes more rigid in the presence of asphaltene particles, as opposed to the case of a monolayer that is composed of asphaltene molecules only. Parts c and d of Figure 10 show that the LB asphaltene films obtained at values of φ ) 0.9 and 1.0 are composed of distinctive asphaltene aggregates of ∼1 µm in size, in contrast to the size of ∼20 nm for LB films deposited at φ ) 0.1 and 0.5. This observation would suggest that, at a higher content of heptane in heptol for φ ) 0.9 and 1.0, the adsorbed asphaltene materials are likely present as particulate aggregates as shown by the brighter areas and as molecular aggregates as

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Figure 11. Tapping mode AFM phase images of monolayer LB films from the high molecular weight asphaltene sample deposited on silicon wafers at π ) 10 mN/m at 20 °C at heptol-water interfaces for φ ) 0.5 and 0.9.

shown by the dark areas in the AFM images (Figure 10c,d). It should be mentioned that AFM topographic images obtained in tapping mode (Figure 9a) and contact mode (Figure 10d) show identical topographic features. Figure 11 shows closeup phase images (0.5 µm × 0.5 µm) taken from dark areas of Figure 10b,d. Figure 11a indicates that for φ ) 0.5 the deposited asphaltene LB film consists of homogeneous asphaltene molecular aggregates. Similar results were also observed for LB films deposited at the heptol-water interface for φ ) 0.1. The phase image in Figure 11b shows features very similar to those seen in Figure 11a, indicating that the dark areas in Figure 10d are also composed of asphaltene molecular aggregates. Similar results were observed for LB films from the low molecular weight and whole asphaltenes. 3.4. Contact Angle of Deposited Asphaltene Films. Static contact angles (θ) were measured against water in air on monolayer LB films of asphaltenes deposited from the heptane-water interface. The measured contact angle values were plotted in Figure 12 as a function of the interfacial pressure (π) at which deposition was performed. For both high and low molecular weight asphaltene films deposited from a heptane-water interface, the contact angle value increases with increasing interfacial pressure of deposition, indicating a closer packing of the hydrocarbon side chains of the surface active asphaltene molecules. At a given interfacial pressure, the contact angle value of an LB film from the high molecular weight asphaltene sample is higher than that from the

Figure 12. Static contact angle θ against water as a function of deposition pressure π for monolayer LB films deposited from high and low molecular weight asphaltenes at the heptane-water (φ ) 1.0) interface at 20 °C.

low molecular weight sample. The longer hydrocarbon side chains in the high molecular weight asphaltene appear to favor a denser molecular packing. A similar trend was observed for LB asphaltene films deposited at the air-water interface.17 At the heptane-water interface, the packing of the high molecular weight asphaltene molecules and precipitates may be denser than that of the low molecular weight asphaltene molecules and precipitates. The contact angle values of 82-94° for LB asphaltene films deposited from the heptane-water interface are larger than those of 69-75° for LB asphaltene films deposited from the air-water interface,17 indicating that packing of the asphaltene molecules or precipitates is


Figure 13. FTIR spectra of monolayer LB high molecular weight asphaltene films deposited at heptol-water interfaces at φ ) 0.1, 0.5, 0.9, and 1.0 and π ) 10 mN/m at 20 °C. Spectrum of powders of the same asphaltene sample is also shown for comparison. Table 2. General Features of the Infrared Spectral Bands of Monolayer LB Asphaltene Films band ν, cm-1

structural feature

3700-2700 1647 1528 1120 1036-1028 894, 849, 738

bonded O-H stretching (strong) amide CdO stretching aromatic CdC stretching COH stretching S-O stretching aromatic C-H bending

closer at the heptane-water interface than that of asphaltene molecules at the air-water interface. The presence of heteroatom moieties such as N, O, and S allows the hydrocarbon side chains to arrange themselves more favorably. For LB films of the high and low molecular weight asphaltenes deposited at π ) 5 mN/m from the heptolwater interfaces for φ ) 0.1, 0.5, 0.9, and 1.0, a contact angle value of ∼90° is observed. The contact angle value appears to be independent of heptane content in heptol. This finding suggests that the presence of solid asphaltene precipitates in the asphaltene films does not affect the measured contact angle values. At a given φ, the measured contact angle value for low molecular weight asphaltene LB films is slightly lower than that for LB films of high molecular weight asphaltene. The contact angle value is slightly lower for films deposited at π ) 5 mN/m than at π ) 10 mN/m. This variation is expected since a higher interfacial pressure causes a closer packing of the alkyl side chains of the surface active asphaltene molecules and hence a more hydrophobic surface with a larger contact angle value. 3.5. FTIR Spectra of Deposited Asphaltene Films. FTIR spectra of a monolayer of high molecular weight asphaltene LB films transferred from the heptol-water interfaces are shown in Figure 13. For comparison, the spectrum obtained from asphaltene powders is also shown. The spectra of the films were normalized with the strongest peak at 889 cm-1. The band assignments are listed in Table 2. The band assignments for asphaltene powders can be found in Zhang et al.17 It is evident that the spectrum of the asphaltene film is different from that of the asphaltene powders. However, the spectra of the asphaltene films obtained at the heptolwater interfaces of varying φ are almost identical, and thus are independent of the heptane volume fraction φ. A very strong and broad band at 3700-2700 cm-1 together with a band at 1120 cm-1 indicates the presence of bonded O-H groups in the deposited films. This

broad band shadows the C-H stretching bands at 2930-2700 cm-1 as shown in the spectrum for asphaltene powders. The carbonyl CdO stretching band shifts from 1606 cm-1 for asphaltene powders to 1528 cm-1 for deposited asphaltene films. The three distinctive bands due to aromatic C-H bending also shift from 862, 814, and 747 to 894, 849, and 738 cm-1, respectively, indicating that the aromatic rings are more constrained in the deposited film than in the powder form. For LB films deposited at varying interfacial pressures from a heptol-water interface for a given φ, the FTIR spectra obtained are similar to those shown in Figure 13. FTIR spectra of LB films from different samples such as low molecular weight and whole asphaltenes from a heptol-water interface are also similar to those of LB films deposited from the high molecular weight asphaltene. It should be mentioned that the deposited asphaltene film could not be dissolved in toluene after soaking in toluene for 30 min. The FTIR spectra of an LB asphaltene film after soaking in toluene for 30 min are identical to those of the same sample before soaking. This would suggest that asphaltene molecules at the heptol-water interface are bonded with water such that the bonded asphaltenes are insoluble in toluene. Yang and Czarnecki21 reported that asphaltene is insoluble in water and accumulates at oil-water interfaces. Starlike, optically anisotropic asphaltene particles were observed in bulk only in the presence of water, suggesting some type of irreversible interaction between asphaltene and water. Our FTIR results shown in Figure 13 indicate that asphaltene molecules or precipitated particles are indeed bonded with water, substantiated by the presence of strong broad bands over the 3700-2700 cm-1 spectral region. Once asphaltene molecules and/or precipitates are deposited on a solid substrate, they are no longer soluble in toluene. The deposited asphaltene films cannot be washed from the substrate by the bulk organic phase when pulling a solid substrate through the organic phase during the LB deposition process. Initially, asphaltenes were dissolved in and spread from toluene to form monolayers at the organic-water interfaces. However, the asphaltene interfacial films are different from the originally extracted bulk solid samples. Asphaltene molecules and/or precipitates become bonded with water at the interface, and consequently their chemical compositions become different as is shown in the FTIR spectra. The reason for asphaltene molecules to stay at the heptol-water interfaces is that they are bonded with water. Hence, they will not leave the heptol-water interface and migrate into the bulk heptol phase. Even though the originally extracted asphaltene solid powders are completely soluble in heptol for φ e 0.5, asphaltene molecules do not migrate from the interface to the bulk heptol phase. These findings are consistent with the results of pressure-area isotherms as shown in Figure 6, that an asphaltene film is always present at the heptol-water interfaces regardless of the heptol composition. 4. General Discussion A rigid asphaltene monolayer located at an oil-water interface prevents coalescence of the dispersed water droplets in water-in-oil emulsions. The interfacial pressure-area isotherms demonstrate that rigid asphaltene monolayers can form at the heptol-water interfaces

1172


with a heptane volume fraction of φ g 0.9. Although the bulk asphaltene materials are completely soluble in heptol when φ e 0.5, a monolayer of molecular asphaltene aggregates can still form at the heptol-water interface, which is a clear indication that surface active asphaltene molecules strongly adsorb at the heptolwater interfaces. This finding indicates that asphaltene molecules, once adsorbed at the oil-water interface, will not leave the interface and partition between the bulk solvent phase and the interface. With increasing heptane content in heptol, the highest attainable interfacial pressure and the mass of adsorbed asphaltene molecules and/or particulate aggregates also increase for all three asphaltene samples studied. There appears to be a strong correlation between the highest attainable interfacial pressure for an asphaltene monolayer at a heptol-water interface and the stability of water-in-oil emulsions. Gafonova and Yarranton7 found that the stability of water-in-diluted bitumen emulsion decreases with decreasing heptane content in heptol. Our results show that the highest attainable interfacial pressure and consequently the rigidity of interfacial asphaltene film decreases with decreasing heptane content in heptol (see Figure 6), suggesting that the stability of water-in-heptol emulsion is linked with the rigidity of the interfacial asphaltene film. Asphaltene colloidal particles alone are capable of stabilizing water-in-oil emulsions. Our contact angle data on the deposited LB films for the three asphaltene samples suggest that a contact angle of g90° for particulate asphaltene colloids is not unexpected. It is generally accepted that solid particles with a contact angle greater than 90° will stabilize water-in-oil emulsions, while a contact angle of

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