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MATERIALS AND INTERFACES Asphaltene-Silica Interactions in Aqueous Solutions: Direct Force Measurements Combined with Electrokinetic Studies T. Abraham,†,# D. Christendat,† K. Karan,‡ Z. Xu,† and J. Masliyah*,† Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6, and DBR Research Inc., 9419-20 Avenue, Edmonton, Alberta, Canada T6N 1E5
An atomic force microscope (AFM) was used to study the interaction between asphaltenes and silica surfaces in aqueous solutions directly. Electrokinetic measurements were performed on similar systems to complement the AFM results. Asphaltene-silica interactions in aqueous solutions exhibited a time-dependent characteristic. A repulsive force component developed with increasing sample incubation time, suggesting molecular rearrangements at the asphaltenewater interface. Upon the addition of 1.0 mM KCl, the repulsive force was suppressed, and the measured force became overall attractive, implying an electrostatic nature for this repulsive force. An increase of the solution pH in the same system eliminated the attractive force regime through the development of a stronger repulsive force component. The increase in the range and magnitude of the asphaltene-silica interaction with increasing solution pH clearly indicates the presence of pH-dependent ionizable groups on the asphaltene surface. The measured longrange force can be very well accounted for by the Poisson-Boltzmann (PB) equation using a constant Stern layer potential boundary condition. The fitted Stern layer potentials of asphaltene surfaces (ψas) became more negative with increasing pH. In the case of asphaltenes at high solution pH values, a satisfactory agreement was obtained between the fitted Stern layer potential based on the AFM force measurements and the electrokinetic potential measured using the electrophoresis method. The swelling/stretching of surface asphaltene molecules at high solution pH and the sensitivity of such stretched layers to added salt were also evident from the forcedistance data. The measured adhesion force correlated well with the long-range force behavior. Introduction Asphaltenes, a solubility class present in crude oils, have been studied extensively in recent years to obtain a better understanding of their complex nature.1-6 Because of their amphiphilic nature, asphaltenes play a prominent, often hindering, role in the extraction and utilization of hydrocarbon resources. For instance, in the Clark Hot Water Extraction (CHWE) process, commonly used to extract bitumen from oil sand ores,7 a water phase under controlled pH and temperature is contacted with oil sand ores in a digester to facilitate the disengagement of bitumen from the sand grains. The efficiency of this process depends on many factors, including the interactions between the surface components of the bitumen and the sand grains. It is generally believed that surface-active asphaltene molecules,8 a component of bitumen, interact with clays and mineral fines (silica surfaces) primarily through their polar functional groups with the hydroxyl groups present on * To whom correspondence should be addressed. Tel.: (780) 492-4673. Fax: (780) 492-2881. Email: jacob.masliyah@ ualberta.ca. † University of Alberta. # Current address: Nanotechnology Research Institute, Bionanomaterials Group AIST, Tsukuba 305-8565, Japan. ‡ DBR Research Inc.
the silica surfaces.1 The nature of such interactions and their dependence on various process conditions, such as pH, salt concentration, temperature, etc., are among the determining parameters for oil recovery. Such interactions are also important in other relevant applications such as the in situ recovery of crude oil using water flooding or enhanced oil recovery processes used to displace oil from the pore of reservoir. Surface force measurements have been applied only to a limited extent in studying the fundamentals involved in the extraction and utilization of hydrocarbon resources.9,10 Using a surface forces apparatus (SFA), Yoon et al.9 measured the forces between two bitumencoated mica surfaces in aqueous solutions as a function of solution pH, temperature, and added salt concentration. In their study, the bitumen surfaces were prepared using the Langmuir-Blodgett (LB) technique. They found that the range and magnitude of the purely repulsive force increased with increasing solution pH, added salt concentration, and temperature. They arguably attributed the repulsive force to steric repulsion developed as a result of the interactions between the tails of polymeric asphaltene molecules protruding from the bitumen surfaces. They also demonstrated that asphaltene-coated surfaces exhibit interaction characteristics similar to those of bitumen-coated surfaces. Yoon and Rabinovich10 further explored the interaction
10.1021/ie0107690 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002
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characteristics of bitumen- and asphaltene-coated surfaces using scanning force microscopy (SFM). In that work, the authors further emphasized that the bitumenand asphaltene-coated surfaces exhibited similar interaction characteristics and argued that such interactions were solely due to the brush-like asphaltene polymers. To our knowledge, data concerning direct measurements of interactions between asphaltenes and silica surfaces are not available in the literature. Asphaltenes are known to adsorb onto silica surfaces from nonaqueous solvent such as toluene.11 Few adsorption studies1 have emphasized the kinetics of adsorption and the structure of the adsorbed surfactant aggregates in nonaqueous systems without revealing the exact nature of the interactions. The main objective of the present study is to measure directly the interactions between silica and asphaltene surfaces in aqueous systems under different solution pH values and added salt concentrations. This nanoscale investigation, in general, will improve our current understanding of systems involving hydrocarbon resources and silica and help in optimizing relevant system performance. Experimental Section Materials. Sodium hydroxide and potassium chloride of ultrapurity grade (>99.99%) were purchased from Sigma-Aldrich Canada. Silicon(100) wafers were purchased from MEMC Electronic Materials, Novara, Italy. Silica spheres (colloidal probes) were obtained from Bang Laboratory, Fishers, IN. In all experiments, Millipore water of resistivity 18.2 MW cm was used as the aqueous medium. The asphaltene sample used in this work was prepared from the n-pentane-insoluble fraction of Alberta Rangeland crude oil according to the following procedure. A 400-mL portion of n-pentane was slowly added with gentle stirring to 600 mL of light crude oil. The crude-pentane mixture was immediately stored in dark environment for about 12 h with occasional stirring to avoid decomposition. The resulting asphaltene precipitate was filtered from the solution with 0.2-µm filter paper by vacuum filtration. The precipitate was further washed with excess n-pentane until the filtrate became colorless. The resulting product was vacuum-dried at room temperature for 2 h. Preparation of Asphaltene-Coated Silicon Wafers. Silicon wafers were cut into 1.5 × 1.5 cm square pieces and used as the substrates for the preparation of asphaltene-coated surfaces. In each test, the cut wafers were first cleaned with chloroform in an ultrasonic bath for 10 min in an attempt to remove all organic surface contaminants. An asphaltene-in-toluene stock solution of 1.0 mg/mL concentration was prepared. This solution was centrifuged at 20 000 rpm for 20 min to remove any inherent fine solids. In this experiment, samples from the top of the centrifuge tube were used in the spin coating of asphaltenes on the silicon wafers. A portable precision spin coater purchased from Speed Line Technologies, Franklin, MA was used. The spin coating was performed at 2000-3000 rpm. To ensure asphaltene coating on the wafer, the contact angles of water on the wafer were measured before and after the spin coating. The contact angles for silicon wafers and asphaltene-coated silicon wafers were found to be in the range of 13-15° and 90-95°, respectively. Scanning force microscopy (SFM) was used to characterize the spin-coated asphaltene surface over an area of 5 × 5
Figure 1. Schematic of force measurement between a modified spherical silica tip and an asphaltene-coated silicon wafer using scanning force microscope (AFM).
mm under a z range of 200 nm. The substrate and the spin-coated surfaces were found to be smooth, with an overall roughness of less than 1.0 nm. Surface Force Measurements. A Nanoscope E atomic force microscope (AFM) from Digital Instruments (Santa Barbara, CA) was used in the force measurements. Gold-coated silicon nitride cantilevers used in the AFM experiments were also obtained in wafer form from Digital Instruments. Cantilevers of the lever type that were 100 mm wide and had spring constants of 0.58 N/m were used for the force measurements. The configuration of the experiment is shown in Figure 1. The cantilevers were modified for the force measurements by attaching a colloidal probe (silica spheres) at the apex of each AFM cantilever using an extremely small quantity of epoxy resin. Scanning electron microscopy (SEM) images indicated that the glue was always confined to the interface between the sphere and the cantilever. The radius of the spheres used in each of the experiment was also obtained from the SEM micrographs. A detailed description of the use of AFM in force measuring mode is provided elsewhere.12,13 Briefly, the surface is approached toward the colloid probe, and the deflection of a cantilever of known spring constant under the influence of the approaching surface is measured using a laser reflected off the cantilever onto a positionsensitive split photodiode. The onset of the constantcompliance region is defined as zero separation distance, where the deflection of the cantilever is linear with respect to the approaching surface displacement. The surface separation is then estimated from the displacement of the lower surface relative to this constantcompliance region. The force acting between the probe and the surface is simply determined from the deflection of the cantilever by using Hooke’s law, F ) kx, where x represents the deflection, k is the spring constant of the cantilever, and F is the force. Force measurements were performed in a solution cell where the colloidal probe (silica sphere) interacted with a flat, asphaltene-coated silicon wafer in an aqueous solution under different experimental conditions. All force measurements were conducted at room temperature, 22 ( 1 °C, after an incubation time of 15 min unless otherwise stated. The measured force was normalized by the probe radius to ensure a quantitative comparison with theory. The force-distance profiles were analyzed using a numerical solution to Poisson-Boltzmann equation14
P ) kBT[
∑i ni,(D/2) - ∑i ni,b]
(1)
where P is the surface pressure, ni,(D/2) is the counterion concentration at the midplane (distance D/2 from the
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Figure 2. Normalized force (F/R) versus distance (D) measured between a silica probe and an asphaltene-coated surface in water at pH 5.8 after different incubation periods: ()) 15 min, (0) 30 min, (∆) 60 and 120 min. The inset shows the measured adhesive force (Fad/R) when the surfaces were pulled apart as a function of incubation time.
surface), and ni,b is the counterion concentration in the bulk. A relaxation method based on a finite mesh was used as the numerical procedure.14 Theoretical electrostatic force curves for similar and dissimilar surface interactions were generated by employing user-entered parameters of Stern layer potentials (i.e., the electrical potential where the diffuse double layer starts) and experimentally measured decay lengths. Electrokinetic Measurements. The fundamentals of electrokinetic measurements are well described elsewhere.15,16 The measurements of the zeta potentials of asphaltenes and silica were carried out with a Zetaphoremeter III instrument purchased from SEPHY CAD, LES ESSARTS LE ROI, France. The temperature was maintained at 22 ( 1 °C throughout the measurements. This instrument is equipped with a rectangular electrophoresis cell containing a pair of hydrogenated palladium electrodes, a laser illuminator, and a CCD camera. The computerized operating system allows for accurate positioning of the view field at the stationary layer for accurate measurement of electropheric mobility. Initially, the electrophoresis cell was filled with ∼30 mL of prepared suspensions in water. Through the laser-illuminating and video-viewing system, the movements of the 50-100 particles in the stationary layer were traced, five times for each direction, by alternating positive/negative electrode potentials. The captured images were then analyzed by built-in image-processing software. The distribution histogram of electric mobility and corresponding average values were determined and converted to zeta potential (ζ) values. The pH’s and ionic strengths of the suspension were continuously monitored during the measurements. Other experimental details have been well described elsewhere.17 Results and Discussion Asphaltene-Silica Interactions and Their Time Dependence. In Figure 2, the force-distance profiles between a silica colloid probe and an asphaltene-coated flat surface in pure water are shown as a function of incubation time. The pH of the system was found to be ∼5.8. As can be seen from Figure 2, a repulsive force
Figure 3. Normalized force (F/R) versus distance (D) measured between a silica probe and an asphaltene-coated surface in water at pH 5.8 with and without added salt: (∆) without salt, (×) with 1.0 mM KCl.
component developed with increasing incubation time. Initially (after 15 min of incubation), the measured force was purely attractive and extended up to about ∼30 nm. In this case, the attractive force dominated the interaction. In general, when the gradient of the attractive force vs distance exceeds the spring constant of the cantilever, the surfaces jump instantaneously into contact, during which detailed data can not be obtained by the instrument. In our experiment, however, we noted that the instrument continued to record data during the jump. As such data are nonequilibrium by definition, they were ignored in the analysis. After 30 min of incubation, a repulsive force regime extending up to ∼60 nm was measured. As a result, the attractive force component became dominant at a much shorter separation distance of ca. 10 nm. About 60 min of incubation led to a repulsive force stronger in magnitude but of approximately the same range. Meanwhile, the attractive force regime was further reduced. A prolonged incubation time beyond 60 min (i.e., 120 min) showed little impact on the measured force profile. At this point, the force measurements were continued to investigate the effects of electrolyte conditions and pH. The abovedescribed time-dependent behavior was not observed in these measurements as the asphaltene surfaces used in these cases had been already incubated in the previous measurements. The measured adhesive force when the surfaces were pulled apart followed the same trend (see inset in Figure 2). The normalized adhesive force (Fad/R) decreases significantly with increasing incubation time. At this stage (i.e., after an incubation of 120 min.), 1.0 mM KCl was added to the system, and the measured force became overall attractive, as shown in Figure 3. This observation clearly suggests that the measured repulsive force (built up over a period of time) is indeed electrostatic in origin. Many authors have reported that the asphaltene molecules contain carboxylic and/or sulfonate/sulfate acid groups.2 Therefore, this charge buildup can be attributed to the migration and orientation of spin-coated asphaltene molecules at the asphaltene-aqueous interface. Presumably, the spin coating yields randomly oriented amphiphilic asphaltene multilayers. The asphaltene molecules can reorient themselves once they come into contact with a polar medium
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Figure 4. Normalized force (F/R) versus distance (D) measured between a silica probe and an asphaltene-coated surface in 1.0 mM KCl solution at different pH values: (×) pH 5.8, ()) pH 6.8, (∆) pH 6.8 and 7.8, (0) pH 8.8, (O) pH 9.8 and pH 10.5. The inset shows the measured adhesive force (Fad/R) when the surfaces were pulled apart as a function of pH.
(water), exposing the ionic polar groups toward the water phase to attain their energetically preferred conformation at the interface. This orientation inevitably increases the surface charge density, which, in turn, causes a more pronounced electrostatic double-layer repulsive force. For a highly solidlike asphaltene surface, this reorientation takes time, showing a timedependent behavior, as observed in our surface force measurements. This set of measurements clearly suggests molecular rearrangement of asphaltene molecules at the asphaltene-water interface and its role in tuning the interaction forces between silica and asphaltene surfaces in aqueous solutions. Asphaltene-Silica Interactions and Their pH Dependence. The force-distance profiles obtained for aspahltene-silica interactions in 1.0 mM KCl as a function of pH are shown in Figure 4. The data at pH 5.8 are taken from Figure 3. At pH 5.8, the measured force was purely attractive. The measured force became almost undetected down to a separation distance of 4 nm when the solution pH was increased from 6.8 to 7.8. With a further increase in solution pH above 8.8, the measured force profile became purely repulsive, extending up to ∼50 nm with a magnitude of ∼1.1 mN/m. Under such conditions, no inward jump could be detected. Instead, the surfaces moved abruptly from an exponential repulsive regime to a hard-wall (constantcompliance) state. At pH 9.8, the range and magnitude of the measured repulsive force were higher than those at pH 8.8. On further increase in solution pH (up to 10.8), the force profiles were similar to those observed at pH 9.8. The measured adhesive force, which decreases with increasing pH, agrees with the observed trend of the long-range force profile: a more repulsive force profile corresponds to a weak adhesion force (see inset in Figure 4). The observed pH dependence of the repulsive forces largely reflects the pH-dependent ionization characteristics of the surface-active asphaltene molecules, as will be shown later. As a reference, force measurements were performed under similar conditions between two silica surfaces
Figure 5. Normalized force (F/R) versus distance (D) measured between a silica probe and a silica surface in 1.0 mM KCl solution at pH 5.8-10.5. The solid line represents a theoretical fit based on the Poisson-Boltzmann equation under constant Stern layer potential boundary conditions. Fit value: ψsi ) -55 mV.
(silica sphere-silicon wafer). For solution pH values between 5.8 and 10.5, the magnitude and distance dependence of the long-range repulsive force regime were invariant. In this case, the measured force profile can be very well described by the Poisson-Boltzmann equation using a constant Stern layer potential boundary condition (Figure 5). The Stern layer potential (ψsi), i.e., the electrical potential where the diffuse double layer starts, obtained from the best fit was found to be -55 mV. The fitted decay length of about ∼8.2 nm corresponded well to the Debye length calculated using a bulk salt concentration of 1 mM/L KCl.19 However, our electrokinetic potential measurements with the silica particles showed a slight increase in the zeta potential (ζsi) from -46.5 to -50.3 mV with increasing pH. We will discuss these issues further in the following sections. Considering the less sensitive nature of silica-silica interactions from pH 5.8 to 10.5, the increase in the range and magnitude of asphaltene-silica interactions with increasing pH clearly implies the ionizable and/or polarizable nature of asphaltene surfaces over this pH range. As mentioned earlier, amphiphilic asphaltene molecules contain carboxylic groups.2 Thus, the increase in the electrical double-layer repulsive force with pH between the negatively charged silica sphere and the asphaltene-coated surface can be explained in terms of both the properties of the electrical double layer and the degree of carboxylic acid dissociation on the asphaltene surface. The diffuse electrical double layer near the negatively charged spherical silica surface contains accumulated positively charged counterions. The carboxylic acid groups at the asphaltene-covered surface dissociate progressively with increasing solution pH, so that the net surface charge on the asphaltene surface becomes more negative. As a result, a diffuse double layer with net positively charged counterions forms. The consequence of this characteristic is a higher local concentration of H+ (and K+) and a lower local concentration of OH- (and Cl-) in both the silica and asphaltene diffuse layers. Qualitatively, the repulsive force between the negatively charged silica probe and the asphaltene increases with pH. Beyond pH 9.8, there is no further change in the measured force profile. This is
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Figure 6. Normalized force (F/R) versus distance (D) measured between a silica probe and an asphaltene-coated surface in water with 1.0 mM KCl at pH 8.8 and 9.8 on a semilogarithmic plot: (O) pH 8.8, (0) pH 9.8. The solid lines represent a theoretical fit based on Poisson-Boltzmann equation under constant Stern layer potential boundary conditions. Fit values: at pH 8.8, ψas ) -49.5 mV, ψsi ) -55.0 mV; at pH 9.8, ψas ) -58.5 mV, ψsi ) -55.0 mV. Note that the silica Stern layer potential (ψsi) is independent of pH from 5.8 to 10.
because the ionization has reached a plateau above pH 9.8. The pH dependence of the measured forces confirms the ionizable nature of asphaltene surfaces in aqueous solutions. Control of asphaltene ionization would allow for the manipulation of the colloidal behavior of asphaltene particulates. Attempts were made to describe the long-range repulsive forces observed for dissimilar surfaces (silicaasphaltene) using the Poisson-Boltzmann equation with a constant Stern layer potential boundary condition (Figure 6). Using surface force measurements of a symmetrical silica-silica system under the same solution conditions, the Stern layer potential of silica was accurately determined to be -55 mV (see Figure 5). Note that the fitted Stern layer potential of silica is relatively insensitive to pH over the pH range studied. The fitted Stern layer potential of asphaltene (ψas) was found to become more negative with increasing solution pH. At pH 8.8, the fitted ψas value was -49.5 mV and at pH 9.8, it becomes more negative (ψas ) -58.5 mV). The increase in the negative magnitude of surface potential translates into a significant increase in the surface charge density through progressive ionization of the surface acid groups on the asphalltene surface with increasing pH. According to a theoretical description,18 two surfaces with the same sign of charge but different Stern layer potentials would exhibit a repulsive force at long range but attractive at short range if one assumes that the Stern layer potentials are fixed as they approach each other (i.e., constant Stern layer potential boundary conditions). The proposed physical mechanism for this type of force profile is that the constant potential boundary condition imposes on the surface having the lower-magnitude Stern layer potential a change in the sign of its surface charge when the two surfaces are brought sufficiently close to each other. In the current system of dissimilar Stern layer potentials, this type of force profile was not observed. There are several factors that could account for the observed deviation. Among these are the ill-defined location of the plane of surface
Figure 7. Comparison between zeta potentials (ζas) and fitted electrical diffuse double-layer potentials of asphaltene surfaces from AFM (ψas) as a function of pH in 1.0 mM KCl solution. Zeta potentials of silica surface (ζsi) as a function of pH in 1.0 mM KCl solution are also shown. (0) ζsi vs pH, (9) ψsi vs pH, ()) ζas vs pH, (() ψas vs pH. The lines are guides for easy viewing.
charge resulting from the roughness of the silica probe and coated asphaltene surfaces and the exclusion of the solvent repulsion term (hydration forces) or short-range steric force from theoretical considerations. The solvent repulsion term or short-range steric force can be attributed to the stretching of asphaltene molecules as a result of the interaction between the ionizable (carboxylic) groups and water molecules. The presence of such a short-range force between two asphaltene-coated surfaces, usually much stronger than double-layer forces, was reported earlier by Yoon and Rabinowitch.10 It is interesting to note that the measured adhesion force was reduced with increasing solution pH, as shown in the inset of Figure 4. The change of the adhesion force with pH correlates well with the increase in the measured long-range repulsive forces with pH, indicating that the charge built up on the surface plays an important role in determining the adhesion force. To establish a link between the electrokinetic potential and the fitted Stern layer potential values from the AFM measurements, it is now of interest to examine the electrokinetic properties of these surfaces. Figure 7 shows the results of zeta potential measurements using silica particles (those used in the force measurement experiments) and asphaltenes under the same electrolyte conditions. For comparison, the fitted Stern layer potential values (ψas) obtained from AFM force profiles are also shown in this figure. In the case of silica particles (open squares), the zeta potential values (ζsi) varied only slightly from -46 to -50 mV over the pH range from 5.8 to 10, which is in qualitative agreement with the fitted Stern layer potential value (ψsi ) -55 mV) from the AFM measurements performed under identical solution pH conditions (see Figure 7). In the case of asphaltenes (diamonds), reasonably good agreement was obtained between the electrokinetic measurements and the fitted Stern layer potentials from the AFM force measurement results at a solution pH higher than 8. Both the ζas and ψas values obtained in the highpH solutions approached a plateau, even though the magnitudes of the two were slightly different. In lowerpH solutions, however, a significant difference was found between the fitted Stern layer potentials (ψas) and the measured electrokinetic potentials (ζas).
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To explain the increase of surface potential with increasing solution pH, a theoretical model that accounts for the effect of pH on surface dissociation is needed. In this study, a site-binding model was adopted.20 Considering an asphaltene-liquid interface, the asphaltene molecules (which bear carboxylic acid groups, RAH) undergo surface dissociation as follows + RAHinterface ) RAinterface + Hsolution
(2)
The equilibrium constant of surface dissociation is given by
KR )
(1 -R R)[H ] +
interface
(3)
i.e.
pKR ) pHinterface - log
(1 -R R)
(4)
where R is the fraction of carboxylic acid groups dissociated. [H+]interface represents the hydrogen ion concentration at the interface and is related to the bulk hydrogen ion concentration, [H+]bulk, through the wellknown Boltzmann distribution16
[H+]interface ) [H+]bulke(-eψ0/kT)
(5)
Figure 8. Normalized force (F/R) versus distance (D) measured between a silica probe and an asphaltene-coated surface at pH 9.8 under different KCl concentrations: (O) 1.0 mM, (0) 10 mM, (∆) 100 mM. The solid lines represent a theoretical fit based on the Poisson-Boltzmann equation under constant Stern layer potential boundary conditions. Fit values: 1.0 mM, ψsi ) -55.0 mV, ψas ) -58.5 mV; 10 mM, ψsi ) -25.0 mV, ψas ) -17.5 mV; 100 mM, ψsi ) -21.0 mV, ψas ) -14.0 mV. ψsi was derived from separate silica-silica interaction measurements (See Figure 9). The inset shows the measured adhesive force (Fad/R) when the surfaces were pulled apart for different KCl ionic strengths (CS).
i.e.
ψ0 pHinterface ) pHbulk + 2.303kT
(6)
where e is the elemental charge of an electron, k is the Boltzmann constant, and ψo is the electric potential at the surface. The combination of eqs 4 and 6 gives
log
(1 -R R) ) pH
bulk
- pKR +
ψ0 2.303kT
(7)
Equation 7 is the theoretical model relating the degree of ionization of the carboxylic acid groups at the surface and the surface potential. If the electrostatic factor, i.e., ψo/2.303kT is not included, eq 7 becomes identical to the relation for equilibrium in bulk aqueous solutions. The experimental data in the current study show that the complete dissociation of surface carboxylic acid groups occurs at pH ∼9.8, where the maximum surface electric potential (ψas ) -58.5 mV, the Stern layer potential in this case) is reached. At pH 8.8, the majority of the surface acid groups were dissociated, and the surface potential was found to be -49.5 mV. Below pH 8.8, only a fraction of the surface acid groups were dissociated, resulting in a low surface potential. It is important to note that the surface potential variations extend over a wider range of pH (from 5.8 to 10.5) than those observed for monocarboxylic acids in aqueous solutions.21 Such characteristics, in contrast to the solution behavior, can be accounted for by the following three mechanisms. First, because of the large electrostatic repulsion that the neighboring ionized carboxylate groups experience at the asphaltene-solution interface, deprotonation of all carboxylic acid groups at a given pH are energetically less favorable. Second, the strong lateral intermolecular hydrogen bonding among the carboxylic acid groups on the surface causes the protons to be held more tightly on the surface. Finally, for such
complex molecular assembles as asphaltene, it is not unlikely that the acids consist of a suit of homologues, each exhibiting a slightly different dissociation characteristic. Asphaltene-Silica Interactions and Their Salt Dependence. In Figure 8, the measured force profiles for asphaltene-silica interactions at pH 9.8 with different KCl concentrations are shown. The addition of salt caused a significant reduction in the range and magnitude of the forces, suggesting an electrostatic nature for the interaction force. With 1 mM KCl, the measured repulsion originated at about 60 nm, whereas increasing the KCl concentration to 10 mM reduced the range of the measured repulsive force to ∼20 nm. A further increase in KCl concentration to 100 mM caused a further reduction in the range of interaction forces. Clearly, the asphaltene surfaces in aqueous solutions exhibited typical characteristic of charged surfaces in terms of their response to increasing salt concentrations.19 To have a reference framework, the forces between two silica surfaces were also measured under similar conditions (Figure 9). The magnitude and distance dependence of the long-range repulsive force decreased with increasing salt concentration. The force profile can be very well described by the Poisson-Boltzmann equation using a constant Stern layer potential boundary condition (Figure 9). The fitted Stern layer potential as a function of KCl concentration is shown in Figure 10. As anticipated, a decrease in negative magnitude of the Stern layer potential with increasing supporting electrolyte concentration was observed. The Poisson-Boltzmann equation fits to the experimental force data (asphaltene-silica) are shown in Figure 8. The Stern layer potentials of silica were fixed using the values derived from silica-silica force profiles under similar conditions. In all cases, the theoretical calculations (solid curves) showed a good fit to the
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electrostatic repulsions among the ionized carboxylic groups. The addition of salt screens the intramolecular electrostatic repulsion, which eventually results in the collapse of aspahaltene molecules protruding from the surface. Again, the reduction in long-range electrostatic repulsion with increasing electrolyte concentration results in an increase in the measured adhesion forces, as shown in the inset of Figure 8. Summary and Conclusions
Figure 9. Normalized force (F/R) versus distance (D) measured between a silica probe and a silica surface at pH 9.8 under different KCl concentrations: (O) 1.0 mM, (0) 10 mM, (∆) 100 mM. The solid lines represent theoretical fit based on numerical solution to a Poisson-Boltzmann equation under constant Stern layer potential boundary conditions. Fit values: 1.0 mM, ψsi ) -55.0 mV; 10 mM, ψsi ) -25.0 mV; 100 mM, ψsi ) -21.0 mV.
Figure 10. Stern layer potentials of silica (ψsi) and asphaltene (ψas) surfaces derived from AFM measurements as a function of ionic strengths (KCl): (0) asphaltene, (b) silica. The lines are guides for easy viewing.
experimental force profiles over a wide range of separation distances. The fitted Stern layer potentials are shown in Figure 10. The reduction in the negative magnitude of the Stern layer potentials with increasing salt concentration further confirms the electrostatic nature of the measured long-range force. It should be noted that, in the case of 1 mM KCl, the theoretical fit deviates from the measured force profile at a separation distance below ∼20 nm, suggesting the presence of a short-range steric repulsive force. In contrast, in the presence of 10 and 100 mM KCl, the theoretical fits deviated from the measured force profiles only at the much closer separation distance of about ∼2-3 nm, suggesting the collapse of steric repulsive forces due to the added electrolytes. Considering both the pH and electrolyte-concentration dependences of the measured force profiles, it is clear that the surface asphaltene molecules in contact with aqueous solutions under highpH conditions swell, protrude, and stretch. Such swelling and stretching are sensitive to the added salt concentrations. This finding signifies that the stretching and/or swelling of asphaltene molecules are due to
Interactions between asphaltenes and silica surfaces in aqueous solutions were directly measured using an atomic force microscope. The electrokinetic properties of these surfaces were determined to establish a link between conventional electrokinetic studies and AFM results. Asphaltene-silica interactions in aqueous solution exhibited a time-dependent characteristic. An electrostatic repulsive force developed with increasing incubation time, suggesting molecular rearrangements at the asphaltene-water interface. An increase in pH caused an increase in the range and magnitude of repulsive asphaltene-silica interactions, implying the presence of pH-dependent ionizable/ polarizable groups on the asphaltene surface. Fitting the measured force-distance profile with diffuse doublelayer theory revealed an increase of surface charge density on the asphalltene surface with increasing pH. Reasonably good agreement was obtained between the fitted diffuse layer potentials and the measured zeta potential at alkaline pH. The addition of salt caused a decrease in the range and magnitude of the forces, suggesting an electrostatic nature for the forces. The decrease in negative magnitude of the fitted Stern layer potentials with increasing salt concentration confirms the electrostatic nature of the measured long-range force. In addition, the swelling or stretching of surface asphaltene molecules at high solution pH and the sensitivity of these stretched layers to added salt were also evident from the force-distance data. The measured variation of adhesion forces with incubation time (at a pH of 5.8), solution pH, and electrolyte concentration correlated well with the measured long-range force profiles, i.e, a strong long-range repulsive force corresponded to a weak adhesion force. This study, in general, contributes to our current understanding of systems involving hydrocarbon resources and silica. It provides a scientific basis for optimizing relevant system performance. It should be mentioned, however, that real systems that includes asphaltenes, silica, etc., are more complex than the system studied here in terms of various dynamic effects associated with their constituents. Acknowledgment Financial support from NSERC- Syncrude Industrial Research in Oil Sands and from Albian Sands are greatly appreciated. Literature Cited (1) Sheu, E. Y.; Mullins, O. C. Asphaltenes, Fundamentals and Applications; Plenum Press: New York, 1995 and reference therein.
Ind. Eng. Chem. Res., Vol. 41, No. 9, 2002 2177 (2) Sheu, E. Y.; Mullins, O. C. Structure and Dynamics of Asphaltenes; Plenum Press: New York, 1998 and reference therein. (3) Yarranton, H. W.; Hussein, H.; Masliyah, J. H. Water-inHydrocarbon Emulsions Stabilized by Asphaltenes at Low Concentrations. J. Colloid and Interface Sci. 2000, 228, 52. (4) Yarranton, H. W.; Masliyah, J. H. Molar Mass Distribution and Solubility Modeling of Asphaltenes. AIChE J. 1997, 42, 9960. (5) Mannistu, K. D.; Yarranton, H. W.; Masliyah, J. H. Solubility modeling of asphaltenes in organic solvents. Energy Fuels 1997, 11, 615. (6) Fenistein, D.; Barre, L. Experimental measurements of the mass distribution of petroleum asphaltene aggregates using ultracentrifugation and small-angle X-ray scattering. Fuel 2001, 80, 283. (7) Clark, K. A.; Pasternak, S. D. Hot water separation of bitumen from Alberta tar sands. Ind. Eng. Chem. 1932, 24, 1410. (8) Ese, M. H.; Yang, X.; Sjoblom, J. Film forming properties of asphaltenes and resins. Colloid Polym. Sci. 1998, 276, 800. (9) Yoon, R. H.; Guzonas, D.; Aksoy, B. S.; Czarnecki, J.; Leung, A. Role of surface forces in tar sand processing. In Processing of Hydrophobic Minerals and Fine Coal; Laskowski, J. S., Ed.; Metallurgical Society of Canada: British Columbia, Canada, 1995; p 277. (10) Yoon, R. H.; Rabinovich, Y. I. Role of surface forces in tar sand processing. In Polymers in Mineral Processing; Laskowski, J. S. Ed.; Metallurgical Society of Canada: British Columbia, Canada, 1999; p 247. (11) Acevedo, S.; Ranaudo, M. A.; Garcia, C.; Castillo, J.; Fernandez, A.; Caetano, M.; Goncalvez, S. Importance of asphaltene aggregation in solution in determining the adsorption of this
sample on mineral surfaces. Colloids Surf. A 2000, 166, 145. (12) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Direct measurements of colloidal forces using AFM. Nature 1991, 353, 239. (13) Xu, Z.; Chi, R.; DiFeo, A.; Finch, J. A. Surface forces between sphaerite and silica particles in aqueous solutions. J. Adhes. Sci. Technol. 2000, 14, 1813. (14) Grabbe, A. Double layer interactions between silylated silica surfaces. Langmuir 1993, 9, 797. (15) Hunter, J. R. Zeta Potential in Colloidal Science; Academic Press: London, 1981. (16) Masliyah, J. H. Electrokinetic Transport Phenomena; AOSTRA: Edmonton, Alberta, Canada, 1994. (17) Vergouw, J. M.; DiFeo, A.; Xu, Z.; Finch, J. A. An agglomeration study of sulphide minerals using zeta-potential and settling rate. Part II: Sphalerite/pyrite and sohalerite/galena. Miner. Eng. 1998, 11, 605. (18) McCormack, D.; Carine, S. L.; Chan, D. Y. C. Calculations of electric double layer force and interaction free energy between dissimilar surfaces. J. Colloid Interface Sci. 1995, 169, 177. (19) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (20) Healy, T. W.; White, L. R. Ionizable surface group models of aqueous interfaces. Adv. Colloid Interface Sci. 1978, 9, 303. (21) Lide, D. R. CRC Handbook of Chemistry and Physics, 71st ed.; CRC Press: Boca Raton, FL, 1990.
Received for review September 11, 2001 Revised manuscript received February 8, 2002 Accepted February 11, 2002 IE0107690
