Interactions between Mica Surfaces across Crude Oil and Asphaltene

10 Feb 2009 - The behavior of adsorbed surface active molecules at mica surfaces from crude oil and from their extracted asphaltenes dissolved in diff...
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J. Phys. Chem. C 2009, 113, 3660–3665

Interactions between Mica Surfaces across Crude Oil and Asphaltene Solutions Kathy Vuillaume† and Suzanne Giasson*,†,‡ Department of Chemistry and Faculty of Pharmacy, UniVersite´ de Montre´al, C.P. 6128 succursale Centre-Ville, Montreal, Quebec, Canada, H3C 3J7 ReceiVed: July 16, 2008; ReVised Manuscript ReceiVed: October 10, 2008

The behavior of adsorbed surface active molecules at mica surfaces from crude oil and from their extracted asphaltenes dissolved in different solvents was investigated. The interactions between mica surfaces across a crude oil and across asphaltene solutions using different solvents and humidity conditions were measured using the surface forces apparatus. The nature of the interactions between the adsorbed layers strongly depends on their composition and on the presence of dissolved water. The results clearly indicate that the adsorbed layer/oil interface for a crude oil is significantly different from that for an asphaltene/toluene interface. Introduction Asphaltenes are a solubility class of molecules found in the low-volatility portion of crude oil that is soluble in toluene but insoluble in n-alkanes. They are the most heavy and polar components of crude oils. They are formed by condensed polyaromatic structures containing small aliphatic side chains, polar heteroatoms (such as oxygen, sulfur, and nitrogen), and some metals.1 They represent about 0-20% of the crude oils. The other fractions in order of increasing molecular weight are the saturates, the aromatics, and the resins. Many studies have indicated the presence of carboxylic acid, carbonyl, phenol, pyrrole, and pyridine functional groups in asphaltenes.1-3 Due to their hydrophilic and hydrophobic functional groups, asphaltenes exhibit an important surface activity and have been shown to be able to stabilize emulsions formed during crude oil production4-7and diluted bitumen emulsions.8-10 In crude oil, the asphaltenes often associate into large structures or aggregates which are stabilized by resins, the latter being polar species soluble in n-pentane.11,12 Among the most polar components in a crude oil, asphaltenes are generally assumed to have the greatest impact on wetting.13 The wetting phenomena in oil reservoirs and on mineral surfaces have been the subject of several reviews.14 Techniques such as adsorption isotherms, contact angle measurements, and surface force measurements have provided important information about possible mechanisms involved in the asphaltene adsorption. The adsorption can be very complex as it involves asphaltene/solid interactions that depend on several factors including solvent composition, surface composition, pH, ionic strength of the medium, temperature, pressure, and amount of water present in the solvent. To study the role of asphaltenes in wetting phenomena, the asphaltenes are generally extracted from the original crude oil and dissolved in various solvents. However, extracted asphaltenes in good solvents do not behave as asphaltenes in their native solvent, that is, crude oil. Other polar and surface active molecules, in addition to asphaltenes, are present in the crude oil, making the crude oil/surface interactions very different from the asphaltene/surface interactions.15,16 Because of the very complex and variable composition of crude * To whom correspondence [email protected]. † Department of Chemistry. ‡ Faculty of Pharmacy.

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oils and extracted fractions, the wettability and surface property changes upon adsorption of asphaltene and other crude oil components are extremely challenging problems which have defied many attempts at description or generalization. In this study, we compare the behaviors of adsorbed surface active molecules at mica surfaces from crude oil and from their extracted asphaltenes dissolved in different solvents. Mica is a mineral exposing a surface with an aluminosilicate structure similar to several clay surfaces. The interactions between surfaces bearing adsorbed molecules in different solvent conditions measured using the surface forces apparatus are presented. The nature of the interactions involved between water, asphaltenes, crude oil, and solvents is addressed. Experimental Section Materials. The crude oil, supplied by IFP France, came from Campos Basin and contained 5 wt % asphaltenes. The asphaltenes were extracted from the crude oil by precipitation in an excess of n-heptane, according to the NF T60-115 method. Following this method, 4 g of crude oil was contacted with 160 cm3 of n-heptane and the mixture was heated at reflux temperature for 20 min. The precipitated material was then filtered through 0.45 µm cellulose ester filters and washed several times with hot heptane. The solvent excess in the retentate was removed by slow evaporation in an oven at 80 °C until a constant weight was obtained. Characteristics of the asphaltene aggregates in toluene have been determined using small-angle X-ray scattering for concentrations of a few weight percent. The experimental setup and the data analysis are reported elsewhere.17 Briefly, the Zimm approximation for dilute solutions in the so-called “Guinier region” enables estimation of the size and the mass of aggregates. For the asphaltenes investigated in this study, mean values of 6 nm and 105 Da were obtained for the aggregate size and mass, respectively.17 However, it is known that asphaltenes are very polydispersed systems and very large aggregates; much larger ones than 6 nm were clearly isolated.17 For similar mean values, the size and mass distribution of asphaltene aggregates extracted from Safanyia crude oil were determined using fractions obtained from ultracentrifugation and values as large as 30 nm and 106 Da were reported.18 Phosphorus pentoxide was purchased from Sigma-Aldrich. Tetrahydrofuran (THF), toluene, and heptane were purchased

10.1021/jp806298a CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

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from VWR International Inc. All organic solvents used for cleaning were filtered prior to use. Anhydrous toluene was obtained by fractional distillation under argon. Milli-Q quality water was obtained from a Millipore Gradient A 10 purification system (resistance 18.2 MΩ · cm, TOC ) 4 ppb). Ruby mica sheets were provided by S & J Trading Inc. (Glen Oaks, NY). Surface Forces Apparatus (SFA) Technique. The interaction forces between two mica surfaces across asphaltene solution or crude oil were measured using a surface forces apparatus (SFA-2000), whose principle is well described in the literature.19 Briefly, the SFA enables the force as a function of surface separation between two surfaces to be determined by measuring the deflection of a variable stiffness spring that supports the lower surface. Back-silvered mica surfaces were glued silver side down on the SFA cylindrical disks (curvature radius of 2 cm) using UV glue (Norland Products Inc., USA). The two disks were mounted in the SFA chamber into cross-cylinder geometry under a particle-free atmosphere. The distance between the two opposing back-silvered mica substrates was measured via an interferometry technique using fringes of equal chromatic order (FECO).19 The reference distance (D ) 0) was set as the adhesive contact between the two bare mica surfaces in air prior to putting asphaltene solution or crude oil between the surfaces. The normal force F(D) was measured at different separation distances D by approaching or separating the surfaces step by step at a constant velocity of 0.2 nm/s. The time required for separation distance to equilibrate between each step varied from 5 to 20 min. The equilibrium distance was arbitrarily set as the distance for which change in separation distance was less than 1 nm/min at rest. The force profiles measured on separation were achieved 1 h after the surfaces were brought, on the approach runs, to the minimum separation distance. The interaction force F(D) was normalized by the curvature radius of the surfaces R (2 cm) in order to obtain the free energy of interaction per unit area between two infinite flat surfaces W using the Derjaguin approximation:20

F(D) ) 2πRW(D)

(1)

Sample Preparation. The crude oil and the extracted asphaltenes were stored in a dark room at 25 °C. Prior to use, the crude oil was centrifuged at 4000 rpm for 1 h at 25 °C to remove any dust particles that could interfere with the measurements. The asphaltene solutions were prepared by dissolving the extracted asphaltenes in a solvent at room temperature. The different solvents used were toluene, THF, and heptane. For the force measurements involving highly concentrated samples, a droplet of the sample was placed on the lower mica surface while the separation distance between the surfaces was set at ca. 100 µm in order for the sample to be in contact with the two opposing mica surfaces. P2O5 desiccant, water, or solvent was placed in a receptacle in the bottom of the SFA chamber 24 h prior to force measurements to reach humidity-free, humid, or saturated solvent vapor condition, respectively. For the experiments involving dilute solutions of extracted asphaltenes, the main chamber was completely filled with the asphaltene solutions while the two mica surfaces were separated at a distance of 1 mm. For the anhydrous asphaltene solutions, the extracted asphaltenes were stored under vacuum in desiccators containing P2O5 for several days prior to their dissolution in anhydrous toluene. For the humid conditions, the diluted samples were prepared with nonanhydrous solvents. The experiments were carried out at 28 °C. The amount of water present in anhydrous (in the presence of P2O5 desiccant for 16 h) and

nonanhydrous solvents was determined using the Karl Fischer technique at 25 °C. The amount of water present was 600 and 250 ppm for nonanhydrous and anhydrous THF, respectively, and 35 and 1.5 ppm for nonanhydrous and anhydrous toluene, respectively. Atomic Force Microscopy (AFM). AFM (Nanoscope III, Digital Instruments, USA) was used for qualitative analysis of the asphaltene layer and the crude oil components adsorbed on mica surfaces. The asphaltenes or any surface active molecules present in crude oil were allowed to adsorb from the solution of extracted asphaltenes in toluene or from the crude oil under humid conditions for 24 h at 28 °C. Prior to imaging, the surfaces were rinsed with toluene and dried under dry nitrogen. AFM measurements were achieved in tapping mode in air at 25 °C under humid atmosphere. Imaging was performed using standard silicon probes provided by Digital Instruments with a resonance frequency of 216 kHz. All AFM images were recorded under a slow scan rate (1 Hz) to avoid sample damaging. Images were recorded in height mode. Three different areas were analyzed on each sample. Data analysis was performed using the NanoScope III software (version 512r2). Results and Discussion AFM Measurements. AFM images of an asphaltene-coated mica surface and a mica surface bearing adsorbed molecules from the crude oil are illustrated in Figure 1b and 1c, respectively. As reference, an AFM image of a freshly cleaved mica substrate is also presented (Figure 1a). The root-meansquare roughness is 9 and 15 nm for asphaltene-coated and crude oil-coated surfaces, respectively, compared to 0.22 nm for the very smooth and freshly cleaved mica substrate. The topographies of the coated surfaces exhibit heterogeneous patterns. The two-dimensional size of the aggregates varies from 25 to 100 nm for the asphaltene-covered surface and from 100 to 250 nm for the crude oil coated surface. These dimensions are significantly larger than the average size of the aggregates measured in toluene, i.e., 6 nm, suggesting an important surface aggregation. Moreover, as the AFM images were taken in air, the surface aggregation phenomenon is expected to be different from the one occurring in the oil phase. These large entities might be the result of an aggregation phenomenon occurring at the interface upon adsorption, or they might correspond to the largest aggregates present in the medium. Indeed, it has been shown in a recent study that the large asphaltene aggregates in an oil/ water emulsion preferentially adsorb to the oil/water interface, leaving the smaller ones in the continuous medium.21 Surface Force Measurements. The force profiles, i.e., the normalized interaction forces F(D)/R as a function of surface separation distance D, were measured at different times after the crude oil was injected between the two surfaces in order to follow any time evolution in the profiles toward equilibrium. Only the profiles obtained after the force profiles did not significantly change with time are presented. These profiles were measured about 24 h after the sample was deposited between the two surfaces. However, it is known that even when the amount of adsorbed layer is relatively stable, slow molecular rearrangements can occur within the layer for weeks or months before any configurational equilibrium is reached.22-24 Measurements were achieved at 28 °C on three different positions for each surface pair and for two different pairs of mica surfaces. The reproducibility of the force profiles was investigated over 3-4 days for each surface pair and indicated the absence of significant changes in the adsorbed amount or in the conformation of the adsorbed layer. As the asphaltenes in the crude oil

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Figure 1. AFM images (2 µm × µm 2) taken at 25° in air in topographic height mode of (a) mica surface, (b) asphaltene-coated mica surface, and (c) crude oil-coated surface. The asphaltenes and any other surface active molecules present in crude oil were allowed to adsorb for 24 h at 28 °C from a solution of extracted asphaltenes in toluene or from the crude oil under humid conditions. Prior to imaging, the surfaces were rinsed with toluene and dried under dry nitrogen.

Figure 2. Force profiles measured on approaching (filled symbols) and on separating (open symbols) two mica surfaces at 28 °C across a drop of asphaltene solution (5 wt % in toluene) saturated with nonanhydrous toluene vapor (9), a drop of crude oil (without added solvent) containing 5 wt % asphaltenes in humidity-free conditions (with P2O5 desiccant in the SFA chamber) ([), and a drop of crude oil in humid conditions (with water present in the SFA chamber) (2). Measurements were carried out 24 h after the surfaces were brought into contact with the samples. The adsorbed layer thickness did not significantly change with time over a period of 4 days. The reference distance (D ) 0) corresponds to the adhesive contact between the two bare mica surfaces in air prior to force measurements.

are not the only fraction containing surface active molecules, the material adsorbed on the mica surfaces from the crude oil most probably contained resins and/or lower molecular weight crude oil components. The force profiles between the mica surfaces across the crude oil are illustrated in Figure 2 and present long-ranged (200 nm) attractive interaction forces under humidity-free condition. These profiles are different from those reported in the literature for similar systems using different crude oils.25,26 These studies report smaller ranged interactions (10-80 nm) with no or with only a weak attraction on the approach force profiles. Because of the very complex and variable compositions of the different crude oils, the surface property changes upon adsorption can also be variable. Despite the discrepancy observed with the literature, the range of interactions observed in Figure 2 is in agreement with the size of the aggregates measured using AFM, i.e., 100-250 nm (Figure 1). The force profiles across the crude oil can be quite well explained by van der Waals forces at large separation distances and steric polymer-like forces at shorter

range. Indeed, the absolute minimum, or attractive well, observed at D ≈ 115 nm in the presence of P2O5 desiccant (i.e., humidity-free condition) can be ascribed to van der Waals interactions between two asphaltene layers across oil using a Hamaker constant of 10-20 J and considering refractive indexes of 1.7 and 1.5 for asphaltenes and oil, respectively.27 The attractive minimum corresponds to an energy per unit area (F/ (2πR)) of 0.25 mJ/m2. This energy would arise for a separation distance between two adsorbed layers of ca. 1 nm. Therefore, the position of the attractive well (115 nm) suggests the presence of an adsorbed layer of ca. 57 nm thick on each surface. This value ranges between the size of the isolated asphaltene aggregates measured in toluene and the size of the aggregates adsorbed on mica measured with AFM. As the asphaltenes are miscible in the crude oil, the noncompressed adsorbed asphaltene layer is diffuse and becomes compressed as the separation distance between the surfaces decreases. The shape of the force profile in the repulsive regime is similar to that encountered for the polymer-like repulsions. The polymer-like interactions vary quasi-exponentially with the distance and rise more steeply when the adsorbed layer is compressed, where excluded volume effects dominate. At the separation distances where the adsorbed layer is compressed, the steric-entropic repulsions overcome the van der Waals attractive forces so that the profile is purely repulsive. The gradual increase in interaction forces with decreasing separation distance for distances smaller than 115 nm together with the position of the hard wall, i.e., ca. 30 nm, is indicative of a very compressible diffuse adsorbed layer. A period of 1 h prior to surface separation was allowed for the compressed adsorbed layer to equilibrate. During that time, the hard wall position moved from 30 to 22 nm, indicating a rearrangement of the molecules within the adsorbed layer. In the presence of water vapor in the SFA chamber, a decrease in the attractive forces between the surfaces upon surface approach and an increase in the adsorbed layer thickness (i.e., larger distance for the position of the hard wall) were measured in comparison with the results obtained in a humidity-free condition. These observations are in agreement with previous studies on crude oils showing an increase in the thickness of the adsorbed layer in the presence of water brought about by the expected increased attraction of hydrated polar groups to the surfaces.25,26 The decrease in the attractive well can be ascribed to a decrease in the van der Waals forces probably due to a strong modification of the force field contribution in the presence

Interactions between Mica Surfaces and Crude Oil of water within the adsorbed layer. The forces in the repulsive regime are representative of a compressible adsorbed layer, but their range is longer compared to that measured in humidityfree conditions, suggesting a more swollen diffuse adsorbed layer. While the attractive forces were inhibited on approaching the surfaces in the presence of water vapor, adhesion was measured on the separation profile leading to an increase in the attraction well. The hysteresis in the force profiles indicates significant changes in the conformation and/or amount of adsorbed molecules upon compression even though the hard wall position measured on separation is not significantly different from the one measured on approach (i.e., 62 nm). Rearrangements of the adsorbed molecules upon surface approach cannot be precluded and could favor the presence of polar groups at the asphaltene/asphaltene interface, giving rise to additional attractive forces such as H-bonding or ionic attraction between the opposing polar groups. There is also a possibility of entanglement between the adsorbed molecules upon compression which would result in time-dependent attractive forces on separation. The interactions between asphaltenes adsorbed from extracted asphaltene solutions (5 wt % in nonanhydrous toluene) are significantly different from those measured in crude oil, as illustrated in Figure 2. The approach force profile exhibits longer-ranged and linear attractive forces together with a smaller hard wall position compared to the crude oil (ca. 20 nm compared to 30 nm for the crude oil). The very steep increase in repulsive forces at short range and the position of the hard wall are indicative of a very compact adsorbed layer whose thickness is comparable to the size of asphaltenes measured in toluene. Moreover, the absence of hysteresis in the force profiles as well as in the hard wall position is indicative of the absence of rearrangement of the adsorbed molecules upon compression, which is quite different from the behavior observed with the crude oil. The long-ranged linear attractive forces are characteristic of capillary condensation forces as previously observed between hydrophilic surfaces across aqueous semidilute polymer solutions28 or across bicontinuous microemulsions.29 If water were present in the system, capillary forces could arise when water droplets condense out from the bulk phase in between the surfaces and would give rise to a water bridge leading to capillary attractive forces. The molar fraction of water in watersaturated toluene determined using the Karl Fischer technique is 35 ppm at 20 °C (see the Experimental Section). According to our experimental results (Figure 2), such water traces are sufficient to induce very long ranged capillary condensation. The fact that linear long-ranged attractive forces in crude oil were not observed clearly indicates that the adsorbed layer/oil interface for the crude oil system is significantly different from the asphaltene/toluene interface. Indeed, it has been shown that more oil-wet conditions can be produced by a crude oil than by adsorption from asphaltene solutions.30 In order to investigate the role of water in these long-ranged linear attractive forces, anhydrous conditions (or humidity free) were used to prepare extracted asphaltene solutions of different concentrations and to measure the interaction forces between the surfaces (see the Experimental Section for details). The resulting force profiles are compared with nonanhydrous conditions in Figure 3 for different asphaltene concentrations. However, the adsorption was achieved at a constant asphaltene concentration of 0.5 wt %. In nonanhydrous toluene, the longranged and linear attractive forces are present and do not depend on the asphaltene concentration (Figures 3 and 1). However, in anhydrous toluene (1.5 ppm water) the linear forces are present

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Figure 3. Force profiles measured between two mica surfaces across asphaltene solution at 28 °C. A droplet of 0.5 wt % asphaltene solution was placed between the two surfaces for 18 hours. The final concentration within the chamber was then adjusted by filling the SFA chamber with asphaltene solution or pure toluene. The final concentration within the chamber was (9) 0.5% under humidity-free conditions, ([) 0.0005% under humidity-free conditions, (2) 0.5% under humid conditions, and (O) 5 ppm under humid conditions. See methodology for details regarding solution preparation and humidity control. The illustrated curves correspond to the profiles measured on the surface approach.

at low asphaltene concentration and annealed at 0.5 wt %. As it is impossible to remove all dissolved water in the extracted asphaltenes as well as in toluene by using P2O5 desiccant, one cannot preclude the presence of water traces under humidityfree conditions. As previously mentioned, it is known that asphaltenes are able to stabilize water/oil emulsions. Therefore, the absence of capillary forces above a certain asphaltene concentration (0.5 wt %) can be associated with the capacity of the asphaltenes to encapsulate and stabilize water droplets. This emulsification phenomenon would prevent contact between the free water molecules and the adsorbed layer surfaces and therefore could prevent water condensation in between the surfaces. A similar emulsification process could also explain the absence of long-ranged linear attractive forces across the crude oil. The comparison between force profiles performed in anhydrous and nonanhydrous conditions strongly suggests that the linear long-ranged forces are due to the presence of water in the system and the ability of surface active asphaltenes to stabilize water droplets in oil. In the presence of water, the hard wall position is slightly larger than under humidity-free conditions, indicating the presence of a swollen adsorbed layer under humid conditions (Figure 3). Under both humidity-free and humid conditions, the decrease in the position of the hard wall with a decrease in the concentration of asphaltenes from 0.5 wt % to 5 ppm is only 2 nm (data not shown). This observation suggests a slight desorption of asphaltenes upon dilution. Figure 4 illustrates the force profiles between asphaltenecoated mica surfaces across different nonanhydrous good solvents, i.e., solvents in which asphaltenes can be solubilized (solubility of asphaltene being larger in THF than in toluene). Asphaltene adsorption was allowed to occur for 18 h from a droplet of 5 wt % in nonanhydrous toluene deposited between the two mica surfaces. Then the SFA chamber was filled with solvents to reach a final concentration of 5 ppm. All force profiles exhibit capillary condensation forces. As the water solubility in THF is larger than in toluene, capillary interactions are not expected to be annealed if the adsorbed molecules exposed to adsorbed layer/solution interface are the same in both solvents. However, the position of the hard wall position slightly decreases as the quality of solvent increases, i.e., from 38 to 23 nm, indicating a decrease in the adsorbed amount as the quality of solvent increases. The slight decrease in the hard

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Figure 4. Force profiles measured on approaching (filled symbols) and on separating (open symbols) two mica surfaces across asphaltene solution at 28 °C. A droplet of 0.5 wt % asphaltene solution was placed between the two surfaces for 18 h before filling the SFA chamber with (9) 100% toluene, (2) 50%/50% toluene/THF, and ([)100% THF. The final concentration within the chamber was 5 ppm, and the solvents were nonanhydrous. The reference distance (D ) 0) corresponds to the adhesive contact between the two bare mica surfaces in air prior to force measurements.

Figure 5. Force profiles measured on approaching (filled symbols) and on separating (open symbols) two mica surfaces across asphaltene solution at 28 °C. A droplet of 0.5 wt % asphaltene solution was placed between the two surfaces for 18 h before filling the SFA chamber with (9) 100% toluene, (2) 75% toluene/25% heptane, and ([) 50% toluene/ 50% heptane. The final concentration within the chamber was 5 ppm, and the solvents were nonanhydrous. The reference distance (D ) 0) corresponds to the adhesive contact between the two bare mica surfaces in air prior to force measurements.

wall position occurring between the approach and separation force profiles suggests slow molecular rearrangements within the adsorbed layer due to its relatively diffuse conformation. Figure 5 illustrates the force profiles between asphaltenecoated surfaces across different nonanhydrous bad solvents and also across nonanhydrous toluene as a reference profile for good solvent conditions. The asphaltene adsorption was allowed to proceed using the same procedure as that used for good solvents. All force profiles were measured under similar conditions and similar asphaltene concentrations. An attractive linear force regime is observed for all force profiles. This result was also expected because the water solubility in these different solvents is not expected to be significantly different. The hard wall position and the range of repulsive forces increase as the quality of solvent decreases (i.e., from 75 to 83 nm), indicating an increase in the amount of adsorbed asphaltenes as the quality of solvent decreases. The hard wall position did not change between the approach and separation profiles in the bad solvents,

Vuillaume and Giasson indicating very compact adsorbed layers compared to good solvents. For solution containing more than 50 vol % heptane, aggregation of asphaltenes occurred between the two surfaces, making force measurements unreliable. In summary, the range and magnitude of the long-ranged linear attractive forces measured between asphaltene layers are independent of the quality of solvent. This is in agreement with capillary-induced forces as these mainly depend on the interfacial energy between the condensed material (water) and the bulk solution (oil), which is not expected to vary significantly for the different solvents used in this study. As previously mentioned, these capillary forces can arise from water condensation in an apolar medium or from a phase separation occurring between the surfaces as the separation distance decreases. The usual approach to describe the capillary condensation forces is based on the Kelvin equation, which gives the curvature of the meniscus of the condensed phase and the Laplace pressure across the condensed and bulk phases. The free molecules of water in the solvent are expected to condense in between the surfaces, forming a bridging lens with a constant Kelvin radius. However, there is also another approach based on thermodynamic calculations which gives rise to equivalent results but better illustrates the driving force behind the condensation or phase separation phenomena.29 According to the latter approach, the concentration of asphaltenes and the water content in the adsorbed layer are expected to be larger than in the bulk solution in which the mica surfaces are immersed. As the separation distance between the surfaces decreases, the composition of the confined material between the surfaces changes toward that of the adsorbed material. Therefore, the asphaltene/oil interface is most probably replaced by the asphaltene/water interface which is energetically more favorable at small distances where the overall composition is rather poor in oil but rich in asphaltenes and water. For the equilibrium case, i.e., using a constant Kelvin radius throughout all separation distances, the resulting equation for capillary forces can be expressed as29

(

F ) -4πR0γ cos θ 1 -

h0 2RK

)

(2)

where R0 is the radius of the curved mica surfaces, γ is the interfacial energy between the condensed phase and the bulk solution, θ is the contact angle of the condensed phase on the surface, h0 is the minimal distance for condensation to occur, and RK is equivalent to the Kelvin radius. The initial value of the water contact angle measured in the oil phase on adsorbed asphaltenes θ is unknown. The interfacial energy between water and the different solvents γ is not expected to vary significantly and ranges between 30 and 40 mN/m. Therefore, the slope of the linear part of the force profiles is not expected to depend on the solvent quality or asphaltene concentration as observed experimentally but rather on the contact angle between water and the adsorbed layer. The parameter RK mainly sets the range of the capillary interaction forces and was fixed as the inset of interaction forces. Therefore, by setting θ and γ as the only variables, the experimental profiles could be very well represented using eq 2. The two fitted parameters θ and γ are presented in Table 1. The similarity among the values obtained for γ is in good agreement with the expected interfacial energy between water and oil. Moreover, the increase in θ as the quality of solvent decreases strongly suggests a reorientation of the molecules at the adsorbed layer/ solvent interface upon change in solvent quality. The adsorbed

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TABLE 1: Parameters from the Linear Fit of Force Profiles Illustrated in Figures 3, 4, and 5 Using Equation 2 asphaltene and nonanhydrous solvent, wt % 0.5%, toluene 5 ppm, 50% toluene/50% THF 5 ppm, THF 5 ppm, 75% toluene/25% heptane 5 ppm, 50% toluene/50% heptane

RK or inset of interactions, nm θ° γ, mN/m 180 225 170 204 130

11 0 18 10 70

35 39 35 41 38

molecules expose preferentially their nonpolar parts to the oil phase in bad solvent conditions (50% heptane). Despite the fact that these fits cannot unequivocally demonstrate how the surface energy changes with solvent quality, by analyzing the results in terms of this picture, one demonstrates that the linear attractive forces can be solely attributed to the capillary condensation forces. Conclusion Force measurements between aluminosilicate surfaces across asphaltene solutions and crude oil have been carried out to better understand the interactions that can occur between crude oil components and mineral surfaces in the presence and absence of water. Asphaltenes are known to associate into large structures or aggregates in oil. Their adsorption to surfaces, which is significantly affected by their capacity to associate, led to larger colloidal-sized entities. The interactions between crude oil coated mica surfaces are mainly determined by van der Waals and polymer-mediated forces, whereas the interactions between asphaltene-coated mica surfaces are mainly dominated by long-ranged capillary forces which strongly depend on the water content in the surrounding medium. The results clearly indicate that the adsorbed layer from the crude oil system is significantly different from that from extracted asphaltene solutions. Therefore, the asphaltenes are certainly relevant components with respect to wetting alternation tendencies but are not necessarily representative of the surface active components of crude oils. Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council (NSERC). We thank Prof. R. E. Prud’homme for providing access to his AFM facilities and Thierry Palermo and Loic Barre´ from IFP for fruitful discussions. References and Notes (1) Ignasiak, T.; Strausz, O. P.; Montgomery, D. S. Oxygen distribution and hydrogen bonding in Athabasca asphaltene. Fuel 1977, 56 (4), 359– 65. (2) Moschopedis, S. E.; Speight, J. G. Investigation of hydrogen bonding by oxygen functions in Athabasca bitumen. Fuel 1976, 55 (3), 187–92. (3) Petersen, J. C. Infrared study of hydrogen bonding in asphalt. Fuel 1967, 46 (4-5), 295–305. (4) Ali, M.; Alqam, M. The role of asphaltenes, resins and other solids in the stabilization of water in oil emulsions and its effects on oil production in Saudi oil fields. Fuel 2000, 79 (11), 1309–1316. (5) McLean, J.; Kilpatrick, P. Effects of asphaltene aggregation in model heptane-toluene mixtures on stability of water-in-oil emulsions. J. Colloid Interface Sci. 1997, 196 (1), 23–34. (6) Yarranton, H.; Hussein, H.; Masliyah, J. Water-in-hydrocarbon emulsions stabilized by asphaltenes at low concentrations. J. Colloid Interface Sci. 2000, 228 (1), 52–63.

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