Article pubs.acs.org/EF
Heterogeneity of Asphaltene Deposits on Gold Surfaces in Organic Phase Using Atomic Force Microscopy Atoosa Zahabi and Murray R. Gray* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
Tadeusz Dabros CanmetENERGY, Devon, Alberta, Canada T9G 1A8 ABSTRACT: Atomic force microscopy (AFM) was used to measure the heterogeneity of asphaltenes adsorbed on gold for pentane-to-oil ratios (S/O) ranging from 0 to 0.5. Both adsorption of asphaltenes and AFM measurements were conducted in the same solvent mixture of toluene and pentane. The averaged approach and retraction heights of the deposited asphaltene material increased from 2 nm on washed surfaces to 8 nm for S/O = 0.43. The average adhesion forces during retraction of the AFM tip were greater than zero for S/O > 0.2. Repetition of the approach and retraction measurements at different locations on gold substrates showed that the deposited asphaltene aggregates are heterogeneous in height and elasticity. The extent of the heterogeneity is more evident at the onset of asphaltene precipitation (S/O = 0.43).
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INTRODUCTION The asphaltene solubility class of petroleum contains large and polar molecules that are rich in heteroatoms such as nitrogen, sulfur, oxygen, and metal components such as vanadium and nickel. The stable dispersion of these asphaltenes in crude oil can be destabilized by adding a paraffinic solvent, such as pentane or hexane, resulting in precipitation.1 Precipitated asphaltene can flocculate water droplets and solid particles, which is extremely useful in producing dry and solids-free bitumen from a water rich froth.2 The same mechanism has potential for removal of fine solids from refinery streams3 and for retaining solids in commercial processes for solvent extraction of oil sands. This flocculation and precipitation behavior is governed by the forces among the asphaltene nanoaggregates, solid surfaces, and oil−water interfaces. In our previous work,3 we showed that destabilization of a suspension of silica particles in a model oil solution occurred when the ratio of added pentane was well below the amount required for the onset of asphaltene precipitation. The silica surfaces in this case were modified by adsorption of asphaltenes. Such adsorption appears to be irreversible, or very slowly reversible in solvent mixtures, and gives long-term modification of the surface properties.4 van der Waals and steric forces are the main forces that are observed between such asphaltene coated surfaces in organic solvents.5 Many researchers have measured the colloidal forces between surfaces in aqueous systems using atomic force microscopy (AFM). However, force measurement in nonaqueous systems has been studied less due to difficulties with the liquid cell of AFM. There are only a few studies applying force measurements between asphaltene coated surfaces in nonaqueous media.5−7 Wang et al.5,7 measured force curves between asphaltenes deposited on oxidized silica wafers and silica spheres using AFM in organic solvents. The organic phase was a heptane−toluene mixture at different ratios (Heptol). They © 2012 American Chemical Society
used two methods for preparing asphaltene layers: the deposition of a Langmuir−Blodgett film on the solid substrate and dip-coating from a 2 g/L solution of asphaltenes in toluene. In pure toluene, the AFM approach mode detected steric longrange repulsion between asphaltene coated surfaces. As the fraction of heptane in the solvent phase was increased, the distance for the steric repulsion decreased from roughly 50 nm in pure toluene to 3 nm in pure heptane. The coating preparation method affected the magnitude and range of these repulsive forces. Using the LB method, the compacted and organized deposited asphaltenes on the surface resulted in larger repulsive forces over larger distances, as compared to the interaction between asphaltene surfaces prepared by the dip coating method. The coating of asphaltenes on surfaces was more organized when the LB method was used for coating the substrate. The applied force in LB method would compel the asphaltene molecules to orient in organized layers on the substrate. This force is absent in the dip coating method. At a toluene volume fraction in heptane 0.2, becoming gradually stronger with increasing pentane concentration, then jumping to a much higher value of 1.5 mN/m after the onset of asphaltene precipitation at S/O = 0.43. Some of the results of Wang et al.7 were obtained for dipcoated samples in toluene, which can be compared to the results of this study. They reported that the characteristic distance for the approach and retraction curves was 36 nm in toluene for the case where both the sample and the AFM tip were coated. Some hysteresis was observed between approach and retraction curves, on the order of 5−10 nm. For deposition on only one surface (not on the AFM tip), the Wang et al.7 results would suggest a distance of ca. 18 nm. In the case of our work, for S/O = 0, the threshold distances on the approach and retraction curves were 2.9 and 2.6 nm (Figure 5), which were only slightly larger than the values for the cleaned surfaces. Three differences could contribute to the discrepancies; our surfaces were rinsed with toluene before use, which could remove weakly attached material and reduce the film thickness, our asphaltenes had a different thermal history, and we used gold rather than silica. In our opinion, the difference in washing is the most likely reason for the much smaller distance of interaction. We would anticipate two stages of deposition in our experiments, which immersed the gold surface in asphaltene solution for 2 h. First, asphaltene nanoaggregates would directly adsorb on the surface of the substrate through several kinds of intermolecular interactions to form a single layer of adsorbed materials. Subsequent deposition would result from asphaltene−asphaltene interactions. The subsequent solvent rinse would remove some of this secondary material. Wang et al.7 did not observe adhesion on the retraction curve in pure toluene, consistent with the data of Figure 6. For Langmuir−Blodgett films, Wang et al.5 reported the adhesion forces for 20−70% heptanes in toluene to be in the range of 0.13 ± 0.7 mN/m, with no significant dependence on concentration. They did not comment on the unusual variability in their results. We attribute the 10-fold difference in the adhesive forces to the use of a gold-coated AFM tip in our study, rather than two asphaltene-coated surfaces in Wang et al.5 The results in Figure 6 suggest that the interaction of asphaltene films with clean surfaces, such as clay minerals, would be much stronger than the asphaltene−apshaltene forces measured by Wang et al.5 This adhesion force increased dramatically near the onset concentration of 0.43, then remained almost constant at S/O = 0.5.
Figure 5. Threshold distance for repulsion and for extension by the deposited materials on the gold substrate for various S/O values (weight ratio). The error bars show the standard deviation on 10 points on the gold surface from two different substrate samples for each value of S/O. The dashed line shows the deflection distance due to residual adsorbed components on a substrate washed with toluene and chloroform (Figure 3).
The mean threshold distances in the approach and retraction curves are comparable for each S/O, showing that the deposited aggregates have similar mean elongation due to retraction of the AFM tip, and distance extended into the solvent phase (approach curve), within the standard deviation of the means. In these experiments, we did not attempt to determine the absolute thickness of the deposits at full compression by the AFM tip by masking a portion of the surface or by attempting to remove the asphaltenes from a section of the surface. The threshold distances below S/O = 0.43 approach a value of 2 nm, which is very close to the result from the washed surface. These distances increase significantly after S/O = 0.43 (8−12 nm), due either to material extending from the aggregates into the solution or due to swelling of the asphaltenes to increase the distance between the initial interaction of the tip and full compression of the deposit. Natarajan et al.16 used a surface force apparatus to measure absolute thicknesses of films of asphaltenes, and confirmed swelling behavior in toluene. The data of Figure 6 show the adhesion forces between the tip and asphaltene coated substrates at various S/Os. In pure toluene, the force of interaction was not significantly different from zero. With addition of pentane, the results show that the asphaltene aggregates attach to the gold tip of the AFM probe in the oil solution and extend in the solution as the tip moves away from the surface (retraction curve). Nonzero adhesion 2895
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Heterogeneous Deposition of Asphaltene on Gold Substrate. Fundamentally different types of force curves in the approach and retraction curves were observed depending on the location of the tip on the surface. Examples of force curves that were observed at S/O = 0.43 at different locations on the same substrate are shown in Figure 7. The distance that the
Figure 8. Distribution for the type of the adhesion forces in the retraction curves, with deposition at S/O = 0.43 and S/O = 0.2 (by weight), measured in pentane−toluene = 0.43 (n = 30) and pentane− toluene = 0.2 (10 points), respectively.
Figure 7. Representative force measurements between the AFM tip and the asphaltene coated surface prepared in oil−pentane solution at S/O = 0.43 (by weight) for the (a) approach and (b) retraction curve. The medium was pentane−toluene = 0.43.
Figure 9. Normalized frequency of adhesion forces during retraction of the AFM tip, measured in pentane−toluene = 0.43 and 0.2, based on ranges of 0.4 mN/m.
cantilever started deflecting during the approach curve varied with location, and the shape of the retraction curves also varied with location. Clearly, the mean values in Figure 5 and Figure 6 fail to describe the true variability of the surface deposits. The data of Figure 8 show the histogram for the frequency of the three different types of force curves at S/O of 0.2 (far below the onset of asphaltene precipitation) and 0.43 (onset of asphaltene precipitation). The discrepancy in the shape of the retraction curves (Figure 8) at different positions on the same substrate shows that the structure and behavior of the asphaltenes on the surface differs from point to point. The main observed pattern of the adhesion forces on the retraction curves for S/O = 0.2 were the simple structure, while a few points showed loose structure. For S/O = 0.43 the majority of the retraction curves exhibited loose or multiple adhesions. The variation in the retraction curves shows that the tip has attached to different types of deposited materials on the substrate, giving a strong indication of heterogeneous properties of the deposited material. The data in Figure 9 show that aggregates with both strong (2.4 to 2.8 mN/m) and weak (0.4 to 0.8 mN/m) attachment to the tip have been observed at different locations on the substrate for S/O = 0.43. The range in the adhesion forces was significantly lower for S/O = 0.2
compared to S/O = 0.43, and values were weaker (0 to 1.5 mN/m) compared to S/O = 0.43 (0.4 to 2.8 mN/m). The two sets of data have different numbers of measurements (n = 30 versus n = 10) which limits statistical comparison, but the data clearly show significant differences in the nature of the deposits in each case, and for data for adhesion and forces and distances of interaction, as discussed below. The data of Figure 10 and Figure 11 compare the threshold distances for the interactions of the AFM tip with the deposited asphaltenes on the approach and retraction curves, respectively. For S/O = 0.43, both distributions of the approach and retraction distances show a mode at 6−8 nm, specifically 7.4 ± 0.2 nm in the approach curve and 6.8 ± 0.2 nm in the retraction curve. However, the maximum distances for adhesion (16−18 nm) were larger than the maximum distances for repulsion (14−16 nm). This result suggests that the asphaltene aggregates elongate more due to the retraction of the AFM tip than they would normally extend into the solution. The threshold distances in the approach curve (Figure 10) and the retraction curve (Figure 11) have a narrower range for S/O = 0.2 (0−8 nm) in comparison to S/O = 0.43 (0−16 nm). 2896
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both in thickness and in adhesion strength, and that the two features were not highly correlated. In addition, the variety of the patterns of adhesion forces (simple, loose, and complex) on the gold substrate suggests distinctly different structures of nanoaggregates on a single solid substrate. The heterogeneity was most pronounced at and above the onset of precipitation (higher S/O). The approach curves show patches of aggregates with different heights and pronounced steric repulsion, while the retraction curves showed different structures of deposited materials with a range of elasticity and lengths with location. Amin et al.9 also observed more heterogeneity in asphaltene deposits when unstable material was rapidly adsorbed on the surface than when the asphaltenes were stable in solution. The height of the tallest surface features at the onset of asphaltene precipitation was reported to be 600 nm, reducing to 400 nm with a stronger solvent solution.9 Similar to their work, we observed lower heterogeneity (i.e., all shorter aggregates with a narrower distribution, Figure 10) on the gold substrate when asphaltene has a higher solubility in the solution (S/O = 0.2) as compared to the case of a higher solvent ratio with less stable asphaltene (S/O = 0.43). The fact that Amin et al.9 used heptane, rather than the pentane solvent in this study, did not affect the similar trend. For all the S/Os, the force measurement was done at different positions on the substrate. In all the positions, there was no evidence of negative deflection, that is, attraction interaction in the approach curve (the main interaction between uncoated gold−gold is attractive VDW forces shown in Figure 2). Therefore, we conclude that in all the cases the probe contacted a position was been covered with adsorbed asphaltene, either aggregates or residual molecular material, and no bare points were detected by force measurement. The diameter of the AFM tip that we used was 42 nm, and the measurement of heights in the approach and retraction curves shows aggregates with sizes less than 18 nm at the highest S/O. Due to the larger size of the tip compared to the aggregate size, we can assume that the tip could contact multiple features on the surface simultaneously, which would result in some averaging while approaching and retracting from the substrate. We showed that the deposition of asphaltene on gold substrate is heterogeneous; however, by AFM measurement, we cannot elaborate on the origin of heterogeneity. We measured the height and adhesion forces of the deposited aggregates; however, it is not clear whether the measurements are on single patch of deposited aggregates with different adhesion forces or on thicker deposits that lead to the formation of the patches. Therefore, we cannot define from these data whether the heterogeneity in measured forces is due to variation in chemical composition of the aggregates or to variations in the thickness of deposition. One motivation for measuring force curves by AFM is to predict how particles or emulsion droplets will interact in different solutions of crude oil and solvent. For example, silica suspensions in the same oil solution were stabilized by means of steric repulsion due to the asphaltene materials adsorbed on particle surfaces.3 By adding pentane, more asphaltene precipitate starts forming in the solution, which adsorbs on the silica particles and coprecipitates suspended particles as well. Upon precipitation, a large portion of silica and asphaltene aggregates settle rapidly, with rapid settling of silica particles above S/O = 0.33, even before the onset of precipitation. Figure 12 compares the residual silica concentration after settling at a given S/O value, measured as wt % ash in the
Figure 10. Normalized frequency of the threshold distance during approach (the distance where the cantilever starts deflecting) measured in pentane−toluene for S/O = 0.43 (n = 30) and S/O = 0.2 (n = 10). This distribution of the distances on the approach curve shows presence of patches of asphaltene with different heights on the gold substrate.
Figure 11. Normalized frequency of the threshold distance during retraction (returning of the cantilever to zero deflection) measured in pentane/toluene for S/O = 0.43 (n = 30) and S/O = 0.2 (n = 10). This distribution of the distances for adhesive forces shows that the asphaltene material attached to the tip extends and breaks at different distances from the gold surface.
The narrower distribution for the threshold distances on the approach curves shows a more uniform deposition of material for S/O = 0.2. For S/O = 0.43, the threshold distances in the approach curves have a broad distribution (2−16 nm), which implies thicker deposits in some locations. The range in threshold distances for the retraction curve for S/O = 0.43 (2− 18 nm) is higher than S/O = 0.2 (0−8 nm) as well, showing that the elongation of some of the aggregates depends strongly on the solvent conditions. For S/O = 0.43 (by weight), the approach and retraction curves varied significantly at different locations on the gold substrate. The correlation coefficient was calculated between the threshold distances on the approach and the threshold on retraction, and the value was 0.61. This value shows that the differences in the maximum separation distance for the adhesive forces from point to point were not simply due to the thickness of the asphaltene coating at each location. From this result, we conclude that the deposited asphaltenes were heterogeneous 2897
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particles. Although the mean values for interaction distances and forces followed a definite pattern, probing the gold substrate at different locations at S/O = 0.43 showed significant differences between the measured parameters from point to point in terms of the height and elasticity of the asphaltene aggregates. Less heterogeneity was observed at S/O = 0.2.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wiehe, I. A. Process Chemistry of Petroleum Macromolecules; CRC Press: Boca Raton, FL, 2008; Vol. 121, p 427. (2) Long, Y.; Dabros, T.; Hamza, H. Stability and Settling Characteristics of Solvent-Diluted Bitumen Emulsions. Fuel 2002, 81, 1945−1952. (3) Zahabi, A.; Gray, M. R.; Czarnecki, J.; Dabros, T. Flocculation of Silica Particles from a Model Oil Solution: Effect of Adsorbed Asphaltenes. Energy Fuels 2010, 24, 3616−3623. (4) Jin, Y.; Liu, W. K.; Liu, Q.; Yeung, A. Aggregation of Silica Particles in Nonaqueous Media. Fuel 2011, 90, 2592−2597. (5) Wang, S. Q.; Liu, J. J.; Zhang, L. Y.; Masliyah, J.; Xu, Z. H. Interaction Forces between Asphaltene Surfaces in Organic Solvents. Langmuir 2010, 26 (1), 183−190. (6) Long, J.; Xu, Z. H.; Masliyah, J. H. Single Molecule Force Spectroscopy of Asphaltene Aggregates. Langmuir 2007, 23 (11), 6182−6190. (7) Wang, S. Q.; Liu, J. J.; Zhang, L. Y.; Xu, Z. H.; Masliyah, J. Colloidal Interactions between Asphaltene Surfaces in Toluene. Energy Fuels 2009, 23 (1), 862−869. (8) Lord, D. L.; Buckley, J. S. An AFM Study of the Morphological Features that Affect Wetting at Crude Oil−Water−Mica Interfaces. Colloids Surf., A 2002, 206, 531−546. (9) Amin, J. S.; Ayatollahi, S.; Alamdari, A. Fractal Characteristics of an Asphaltene Deposited Heterogeneous Surface. Appl. Surf. Sci. 2009, 256, 67−75. (10) Zahabi, A.; Gray, M. R.; Dabros, T., Kinetics and Properties of Asphaltene Adsorption on Surfaces. Energy Fuels 2011, Submitted September 2011, 18 pp. (11) Heinz, W. F.; Hoh, J. H. Spatially Resolved Force Spectroscopy of Biological Surfaces using the Atomic Force Microscope. Trends Biotechnol. 1999, 17, 143−150. (12) Zahabi, A.; Gray, M. R.; Dabros, T. Kinetics and Properties of Asphaltene Adsorption on Surfaces. Energy Fuels 2012, 26 (2), 1009− 1018. (13) Suzuki, A.; Ho, N. F. H.; Higuchi, W. I. Predictions of Particle Size Distribution Changes in Emulsions and Suspensions by Digital Computation. J. Colloid Interface Sci. 1969, 29, 552−564. (14) Yang, C.; Dabros, T.; Li, D. Kinetics of Particle Transport to a Solid Surface from an Impinging Jet under Surface and External Force Fields. J. Colloid Interface Sci. 1998, 208, 226−240. (15) Masliyah, J. H.; Bhattacharjee, S. Electrokinetic and Colloid Transport Phenomena; John Wiley and Sons: New York, 2006. (16) Natarajan, A.; Xie, J.; Wang, S.; Liu, Q.; Masliyah, J. H.; Zeng, H.; Xu, Z. Understanding Molecular Interactions of Asphaltenes in Organic Solvents Using a Surface Force Apparatus. J. Phys. Chem. 2011, 115, 16043−16051.
Figure 12. Concentration of silica particles remaining after settling versus adhesive force measured by AFM at the same S/O. Force data are from Figure 6, without error bars for clarity, while data on silica, measured as ash content, are from Zahabi et al.3
suspension, against the adhesive force measured by AFM on gold substrate at the same S/O value. The data suggest that, above a critical adhesive force of 0.4 mN/m, based on AFM results, the flocculation and sedimentation of the silica particles is rapid. This condition corresponds to S/O = 0.33. At this point, the interaction between the silica particles is positive enough to develop stable flocs, giving fast settling rates. Higher S/O gives stronger adhesion but no further improvement in sedimentation. In addition, we can say that the VDW forces have become dominant over the steric repulsion between the silica particles. AFM results indicate that, at higher S/O, asphaltene deposits with a more heterogeneous structure in terms of length and elastic characteristics would form on solid surfaces. The average height of the deposited material on the gold substrate changed little for 0.2 < S/O < 0.33. Therefore, steric repulsion does not change, and there is no apparent collapse in the barrier to contact at S/O = 0.33 (unstable suspension) compared to S/O = 0.2 (much more stable suspension). The adhesive forces between surfaces, on the other hand, increased progressively with pentane concentration in the range 0.2−0.5. Given heterogeneous deposition of asphaltene on the surfaces, the conditions for attachment would depend on the local deposits, rather than the mean values in Figure 12. Above S/O = 0.33, the interaction between the asphaltene with longer and more adhesive structure adsorbed on silica particles with heterogeneous surfaces would overcome the steric repulsion between the particles, leading to bridging and flocculation.
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CONCLUSIONS The average height of the asphaltene aggregates on the approach (showing the extension of the aggregates in the solution) and retraction curve (showing the elongation of the aggregates due to AFM tip) were around 3−5 nm in pentane− oil solutions, for S/O increasing from 0 to 0.33. The average height of the aggregates increased to 8 nm at the onset of asphaltene precipitation at S/O = 0.43. The adhesion forces during retraction were less than 0.5 mN/m at low S/O, and increased to 1.5 mM/m at the onset point of precipitation. Prior data for stability of particle suspensions stabilized by asphaltenes indicated that an attractive force of 0.4 mN/m was sufficient to cause flocculation and sedimentation of the fine 2898
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