Study of Asphaltene Adsorption on Kaolinite by X-ray Photoelectron

Article pubs.acs.org/EF

Study of Asphaltene Adsorption on Kaolinite by X‑ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectroscopy Shanshan Wang,†,‡ Qi Liu,‡ Xiaoli Tan,‡ Chunming Xu,† and Murray R. Gray*,‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

‡

ABSTRACT: The interaction between asphaltenes and clay is crucial in understanding wettability changes in petroleum reservoirs and in oilsands production. In this study, we report the changes in surface properties and composition of kaolin clay as a result of exposure to solutions of asphaltenes. Adsorption experiments were conducted at 25 °C with solutions of asphaltenes in toluene at concentrations ranging from 0.05 to 5 mg/mL. The wettability of the modified kaolinite surface was characterized by contact angle measurement. Chemical composition changes of the surface because of asphaltene adsorption were assessed using time-of-flight secondary ion mass spectroscopy (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and elemental analysis. The contact angle data indicated that, upon asphaltene adsorption, the clay particles changed from water-wet to bi-wet. Both ToF-SIMS and XPS measurements indicated that the kaolinite surface was never completely covered by asphaltenes based on the concentration of Al and Si on asphaltene-treated kaolinite surfaces. ToF-SIMS analysis indicated that, with more asphaltenes covered on the kaolinite surface, the relative intensities of C3H5+/Al+ and C3H5+/Si+ increased and an inverse linear correlation between the contact angle and surface Si+ concentration was observed. The maximum thickness of adsorbed asphaltenes, assuming complete surface coverage, was estimated to be 11 nm based on XPS depth profile results on a model silica surface, with a mean value of 3 nm. The XPS depth profile also indicated no preferential adsorption of nitrogen- or sulfur-rich species at the interface between asphaltenes and kaolinite.

1. INTRODUCTION The adsorption of asphaltenes on the surface of various clay minerals plays an important role in the oil industry. The strong adsorption of asphaltenes can render the mineral surfaces hydrophobic, changing the wettabilility from water-wet to oilwet and, thereby, changing the location and distribution of reservoir fluids.1−5 In bitumen extraction from oilsands, the wettability of clays and other fine solids determines whether the particles remain in the aqueous phase, partition to the interface to stabilize water-in-oil emulsions,6−8 or contaminate the bitumen product in solvent extraction processes.9,10 Asphaltenes are defined as the fraction of petroleum soluble in toluene but insoluble in alkanes. These components are the most polar and heterogeneous fraction in petroleum and include condensed aromatic rings, alkyl chains, and various functional groups based on nitrogen, oxygen, sulfur, and trace amounts of vanadium and nickel. Because of those polar functional groups, asphaltenes can interact strongly with each other to form 2−10 nm aggregates in the crude oil,8,11 at oil−water interfaces, and with mineral surfaces, as noted above. The forces that can contribute to adsorption on mineral surfaces may include electrostatic, charge-transfer, van der Waals, and hydrogenbonding interactions.12,13 A number of studies have examined asphaltene adsorption on clay minerals, glass, and metallic surfaces.14−19 Both mono- and multilayer adsorption have been reported by different researchers, and the effects of different factors on asphaltene adsorption have been investigated.13 Dudasova et al.16 studied the adsorption of asphaltenes from five different petroleum sources onto different organic surfaces and observed a saturated Langmuir-type adsorption. They concluded that the adsorption © 2013 American Chemical Society

depends more upon the solid surface than upon the origin of the asphaltenes, but they correlated the amount of nitrogen in the asphaltene samples and its adsorption density on the solids. Gonzalez et al.20 concluded that the heteroatomic species in asphaltenes were important for surface adsorption. The majority of adsorption studies did not observe a simple equilibrium; for example, Zahabi et al.21 reported that the adsorption of asphaltenes on solid surfaces at different concentrations showed multilayer deposition without reaching equilibrium after 16 h. Another important factor that may affect the adsorption of asphaltenes is water. Water can easily be adsorbed on mineral surfaces to form a water film between asphaltenes and the mineral surface, even in the presence of solvents.22 Saada et al.23 compared the hydrophilicity/hydrophobicity of illite and kaolinite and showed that a larger amount of asphaltenes was adsorbed on kaolinite, while illite showed more affinity for water. Surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), offer ways to determine the nature of chemical species present on the particle surface on an area average basis (XPS) or on a particle-by-particle basis.24 XPS is a quantitative surface analysis technique that can measure the elemental composition and chemical states of elements.7,25,26 These methods have the potential to define the chemical groups in the asphaltene material that mediate adhesion to surfaces, the thickness of surface layers, and the extent of the Received: January 23, 2013 Revised: April 13, 2013 Published: April 16, 2013 2465

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particles, and then rinsed with ca. 20 mL of toluene until the filtrate was colorless. The separated mineral particles were then placed in a vacuum oven at 80 °C overnight for later analysis. The equilibrium concentrations of asphaltene were determined with an ultraviolet− visible (UV−vis) spectrophotometer (Cary 50, Varian). The amount of adsorbed asphaltenes was calculated from the difference of the concentration of the asphaltenes in solution before and after adsorption. This technique has been widely used to measure the adsorption of asphaltene.11,13 Because of the spectrophotometer setup and the very high absorption of asphaltene−toluene solutions at higher concentrations, various dilutions were prepared from the initial concentrations. Three different calibration curves at different wavelengths (350, 450, and 600 nm) were necessary to cover the wide range of solution concentrations, always keeping the solution concentration in the range for application of the Beer−Lambert law; i.e., absorbance < 1.5. 2.2.2. Asphaltene Adsorption on a Silicon Wafer. Silicon wafers were used to provide a flat silicon dioxide surface. The silicon wafer disks were cut into 1 cm2 pieces, which were sonicated in toluene and then ethanol to clean and remove any surface contamination. Thereafter, the silicon wafer pieces were immersed in a 5 mg/mL asphaltene solution at room temperature, and the container was sealed to avoid solvent evaporation. After 48 h, the disks were removed from asphaltene solution. Before drying, some silicon wafer disks were washed by toluene, while some were not washed. All dried silicon wafer pieces were then placed in a sample container. 2.3. Contact Angle Measurement. The original kaolinite and the dried asphaltene-treated kaolinite samples from adsorption experiments were pressed into flat pellets with a diameter of 10 mm under 3500 psi. The contact angles of kaolinite samples were determined by the sessile drop method; i.e., water drops were placed on the pressed kaolinite pellets, and the contact angles were measured by the outline of the water drops. The measurements were made using a contact angle goniometer (FTA200, Portsmouth, VA) equipped with an optical microscope and illumination system. For each sample, contact angles were measured 3 times and the average value was reported. 2.4. Elemental Analysis. A Vario MICRO cube elemental analyzer (Elemental Analysis, Hanau, Germany) was used to analyze the carbon, hydrogen, nitrogen, and sulfur of asphaltenes and asphaltene-treated kaolinite. The sample was catalytically combusted at 1200 °C. Asphaltene samples (2−5 mg) or asphaltene-treated kaolinite samples (15−30 mg) were weighted for this analysis. 2.5. XPS and XPS Depth Profile Measurement. Pellet samples for XPS and ToF-SIMS measurements were prepared by the same method as described above for the contact angle. X-ray photoelectron spectroscopic analysis was performed using a Kratos Axis 165 spectrometer with monochromatic Al Kα radiation (hυ = 1486.71 eV). The spectrometer was calibrated by the binding energy (84.0 eV) of Au 4f7/2 with reference to the Fermi level. The pressure of the analysis chamber during experiments was less than 5 × 10−10 Torr. A hemispherical electron-energy analyzer working at the pass energy of 20 eV was used to collect core-level spectra, while the survey spectrum within a range of binding energies from 0 to 1100 eV was collected at an analyzer pass energy of 160 eV, a step size of 0.33 eV, and a dwell time of 31 ms (average of two scans). High-resolution spectra over the C 1s, S 1s, N 1s, O 1s, Si, and Al regions were acquired with a pass energy of 40 eV and a dwell time of 160 ms (average of six scans). Nonlinear optimization using the Marquardt algorithm (Casa XPS) was used to determine the peak model parameters, such as peak positions, widths, and peak intensities. The model peaks to describe XPS core-level lines for curve fitting were a product of Gaussian and Lorentzian functions. A depth profile of the XPS spectrum was obtained by combining a sequence of ion gun etch cycles interleaved with XPS measurements. A Bi3+ ion gun was used to etch the material for a period of time, while XPS spectra were acquired. Each ion gun etch cycle exposes a new surface, and the XPS spectra provide the means of analyzing the composition of these surfaces. For comparison, the method was applied to both a pellet sample and an oriented-film sample prepared by the filter-transfer method, following Moore and Reynolds.34

exposed mineral surface. For example, in a study on adsorption of asphaltenes on stainless steel, Abdallah and Taylor25 identified various functional groups associated with the adsorbed asphaltenes, such as carboxylic acid, pyrrole, pyridine, thiophene, and sulfide. Rudrake et al.1 studied asphaltene− metal interactions by XPS and estimated that the thickness of asphaltenes was 6−8 nm. Durand and Beccat27 analyzed reservoir sandstone with XPS and demonstrated that XPS seemed to be an attractive tool to evaluate wettability. ToFSIMS is another widely used surface technique, which has the advantage to recognize molecules through their characteristic fragments and its high sensitivity for elements. Priest et al.28 inferred wettability of heterogeneous surfaces by ToF-SIMS and correlated surface coverage of each component with ToFSIMS analysis and contact angle. Their results demonstrate that ToF-SIMS analysis can provide a quantitative analysis of the surface coverage of an octadecylphosphonic acid monolayer on mica, and the technique may have application to a range of mineral surfaces. Bensebaa et al.29 characterized different organic-treated solids in Athabasca bitumen with ToF-SIMS by comparing the relative intensity of Al+ and C2H3+; the amount of hydrocarbon in the top surface layer of different solids was estimated. In oilsands deposits, kaolinite is the most abundant clay mineral.30,31 Kaolinite is a layered aluminosilicate, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedral, with successive 1:1 layers hydrogen-bonded to form pseudo-hexagonal platy particles.32 Because of the two types of external basal face and the broken bonds at particle edges combined with cation exchange from isomorphous substitution near the surfaces, kaolinite can exhibit a strong affinity for organics.32 In this study, XPS and ToF-SIMS are employed to study the interactions between asphaltenes and kaolinite. Our objectives were to study how asphaltene adsorption changes the kaolinite surface and to establish a correlation between wettability changes in the clay and the chemical composition of the asphaltenes.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagent-grade toluene and n-pentane purchased from Fisher Scientific (Mississauga, Ontario, Canada) were used as received. Kaolinite from Ward’s Natural Science (China clay, item 460005, d(0.5):0.377 μm) was used as the adsorbent. The surface area of the particles is approximately 20−25 m2/g based on particle size analysis (Mastersizer 2000, Malvern Instruments, Westborough, MA). P-type silicon dioxide wafers (100) with a 4 in. diameter and a resistivity of 1−35 Ω cm were purchased from University Wafer (Boston, MA). The Athabasca asphaltenes used in this study were derived from the bottom stream of a deasphalting unit processing bitumen from a steam-assisted gravity drainage (SAGD) operation.33 The asphaltenes were prepared by dissolution in toluene and filtration through a 0.22 μm Millipore filter to remove toluene insoluble. The asphaltenes were precipitated with n-heptane and then dried in an oven at 80 °C overnight. 2.2. Adsorption Experiments. 2.2.1. Asphaltene Adsorption on Kaolinite. Adsorption experiments were carried out at a fixed ratio of kaolinite mass/solvent volume, i.e., 40 mg/mL solution. Asphaltene solutions at different concentrations (initial concentrations in the range of 0.05−5 mg/mL) were prepared by dissolving dried asphaltenes in toluene followed by sonication for 30 min. The asphaltene−clay−solvent mixture was shaken in a circular shaker for 24 h at 25 °C. Afterward, the suspension was allowed to settle for 12 h, then filtered through 0.22 μm Millipore filters to separate the mineral 2466

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2.6. ToF-SIMS Measurement. ToF-SIMS spectra were obtained from a ToF-SIMS IV instrument (ION-ToF GmbH). Bi3+ ions were used as an analytical source, operated at 25 kV in a static mode. An area of 64 × 64 μm per sample was analyzed with collecting pixels of 128 × 128. The positive-ion spectra, as a function of mass, were calibrated using the H+, Na+, and C3H5+ peaks. Negative spectra were calibrated with C−, O−, and C2−.

3. RESULTS Figure 1 presents the adsorption of asphaltene on kaolinite as a function of the asphaltene concentration, which suggests the

Figure 2. Carbon content and amount of asphaltene adsorbed on kaolinite by UV−vis absorbance are linearly correlated. The line shows the linear regression of the data (R2 = 0.99974).

Characterization of the surface of kaolinite, KA1, KA2, KA3, KA4, and asphaltenes, to a depth of 2−5 nm, was carried out by ToF-SIMS. Negative-ion (m/z 11−38) and positive-ion (m/z 21−44) spectra for kaolinite, KA2, KA4, and asphaltenes were presented in panels a and b of Figure 3, respectively. All spectra were run under high-resolution conditions, but only highintensity fragments for all elements were recorded. In the negative-ion spectra, the possible heteroatom ions detected in m/z 11−38 were O− (m/z 16), OH− (m/z 17), CN−(m/z 26), Si− (m/z 28), S−/O2− (m/z 32), and SH− (m/z 33). In kaolinite, peaks at 35 and 37 were assigned to chlorine impurities, which do not exist in asphaltenes. Hydrocarbonderived ions, such as C− (m/z 12), CH− (m/z 13), C2− (m/z 24), C2H− (m/z 25), and C2H2− (m/z 26), were detected in all samples, which indicated the existence of adventitious organic matter even on the untreated kaolinite surface. Kaolinite and asphaltene-treated kaolinite samples (KA2 and KA4) showed relatively strong peaks for O− and OH−, while the spectra for asphaltenes alone are dominated by carbon-containing fragments (CH−, C2−, C2H−, and CN−).The possible positive inorganic ions in m/z 21−44 were Al+ (m/z 27), Si+ (m/z 28), SiH+ (m/z 29), K+ (m/z 39), and Ca+ (m/z 40), and the strongest organic peaks were C3H5+ (m/z 41) and C3H7+ (m/z 43). Peaks identified in KA1 and KA3 were similar to those in KA2 and KA4. XPS analysis was performed on kaolinite, KA1, KA2, KA3, KA4, and asphaltene. The presence or absence of elements associated with polar functional groups on kaolinite and asphaltene-adsorbed kaolinite may indicate their role in asphaltene−kaolinite interactions. The atomic surface elemental concentrations for all samples are summarized in Table 2.

Figure 1. Amount of asphaltene adsorbed as a function of the solution concentration.

typical type I isotherm. In the case of asphaltenes, however, we must be cautious in treating this relationship as a reversible equilibrium relationship because of prior observations of multilayer and irreversible adsorption.35,36 Table 1 gives the contact angle, adsorbed amount of asphaltenes, C, H, N, and S contents of pure kaolinite, pure asphaltenes, and four asphaltene-treated kaolinite samples. KA1, KA2, KA3, and KA4 were obtained at increasing initial asphaltene concentrations of 0.1, 0.5, 1.0, and 5.0 mg/mL, respectively. On the basis of data in Figure 1 and Table 1, sample KA4 with an adsorbed amount of 55 mg/g of asphaltenes seems to have apparently reached saturation. The carbon content in these four asphaltene-treated kaolinite samples was linearly proportional to the amount of asphaltene adsorbed on the basis of the spectroscopic analysis of the liquid solution, as illustrated in Figure 2. The contact angles for the series of samples (Table 1) indicated that kaolinite changed from hydrophilic to more hydrophobic as a result of adsorption of asphaltenes. When 13 mg of asphaltenes was adsorbed on 1 g of kaolinite, the contact angle increased by 47°, but a further increase of only 24.6° was detected when the adsorbed amount increased from 13 to 43 mg/g. This result indicated that the contact angle was more sensitive at lower density surface coverage by asphaltenes.

Table 1. Contact Angle, Adsorbed Amount, and Elemental Composition of Kaolinite, KA1, KA2, KA3, KA4, and Asphaltenes sample kaolinite KA1 KA2 KA3 KA4 asphaltene

contact angle (deg)

adsorbed amount (mg/g)

C (wt %)

H (wt %)

N (wt %)

S (wt %)

± ± ± ± ±

0 3 13 43 55

0.36 0.56 1.26 3.32 4.14 79.98

1.58 1.61 1.62 1.83 1.91 7.51

0.16 0.15 0.19 0.18 0.27 1.21

0.04 0.01 0.02 0.03 0.03 7.97

17.4 28.9 63.9 88.5 91.5

1.8 0.9 0.7 1.2 0.7

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Figure 3. ToF-SIMS spectra for kaolinite, KA2, KA4, and asphaltene samples [(a) negative ions and (b) positive ions] showing peak assignments for kaolinite and asphaltenes.

Table 2. Atomic Concentration of Elements for Kaolinite and Asphaltene Surfaces as Determined by XPSa concentration (atom %)

a

surface coverage of asphaltene (%)

sample

O 1s

N 1s

C 1s

S 2p

Si 2p

Al 2p

C 1s

Si 2p

Al 2p

kaolinite KA1 KA2 KA3 KA4 asphaltene

62.0 61.0 56.4 41.6 38.3 5.9

0.4 0.2 0.4 0.5 0.5 1.1

11.7 13.6 19.3 28.9 43.3 89.3

0.0 0.0 0.2 0.8 0.9 2.3

13.0 12.8 11.8 9.3 8.6 0.8

12.3 12.0 11.4 8.8 8.0 0.6

0 2.4 9.8 35.0 40.8 100

0 1.7 9.4 30.3 36.3 100

0 2.9 7.5 30.4 36.7 100

Surface coverage was calculated from eq 1.

kaolinite was relatively stable. Figure 4b gives the trends for C, O, Si, and Al in KA4, while Figure 4c gives the changes in N and S in KA4. The analysis was repeated for an oriented-film sample of KA4 prepared by the filter-transfer method.34 The XPS signals from the kaolinite film as a function of time were equivalent to the data of Figure 4b for a pellet sample (data not shown). In the first 300 s, Si and Al concentrations reached a nearly constant level, while the O concentration continued to rise for 1300 s and the C concentration was reduced for a period of 1700 s. In the depth profiling measurements, each etch cycle with the ion gun exposes a fresh surface. When the ion gun reached the bulk kaolinite in a particle, the concentrations of all elements would not change. The sample consisted of randomly oriented particles; therefore, as the sputtering proceeds, the XPS detects not only the planar faces of kaolinite particles but also material on the edges of the particles and/or sandwiched between particles. The etching rate in these experiments was 2.2 nm/ min based on sputtering a standard SiO2 sample of known thickness. Assuming that the etching rates of asphaltenes and kaolinite were similar, the data of Figure 4b showed that the

The concentrations of C, O, Al, Si, N, and S in this series of samples were determined using the electrons from the orbitals listed in Table 2. In comparing the five different samples, kaolinite showed the highest atomic concentration of O and the lowest content of C, while asphaltene showed the highest atomic concentration of C and the lowest content of O. As in the XPS analysis, the atomic concentration of carbon on the untreated kaolinite of 11.7% was due to adventitious of organic matter on the surface. The atomic concentrations of N and S were relatively low as expected, especially in the pure kaolinite sample. As the amount of asphaltenes on the kaolinite surface increased, C, N, and S contents increased, while the concentration of O, Al, and Si decreased. Pure kaolinite, asphaltene-treated kaolinite sample KA4, unwashed silicon wafer (USW), and washed silicon wafer (WSW) were analyzed by XPS depth profiling. In Figure 4, the XPS compositions are plotted as a function of the sputtering time. The composition changes in kaolinite versus sputtering time are shown in Figure 4a. The content of O, Si, and Al increased with sputtering time, while the C content was reduced during the first 50 s. After 50 s, the composition of 2468

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thickness of the asphaltene material on the surface was at least 11 nm, although the latter value was likely due to detection of asphaltenes on the edges of randomly oriented particles of kaolinite. Analysis of a kaolinite film sample gave the same profiles as illustrated in Figure 4b; therefore, we conclude that pellet preparation did not affect the surface composition of the sample. Scanning electron microscopy (SEM) imaging of the film showed random orientation of the particles; therefore, this sample preparation method did not allow for a comparison of face versus edge coverage of clay crystallites by asphaltenes. The concentrations of both nitrogen and sulfur were reduced to the noise level when the exposed faces of the kaolinite were cleaned of the asphaltene deposits. There was no maximum in the concentration of N between 300 and 1700 s as the surfaces were cleaned of the asphaltene material; therefore, there is no evidence from these data for the preferable adsorption of these elements at the interface between asphaltenes and kaolinite. A silicon wafer provides a flat surface, which provides a more accurate measurement of the thickness of the asphaltene coating. Composition changes by XPS on two kinds of asphaltene-treated silicon wafer, USW and WSW, while sputtering are shown in panels a and b of Figure 5, respectively. For USW, the initial atomic concentration of Si and O is almost 0, while the C concentration is around 96%. During sputtering, Si and O concentrations gradually increased and C was reduced. After sputtering for about 1250 s, the atomic concentration of Si does not change any more. For the WSW, the initial atomic concentrations of Si, O, and C are 4.3, 13.0, and 79.1%, respectively. The intensities of Si and O increased, while the intensity of C dropped dramatically in a short time of sputtering, as compared to UWS. At the end of sputtering, the concentrations of all three elements are the same for both UWS and WSW, indicating that silicon was exposed. Si, O, and C reached final atomic concentrations of 30, 70, and 0%, respectively. This result indicated that the XPS depth profile data are consistent for different surface coatings. Given that the etching rate in these experiments was 2.2 nm/ min and assuming that the etching rates of asphaltenes were similar, the data for Figure 5a indicated a maximum thickness of asphaltenes on the surface of 55 nm, while the data of Figure 5b gave a much lower estimate of 11 nm. Furthermore, the UWS sample gave no significant Si for 500 s, indicating a minimum thickness of 18 nm of asphaltenes on this sample. In contrast, the washed sample showed a significant signal from Si immediately, indicating exposed SiO2 on the surface.

4. DISCUSSION 4.1. Calculation of the Adsorbed Amount of Asphaltenes. In adsorption studies, analysis of the liquid solution is commonly used to calculate the amount of asphaltene adsorbed, but there are some limitations with this method. When UV−vis absorbance of asphaltenes is employed to measure the asphaltene concentration, linear calibrations of absorbance versus asphaltene concentration are required in a given solvent. The slope or apparent extinction coefficient correlates to the average associated molar mass of the asphaltenes. Therefore, any change in composition or molar mass of asphaltenes may change the calibration curve. The corresponding change in the calibration constants can lead to errors of 5−25% in the estimated concentration.37 For adsorption on kaolinite, the composition of asphaltene solution after adsorption may be different from the initial composition. Another important parameter in the spectrometric method is

Figure 4. Composition in atom percent by XPS as a function of sputtering time for (a) kaolinite, (b) KA4, and (c) N and S in KA4. 2469

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showed the highest content of carbon and lowest contents of oxygen, silicon, and aluminum, on the basis of XPS results (Table 2). The Si/Al ratio was about 1.06 for both kaolinite and asphaltene-treated kaolinite samples, which demonstrated that neither the alumina nor the silica surfaces of kaolinite were preferentially covered by asphaltenes. For KA4, which has reached apparent adsorption saturation, as described before, there are obvious Si+ and Al+ signals, as shown in Figure 4b. The same results were obtained from an oriented film sample; therefore, the exposure of the mineral surface was not an artifact of pellet preparation. At the same time, XPS data showed that atomic concentrations of Si 2p and Al 2p are 8.6 and 8.1%, respectively. Because Si and Al do not occur in the asphaltenes, this result suggested that the kaolinite surface was not completely covered by asphaltenes, even when the surface was saturated with asphaltenes. This result is consistent with the conclusion by Bensebaa et al.29 that coverage of organic matter on solids was patchy rather than continuous in the organic-treated solids separated from Athabasca bitumen. Both ToF-SIMS and XPS measurements can detect elements at depths up to 10 nm on sample surfaces but are normally limited to the top 2−5 nm.7,9 The data of Figure 4b indicate that the asphaltene domains on the surface had a maximum thickness of ca. 11 nm, but some areas had a thinner coating. Given the similarity in the length scales, can we definitely conclude that portions of the kaolinite surface were always exposed? Two pieces of evidence indicate that kaolinite is only partially covered by asphaltenes, even at the highest asphaltene concentration. On the basis of the atomic concentration of elements measured by XPS (Table 2), the surface coverage of asphaltene on kaolinite can be estimated using the following equation: θ=

Ccoated − C0 C bulk − C0

(1)

where θ is the surface coverage of adsorbed asphaltene and Ccoated, C0, and Cbulk are the atomic concentrations of elements on asphaltene-coated kaolinite, kaolinite, and bulk asphaltene, respectively. The results in Table 2 indicated that the surface coverage increased linearly with an increasing asphaltene concentration (R2 = 0.996 for coverage based on C 1s). The maximum surface coverage of asphaltene on kaolinite after apparent saturation of adsorption was 36−40%, which means that only part of the kaolinite surface was covered by asphaltene. The contact angle was much more sensitive to low levels of asphaltene loading (Figure 6), and it gave the same trend with surface coverage. The data for the USW (Figure 5a) are inconsistent with the results for kaolin. This sample gave complete coverage of the silica by asphaltene, with a minimum thickness of 18 nm. We attribute this result to a combination of adsorption and deposition, as observed in quartz crystal microbalance (QCM) studies, which are carried out in a stagnant liquid phase.40 In contrast, the kaolinite particles were mixed in the solution and then rinsed with solvent. Consequently, we focus our comparison on the WSW and the kaolinite samples. If we assume that the first sputter interval removes the adventitious carbon from the surface, as illustrated in Figure 4a, then the “initial” surface composition from Figure 5b is 18% Si, 17% O, and 62% C. The difference between this composition and the composition at long sputtering time (28% Si, 68% O, and 0% C) gives an estimate of surface coverage. Hence, asphaltenes

Figure 5. XPS composition of the asphaltene film coated onto silica as a function of sputtering time for (a) USW and (b) WSW.

the wavelength used to obtain the calibration curve. In different studies, wavelengths from 336 to 800 nm have been used.1,32,38 Alboudwarej et al.39 suggested that a short wavelength gives better accuracy at relatively low asphaltene concentrations (below 0.1 kg/m3), while a long wavelength is more preferable for high asphaltene concentrations (above 0.1 kg/m3). This approach is simply a method to keep the absorbance in a range for application of the Beer−Lambert relationship. The data of Figure 2 showed that, in this study, the measurement of the asphaltene content by UV−vis absorbance was linearly correlated with the carbon content of the samples. The linear correlation showed that neither the absorbance of the asphaltenes nor the elemental composition was changed by the adsorption process. Although the asphaltenes are a complex mixture, the correlation data indicate constant extinction coefficients at a given wavelength as asphaltenes are adsorbed from the solution. This plot gives confidence in the calculated surface concentration of the asphaltenes. 4.2. Surface Coverage of Kaolinite by Asphaltene Adsorption. In comparing the four asphaltene-treated kaolinite samples, KA1 has the highest contents of oxygen, silicon, and aluminum and lowest content of carbon, while KA4 2470

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covering the surface, the ratio of C3H5+/Al+ only increased from 0.47 to 0.59, while C3H5+/Si+ showed a more dramatic increase from 0.8 to 2.5. This result indicates that, in ToFSIMS, generation of Si+ is more sensitive than Al+ to the changes in asphaltene coverage. This difference might be taken to indicate preferential sorption on the silica surface of the kaolinite, except that the ratio of the two elements was constant in all of the treated samples, as determined by XPS. The spectra of Figure 6 show that Al+ ions are generated at a higher concentration than Si+, even though the elemental abundance is the same, which suggests that the details of mechanisms of ion generation could account for the greater sensitivity of Si+ to the presence of asphaltenes. The data showed that the KA4 surface carried 3 times as much asphaltene on an area and a mass basis as the KA1 surface. With more asphaltenes adsorbed on the kaolinite surface, the contact angle increased from 17.4° to 91.5°, which indicated that kaolinite changed from water-wet to neutrally wet. However, the most noticeable feature is that the contact angle increased dramatically when the surface coverage was low. Beyond a contact angle of ca. 75°, a higher asphaltene coverage on the surface had a much smaller effect on wettability. The surface of a pellet of coated kaolinite is heterogeneous in surface roughness on a micrometer length scale and heterogeneous in hydrophobicity at a sub-micrometer length scale because of partial coverage of the mineral surface by asphaltene aggregates. The role of such non-uniform surfaces could cause the sensitivity of the contact angle to low concentrations of asphaltenes. Similarly, the roughness and variation of the surfaces of kaolinite particles could dominate in the physical adsorption of asphaltene aggregates, so that chemical functional groups, such as nitrogen, are much less important than has been observed on adhesion to metals.43,44 To compare the two surface analysis methods, the measured contact angle was plotted against Si+ as determined from ToFSIMS in Figure 7 and Si from XPS in Figure 8. A good linear

Figure 6. Relationship between the relative intensity of C3H5+/Al+, C3H5+/Si+, contact angle, and asphaltene adsorbed amount on kaolinite.

covered 62% of the initial surface based on the carbon content and 61% of the initial surface based on Si and O contents. The estimate based on carbon is most reliable because Si and O will be detected even at a depth of 2−5 nm under an asphaltene layer, as discussed above. As discussed above, the maximum thickness of asphaltene domains or nanoaggregates was approximately 11 nm based on the time for the clean silica surface to emerge. Using the data for carbon between 30 and 300 s of sputtering, the mean thickness was 3 nm on an areaweighted basis and followed an approximately exponential distribution. This mean value and the maximum sizes for asphaltene aggregates compare well to the literature. Zahabi et al.21,41 used atomic force microscopy and determined that the thicknesses of asphaltenes on silica were in the range of 2−8 nm. By combining XPS and QCM analyses on asphaltene adsorbed on a gold surface, Rudrake et al.1 estimated that the thickness of adsorbed asphaltene ranged from 6 to 8 nm. Sztukowski et al.42 reported that the thickness of the asphaltene monolayer on the water−oil interface was 2−9 nm. The thickness of asphaltene aggregates adsorbed on kaolinite should be similar to that on the WSW. 4.3. Correlation between Surface Composition and Wettability. The ToF-SIMS results show that inorganic fragments, especially Al+ and Si+, dominate in kaolinite, while organic fragments are dominant in asphaltenes. Because Al+ and Si+ are the highest intensity inorganic peaks as a result of kaolinite and C3H5+ is the highest intensity organic peak from the asphaltenes, the relative intensities of C3H5+/Al+ and C3H5+/Si+ were used to characterize the kaolinite surface and the relationship between asphaltene adsorption density and contact angle, as illustrated in Figure 6. With more asphaltenes

Figure 7. Correlation between the contact angle and Si+ intensity in ToF-SIMS. The data show a linear regression of the data, with R2 = 0.93.

relationship of the contact angle with Si+ was observed (R2 = 0.94), suggesting that ToF-SIMS can be used as a method to investigate at the level of the surface composition. In contrast, the concentration of Si 2p on the surface from XPS measurement gave a more nonlinear relationship to the contact angle (R2 = 0.77), functionally similar to the total asphaltene concentration in Figure 6. The use of ToF-SIMS to estimate 2471

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Imperial Oil Limited and the Centre for Oil Sands Innovation for providing financial support and the Alberta Centre for Surface Engineering and Science at the University of Alberta for providing access to make use of their instruments. The authors also thank Dr. Quan Shi for valuable suggestions and helpful discussion.

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Figure 8. Relationship between the contact angle and Si concentration by XPS.

material wettability has been reported by Skinner et al.24,28,45 based on data for octadecylphosphonic acid (OPA) adsorption on mica. They selected Si+ to represent the mica substrate, while PO2− and PO3− were measures of the adsorbed OPA. Their results showed that there was a good linear correlation between the contact angle and total POx, which demonstrates that ToF-SIMS can be used to infer the wettability of a heterogeneous planar surface within reasonable error.28 Although the use of ToF-SIMS to determine wettability is not a practical alternative to simple contact angle measurement, it does offer the opportunity to understand how the range of surface properties in a particulate sample gives rise to the observed macroscopic contact angle. In the case of reservoir materials and oilsands, a range of minerals may be present that give different interactions with the asphaltenes and, hence, contribute in different ways to the observed macroscopic behavior. The ability of ToF-SIMS to give data at the length scale of 1 μm or less opens up new opportunities to understand how the various particle surfaces contribute in more complex mineral mixtures.

5. CONCLUSION In this study, a method using the carbon content in samples of kaolinite to calculate the adsorbed amount of asphaltene is demonstrated. Both ToF-SIMS and XPS showed that the kaolinite surface was not completely covered, even at high asphaltene concentrations, based on the existence of Al and Si in the surface layers. After asphaltene adsorption, kaolinite surface wettability changed from water-wet to neutrally wet. The thickness of asphaltene coating was estimated to be around 11 nm based on XPS depth profile measurement results. The XPS depth profile measurement also indicated that there was no preferential adsorption of nitrogen- or sulfur-rich polar species from asphaltenes at the interface of asphaltene and kaolinite. A good inverse linear correlation was observed between the contact angle (wettability) of the asphaltenecovered kaolinite surface and the intensity of the Si+ ions detected by ToF-SIMS.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: 780-492-2280. Fax: 780-248-1565. E-mail: gray@ ulaberta.ca. 2472

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