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Energy & Fuels 2007, 21, 1226-1230
Ellipsometry Study of the Adsorption of Asphaltene Films on a Glass Surface† Henry Labrador,*,‡ Yackelin Ferna´ndez,‡ Jose´ Tovar,‡ Rafael Mun˜oz,§ and Juan Carlos Pereira‡ Laboratorio de Petro´ leo, Hidrocarburos y DeriVados (PHD), Departamento de Quı´mica and Laboratorio Espectroscopia O Ä ptica de Superficies, Departamento de Fı´sica, Facultad Experimental de Ciencias y Tecnologı´a, UniVersidad de Carabobo, Valencia, Estado Carabobo, Venezuela ReceiVed August 12, 2006. ReVised Manuscript ReceiVed NoVember 29, 2006
Asphaltene adsorption from a toluene solution on a glass surface was studied as a function of concentration, at ambient temperature and atmospheric pressure. The thickness of the asphaltene film measured by ellipsometry was found to be in the 20-298 nm range. The film thickness was found to increase by 38-70 nm after 24 h. Solutions containing asphaltene mixtures from different crude oils result in significant variations in film thickness when compared with single asphaltene cases. For CN-Ceuta and DM153-Ceuta mixtures, a diminution was observed, whereas an increase was found with the DM153-Furrial mixture. Finally, the results attained with asphaltenes in which low molecular weight compounds have been removed suggest a film swelling effect due to the alkyl-type resins which are found to modify the colloidal properties of the aggregated asphaltenes in agreement with the literature.
Introduction Asphaltenes are the heaviest components in crude oil, being highly polar, by definition, and belong to a solubility class of material that is soluble in toluene but insoluble in alkanes such as n-heptane. Asphaltenes have received great attention by petroleum chemists over the past decade,2-4 mainly due to their negative impact on the petroleum industry. Studies of their colloidal4-6 and molecular7-9 properties are popular research topics. These asphaltenes are very complex mixture of compounds, which exist in petroleum as colloidal dispersions. Ekholm et al.10 observed the adsorption of asphaltenes and resins onto gold surfaces using a quartz crystal microbalance with dissipation measurements. These results indicate that asphaltenes adsorb in large quantities, and the mass and dissipation data demonstrate the presence of aggregates on the surface. Zhang et al.11 showed that asphaltenes can form a monolayer at an † Presented at the 7th International Conference on Petroleum Phase Behavior and Fouling. * Corresponding author. E-mail:
[email protected]. ‡ Departamento de Quı´mica. § Departamento de Fı´sica. (1) Acevedo, S.; Rodrı´guez, P.; Labrador, H. Energy Fuels 2004, 18, 1757-1763. (2) George, M.; Al-Sri, M.; Khan, Z.; Ali, F. Energy Fuels 2005, 19, 1598-1605. (3) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquim, G. Energy Fuels 2003, 17, 1233-1338. (4) Strauz, O.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809-822. (5) Sheu, E.; Storm, D. Asphaltenes: Fundamentals and Application; Sheu, E., Mullins, O., Eds.; Plenum Press: New York, 1995; Chapter 4, p 131. (6) Acevedo, S. ReV. Soc. Venez. Quı´m. 2001, 24, 33-43. (7) Ingnasiay, T.; Kemp-Jones, A.; Strausz, O. J. Org. Chem. 1977, 42, 312-320. (8) Platanov, V.; Proskuryakov, V.; Klyavina, O.; Kliabina, N.; Russ. J. Appl. Chem. 1994, 67, 440-443. (9) Strausz, O.; Mojelsky, T.; Lown, T. Energy Fuels 1999, 13, 228247. (10) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I.; Sjoblom, J.; Kornfeldt, A. J. Colloid Interface Sci. 2002, 247, 342-350.
air-water interface, indicating that asphaltenes act as surface active molecules. Flocculation of these colloids leading to the clogging of pipes and other production facilities is a matter of extraordinary importance, both from the industrial and academic point of view, owing to its negative impact on the petroleum industry. Chemical analysis of these pipe deposits confirms the presence of large amounts of asphaltenes and resins and lower but significant amounts of inorganic material.12 Porte et al.13 observed that precipitation eventually occurs, determined by van der Waals attractions between aggregates, when the solubility parameter of the solvent is shifted; in his descriptions, asphaltene aggregation and precipitation are distinct steps in a completely reversible process. The presence of inorganic material in these deposits is interesting in terms of their role during the formation of the deposit itself. The free energy of asphaltenes in crude oils is probably related to the stability of their colloidal aggregates or micelles.14 On the other hand, instability accounts for the tendency of asphaltenes to precipitate from crude oils under a variety of circumstances and to be adsorbed on an interface of any kind. This is a general problem related to nucleation and particle growth. The surface flocculation will be enhanced if the solute is being adsorbed on such a surface and more so if a multilayer can be formed since, in such a case, barriers due to surface saturation would be reduced. According to the foregoing arguments, adsorption and multilayer formation by asphaltenes on a surface should be of paramount importance since surface flocculation could be the beginning of the clogging problems mentioned above. Although many studies have reported the adsorption of asphaltenes on a variety of surfaces, these were performed under highly diluted conditions where (11) Zhang, L.; Lawrence, S.; Xu, Z.; Masliyah. J. Colloid Interface Sci. 2003, 264, 128-140. (12) Acevedo, S.; Ranaudo, M.; Escobar, G.; Gutie´rrez, L.; Ortega, P. Fuel 1995, 74, 595-598. (13) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40-47. (14) Acevedo, S.; Ranaudo, M.; Pereira, J.; Castillo, J.; Ferna´ndez, A.; Pere´z, P.; Caetano, M. Fuel 1999, 78, 997-1003.
10.1021/ef060375r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007
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Langmuir type adsorption is generally found, and no evidence for multilayer formation is reported;15 but, there is also evidence of multilayer formation on glass surfaces that have been studied by using photothermal surface deformation spectroscopy.14 However, in general, when the usual optical absorbance technique was employed, we found serious difficulties in measuring the amount of material adsorbed. These problems were overcome by measuring the thickness, using ellipsometry (reflection ellipsometry), which measures the changes in the state of polarization of light upon reflection from a surface.16 Ellipsometry measures the refraction index n2, the extinction coefficient k2, and the thickness d of a film on a substrate of refraction index n3 (see Figure 1a) studying the change in amplitude (Ψ) and phase (∆) of polarized light (wavelength λ) reflected from the sample. It will be assumed that all media are isotropic and homogeneous and that films are of uniform thickness. The ellipsometric parameters ψ and ∆ are related to the optical constants (n and k) and the thickness d of the film, through the ratio of complex total reflection coefficients Rp and Rs, from parallel, p, and normal, s, polarized light, respectively,17
tan Ψ exp(i∆) ) RP/RS R ) p
Rs )
rp12 ) rs12 )
rp12 + rp23 expD 1 + rp12rp23 expD rs12 + rs23 expD 1 + rs12rs23 expD
(n2 - ik2)cos φ1 - n1 cos φ2 (n2 - ik2)cos φ1 + n1 cos φ2 n1 cos φ1 - (n2 - ik2)cos φ2 n1 cos φ1 + (n2 - ik2)cos φ2 d)
4πi(cos φ2)n2d λ
n1 sin φ1 ) (n2 - ik2)sin φ2
(1) (2)
(3)
(4)
(5)
(6) (7)
In practice, at very low concentrations of asphaltenes, the thin films were weakly absorbing so, in eqs 4 and 5, it is possible to use k2 ) 0. In this work, the interest is focused on studying the behavior of the asphaltenes and their mixtures in contact with a glass surface, measured with the ellipsometry technique. Materials and Methods Materials. Analytic grade analytic acetone, toluene, n-heptane, methanol, and tetrahydrofuran were all purchased from J. T. BAKER. Two glass plates were used as adsorbents: one is a circular plate, supplied by Glass Forms, with 32 mm of diameter and 1 mm of thickness, ground by one of their surfaces and polished in the other one, to guarantee a better reading in the surface of the film; and the other is a basis precleaned microscope glass slide supplied by Fisher Scientific Co., without pretreatment, and cut in (15) Gonza´lez, G.; Middea, A. J. Dispersion Sci. Technol. 1987, 8, 525548. (16) Aazam, R.; Bashara, N. Ellipsometry and Polarized Light; Elsevier Science Publishers: Netherlans, 1987. (17) 439PCS11 A Basic Program For The analysis of Ellipsometric Measurements; Rudolph Instruments, Inc.: New Jersey, 2000.
Figure 1. (a) Film covered surface under study with n1, n2, and n3 being the refraction indexes of the medium, film, and substrate, respectively. (b) Basic components of an elipsometer: light, polarizer, compensator, sample, analyzer, and photodetector.
pieces of approximately 10 × 25 mm. These absorbents were submerged in tetrahydrofuran (THF) to eliminate the possible remains of organic material; they were cleaned with a soapy solution, rinsed with abundant distilled water, and finally washed with acetone. Asphaltenes. The asphaltenes were obtained from crude oils by addition of 40 volumes of n-heptane. The heptane-diluted crude oil was stirred for 8 h and was left to rest for 24 h. Then, we proceeded to filter, with a filter paper, the asphaltenes; the resins in the solution coprecipitated, and they remain as solids. The maltene fraction remains in the solution. The previous solid was placed in a Soxhlet extractor using n-heptane as solvent for 48 h to extract the resins that coprecipitated with the asphaltenes. The solid was placed in reflux for 12 h. The remaining solids were filtered, dried, weighed, and stored under an inert atmosphere. Solution Preparation. For each one of the asphaltenes (Furrial, Ceuta, and DM-153), an asphaltene solution (around 10 000 mg/ L) was prepared, by dissolving appropriate amounts of dried asphaltenes in toluene. The solution was left in a thermostated ultrasonic bath for 30 min to ensure a complete dissolution. Finally, the solution was diluted to a desired volume using an appropriate amount of toluene. Starting from this solution, diluted solutions between 200 and 10 000 mg/L were prepared. The following mixtures of asphaltenes were prepared at a mass ratio of 1:1, to the same concentrations (200 and 6000 mg/L): Cerro Negro/Furrial, Cerro Negro/Ceuta, Furrial/DM-153, Furrial/Ceuta, and Ceuta/ DM-153. Preparation of the Films and Measurements of the Thickness. The glass plates were placed inside the solutions of asphaltenes contained in glass flasks at room temperature that were closed to avoid the evaporation of the solvent and placed in a dark area. After 24 and 48 h, the glass plates were extracted and placed vertically onto absorbent paper in a toluene atmosphere until the solution excess was eliminated and the thickness for ellipsometry was determined. Ellipsometry was done with a manual Null Type model 439 Rudolph Instruments ellipsometer with a helium neon laser (632.8 nm) in the PCSA (polarizer-compensator-sample-analyzer) configuration (see Figure 1b). The angles ∆ and ψ were measured at two angles of incidence φ ) 60° and φ) 70°, measured from the surface normal. The compensator was a quarter wave plate set with the optical axis at 45° to the plane of incidence, and measurements were made in four zones to correct for any instruments misalign-
1228 Energy & Fuels, Vol. 21, No. 3, 2007
Labrador et al.
Table 1. Properties of Asphaltenes and Crude Oil Used crude oil
oAPI
Mnb (g/mol)
Furrial CN Ceuta DM-153 Guafita
21 8 21 14 22
979 1287 850 1184 ndc
% asphaltene p/v
avg diametera (nm)
7 11 2 9 2
7.05 9.25 8.50 7.09 nd
Table 2. Maximum and Minimum Thickness Values of the Asphaltene Films asphaltene Ceuta Furrial DM-153
a
See ref 1. b Molecular weight in number, at VPO, nitrobenzene 130 °C. nd: not determining. c
ment. The instrument was calibrated by using a fused silica sample provided by the manufactures. Three different areas on the film surface were sampled around the surface center to obtain information on the film thickness uniformity at λ ) 632.8 nm, and the index of refraction of the glass substrate was measured giving n ) 1.51. The analysis of data was made with the Rudolph Instrument 439PCSII software, considering experimental errors E∆ ) 0.015 and Eψ ) 0.010. Extraction Procedure for Treaty Asphaltenes. Solutions of asphaltenes were prepared in toluene, dissolving 1 g of asphaltenes in a 250 mL solution, and the silica was added, previously activated at 100 °C for 24 h; the asphaltene/silica mass ratio was 1:30. The mixture was stirred for 12 h and filtered for gravity, and the filtrate was discarded. The solid was placed in the Soxhlet and extracted with a chloromethane/methanol solution at a volume ratio 7:3, until the solvent mixture was colorless. The mixture of solvents with the asphaltenes extracted in solution was filtered again to separate some remains of silica that had been able to pass through the Soxhlet during the washing. Subsequently, the solution was concentrated to a minimum volume, and the asphaltenes precipitated, adding 150 mL of heptane, agitated for 6 h, filtered, and dried. Treated asphaltenes solutions were prepared in toluene, at 200, 500, 1 000, 2 500, 5 000, 7 500, and 10 000 mg/L, placing these in ultrasound, to dissolve the treated asphaltenes. The same proceeding was carried out previously using with the asphaltenes.
Results and Discussion Asphaltenes. In this section, the results obtained are presented and analyzed in the study of adsorption on glass of different asphaltenes (Furrial, Ceuta, DM-153, Cerro Negro, and Guafita), using the technical ellipsometry to determine the thickness of the asphaltene films. The formation of asphaltene films on glass plates happens in a process of adsorption, being the asphaltenes the adsorbate and the glass the adsorbent. This type of adsorption between the glass and the asphaltene is a physical adsorption, since the links of adsorption are not ionic or covalent but weak links.18 Once the asphaltene films on the glass plates are formed, the thickness of each of them was determined. Some characteristics of asphaltenes studied are shown in Table 1. Maximum and minimum film thickness values are showed in Table 2. In Table 1, the average diameters obtained are reported by electronic transmission microscopy of the asphaltenes studied;1 the diameters showed suggest that the films thickness values of the films could be related to the multilayer adsorption by the asphaltene films and their mixtures. The thickness of the asphaltene films could be a product of the adsorption of the “free molecules” of asphaltenes, or mainly the adsorption of aggregates present in a given solution, due to the concentrations used in this work. In other words, in aromatic solvents, such as toluene, the asphaltenes are aggregate. Some investigators report (18) Akhlaq, M. S.; Go¨tze, P.; Kessel, D.; Dornow, W. Colloids Surf. A: Physicochem. Eng. Aspects 1997, 126, 25-32. (19) Tanaka, R.; Sato, E.; Hunt, J.; Winans, R.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1405-1413.
time (h)
min thickness (nm (5)
max thickness (nm (5)
24 48 24 48 24 48
20 27 48 69 33 53
95 168 118 165 260 298
Table 3. Maximum and Minimum Thicknesses Values of the Films of Asphaltenes Mixtures mixtures CN-Ceutaa CN-Furrial Furrial-Ceuta Ceuta-DM-153 Furrial-DM-153 a
time (h)
min thickness (nm (5)
max thickness (nm (5)
24 48 24 48 24 48 24 48 24 48
10 21 61 60 24 55 15 26 115 150
138 254 144 166 116 121 134 166 390 395
CN: Cerro Negro.
that these begin at low concentrations, 50-100 mg/L.14 Therefore, at the concentrations used (200-10 000 mg/L), this effect is significant, and the adsorption of asphaltene films on glass will be a function of the aggregates in solution. The adsorption of asphaltenes on the glass surface was monitored by measuring the film thickness. It was observed that the film thickness increases in proportion to the concentration of asphaltenes in solution. The interval of thickness of the obtained films was 20-298 nm (see Table 2). The thickness values measured with concentrations under 1000 mg/L mainly suggest the adsorption of primary particles, in accordance with the data reported by other investigators.1 For concentrations higher than 5000 mg/L, thickness values over 80 nm were obtained; this suggests the adsorption of fractal aggregates15,18 which are constituted of primary particles, including solvent. It can be proposed that the thickness of the asphaltene films changes with time, for each the samples (see Table 2). It was determined that the longer the immersion time of the glass plates in the asphaltene solution, the thicker the film. Asphaltenes Mixtures. Due to the development of technologies that imply the mixture of crude oils of different composition and origin, it is of interest to study the asphaltenes behavior under these conditions. The stability of the aggregates in these mixtures will depend on diverse factors; as for example the aliphatic/aromatic balance of the asphaltenes structures. The instability of the aggregates products of the mixtures can drive the formation of precipitate during the different operations of the petroleum industry, and their serious consequences. The thickness of asphaltenes films on glass, were evaluated monitoring the stability of the asphaltenes in different mixtures. With regard to the result of the mixture CN-Ceuta, the minimum thickness value obtained was of 10 nm. This indicates the possibility that a layer of asphaltenes was adsorbed on the glass to a concentration of 200 mg/L, indicating that when mixing the asphaltenes, its adsorption behavior on the glass is altered, and will depend on the asphaltene characteristics. In the case of the mixture DM-153/Furrial (see Table 3), the thickness values obtained were highest to 24 and 48 h. These asphaltenes present flocculation problems (Furrial), but DM153 asphaltenes have no flocculation problems. The results of
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Energy & Fuels, Vol. 21, No. 3, 2007 1229
Figure 2. Thicknesses of asphaltenes films as a function of concentration (miligrams per liter) a t ) 24 h. (a) Ceuta asphaltene. (b) Cerro Negro asphaltenes. (c) CN/Ceuta mixture asphaltenes.
the mixtures indicate that their properties were altered by the presence of other asphaltenes. It can occur that in a case is more stable the added asphaltenes state remaining in solution, and in the other case they become unstable preferring the adsorption on the glass surface. Variation of Asphaltenes Films’ Thickness With Concentration. Figure 2a and b shows that the thickness of the asphaltenes films increases with the concentration, indicating the presence of two stages. The first could correspond to the saturation of the surface of the glass, and the other additional stages, to the multilayer formation on the surface of the glass that can be a consequence of the asphaltene-asphaltene interactions. Figure 2 shows that the thickness increases with the concentration; at the beginning, an increase of the thickness corresponds mainly to adsorption of the sample on the surface where, as the concentration increases, the formation of aggregates grows quickly and the capacity of the formation of aggregates on the surface is ended, giving rise to the observed step. In this case of the asphaltenes mixtures, as in the pure asphaltenes, multilayer formation was observed on the glass surface. Observing the minimum and maximum thickness in Table 3, the tendency of the curves of the asphaltenes of Ceuta 24 h and DM-153 24 h compared with the thickness from their asphaltenes mixture to 24 h are completely different; this indicates that the interaction of the asphaltenes alters the properties varying the thickness of the films (see Figure 2c). Asphaltene-asphaltene interactions are seen as important and apparently depend on the properties of each asphaltene, on their structure, and the functional groups presents on each of them. Effect of Resins. The effect of the properties of the resins in the asphaltenes films was studied by ellipsometry. For this purpose, asphaltenes from Guafita crude oil were used. The difficulty in separating the fractions of resins and asphaltenes is very well-known. Even after extensive separation processes, asphaltenes contain resins that can influence notably their properties. In order to of obtain asphaltenes with lower molecular weight (like resins), an adsorption treatment from toluene solutions on silica was carried out. The material was
Table 4. Thicknesses of Asphaltene and Treated Asphaltene of the Guafita Crude Oil Films Absorbed on Glass concentration
thickness of asphaltene (nm (5)
thickness of treated asphaltene (nm (5)
200 500 1000 2500 5000 7500 10000
13 18 17 35 80 101 128
5 10 20 34 56 76 110
extracted with a polar mixture of chloroform-methanol (7:3), given the high polarity of the asphaltenes regarding the resins. Results reported by Acevedo et al.14 indicate that the resins are adsorbed in small quantities on this solid when the solvent is toluene. Thus, it was expected to minimize the content of these compounds in the asphaltene fraction. The asphaltenes obtained in this way were denominated “treaty asphaltenes”. The thickness data are shown in Table 4. The treaty asphaltenes obtained were thinner, regarding the asphaltenes. In accordance with the above-mentioned analysis, the content of resins was smaller in the treaty asphaltenes. It was expected that the components present in this sample were more aromatic, and its capacity to be organized in a film is possibly greater. The presence of aliphatic structures by steric effects stops this distribution. This phenomenon has been observed by diverse investigators20 and has notable influences on properties of the asphaltenes in solution, such as their solubility and aggregation. In the case of film thickness adsorbed on glass, it is possible that the presence of aliphatic components, such as resins, in the asphaltenes increases the thickness. These results agree well with the acquaintance fact, that the resins peptizing the asphaltenes allow their dispersion in crude oils. Conclusions The present study reveals the possibilities to study the thickness of asphaltenes films adsorbed on glass plates using (20) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972-978.
1230 Energy & Fuels, Vol. 21, No. 3, 2007
ellipsometry. The thickness intervals obtained for asphaltenes were 20-298 nm, and the mixtures of asphaltenes had film thicknesses of 10-395 nm. We propose that in the interval of concentrations used 200-10 000 mg/L, from the solution in toluene primary aggregates up to fractal aggregates of large size are adsorbed on the glass surface. These data together with the forms of the curves of thickness as a function of the concentration corroborate multilayer formation on glass. In the case of the mixtures, the results suggest a change in the behavior of the aggregate asphaltenes, for the stability and adsorption on
Labrador et al.
the glass. Due to its more aliphatic structure than that of the asphaltenes, resins tend to increase the thickness of the asphaltenes films. Acknowledgment. We would like to thanks the Experimental Faculty of Sciences and Technology, University of Carabobo (Grants 407), for its cooperation. We also thank the technical support of Mrs. Marı´a Be´len Martı´n. EF060375R
