Precipitation of Asphaltenes and Resins at the Toluene–Silica

Article pubs.acs.org/EF

Precipitation of Asphaltenes and Resins at the Toluene−Silica Interface: An Example of Precipitation Promoted by Local Electrical Fields Present at the Silica−Toluene Interface Sócrates Acevedo,* Jimmy Castillo, and Edgar Hernán Del Carpio Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1041-A, Venezuela ABSTRACT: Adsorption and desorption of asphaltenes and resins occurring at the silica−toluene interface were used as a tool to investigate phase separation or precipitation promoted by the surface. Adsorption isotherms for this system were obtained as usual, employing commercial silica plates immersed in toluene solutions. After the adsorption of the sample (asphaltenes, asphaltene subfraction A2, resins I, and resins II), the plate was removed, contacted with and retracted from toluene to remove trapped solution, and then immersed again in fresh toluene to measure the desorption. In all cases, the amount desorbed was either insignificant or a small fraction of the amount expected. This could be analyzed in terms of precipitation at the interface promoted by local electrical fields present at the silica−toluene interface; these fields promote molecular polarization, dispersion interactions, and piling up, leading to precipitation. As a result, when the plate with the adsorbed precipitate (material of very low solubility in toluene) is contacted with fresh toluene, desorption is either insignificant or very low. The combine effects of the local electrical fields, molecular polarization, and dispersion interactions are so effective that resin precipitation at the above interface was also obtained. These ideas were found coherent with preliminary atomic force microscopy (AFM) measurements performed on asphaltenes deposited on a glass surface.

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pyridinic-, and pyrrolic-type functional groups.12 Similar results, with regard to the presence of these functional groups, were reported earlier by Abdallah and Taylor using XPS and an stainless-steel−toluene interface.13 The presence of a pyrrolic functional group in asphaltenes is an interesting finding, probably related to metal porphyrines. Step-wise adsorption isotherms for asphaltenes dissolved in toluene using several mineral rocks as substrates were reported by Mendoza de la Cruz and collaborators.14 These stepwise adsorption isotherms were reported for the first time by Acevedo and co-workers.15 These may be the result of either adsorption of aggregates or cluster formation at the interface (see the discussion below). Film thickness in the 20−298 nm range, measured by ellipsometry, was reported by Labrador and collaborators using the toluene−asphaltene solutions and glass plates as the surface.16 These thicknesses were somewhat supported by the report of Castillo et al. on a similar system (chloroform− asphaltenes/glass plates), where stalagmite-type columns up to 200 nm were measured using white light interferometric microscopy.17 The adsorption behavior of Athabasca bitumen C7 asphaltene over a macroporous silica−alumina, kaolin, at room temperature was compared to polynuclear heteroatom aromatic model compounds. Authors found that up to one point in concentration adsorption was independent of the heteroatom content.18 A higher molar mass adsorption for resins compared to asphaltenes was reported for toluene− heptane solutions using powdered metals as substrates, a result related to surface morphology.19 A strong dependence of asphaltene adsorption kinetics with concentration was

INTRODUCTION Adsorption of asphaltenes at both organic and inorganic interfaces is a well-known phenomenon reported many times in the literature.1−30 Dynamic adsorption of asphaltenes on quartz and calcite packs and their effect on wettability were documented.1 Several thermodynamic models were proposed to account for asphaltene adsorption on mineral surfaces.2,4,6−8 X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry−time of flight (SIMS−TOF) were used by Gray et al. to study the surface after asphaltene adsorption on kaolinite.3 Maximum thickness of the asphaltene layer was estimated at 11 nm, and no preferential adsorption of nitrogen or sulfur compounds was observed.3 NiO nanoparticles supported on nanoparticulated alumina were employed as an asphaltene adsorbent.4 According to these authors,4 complete effective adsorption times were observed after 2 min. Thermocatalytic reactions of asphaltenes adsorbed on nanoparticulated metal oxides were reported by Nassar and coworkers.5 The use of nanoparticles has been suggested for the fast removal of asphaltenes from oils.6 It was reported that in situ preparation of NiO nanoparticles in crude oil led to asphaltene effective removal close to 2.8 g of asphaltenes/g of NiO.9 Massive precipitation of asphaltenes from crude oil, promoted by chromatography-grade commercial silica, was reported some time ago;10 thus, these high uptakes of asphaltenes by nanoparticles are expected. Correlations between molecular parameters and adsorbate uptakes were documented by Lopez-Linares et al.11 working with vacuum and visbroken bitumen, using modified kaolinites. A combined quartz crystal microbalance (QCM) and XPS method was employed to explore the adsorption of asphaltenes on a gold surface.12 XPS analysis of adsorbed and bulk asphaltene demonstrated the presence of carboxylic-, thiophenic-, sulfidic-, © 2014 American Chemical Society

Received: April 10, 2014 Revised: July 2, 2014 Published: July 22, 2014 4905

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with respect to crude oil for asphaltenes and resins I were about 15 and 10%, respectively. Separation of Resins II. These were obtained from the DAO above using the following chromatography procedure. For this, a continuous extraction chromatography was assembled by placing commercial silica gel (100 g, 130−270 mesh) soaked with n-heptane within a Soxhlet extractor. The DAO (5 g) dissolved in 10 mL of n-heptane was placed at the top of the column, and the extraction was accomplished by employing boiling solvent and using the following solvent gradient: heptane, heptane/toluene (1:1), toluene, toluene/chloroform (1:1), chloroform, and chloroform/methanol (1:1). The complete procedure lasted for about 98 h to afford 25% of resins II. Recovery was about 96%. Thin-layer chromatography plates or TLC glass plates (2 cm length and 1 cm width), covered with silica 60 with a pore diameter of 60 Å, pore volume of 0.8 mL g−1, and mean diameter size of 10−12 μm, obtained from Merck, were used here. The available area, close to 35 m2 g−1, was determined using solutions of p-nitrotoluene as described earlier.21 Asphaltene subfraction A2 was obtained from asphaltenes using the reported PNP method.32 Solubility of this fraction in toluene, close to 10%, is similar to the fraction for asphaltenes. Methods. Calibration Lines and Calculation of Both Solution and Surface Concentrations. Both surface and solution concentrations were calculated using procedures similar to others reported earlier.21 The concentrations in the range from 0 to 3000 mg L−1 for asphaltenes and asphaltene subfraction A2 and in the range from 0 to 1000 mg L−1 for resins were used. To avoid saturation of the ultraviolet (UV) detector, concentration−absorbance calibration curves were obtained with four different cell paths (cm) and four different concentration ranges (mg L−1) as follows: (1) 1, 0−400; (2) 0.5, 400−600; (3) 0.2, 600−1500; and (4) 0.1, 1500−3000. For these paths and ranges, straight calibration lines with r2 > 0.99 were obtained. Both adsorption and desorption measurements were made by duplicates and at different times (24, 48, and 72 h). Adsorption isotherms were measured by placing 7 mL of the toluene solution in small stoppered vials. The silica TLC plates were then immersed in toluene and contacted with the solution during the required time. Desorption isotherms were obtained after the adsorption experiments as follows: After an adsorption contact time of 72 h and at selected surface concentrations νF, the TLC plate was removed from the toluene solution, immersed for a few seconds (5−10 s; removing time, see below) in fresh toluene to remove small quantities of solution attached to the plate, and then reimmersed in fresh toluene to allow for desorption for 24 h. After this time, absorbance was measured and the corresponding solution concentration was plotted against the above surface concentration νF. Because in all cases desorption was very small, values of νF corresponding to the adsorption process were used for plotting purposes. To check reliability of desorption results, for some νF values, the removing time was varied within a 30 s period. No significant change in the desorption measurements was detected. AFM Measurements. For AFM measurement of electrical potential range coming from glass surface alone, the instrument tip was placed at several vertical locations from the glass plate and the corresponding potential was recorded. Zero reference of the vertical axis was taken equal to the axis corresponding to the potential minimum. In this way, values of electrical potential against vertical distance were plotted. For asphaltenes, a 10 mg L−1 solution in toluene was prepared and a drop of this (about 0.2 mL) was placed on the glass plate. Evaporation to dryness of this solution under nitrogen flow was allowed, and the deposited sample was examined under the AFM instrument. Results were obtained and plotted as described above for the glass plate alone. Profiles or cross-section heights of deposited sample, measured from the glass surface with the instrument tip, afforded values close to 200 nm high.

accounted for in terms of aggregate formation in solution. The results for the toluene/silica system were adjusted to a secondorder irreversible process.20 This was coherent with the very slow desorption process measured earlier.21 A QCM method was employed to examine adsorption of asphaltenes, resins, and asphaltene−resin mixtures dissolved in heptane−toluene mixtures adsorbed onto gold.22 Adsorption of resins on asphaltene surfaces lead to both multilayer formation and asphaltene dispersion into heptane.23 Such formation of the resin multilayer was coherent with previous results.24 Steps and other features of the isotherms, apparently consistent with micelle and hemimicelle aggregates, were reported when the adsorption on rock minerals from toluene solutions was studied.25 Adsorption on several clays, such as kaolinite, ilite, and montmorillonite, from toluene solutions was reported.26 Several techniques, such as N2 adsorption, X-ray diffraction, small-angle X-ray scattering (SAXS), and others, were employed to analyze the results.26 Adsorption of asphaltene functionalities, such as carboxylic, pyrrolic, and other, on goethite (FeOOH) suspended in heptol was reported.27 Irreversible adsorption of much diluted solutions of asphaltenes filtered on freshly cleaved mica was reported and studied using atomic force microscopy (AFM).28 When unfiltered samples were tried, large objects with finger lengths in the few micrometer range were observed.28 The adsorption of asphaltenes and resins from toluene solutions onto quartz and feldspar and their effect on the mineral−aqueous properties were reported; after the adsorption, no change in eletrophoretic mobility of these minerals was detected.29 Studies of adsorption of asphaltene solutions in toluene onto clay minerals in the presence of water show that the adsorption was reduced but not eliminated by the presence of water.30 A very recent review on asphaltene adsorption and other interesting relevant issues have been reported.36 Because of compliance of obtained isotherms to simple adsorption models, such as Langmuir and others, it has been assumed that asphaltene adsorption at the toluene−silica interface is a reversible process. However, as shown in a previous adsorption−desorption study, desorption into toluene from a silica surface of previously adsorbed asphaltenes was insignificant, thus suggesting an irreversible adsorption process.20,21 In fact, massive asphaltene precipitation was promoted by contacting the crude oil (25 mL) with commercial silica (5 g).10 The amount of asphaltenes precipitated in this way approached the total asphaltene content when the temperature approached 150 °C. Very significant quantities of precipitate were observed even at 0 °C.10 In addition to asphaltenes, in this work, the adsorption− desorption studies were extended to include the asphaltene subfraction A2, resins I, and resins II. As we did earlier,31 we call resins I the subfraction co-precipitated with asphaltenes and resins II the subfraction that remains in the deasphalted oil or DAO; asphaltene subfraction A2 was obtained from asphaltenes using the p-nitrophenol or PNP method.32

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EXPERIMENTAL SECTION

Materials. Asphaltenes and resins I were obtained from extra-heavy Cerro Negro oil [American Petroleum Institute (API) gravity = 8.3°] by dilution with 40 volumes of n-heptane using reported procedures.31 In this case, rather than diluting with toluene, the oil was heated (60 °C) and n-heptane was added under mechanical stirring. After working up, the precipitated solid (asphaltenes + resins I) was placed within a Soxhlet and the resins I were extracted as described earlier.31 Yields 4906

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Table 1. Yieldsa,b and Elemental Analysisc of Samples Studied

a

sample

%

C

H

N

S

H/C

asphaltenes asphaltene subfraction A2 resins I resins II

15 20−30b 18 17

82.34 81.88 83.1 80.8

8.16 8.00 10.1 9.6

1.4 1.4 0.8 0.7

5.64 4.78 4.1 4.7

1.19 1.17 1.46 1.43

With respect to the crude oil (this work). bWith respect to the asphaltenes. cElemental analysis for resins is taken from ref 31.

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RESULTS Yields and elemental analysis are shown in Table 1. As shown, elemental analysis is similar for resins I and resins II. Other measurements reported for these resins, such as molecular mass distribution [determined by size-exclusion chromatography (SEC)] and Mn [vapor pressure osmometry (VPO), with toluene], were also similar.31 As shown, no significant differences between asphaltenes and asphaltene subfraction A2 were found. Figure 1 shows the toluene solution−silica isotherm measured for both asphaltenes and asphaltene subfraction A2

concentration range. As shown, general trends of both samples in Figure 1 are similar and any possible structural difference between asphaltenes and asphaltene subfraction A2 is not reflected in these isotherms. A comparison of adsorption isotherms for resins I and resins II, plotted in Figure 2, shows no significant differences. These isotherms were measured after 24 h. As shown, large quantities of sample were also adsorbed in both cases.

Figure 2. Comparison of adsorption isotherms measured at the silica− toluene interface for sample resins I and resins II, measured after 24 h.

Table 2 shows results for adsorption and desorption for asphaltenes. Here, the first and second columns correspond to Figure 1. Isotherm measured at the toluene−silica interface corresponding to asphaltene and asphaltene subfraction A2 samples, registered after 72 h of contact at room temperature. Isotherms are Htype, meaning that, at high dilution, the final solution concentration is insignificant (see the text).

Table 2. Adsorption and Desorption Results for Asphaltenes

in the 0−3000 mg L−1 range after 72 h (see the Experimental Section). Isotherms were also measured at 24 and 48 h; apart from affording smaller surface concentrations, these show no significant differences with the isotherm measured at 72 h, and hence, they are not shown here. According to the literature,33 these isotherms are of type H, meaning that, at final solution concentration CF ≈ 0, the final surface concentration νF > 0. This is emphasized by the vertical line drawn at CF = 0. This revels that affinity between the surface and solute is very high.33 Using the method described,21 we estimate that a νF close to 7 mg g−1 would be required to saturate the silica surface with a monolayer of asphaltenes; as seen in Figure 1, this is close to values of νF corresponding to CF = 0. Thus, adsorption equivalent to multilayers is expected at any CF, except for extreme diluted concentrations (less than 10 mg L−1). Very large values of νF were observed in the investigated CF

CIa (mg L−1)

CFb (mg L−1)

νFc (mg g−1)

CDd (mg L−1)

400 600 800 1000 1500 2000 2500

238 361 564 706 1124 1333 2356

52.5 57 67.8 78.7 106.6 115.6 166

6.5 7.2 9.5 12.2 16.4 19.6 23.9

a Initial solution concentration. bFinal solution concentration corresponding to the adsorption process, measured after 72 h. cFinal surface concentration corresponding to the adsorption process, measured after 72 h. dFinal desorption concentration measured after 24 h.

adsorption experiments, whereas the third column corresponds to concentrations measured during desorption. Because desorption was very small and for plotting purposes, we use νF measured in the adsorption experiments. Desorption of adsorbate could not be detected below initial concentrations of 400 mg L−1 (corresponding to νF = 238 mg L−1 for asphaltenes and below 200 mg L−1 for resins I and resins II; see Table 1). 4907

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Figure 3 is a comparison of adsorption and desorption experiments performed as described above. Desorption experi-

Figure 4. Comparison of AFM measurements of electrical potential of a glass plate alone with the electrical potential corresponding to asphaltenes deposited on a similar glass plate. Vertical lines mark the range of potential measured (minimum to zero), and the arrows illustrate the potential range in each case.

asphaltenes is about 4.4-fold the range measured for the glass surface alone.

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DISCUSSION The results above are consistent with precipitation of sample at the interface. As mentioned above, massive asphaltene precipitation was observed when crude oils were contacted with powdered silica10 (see the Introduction); thus, asphaltene precipitation at the interface could be predicted. However, resin and asphaltene subfraction A2 precipitations are new results. Apparently, the surface promotes the formation of clusters or aggregates at the interface, which eventually leads to partial sample precipitation. The precipitated sample would then be in equilibrium with a very small quantity in solution, as suggested by the results of the desorption experiments (see Figure 3). Apparently, the electrical field prevailing at the surface tends to align the sample molecules in ways that enhance their interaction. The large amount of precipitated material, equivalent to many molecular layers, strongly suggests that the field effect extends into the solution well beyond the interface itself. This is coherent with the AFM results, where the field effect propagates up to 560 nm (see Figure 4 and below). Although in the asphaltenes case aggregate adsorption from the solution could not be discarded, this is not the case for resins, where aggregates in toluene have never been reported. Thus, the main effect should be precipitation promoted by the surface. Dipole moments, in the range from 4 to 8 D for asphaltenes and between 2 and 4 D for resins have been reported by Goal and Firoozabadi;34 thus, dipolar interactions with the silica surface could be predicted. Apparently, interaction with the field aligns the sample molecules in such a way to enhance polar and dispersion interactions, which propagates away from the surface, leading to precipitation. Thickness of the interface will thus grow until field effects become small compared to RT. Figure 5 depicts a crude simulation of the interactions described.

Figure 3. Comparison of adsorption and desorption isotherms for asphaltenes (top) and resins I (bottom), both measured at the silica− toluene interface, with adsorption measurements after 72 h for asphaltenes and 24 h for resins, at room temperature, and desorption measured after 24 h.

ments for the asphaltene subfraction A2 yielded results similar to asphaltenes and are not shown here. Operationally, the desorption experiment is the result of placing a silica plate loaded with νF (mg g−1) of sample in contact with fresh toluene, and then after 24 h, the small quantity desorbed from the plate will give the CD shown by the red points in Figure 3. In all cases, these desorbed values were very small, being a bit higher for resins. It is very important to note that νF, the final adsorbed quantity, is in equilibrium with CD rather than with CF, as should be the case for any adsorption process. AFM results for both the glass plate surface and asphaltenes deposited over the glass plate surface are compared in Figure 4. Attraction and repulsion between the surface and instrument tip are shown to the right and left of the minimum, respectively. It is important to note that, for the asphaltene case, the potential is both deeper and wider than the case measured for the glass surface alone. Thus, the vertical attraction potential range for 4908

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of aggregates formed in solution is not a condition for further clustering at the interface. Clustering at the interface, followed by sample precipitation, apparently accounts for the results presented here for both asphaltenes and resins. The above results and comments have both industrial and academic importance; from the academic point of view, it open a question on thermodynamic models currently used to simulate asphaltene adsorption on rock mineral because all of them assume adsorption equilibrium. As shown above, the quantity of asphaltene in solution, which is in equilibrium with the solid at the surface, is a minute quantity unrelated to the quantity predicted by the adsorption isotherm. From the industrial point of view, as shown by the AFM results, asphaltene deposition on the mineral surface extends field effects to distances several fold compared to the original surface, suggesting that, under favorable conditions, asphaltene and resin precipitation could grow unlimited, leading to the well-known asphaltene separation during production and other industrial operations. On the other hand, total precipitation by mineral surfaces could be used for DAO preparation and as an alternative method for asphaltene quantification in crude oils.

Figure 5. Model for arrangement of molecules of sample (asphaltenes or resins) at the toluene−silica interface. The figure represents a side view of the multilayer. As shown here, the local electric field aligns molecules with permanent or induced dipole, up to a height marked by the dashed line, where the multilayer reaches a thickness d and where the field effect either vanishes or becomes smaller compared to RT. The figure is not to scale and highly schematic.

These arguments are coherent with the AFM measurements plotted in Figure 4, where electrical potential measurements for both glass alone and asphaltenes deposited on glass are compared. As shown, the electrical potential for the asphaltene case is stronger and extends well beyond the interface up to a distance about 500 nm above the surface. Thus, asphaltene deposition enhances the electrical potential several fold, and such enhancement is coherent with the observed precipitation at the interface and with piling up effects (see below). Although precipitation of asphaltenes is an expected result10 (see Introduction), it is interesting that the same evidence for precipitation is observed for resins (see Figure 5). We believe that, in both asphaltenes and resins, besides polar interactions, dispersion interactions are essential for the observed precipitation at the interface. This should come about through induced polarization by the surface field. Once the first molecular layers are properly aligned by the electric field near the interface, precipitation will be enhanced by the action of dispersion interactions. The high molecular mass of asphaltenes and resins and their main structural features (polycyclic aromatic and aliphatic rings) lead to high polarizability and, hence, significantly induced polarization near the interface. It this well-known that polarizability increases with molecular volume.35 Extreme piling up of asphaltenes adsorbed at the glass− chloroform interface was reported by Castillo et al., where stalagmite-type columns of asphaltenes, with heights up to 200 nm, scattered over the entire glass surface were observed.17 This important finding is coherent with the results and discussion described above. A reasonable account of these findings17 would be that the stalagmite dimensions, height and width, should be determined by the height (distance d in Figure 5) and the width of the electrical local field. As mentioned above, using both ellipsometric16 and AFM methods, lengths in the 200 nm to a few micrometers28 range have been found for asphaltene clusters at the corresponding interfaces. These arguments and findings are coherent with the present AFM results (see above and Figure 4).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support provided by projects FONACIT (G2005000430) and CDCH (AI-03-12-5509-2004, PG-03-005732-2004, and PI-03-00-5648-2004) is gratefully acknowledged. The authors also thank Lic. Betilde Segovia for administrative assistance.

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REFERENCES

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FINAL COMMENTS The combined effects of the surface local fields, polarity, and polarizability of both asphaltenes and resins act synergistically to produce sample precipitation at the toluene−silica interface. Because precipitation was also apparent for resins, adsorption 4909

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