Asphaltene and Maltene Adsorption into Graphene - American

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Asphaltene and Maltene Adsorption into Graphene Estrella Rogel, Michael E. Moir, Matthew Hurt, Toni Miao, and Eddy Lee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01681 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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Asphaltene and Maltene Adsorption into Graphene Estrella Rogel,* Michael E. Moir, Matthew Hurt, Toni Miao, Eddy Lee Chevron Energy Technology Company. 100 Chevron Way. Richmond. CA 94801

*

To whom correspondence should be addressed. Telephone: (510)-242-1725. E-mail:

[email protected]

ACS Paragon Plus Environment

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ABSTRACT

In this work, the mechanism by which insoluble asphaltenes represented by small graphene fragments interact with different components of crude oils is explored. Adsorption isotherms of maltenes and asphaltenes into graphene from toluene were determined by the depletion method. The results indicate that the adsorption of these heavy fractions occurs into two steps: after the initial adsorption, a plateau is reached that seems to follow a Langmuir type adsorption isotherm. In the second step, at higher concentrations, the adsorbed amount starts to increase sharply after an inflection point indicating strong interactions between the adsorbates. A multilayer adsorption model was successful in describing this adsorption behavior. Dispersion of graphene in the solution was observed for some of the samples in the concentration range studied. It was found that asphaltenes can disperse graphene at lower concentrations than maltenes. From the same crude oil, it was observed that heptane extracted asphaltenes disperse graphene at lower concentrations than the pentane extracted ones. Mass spectrometry analysis of the species of molecules left in the solution after adsorption shows the preferential adsorption of highly aromatic molecules, high molecular weight molecules and molecules containing several heteroatoms. A reduction of 92 % on the relative abundance of porphyrins points out to a great affinity of these species to graphene. These results indicate that graphene-like molecules in hydrocarbons cannot remain in solution unless they are dispersed by other components. Therefore, the existence of a critical nanoaggregate concentration for these crude oil components is unlikely.


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INTRODUCTION In 1938, based on X-ray testing and oxidation reactions, F. J. Nellensteyn1 concluded that graphitic carbon is present in asphaltenes. Later, studies using X-ray diffraction2,3 and Raman spectrometry4,5 demonstrated that the core of the asphaltene aggregates consists of stacked aromatic sheets. The structure of the aggregates is maintained at least up to 300o C3 which is higher than the glass transition temperatures published for asphaltenes.6,7 On the other hand, XRD studies show that the distance between adjacent sheets is 3.6 Å independently of the type of asphaltenes. These results indicate that the formation of stacked structures is driven mainly by the aromatic moieties present in asphaltenes and that these interactions persist even at considerably high temperatures.2,3 For polyaromatic compounds, it has been shown that an increase in the surface area of the molecule leads to a more pronounced stacking tendency and reduces solubility.8,9 Similarly, it has been shown that the incorporation of aliphatic chains in large polyaromatic molecules increases their solubility.10,11 The increase in solubility depends on the number and volume of the chains.8,12 Also, different chain size and chain number induce different aggregation modes.13,14 These results with analog polyaromatic molecules agree with experimental data gathered for asphaltene fractions. Separation of asphaltenes in solubility fractions has shown that as the fractions become less soluble in hydrocarbons, their H/C molar ratios decrease and aromaticities increase.15-21 It has been reported that the solubility in toluene of some separated asphaltene fractions is around 1000 times lower than the solubility of the whole asphaltenes.22 These results, as well as other studies,21,23 point to the presence of molecules with very low solubility in the crude oil. It also indicates that the absence of molecules to act as a material of intermediate solubility between these low soluble asphaltenes and the dispersion medium can induce asphaltene precipitation.23


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Rather recently, images of asphaltenes obtained using Atomic Force Microscopy24 revealed the presence of molecules that resembled small graphene fragments. One of these molecules is shown in Figure 1. Based on their high aromaticity, we can expect these molecules to have a significantly low solubility in hydrocarbons or even to be insoluble in the crude oil if other components were not present. While the effect of asphaltene fractionation and solubility distributions on asphaltene behavior have been studied, information about the nature of the interactions between different crude oil fractions and their relationship to aggregation is scarce. A few studies have analyzed the interactions and aggregation effects when asphaltenes and resins are mixed,25-27 but clearly, more experimental and theoretical work is needed to uncover the synergistic effects on solubility and aggregation due to the blending of different crude oil components. In particular, a worthwhile objective is to understand how insoluble asphaltenes can be solubilized by the presence of other components and how the interactions between components can be linked to the structure of the aggregates. Analysis of adsorption data has been instrumental in understanding the dispersion of asphaltenes by resins25 and amphiphiles.28,29 In the present work, we want to explore the mechanism by which insoluble asphaltenes represented by small graphene fragments interact with different components of crude oils. We considered that graphene fragments would act as an extreme case of insoluble asphaltenes. Based on this assumption, we evaluated the adsorption isotherms of different heavy fractions into graphene. Our main goal is to understand the adsorption patterns of these fractions that can be related to the ability to keep the least soluble asphaltenes in solution.


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EXPERIMENTAL SECTION Materials. Asphaltene A was obtained by ASTM D-4055 (n-pentane extraction). Other asphaltenes (B, D, E and F) and maltenes (B, C) were obtained by ASTM D6506 (n-heptane extraction) from different crude oils. Maltene I and asphaltene fractions I, II, and III were obtained from a Heavy Crude Oil Vacuum Residue (HVCOR) according to a previously published procedure.20 Characteristics for maltenes, asphaltenes, and asphaltene fractions are shown in Table 1. Graphene Nanopowder (1-5nm) was obtained from Sky Spring Nanomaterials Inc. Surface Area for the graphene was determined to be 756 m2/g by the Brunauer–Emmett–Teller (BET) method using N2.30 A comparison between the relative sizes of an asphaltene molecule identified in a recent study using Atomic Force Microscopy24 and the approximated size range of the graphene sheets used in this study is shown in Figure 1. The asphaltene molecule shown in Figure 1 was one of several images of heptane asphaltene molecules obtained from a heavy crude oil. HPLC grade toluene was used as a solvent in the adsorption experiments and was obtained from Fisher Scientific and used without further purification.


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Table 1. Characteristics of maltenes and asphaltenes used in the study. Extracted from: Maltene B Maltene C Asphaltene A (n-pentane extracted) Asphaltene B

Heavy Oil B (9.4 oAPI) Vacuum Residue C (4.8 oAPI) Heavy Oil A (7.7 oAPI)

Asphaltene D Asphaltene E Asphaltene F Maltene I Asphaltene fraction I Asphaltene fraction II Asphaltene fraction III *N.A. not available.

Heavy Oil B (9.4 oAPI) Heavy Oil A (7.7 oAPI) Crude Oil E (24.9 oAPI) Crude Oil F (32.2 oAPI) Atmospheric Residue G (6.0 oAPI)

Carbon (wt%) 83.03

Hydrogen (wt%) 9.93

Nitrogen (wt%) 0.35

85.14

11.38

0.36

83.94

8.22

1.58

81.22

7.76

2.01

82.24

7.78

1.92

82.61

7.58

1.23

88.84

6.27

0.58

82.47

9.31

N.A.

82.54

8.45

1.61

82.47

7.90

1.89

82.43

7.71

1.99


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Asphaltene Molecule

Figure 1. Comparison between an asphaltene molecule identified using AFM24 and graphene sheets of sizes between 1 and 5 nm. Characterization of the Adsorbates. Elemental analysis of the samples (carbon, hydrogen, and nitrogen) analysis was carried out with a Carlo Erba model 1108 analyzer. For some of the adsorbates, the molecular weight was determined by size exclusion chromatography (SEC), using a 30 cm × 7.5mm PLgel “mixed E” column. Adsorbate solutions (10 ppm) were prepared using methylene chloride. In these experiments, a 90/10 methylene chloride/methanol blend was used as eluent with a flow rate of 1.0 mL/min. The HPLC system used to carry out the determinations consisted of a HP Series 1100 chromatograph and an Alltech ELSD 2000 detector. The molecular weights (MW) were calculated based on a calibration that uses porphyrins, dyes, and polyaromatics as standards as published previously in greater detail.31 Molecular weights for the adsorbates are shown in Table S1 (Supporting Information) Adsorption Isotherms. Adsorbed amounts were measured by the depletion method following the changes in the concentration of solute in toluene. In a typical experiment, around 0.01 g of graphene is put into contact with toluene solutions containing different concentrations of


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maltenes, asphaltenes or asphaltene solubility fractions (100 to 30,000 ppm). After 72 hours, the solutions are separated from the adsorbent. Preliminary kinetic experiments indicate that this time is enough to reach equilibrium concentrations. An example of the change in adsorbate concentration as a function of time can be found in Figure S1 (in Supporting Information). Taking care not to disturb the sedimented particles, the supernatant is decanted into an empty centrifuge tube. The resulting solution was centrifuged at 2000 rpm for 2 h to ensure that no solid particles remained in the liquid. Concentration in the liquid is determined using high-performance liquid chromatography (HPLC). The same HPLC setup described in previous paragraphs was used. Calibration curves were prepared for all the adsorbates used in this study. An example of a calibration curve can be found in the Supporting Information (Figure S2, Supporting Information). The amount of material adsorbed at equilibrium is given by: G = V(Co-Cf)

(1)

V is the volume of the adsorbate solution, Co is the initial concentration of adsorbate and Cf is the final concentration or equilibrium concentration of the adsorbate. Fourier Transform Infrared Spectroscopy. Infrared spectra were obtained using a Varian 7000e FT-IR infrared spectrophotometer. Transmission measurements were carried out with a Diamond Anvil Cell (DAC), which is used to compress the sample for transmission analysis. Spectra were measured from 4000 cm-1 to approximately 400 cm-1 using a Deuterated Triglycine Sulfate (DTGS) detector as the average of 32 scans acquired at a resolution of 4 cm-1. We used the infrared spectra results as a measurement of the presence of carbonaceous material in the adsorbate solution. If the graphene is dispersed in the solution, its concentration in solution can be measured


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semi- quantitatively using the height of the baseline at 2000 cm-1. A calibration curve prepared using blends of asphaltenes and graphene is shown in the Supporting Information (Figure S3). Mass Spectrometry Analysis. Solutions before and after adsorption were examined to determine the type of species that preferentially adsorbed into the graphene. The samples were analyzed using a LTQ Orbitrap XL (Thermo Scientific) equipped with a high-resolution detector (100,000 resolution at m/z 400). Atmospheric Pressure Photoionization (APPI) was the ionization technique used in the analyses. APPI in positive mode can efficiently ionize polycyclic aromatic compounds including those with heteroatoms.32,33 Double-bond- equivalents (DBE) were calculated using the standard equation.34 Petro-org software was used for the processing of the high-resolution data. RESULTS AND DISCUSSION Adsorption of Maltenes into Graphene. Adsorption isotherms of maltenes B and C are shown in Figure 2. In these adsorption isotherms, two regions can be observed. At low concentrations of maltenes in the solution, the maltenes are completely adsorbed. At these initial concentrations, the adsorption isotherm is practically vertical. After the initial adsorption, a plateau is reached that seems to follow a Langmuir type adsorption isotherm. In the second step, at higher concentrations, the adsorbed amount starts to increase again after an inflection point. Adsorption isotherms shown in Figure 2 have the characteristics of what is called “cooperative adsorption.” In this type of adsorption, molecules adsorbed by interaction with other adsorbed molecules and not based on interaction with the substrate. This behavior has been previously observed for the adsorption of ionic alkyl surfactants on graphitic carbons from aqueous solutions.35 Similarly, amphiphiles28,36 and resins29 in n-heptane have shown a similar behavior


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when adsorbed into asphaltenes. At the concentrations studied in this work, it was found that when Maltene B equilibrium concentration increases above ~7000 ppm, dispersion of graphene in the solution starts. The dispersion is experimentally observed as the measured equilibrium concentrations start to decrease and at high concentrations, they become larger than the initial concentration of the adsorbate in the solution. In the studied concentration range, Maltene C did not disperse the graphene in observable amounts. The adsorption behavior of the maltenes can be described using the Aranovich-Donohue equation37 developed to describe multilayer adsorption. This equation contains two terms: The first term described the formation of a monolayer corresponding to the first plateau in the adsorption isotherm. The second term corresponds to the formation of a multilayer. Both terms are independent and because of this independence, the first term can be the Langmuir, Freundlich or any other model describing a monolayer formation.37 In this work, we considered that the first plateau is described by the Langmuir model. Based on this consideration, the form of the equation is:38 S=K1(K2C/(1+K2C))(1/(1-K3C))

(1)

Where S is the adsorbed amount, while C is the equilibrium concentration. In the first term, K1 and K2 are the constants common to the Langmuir equation, while the second term represents the sigmoidal shape characteristics of the fast increase in adsorption after the first plateau. K3 and  are constants. The fitting of this equation to the experimental data is shown in Figure 2. Fitting parameters for the maltene isotherms can be found in Table S2 (Supporting Information)


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Figure 2. Adsorption isotherms of Maltenes B and C into graphene. Solid lines were obtained fitting the data to the Aranovich-Donohue equation.37 Adsorption of Asphaltenes into Graphene. Adsorption isotherms for heptane extracted asphaltenes are shown in Figure 3. In this figure, all the isotherms show a plateau. A further increase in concentration leads to, in the studied cases, the dispersion of graphene in the solution. The measurement of the isotherms was stopped when the dispersion of the graphene was detected. Based on the comparison of these isotherms, different asphaltenes start dispersing graphene at different concentrations. As in the maltene adsorption, the adsorption at low concentrations is very significant and, the remainder adsorbate in solution cannot be quantified. Two of the tested asphaltenes (B and E) also show a sharp increase in the concentration after a plateau like the one exhibit by the tested maltenes in the previous section. These asphaltenes are the ones that show graphene dispersion at higher concentrations. As in the case of the maltenes, asphaltene adsorption can be described using the Aranovich-Donohue equation37 as shown in Figure 3. This equation


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describes Langmuir behavior when an increase in adsorption could not be observed because graphene dispersion as is the case of asphaltenes D and F. Fitting parameters for the equations are shown in Table S2 (Supporting Information).

Figure 3. Adsorption isotherms for n-heptane extracted asphaltenes. Lines are included to help visualization. Solid lines were obtained fitting the data to the Aranovich-Donohue equation.37 Figure 4 shows a comparison between pentane extracted and heptane extracted asphaltenes (A and D). In this figure, both adsorption isotherms look quite similar. However, the adsorbed amount of pentane asphaltenes is lower. This behavior is the product of the relative solubility of both asphaltenes in toluene. It is expected that pentane extracted asphaltenes (A) are more soluble than those extracted using heptane (D) from the same crude oil. For instance, it is possible to prepare more concentrated solutions of pentane asphaltenes than of heptane asphaltenes in toluene without observing precipitation. As shown in Table 1, the heptane asphaltenes are more hydrogen


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deficient and, therefore less soluble than the pentane asphaltenes. Fitting parameters for pentane asphaltenes using Langmuir equation are reported in Table S1 (Supporting Information).

Figure 4. Comparison of adsorption isotherms for n-pentane and n-heptane extracted asphaltenes Solid lines were obtained fitting the data to the Aranovich-Donohue equation.37 Both pentane and heptane asphaltenes disperse graphene in the solution once they reach a certain concentration. However, pentane asphaltenes (A) begin to disperse graphene at a larger concentration than heptane asphaltenes (D) coming from the same crude oil. A similar behavior to the one observed in Figure 4 was obtained when adsorption behaviors of maltenes B and asphaltene B are compared (see Figure 4S in Supporting Information). The adsorbed amount of maltenes is lower than the adsorbed amount of the corresponding asphaltenes at the same equilibrium concentration. Furthermore, graphene dispersion by asphaltene B starts at around half the equilibrium concentration observed for graphene dispersion by maltene B.


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To verify the dispersion of the graphene in asphaltene solutions, the material in these solutions was recovered after evaporation for asphaltene A and analyzed using FTIR. The increase of carbonaceous material in the region where dispersion was noticed indicated that around 9-10 % of the material in solution is graphene, this will correspond to an amount in the range 0.1 to 1 mg/mL of graphene depending on the concentration of asphaltene A in the solution. Comparison of the maximum adsorbed mass densities at the plateaus observed in Figures 3 and 4 indicates values between 0.37 to 0.60 mg/m2, much lower than values reported for asphaltenes adsorbed on iron surfaces and minerals (1 to 5 mg/m2).39-41 If the observed plateaus in the isotherms correspond to the formation of a monolayer, values of area per asphaltene molecule can be calculated based on the BET value obtained for graphene. Using the molecular weights determined for asphaltenes (See Table S1, Supporting Information), the number of moles of adsorbate per unit area of graphene was calculated, and the area occupied by each adsorbate molecule was obtained. The values obtained indicated that the areas occupied in average for the studied asphaltenes are in the range: 200 to 300 A2 per molecule. These asphaltene areas are similar to values measured at the liquid air interface using surface tension measurements.42,43 This finding does not necessarily indicate that asphaltenes are actually forming a monolayer at the graphene surface, but it might mean that adsorption at the liquid air interface show similar characteristics to the graphene/toluene interface. It is noteworthy that the Langmuir monolayers of asphaltenes at the interface solvent-air have similar areas per molecule to the values reported in this work.44 To establish the characteristics of the molecules that preferentially adsorbed on graphene, for some solutions recovered after adsorption, the solvent was evaporated, and the material left in solution analyzed. Figure 5 shows a comparison between the molar hydrogen to carbon ratio before and after adsorption for asphaltenes A and D and maltenes B and C. In the four cases, initial


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concentrations were 1000 ppm. This plot also contains a comparison between the nitrogen contents. The plot shows that the material that remains in solution after adsorption is composed of molecules that contain more hydrogen and, therefore, indicates that the more hydrogen deficient (or more aromatic) molecules are the ones that preferentially adsorbed onto the graphene. As well, the molecules that are left in the solutions contain less nitrogen as expected, since nitrogen is usually associated with aromatic structures. Figure 6 shows the comparison of DBE vs C plots for asphaltenes A and F before and after adsorption. In both cases, initial concentrations were 1000 ppm. Figure 6 shows that highly aromatic molecules are adsorbed, but also high molecular weight molecules with low DBE disappear from the solution after adsorption, showing that these types of molecules adsorb preferentially into the graphene. These results support the conclusion previously stated that molecules and polymers exhibiting delocalized π-systems emanating from single to multiple ring structures have favorable affinities for graphene.45 However, the results show that large molecules, mainly aliphatic, in character adsorbed into the graphene. Studies of long n-alkanes adsorption on graphite have shown the cooperative adsorption of such molecules on graphitic surfaces.46


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Figure 5. Comparison of characteristics of some of the adsorbates before and after adsorption. A. H/C Ratio. B. Nitrogen Content


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Figure 6. Compositional distribution of species before and after adsorption. APPI positive Figure 7 shows a comparison of the class distributions for asphaltenes A and F before and after adsorption. These class distributions are based on weighted average intensities considering radical cations as well as protonated species generated by positive APPI. This plot shows the preferential adsorption of molecules containing several heteroatoms. This finding is in agreement with studies that indicated the preferential adsorption of molecules containing heteroatoms heteroatoms on mineral surfaces.47,48 In a separate study, it was shown that molecules with more than two heteroatoms preferentially precipitated out of solution,49 so this can be an effect of low solubility. Additionally, density functional theory calculations of the adsorption of aromatic molecules into graphene showed that the presence of heteroatoms in aromatic rings favors adsorption.50 Particularly interesting is the almost complete disappearance of porphyrins of the solution (92 % of reduction on relative abundance) for asphaltene A that points out to their great


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affinity to graphene. Previous research indicates that the adsorption of vanadyl etioporphyrin into graphene is a favorable process.51 The preferential adsorption of porphyrins into graphene has led to numerous studies in the electro-optical properties of graphene and nanocomposites. For instance, porphyrins have been used in the exfoliation of graphite to produce graphene,52 and in the control of the morphology of graphene hybrid materials.53 Interactions between porphyrins and graphene have been attributed to pi-pi stacking in different solvents.54,55 In addition, studies of porphyrins in petroleum have shown that porphyrins are associated with asphaltene aggregates.5658

On the other hand, evidence has been published about their activity as asphaltene inhibitors.59

This reported activity of porphyrins might be related to their properties that allow their use as exfoliators of graphite to produce graphene. Adsorption of Fractions Separated by Solubility. In an attempt to evaluate how the composition of different fractions of the asphaltenes affect their adsorption into graphene, Figure 8 shows the adsorption isotherm of maltene I onto graphene, while Figure 9 compares the adsorption isotherms of asphaltene fractions I, II and III. Fraction I contains asphaltenes having the highest solubility while Fraction III contains asphaltenes having the lowest solubility. The curves observed for maltene I and asphaltene fraction I are similar to the ones obtained for maltenes from different crude oils (see Figure 2) with a sharp increase after a plateau. In the range of adsorbate concentration studied, no graphene dispersion was observed for these two fractions. In contrast, fraction II and III exhibit graphene dispersion in the same range before a sharp increase in adsorption can be measured. Areas of the molecules calculated based on the formation of a monolayer in the plateau varied from 126 A2 (Maltene I) to 329 A2 (Asphaltene Fraction I).


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Figure 7. Class distributions of asphaltenes A and F before and after adsorption. Initial concentration is 1000 ppm. On the other hand, the equilibrium concentration needed to start the dispersion of graphene in the solution is lower for asphaltene fraction II than for III, indicating that there is an optimal composition of molecules to induce graphene dispersion at lower concentrations. Based on the


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results, asphaltene fraction II has a better activity as a dispersant of graphene that the rest of the fractions. Solubility parameters for the fractions increase from fraction I to fraction III (from 19.8 to 20.4 MPa0.5).20 The use of solubility parameters within the Flory approach60 to describe the dispersion behavior of exfoliated two-dimensional graphene nanosheets61 has been successful. Based on this approach, it is possible to hypothesize that, in a similar way to polymers, the concentration of dispersed graphene as a function of the solubility parameter of the asphaltenes follows a gaussian distribution with a maximum dispersion power around the solubility parameter of fraction II. Another aspect of interest is that molecules like the one shown in Figure 1 that resemble graphene cannot be dispersed into a petroleum system without the presence of other molecules that act as dispersant agents. This behavior seems to indicate that a critical nanoaggregate concentration is unlikely to exist in systems containing this type of molecules. After all, to be dispersed or stabilized in the fluid, they need to form part of aggregates. This finding also supports a recent hypothesis62 that indicates that precipitation or phase separation of a low solubility asphaltene subfraction can be suppressed by the presence of a higher solubility asphaltene subfraction. According to this hypothesis, the intercalation of a high solubility subfraction allows nuclei dispersion.62


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Figure 8. Adsorption isotherm of Maltene I into graphene. Solid lines were obtained fitting the data to the Aranovich-Donohue equation.37

Figure 9. Adsorption isotherms of asphaltene fractions into graphene. Solid lines were obtained fitting the data to the Aranovich-Donohue equation.37


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CONCLUSIONS Crude oil asphaltenes and maltenes adsorb at the graphene-toluene interface in two steps. In the first step, they reach an adsorption plateau that can be described using the Langmuir model. In the second step, as their concentration increases, the adsorbed amount increases sharply. This behavior indicates the formation of a multilayer, showing the existence of strong interactions between adsorbate molecules. Modeling of the adsorption isotherms can be achieved using the Aranovich-Donohue equation. At the concentrations studied, it has been shown that asphaltenes and some maltenes can disperse graphene in toluene. For some of the adsorbates, dispersion of graphene seems to be associated with multilayer formation. However, this could not be observed for several of the asphaltenes. In these cases, dispersion of graphene starts before multilayer formation could be detected, but it happens after the adsorbed amount is constant and a well-defined plateau is reached. This might indicate that the incipient formation of a multilayer could be the trigger for graphene dispersion. Dispersion of graphene by asphaltenes and maltenes depends on the characteristics of the adsorbate. In general, asphaltenes are better dispersants for graphene than maltenes as they work at lower concentrations. Additionally, not all the tested asphaltenes act similarly as graphene dispersants. Minimal concentrations to initiate graphene dispersion vary depending on the asphaltene characteristics. It was found that an intermediate asphaltene fraction has the lowest concentration to start dispersion indicating that this fraction contains an optimal composition of dispersant molecules.


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Mass spectrometry analysis of the species of molecules left in the solution after adsorption show the preferential adsorption of highly aromatic molecules, but also high molecular weight molecules with low DBE disappear from the solution after adsorption. Additionally, the preferential adsorption of molecules containing several heteroatoms was observed. Asphaltenes containing nitrogen are preferentially adsorbed compared with those that do not contain heteroatoms or contain sulfur. Particularly interesting is the reduction of 92 % on the relative abundance of porphyrins in solution after adsorption, indicating a great affinity of these species to graphene. Finally, these results seem to indicate that the solubilization or stabilization of graphenelike molecules in hydrocarbons requires the presence of other molecules that can act as dispersants. This evidence puts in doubt the existence of critical nanoaggregate concentration as these graphene-like molecules cannot remain in solution unless they are dispersed by other components.

ACKNOWLEDGMENT

The authors wish to thank Chevron ETC for financial support and for permission to publish this work.

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Asphaltene and Maltene Adsorption into Graphene - American

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