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Retention of alkane compounds on asphaltenes. Insights about the nature of asphaltene-alkane interactions Miguel Antonio Orea, Maria Antonieta Ranaudo, Patricia Lugo, and Liliana López Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01152 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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Retention of alkane compounds on asphaltenes. Insights about the nature of asphaltene−alkane interactions Miguel Orea,*†§ María A. Ranaudo, † Patricia Lugo, ‡ and Liliana López. ‡ †

Escuela de Química. ‡Instituto de Ciencias de la Tierra, Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1041 A, Venezuela.

§

Consultores Analíticos Integrales, ChemiConsult, C.A. Carrizal, Estado Miranda. Venezuela.

KEY WORDS: Asphaltenes, alkanes, wax, interactions, adsorption, retention.

ABSTRACT.

Retention of alkane compounds on asphaltenes was carried out in organic solutions. The distribution of retained and non-retained alkanes was assessed by GC/MS and interpreted from the perspective of reverse phase liquid chromatography (RPLC). Results showed that asphaltenes from unstable and stable Venezuelan crude oils behave like an alkyl-bonded phase to retain alkanes. The retention is influenced by the structural features of the studied asphaltenes, as well as by the own structure of the interacting alkanes. Asphaltenes from the unstable Furrial crude oil presented shorter but more abundant alkyl chains than asphaltenes from the stable Ayacucho

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crude oil; and consequently a greater capacity to retain alkane compounds. Nevertheless, the preference for a particular group of alkanes was very similar in both asphaltene samples: nalkanes were the strongest retained, followed by cyclic isoprenoids (steranes and terpanes). Acyclic isoprenoids, including pristane and phytane were the least retained of the three alkane families. The exclusion of methylene groups −a phenomenon observed in RPLC− was also observed in asphaltenes. This phenomenon could be responsible for the appearance of liquidcrystal-like structures in asphaltene-alkane composites and for the destabilization of asphaltene assemblies. The approach used in this work opens a new window to interpretation of the interactions of asphaltenes and alkanes at a molecular level and to understand the roles of both compound classes in the mechanisms of asphaltene precipitation and wax crystallization.

INTRODUCTION Background. Asphaltene precipitation in production, transportation, and storage of crude oils is of great concern to the oil industry.1,2 The problem has prompted a vast number of research works aimed to understand the phenomenon at a molecular level and to identify the most important factors that control aggregation, growth, and precipitation of asphaltene particles.1−6 It is currently known that changes in pressure, temperature and composition can initiate asphaltene precipitation; and that some crude oils are more susceptible to these changes than others.2,7 Asphaltene precipitation leads to the formation of solid deposits in porous media, wellbores, pumps, pipelines, storage tanks, and surface facilities.1,2 Asphaltenic solid deposits found in Venezuelan oil wells and surface facilities not only comprised asphaltene constituents, but also included contents of high-molecular-mass (HMM) alkanes ranging between 1.2 to 31.8 % m/m, lower quantities of aromatic hydrocarbons and


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resins, and in some cases, lower content of inorganic materials.2 In most of these deposits, alkane compounds were mainly linear with chain lengths ranging between n-C26 to n-C160.2,8 HMM alkanes present in asphaltene deposits have encouraged several researchers to focus their attention on understanding this phenomenon. Most investigations have been devoted to studying macroscopic effects (e.g., changes in the asphaltene precipitation onset,9 variations in wax appearance temperature,10−12 rheological behavior of oils,13 and effects on the performance of inhibitors10) rather than addressing the issue at a molecular level. Indeed, few studies have been published in this regard and they have shown very interesting results. For example, Rogel14 concluded, on the basis of molecular thermodynamics calculations, that the interaction of asphaltenes with alkanes is a favorable process; while Mahmoud et al.,15 Stachowiak et al.,16 and Nikooyeh et al.17 found experimentally that the mixing of n-alkanes with asphaltenes leads to exothermic effects. Furthermore, it has been shown that solid asphaltenes swell when they are in contact with n-alkanes,17,18 and that large n-alkanes quasi-crystallize at the periphery of flocculated asphaltenes through the formation of a liquid- crystal-like structure.15,17,19 Although these phenomena might involve a direct participation of the alkyl chains attached to the aromatic cores of asphaltenes, the evidence is not enough to assert this fact neither to fully depict the pathways followed by alkane compounds to interact with asphaltenes. Questions like why long n-alkanes are preferably engaged with asphaltenes during precipitation or how different alkane compounds are accommodated onto the asphaltene structure to form liquid-crystal-like complexes demand explanation to fully understand the mechanism of asphaltene precipitation in the presence of HMM alkanes, as well as the mechanism of wax crystallization in the presence of asphaltenes.


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Considering a probable participation of asphaltene alkyl chains in the interaction with alkanes, it is valid to think that asphaltenes might behave like an alkyl-bonded phase (similar as those used in RPLC) to retain alkane hydrocarbons. Under this perspective, it is possible to obtain a reasonable explanation for the preferential association of asphaltenes with HMM n-alkanes and to determine if the interaction also occurs with branched and cyclic alkanes or if it is exclusive to the n-alkanes. Based on a thermodynamic framework developed for a chromatographic approach, in this paper we report a method to confirm the participation of asphaltene alkyl chains in the interactions with alkanes and to assess the effects of asphaltene and alkane structural features on their mutual attractions. Chromatographic approach. Liquid-crystal-like structure formation is a very distinctive phenomenon that occurs when asphaltenes are mixed with linear, high-molecular-mass alkanes.15−17,19 This phenomenon seems to be related to a partial immobilization of the n-alkane molecule on the asphaltene network;15−17,19 so that the immobilized part of the n-alkane behaves like a solid crystal, while the free part of it can rotate and vibrate like a liquid. The consequence of this state is the appearance of a birefringent surface when particles of asphaltene-n-alkane composites are exposed to polarized light.15−17,19 In composites formed by asphaltenes and neicosane (n-C20H42) the number of methylene groups (−CH2−) of the n-alkane involved in the interaction (i.e., the immobilized part of the molecule) has been estimated between seven and ten.15 This means that about 45% of the total molecular length of n-eicosane is immobilized on the asphaltene periphery; whereas the remaining 55% is free to rotate and vibrate, like a liquid. The partial immobilization of straight alkyl compounds also occurs in RPLC and it is known as the “methylene group exclusion” phenomenon. This phenomenon, which is quite similar to the


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one observed in asphaltenes, was reported by Tchapla et al.20 in 1984 while studying the separation of homologous series of n-alkyl derivatives on different n-alkyl-bonded stationary phases. In chromatography, the distribution of an analyte between the stationary and mobile phases is often described by the partition coefficient K, which is defined as the molar concentration of the analyte in the stationary phase divided by its molar concentration in the mobile phase.21 On the other hand, a measure of the time the analyte resides in the stationary phase relative to the time it resides in the mobile phase is given by the capacity factor k’:21 k’ = tR/tM

(1)

Where tR is the retention time of the analyte and tM is the time taken for the mobile phase to pass through the column. If K is independent of the analyte concentration, then k’ becomes equal to the ratio of the contents of the analyte in the stationary and mobile phases.21 If the mole fraction of the analyte in the mobile phase is xm, then the mole fraction in the stationary phase is 1−xm.Thus: k’ = (1−xm)/(xm) (2) Equation (2) indicates that the capacity factor of a given analyte can be determined from its retained and non-retained mole fractions, as long as the partition coefficient K is independent of the analyte concentration.21 Under this condition, k’ becomes independent of the flow rate of the mobile phase and of some physical properties related to the stationary phase such as its particle size, porosity, and volume. That is, k’ will only depend on temperature and on the chemical


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features of the three elements involved in interactions; namely, the analyte, the mobile phase, and the stationary phase.21 Tchapla et al.20 found a linear relationship between log k’ and the length of the alkyl chain of R−X derivatives (R= a straight alkyl chain and X= −CH3, −OH, −COOH, or −C5H5) given by the following equation: log kn’ = nc × (αCH2) + q

(3)

Where kn’ is the capacity factor of the nth homolog, nc is the length of the alkyl chain (expressed as number of carbon atoms), αCH2 is the methylene group selectivity, and q is a constant that depends on the chemical nature of the ending group –X. The methylene group selectivity measures the contribution of an individual methylene moiety of the alkyl chain to the overall retention of the derivative; whereas q measures the contribution of the ending group to retention.20,21 The linear increase of log k’ with nc was observed as long as the chain length of the derivatives did not overpass the length of the n-alkyl ligand of the stationary phase; otherwise, a discontinuity (an inflection) appeared in the plot of log k’ vs. nc.20 In the separation of even nalkanes ranging from C8 to C16 on an n-dodecyl (RP C12) bonded phase, it was observed that log k’ increased linearly with nc just up to nc=12. Above this value, the curve showed an inflection, because the incorporation of additional methylene moieties in the n-alkane structure (i.e., as in n-C14 and in n-C16) did not contribute to the overall retention.20 As those additional methylene groups did not find an available adsorption site on the alkyl chain of the stationary phase, they


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remained “excluded” from the interaction. The break point was better defined when αCH2 (calculated as the ratio of log k’ of two successive homologues) was plotted against nc.20 The phenomenon was also observed at nc values of 6, 8 and 18 when n-hexyl (RP C6), n-octyl (RP C8) and n-octadecyl (RP C18) bonded phases were used as stationary phases.20 This behavior indicates a direct dependence between the nc value at which the discontinuity appears and the ligand chain length of the stationary phase.20,21 The comparison of the “methylene group exclusion” phenomena observed in RPLC with the formation of liquid-crystal-like structure in asphaltene−n-alkane composites reveals some similarities. Both phenomena can be interpreted as the “partial immobilization” undergone by an n-alkyl molecule when only a part of its total chain length becomes engaged in the interaction with a single n-alkyl ligand. Thus, the engaged part of the n-alkyl molecule becomes “quasicrystallized” or “partially immobilized“ by the n-alkyl ligand, whereas the other part −the one that is “excluded from” or “unengaged in” interactions− is free to rotate and vibrate like a liquid. This simple comparison opens a new window to studying asphaltene−alkane interactions by means of the thermodynamic framework developed by Tchapla et al.20 if asphaltenes are considered as an alkyl-bonded phase. A first attempt to undertake a study of this nature might consist of packing a column with solid asphaltenes and performing some runs with selected alkane standards in order to determine their retention times after elution with a suitable mobile phase. However, by using this experimental setup, it would be difficult to recognize the participation of asphaltene alkyl chains in the retention process. Carbognani et al.22 have demonstrated that the separation of saturated compounds on columns packed with solid asphaltenes follows an occlusion−exclusion


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mechanism that is driven by the porosity of asphaltenes rather than their attached alkyl chains. Additionally, the presence of saturated compounds mixed with asphaltenes could improve the solubility of the latter in the mobile phase;23 so the results obtained under flowing conditions would yield insufficient detailed information as to build a picture of the asphaltene−alkane interaction process at a molecular level. A more promising approach would be to carry out the interaction process under static conditions and with asphaltenes dissolved in a suitable solvent. As the capacity factor does not depend on the flow rate of the mobile phase, it is irrelevant to perform retention tests of alkane compounds on asphaltenes using flow or static conditions. This interesting property of k’ could facilitate the development of a simple methodology that overcomes the drawbacks of using solid asphaltenes contained in packed columns. In this paper we describe a method to assess asphaltene−alkane interaction under static conditions and by using asphaltenes in solution rather than in a packed column. The method consists of: i) the formation of asphaltene−alkane complexes in solution, ii) the separation of saturated compounds not retained by asphaltenes from the asphaltene−alkane complexes, and iii) the disruption of the asphaltene−alkane complexes with the subsequent recovery of the retained saturated compounds. These three unsophisticated steps afford the quantitative separation of saturated compounds retained and non-retained by asphaltenes through simple procedures routinely used in most organic chemistry laboratories. GC/MS analyses of retained and nonretained saturated fractions allow alkane identification and quantification; hence, the calculation of k’. This approach could contribute to understanding the interaction process between


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asphaltenes and alkanes at a molecular level and to provide some answers to the questions stated above. EXPERIMENTAL SECTION Materials and samples. HPLC grade solvents (n-hexane, n-heptane, and chloroform) were provided by J.T. Baker®. Alumina (Riedel de Haën, 50-200 µm particle size) and silica gel (Merck, 230−400 mesh) were activated at 200 °C for 24 h before use. Ayacucho and Furrial crude oils, from the Eastern Venezuela Basin, were selected for the study. These crude oils are significantly different in their API gravities and in their asphaltene contents and stability. Ayacucho is a biodegraded,24 extra heavy crude oil (8 °API gravity25) with 16 % m/m of n-C7 asphaltenes and a low tendency to asphaltene precipitation.25,26 Furrial is a medium (21 °API gravity25), non-biodegraded crude oil,27 with an n-C7 asphaltene fraction (5.3% m/m) that shows a strong tendency to flocculate and precipitate.25,26,28 These remarkable differences in crude oil characteristics make these samples of great interest for studying asphaltene−alkane interactions. Asphaltenes were obtained by adding 40 volumes of n-heptane to the crude oils as described elsewhere.29,30 The resulting mixture was kept under constant stirring (125 rpm) for 30 min and allowed to stand for 24 h at 4 °C. Then, the asphaltene fraction was separated from the maltenes by centrifugation and further extracted at 85°C with n-heptane in a Soxhlet system to eliminate the co-precipitated material.29,30 The saturate fraction employed in this study was obtained from Furrial crude oil. The reason of using this particular fraction to study asphaltene−alkane interactions is because alkane


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distribution of this crude oil has remained unaltered and free from the effects of biodegradation during the time the crude oil has been trapped in the reservoir; which facilitates compound identification by GC/MS. The saturate fraction was obtained from the maltenes by adsorption chromatography separation in an open glass column (50 mm length × 20 mm i.d.) dry packed with activated neutral alumina. The mass ratio of alumina to sample was 60:1. Saturate fraction was eluted with 100.0 mL of n-hexane and further recovered after solvent evaporation.29,30 Characterization of asphaltenes by FTIR and determination of structural indexes. Asphaltenes from Ayacucho and Furrial crude oils were analyzed by infrared spectroscopy in order to correlate their structural features with their ability to retain alkane compounds. Quadruplicates of asphaltene films of about 20 µm thick were obtained after drop-wise deposition of 500 µL of 1% m/v asphaltene−CHCl3 solution on KBr plates. Spectra were recorded in transmittance mode on a Varian 640 IR spectrometer with a spectral resolution of 4 cm-1 over the range of 4000−400 cm-1 and 32 scans. The OMNIC 7.3 software was used for data processing. The mathematical deconvolution of the IR spectra was performed using the "AutoFourier deconvolution” function available in the software, and the spectral signals were valleyto-valley integrated in order to calculate some structural indexes according to methodologies reported in the literature.31−36 Method to assess saturated hydrocarbon retention by asphaltenes in solution. In order to verify the absence of alkane compounds that could interfere in the retention experiments, we carried out a series tests on materials, solvents, and asphaltene samples before use. Solvent purity (chloroform and n-heptane) was assessed through residue formation after total evaporation of 100 mL at reduced pressure. Silica gel (adsorbent) purity was tested by packing an opened glass


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column (250 mm ×20 mm i.d.) with different quantities of the adsorbent and percolating 100 mL of chloroform at 20 °C or n-heptane at 70 °C. Percolated solvents were recovered and evaporated at reduced pressure to obtain the evaporation residue. For asphaltenes samples, the test consisted in loading concentrated solutions (100.0 or 200.0 mg dissolved in 20.0 mL of chloroform) in a glass chromatography column packed with silica gel. Volumes of 50.0 or 100.0 mL of both chloroform at 20 °C and n-heptane at 70 °C were passed through the column. Effluents were collected and treated as indicated above to obtain the evaporation residue. The presence of interferences was not detected in any of the tested materials. This set of experiments was also used to optimize the quantities of silica gel, asphaltenes, and solvents needed to carry out the retention experiments. Additionally, systematic errors associated to unwanted fractionation of alkanes either by temperature or by size exclusion mechanisms8 were also evaluated. In the first case, a 3000 mg L−1 solution of Furrial saturates dissolved in chloroform was kept in darkness at 20 ± 2 °C for seven days (168 h) and visually inspected on a daily basis for temperature-induced-precipitate formation. Under these conditions, no cloudy appearance of HMM alkane precipitates was observed; which means that the solution remained stable. In the second case, the influence of size exclusion mechanisms was evaluated by means of the percentage recovery obtained after percolating Furrial saturates through a silica-gel-packed column. In this experiment, 10.0 or 50.0 mg of saturates previously dissolved in 20.0 mL of chloroform were eluted with 100 mL of chloroform at 20 °C, with 100.0 mL of n-heptane at 70 °C, and with a solvent sequence of chloroform (100 mL, 20 °C) followed by n-heptane (100.0 mL, 70 °C). For all cases, the recovery ranged between 99.0 and 100.5%, showing no evidence of unwanted alkane fractionation under the experimental conditions.


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The retention experiments were carried out in asphaltene solutions as follows: pure asphaltenes (100.0 mg) contained in a 50-mL-screw-cap vial were dissolved in 10.0 mL of chloroform. Then, 10.0 mg of the saturate fraction was dissolved in 2.0 mL of chloroform and quantitatively added to the asphaltene solution. The final solution volume was completed to 20.0 mL. The resulting concentration of asphaltenes and saturates were 5000 mg L−1 and 500 mg L−1 respectively. A schematic of the experimental setup is show in Figure 1. In order to investigate the effects of the concentration of saturates on the retention capacity of asphaltenes, the same procedure was repeated with 20.0, 40.0 and 60.0 mg of the saturate fraction to obtain solution concentrations of 1000, 2000, and 3000 mg L−1, respectively. To evaluate the retention capacity of asphaltenes at lower concentrations of saturates, we prepared two additional mixtures for Furrial asphaltenes and one for Ayacucho asphaltenes. In the case of Furrial, the first mixture contained 5000 mg L−1 of the asphaltenes and 50 mg L−1 of saturates, while the second one contained 5000 mg L−1 of the asphaltenes and 100 mg L−1 of saturates. Each mixture was prepared by directly mixing 100.0 mg of Furrial asphaltenes with 20.0 mL of a chloroform solution containing either 50 or 100 mg L−1 of the saturates. Saturate solutions were previously prepared by successive dilution of a 1000 mg L−1 stock solution of Furrial saturates. In the case of Ayacucho, the prepared mixture contained 5000 mg L−1 of the asphaltenes and 50 mg L−1 of the saturates. All the vials were properly capped to avoid solvent losses, gently shaken, and kept in darkness at 20 ± 2 °C for seven days (168 h). Asphaltene−alkane complexes were formed under this condition.


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As observed by several researchers, the state of asphaltenes aggregation can change their adsorption properties37,38 and their interactions with surrounding alkanes.11,12,23 To control the development of this potential interference, a chloroform solution of asphaltenes and saturates from Furrial crude oil (5000 and 3000 mg L−1, respectively) was prepared and kept in darkness at 20 ± 2 °C for seven days (168 h). Daily inspection through an optical microscope (Carl Zeiss, model Axio Lab A1 at 100× magnification) of this control solution did not show any asphaltene particles formation. After completing the time required for asphaltene−alkane complex formation, the mixtures were quantitatively transferred to opened glass columns (250 mm ×20 mm i.d.) packed with 50.0 g of silica gel in order to adsorb asphaltenes and asphaltene−alkane complexes onto the silica gel surface. The free saturates (those not retained by asphaltenes) were easily separated after eluting with 100.0 mL of chloroform (4 × 25.0 mL) at 20 °C. These compounds were collected as fraction f1; while asphaltenes and asphaltene−alkane complexes remained adsorbed on the top of the columns. Recovery of saturates retained by asphaltenes required the disruption of the asphaltene−alkane complexes. The disruption was induced by adding n-heptane at 70 °C to the packed columns. Released saturated compounds were recovered at the bottom of the columns as fraction f2 after eluting with 100 mL (4 × 25.0 mL) of n-heptane at 70 °C. Fractions f1 and f2 were quantified gravimetrically after solvent evaporation at reduced pressure. Alkane distributions (n-alkanes, pristane, phytane and steranes) were determined in these two fractions by GC/MS.


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GC/MS Characterization of saturated hydrocarbons retained and non-retained by asphaltenes. Gas chromatography−mass spectrometry analysis was carried out on a 6890N Agilent Technologies network gas chromatograph provided with a DB-1 fused capillary column (60 m × 0.25 mm × 0.25 µm) and coupled to a 5975 Agilent Technologies mass spectrometer operated in single ion monitoring mode. Monitored ions were m/z 113 for determination of nalkanes and acyclic isoprenoids, and m/z 217 and 191 for steranes and terpanes, respectively.29−30 Capacity factor (k’). To determine the capacity factor of individual alkane compounds involved in the interaction with asphaltenes, their molar fractions in f1 and f2 were determined from individual peak areas of the mass chromatograms. k’ was calculated according to equation (2).20,21 Graphical representations of log kn’ as a function of the chain length (nc) of retained nalkanes were constructed for every initial concentration (C0) of saturates. Methylene group selectivity (αCH2) was also plotted as a function of nc, after being calculated from equation (4).20,21 αCH2= [log (k’(i+1))] / [log (k’i)]

(4)

Where k’i is the capacity factor of n-alkane with chain length nc = i and k’(i+1) is the capacity factor of n-alkane with chain length nc = i+1. RESULTS AND DISCUSION Structural features of asphaltenes from Ayacucho and Furrial Crude oils. Structural characteristics of Ayacucho and Furrial asphaltenes were determined by FTIR. The corresponding spectra and assignment of absorption bands are summarized in the supplementary material that accompanies this paper. To estimate the structural differences between Ayacucho


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and Furrial asphaltenes, FTIR structural indexes were calculated as the average of four determinations using the area of absorption bands after the deconvolution of spectra in the 4000−400 cm-1 region (Figure 2). The adopted methodology was reported by Permanyer et al.,31 and Abbas et al.32 The structural indexes used in this work are listed in Table 1. In order to aid the discussion, it is convenient to take one of the asphaltene samples as the basis of comparison and describe the other sample with respect to it. Therefore, the differences between the FTIR indexes of Furrial and Ayacucho asphaltenes were expressed as percentages on the basis of the properties of Ayacucho asphaltenes. The results are listed in Table 2. The percentage difference obtained for the aromaticity index A indicates that Furrial asphaltenes are 50.0% more aromatic than Ayacucho asphaltenes. Similarly, the aromatic ring index AR indicates that the population of aromatic rings in Furrial asphaltenes is 38.0% higher than in Ayacucho asphaltenes; while the degree of condensation, given by the aromatic condensation index AC, is also higher by 32.4%. The relative distribution of peri-condensed and cata-condensed aromatic structures can be assessed through the As2 and As3 indexes, respectively. In this regard, a difference of only 0.3% of the As2 index reveals that the contents of peri-condensed structures in Furrial and Ayacucho asphaltenes are very close. However, in the case of the As3 index, a difference of −7.7% expresses an important deficit of cata-condensed structures in Furrial asphaltenes. Regarding the aliphatic groups, the CH2/CH3 index −used to estimate the average length of alkyl chains attached to aromatic cores31,32− indicates that these substituents are 10.3 % shorter in Furrial asphaltenes than in Ayacucho asphaltenes. On the other hand, the long chain index (LC), which is used to assess the number of alkyl chains longer than four carbon atoms,31,32 shows that


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population of these substituents in Furrial asphaltenes is increased in 47.4% respect to Ayacucho asphaltenes. According to these results, Furrial asphaltenes are more aromatic than Ayacucho asphaltenes; which agrees with 13C-NMR data reported by Acevedo et al.39 for the same asphaltene samples. Aromatic cores in Furrial asphaltenes are more condensed and their attached alkyl chains are shorter but more abundant in comparison to Ayacucho asphaltenes. These results are in agreement with the notion that asphaltenes from unstable crude oils are structurally different than corresponding asphaltenes from stable crudes.2,39 Such structural differences are also expected to have a strong effect on the capacity of asphaltenes to retain saturated compounds.

Retention of saturated hydrocarbons by asphaltenes. As stated in the introduction section, the method reported in this paper consists of three stages: i) asphaltene−alkane complex formation, ii) recovery of non-retained saturates, and iii) recovery of retained saturates after disrupting the asphaltene−alkane complexes. Each of these stages was optimized to minimize systematic errors and interferences during the process of asphaltene−alkane complex formation and during the recovery of retained and non-retained saturate fractions. A summary of results is shown in Table 3. Stapf et al.40 have demonstrated, −on the basis of NMR relaxation measurements− that the formation of asphaltene−alkane complexes takes place at 20 °C in oil media. Mahmoud et al.15 reported that interactions that give rise to asphaltene−alkane complexes take place irrespective of the asphaltene state and occur either with precipitated asphaltenes or in solutions containing dissolved asphaltenes. In our case, asphaltene−alkane complexes were formed in solution, using chloroform as a solvent.


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The use of chloroform has several advantages. First, its high polarity diminishes self-association of asphaltene,41−43 preventing the alteration of their adsorption properties. Second, the interaction between alkanes and asphaltenes is enhanced, since accessibility to potential interaction sites is considerably improved;11 and finally, its low boiling point facilitates evaporation of the separated saturate fractions. Volumes of chloroform and n-heptane required for the recovery of fractions f1 and f2 were also adjusted to minimize any unwanted fractionation of saturates caused by size-exclusion mechanisms that take place in the silica gel adsorbent.8 Thus, in absence of asphaltenes, the elution with 100.0 mL of either chloroform at 20 °C or n-heptane at 70 °C was sufficient to recover 99-100.5% of Furrial saturates from the silica gel column (Table 3, runs 12, 13, and 15 for chloroform and 14, 16, and 17 for n-heptane). After sequential elution with 100.0 mL of chloroform followed by 100.0 mL of n-heptane at the indicated temperatures, a saturate fraction was obtained from the chloroform affluent with a recovery between 99.5−99.7% (runs 18 and 16; Table 3). In the presence of asphaltenes, the elution with n-heptane yielded a second saturate fraction after the one obtained by eluting with chloroform (Table 4). The recovery of two fractions (f1 and f2) under these conditions is a consequence of the mutual interactions between alkanes and asphaltenes in solution and not an artifact produced by a possible intervention of silica gel. Certainly, it must be considered that during the recovery of f1 and f2, an additional mechanism could have taken place involving asphaltenes in adsorbed state rather than dissolved. Simon et al.38 observed that asphaltenes modify their fractal dimension and adopt new conformations when they adsorb from solutions onto the surface of silica gel. Some of these configurations


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might facilitate the interaction with alkanes and also contribute to the retention process carried out in solution. Table 4 shows the results obtained during the tests of hydrocarbon retention on asphaltenes. Mass fractions of alkanes not retained (Xf1) and retained (Xf2) by Furrial and Ayacucho asphaltenes and the percentage of recovery for each run are summarized. Duplicates of some runs (identified with an asterisk) were carried out in order to test repeatability of the method. Recovery percentages of the separation procedure ranged between 95-104%. To measure the capacity of asphaltenes to retain saturated hydrocarbons, we defined the parameter M as the amount in milligrams of alkanes retained by 100 mg of asphaltenes. The graphical representation of the parameter M as a function of the initial concentration of saturated hydrocarbons (C0) is shown in Figure 3. The following was observed in the graph: i) Furrial asphaltenes show a higher capacity to retain alkanes compared to Ayacucho asphaltenes; and ii) As C0 increases, the parameter M increases, but at concentrations higher than 500 mg⋅L−1 reaches a plateau of 2.5 mg/100mg in Furrial asphaltenes and of 0.95 mg/100mg in Ayacucho asphaltenes. The first observation seems to be directly related to the intrinsic structural characteristics of each asphaltene sample. FTIR results discussed above revealed that Furrial asphaltenes are 50% more aromatic than Ayacucho asphaltenes and with a 47.4% higher population of attached alkyl chains. These structural differences make Furrial asphaltenes more suitable for retention of alkanes owing to: i) an increased number of potential adsorption sites (the attached alkyl chains), and ii) an optimal molecular geometry given by the planarity of more condensed aromatic systems. Planarity of the aromatic cores favors the radial extension of substituent alkyl chains; which in turn, maximizes the exposure of these substituents to a probable interaction with surrounding alkanes.


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Regarding the second aspect observed in Figure 3, the fact that the parameter M progressively increases with C0, and then reaches a plateau suggests that the potential sites available for retention, are fully occupied at C0 > 500 mg⋅L−1. These results show that the capacity of asphaltenes to retain alkanes is influenced by the concentration of the latter. Nevertheless, a detailed inspection of the mass chromatograms showed in Figures 4 to 7 revealed that despite the parameter M remained constant at concentrations higher than 500 mg⋅L−1, the molecular composition of the alkane fraction retained by asphaltenes (f2) did change according to C0 increments. For example, in the m/z 113 mass chromatograms (Figures 4 and 5) it is observed that signals related to some acyclic isoprenoids (labeled with an asterisk) dominate the chromatograms over the corresponding signals of n-alkanes at C0 =500 mg L−1. After increasing C0 from 500 to 3000 mg L−1, signal intensity of acyclic isoprenoids decreases and that of nalkanes augments. This dynamics gives the impression that acyclic isoprenoids are being detached from asphaltenes and replaced by high-molecular-mass n-alkanes as C0 increases. The parameter R1, defined as the ratio of linear to branched alkanes retained by asphaltenes (R1= (nCn)/(i-Cn); Figures 4 and 5) supports this observation. In Furrial asphaltenes, R1 increases from 0.43 to 1.40 (Figure 4), whereas in Ayacucho asphaltenes, it increases from 0.45 to 1.07 (Figure 5). In both cases, the progressive increase of R1 indicates that at lower C0 the retention of branched alkanes dominates since the concentrations of HMM n-alkanes are insufficient to compete for the potential retention sites. However, at higher C0 values HMM n-alkanes are preferentially retained because their concentrations become more significant. A similar trend is observed in the m/z 217 mass chromatograms of retained steranes (Figures 6 and 7), but in this case, steranes ranging from 21 to 22 carbon atoms (C21−C22 pregnanes) seem to be replaced by higher steranes (C27−C29 steranes). The same replacement pattern occurred in the m/z 191 mass


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chromatograms of retained terpanes (not included), but in this case members from C21 to C24 were progressively replaced by C27 to C31 members. The relative retention of linear, branched, and cyclic alkanes can be assessed by comparing the magnitude of the capacity factor. This parameter has been found to increase according to the strength of retention in RPLC.20,21 So, a similar behavior is also expected in asphaltenes. Table 5 shows k’ values for a representative group of alkanes retained by the studied asphaltenes. As observed, k’ of n-alkanes are about one to three orders of magnitude higher than those of cyclic (steranes and terpanes) or acyclic (pristane and phytane) isoprenoids. This means that among the three groups of saturated compounds, n-alkanes are the strongest retained by asphaltenes. The order of retention is as follows: n-alkane>>> steranes≈terpanes > acyclic isoprenoids. A closer inspection of results shown in Table 5 reveals that some structural factors such as steric hindrance and molecular flexibility of alkanes seem to condition their retention on asphaltenes. Steric effects caused by either the number, the size, and the relative position of branches in the molecular structure of alkanes have been found to be responsible for their lower retention in nalkyl bonded phases.44 These effects explain why the acyclic isoprenoids pristane (C19) and phytane (C20) are weakly retained by the asphaltenes when compared to their equivalent nalkanes. In the case of steranes, the reduced flexibility of their cyclic structures seems to favor the interaction with the retention sites of asphaltenes. A similar behavior has also been observed in the RPLC separation of cyclic alkanes.45 There is another fact related to the retention of steranes that is immediately apparent in Table 5; those member with the same number of carbon atoms but with dissimilar stereochemical configurations show important differences in their k’ values. For example, steranes with S


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stereochemistry at C-20 (Table 5) are more strongly retained by asphaltenes than the corresponding R isomers. These observations reveal the existence of a sort of chiral control in the interaction of complex polycyclic alkanes with asphaltenes. In general terms, the interpretation of the retention behavior of the three different groups of alkanes (namely, n-alkanes, cyclic and acyclic isoprenoids) leads to the conclusion that asphaltene structure is not the only factor that plays an important role in the interaction with alkanes. The structure and the stereochemical configuration of the interacting alkanes also have an important effect on the entire process. Concerning the preference of asphaltenes to retain HMM alkanes, the assessment of parameters R2 and R3 can be an amenable approach for a systematic interpretation of this phenomenon. The parameter R2 (included in Figures 4 and 5) is defined as the ratio of n-alkanes higher than n-C22 to n-alkanes lower than n-C22 (R2= (>n-C22)/(< n-C22)); whereas the parameter R3 (included in Figures 6 and 7) is defined as the ratio of C27−C29 steranes to C21−C22 pregnanes (R3= (ST2729)/(ST21-22)).

As observed in Figures 4 and 5, the increase of R2 with C0 makes evident the preference of asphaltenes to retain n-alkanes higher than C22. In Furrial asphaltenes, this parameter increases from 1.04 to 1.86 (Figure 4), but in Ayacucho asphaltenes the increment is more noticeable, from 1.82 to 7.80 (Figure 5). These remarkable differences suggest that Ayacucho asphaltenes are more selective towards the retention of HMM n-alkanes compared to Furrial asphaltenes. A more dramatic preference is observed in the retention of steranes. In the case of Furrial asphaltenes, the parameter R3 increased from 0.8 to 2.76 (Figure 6); but in Ayacucho asphaltenes


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it increased alarmingly from 1.22 to 54.56 (Figure 7). Despite having the lowest capacity to retain alkane compounds, Ayacucho asphaltenes show a greater affinity for HMM alkanes. A possible explanation for this behavior can be obtained from the structural information provided by the FTIR indexes. According to these results, Ayacucho asphaltenes have a lower population of alkyl chains than Furrial asphaltenes. Nevertheless, the average length of their alkyl chains is 10.3% greater. It is known that the physical adsorption capacity of a surface depends on the number of available adsorption sites46 and the strength of London dispersive forces.46 This latter depends upon the amount of surface area or molar volume involved in the interaction, the polarizability of each interacting entity, the intimacy of their contact, and finally, on their ionization potentials.46 If asphaltene alkyl chains participate in the retention of alkanes, a low population of such substituents would reduce the retention capacity of the whole asphaltene sample, but an increase of their average length would improve the interactions with the surrounding alkanes. Hence, the final balance between the number and the length of the alkyl chains will condition the retention behavior of a given asphaltene sample. In the case of Ayacucho asphaltenes, the deficit of alkyl chains is overcome by making the most of their chain lengths; which is, maximizing the contact through the selective retention of HMM alkanes. However, in Furrial asphaltenes alkyl chains are shorter, so the interactions with long alkanes are less effective than in Ayacucho asphaltenes. This leads to the conclusion that the population of alkyl chains in asphaltenes controls the capacity of alkane retention, while their lengths control the selectivity towards high molecular mass alkanes.


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Although the discussed results strongly suggest that asphaltene alkyl chains participate in the interaction with alkanes, the gathered evidences might be regarded not as direct, but as circumstantial. In order to strongly support all these findings, it is mandatory to demonstrate that asphaltenic substituents indeed participate in the interaction with saturated compounds. The determination of the capacity factor k’ and the selectivity of methylene groups (αCH2) of homologous n-alkanes can be appropriated for this purpose. However, at this point of discussion it is important to consider some aspects of asphaltenes before adopting the thermodynamic framework reported by Tchapla et al.20 It is worth mentioning that such a framework was developed for a RPLC systems under flowing conditions, using rigid silica gel particles coated with different alkyl bonded phases.20,21 However, if the experiment had been conducted under static conditions −that is, by dispersing the adsorbent particles in a solution containing the alkyl derivatives and then separating the retained from the non-retained derivatives− the result would have been the same, because neither flowing conditions, nor physical properties of the adsorbent particles (size, porosity and volume) affect the partition equilibrium attained by the derivatives.20,21 This hypothetical experiment has common features with the case of asphaltenes retaining alkanes; except for two aspects: i) asphaltenes are not rigid particles and ii) the distribution of their alkyl substituents (the potential retention sites) cannot be described by a single chain length. Regarding the first aspect, it has been shown that asphaltene structures are flexible enough to adopt some spatial conformations.47 Such conformations could impose preferential orientations on the alkyl chain substituents and affect their exposure to the interaction with surrounding alkanes, especially if aggregate growing occurs in an uncontrolled manner.11


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Concerning the substituent alkyl chains, analyses of pyrolysates48 and oxidation products of asphaltenes obtained from the ruthenium-ion-catalyzed oxidation (RICO) reaction49 showed polydispersed distributions ranging from 1 to more than 35 carbon atoms. Average lengths were found between C4 and C6, with most of the longer chains (> C9) merely comprising 20-25% of the total population of the alkyl chain substituents.49 If we consider that the interaction with alkanes is driven by the maximization of London dispersive forces, then only a reduced population of asphaltene alkyl chains with the proper orientation and favorable lengths will contribute to this process. The inconveniences caused by the possible aggregation of asphaltenes, even over time,50 can be avoided by the wise selection of a solvent that ensures a good dispersion of these materials. However, complications caused by the polydispersed character of alkyl substituents cannot be avoided, because it is an intrinsic structural feature of asphaltenes. Yet, if all necessary precautions are taken, the partition equilibrium attained between alkanes retained by asphaltenes and the free ones in the solvent can still be described, in a very good approximation, by the thermodynamic framework used by Tchapla et al.20 Figures 8 and 9 show the behaviors of log k’ and methylene group selectivity (αCH2) as a function of the chain length (nc) of n-alkanes retained by the studied asphaltenes. Again, graphs are arranged in increasing order of C0. From these graphs it is observed that log k’ increases linearly with nc up to a critical value where the slope changes abruptly. The inflection occurs at nc= 19 in Furrial asphaltenes and at nc= 21 in Ayacucho asphaltenes for all the range of tested C0 values. The nc values at which the inflection point appears are better defined in the graphs of αCH2.


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In all cases, the shapes of the log k’ and αCH2 curves are very similar to those obtained by Tchapla et. al. during the separation of n-alkyl homologues by RPLC.20 This is a good indication that the chromatographic approach works quite well to describe the retention equilibrium of alkanes. Consequently, the observed inflection points in Figures 8 and 9 indicate that the “methylene group exclusion” phenomenon also occurs in asphaltenes. That is, Furrial and Ayacucho asphaltenes do behave as an alkyl-bonded phase to retain alkanes through the direct participation of their substituent alkyl chains. The difference between the two nc values at which the inflection points appear in asphaltenes is equivalent to two methylene groups. If we assume that the exclusion of methylene groups is related to the length of the asphaltenic alkyl chains, then we could speculate that alkyl chains available for retention in Furrial and Ayacucho asphaltenes have an average length of 19 and 21 carbon atoms respectively, thus evidencing the participation of longer alkyl chains in the retention process. On the other hand, given that alkyl chains longer than C9 merely comprise 2025% of the total population of alkyl chains in asphaltenes,49 and considering the values of the parameter M reported in Table 4, it is inferred that only a small fraction of the longer, asphaltenic alkyl chains participate in the interaction with surrounding alkanes under the experimental conditions. Moreover, the fact of observing the “methylene group exclusion” phenomenon in asphaltenes strongly suggests that, at least, the interaction with n-alkanes is guided by a clustering process where n-alkanes have to align their molecular axes parallel to the ones of asphaltenic alkyl chains. Only those n-alkanes whose chain lengths are longer than the average length of the participating asphaltenic chains take part in the “methylene group exclusion” phenomenon.


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So, far, the foregoing discussion demonstrates that the interplay of asphaltenes and alkanes is extremely complex. As C0 increases, the proportion of low molecular mass alkanes retained by asphaltene decreases, while that of HMM alkanes increases. Moreover, the fact that the interaction of asphaltenes with alkanes involves a small number of asphaltene alkyl chains suggests that these substituents can accommodate varying amounts of alkanes of different molecular masses in order to strengthen the interaction. For example, an asphaltene alkyl chain with average length of 21 carbon atoms can accommodate, along its molecular axis, up to 3 molecules of n-hexane (n-C6). The replacement of two of the three retained n-C6 molecules by one of n-dodecane (n-C12) does not alter the total number of carbon atoms retained by the asphaltenic alkyl chain, but it significantly minimizes the effects of steric repulsion caused by the terminal methyl groups of each n-C6 molecule and improves the effectivity of the interaction. At the same time, the fraction of n-C6 molecules is reduced by 66.6%, while the fraction of n-C12 molecules increased in 33.3%. All these changes improve the interaction between alkanes and asphaltenes; however, the replacement of two of the three retained n-hexane molecules by one of n-dodecane just produces a variation of two units in the total mass of retained alkanes. The magnitude of this variation is so subtle that it becomes practically imperceptible from a gravimetric point of view. Therefore, the idea of a large number of retained alkanes with low molecular mass being replaced by a low number of HMM alkanes would provide a

reasonable explanation for the compositional

dynamics observed in the f2 fraction while the parameter M remains constant at C0 values higher than 500 mg⋅L−1. An important aspect to evaluate is the potential effect that the alkane retention phenomenon might have on the colloidal stability of asphaltenes. Pahlavan et al.51 made quantum−mechanical


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calculations using density functional theory (DFT) to evaluate the strength of interactions between asphaltene unit sheets in the presence and absence of waxes. Their results revealed that paraffin waxes destabilize the asphaltene assembly by weakening the interaction between the asphaltene unit sheets. On the other hand, García and Carbogani23 studied the dissolution kinetics of asphaltene−alkane composites using asphaltenes from stable and unstable crude oils mixed with macro and microcrystalline waxes. Composites from stable asphaltenes showed no significant variation in their dissolution kinetics when compared to the original asphaltene sample; but composites from unstable materials showed synergistic effects that contributed to a faster dissolution. Recently, Rogel et al.52 used filtration methods to separate a mixture of precipitated waxes and asphaltenes from paraffinic crude oils at room temperature. Asphaltenes from the waxy precipitates were recovered and compared to the whole asphaltene fraction presented in the original crude oils in terms of their solubility parameter profiles. Results showed that asphaltenes from the waxy precipitates were enriched in the least soluble asphaltene constituents. In a second experiment, these authors added C5-asphaltenes to the filtered crude oils, sonicated for several hours and allowed to stand for seven days. After that time, a new precipitate composed of waxes and asphaltenes appeared in the spiked crude oils. Results showed again that asphaltenes from the waxy precipitate were enriched in the least soluble asphaltene constituents when compared to original C5-asphaltenes added to filtered crude oils. On the basis of these experiments, these authors concluded that the selective precipitation of the least soluble asphaltenes occurs in presence of HMM alkanes. The results reported by Pahlavan et al., García and Carbogani, and Rogel et al. indirectly converge at a common point: the formation of asphaltene−alkane complexes. If these findings


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are interpreted in an integrated fashion, and considering that asphaltenes are multicomponent assemblies formed by individual molecules having different solubilities,52 then it is correct to think that the interactions with alkanes will produce the aggregate disassembling through the solubilization of a portion of asphaltenes (the most soluble) and the precipitation of the other portion (least soluble). In this regard, the structural model of asphaltenes reported by Acevedo et al.,47 and Gutiérrez et al.53 can be of great utility in providing a consolidated ground to interpret the effects of asphaltene−alkane interactions on the stability of crude oils. This model considers that the supramolecular structure of asphaltenes is formed by two asphaltene subfractions −A1 and A2− with different solubilities. Molecules comprising each subfraction are arranged in such a way that least soluble asphaltenes (A1) are located in the central part of the structure and are surrounded by the most soluble components (A2). This spatial arrangement allows the most soluble asphaltenes to form a peripheral layer that contributes to the dispersion of the system. According to this model, the most soluble asphaltenes are directly involved in the interplay with molecules from the surroundings, including alkanes. Hence, they are responsible for the formation of liquid-crystal-like structures that result from the occurrence of the “methylene group exclusion” phenomenon. Indeed, this phenomenon is seen as a process where the aliphatic character of the participating asphaltenes is increased by the attachment of external alkyl appendages, which in turn enhances their solubility properties in the oil phase. As a result, the most soluble asphaltenes are more likely to escape from the asphaltene molecular assembly, leaving the least soluble asphaltenes unprotected and free to aggregate, flocculate and eventually act as nucleation sites for the building up of HMM alkane crystals.


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Undoubtedly, the disassembly of the asphaltenic supramolecular aggregate is controlled by the intrinsic structural features of the own asphaltenes and by their capacity to retain alkanes. This is supported by the experimental results showing that asphaltenes from the unstable Furrial crude oil are more aromatic and have a higher capacity to retain alkanes compared to the stable Ayacucho crude oil. This interpretation leads to the conclusion that formation of asphaltene−alkane complexes is one of the initial steps that destabilize the asphaltenic aggregate assembly and initiate the process of flocculation and further precipitation.

CONCLUSIONS Asphaltenes from Furrial and Ayacucho crude oils behave as an alkyl-bonded phase to retain saturated hydrocarbons. The retention takes place on the alkyl chains of asphaltenes, and the nalkane compounds are the strongest retained. Cyclic and acyclic isoprenoids followed in this order of retention. The entire process is controlled by structural features of both asphaltenes and the interacting alkanes, as well as by the initial concentration of the latter in the solution media. It was found that asphaltenes from the unstable Furrial crude oil showed a higher capacity to retain alkanes than asphaltenes from the stable Ayacucho crude oil. However, in both asphaltene samples the retention capacity progressively increased with C0, and then reached a plateau at C0 > 500 mg⋅L−1. The occurrence of this plateau might be related to a complex intermolecular dynamics involving the replacement of a large number of retained alkanes with low molecular mass by a low number of HMM alkanes.


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Despite the lower capacity of Ayacucho asphaltenes to retain alkanes, their selectivity towards the retention of HMM alkanes was considerable higher than in Furrial asphaltenes. These trends are explained in terms of the difference in population and length of the asphaltenic alkyl chains. The phenomenon of “methylene group exclusion”, which is believed to be the responsible for the formation of liquid-crystal-like structures in asphaltene-alkane composites, was observed in Furrial and Ayacucho asphaltenes at alkyl lengths equivalent to 19 and 21 carbon atoms. This result suggests that only the longer asphaltene alkyl chains participate in the interplay with straight alkanes. Asphaltene−alkane complex formation is believed to promote the disassembly of the supramolecular structure of asphaltenes by modifying the aliphatic character of the most soluble asphaltene constituents and further increasing their solubility in the oil media. The structural features of asphaltenes, their capacity to retain alkanes (the parameter M) and the “methylene group exclusion” phenomenon are thought to play crucial roles in this process. The approach used in this work opens a new window to interpret the interactions of asphaltenes and alkanes at molecular levels. It can provide better understanding of the role that each compound class plays in asphaltene aggregation−flocculation−precipitation mechanisms and in wax crystallization. Corresponding Author *E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors gratefully acknowledge all the support given by the Specialist Ana K. Faraco and Professors Gastón Escobar, Lola De Lima, and Salvador Lo Mónaco. The financial support provided by projects FONACIT G-2001000874, FONACIT 2012002299, CDCH-UCV: PG03.8205-2011/2, and PG-03.051.2012 is gratefully acknowledged. The authors are grateful to the anonymous reviewers for their critical reviews of the manuscript, which contributed to improving the original manuscript. Supporting Information Available: Asphaltene FTIR Spectra and band assignations, list of identified steranes and diasteranes retained by asphaltenes, and k’ values of n-alkanes retained by asphaltenes. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1) Park, S. J.; Escobedo, J.; Mansoori, G. L. Asphaltene and other heavy organic depositions. In Yen, T.F.; Chilingarian, G.V. (Editors) Asphaltenes and Asphalts 1: Developments in Petroleum Science 40; Elsevier: Amsterdam, The Netherlands, 1994; pp 179−205. (2) Carbognani, L. Espidel, J. Izquierdo, A. Characterization of asphaltenic deposits from oil production and transportation operations. In Yen, T.F.; Chilingarian, G.V. (Editors) Asphaltenes


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and Asphalts 2: Developments in Petroleum Science 40B; Elsevier: Amsterdam, The Netherlands, 2000; pp 335−362.

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Figure 1. Schematic representation of the experimental method for separation of saturated hydrocarbons retained by asphaltenes.


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Figure 2. Original (blue) and deconvoluted (red) FTIR spectra of Ayacucho asphaltenes. Peak areas used for the calculation of molecular indexes are indicated.


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Figure 3. Capacity of asphaltenes to retain saturated hydrocarbons.


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Figure 4. m/z 113 mass chromatograms of n-alkanes and acyclic isoprenoids pristane (Pr) and phytane (Ph) non-retained (f1) and retained (f2) by Furrial asphaltenes at different initial concentrations (C0) of saturated hydrocarbons. Alkanes are identified with numbers indicating their chain length in terms of carbon atom number. Branched compounds are labeled with an asterisk (*). R1= (n-Cn)/(i-Cn) and R2= (>n-C22)/(n-C22)/(

Retention of Alkane Compounds on Asphaltenes ... - ACS Publications

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