Studies of Athabasca asphaltene Langmuir films at air-water interface

Asphaltenes are present in heavy oils and bitumen. They are a mixture of hydrocarbons having complex structures of polyaromatic rings and short side chains. In general, the high-molecular-weight asphaltene is the most aromatic fraction with the highest number of side chains and the low-molecular-weight asphaltene contains the lowest number of side chains, while the number of side chains of the whole asphaltene fraction lies in between. In this study, asphaltenes were extracted and/or fractionated from Athabasca oil sand bitumen. Subfractions of high and low molecular weight and the whole asphaltenes were characterized using a Langmuir trough and complementary techniques such as VPO, FTIR, AFM, and contact angle measurements. At an air-water interface, amphiphilic asphaltene molecules can form a monolayer. Various fractions (high, low, and whole) of the asphaltene molecules behave similarly at the air-water interface, characterized by close resemblance of their surface pressure-area, hysteresis, and relaxation isotherms. The high-molecular-weight asphaltene is the most expanded fraction, while the low-molecular-weight asphaltene fraction is the most condensed, with the whole asphaltene lying in between. At the air-water interface a monolayer of the low-molecular-weight asphaltene relaxes at a faster rate than one of the high-molecular-weight asphaltene.     

Model molecules mimicking asphaltenes

Asphalthenes are typically defined as the fraction of petroleum insoluble in n-alkanes (typically heptane, but also hexane or pentane) but soluble in toluene. This fraction causes problems of emulsion formation and deposition/precipitation during crude oil production, processing and transport. From the definition it follows that asphaltenes are not a homogeneous fraction but is composed of molecules polydisperse in molecular weight, structure and functionalities. Their complexity makes the understanding of their properties difficult. Proper model molecules with well-defined structures which can resemble the properties of real asphaltenes can help to improve this understanding. Over the last ten years different research groups have proposed different asphaltene model molecules and studied them to determine how well they can mimic the properties of asphaltenes and determine the mechanisms behind the properties of asphaltenes.     

塔河常压渣油沥青质含硫官能团形态与其性质的关系研究

以塔河常压渣油为原料,分离正庚烷沥青质,并将正庚烷沥青质分成三个极性不同的亚组分。测定沥青质亚组分的平均偶极矩,利用¹H-NMR谱关联得到沥青质平均结构参数以分析沥青质亚组分缔合性质,采用X射线吸收近边结构谱(XANES)表征沥青质亚组分中含硫官能团形态,分析硫原子存在形态对沥青质性质的影响。结果表明,随着沥青质亚组分极性降低,H/C原子比增大;沥青质极性增加,缔合性增强;沥青质中还原态硫主要以噻吩硫形式存在,其次为硫醚;亚砜、砜和磺酸盐是主要的含氧硫化物。噻吩、砜和磺酸盐等官能团的存在对沥青质性质具有影响,但对沥青质极性及缔合性的影响不明显,杂原子硫不是影响沥青质极性和缔合性的主导因素。     

A DPD study of asphaltene aggregation: The role of inhibitor and asphaltene structure in diffusion-limited aggregation

The kinetic effects of DBSA (dodecyl benzene sulfonic acid) and a linear amphihile on asphaltene aggregation was investigated, using dissipative particle dynamics molecular simulations. The simulation results indicated that without inhibitor, diffusion-limited asphaltene aggregation can be initiated by a kinetic/diffusive capture process between polar side chain groups rather than by interaction between polyaromatic rings. The most likely reason for this is that the side chains have higher diffusive mobility than the more massive aromatic ring structures. The DBSA acidic head groups adhered to the asphaltene side chain polar groups (the basic functional groups), resulting in lowered mobility of the side chain/DBSA complexes, thereby suppressing asphaltene aggregation initiation. A more mobile amphiphilic inhibitor without the aromatic ring gave a higher asphaltene aggregation rate. Adsorption of asphaltenes on a solid surface was suppressed with DBSA, due to an adsorbed mono-layer of DBSA that occupied a significant fraction of the surface area.     

DPD Molecular Simulations of Asphaltene Adsorption on Hydrophilic Substrates: Effects of Polar Groups and Solubility

Adsorption of asphaltenes onto a polar substrate (e.g., a mineral) was modeled with dissipative particle dynamics (DPD) simulations, using continental asphaltene models. The adsorption mechanisms in 10–20% wt, of asphaltene in toluene/ heptane solutions were studied (well above the solubility limit). The structure in the adsorbed layer was highly sensitive to the presence of polar groups in the alkyl side chains and heteroatom content in the aromatic ring structure. Four types of asphaltene models were used: completely apolar (zero adsorption), apolar chains and polar heteroatoms, polar chains and no heteroatoms, and polar chains and heteroatoms (maximum adsorption). One hundred asphaltene monomers were distributed homogeneously in the solvent initially, in a ～(10 nm)³ domain.

Asphaltene monomers adsorbed irreversibly on the substrate via the polar group in the side chains, resulting in an average perpendicular orientation of the aromatic rings relative to the substrate. More frequent π–π stacking of the aromatic rings occurred for less solubility (more heptane), as in aggregates. With apolar side chains, only the heteroatoms in the aromatic ring structure had affinity to the substrate, but the ring plane did not have any preferred direction.

An important finding is that the aromatic ring assemblies “shielded” the substrate and polar groups that were anchored to the substrate, resulting in an effective non-polar surface layer seen by asphaltenes in the bulk, leading to much lower adsorption probability of the remaining asphaltenes. This “adsorption termination” effect leads to mono-layer formation. Continued adsorption with multilayering and reversible nanoaggregate adsorption occurred when both side chains in the model asphaltene (located on opposite sides of the aromatic sheet) contained polar groups, with a higher probability of exposing further polar groups to the bulk asphaltene. The general conclusion is that the number and position of the polar groups in side chains determine to a large degree the adsorption and aggregation behavior/efficiency of (continental) asphaltenes, in line with experimental evidence. The heteroatoms in the aromatic ring structure plays a more passive role in this context, only by providing organization via more π–π stacking in the adsorbed layer, and in aggregates.     

石油沥青质的NMR测定及其模型分子推测

从6种不同原油中分离提取了正己烷不溶的沥青质，测定了沥青质的1H NMR(Nuclear Magnetic Resonance)和13C NMR谱，从不同类型氢和碳原子的质量分数计算得到了一系列平均结构参数，结合相对分子质量测定和元素分析，给出了沥青质基本结构单元的平均分子式，推测了模型分子的结构。结果表明，沥青质的基本结构单元可以用稠环芳烃连接环烷烃和烷基侧链并含氧、氮和硫等杂原子的单元表示，结构单元之间形成缔合体，缔合数为4～6。     

Study of a supramolecular structure of continental type petroleum asphaltenes

Supramolecular, electronic, and chemical structures of petroleum asphaltene molecules are studied. The investigations are carried out by quantum chemistry and molecular mechanics methods. The quantum chemical calculation of the structure-chemical parameters of dimers and trimers of petroleum asphaltenes is made using DFT/B3LYP. The refined values of the ionization potential and electron affinity of petroleum asphaltene molecules, their dimers and trimers agree well with the electron spectroscopy data. The results of the study of geometric structures of petroleum asphaltene dimers and trimers confirm the non-planar structure of asphaltenes.     

Effects of petroleum resins on asphaltene aggregation and water-in-oil emulsion formation

Asphaltenes from four crude oils were fractionated by precipitation in mixtures of heptane and toluene. Solubility profiles generated in the presence of resins (1:1 mass ratio) indicated the onset of asphaltene precipitation occurred at lower toluene volume fractions (0.1–0.2) than without resins. Small-angle neutron scattering (SANS) was performed on solutions of asphaltene fractions in mixtures of heptane and toluene with added resins to determine aggregate sizes. Water-in-oil emulsions of asphaltene–resin solutions were prepared and separated by a centrifuge method to determine the vol.% water resolved. In general, the addition of resins to asphaltenes reduced the aggregate size by disrupting the π–π and polar bonding interactions between asphaltene monomers. Interaction of resins with asphaltenic aggregates rendered the aggregates less interfacially active and thus reduced emulsion stability. The smallest aggregate sizes observed and the weakest emulsion stability at high resin to asphaltene (R/A) ratios presumably corresponded to asphaltenic monomers or small oligomers strongly interacting with resin molecules. It was often observed that, in the absence of resins, the more polar or higher molecular weight asphaltenes were insoluble in solutions of heptane and toluene. The addition of resins dissolved these insolubles and aggregate size by SANS increased until the solubility limit was reached. This corresponded approximately to the point of maximum emulsion stability. Asphaltene chemistry plays a vital role in dictating emulsion stability. The most polar species typically required significantly higher resin concentrations to disrupt asphaltene interactions and completely destabilize emulsions. Aggregation and film formation are likely driven by polar heteroatom interactions, such as hydrogen bonding, which allow asphaltenes to absorb, consolidate, and form cohesive films at the oil–water interface.     

Non-Exponential 1H and 2H NMR Relaxation and Self-Diffusion in Asphaltene-Maltene Solutions

The distribution of NMR relaxation times and diffusion coefficients in crude oils results from the vast number of different chemical species. In addition, the presence of asphaltenes provides different relaxation environments for the maltenes, generated by steric hindrance in the asphaltene aggregates and possibly by the spatial distribution of radicals. Since the dynamics of the maltenes is further modified by the interactions between maltenes and asphaltenes, these interactions—either through steric hindrances or promoted by aromatic-aromatic interactions—are of particular interest. Here, we aim at investigating the interaction between individual protonic and deuterated maltene species of different molecular size and aromaticity and the asphaltene macroaggregates by comparing the maltenes’ NMR relaxation (T1 and T2) and translational diffusion (D) properties in the absence and presence of the asphaltene in model solutions. The ratio of the average transverse and longitudinal relaxation rates, describing the non-exponential relaxation of the maltenes in the presence of the asphaltene, and its variation with respect to the asphaltene-free solutions are discussed. The relaxation experiments reveal an apparent slowing down of the maltenes’ dynamics in the presence of asphaltenes, which differs between the individual maltenes. While for single-chained alkylbenzenes, a plateau of the relaxation rate ratio was found for long aliphatic chains, no impact of the maltenes’ aromaticity on the maltene–asphaltene interaction was unambiguously found. In contrast, the reduced diffusion coefficients of the maltenes in presence of the asphaltenes differ little and are attributed to the overall increased viscosity.     

Molecular representation of coal-derived asphaltene based on high resolution mass spectrometry

The asphaltene separated by solubility in small molecular alkanes and toluene is the most structurally diverse and complex components in heavy oil, such as vacuum residue and coal tar. The coal-derived asphaltene is always regard as a succession of maltene fraction from small molecules to large molecules, and also a continuum of island- and archipelago-type structures, which is difficult to be identified accurately through current characterization methods. This limits the further study of molecular dynamics and reaction dynamics simulation of asphaltenes. In this work, a representation model of molecular composition and structure for coal-derived asphaltene is developed mainly based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with collision induced dissociation (CID) and traditional methods of nuclear magnetic resonance spectroscopy (¹³C NMR), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS). Island- and archipelago-type structures are considered qualitatively in the representation of asphaltene. The asphaltene molecules are systematic assembled using stochastic algorithms and optimized by simulated annealing algorithm according to the group contribution method. The bulk properties for simulating asphaltenes are in good agreement with the experimental results giving acceptable predictions for the composition and structure of the asphaltenes. Moreover, the representative average structure asphaltene molecules are obtained using the developed molecular similarity function, which could be applied in the further study of molecular aggregation simulation and reaction kinetics simulation.     

Asphaltene self-association and water-in-hydrocarbon emulsions

The configuration of asphaltenes on the water-oil interface was evaluated from a combination of molar mass, interfacial tension, drop size distribution, and gravimetric measurements of model emulsions consisting of asphaltenes, toluene, heptane, and water. Molar mass measurements were required because asphaltenes self-associate and the level of self-association varies with asphaltene concentration, the resin content, solvent type, and temperature. Plots of interfacial tension versus the log of asphaltene molar concentration were employed to determine the average interfacial area of asphaltene molecules on the interface. The moles of asphaltenes per area of emulsion interface were determined from the molar mass data as well as drop size distributions and gravimetric measurements of the model emulsions. The results indicate that asphaltenes form monolayers on the interface even at concentrations as high as 40 kg/m(3). As well, large aggregates with molar masses exceeding approximately 10,000 g/mol did not appear to adsorb at the interface. The area occupied by the asphaltenes on the interface was constant indicating that self-associated asphaltenes simply extend further into the continuous phase than nonassociated asphaltenes. The thickness of the monolayer ranged from 2 to 9 nm.     

Aggregation and solubility behavior of asphaltenes and their subfractions

Asphaltenes from four different crude oils (Arab Heavy, B6, Canadon Seco, and Hondo) were fractionated in mixtures of heptane and toluene and analyzed chemically, by vapor pressure osmometry (VPO), and by small angle neutron scattering (SANS). Solubility profiles of the asphaltenes and their subfractions indicated strong cooperative asphaltene interactions of a particular subfraction that is polar and hydrogen bonding. This subfraction had lower H/C ratios and modestly higher N, V, Ni, and Fe contents than the less polar and more soluble subfraction of asphaltenes. VPO and SANS studies indicated that the less soluble subfractions formed aggregates that were considerably larger than the more soluble subfractions. In general, asphaltene aggregate size increased with decreasing solvent aromaticity up to the solubility limit, beyond which the aggregate size decreased with heptane addition. The presence of a low wavevector Q feature in the scattering curves at 25 degrees C indicated that the individual aggregates were flocculating; however, the intensity of the feature was diminished upon heating of the samples to 80 degrees C. The solubility mechanism for Canadon Seco asphaltenes, the largest aggregate formers, appears to be dominated by aromatic pi-bonding interactions due to their low H/C ratio and low nitrogen content. B6 and Hondo asphaltenes formed similar-sized aggregates in heptol and the solubility mechanism is most likely driven by polar interactions due to their relatively high H/C ratios and high nitrogen contents. Arab Heavy, the least polar asphaltene, had a H/C ratio similar to Canadon Seco but formed the smallest aggregates in heptol. The enhancement in polar and pi-bonding interactions for the less soluble subfraction indicated by elemental analysis is reflected by the aggregate size from SANS. The less soluble asphaltenes contribute the majority of species responsible for aggregation and likely cause many petroleum production problems such as pipeline deposition and water-in-oil emulsion stabilization.     

Polarization of Fluorescence of Asphaltene Containing Systems

Formation of supermolecular structures in petroleum disperse systems is determined by interactions of asphaltenes. Petroleum systems are lyophilic oleocolloids with low polar dispersive media which is in dynamic balance with elements of disperse structure. Supermolecular scale of organization is most important for determining the macroscopic parameters. Levshin-Perrin equation for depolarization of fluorescence was modified for polydispersed systems. Interfacial tension coefficient of model asphaltene solution was calculated in a case of Volmer's function. For technogenic mixtures the temperature dependences of the sizes near the point of phase transition are presented. Enthalpy of asphaltene association per one molecule was calculated. Polarization degree together with macroscopic parameters correlates with appearance of asphaltene dispersed phase in system.     

A model to calculate the average interaction energy and adhesion force between petroleum asphaltenes and some metallic surfaces

A model is proposed to account for the interaction energy and adhesion force between petroleum asphaltenes and metallic surfaces. It is assumed that the total molecule-surface interaction potential may be constructed through superposition of corresponding interactions with a relevant number of atomic layers forming the substrate and resorting to the Lindhard continuum planar potential (CPP) approximation, which requires only of knowledge of binary molecule-atom interactions. Molecular mechanics (MM) calculations are performed to generate the average binary interaction between the asphaltene molecule and an atom in the substrate, which in turn is represented by a parameterized analytical--physically sound--expression. The resulting CPP yields an analytical expression representing the interaction between the asphaltene molecule and each substrate layer. To validate the method, pilot calculations are performed for a sample asphaltene molecule with a fixed orientation relative to metallic surfaces of iron, aluminum, and chromium. Comparison between corresponding CPP and MM calculations for the asphaltene-plane (A-P) and asphaltene-substrate (A-S) interactions indicate reasonable agreement pointing to the adequacy of the CPP method to represent molecule-surface interactions. Also, the effect of a surrounding (i.e., solvent) medium is addressed with the use of a dielectric constant, epsilon, incorporated in the molecule-atom potential. Finally, a discussion is presented on the applicability of the method to generate an analytical universal expression for asphaltene-metallic wall interactions.     

超声波处理对加氢反应前后沥青质单元分子结构的影响

以胜利减渣和沙轻减渣为原料,研究了超声波处理对加氢反应前后沥青质单元分子结构的影响,并结合¹H-NMR数据、沥青质单元分子参数变化和红外光谱分析等结果,用Chem Bio Draw Ultra 2012模拟出不同条件下两种沥青质单元分子的结构。结果表明,超声波处理减少了沥青质的缔合数,使沥青质单元分子发生开环反应和脱烷基侧链反应加剧,改变了沥青质单元分子的结构,对加氢后沥青质单元分子的结构和组成产生重要影响。沥青质单元分子模型可形象体现超声波处理对加氢反应前后沥青质单元分子化学结构的影响,有助于在分子水平上解释超声波处理影响沥青质单元分子的原因。     

FUSED RING AROMATIC SOLVENCY IN DESTABILIZING WATER-IN-ASPHALTENE-HEPTANE-TOLUENE EMULSIONS

The role of asphaltenes in stabilizing water-in-crude oil emulsions is extremely well established. The mechanism appears to be one in which planar, disk-like asphaltene molecules aggregate through lateral intermolecular forces to form primary aggregates or micelles which are interfacially active. These aggregates — upon adsorbing at the oil-water interface — crosslink through physical interactions to form a viscoelastic network, which has been characterized by some as a “skin” or a “plastic film”. The strength of this film, as gauged by shear and elastic moduli, seems to correlate well with water-in-oil emulsion stability. What is still relatively unknown is the role of chemistry in governing the strength of these lateral inter-asphaltene interactions. The candidate interactions include π-bonding between the delocalized electrons in the fused aromatic ring core, H-bonding between proton donors and acceptors imbedded in the asphaltenic cores, and metal-electron interactions between, for example, heavy metal ions such as vanadium or nickel and electron pairs in pyrrolic or porphyrin functional groups. We have probed these interactions indirectly by studying the destabilization of water-in-oil emulsions by a variety of aromatic solvents. In this paper, we review our previous results on both water-in-crude oil systems, as well as water-in-model oil (heptane-toluene-asphaltene mixtures) systems, in which the emulsions were progressively destabilized by addition of aromatic solvents. We also present new results with fused ring aromatic solvents, specifically methyl-naphthalene, phenanthrene, and phenanthridine. Our results suggest that fused ring aromatic solvents are considerably more effective at destabilizing asphaltene emulsions and proton-accepting fused ring aromatic solvents are most effective. These results indicate that both π-bonding and H-bonding play significant roles in mediating the aggregation of asphaltenes in oil-water interfacial films.     

Interaction between asphaltenes and fatty-alkylamine inhibitor in bulk solution

In the present work, the mechanism of interaction between asphaltenes and a commercial fatty-alkylamine inhibitor was investigated by a combination of techniques. The “macro” properties, like the asphaltene precipitation onset and the amount of asphaltenes precipitated, were measured by near-infrared (NIR) and UV-vis spectroscopy, respectively, while the interaction enthalpy between asphaltenes and inhibitor was measured by isothermal titration calorimetry (ITC). Asphaltenes subfractions and derivatives were also used to identify the mechanism.

ITC indicated that only a small fraction (～6%) of asphaltenes interacts strongly with the inhibitor. The proportion of interacting species was found to be higher in irreversibly adsorbed asphaltenes subfraction. These 6% are mostly composed of acidic asphaltenes, as indicated by measurements involving ester asphaltenes. However, the measurement of precipitation onset and amounts precipitated for whole and ester asphaltenes indicated that the acid–base interaction was not the main interaction responsible for the inhibitory action. Other type(s) of interaction is/are responsible for the inhibition properties of the inhibitor, which are not detected by ITC. The nature of other interactions is not known for the moment, but it was shown that irreversibly adsorbed asphaltene fraction contains a higher concentration of the functionality (ies) responsible for the “other” type of interaction.     

Two-step laser mass spectrometry of asphaltenes

Defined by their solubility in toluene and insolubility in n-heptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and most polar fraction of crude oil. Although asphaltenes are critically important to the exploitation of conventional oil and are poised to rise in significance along with the exploitation of heavy oil, even as fundamental a quantity as their molecular weight distribution is unknown to within an order of magnitude. Laser desorption/ionization (LDI) mass spectra vary greatly with experimental parameters so are difficult to interpret: some groups favor high laser pulse energy measurements (yielding heavy molecular weights), arguing that high pulse energy is required to detect the heaviest components of this mixture; other groups favor low pulse energy measurements (yielding light molecular weights), arguing that low pulse energy is required to avoid aggregation in the plasma plume. Here we report asphaltene mass spectra recorded with two-step laser mass spectrometry (L2MS), in which desorption and ionization are decoupled and no plasma is produced. L2MS mass spectra of asphaltenes are insensitive to laser pulse energy and other parameters, demonstrating that the asphaltene molecular weight distribution can be measured without limitation from insufficient laser pulse energy or plasma-phase aggregation. These data resolve the controversy from LDI, showing that the asphaltene molecular weight distribution peaks near 600 Da and previous measurements reporting much heavier species suffered from aggregation effects.