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Understanding the Liberation of Asphaltenes on Muscovite Surface Xingang Li, Yun Bai, Hong Sui, and Lin He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02278 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017
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Graphic abstract 66x36mm (300 x 300 DPI)
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Understanding the Liberation of Asphaltenes on
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Muscovite Surface
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Xingang Li1,2,3, Yun Bai1,3, Hong Sui1,2,3*, Lin He1,3*
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China.
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China.
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3
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School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072,
National Engineering Research Center of Distillation Technology, Tianjin, 300072,
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
300072, China.
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ABSTRACT: Separation of heavy hydrocarbons from mineral surfaces is the key step
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for unconventional oil production and remediation of oil contaminated soils. The
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presence of asphaltene and the coexistence of mineral rocks are considered as the most
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challenge during the above separation processes. Herein, the liberation of asphaltenes
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(and/or heavy oil) on muscovite (KAl2(Si3Al)O10(OH)2)) surface has been
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systematically investigated through instrumental characterization and molecular
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dynamics (MD) simulation. It is observed that, quite different from that on silica surface,
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asphaltenes can flake off from the muscovite surface due to the weaker adhesion force
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between asphaltenes and muscovite surface. This liberation pattern was also found to
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be influenced by the addition of other oil fractions. The micro force measurements by
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AFM show that the adhesion force between asphaltenes and muscovite is weaker than
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that between asphaltenes and silica in both air and water. Assisted by the MD simulation,
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it is found that the detachment of asphaltenes are highly dependent on the mineral types
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and the presence of water film on the mineral surfaces. Although the van der Waals
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force is found to be the main force between asphaltenes and mineral surfaces, the
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presence of potassium ions (K+) on muscovite surface could increase the percentage of
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the electrostatic forces in the total force. Furthermore, the presence of a 0.4 nm water
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layer (in the air) between asphaltenes and muscovite surface could reduce their
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interactions dramatically compared with that in vacuum state. This finding suggests that
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the presence of water between mineral surface and oil is beneficial for the separation of
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oil from mineral surface. In addition, the asphaltene molecules are found to contact with
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silica surface by face-to-face (aromatic ring) form, while a much more perpendicular 2
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orientation of the asphaltene molecules happens on muscovite surface.
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Keywords: Asphaltene, adsorption, liberation, muscovite, molecular dynamic
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simulation
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1. INTRODUCTION
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Energy and environment are two hot topics in 21st century.1 However, as known, the
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traditional fossil fuels are running out and the environment is suffering from heavy
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pollutions including soil contamination, water pollution, and air pollution. Herein, we
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will talk about something in common during the unconventional oil production and
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remediation of heavy oil-contaminated soils: liberation or separation of heavy oil from
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mineral surfaces.
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The unconventional oils (including heavy oil, oil/tar sands bitumen, shale oils, asphalt
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rocks, etc.) are important parts of the fossil fuels, which are considered as alternative
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for the traditional crude oil.2-3 However, different from traditional crude oil, the
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unconventional oils are more viscous with higher content of heavy components and
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more complex due to the coexistence with different kinds of mineral solids, including
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quartz sands, carbonate rocks (e.g., calcite, dolomite, feldspar, magnesite, etc.), clays,
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etc. These natural properties lead to higher difficulty in the exploitation of
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unconventional oils.2 Although many different kinds of methods have been proposed to
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unlock this kind of energy source, the widely used method in industry is the water
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flooding. In this process, the liberation of heavy oil from their host rocks is considered
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as the controllable step. The success of this step is highly dependent on the water 3
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chemistry, oil composition and mineral composition, etc.2,4-5 During the past decades,
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great efforts have been made to understand and enhance the oil liberation from their
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host rocks by water chemistry control,6-8 external chemicals addition,9-12 temperature
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changing, solid wettability modification,12 etc. It is concluded that asphaltenes, one of
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the SARA (saturates, aromatics, resins and asphaltenes) fractions in petroleum, play an
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essential role in influencing the above processes through viscosity/density changing,
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interfacial properties alteration.2,13 However, to the best of our knowledge, little work
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has been published on how the asphaltenes liberate from their host rock surfaces. The
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same thing happens during the remediation of soils which are contaminated by
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petroleum hydrocarbons. Although most of the light petroleum components (e.g., PHAs,
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light hydrocarbons of kerosene, gasoline, etc.) could be relatively easily removed from
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the soils, the heavy hydrocarbons, especially the resins, asphaltenes, are much difficult
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to be separated and stay attached on the soil surfaces.14 Accordingly, understanding the
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liberation profiles of asphaltenes is also crucial for the remediation of oil-contaminated
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soils.
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Mineral type is considered as another important factor influencing the separation
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efficiency.15 It is reported that there are tens of different minerals existing in
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unconventional oil ores and soils, including the silica, silicate minerals, carbonate
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minerals, and some other metal oxides, etc. Many public literatures show that these
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minerals appear big difference in physicochemical properties, making the recovery of
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oil from the ores quite different.16 Most of previous works have been focused on the
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silica. However, little attention has been paid on another important mineral, muscovite, 4
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a typical aluminosilicate mineral in soils and unconventional oil ores, to obtain how oil
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components liberates from muscovite surface, as well as its mechanisms at molecular-
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level until now.
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Accordingly, in this paper, asphaltene is selected as a heavy oil representative to
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investigate the oil liberation on muscovite surface. The purpose of the present study is
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to: i) investigate the liberation behaviors of asphaltenes on muscovite surface; ii)
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understand the molecular interactions between asphaltene and muscovite; iii) obtain the
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influence of water film on the asphaltene-solids interactions at molecular-level.
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2. Experimental Section
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2.1 Materials
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Toluene and n-heptane (>99%), provided by Tianjin Jiangtian Technology Co. Ltd.,
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China, are used as solvents. The silica glass and muscovite plates, supplied by
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Zhongjingkeyi Technology Co., Ltd., China and Yasheng electronic technology Co.,
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Ltd., China, are used as substrate for preparing bitumen and asphaltene coatings. All of
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the substrates are in the form of circular slices with a diameter of 15mm, with the
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surface roughness less than 1 nm. During the surface force measurement, silica
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microspheres (~Φ10 µm) (Knowledge & Benefit Sphere Tech. Co., Ltd., China.) are
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used in preparation of asphaltene probes with dip-coating technique.17-18
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The bitumen sample (with asphaltenes content at 13.9 wt%) was extracted from
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Athabasca oil sands by toluene using the standard Dean Stark method.19 The
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asphaltenes were precipitated from bitumen by dilution with n-heptane. The water used
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in the whole experiment was at an ultrapure level (18 MΩ). 5
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2.2 Oil liberation on solid surfaces in water
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(1) Substrates preparation
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The silica substrates were washed by the Piranha solution (VH2SO4 (96 wt.%) : VH2O2 = 7:3)
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and ultrapure water followed by drying with high purity nitrogen gas. The muscovite
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substrate was cleaved freshly in air, with its crystallographic plane (001) exposed.
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(2) Bitumen and asphaltene coating
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A model oil-solid system has been prepared to simulate the oil sands surface using the
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following procedures. The bitumen was diluted by toluene to form a solution (10 wt.%
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bitumen) which was dropped on the substrate by an accurate micropipette. The substrate
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was initially placed on a spin coater (KW-4A, Institute of Microelectronics, CAS). A
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total of 1mL diluted bitumen was pipetted as 25 drops at the stirring rate of 2000 rpm
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within 20 s, followed by a speed of 5000 rpm for 60 s to obtain a smooth and uniform
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oil surface.20 The same procedure was used to spin-coat asphaltene on the substrates
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using 2 wt% asphaltene in toluene solution. Subsequently, the prepared substrates with
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different coatings were dried for 3 hours in a vacuum oven at 0.08 MPa, 25 ℃ to
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volatilize the toluene. Consequently, four types of surfaces were obtained: muscovite
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coated by bitumen, silica coated by bitumen, silica coated by asphaltene, and muscovite
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coated by asphaltene.
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(3) Liberation visualization and analysis
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The prepared substrates were placed into the water at ambient conditions. The recession
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of bitumen and/or asphaltenes on the substrate was captured and recorded by an optical
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microscope (Motic-206A, Motic) equipped with a camera successively in 20 min. The 6
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captured images of oil liberation on the substrate were sent for quantitative analysis
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using our developed image processing method.21
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2.3 Surface force measurement by AFM
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(1) Probe particle and substrate preparation
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The atom force microscope (AFM) has been applied to accurately measure the
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interaction force between mineral surfaces and asphaltenes. A silica sphere (Φ10 μm)
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was glued onto a silicon nitride V-shaped cantilever by A&B adhesives. Subsequently,
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the silica sphere immobilized on the probe was dipped in asphaltene-toluene solution
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(2 wt.%) for 1 minute to deposit the asphaltenes. After the successful deposition of
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asphaltenes (shown in Fig. SI-1), the cantilever was taken out and placed in fume hood
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for 3 hours to volatilize the toluene. The clean silica and muscovite surfaces were
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prepared as mentioned in section 2.2 (1).
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(2) Force measurements
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The interaction forces between asphaltenes and two clean substrates (i.e., silica and
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muscovite) were measured using AFM (Bruker, Multimode 8) at room temperature both
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in air and water. Under each condition, about 100 force-distance curves at 10 different
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spots were collected. The average adhesion force was analyzed and compared between
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the two asphaltene-substrate systems. A detailed description of using AFM for force
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measurements can be found elsewhere.16,22 In this study, the SNL-10 probe was applied.
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First of all, the deflection sensitivity of cantilevers was calibrated by measuring force
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curves between probes and monocrystalline silicon. The spring constants of cantilevers
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were calibrated through the analysis of their thermally-induced fluctuation. And typical 7
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probe rates of 3 Hz and 1 Hz were used in air and water, respectively.
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3. Molecular dynamics (MD) simulation
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Molecular dynamics (MD) simulation is a powerful method to investigate the
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adsorption and liberation behavior of molecules on mineral surfaces at microscopic
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molecular level.23-27 In this study, the MD simulation method is applied to understand
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the exact interactions between asphaltenes and muscovite surface.
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3.1 Molecular structures and models
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The construction of mineral surfaces and selection of oil molecules are key steps of the
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molecular dynamic simulation. Here, the monoclinic α-quartz and muscovite crystal
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structure were used as the initial input structures for the MD simulation. The
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crystallographic cells were geometrically optimized by the density functional theory
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(DFT) with the CASTEP code. The lattice parameters (a, b, c) and the crystal plane
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angles (α, β, γ) were obtained through the computational simulation, which were close
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to the experimental values (Table 1).28-29
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Table 1. Crystallographic cell parameters of silica and muscovite obtained from DFT
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calculation Lattice parameters a(Å) b(Å) c(Å) α(degrees) β(degrees) γ(degrees)
silica experiment 4.916 4.916 5.405 90.00 90.00 120.00
muscovite DFT 5.110 5.110 5.599 90.00 90.00 120.00
experiment 5.202 9.024 20.078 90.00 95.756 90.00
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DFT 5.231 9.118 20.514 90.00 95.481 90.00
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The model surfaces of silica and muscovite were cleaved from the optimized unit
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cells along the (001) crystallographic orientation. The newly built silica and muscovite
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supercells were further converted into 3D periodic cells by building vacuum slabs. The
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cross-section of the surfaces was determined to be 1108 Å2 and 1145 Å2 for silica and
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muscovite, respectively. To mimic the real state of the mineral surfaces, the silica was
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modified with hydroxyls. The number of Si-OH groups per square nanometer in the
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hydroxylated surface amounts to 4.4 OH/nm2, similar to the experimental observed
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values (5 OH/nm2).30 Before the MD simulation, geometric optimization of the model
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surfaces was conducted using the Forcite module to get relaxed surfaces. The charge of
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atoms in the two surface models were modified with the charge equilibration (QEq)
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method, which was proven to be a reasonable and rapid charge distribution method.31
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Asphaltenes are the heaviest and most complex fraction in bitumen or petroleum.
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During the past decades, quantities of different asphaltene molecules have been
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reported through instrumental detection and many of them have been used as models in
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MD simulation. Violanthrone-79 (VO-79, C50H48O4, MW: 713), one of the published
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asphaltene models, was selected in this MD simulation, as shown in Fig. 1. There are a
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big aromatic fused ring and two side alkane chains, with carbonyl oxygen and ether
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linkage as the polar groups in the molecule. This type of asphaltene molecule has been
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successively applied as model molecule in many molecular simulations, including self-
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aggregation of asphaltenes, emulsion-stabilization, and adsorption.13,32-33
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Fig. 1. The two-dimensional structure of VO-79 molecule (C50H48O4, MW: 713) 33
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3.2 MD Simulations
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The goal of this study is to get to know the adsorption and liberation behaviors of
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asphaltenes on muscovite surface. During the MD simulation process, the adsorption
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of asphaltenes on both silica and muscovite surfaces was conducted in two different
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conditions: i) the ideal adsorptions of VO-79 molecules on the two surfaces in vacuum
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state (the asphaltenes contact with the mineral surface directly), and ii) the real
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adsorptions of VO-79 molecules on the two surfaces with a layer of water between
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asphaltenes and solid surface.
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In vacuum state, when building up the asphaltene layer, five VO-79 molecules were
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placed into a box with the same length and width to the corresponding solid surface.
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However, it should be mentioned that during the liberation experiments and AFM tests,
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both the silica and muscovite surfaces were covered with thin water films due to their
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strong capacity in adsorbing water from the air.34-38 The amount of water adsorbed on
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the silica and muscovite surfaces is found to be positively dependent on the humidity.
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At the relative humidity (RH) of ~ 70%, the thickness of water layers on the silicon and
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muscovite surfaces reaches 0.4 nm.34 Therefore, to simulate the real adsorption of
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asphaltenes on the silica and muscovite surfaces, a ternary system was adopted, where 10
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the asphaltene layer was placed on preconditioned surfaces with a water film at the
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thickness of 0.4 nm (shown in Fig. SI-2).
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After the completion of the initial state setting, the selection and determination of the
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simulation parameters will be taken into consideration. In this study, the MD
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simulations were carried out using Forcite modules in Materials Studio of Accelrys Inc.
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Three dimensional periodic boundary conditions were applied, and all energy
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expressions and interatomic interaction parameters were taken from the COMPASS
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force field.39 All MD simulations were carried out at NVT-ensemble (constant number
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of atoms (N), volume (V) and temperature (T)) for the established adsorption system at
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constant T= 298 K and P =1 atm. The temperature was controlled by the Berendsen
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thermostat.40 The van der Waals interactions were calculated by the Atom based method,
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while the electrostatic interactions were calculated by the Ewald method. Both the
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cutoff distances were 12.5 Å. The simulation time used for the individual adsorption of
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asphaltene and water molecules was 200 ps, while the adsorption of the ternary system
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was run for 1500 ps. All of the simulations were set with a time step of 1 femtosecond
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(fs). During the simulation, both the silica and muscovite surfaces were considered as
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ideal planes and they were frozen up. The system was equilibrium when the system
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energy and motions of molecules remained relatively stable.
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3.3 Data analysis
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After the adsorption system reached the equilibria state, the interaction energies
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between adsorbates and surfaces, the structure and the diffusion coefficient of adsorbed
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molecules on the surfaces will be calculated, shown in supporting information. In 11
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addition, the density distributions of asphaltene and water molecules along the z-axis
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in the ternary systems are also analyzed.
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Molecular Orientation. The orientation of the asphaltene molecules is described by
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the dihedral angle (α) between the asphaltene polyaromatic plane and mineral surface.
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As shown in Fig. 2, to obtain the angle distribution (α) of the asphaltene molecules to
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the solid surface, the angle between the vector a (normal to the polyaromatic ring
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plane of asphaltene molecule) and the vector n (perpendicular to the surfaces) is
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calculated accordingly. The direction of vector n is parallel to the z-axis 41. After the
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adsorption system reaches equilibria state, the angle calculation is conducted for the
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five asphaltene molecules for each simulation.
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Fig. 2. The schematic representation of the angle distribution (α) of the polyaromatic
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plane of asphaltene molecule at the asphaltene-solid interface.
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4. Results and Discussions
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4.1 Liberation tests
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Fig. 3a shows the liberation behaviors of asphaltenes on two different surfaces. It is
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observed that the asphaltenes appeared to flake off from the muscovite surface directly.
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However, almost no changes happened to the asphaltenes on silica surface. The flaking 12
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of the asphaltenes was mainly contributed to the superhydrophilicity of the muscovite
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surface. The water contact angles measured on muscovite and silica were 1° and 13.5°
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respectively, indicating that the muscovite surface is more hydrophilic and water
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exhibits higher affinity with it. As a result, it is easier for water to replace asphaltenes
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on muscovite. While, a relatively stronger affinity of asphaltenes to silica surface would
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be the reason of no liberation of asphaltenes from silica surface. Additionally, the high
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rigidity of the asphaltene layer caused it to peel off in form of slices rather than recess
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into a droplet.42
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Fig. 3. Surface morphologies of (a) asphaltene film and (b) bitumen film liberating on 13
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silica and muscovite surfaces in water at 25 °C. (c) Degree of bitumen liberation as a
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function of the time on silica and muscovite surfaces in water at 25 °C. Inset shows the
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liberation rate and ultimate liberation degree of bitumen on silica and muscovite
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surfaces.
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To further understand how the other oil fractions influence the asphaltene liberation,
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the bitumen (with maltenes and asphaltenes) was used instead of asphaltenes as coating
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oil on the solid surfaces for liberation tests. Snapshot images in Fig. 3b show that the
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liberation profiles of bitumen are quite different from those of asphaltenes. The bitumen
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liberation is being conducted based on the formation of some holes on the solid surfaces,
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which further facilitates the recession of the bitumen. This difference in oil liberation
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is ascribed to the reduced viscosity of the asphaltenes mixed with maltenes compared
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with the asphaltenes alone. In addition, the bitumen liberation on muscovite surface is
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also found to be largely different from that on silica surface. Many small holes are
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formed on the silica surface, which accelerates the further recession of the bitumen.
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However, only several large holes are quickly formed on muscovite surface, resulting
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in a faster recession of bitumen compared with that on silica surface. At the equilibrium
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state, a nearly complete liberation of bitumen on muscovite surface was obtained. A
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simple first-order kinetic model has been applied to fit the liberation data (shown in Fig.
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3c).9,20
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R R (1 e kt )
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Where R donates the degree of bitumen liberation as a function of time (%), R ∞
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represents the ultimate liberation degree (%), k is the liberation rate constant, referring 14
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to the rate to reach the equilibrium state (s-1), and t is the liberation time (s).
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The liberation rate of bitumen on muscovite surface (23.85×10-3 s-1) was found to be
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more than 5 times faster than that on silica surface (4.4×10-3 s-1). The above results
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suggest that, although oil composition dependent, the mineral surface properties play a
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vital role in oil liberation from their host rocks surfaces.
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4.2 Asphaltene–solids interactions
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The adhesion force distributions and representative normalized force-distance curves
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in air for asphaltene-silica and asphaltene-muscovite interactions are shown in Fig. 4a
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and Fig. 4b, respectively. Both of the approach curves are similar. However, when
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retracting the probe from the solid surface, an obvious adhesion force appears and
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increases along with the increase of detaching distance, reaching to the maximum pull-
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off force. After the pull-off force is reached, the force disappeared suddenly when the
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separation distance continues to increase. On the silica surface, during the probe
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retraction, the pull-off force is determined to be averaged at ~18.01 mN/m at the
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separation distance of about 200 nm. While, under the same conditions, the pull-off
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force (attractive force) between asphaltenes and muscovite (averaged at ~10.03 mN/m)
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is found to be located at the separation distance of about 130 nm, which is much smaller
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than that of silica. This retraction force pattern is quite different from that during the
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approaching of the probe. Different from the single van der Waals force (normally
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acting in a short distance,
