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Layer-by-Layer Assembled Film of Asphaltenes/ Polyacrylamide and its Stability of Water-in-Oil Emulsions: A Combined Experimental and Simulation Study Ming Duan, Xianyu Song, Shuangliang Zhao, Shenwen Fang, Fen Wang, Cheng Zhong, and Zhaoyang Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12168 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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The Journal of Physical Chemistry

Layer-by-Layer

Assembled

Film

of

Asphaltenes/Polyacrylamide and its Stability of Water-in-Oil Emulsions: A Combined Experimental and Simulation Study Ming Duana,b*, Xianyu Songc*, Shuangliang Zhaod, Shenwen Fanga,b, Fen Wange, Cheng Zhongc, Zhaoyang Luoc

a

College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China b Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Chengdu, 610500, P. R. China c Department of Mechanical and Electrical Engineering, Dazhou Vocational and Technical College, Dazhou, Sichuan 635000, P. R. China d State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China e School of Chemistry and Chemical Engineering, Sichuan University of Arts and Science, Dazhou, Sichuan 635000, P. R. China

*To whom correspondence should be addressed: E-mail: [email protected] (Ming Duan) E-mail: [email protected] (Xianyu Song) Telephone number：+8602883037346 Fax number：+8602883037346 Present Addresses: 8 Xindu Avenue, Xindu District, Chengdu, Sichuan 610500, P. R. China

1

ACS Paragon Plus Environment


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Abstract Emulsions with interface-active components at water/oil (w/o) interface are of fundamental and practical interest in many fields. Here we investigate the interfacial films of asphaltenes and asphaltenes/polyacrylamide (PAM) at the w/o interface of water-in-crude oil emulsion. With the combination of dissipative particle dynamics simulation and experimental observation, the molecular interactions of asphaltenes and asphaltenes/PAM at the w/o interface are extensively analyzed. We show that the rigid mechanical film of asphaltenes originates from rigid structure of polycyclic aromatic hydrocarbons (PAHs) and the π-π bonding interactions between the PAHs of asphaltenes, and at a higher concentration of asphaltenes, the nanoaggregates of asphaltenes, acting as space fortress at the w/o interface, make the drop-drop coalescence becoming more difficult. In addition, a layer-by-layer assembled architecture film of asphaltenes/polyacrylamide formed at the w/o interface is identified, and we observe that the inner layer is composed of PAM with network structure and the outer layer is composed of rigid asphaltenes. While the rigidity and stability of this film is attributed to the viscoelasticity and rheology of PAM and the “synergy effect” between asphaltenes and PAM, its presence greatly enhances the stability of water-in-oil emulsions. We further conclude that PAM with higher concentrations and molecular weights can generate more ordered network structure, leading to a more stable asphaltenes/PAM film at the w/o interface. This combined study provides helpful insight into the demulsification of water-in-oil emulsion.

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1. INTRODUCTION Formation of stable water-in-crude oil (w/o) emulsions is detrimental for the petroleum industry because these emulsions can cause flow assurance problems due to their high viscosity and thus require significant efforts to separate the emulsified water from the crude oil.1 Indigenous components of crude oil such as asphaltenes, resins, naphthenic acids, and fine solids play important roles in stabilizing these emulsions.2,3Among these indigenous components, asphaltenes are of increasing importance because of their unique chemical structure.4,5Asphaltenes, constituting of sheet like polyaromatic hydrocarbons, are built up from carbon and hydrogen, together with varying amounts of heteroatoms such as nitrogen, oxygen, and sulfur atoms

6,7

, thus present interface-active properties due to the inclusion of

polar groups such as acid or base.8,9It is generally accepted that the formation of rigid mechanical film with thickness ranging from 2 to 9 nm at the w/o interface is responsible for the stability of emulsions, resulting into hindering droplet-droplet coalescence.10,11Several researchers12,13 have proposed that stable emulsions are largely related to the formation of a kind of cross-linked gel phase at the w/o interface. Furthermore, the colloidal asphaltene aggregates at the w/o interface are also thought to increase the stability of emulsions.14,15Though large number of aforementioned studies has been reported, a further understanding of the interfacial behaviors and emulsion stability is still needed. Polymer flooding, a mature technology of enhanced oil recovery (EOR), has been widely applied in oil fields.16 Hydrolyzed polyacrylamide (HPAM) is the most commonly used polymer in polymer flooding.17,18 It has the advantages of high viscosity(thus significantly reducing the mobility of the aqueous phase) low adsorption loss, insensitivity to bacterial infringement, and obviously enhanced oil recovery.19,20 However, polymer flooding would increase the viscosity of water phase and the relative motion resistance between oil drops and water drops, resulting in the formation of more stable emulsions.21,22Despite many 3



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published results, considerable efforts are still required for better understanding the structure and chemical nature of asphaltenes and asphaltenes/HPAM film at the w/o interface, clarifying the elucidate mechanisms of emulsions stability, and proposing experiment strategies for breaking these emulsions, especially the ones formed with heavy crude oils. In this work, we firstly employed dissipative particle dynamics (DPD) simulations for studying the emulsion morphologies with different concentrations of asphaltenes at mesoscopic scale. The formation of layer-by-layer assembled architecture film of asphaltenes/polyacrylamide was investigated in the presence of PAM with different molecular weights and concentrations. The synergistic effects between asphaltenes and PAM were examined by accessing the radial distribution function. Afterward, by using the interfacial dilational modulus, we characterized the emulsion stability of asphaltenes/PAM film in views of experiments. By combining the simulation and experimental observations, we analyzed the morphologies of the crude oil emulsion at mesoscopic scale.

2. METHODS AND MATERIALS 2.1

EXPERIMENT

2.1.1 Materials The asphaltenes were extracted by precipitation with 20 volumes of heptane to 1 volume of crude oil from the Lvda oil field, stirring overnight at room temperature condition, followed by filtration and rinsing with heptane.4,23DI water, NaCl, and toluene were reagent grade without further purification. Hydrolyzed polyacrylamides with different molecular weights were obtained from Southwest Petroleum University and the viscosity-average molecular weights were determined using viscometry method.19

2.1.2 Dilational Viscoelasticities Measurement Actually, studying of the dilational viscoelasticity which accounts for the viscoelastic 4


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properties of an interface subjected to dilational stresses is very useful to access the adsorption kinetic mechanism of soluble surfactants and the processes occurring in the adsorption layer. Moreover, these properties play a fundamental role in the physics of interfacial films between droplets and bubbles, in particular in their equilibrium and stability conditions.24,25 The dilational viscoelasticities were measured by a drop shape analyzer (DSA30, KRÜSS GmbH CO.) according to ref 24.The device is made up of a glass capillary, a thermostatic water bath, and a microliter syringe, which controls the size and shape of a water drop formed at the tip of the capillary (inner diameter: 0.514 mm). The extracted asphaltenes was dilute to different concentrations with toluene. The oil phase is the asphaltenes and toluene, and aqueous phase is the 1.0 wt% of NaCl solution.25 The interfacial dilational modulus ε at a special particular frequency is quantified by the absolute value ε and phase angle θ describing the phase difference between the variation of dynamic interfacial tension and the variation of interfacial area:11,12,18,26

ε=

dσ = ε d + iωη d , d ln A

(1)

where the dσ and d ln A are the area and interfacial tension variations. The dilational modulus is a complex quantity, of which the real part (storage modulus) represents the elastic energy stored in the interface and is known as the dilational elasticity ε d , and the imaginary part (loss modulus) may be expressed in terms of the interfacial dilatonal viscosityηd because it accounts for the energy dissipated in the relaxation process:8,18,26

ε d = ε cosθ ;

(2)

ε sinθ , ω

(3)

ηd = where θ is the loss angle of the modulus.

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2.2

DPD SIMULATION

2.2.1 DPD Method Dissipative particle dynamics (DPD) were used to simulate the film structures and interfacial properties of asphaltenes and asphaltenes/PAM at the oil/water interface. It is in principle viable to study the interfacial properties at atomistic level by using molecular dynamics.27 However, oilfield emulsions have the droplet size exceeding 0.1µm and may be larger than 50 µm.28 It is obvious that the crude oil emulsions take place at mesoscopic level. In contrast to molecular dynamics, DPD is an excellent method for the simulation of coarse-grained systems over considerable length and time scales up to mesoscopic level. Consequently, DPD has been used to simulate the different kinds of the w/o interfaces6,29,30,31 and crude oil emulsion systems.32,33,34We have demonstrated that DPD method can be applied to study the demulsification process of crude oil emulsions in present of different block polyether33 and the asphaltene aggregates behavior under shear35 with different coarse-grained levels. Here we focus on the study of the interfacial properties of asphaltenes/PAM and its’ stability of crude oil emulsions. In the DPD method, each bead represents a group of molecules or atoms. All beads in a system interact with each other through three kinds of forces, and they are conservative repulsive forces, dissipative forces and random forces. A extensive description on DPD simulation can be found else where,36-38 the detailed description of DPD technique is also presented in the Supporting Information (SI) of this work. In our previous study, by introducing the bonded potentials including bond potential, angle potential and inversion angle potential, the coarse-grained model of asphaltenes is constructed successfully.35 Here we adopt the same method, as depicted in the SI for constructing the coarse-graining model.

2.2.2 Coarse-Graining Method It was common to take the idea that asphaltene molecules are island architecture and 6


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archipelago architecture. The archipelago architecture model which was thought to be composed of several small fused aromatic rings connected by bridge chains of small molecular weights is more applicable to heavy or ultra-heavy oil rather than to light crude oil.35,39In the present work we use archipelago architecture model to investigate the stability mechanism of heavy oil emulsions. As shown in Figure 1a, a coarse-grained model of the fused aromatic rings is constructed by creating the rigid sheet of the hexa-particle ring. To effectively coarse-grain the asphaltenes, toluene and water, four different types of beads have been identified as the building blocks of the different molecular components, as shown in

Figure 1. The typical beads, B represents the moiety of aromatic rings, which is denoted by benzene molecule; H corresponds to the alkyl chain, which is defined as butane molecule, T is the functional group containing heteroatoms, we take thiourea as T bead.35Three water molecules are took as one bead (W bead),6,38while the toluene molecule is clustered into two different coarse-grained particles which are represented as B bead and H bead.6,35The archipelago architecture coarse-grained model of asphaltenes are presented in Figure 1e, which was reported by Zhang et al.35,38 four PAM monomers are defined as one AM bead.40 In the DPD simulations, in general a bead corresponds to Nm water molecules. The number

Nm (degree of coarse graining) can be viewed as a real-space renormalization factor.6,38In the present work, N m = 3 , and this treatment has already been confirmed to produce ideal crude oil systems.6,38The length scale Rc in angstroms, mass scale m and the time scale τ in

picoseconds can be evaluated, and they are Rc ==3.107 ( ρ N m )1/3 Å, m = N m ⋅ mwater amu, τ =(1.41±0.1)Nm5/3 ps,6,36,38 where ρ is the DPD number density and mwater is the mass of the water molecule. In practice, since the number of bead-bead interactions increases with density, the DPD algorithm is most efficient when the density ρ is set to 3.0.36The length and time scales in physical units are Rc = 6.46 Å, and m = 54 amu, τ = 8.8ps with Nm=3. 7



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Figure 1. Coarse-grained model molecules of (a) fused aromatic rings (or polycyclic aromatic hydrocarbons), (b) chain alkanes, (c) water molecules, (d) toluene, (e) archipelago architecture model of asphaltenes, (f) corresponds to the schematic representation of coarse-grained beads in simulations.

2.2.3 DPD Parameters and Simulation Details The most important parameter in DPD method is the conservative repulsive force, namely, the bead-bead interaction parameters. They can be determined by the equation: aij = aii + 3.27 χ ij ,36,41 where aii = 78.0 with the mapping of three water molecules

perbead.41,42The values of χ ij can be calculated from the solubility parameters following the equation: χij =

ν ij RT

(δ i − δ j ) 2 ,42 where ν ij is the average of molar volumes of two beads; δ i

and δ j are the solubility parameters of component i and j , respectively. Employing the Hansen solubility parameters for δ i , χ ij can be calculated, and this method has already been confirmed to produce ideal interfacial tensions.27Moreover, Hansen solubility parameters used in DPD simulations can provide an interpretation on the experimental results on diffusion coefficient and interlayer distance of asphaltenes in toluene.35Combining with the coarse-graining method described in part 2.2.2, the Hansen solubility parameters43and conservative repulsive forces in DPD simulations were calculated and collected in Table 1 and Table 2, respectively. The classes and concentrations of salts in aqueous solution have a 8


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great influence on the solubility parameters of PAM. We take the value of 0.11 for χ water − AM without considering the hydrolyzed and hydrophobic properties, and the end-to-end distance of the PAM chains from DPD simulations is consistent with the experimentaldata.40

Table 1.Hansen Solubility Parameters (J/cm3)1/2and molar volume (cm3/mol) at 298K40,43 molecule water (W) benzene (B) butane (H) thiourea(T) polyacrylamide (AM)

Hansen Solubility Parameter (J/cm3)1/2 47.81 18.51 14.10 33.01 45.37

Molar Volume (cm3/mol) 18.00 89.40 101.4 72.8 73.90

Table 2. Parameters of Conservative Force bead

water (W) water(W) 78.0 benzene (B) 138.7 butane (H) 161.9 thiourea(T) 91.10 polyacrylamide(AM) 78.36

benzene (B)

butane (H)

thiourea (T)

polyacrylamide (AM)

78.0 80.4 100.5 84.59

78.0 119.1 85.11

78.0 85.28

78.0

All the DPD simulations were carried out using the Mesocite module embedded in the Materials Studio 6.1 package from Accelrys, Inc.44In specific, all simulations were performed in a cubic box with a size of 100× 100× 100Rc3 with periodic boundary conditions applied along three direction. The system temperature is set as 298K. To simulate the water-in-crude oil emulsions, the cubic box was divided into asphere with radius of 30Rc in the center of box, as depicted in Figure S1 of the SI. For the aqueous phase, all water molecules were placed inside the sphere, and the remaining part of box was filled with toluene and asphaltenes molecules, and referred as oil phase. The asphaltenes concentration is defined as the number ratio of asphaltenes beads to total beads in the oil phase. The total number of beads is 1.2428×104 in the box when the density of all systems is set to 3.0 in reduced units. A total of 10 × 1010 DPD simulation steps were carried out with a time step ∆t =0.005 τ .The scales used in DPD simulations were as follows: length scale, 6.46 Å; mass scale, 54 amu; energy 9



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scale, 0.59191 kcal/mol; time scale, 3.0158 ps.6 The total real dynamic time ttotal = N steps ⋅ ∆t , namely, 15.08 ns.

3. RESULTS AND DISCUSSION To ensure the simulations are completely equilibrated, we check the time evolution of temperature and total potential energy of the system. The results are shown in Figure S2 in the SI. Figure S2 shows that the temperature and total potential energy in terms of simulation time decrease dramatically and reach equilibrium quickly. Figure S2 implies that the simulation time of 10× 105steps is sufficiently long for our systems to reach equilibrium.

3.1 Validation of the DPD Method As a first step, we validate our DPD calculations by accessing the diffusion coefficient and interlayer distance of asphaltenes. The calculated diffusion coefficient and interlayer distance of asphaltenes agree well with the available experiment data.38,45 The diffusion coefficients D, calculated from the slopes of the mean square displacements (MSD) in the long time limit using follow equation: 46 D=

1 2Nd

N

lim ∑  r（t）- r（t）  t →∞

d dt

2

i

i

(1)

i =1

where Nd is the dimensionality (Nd = 3 for the simulations),ri(t) and [ri(t)- ri(0)2] are the position and squared displacement of given molecules at time t, respectively. The simulation results of mean square displacements were given in Figure 2a. By calculating the slope of mean square displacements versus time, the diffusion coefficients D can be calculated, as shown in Figure 2a. The diffusion coefficient of diluted asphaltene in toluene is 3.41∼5.73× 10-10 m2s-1 which is consistent with the experimental measurement 2.2∼6.3 ×10-10 m2s-1.45 The interlayer distance value is calculated from radial distribution function. The radial 10


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distribution function is computed for all pairs of beads or centroids in the set which are closer than the cutoff value. Thus, it could illustrate the interlayer distance between asphaltenes in aggregation process.35 Radial distribution function can be calculated using following equation33,35,37 g ij ( r ) =

{∆N （ij r → r + ∆r）}V

(2)

4π ⋅ r 2 ∆rN i N j

where {∆Nij(r→r+∆r)} is the ensemble averaged number of j around i within a shell from r to r+∆r, V is the system volume, Ni and Nj are number of i and j, respectively. We take the first peak of radial distribution function as interlayer distance. As shown in Figure 2b, the interlayer distance value from DPD simulations is about 5.05 Å in our calculation, which is slightly larger than the experimental value (∼3.55Å)47 and other DPD simulations predictions (3.75-4.05 Å).38 Actually, the diffusion coefficient and interlayer distance of asphaltenes are appropriate to evaluate the DPD calculations, and this method were used in our previous study.35

700 600

2

(a)

MSD(Angstrom )

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5% 8% 14% 18%

500 400

-10

2

K5%=34.191×10 m /s

-10

2

-10

2

K8%=34.392×10 m /s

300 200

K14%=20.045×10 m /s

100

-10

2

K18%=21.299×10 m /s

0 0

500

1000 Time(ps)

1500

11


2000


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(b)

180 160 140 g(r) 120 100 80 60 40 20 0 5% 8% 14% 18%

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N=1

N=2 N=3 N=4

0

4 3 2 (nm) r

1

5

interlayer distance

Figure 2. (a) The MSD of asphaltenes at different concentrations in toluene and (b) the radial distribution functions of asphaltenes at different concentrations in toluene. The slope values K of MSD with time are list in figure (a) and the calculated diffusion coefficients are 5.70×10-10m2/s, 5.73×10-10m2/s, 3.41×10-10m2/s, 3.55×10-10m2/s, respectively. The interlayer distance value is about 5.05 Å.

3.2 Asphaltenes W/O Emulsions 3.2.1 Accumulated Configuration at the W/O Interface Most previous studies on the self-aggregation of asphaltenes at the w/o interface focused on the planar models of liquid-liquid interface (such as oil/water/oil planar interfaces, and oil/water planar interface) by using MD or DPD simulations,6,46,48however, these planar models of liquid-liquid interface only provide small part of interfacial structure information compared to the droplets in emulsions. Several researchers reported the emulsion stability of w/o emulsion model by using MD simulations.49,50 Nevertheless, the large droplets (exceeding 0.1µm and may be larger than 50 µm) exist in stable emulsions droplet at nanoscale

or

micronscale

level

which

matches

to

mesoscopic

level

of

DPD

simulations.32,33,34The adsorption behaviors of asphaltenes are initially performed using w/o 12


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emulsion model and the results are shown in Figure 3. It can be found that the initial disordered asphaltene molecules quickly self-assembly into ordered structure at the w/o interface. Multilayer accumulated structural aggregates consisting of a few asphaltene molecules are formed. More importantly, it is interesting to find that most of the stacked polycyclic aromatic hydrocarbons (PAHs) of asphaltenes prefer to be parallel to the w/o interface. A small part of asphaltenes tends to be perpendicular or slope to the w/o interface. Moreover, the stacked structure of asphaltenes remains essentially stable after 4.02 ns simulation time. It is also observed that the water droplet in oil is wrapped tightly by asphaltenes when the system reaches equilibrium, which produces asphaltenes protective films hindering the droplet-droplet coalescence. Actually, these asphaltenes protective films are so rigid and sturdy that interfacial sliding or shearing is generally required to destabilize the protective interfacial asphhaltene layers which facilitates the coalescence of emulsion drops.51It should be pointed out that the accumulation and orientation of the asphaltenes at the w/o interface are resulted from the heteroatoms’ affinity for water molecules, as shown the red beads of asphaltenes in Figure 3. The asphaltenes are highly oriented in protective films with their PAHs in plane (parallel to the w/o interface), while the alkyl substituents are out of plane which can be verified by experimental results from sum frequency generation.6,38,52

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Figure 3. Morphologies of water-in-oil emulsions at different simulation time with the concentrations of 5% asphaltenes. Different beads in simulations are represented by different colors as can be illustrated from Figure 1f (the same as below). The toluene molecules are suppressed for clarity.

3.2.2 Effects of Concentration The effects of asphaltenes concentrations on adsorbed behavior are also investigated. From the Figure 4, it is observed that most of the stacked PAHs of asphaltenes are aggregated in parallel and most of the fused aromatic ring planes tend to parallel to the w/o interface when the concentrations of asphaltenes lower 15%. At higher concentrations (like 20%), the nanoaggregates of asphaltenes are formed at edge of the asphaltenes protective films, as shown in Figure 4d. the nanoaggregates of asphaltenes act as space fortress (called as “steric effect”) enhancing the emulsions stability. The nanoaggregates of asphaltenes are observed by experiment results53,54 and MD simulations.55The nanoscale aggregates (7-20 nm in characteristic dimension) and aggregate number (6-14) in experimental data53,54are qualitatively in good agreement with simulation results: aggregates size of 3-22 nm and aggregate number of 7-13. 14


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Bi et al.56 used atomic force microscopy investigate asphaltene films evolution at water/xylene interface using asphaltene model compounds, and the experimental results were shown in Figure 4e-4g. When the concentrations of asphaltenes is 0.06 mM, they observed that black spots (composed of asphaltenes and thickness < 10 nm) were formed, and these black spots expanded rapidly, and the last image before film rupture was shown in Figure 4e. Compared to the simulation results (see Figure 4a) the black spots Figure 4e can be confirmed by the region of asphaltenes adsorbed at the w/o interface, as shown in Figure 4a and 4b. At concentration of asphaltenes was 0.2 mM, the black spots evolved, leading to the formation of a thin black film, as shown in Figure 4f (the bright and colored lenses region due to the presence of nanoaggregates in the intervening liquid film). The simulated results also observe the nanoaggregates of asphaltenes, as shown in Figure 4d. With a further increase in asphaltenes concentration to 0.5 mM, the films become thicker as indicated by the grey and white shades. These bright and colored spots observed in asphaltenes films at the w/o interface see (Figure 4f and 4g) are also well agree with the simulation results (the region of asphaltenes adsorbed at the w/o interface, as shown in Figure 4c and 4d). In addition, the experimental results of atomic force microscopy are consistent with the simulation results.

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Figure 4. Morphologies of water-in-oil emulsions at different concentrations of asphaltenes: (a) 5%, (b) 10%, (c) 15%, (d) 20%; (e-g) atomic force microscopy (AFM) images of model asphaltenes at different concentrations of asphaltenes: (e) 0.06mM, (f) 0.2mM, (g) 0.6mM.56 The asphaltenes, namly, N-(1-hexylheptyl)-N’-(5-carbonylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (in brief C5Pe), was used in atomic force microscopy (AFM) experiments. The toluene molecules are suppressed for clarity.

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(a) 50

40 Film thikness(nm)

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30 20 10 0 5

20

10 15 Concentration(%)

(b)

2.5

peak1

2.0

peak2

g(r) 1.5 peak3

1.0 0.5

peak4

0.0 5% 10% 15% 20%

0

1

3 2 ) r(nm

4

5

Figure 5. The film thickness and radial distribution function g(r) at different concentrations ratio of asphaltenes. The film thickness is an important interfacial physical property which can provides a quantitative measure for the size of the interface. According to the density profiles, the interfacial thickness is calculated by the “90-10” criterion, which is defined as the distance along the interface over which the densities of oil from 90% to 10% of their bulk value.57-58The relative density profiles are given in Figure S3 of Supporting information. As shown in Figure S3, relative density profiles for asphaltenes distribute in common boundary 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of density profiles for aqueous phase and oil phase which indicates the asphaltenes adsorb at the w/o interface due to the amphiphilic nature of asphaltenes. The calculated film thickness is presented in Figure 5a. As shown in Figure 5a, the increasing asphaltenes concentration leads to a pronounced expansion of the interfacial thickness, which suggests that surfactant molecules want to be more ordered at high concentration (reflected by Figure 4). The calculated film thickness are range from 22.05 to 46.28 nm which slightly less than experimental dada 15-90 nm.59 The asphaltenes with higher concentrations in experiments may lead to a higher film thickness. In order to further characterize the aggregation structure of asphaltenes at w/o interface, the radial distribution function is introduced to exhibit the configuration and ordered array of the aggregation structure. The higher values of radial distribution function show stronger interactions between molecules. The radial distribution function in this case, calculated for a cut-off radius of 50 Å and an interval distance of 0.05 Å, is shown in Figure 5b. As shown in

Figure 5b, the simulation systems have one sharp peak (about 0.58 nm) and some small peaks (about 1.14 nm, 1.65 nm, 2.18 nm). Interestingly, these peaks observed in Figure 5b increase with arithmetic sequence, and these results can be confirmed by lamellated asphaltenes films. (see Figure 4 and 5)The sharp peak at about 0.58 nm corresponds to the face-to-face stacking, which is formed by the π-π stacking of polyaromatic rings. These small peaks reflect that the asphaltenes films adsorbed at the w/o interface are multi-layers due to the intensively π-π bonding between PAHs of aspahltenes.

6,35

Consequently, the asphaltenes

films are quite rigid and stable.

3.2.3 Stability of Water-in-Oil Emulsions Dilational viscoelasticities are the study of the relationship between interfacial stress and the resultant deformation of the interface. The dilational viscoelasticities are usually used to quantify the stability of crude oil emulsions. The dilational viscoelasticity properties of the 18


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asphaltenes films at the w/o interface with different concentrations are presented in Figure 6. As shown, we can find that the dilational elasticity and dilational viscosity of asphaltenes films gradually increase with the increase of asphaltenes concentrations, suggesting that the emulsions stability of asphaltenes films becomes stronger. The dilational viscosity reveals the deformation blocking ability of the interfacial film, while the dilational elasticity reflects the ability of an interfacial film to recover from deformation.18With the increase of asphaltene concentration, the coverage areas become larger (see Figure 4), thus leading to a higher dilational viscosity. Higher asphaltene concentration and more π-π stacking of polyaromatic rings would result in higher dilational elasticity (reflected by in radial distribution function). Actually, asphaltenes are inherent in special PAHs, and these PAHs are naturally rigid structure.

6,35,38

Moreover, the intensively π-π bonding existed in PAHs of aspahltenes

enhances the rigid mechanical film of asphaltenes. Based on these reasons, the absolute values of dilational viscosity are much higher than dilational elasticity (as shown in Figure

6). (a) 1.8

1.6 1.4 1.2

|ε |(mN/m) d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 Asphaltenes Asphaltenes/20 mg/LPAM Asphaltenes/800mg/LPAM Asphaltenes/150mg/LPAM

0.6 0.4 0.2

50

100 150 200 250 Concentrations of asphaltenes(mg/L)

19



(b) 140

Asphaltenes Asphaltenes/20 mg/LPAM Asphaltenes/80 mg/LPAM Asphaltenes/150mg/LPAM

120 100

|η |(mN/m) d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 40 20 50

100 150 200 250 Concentrations of asphaltenes(mg/L)

Figure 6. (a) Dilational elasticity and (b) dilational viscosity as a function of concentraions of PAM.

3.3 Asphaltenes and PAM W/O Emulsion

3.3.1 Accumulated Configuration at the W/O Interface The DPD simulations are then performed to simulate the asphaltenes and PAM w/o emulsions with different molecular weights of PAM. Taking the ratio of asphaltenes and PAM into consideration, the simulation results are provided in Figure 7. It is found that the initial asphaltene and PAM molecules self-assembly into ordered network structure at the w/o interface. More importantly, it is interesting to find that special layer-by-layer assembled architecture film of asphaltenes/PAM is observed: the PAM adsorb at the w/o interface acting as underwear, and asphaltenes self-assembly at the surface of PAM films looking like an overcoat. The relative density profiles for the simulation systems are given in Figure S4 of Supporting information. As the Figure S4 shown, the density profiles for PAM have a similar to the density profiles for aqueous phase, while the asphaltenes tend to distribute at the w/o interface because the PAM have a stronger hydrophilic properties.40These results also reflect that PAM films exist at the inner layers of bilayer films while the asphaltenes films locate at the outer layers. Moreover, the film thickness of the special bilayer films are larger than the pure asphaltenes film at the w/o interface, as shown in Figure 8, which are consistent with 20


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experimental results.19,20 The PAM molecules prefer to locate parallel to the w/o interface with a special network structure (see Figure 7). In addition, higher molecular weights the PAM molecule has, more obvious the network structure becomes, and sturdier the network structure is. These results can be explained by analyzing the radial distribution function between the PAM molecules, as shown in Figure 8. Clearly, there are four sharp peaks (at about 0.63 nm, 1.19nm, 1.74 nm, and 2.28 nm). The sharp peak reflects the strong attraction between the PAM molecules. Moreover, if the molecular weight of PAM is larger, the peaks become sharper. The feature of multi-peaks also confirms the network structure of PAM molecules. For asphaltenes, most of the stacked PAHs of asphaltenes prefer to be parallel to the PAM films at the w/o interface. Similarly, the alkyl substituents stretch into oil phase. The multi-peaks of radial distribution function of asphaltenes molecules are also observed because of the π-π bonding between the PAHs of asphaltenes. Interestingly, the radial distribution function between asphaltenes and PAM molecules, as shown in Figure 7, presents pronounced peaks than that between asphaltenes. This indicates that a strong interaction exist between asphaltene and PAM molecules, we call this as “synergy effect”, which leads to the formation of more stable bilayer films of asphaltenes/PAM.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Morphologies of water-in-oil emulsions with different concentrations ratio of asphaltenes/PAM: (a, c, e) 5:1，（b, d, f）2:1；the chain length of PAM: (a, b) N=20，(c, d） N=30，(e, f）N=40，the total contents of asphaltenes and PAM are 20% aqueous phase. The toluene molecules are suppressed for clarity.

22


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Asphaltenes/PAM=5:1 Asphaltenes/PAM=2:1

60

(c)

Film thickness(nm)

50

(e)

(b)

40

(f)

(a) (d)

30

20

10

0

PAM3

PAM1

PAM1

Concentration(%)

Figure 8. The film thickness with different concentrations ratio of asphaltenes/PAM: (a, c, e) 5:1，（b, d, f）2:1；the chain length of PAM: (a、b) N=20，(c, d）N=30，(e, f）N=40，the total contents of asphaltenes and PAM are 20% aqueous phase. (a) 10

6 4 2 0

Asphaltenes-PAM PAM-PAM Asphaltenes-Asphaltenes

8 g(r)

g(r)

(b) 10


8

6 4 2

0

1

(c) 10

2

r(nm)

3

4

0

5

1

g(r)

4

2

r(nm)

3

4

5


8

6

6 4 2

2 0

0

(d) 10


8 g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1

2

r(nm)

3

4

5

0

0

1

23


2

r(nm)

3

4

5


(e) 10

(f) 10


8

peak1


8

6

6 g(r)

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4

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peak2

4

peak3 peak4

2 0

2 0

1

2

r(nm)

3

4

5

0

0

1

2

r(nm)

3

4

5

Figure 9. The radial distribution function g(r) of different fractions in the simulation systems with different concentrations ratio of asphaltenes/PAM: (a, c, e) 5:1，（b, d, f）2:1；the chain length of PAM: (a, b) N=20，(c, d）N=30，(e, f）N=40，the total contents of asphaltenes and PAM are 20% aqueous phase.

3.3.2 Stability of Water-in-Oil Emulsions The effect of PAM concentration on the dilational viscoelasticities of asphaltenes/PAM films is analyzed and discussed in Figure 6. It is can be found that the PAM molecules increase the dilational elasticity and dilational viscosity when increasing the PAM concentration. This shows that the addition of PAM into the aqueous phase is beneficial to the crude oil emulsion interfacial film stability; however, it is detrimental for demulsification of stable crude oil emulsion. The increase in dilational elasticity and dilational viscosity is mainly because the reorientation and rearrangement take place when adding PAM into the aqueous phase, 60 which is reflected by Figure 7. Moreover, we can find that the PAM with higher concentrations can result in a more ordered network structure of PAM films (compared with the right column and left column of Figure 7). For a highly rigid interfacial film, if the distance between the molecules in the interfacial film are closer, the deformation blocking ability is much stronger, and the dilational viscosity and elasticity are bigger. The peaks of radial distribution function of asphaltenes and PAM are depicted in Figure 9, indicating the “synergy effect” between asphaltenes and PAM significantly strengthen the films stability.

24


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(a)3.0

2.5

|ε |(mN/m) d

2.0 1.5 1.0 0.1000HZ 0.0330HZ 0.0125HZ

0.5 0.0

0

200

400

600 4 Mη(10 g/mol)

0.0500HZ 0.0200HZ

800

1000

(b) 140

120 100

|ηd|(mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 0.1000HZ 0.0330HZ 0.0125HZ

40 20 0

200

400

600 4 Mη(10 g/mol)

0.0500HZ 0.0200HZ

800

1000

Figure 10. (a) Dilational elasticity and (b) dilational viscosity as a function of molecular weight of PAM with different frequencies. Under the condition of a concentration ratio of asphaltenes (100 mg/L) to PAM (150 mg/L), the changes in the w/o dilational viscoelasticities of the asphaltenes/PAM film and its fractions with changes PAM molecular weight at different frequencies are also measured, as shown in Figure 10. We can see from the Figure 10 that the dilational elasticity and dilational viscosity gradually increase at the first beginning, and then reach to a platform with the changes of frequencies. On one hand, the PAM molecules are inherent in viscoelasticity and rheology properties because of compliant PAM chains which may result in an increase of dilational elasticity and dilational viscosity; on the other hand, as the molecular weights of 25



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Page 26 of 32

PAM increases, the PAM chains adsorbed at the w/o interface become more and more ordered, and this is confirmed by the simulation results (see Figure 7), and thus, the film of asphaltenes/PAM becomes more stable. The radii of gyrations of PAM for different simulation systems are presented in Figure S5 of the SI. As shown in Figure S5, the radius of gyration increases with the increase of molecular weight of PAM. These results agree well with the experimental results.19,20 However, the PAM with super molecular weights would result in a decrease of dilational elasticity and dilational viscosity. This is likely because the sub-layer formed at high concentrations together with the fast exchange between the network structure and PAM monomer near the interface destroy the rigid structure, hence reducing the modulus dilational elasticity and dilational viscosity.19,61,62

4. CONCLUSIONS In this work, by combining computer simulations and experiment measurement, we studied the asphaltenes and asphaltenes/PAM films at the interface of water-in-oil emulsion, and the emulsion stability mechanism was explored in detail. Specifically the simulation results provide an exhaustive interpretation on the experimental results of the diffusion coefficient of asphaltenes in toluene and on the size and number of aggregates of asphaltenes at the w/o interface. All calculations from DPD indicate that the formation of rigid and stable asphaltenes films at the w/o interface results from the rigid structure of PAHs and the π-π bonding interactions between the PAHs of asphaltenes. The effect of the PAM molecular weight on the asphaltenes/PAM film is investigated, and

it

reveals

that:

a

layer-by-layer

assembled

architecture

film

of

asphaltenes/polyacrylamide is formed at the w/o interface; the PAM adsorb at the w/o interface acting as underwear, and the asphaltenes self-assembly at the surface of PAM films as an overcoat. The layer-by-layer assembled architecture film of asphaltenes/polyacrylamide 26


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results in a higher dilational viscoelasticities, and this enhances the stability of water-in-oil emulsions. This film is rigid and stable due to the contribution of viscoelasticity and rheology properties of PAM and the “synergy effect” between the asphaltenes and PAM. Moreover, PAM with higher molecular weights makes the asphaltenes/PAM film more stable because more ordered network structure is formed. Our results provide fundamental understanding for the structure and chemical nature of asphaltene and asphaltene/HPAM film at the w/o interface, and cast molecular insight into the stabilization of water-in-crude oil emulsions.

Supporting Information is available: Detailed information on DPD method, evolution of temperature and total potential energy, and the calculation method of diffusion coefficient and radius of gyration; figures of schematic representation for simulations and radial distribution function as well as density distributions.

AUTHOR INFORMATION Corresponding Author * E-mail [email protected] (M. D.); [email protected] (X. S.); Tel (+1)-608-332-6584. ORCID Xianyu Song: 0000-0003-2320-2599 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by National Key Basic Research Program of China (2014CB748500), and the National Natural Science Foundation of China (21376193 and 91434110), the Blue Fire Project from the National Department of Education (2014-LHJH-HSZX-015) and the Fundamental Research Funds for the Central Universities of China.SZ and Fok Ying Tong Education Foundation (151069), and also acknowledges the support of Scientific Research Fund of Sichuan Provincial Education Department (15ZA0363).

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TOC Graphic


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Polyacrylamide and

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