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Article pubs.acs.org/JPCC

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§ †

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

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‡

S Supporting Information *

ABSTRACT: Emulsions with interface-active components at the 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 emulsions. With the combination of the 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 a 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 a space fortress at the w/o interface, make the drop−drop coalescence 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 a network structure and the outer layer is composed of rigid asphaltenes. While the rigidity and stability of this film are 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 a 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.

1. INTRODUCTION The 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 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 and thus present interface-active properties due to the inclusion of polar groups such as an acid or a base.8,9 It is generally accepted that the formation of a rigid mechanical film © 2017 American Chemical Society

with a thickness ranging from 2 to 9 nm at the w/o interface is responsible for the stability of emulsions, resulting in hindering droplet−droplet coalescence.10,11 Several 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.14,15 Though a large number of the aforementioned studies have 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 Received: December 2, 2016 Revised: February 6, 2017 Published: February 7, 2017 4332

DOI: 10.1021/acs.jpcc.6b12168 J. Phys. Chem. C 2017, 121, 4332−4342

Article

The Journal of Physical Chemistry C

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

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, KRÜ SS 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 were diluted to different concentrations with toluene. The oil phase is the asphaltenes and toluene, and the aqueous phase is the 1.0 wt % 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

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 the water phase and the relative motion resistance between oil drops and water drops, resulting in the formation of more stable emulsions.21,22 Despite many published results, considerable efforts are still required in order to better understand the structure and chemical nature of asphaltenes and asphaltenes/ HPAM films 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 first employed dissipative particle dynamics (DPD) simulations for studying the emulsion morphologies with different concentrations of asphaltenes at the mesoscopic scale. The formation of a 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 the asphaltenes/PAM film in views of experiments. By combining the simulation and experimental observations, we analyzed the morphologies of the crude oil emulsion at the mesoscopic scale.

ε=

dσ = εd + iωηd 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 εd, and the imaginary part (loss modulus) may be expressed in terms of the interfacial dilatonal viscosity ηd because it accounts for the energy dissipated in the relaxation process:8,18,26

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, stirred overnight at room temperature conditions, filtered, and rinsed with heptane.4,23 DI 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 the viscometry method.19 2.1.2. Dilational Viscoelasticities Measurement. Actually, studying of the dilational viscoelasticity which accounts for the viscoelastic properties of an interface subjected to dilational stresses is very useful to access the adsorption kinetic mechanism of soluble surfactants and the processes occurring

εd = |ε| cos θ

(2)

| ε| sin θ ω

(3)

ηd =

where θ is the loss angle of the modulus. 2.2. DPD Simulation. 2.2.1. DPD Method. Dissipative particle dynamics (DPD) was 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 the atomistic level by using molecular dynamics.27 However, oilfield emulsions have the droplet size exceeding 0.1 μm and may be larger than 50 μm.28 It is obvious that the crude oil emulsions take place at the mesoscopic level. In contrast to molecular dynamics, DPD is an 4333


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2.2.3. DPD Parameters and Simulation Details. The most important parameter in the DPD method is the conservative repulsive force, namely, the bead−bead interaction parameters. They can be determined by the equation: aij = aii + 3.27χij,36,41 where aii = 78.0 with the mapping of three water molecules perbead.41,42 The values of χij can be calculated from the solubility parameters following the equation: νij χij = RT (δi − δj)2 ,42 where νij is the average of molar volumes of two beads; δi and δj are the solubility parameters of component i and j, respectively. Employing the Hansen solubility parameters for δi,χij can be calculated, and this method has already been confirmed to produce ideal interfacial tensions.27 Moreover, Hansen solubility parameters used in DPD simulations can provide an interpretation on the experimental results on the diffusion coefficient and the interlayer distance of asphaltenes in toluene.35 Combining with the coarse-graining method described in section 2.2.2, the Hansen solubility parameters43 and conservative repulsive forces in DPD simulations were calculated and collected in Tables 1 and 2, respectively. The classes and concentrations of

excellent method for the simulation of coarse-grained systems over considerable length and time scales up to the mesoscopic level. Consequently, DPD has been used to simulate the different kinds of the w/o interfaces6,29−31 and crude oil emulsion systems.32−34 We have demonstrated that the DPD method can be applied to study the demulsification process of crude oil emulsions in the presence 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. An extensive description on DPD simulation can be found elsewhere,36−38 and a detailed description of the DPD technique is also presented in the Supporting Information (SI). In our previous study, by introducing the bonded potentials including bond potential, angle potential, and inversion angle potential, the coarsegrained 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 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 ultraheavy oil rather than to light crude oil.35,39 In 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),6,38 while the toluene molecule is clustered into two different coarse-grained particles which are represented as B bead and H bead.6,35 The archipelago architecture coarsegrained model of asphaltenes is 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.6,38 In the present work, Nm = 3, and this treatment has already been confirmed to produce ideal crude oil systems.6,38 The length scale Rc in angstroms, the mass scale m, and the time scale τ in picoseconds can be evaluated, and they are Rc = 3.107(ρNm)1/3 Å, m = Nm·mwater amu, and 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 3.0.36 The length and time scales in physical units are Rc = 6.46 Å, and m = 54 amu, τ = 8.8 ps with Nm = 3.

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

Hansen solubility parameter (J/cm3)1/2

molar volume (cm3/mol)

47.81 18.51 14.10 33.01 45.37

18.00 89.40 101.4 72.8 73.90

salts in aqueous solution have a great influence on the solubility parameters of PAM. We take the value of 0.11 for χwater−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 experimental data.40 All of the DPD simulations were carried out using the Mesocite module embedded in the Materials Studio 6.1 package from Accelrys, Inc.44 Specifically, all simulations were performed in a cubic box with a size of 100 × 100 × 100Rc3 with periodic boundary conditions applied along three directions. The system temperature is set as 298 K. To simulate the water-in-crude oil emulsions, the cubic box was divided into a sphere with radius of 30Rc in the center of the 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 to as the oil phase. The asphaltenes concentration is defined as the number ratio of asphaltene beads to the 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 of Δt = 0.005τ. The scales used in DPD simulations were as follows: length scale, 6.46 Å; mass scale, 54 amu; energy scale, 0.59191 kcal/mol; time scale, 3.0158 ps.6 The total real dynamic time ttotal = NstepsΔt, 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 4334


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The Journal of Physical Chemistry C Table 2. Parameters of Conservative Force bead

water (W)

benzene (B)

butane (H)

thiourea (T)

polyacrylamide (AM)

water (W) benzene (B) butane (H) thiourea (T) polyacrylamide (AM)

78.0 138.7 161.9 91.10 78.36

78.0 80.4 100.5 84.59

78.0 119.1 85.11

78.0 85.28

78.0

calculated, as shown in Figure 2a. The diffusion coefficient of diluted asphaltene in toluene is 3.41−5.73 × 10−10 m2 s−1, which is consistent with the experimental measurement 2.2−6.3 × 10−10 m2 s−1.45 The interlayer distance value is calculated from the radial distribution function. The radial 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 the aggregation process.35 The radial distribution function can be calculated using the following equation33,35,37

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 × 105 steps 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, were calculated from the slopes of the mean square displacements (MSD) in the long time limit using the following equation:46 1 d D= lim 2Nd t →∞ dt

gij(r ) =

{ΔNij(r → r + Δr )}V 4π ·r 2ΔrNN i j

(2a)

N

∑ [|ri(t ) − ri(t )|2 ] i=1

where {ΔNij(r → r + Δr)} is the ensemble averaged number of j around i within a shell from r to r + Δr, V is the system volume, and Ni and Nj are number of i and j, respectively. We take the first peak of radial distribution function as the 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 (∼3.55 Å)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 was used in our previous study.35 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 are 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;6,46,48 however, 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 0.1 μm and may be larger than 50 μm) exist in stable emulsions droplet at nanoscale or micronscale level which matches the mesoscopic level of DPD simulations.32−34 The adsorption behaviors of asphaltenes are initially performed using w/o emulsion model, and the results are shown in Figure 3. It can be found that the initial disordered asphaltene molecules quickly self-assemble into an 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 tend to be perpendicular or slope to the w/o interface. Moreover, the stacked structure of asphaltenes remains essentially stable after a 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

(1a)

where Nd is the dimensionality (Nd = 3 for the simulations) and 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 the mean square displacements versus time, the diffusion coefficients D can be

Figure 2. (a) 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 listed in panel a, and the calculated diffusion coefficients are 5.70 × 10−10 m2/s, 5.73 × 10−10 m2/s, 3.41 × 10−10 m2/s, and 3.55 × 10−10 m2/s, respectively. The interlayer distance value is about 5.05 Å. 4335


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Figure 3. Morphologies of water-in-oil emulsions at different simulation times 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.

produces asphaltene protective films hindering the droplet− droplet coalescence. Actually, these asphaltene 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.51 It should be pointed out that the accumulation and orientation of the asphaltenes at the w/o interface result from the heteroatoms’ affinity for water molecules, as shown by 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 3.2.2. Effects of Concentration. The effects of asphaltenes concentrations on adsorbed behavior are also investigated. From 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 be 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 asphaltene 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.55 The nanoscale aggregates (7−20 nm in characteristic dimension) and aggregate number (6−14) in experimental data53,54 are qualitatively in good agreement with simulation results: aggregates size of 3−22 nm and aggregate number of 7−13. Bi et al.56 used atomic force microscopy to investigate asphaltene films evolution at the water/xylene interface using asphaltene model compounds, and the experimental results are shown in Figure 4e−g. When the concentrations of asphaltenes are 0.06 mM, they observed that black spots (composed of asphaltenes and thickness

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