Liquid Interface in

Article pubs.acs.org/EF

“Peeling Off” Mechanism of Asphaltenes from Solid/Liquid Interface in the Presence of a Highly Charged Amphiphilic Macromolecule Ting Chen,†,‡ Fanghui Liu,† Shizhe Huang,† Wei Zhang,† Hui Wang,† Qingfeng Hou,§ Donghong Guo,§ Aiqing Ma,∥ Keji Sun,∥ Hui Yang,*,† and Jinben Wang† †

Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Key Laboratory of Oilfield Chemistry, Research Institute of Petroleum Exploration and Development (RIPED), CNPC, Beijing 100083, P. R. China ∥ Oil Production Technology Research Institute, Shengli Oilfield Branch Company, Sinopec, Dongying, Shandong 257000, P. R. China S Supporting Information *

ABSTRACT: Asphaltene desorption from silica-coated quartz crystal surface, in the presence of a highly charged amphiphilic macromolecule (polyacryloyloxyethyl-N,N-dimethyl-N-benzylammonium bromide, denoted as PASBn), was investigated through a quartz crystal microbalance with dissipation, atomic force microcopy, and ultraviolet spectroscopy measurements. The results showed that asphaltenes were displaced by PASBn even at a low concentration of 0.5 g/L, mainly attributed to the electrostatic, polar, and hydrophobic interactions between asphaltenes and PASBn, as well as the electrostatic interactions between PASBn and binding sites of the solid surface. In order to understand the processes of asphaltene desorption and PASBn adsorption, the random sequential adsorption model was introduced. Unfortunately, this classical model was not consistent with the kinetic process of asphaltenes exposed to PASBn aqueous solution, due to the complex adsorption−desorption processes, including the detachment of asphaltenes, subsequent transport of the detached ones to the bulk, and the adsorption of PASBn. Furthermore, a new kinetic model and a reasonable physical model were proposed to reveal the desorption mechanism of asphaltenes from the solid/liquid interface, providing a new way of improving heavy oil recovery. Over the past few decades, polymer flooding has played an important role in increasing the viscosity of aqueous phase, as a result of improving sweep efficiency during EOR processes.11−13 However, it is hardly recommended for heavy oil due to the mobility reduction both from laboratory and field trials.13,14 More recently, the development of molecular design and academic researches on polymer flooding have been focused on the products with new functions, such as nonionic polymers, as a wettability modifier of solids from oil-wet to water-wet in heavy oil processing, can displace the protective interfacial film at the water/oil interface and decrease the surface hydrophobicity.15 Other examples of low-tension polymer or particle-type polymer flooding also become a promising method for oil recovery from heavy oil reservoirs.16−18 It should be noted that the interplay between asphaltenes and polymers does not depend on the high molecular weight and viscosity of polymers, but the desorption mechanisms of asphaltenes from the solid surface in the presence of polymer solution. Up to now, however, studies on the nanoscale dynamics of heavy oil recovery via polymer floods have still been very limited.15

1. INTRODUCTION As one of the most important fossil energy, about 7.5 × 107 barrels a day of crude oil is consumed worldwide, which provides for modern lifestyles of energy supply in a high proportion.1 However, much of the oil, especially heavy oil, stays underground and adsorbs on rock surfaces, and therefore the approaches to enhance oil recovery (EOR) are technically challenging.2−6 Generally, asphaltenes are defined as the most important fraction of heavy oil, the polarity of which relates to heteroatoms or metal content, being responsible for the aggregation of petroleum fluids on mineral surface or at oil/ water interface. Till now, the adsorption mechanisms of heavy oil components or asphaltenes onto mineral surfaces have been attracted huge attention and proposed four mechanisms: polar interactions, surface precipitation, acid/base, and ion-binding interactions.7−10 Adsorption and wettability alteration in the absence of water mainly occur between polar groups in the crude oil and polar sites on the mineral surfaces. When the oil is a poor solvent, asphaltene precipitation and oil-wet surface are induced after aging. Water has a major role in acid/base and ion-binding interactions. As a Coulombic type, the former interactions take place between charged sites at the mineral surface and oppositely charged sites at the oil/water interface. In the case of latter interactions, divalent and multivalent ions bind to negatively charged polar compound sites at the interfaces. © XXXX American Chemical Society

Received: August 6, 2016 Revised: October 12, 2016

A

DOI: 10.1021/acs.energyfuels.6b01971 Energy Fuels XXXX, XXX, XXX−XXX

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pure water for 1 w, and the final product was obtained by lyophilization. 1 H NMR of ASBn (400 MHz, D2O, ppm): 3.32 (6H, d, N-(CH3)2), 3.56 (2H, m, N−CH2-), 4.52 (2H, m, C6H5CH2N-), 5.31 (1H, m, O− CH-), 6.01 (1H, d, CH2CH−), 6.15(1H, q, CH2CH-), 6.37 (1H, d, CH2CH−), 7.22−7.34 (5H, m, C6H5CH3). The polydispersity and molecular weights of PASBn were measured by GPC, as shown in Table S1. The Mw of PASBn was about 8.0 × 103 g·mol−1 with Pd about 1.30 from GPC results (n = 11−15). PolyacryloyloxyethylN,N,N-trimethylammonium chloride (denoted as PASC1, Scheme 1b) without polar groups, was purchased from Beijing Chemical (Mw = 1.3 × 104 g·mol−1, n = 84−91). 2.2. Extraction and Solution Preparation of Asphaltenes. Extracted from the oil sand (Long Lake, Canada), asphaltenes were used as the adsorbate which was insoluble in n-alkanes but soluble in aromatics. The asphaltenes were precipitated by reflux at a 40:1 volume ratio of n-heptane and crude oil at room temperature. Subsequently, the asphaltic precipitate was collected in toluene under reflux to remove impurities. The asphaltenes were finally obtained by spin steaming and vacuum drying. The results of elemental composition of asphaltenes are provided in the Supporting Information (see Table S2). A stock solution of asphaltenes at the concentration of 1.0 g/L was prepared by dissolving the sample in pure toluene (AR, 99.5%) and placing it on a vibrator for 1 h. The solution was stored in a dark and cool place. 2.3. Measurements. 2.3.1 Quartz Crystal Microbalance with Dissipation. To study the adsorption and removal of asphaltenes from a silica surface, QCM-D (Q-sense E4, Biolin Scientific, Sweden) and a sensor of silica-coated quartz crystal with AT-cut (QSX 303, Biolin Scientific, Sweden) were used. The sensor was with a fundamental resonant frequency of 5 MHz, a diameter of 14 mm, and a root-mean-square (RMS) roughness less than 2.0 nm, mounted in a fluid cell with one side exposed to the solution. Prior to each experiment, the substrates were immersed in toluene and SDS solutions for 2 h, respectively, and rinsed thoroughly with Milli-Q water and blow-dried with nitrogen. Then they were placed in a UV chamber for an ozone treatment for 15 min to remove further hydrocarbon contaminations and organic materials. They were rinsed with excess Milli-Q water and blow-dried with nitrogen again. Besides, the QCM chamber and connecting tubes were rinsed with Milli-Q water and toluene prior to each measurement to obtain a baseline with a fluctuation being less than 1 Hz for 30 min. The asphaltene film was then formed based on the oscillation frequency shift that was induced by the presence of asphaltene solution, and components weakly bound to the surface were removed after being rinsed by pure toluene. Finally, solutions of macromolecules were introduced subsequently to initiate and measure the desorption process of asphaltenes from the solid surface. Flow diagram of typical steps involved in a QCM experiment is shows in Figure 1. The flow rate was set at a constant of 50 μL·min−1 within situ data acquisition. All of the experiments were repeated at least three times and the temperature was kept at 25 °C. The changes in frequency are determined when the frequency is stabilized after the injection of some solution, and then the data of the

With this in mind, we chose asphaltenes, the most important contributor to the viscosity of heavy oil, and synthesized a new kind of amphiphilic macromolecule with polar group in each repeat unit, poly acryloyloxyethyl-N,N-dimethyl-N-benzylammonium bromide. In this work, the interactions between asphaltenes and macromolecules were investigated through measurements of quartz crystal microbalance with dissipation (QCM-D), ultraviolet (UV) spectroscopy, and atomic force microscope (AFM). The apparent adsorbed mass, desorbed mass, adsorption and desorption kinetics, and topography of films were obtained, and at last a novel insight into desorption mechanism of heavy oil recovery in nanoscale was proposed.

2. EXPERIMENTAL SECTION 2.1. Metarials. Acryloyl chloride, 1-benzyl bromide (98%), ammonium persulfate, ammonium iron [II] sulfate hexahydrate, and N,N-dimethyl amino ethyl acrylate (DA) were purchased from Beijing Chemical Corp. (A.R. Grade). Deuterium oxide (99.9%) and chloroform-d (99.9%) were purchased from Acros. All of the reagents except those especially mentioned were used without further purification, and all of the aqueous solutions were prepared with Milli-Q water in this work. The synthetic route of poly acryloyloxyethyl-N,N-dimethyl-Nbenzylammonium bromide (denoted as PASBn) was shown in Scheme 1. Acryloyloxyethyl-N,N-dimethyl-N-benzylammonium bro-

Scheme 1. (a) Synthetic Routes and Chemical Structure of PASBn and (b) Chemical Structure of PASC1

mide (ASBn), as the intermediate monomer, was initially prepared through synthetic route (1) (see Scheme 1a) and used as the reactant of the following synthesis. Benzyl bromide was added dropwise to an acetone solution of DA at a molar ratio of 1.2:1 in a 50 mL flask at ambient temperature. The mixture was stirred for 2 h at 45 °C and cooled to room temperature. The products were washed with diethyl ether repeatedly. PASBn was then synthesized by free radical polymerization in aqueous solution, as shown in the synthetic route (2). The solution was bubbled with nitrogen for 30 min to remove oxygen before polymerization. Then the initiators (ammonium persulfate and ammonium iron [II] sulfate hexahydrate) were added successively at 10 °C. The polymerization was allowed to proceed for 2 h under an atmosphere of N2. The products were dialyzed against

Figure 1. Flow diagram of asphaltenes desorption in the presence of PASBn at solid surface. B


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Figure 2. (a) Frequency and dissipation shift as a function of adsorption time of asphaltenes and PASBn (at 10.0 g/L). (b) Frequency and dissipation shift as a function of adsorption time of PASBn and PASC1 at different concentrations of 0.5, 1.0, 5.0, 10.0, and 20.0 g/L. frequency (Δf) and the dissipation (ΔD) are collected. The curves of Δf and ΔD versus time are evaluated with Q-Tools software. The frequency shift is measurable within ±1 Hz in aqueous medium. During the QCM-D experiments, the frequency and dissipation shifts of all the harmonics (n = 1, 3, 5, ..., 13) are recorded, in which the former provides the information about the mass of adsorbate on the surface and the latter provides the information about the viscoelastic property of the adsorbed film. For the sake of simplicity, we used the third overtone to determine the frequency and dissipation shifts and calculated the corresponding mass uptake. The uncertainty was below 0.5 ng·cm−2. When the dissipation traces are small, indicating that the asphaltene film is relatively rigid and evenly distributed.19 The introduction of such a thin layer leads to a decrease in resonant frequency which is proportional to the mass of the layer. Therefore, the Sauerbrey equation is appropriate for calculating the adsorbed mass on the sensor, as shown in eq 1:20

Δm = −

ρq tq Δf f0 n

= −C

Δf n

β = ζ1

Ed 1 = 2πEs πfτ

α=

(2)

where Ed and Es are the lost and stored energy in the oscillating system, respectively, and τ is the time constant of the amplitude decay of the oscillating crystal as the driving voltage over the crystal is turned off.23 By switching the driving voltage on and off periodically, we can simultaneously obtain a series changes of resonant frequency and dissipation. The decay of energy loss is related to the viscosity and elasticity of the molecular layer on the sensor. In the case of ΔD/(Δf) > 0.2 × 10−6/Hz, the Sauerbrey equation is not valid and will underestimate the mass for a soft film in a Newtonian liquid,24,25 since the film is not fully coupled to the motion of the sensor surface.26 So, an evaluation process based on the Voigt model is introduced, in which Δf and ΔD can be obtained from the imaginary and real parts of the function β by eqs 3 and 4, respectively:27,28 (3)

ΔD = − Re(β)/πftqρq

(4)

ζ1 2πfηf − iμf ζ2 2πfηl

+1

ζ1 2πfηf − iμf ζ2 2πfηl

−1

ζ1 =

−

ζ2 =

i

(6)

(2πf )2 ρf μf + i 2πηf

(7)

2πfρl ηl

(8)

where the subscript f and l refer to the film and liquid, respectively, and d is the film thickness. In this work, both of the corresponding density (ρl) and viscosity (ηl) of the PASBn solutions were measured, as can be seen from Table S3 in the Supporting Information. The density (ρf) of the adsorbed asphaltene film was ∼1000 kg/m3.29,30 2.3.1. UV Spectroscopy. The effluent coming out of the QCM chamber was collected and analyzed by UV spectroscopy (UV 2600, Shimadzu, Japan). After 1 mL of toluene was added to extract asphaltenes in the effluent, it was placed on a vibrator for 24 h. The phase of toluene was transferred to a cuvette with 1 mm optical path and the UV spectrum of background solvent was determined by pure toluene. The amount of asphaltenes was finally determined by a calibration curve at the wavelength of 278.5 nm. 2.3.2. Atomic Force Microcopy. The surface topography was characterized through AFM method in Peakforce tapping mode (FASTSCAN, Bruker Instruments, USA). The images were scanned using silicon cantilever (Fastscan-B, Bruker Instruments, USA) with a nominal spring constant of 4 N/m and at a scan rate of 1.0 Hz. The height distribution of asphaltene layer in air was evaluated by section line analysis of AFM images. The roughness was evaluated from three locations which were randomly selected on a scan area of 10 μm × 10 μm and the average value was statistically analyzed by the image analysis software. All measurements were performed at 25 °C. 2.3.3. Characterization of Critical Aggregation Concentration. The CAC value (around 1 g·L−1) of PASBn in aqueous solutions was obtained through steady-state fluorescence as we reported previously.31 Different from such case, I1/I3 values almost reach a constant value at measured concentration of PASC1, indicating that PASC1 molecules cannot form aggregates due to the lack of polarity segments (Figure S2). 2.3.4. Zeta Potential Measurements. The ζ potentials of PASBn/ asphaltene (sample 1) and PASBn/SiO2 (sample 2) were measured at 25 °C through Zetasizer instrument (Nano-ZS, Malvern, England). Sample 1 was obtained by mixing asphaltene solution and PASBn solutions at a volume ratio of 1:19. Sample 2 was prepared by adding 2 mg of SiO2 powder to 2 mL of PASBn solutions, in which the powder represented reservoir rock. The suspension solution was sonicated for

(1)

Δf = Im(β)/2πtqρq

(5)

where α, ζ1, and ζ2 are defined through eqs 6−8, respectively:

where f 0 is the fundamental frequency, 5 MHz; Δm is the adsorbed mass or mass uptake in ng/cm2; ρq is the density of quartz crystal, 2648 kg·m−3, tq is the thickness of quartz crystal, 3.3 × 10−4 m; n is the overtone number, n = 1, 3, 5, ..., 13; and C is a constant, 17.7 ng·cm−2· Hz−1 (in the case of n = 1 and f 0 = 5 MHz). The dissipation is the energy loss in the molecular layer on the sensor which is calulated by eq 2:21,22 ΔD =

2πfηf − iμf 1 − α exp(2ζ1df ) 2πf 1 + α exp(2ζ1df )

where ρq is the density of quartz crystal, tq is the thickness of quartz crystal, and β depends on the shear elasticity (μ) and shear viscosity (η) as shown in eq 5:19,21 C


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Figure 3. (a) Apparent adsorbed mass on the asphaltene-coated surface after introducing PASBn and PASC1 as a function of concentration. (b) Desorbed amount of asphaltenes as a function of concentration of PASBn and PASC1 through UV spectroscopy.

35.1 Hz and an increase in dissipation by around 15.3 × 10−6 with the concentration ranging from 1.0 to 20 g/L. The turning point is expected to relate to the critical aggregation concentration (CAC) of PASBn (Figure S2). After an asphaltene-coated surface being exposed to PASC1 at a series of concentrations, a monotonous decrease in frequency and increase in dissipation can be obtained, exhibiting a simple adsorption process compared with PASBn system. Changes in frequency and dissipation versus time at different concentrations of PASBn and PASC1 are described in detail in Figure S1. The apparent adsorbed mass, being calculated through Voigt model, remains on the silica surface after introducing PASBn and PASC1 solutions, as shown in Figure 3a. When the concentration increases, almost linear trends of the apparent adsorbed mass of the two kinds of macromolecules are exhibited. High values of adsorbed mass indicates that there are strong interactions between macromolecules and asphaltenes, as well as a big effect of liquid loading which can be neglected under the same adsorption condition of the solvent changing from toluene to water. In order to further understand the desorption process of asphaltenes, the effluent from QCM-D measurements was refined and the desorbed amount was detected through UV methods. The desorbed amount as a function of concentration of PASBn and PASC1 is represented in Figure 3b, indicating that asphaltene desorption takes place after introducing the macromolecules even at a low concentration of 0.5 g/L. The desorbed amount in the presence of PASBn is more than twice as much as that in PASC1 system, due to both of the electrostatic and polar interactions between PASBn and asphaltenes. More interestingly, at higher concentration (1.0− 20.0 g/L), the desorbed amount increases gradually in the case of PASBn system, while it has little change in the case of PASC1. It can be attributed to that, above CAC of around 1.0 g/L, PASBn molecules tend to aggregate to form hydrophobic microdomains which become “pockets” or “hands” and get in touch with asphaltenes. Under such condition, the dominant interaction between asphaltenes and PASBn is deduced to be hydrophobic interactions, besides which electrostatic and polar interactions also contribute to the asphaltene removal from the solid surface. While PASC1 molecules cannot form aggregates due to the lack of polarity segments (Figure S2), and therefore, the interactions of asphaltene-PASC1 is almost limited to electrostatic interactions. To provide more information about adsorption kinetics and conformational changes of the asphaltene-coated layers exposure to macromolecule solutions, the relations of ΔD

at least 2 h and then left overnight before zeta potential measurements. Each ζ potential value was derived from the average of three measurements. 2.3.5. Dynamic Light Scattering. Measurements were carried out through a commercial LLS spectrometer (ALV/DLS/SLS-5022F, China) which employed a multi-τ digital time correlator (ALV- 5000). A cylindrical light (λ = 632.8 nm, Uniphase) from a solid-state He−Ne laser (22 mW) was used as an incident beam. The scattering angle was selected to 90° and the correlation function was analyzed with Contin method. Samples were filtrated through a 0.45 μm filter to leach dust and were laid until stabilization prior to measurements. 2.3.6. Transmission Electron Microscopy (TEM). Negative-staining was used for TEM sample preparation, and uranyl acetate aqueous solution of 0.5 wt % was used as the staining agent. A drop of the sample solution was placed onto a carbon Formvar-coated copper grid (300 mesh) for 20 min, and then a drop of staining agent was placed on the copper grid for 15 min. After drying, the samples were imaged under a JEM-1011 electron microscope (JEOL, Japan) at an operating voltage of 100 kV.

3. RESULTS AND DISCUSSION 3.1. Adsorption and Desorption Processes. The adsorption of a macromolecule solution on asphaltene preadsorbed surface is shown in Figure 2a, taking PASBn of 10.0 g/L for an example. An immediate drop in the resonance frequency by ∼35 Hz and an increase in dissipation by ∼1.2 × 10−6 can be seen when the bare surface is exposed to the asphaltene solution (region I), indicating a formation of rigid film. After being rinsed by pure toluene to remove weakly bound components, a slight increase in frequency and a decrease in dissipation are observed, showing that the residual components are rigidly adsorbed at the surface and the adsorbed mass is about 620 ng·cm−2 through Sauerbrey relation of eq 1, in agreement with previous reports.15,32 Polar fraction with high aromaticity and polarity of asphaltenes are considered to play an important part in asphaltene adsorption.15,33,34 In region II, a big shift around 149.5 Hz in frequency and around 52.8 × 10−6 in dissipation can be observed when the preadsorbed surface is exposed to PASBn aqueous solution, showing the formation of loosely adsorbed layers and the inapplicability of Sauerbrey relation. It indicates that not only the interactions between asphaltenes and PASBn play an important role in this adsorption process but also a liquid loading effect does due to the big differences in viscosity and density between water and toluene. The introduction of PASBn to asphaltene-coated surface leads to an increase in frequency by about 16.6 Hz and a decrease in dissipation by about 1.5 × 10−6 with the concentration ranging from 0.5 to 1.0 g/L (Figure 2b and Figure S1). While there is a decrease in frequency by around D


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PASBn

concentration (g/L) slope of ΔD/Δf (×10‑6 /Hz)

I II

PASC1

0.5

1.0

5.0

10.0

20.0

0.5

1.0

5.0

10.0

20.0

0.058 0.37

0.055 0.48

0.052 0.50

0.058 0.52

0.054 0.47

0.065 0.44

0.054 0.57

0.055 0.68

0.062 0.62

0.053 0.72

Figure 4. AFM images and height profiles of the QCM-D samples: (a) bare substrate; (b) asphaltene-coated film; (c) asphaltene-coated film in the presence of PASBn at 0.5 g/L; (d) asphaltene-coated film in the presence of PASBn at 1.0 g/L; (e) asphaltene-coated film in the presence of PASBn at 5.0 g/L; (f) asphaltene-coated film in the presence of PASBn at 10.0 g/L; and (g) asphaltene-coated film in the presence of PASBn at 20.0 g/L.

versus Δf are listed in Table 1. In region I, all the values of ΔD/ Δf are small and reproducible (about 0.06 × 10−6 /Hz) during all the experiments, suggesting the formation of rigid adsorbed layers of asphaltenes. Subsequent introduction of macromolecules causes a big increase in the slope value at different concentrations (region II), implying the existence of a quite different kinetic process. In region II, in the presence of PASC1, the slope increases gradually with the increase of concentration, because the driving force for the process is mainly electrostatic attraction between macromolecules and asphaltenes. While in the case of PASBn, the slope is smaller than that of PASC1 and close to a constant above CAC, suggesting that the predominant factor is entropy increase which results in a more disorder in the conformations than that in PASC1asphaltene system (Figures 4 and 5). It is attributed to the multiple interplays between PASBn molecules and asphaltenes, PASBn molecules incoming and those adsorbing on the surface, PASBn aggregates, and asphaltenes. Furthermore, combining with the frequency and dissipation shifts, it can be concluded that “soft” layers form and there are more conformational changes at higher concentrations (>CAC), indicating that the interactions between asphaltenes and binding sites of solid surface are weaken to a large extent. And then the detachment

of asphaltenes from the surface and the subsequent transport of the detached ones to the bulk take place, which are also confirmed by UV spectroscopy measurements. Remarkably, the aggregation behavior of PASBn in aqueous solution contributes a lot to the asphaltene desorption, which will be discussed in detail in the following. A smooth and homogeneous surface with an average rootmean-square roughness of ∼0.5 nm can be observed in the bare substrate of Figure 4a. After being coated with asphaltenes, it exhibits fluctuant topography and heterogeneous nature with a roughness of around 1 nm, indicating that asphaltenes are randomly distributed in the form of closely packed nanoaggregates (Figure 4b). Different from the asphaltene preadsorbed surface, in the presence of PASBn at a series of concentrations, the images show more fluctuant and heterogeneous surface morphology, with a higher roughness increasing from ∼1 to ∼2.5 nm and covered with growing “cavities” and “islands” (Figure 4c−g). It can be deduced that asphaltene desorption from the surface makes a contribution to the “cavities”, while the “islands” are mainly related to PASBn adsorption or their formation of intra- and intermolecular aggregates. Therefore, in the presence of PASBn, the asphaltene-coated surface becomes rougher via the interactions E


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Figure 5. AFM images and height profiles of the QCM-D samples of asphaltene-coated film in the presence of PASC1 at the concentration of (a) 0.5, (b) 1.0, (c) 5.0, (d) 10.0, and (e) 20.0 g/L.

of asphaltene-PASBn, PASBn-surface, and PASBn molecules, which is in agreement with the results from the conformation changes in QCM-D measurements. The topographical images of PASBn adsorbed on the bare silica surface are provided in Figure S3 in the Supporting Information, to further compare with the topography of asphaltene film when subjected to amphiphilic macromolecule. It further confirmed the competitive adsorption between asphaltenes and polyelectrolytes. AFM images and height profiles can be observed in the presence of PASC1 (see Figure 5), the roughness of the absorbed layers is smaller and they are less heterogeneous than those in the case of PASBn at the same concentration, implying that there are less conformation changes and short of interactions between asphaltenes and PASC1. The change of wettability on the QCM-D samples before and after desorbing asphaltenes is measured, as shown in Table 2. The contact angle of water on fresh silica surfaces was 27.7°, whereas it was increased to about 79.3° upon exposure to asphaltene solution, which shows a wettability change from hydrophilic to hydrophobic. As was expected, the contact angle

of water on QCM-D samples in the presense of PASBn was consecutive decreased with an increase of concentration, indicating asphaltene desorption as well as PASBn aggregates adsorption on silica surfaces. 3.2. Kinetics Models. To focus on the kinetics of asphaltene adsorption and desorption on the silica surface, kinetic models are introduced to understand the whole kinetic process. Initially, the adsorbed mass of asphaltenes increases with time, and after a transient period, it reaches a final saturation plateau, as shown in Figure 6a. According to the random sequential adsorption (RSA) model, the adsorption process can be redescribed, in which the asphaltenes are assumed to adsorb on a planar surface sequentially at random locations. The kinetics of the adsorption process can be approximated by relating the rate of change of the adsorbed number density with time t to the number concentration c of asphaltenes in solution as shown in eq 9:35

Table 2. Contact Angle of the QCM-D Samples

where ka is the adsorption rate coefficient, c is the concentration of asphaltenes, and B(Γ) is the blocking (or available surface area) function. The initial adsorption rate coefficient is determined here from the plateaus in the smoothed time derivative of the adsorption transient, with the resulting ka of 1.08 × 10−7 m/s. It is lower than the value in the reference of

dΓ = kacB(Γ) dt

PASBn (g/L) samples ο

θ/

bare

asphaltene

0.5

1.0

5.0

10.0

20.0

27.7

79.3

76.8

73.9

67.6

52.6

51.2 F

(9)


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Figure 6. (a) Adsorption kinetics of asphaltenes on silica surface (blue line) and fitting results of classical RSA model (red line). (b) Adsorption− desorption kinetics of asphaltenes exposed to PASBn solution of 0.5 g/L (blue line); fitting results of the classical RSA model (green line); fitting results of the exponential decay function (red line). (c) Linearized fit of the data from panel b in regions I and II.

silica substrates adsorbed DADMAC layers,36 mainly due to the electrostatic interactions between DADMAC molecules and substrates besides dispersion interactions. Moreover, the Langmuir adsorption model suggests the blocking function as shown in eq 10:35 ⎧1 − Γ/Γ0, Γ < Γ0 B(Γ) = ⎨ ⎩ 0, Γ ≥ Γ0

R(t ) = (D/πt )1/2

where D is the center-of-mass diffusion coefficient and R(t) reflects irregularity of the surface and the type of diffusion process. Furthermore, the desorption rate can be modeled as proportional to the product of R(t) and the surface concentration Γ(t), being time-dependent due to the intermittency of surface detachment, as shown in eq 13:37

⎪ ⎪

(10)

dΓ(t ) = −R(t )Γ(t ) dt

where Γ0 is the adsorbed number density at saturation, corresponding to the jamming limit within the RSA model. The prediction of this kinetic model is shown in Figure 6a (red line), in agreement with the experimental profile, which indicates that the results are consistent with the experiment semiquantitatively. Desorption process of asphaltenes exposed to PASBn solution at the concentration of 0.5 g/L versus time is exhibited in Figure 6b, showing an increase of adsorbed mass in elapsed time. From the results, the process does not coincide with the RSA model (green line), whereas it can be fitted with a double-exponential function (red line) as described in eq 11, with the parameters shown in Table S4. It is notable that the process is different from the pure asphaltene adsorption, due to the equilibrium process of asphaltene desorption and PASBn adsorption. y = A1e b1x + A 2 eb2x

(12)

(13)

In consideration of the depletion of the surface concentration, integration of eqs 12 and 13 gives37 Γ(t )/Γ0 ∝ exp[−(t /τoff )β ]

(14)

where Γ0 is the steady-state mass per area adsorbed at t = 0, τoff is the desorption time constant, and β is the exponent in this model. The adsorption of the incoming layer can be described by a similar kinetic model in a symmetric prediction, as shown in eq 15:37 Γ(t )/Γ∞ ∝ 1 − exp[−(t /τon)β ]

(15)

Equation 15 describes the saturation phenomenon, in which Γ(t) approaches a limiting steady-state level and Γ∞ represents the adsorption endure for a long time. From other studies, a displacement of a weakly adsorbed macromolecule (d-PS) by a more strongly adsorbing macromolecule (PI) at an oxidized silicon surface was reported, with a decrease of mass adsorbed per area against time.37,38 In the case of the two kinds of nonionic macromolecules, the desorbed kinetics mainly depends on the diffusion reactions and the depletion of the surface concentration. By contrast, in our present work, the adsorbed mass behaves in an increasing trend in the presence of PASBn in the elapsed time. Interestingly,

(11)

where A1 and A2 are the coefficient and b1 and b2 are the decay length. In order to further understand the kinetics of the complex processes of asphaltene desorption and PASBn adsorption, a theoretical model is adopted. The steady-state flux R(t) per unit concentration from a planar surface shows in eq 12:37 G


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Figure 7. (a) Zeta potential of asphaltene solution (blue line) and SiO2 powder (red line) mixed with PASBn respectively at different concentrations. (b) Proposed model of macromolecule adsorption at the interfaces of solid/water and oil/water. (c) Rh distribution of PASBn solutions at different concentrations from 0.5 to 20.0 g/L. (d) Typical TEM images of PASBn at the concentration of 5.0 g/L. (e) Schematics of asphaltenes displaced by PASBn at solid/water and oil/water interfaces.

from 0.5 to 20.0 g/L. Furthermore, there seems to be network aggregates composed of small spherical micelles of ∼50 nm and interconnected by macromolecule chains, observed from typical TEM images of PASBn at the concentration of 5.0 g/L. Therefore, when all the results are put together, the picture of removal mechanism appears as shown in Figure 7e. In the absence of water, asphaltenes adsorb onto the silica surface mainly due to interactions between polar groups of asphaltenes and polar sites on the solid surfaces, forming a rigid adsorbed layer. Upon the flow of PASBn solution, the macromolecules disperse in water at either molecules or aggregates, and reaches the oil/water interface through electrostatic and polar−polar interactions. After the detachment of asphaltenes from the surface, they touch with PASBn due to the polar−polar interactions, and then asphaltene granules are surrounded or encapsulated by hydrophobic microenvironment formed by PASBn aggregates, in which way asphaltenes are finally transported to the flowing solution. The empty sites on the surface are occupied by incoming PASBn molecules because of electrostatic attractions between PASBn and binding sites, in the formation of a heterogeneous and soft layer. In addition, even at the concentration lower than CAC, the detachment process of asphaltenes from silica surface can also take place, mainly due to the intramolecular aggregation of PASBn molecules.

after plotting the profiles according to eqs 14 and 15, different values of exponent β can be obtained from Figure 6c. The slope value is almost linear at first, showing a fast increase of adsorbed mass. Then, it acts with up-and-down fluctuations and suggests the existence of adsorption−desorption process. It includes the detachment of asphaltenes from solid surface and the subsequent transport of asphaltenes to the bulk due to the polar interactions between asphaltenes and PASBn, as well as that the empty sites left are occupied by PASBn. 3.3. Removal Mechanism. After introducing PASBn solution to the asphaltene-coated surface, the value of ζ potential at the interface of oil/water (i.e., asphaltene-PASBn aqueous solution) becomes positive, and gradually it reaches a platform with the increase of PASBn concentration (see Figure 7a). It indicates that the surface of oil droplets is adsorbed and occupied by PASBn molecules and gradually reaches a saturated adsorption. At the interface of solid/water (i.e., substrate-PASBn aqueous solution), the variation trend of ζ potential is similar to that of oil−water, showing an increase in positive charge of the silica surface. So, it confirms that the detachment of asphaltenes from surface is mainly promoted by the electrostatic repulsion (see Figure 7b). We use DLS and TEM methods to obtain further understanding of aggregation behaviors of PASBn in aqueous solution (Figure 7c,d). The dimension of aggregates is dependent on the solution concentration, exhibiting an increase trend from ∼90 to ∼1200 nm with the concentration increase H


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Energy & Fuels

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4. CONCLUSION In this study, a kind of amphiphilic macromolecule with highly charged density and polar groups, denoted as PASBn, as well as a macromolecule with highly charged density, denoted as PASC1, were introduced to displace asphaltenes from the silica surface as a function of concentration. Upon adsorption, asphaltenes formed a rigid layer on solid surface through polar interactions between asphaltenes and binding sites of the surface. In the presence of PASBn, the absorbed film was in the form of heterogeneity and roughness even at a low concentration of PASBn, indicating strong interactions between asphaltenes and PASBn. Different from the desorption process of asphaltenes in such case, the recovery efficiency in the presence of PASC1 was much lower, suggesting the important role of polar groups of PASBn in removing asphaltenes. In order to further investigate the nanoscale dynamics and desorption mechanism, a new kinetics model was established and fitted well with experimental data. Almost three processes were responsible for the removal behavior, including the detachment of asphaltenes from solid surface, the transport of asphaltenes to the bulk, and the occupation of empty sites by macromolecules. Driving forces of the first two processes were mainly attributed to the electrostatic, polar, and hydrophobic interactions between PASBn and asphaltenes; and the electrostatic interactions between PASBn and the asphaltene-coated surface almost dominated the last process. Overall, the displacement mechanism and kinetic model we proposed will be helpful to shed light on a new way of enhancing heavy oil recovery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01971. Additional materials as described in the text. (PDF)

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the Important National Science and Technology Specific Project of China (2016ZX05013003-004 and 2016ZX05025003-007), the project of PetroChina Exploration and Development Research Institute (201540222-000006), and the Strategic Priority Research Program of CAS (XDB22030102). We thank Dr. Min Wang from Biolin Scientific AB for the help in QCM measurements.

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