Synthesis of Triblock Copolymers via RAFT Polymerization and

Article pubs.acs.org/IECR

Synthesis of Triblock Copolymers via RAFT Polymerization and Their Application as Surfactants for Crude Oil-in-Water Emulsion Jing Huang,† Jun Xu,*,† Kaimin Chen,‡ Tongshuai Wang,† Chao Cui,† Xiaoming Wei,§ Rui Zhang,† Li Li,† and Xuhong Guo*,† †

State-Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China § Petrochina Liaohe Oilfield Company, Panjin 124010, China ‡

S Supporting Information *

ABSTRACT: As surfactants for crude oil emulsions, poly[N,N-(dimethylamino) ethyl methacrylate-b-poly(ethylene glycol) methyl ether methacrylate-b-lauryl methacrylate] (PDMA-b-PPMA-b-PLMA) triblock copolymers were synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization and characterized by gel permeation chromatography and proton nuclear magnetic resonance spectroscopy. The copolymers exhibited high interfacial activity which could be tuned by simply adjusting the solution pH, and remarkably reduced the dodecane/water interfacial tension from 52.8 to 2.1 mN/m, producing stable dodecane-in-water emulsions even at a low concentration of 0.5 mg·mL−1. Utilization of these copolymers in Shengli crude oil further confirmed their emulsification capacity. The apparent viscosity of crude oil reduced from 34 000 to 350 mPa·s after emulsification at 20 °C, and the formed emulsions exhibited long-term stability for above 3 months. Due to the balanced hydrophilicity and hydrophobicity, PDMA22-PPMA27-PLMA36 proved to be the most efficient surfactant, which generated stable O/W emulsion with the smallest dosage.

1. INTRODUCTION Until now, most of the world’s remaining oil resources are heavy and viscous oil, which are abundant1 but difficult and costly to recover, refine, and transport. The high viscosity of heavy oil is one of the factors that most affects the producibility and recovery of oil. One effective way to explore and transport the heavy crude oil is to disperse it into water as emulsion. Formation of oil-in-water emulsion with suitable surfactants effectively decreases the viscosity of heavy oil and facilitates the oil transportation with conventional equipment,2 providing an alternative to the use of diluents or heat in pipelines.3 Since water is the continuous phase, crude oil has no contact with the inner wall of pipes, which reduces the pipe corrosion induced by the oil components and prevents sediments in pipelines. Moreover, as the viscosity of emulsion is far less sensitive to temperature than that of heavy oil, it would be quite advantageous for the pipeline operation even after an emergency shutdown or re-emulsification.3,4 When an interfacial active substance is added, it spontaneously adsorbs on the oil/water interface and decreases the interfacial tension by covering the individual oil droplets with the polar parts of its molecules in contact with water and the hydrophobic parts with oil. This effective barrier formed by emulsifier molecules prevents against droplet coalescence and, hence, promotes the maintenance of discrete hydrocarbon droplets dispersed in a continuous aqueous phase.2,5 Amphiphilic block copolymers, consisting of water-soluble hydrophilic blocks and water-insoluble hydrophobic blocks, have been well investigated as surfactants for the generation of stable emulsions.6−9 They would spontaneously adsorb on the interface driven by the distinct solubility of different segments © XXXX American Chemical Society

in structure and consequently stabilize the dispersed droplets. Compared to low molar mass surfactants, polymeric surfactants offer unique advantages for application, such as lower required concentration for generation of a stable emulsion, lower critical micelle concentration (CMC), and lower molecular mobility which reduces the desorption of surfactants from particle surface and thus improves the emulsion stability.10 Furthermore, the composition of block copolymer can be easily tailored for a given application through the introduction of functional sites, aiming to adjust the interaction between the polymeric surfactants and the system to be stabilized. Among the water-soluble polymers, poly[N,N-(dimethylamino) ethyl methacrylate] (PDMA), a pH-responsive weak polybase (pKa ∼7.2),11−13 and its derivatives are widely studied. Below its pKa, PDMA is hydrophilic since its tertiary amine groups are protonated, while above the pKa, this polymer turns hydrophobic due to the deprotonation of the tertiary amine groups. Researchers have investigated this phenomenon to explore the potential of PDMA-based (co)polymers in applications of dye/pigment dispersion,14 natural protein isolation and purification,15 gene/drug carrying and delivery,16 etc. However, as these polymers are generally synthesized by the conventional free radical polymerization technique, which affords number-average molecular weight (Mn) in a wide range with high molar-mass dispersity (Đ = Mw/Mn),17 it is difficult to obtain homogeneous products with narrow distributed Received: October 24, 2014 Revised: January 15, 2015 Accepted: January 16, 2015

A

DOI: 10.1021/ie504207r Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research molecular weights, appropriate functionality, and predetermined architectures. To overcome these disadvantages, reversible-deactivation radical polymerization (RDRP)18 (recommended by IUPAC to replace the misleading term “controlled living radical polymerization”19−21) techniques are required. Specifically, one such technique, reversible addition− fragmentation chain transfer (RAFT) polymerization has steadily grown in popularity. This technique allows the synthesis of water-soluble copolymers and offers opportunities for chain-end functionalization as well as good control over the molecular architectures.22 The specific reagents for this process comprise metal-free formulations, which thereby exhibit exceptional functional group tolerance.23,24 Since its first report, RAFT polymerization has developed into a versatile technique with respect to the facile reaction procedure and the wide range of applicable monomers.21,25 A variety of PDMA-based copolymers with predetermined structures and narrow distributed molecular weights have been prepared via RAFT polymerization11,26,27 and show potential as surfactants or emulsifiers.13,28−31 However, utilization of them in oil-in-water emulsification is rarely reported. Herein, we describe the RAFT synthesis of PDMA-based block copolymers with both poly[poly(ethylene glycol) methyl ether methacrylate] (PPMA) segments, in which the poly(ethylene glycol) (PEG) was present as side groups, and poly(lauryl methacrylate) (PLMA) segments with varying chain lengths. The PEG groups are well documented nonionic components of surfactants and were chosen as another hydrophilic block with extra steric hindrance,32 in addition to the PDMA parts. The PLMA block was chosen to provide copolymer with hydrophobicity and affinity for oil. The oil/water interfacial activity and emulsification performances of these copolymers were researched in detail with dodecane as a model oil, and their effects on emulsification of Shengli crude oil were subsequently studied. Under low concentration of copolymers, the dodecanein-water emulsion showed long-term stability, and crude oil-inwater emulsion exhibited low apparent viscosity.

Table 1. Properties of Shengli Crude Oli

a

property

value

densitya (g/cm3) viscositya (mPa·s) saturates (wt %) aromatics (wt %) resins (wt %) asphaltenesb (wt %)

0.9672 34000 26.3 40.3 10.4 21.9

Measured at 20 °C. bn-Heptane-precipitated asphaltenes.

methacrylate-b-lauryl methacrylate] triblock copolymer, where n, m, and x indicate the polymerization degrees of each block. 2.2.1. Synthesis of D22 Macro-Chain Transfer Agent (CTA). DMA (1.96 g, 12.5 mmol) was polymerized in 7.8 mL of 1,4dioxane with DTTCP (168.2 mg, 0.42 mmol) as CTA, ACVA (23.3 mg, 0.083 mmol) as initiator, and 1,3,5-trioxane (50 mg, 5 mg·mL−1) as an internal standard for 1H NMR spectroscopy. The sealed reaction vessel was evacuated for 5 min and then purged with nitrogen for 15 min. This procedure was repeated three times as the deoxygenation process. The polymerization proceeded at 80 °C for 6 h under the protection of nitrogen and was quenched by cooling in ice water. An aliquot was taken for 1H NMR analysis to determine the monomer conversion by the relative integrals of trioxane protons at 5.1 ppm and the vinylic protons in residual monomers at 5.6 and 6.1 ppm. The purified polymer (monomer conversion = 81%, Mn,GPC = 3900 g·mol−1, Mw/Mn = 1.17) was acquired by precipitation into excess petroleum ether, and the mean degree of polymerization (DP) of this macro-CTA was calculated to be 22 (i.e., D22 macro-CTA) via 1H NMR spectroscopy by comparing the integrated methylene protons of the dodecyl group at 1.3 ppm with the methyl protons of the dimethylamino group at 2.3 ppm. 2.2.2. Synthesis of D22-P27 Diblock Copolymers. PMA (1.25 g, 2.5 mmol) was polymerized in 8.5 mL of 1,4-dioxane with D22 (196.8 mg, 0.05 mmol) as macro-CTA, ACVA (2.8 mg, 0.01 mmol) as initiator, and 1,3,5-trioxane (50 mg, 5 mg·mL−1) as an internal standard for 1H NMR spectroscopy. The reaction was deoxygenated as mentioned above, and the polymerization proceeded at 80 °C for 6 h under the protection of nitrogen. For kinetic studies, sampling was performed via syringe at regular time intervals for 1H NMR and GPC analysis, and the polymerization was finally quenched by cooling in ice water. Monomer conversion was determined via 1H NMR analysis by the relative integrals of trioxane protons at 5.1 ppm and the vinylic protons in residual monomers at 5.6 and 6.1 ppm. The purified polymer (monomer conversion = 57%, Mn,GPC = 17 500 g·mol−1, Mw/Mn = 1.11) was acquired by precipitation into excess petroleum ether, and the mean DP of PPMA block was calculated to be 27 (i.e., D22-P27) via 1H NMR spectroscopy by comparing the integrated methylene protons of the dodecyl group at 1.3 ppm with the methoxy protons of the methyl ether group at 3.3 ppm. 2.2.3. Synthesis of D22-P27-Lx Triblock Copolymers. In a typical synthesis procedure for the triblock copolymer D22-P27L36, LMA (1.27 g, 5 mmol) was polymerized in 6.9 mL of ethanol with D22-P27 (1.75 g, 0.1 mmol) as precursor, AIBN (3.3 mg, 0.02 mmol) as initiator, and 1,3,5-trioxane (50 mg, 5 mg·mL−1) as an internal standard for 1H NMR spectroscopy. The reaction was deoxygenated, and the polymerization proceeded at 70 °C for 8 h under the protection of nitrogen. For kinetic studies, sampling was performed via syringe at

2. EXPERIMENTAL SECTION 2.1. Materials. N,N-(Dimethylamino) ethyl methacrylate (DMA, 98%) and poly(ethylene glycol) methyl ether methacrylate (PMA, ave. Mn = 500 g·mol−1) were purchased from Aldrich and purified by passing over a basic aluminum oxide column prior to use. Lauryl methacrylate (LMA, Acros, 97%) and 2,2′-azobis(isobutyronitrile) (AIBN, Aldrich, 98%) were purified by recrystallization in hexane and methanol, respectively. 4,4′-Azobis(4-cyanopentanoic acid) (ACVA, Aldrich, 98%), 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic (DTTCP, Aldrich, 97%), deuterated chloroform (CDCl3, Aldrich, 99.96 atom % D), deuterium oxide (D2O, Aldrich, 99.9 atom % D), dodecane (Aldrich, 99%), 1,3,5trioxane (J&K, 99%), 1,4-dioxane (Shanghai Lingfeng Chemical Reagent Co., Ltd., 99.5%), ethanol (Shanghai Titanchem Co., Ltd., 99.7%), tetrahydrofuran (THF, Aldrich, 99%), and petroleum ether (60−90 °C, Shanghai Titanchem Co., Ltd.) were all used as received. The heavy crude oil sample was kindly donated by Sinopec Shengli Oilfield, and its properties are summarized in Table 1. 2.2. Synthesis of Block Copolymer. For the sake of brevity, a shorthand notation is introduced to describe the block copolymers. D, P, and L stand for DMA, PMA, and LMA, respectively, thus Dn-Pm-Lx denotes the poly[N,N-(dimethylamino) ethyl methacrylate-b-poly(ethylene glycol) methyl ether B


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Industrial & Engineering Chemistry Research Scheme 1. Structure of DTTCP and Synthesis of Triblock Copolymer Dn-Pm-Lx via RAFT Polymerization

regular time intervals for 1H NMR and GPC analysis, and the polymerization was finally quenched by cooling in ice water. The final copolymer was purified by precipitation into excess petroleum ether and dried under vacuum to a constant weight. Monomer conversion and mean DP of PLMA block were both determined by 1H NMR analysis. In further synthesis of triblock copolymers (Scheme 1), the target DP of PLMA block was varied by adjusting the molar ratio of the LMA monomer to the D22-P27 precursor, while at a constant monomer concentration. 2.3. Preparation of Oil-in-Water (O/W) Emulsions. Triblock copolymers were dissolved in THF and dispersed in water by the membrane dialysis method (see the Supporting Information (SI)).33,34 The pH of aqueous solution was adjusted to 7 by concentrated HCl or NaOH solution unless otherwise noted. In the case of model oil, the water/dodecane ratio was set at 1:1 by volume, and the emulsion was generated by employing a Kudos SK7210LHC ultrasonic homogenizer (350 W, 35 kHz) for 10 min of sonication at 20 °C. In the case of crude oil, emulsions were generated at 20 °C by dispersing crude oil in aqueous dispersion of copolymer (mass fraction of oil = 70%), using a mechanical stirrer Utra-Turrax T25 (IKAWERKE, Germany) at 20 000 rpm for 15 min. The drop test

was conducted to verify the emulsion type by placing one drop of the emulsion phase into neat water or neat oil. An O/W emulsion droplet would disperse readily in water but not in oil, and vice versa. 2.4. Characterization. 2.4.1. Characterization of Block Copolymer. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a 400 MHz Bruker Avance III spectrometer using CDCl3 or D2O as solvent. Molecular weight and Đ of polymers were measured using a Waters 1515 gel permeation chromatography (GPC) instrument at 40 °C, with tetrahydrofuran (THF) as eluent at flow rate of 1.0 mL·min−1 and a series of poly(methyl methacrylate) standards with low dispersity for calibration. Intensity-average hydrodynamic diameter of copolymer micelles in water was measured by dynamic light scattering (DLS) using a particle sizing system (NICOMP 380 ZLS) with a fixed scattering angle of 90° at 20 °C. Copolymer micelles were prepared by the membrane dialysis method (see the SI).33,34 All aqueous samples were prepared at a concentration of 5 mg·mL−1, and the solution pH was adjusted by NaOH or HCl solutions where appropriate. Each measurement was repeated 3 times, from which an average value was obtained. C


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Industrial & Engineering Chemistry Research 2.4.2. Interfacial Tension. Interfacial tension of dodecane/ water was measured by spinning drop tensiometer SVT20N (DataPhysics Instruments GmbH, Filderstadt) at 20 °C. A capillary tube containing a droplet of light phase (dodecane) within the bulk of dense phase (water phase) was rotated on its longitudinal axis at 7000 rpm, and the interfacial tension was calculated according to the length and width of the oil droplet by means of built-in software. For each measurement, 10 pieces of data were recorded at intervals of 20 s as soon as the rotation speed reaches 7000 rpm, and the average value of them was counted as the interfacial tension. 2.4.3. Polarizing Optical Microscopy. The morphology of emulsion droplets was observed with a Leica DM2500 P microscope. A small drop of emulsion was placed on a microscope slide for observation, and images were captured with a charge-coupled device camera connected to a PC via WT-1000GM imaging board. 2.4.4. Apparent Viscosity. The viscosity of the crude oil-inwater emulsion was measured employing a MCR501 rheometer (Anton Paar GmbH, Austria) equipped with a cylinder and plate geometry, and the temperature was maintained at 20 °C by a Peltier module. Samples were left still in the measurement system for 2 min prior to analysis to achieve equilibrium. For each sample, 30 points were recorded at intervals of 10 s at constant shear rate of 100 s−1, and the average value was counted as its apparent viscosity. 2.4.5. Emulsion Stability. All the samples of O/W emulsions were stored in closed vials at 20 °C. For model oil emulsions, the height of each phase (emulsion, neat water and neat oil) was measured 24 h after emulsification to calculate the original volume percentage of the emulsion phase. The emulsion phase fraction was then checked regularly to characterize the emulsion stability. In the case of crude oil emulsion, the amount of resolved water was determined after 3-monthstanding, and its percentage was used to assess the emulsion stability.

Figure 1. Typical 1H NMR spectra of synthesized polymers (CDCl3).

DPL = 9Ii(Ia − 9Ii)

where DPL is the mean DP of the PLMA block, and Ii and Ia are the integrals of signal i and a at 1.6 and 1.3 ppm, respectively. Molecular weight and Đ of the synthesized polymers were calculated by 1H NMR and GPC, and the results are summarized in Table 2. As expected for RAFT polymerization, all the Đ values are relatively low in the range of 1.11−1.23, demonstrating good control over the polymerization processes. Moreover, kinetic study of polymerization processes of PMA and LMA with target DP = 50 (SI, Figure S1a and c) demonstrated linear increases of ln([P]0/[P]) and ln([L]0/[L]) versus time after short retardation periods, indicating the pseudo-first-order kinetics of the both reactions. The linear relationships between Mn,GPC of products and monomer conversions (SI, Figure S1b and d) also confirmed that the RAFT polymerizations occurred in a well-controlled manner.36 It could be observed that there is a correlation between the target DP of PLMA block, i.e., the initial molar ratio of LMA to D22-P27, and both the LMA conversion and the Đ of the synthesized copolymer; an increase of the target DP results in a descent of the monomer conversion and a slight growth of the Đ. This could presumably be ascribed to the decreases of both the radical flux and the RAFT chain transfer agent, since the monomer concentration is kept constant for all target DPs. 3.2. pH-Sensitivity of D22-P27-Lx. PDMA-based block copolymers are anticipated to exhibit pH-induced micellization in water, as the PDMA segment should behave as a hydrophilic block in acidic medium, while hydrophobic in alkaline medium.37 To examine the pH-sensitivity of the triblock copolymers, DLS was employed to measure the hydrodynamic diameter of copolymer micelles in water with varying pH values at 20 °C. As shown in Figure 2, only the results of D22-P27-L36 and D22-P27-L51 are illustrated, because the micelles formed by D22-P27-L11 and D22-P27-L23 were fairly small to be detected in a wide range of pH. In acidic solution, detectable micelles were generated by D22-P27-L51 with the mean diameter of 22 nm.

3. RESULTS AND DISCUSSION 3.1. Characterization of Triblock Copolymer D22-P27Lx. A series of amphiphilic triblock copolymers based on methacrylate derivatives (DMA, PMA, and LMA) were synthesized using RAFT polymerization technique in three steps (Scheme 1). Polymerizations of DMA and PMA were stopped before the full conversions of monomers so as to retain the high RAFT end-group functionality.27,35 In the third step, the hydrophobic PLMA block was polymerized at a constant monomer concentration of 0.5 mol·L−1, while its target DP was varied from 15 to 80. Typical 1H NMR spectra for the synthesized polymers in all three steps are shown in Figure 1. Characteristic signals g (3.6 ppm) and h (3.3 ppm) attributed solely to the PPMA block are detected in spectra of both the diblock precursor and the triblock copolymer. For the final D22-P27-Lx copolymers, an additional signal i and a much stronger signal a corresponding to the PLMA block are clearly visible at 1.6 and 1.3 ppm, respectively, confirming the successful synthesis of the triblock copolymers. Monomer conversion of LMA was determined via 1 H NMR analysis of the final reaction solution by the relative integrals of trioxane protons at 5.1 ppm and the vinylic protons in residual monomers at 5.6 and 6.1 ppm. Mean DP of PLMA block was estimated by comparison of integrated signal i with signal a following the equation: D


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Industrial & Engineering Chemistry Research Table 2. Experimental Conditionsa and Polymer Characteristics polymer composition

[M]:[CTA]b

conv.c (%)

Mn,theod (kg·mol−1)

Mn,NMRe (kg·mol−1)

Mn,GPCf (kg·mol−1)

Đf

D22 D22-P27 D22-P27-L11 D22-P27-L23 D22-P27-L36 D22-P27-L51

30:1 50:1 15:1 30:1 50:1 80:1

81 57 86 73 68 59

4.2 18.1 21.2 23.5 26.1 29.9

3.7 17.7 20.8 23.8 26.8 31.0

3.9 17.5 20.3 23.5 26.7 30.4

1.17 1.11 1.19 1.21 1.23 1.23

a Polymerization condition: [D] = 1.25 mol·L−1, [P] = 0.25 mol·L−1, [L] = 0.5 mol·L−1, [CTA]:[Initiator] = 5. bM: monomer; CTA: DTTCP in 1st step, D22 macro-CTA in 2nd step, D22-P27 precursor in 3rd step. cMonomer conversions calculated by 1H NMR spectra using trioxane as internal standard. dTheoretical molecular weight Mn,theo = ([M]/[CTA])Mn,M Conv./100 + Mn,CTA, where the Mn,M and Mn,CTA are the molecular weights of monomer and CTA respectively in each step. eCalculated by mean DP of monomer from 1H NMR data. fDetermined by GPC (THF) with PMMA calibration.

Figure 2. Intensity-average hydrodynamic diameter of (a) D22-P27-L36 and (b) D22-P27-L51 under different pH conditions. The solution pH was adjusted by 0.1 M HCl or NaOH.

Figure 3. 1H NMR spectra of D22-P27-L36 with D2O as solvent under different pH conditions: (a) pH 3, (b) pH 7, and (c) pH 11.

Adjusting the solution pH to 7, larger micelles of 19 and 27 nm were formed by D22-P27-L36 and D22-P27-L51, respectively, and grew remarkably to 38 and 40 nm when the pH was further increased to 11. We believe that further micellization occurs with increasing pH value, due to the deprotonation of PDMA segments, and attributes to the growth of the micelle size. It is also well noted that larger micelles are invariably obtained when the chain length of the core-forming block is increased.38 The pH-induced self-assembly of the triblock copolymers was further investigated by 1H NMR spectroscopy in D2O (10 mg·mL−1), using 0.1 M DCl and NaOD for adjusting the solution pH. A typical result of D22-P27-L36 is shown in Figure 3. Generally, the characteristic signals i and a for the PLMA segment are completely invisible regardless of the solution pH, due to the hydrophobicity of this block. At pH 3, all the signals expected for PDMA and PPMA blocks are intensely detected, confirming the full protonation and solvation of the PDMA segment. It suggests that PLMA-core micelles are generated with cationic PDMA and neutral PPMA blocks forming the coronas (Figure 4). With the increase of pH value, PDMA

signals e (2.7 ppm) and f (2.3 ppm) decrease significantly and at last totally disappear at pH 11, indicating the complete deprotonation and insolubility of the PDMA block. Under this alkaline circumstance, PLMA/PDMA-mixed-core micelles are formed with PPMA as corona. 3.3. Interfacial Activity. The interfacial activity of the triblock copolymer was studied by measuring the interfacial tension of a dodecane droplet within the bulk of the aqueous solution of copolymer. n-Dodecane was selected as the model oil because it is highly nonpolar, nonvolatile, and inexpensive. The interfacial tension as a function of copolymer concentration (pH 7) is plotted in Figure 5. Interfacial tension of bare dodecane/water without any additive was measured to be 52.8 mN·m−1. With the addition of triblock copolymers, it was reduced by more than a half even at a low concentration (1 mg·mL−1, except for D22-P27-L11), revealing the efficient interfacial activity of those copolymers. Upon increasing the fraction of the PLMA segment, the interfacial tension declined more sharply with the addition of copolymer and resulted in a lower plateau value, which E


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Figure 4. Representation of pH-induced self-assembly of triblock copolymers.

Figure 5. Oil/water interfacial tension as a function of copolymer concentration at pH 7.

Figure 6. Oil/water interfacial tension versus pH for aqueous solutions of D22-P27-L11and D22-P27-L51. The copolymer concentration was 5 mg·mL−1.

indicated the enhanced interfacial activity of the copolymer. Benefiting from larger hydrophobic PLMA blocks, the copolymers are driven to adsorb more firmly at the dodecane/water interface and constitute a monolayer with denser hydrophobic chains at the oil side of the interface. Therefore, more potent affinity for oil molecules is provided, and a consequent decrease of the interfacial tension results. Copolymer D22-P27-L51 exhibited the highest interfacial activity. The dodecane/water interfacial tension was significantly reduced from 52.8 to approximately 3.3 mN·m−1 at a fairly low D22-P27-L51 concentration of 1 mg·mL−1 and reached the valley value of 2.1 mN·m−1 at 5 mg·mL−1, which is far more pronounced than in most other studies of polymer surfactants.10,28,39 The significant decrease of interfacial tension observed herein reflects the efficient interaction ability of the triblock copolymers with oil and water on each side of the interface.40 The influence of pH on the interfacial activity of copolymers was also investigated, and the results of D22-P27-L11 and D22P27-L51 are illustrated as instances in Figure 6. Coincident with

the observations in other research,13,31,41 interfacial activity of D22-P27-L11 was facilitated as the solution pH was increased. It is believed that at pH lower than 5, the PDMA blocks are almost completely protonated. These highly cationic blocks cause the electrostatic repulsion among copolymers, and the adsorption of copolymers at the interface is minimal under this condition. As the solution pH is increased, the deprotonated PDMA blocks turn hydrophobic and tend to adsorb firmly at the interface, which enhances the interfacial activity of copolymers. However, with the highest PLMA fraction, D22P27-L51 exhibited a contrary dependence of its interfacial activity on solution pH. As the concentration (5 mg·mL−1) is much higher than the CMC of D22-P27-L51 (∼1 mg·mL−1, see Figure 5), there should be numbers of micelles formed in solution. On lowering the solution pH, micelles with protonated PDMA residues presumably act as additional salts screening the electrostatic repulsion among adsorbed copolymers,13 and thereby more compact adsorption of D22-P27-L51 at the interface is enabled, consequently leading to lower interfacial F


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stable O/W emulsions bearing 3-month-standing without coalescence were obtained, although the required amount of each copolymer and the proportion of emulsion varied slightly. Consistent with their interfacial activities, D22-P27-L36 and D22P27-L51 exhibited evidently higher emulsification performances compared to the other two copolymers with shorter hydrophobic blocks. To form stable emulsions, smaller amounts of the former two copolymers were needed than those of the latter. These distinct performances are supposed to be ascribed to the different hydrodynamic volumes of copolymers. Since D22-P27-L11 and D22-P27-L23 have smaller hydrodynamic volumes than D22-P27-L36 and D22-P27-L51, they provide less coverage of the dispersed droplets, which results in the need for larger amounts of copolymers to generate stable oil-in-water emulsions.10 It is interesting to note that D22-P27-L51, although the most efficient interfacial-tension-reducer it was, proved to be a relatively less effective emulsifier plus stabilizer than D22-P27L36. The distinct emulsification and stabilization efficiency reflects the differences in the interfacial monolayers formed by the two copolymers. As D22-P27-L36 has a lower molecular weight and a higher hydrophilic proportion, a larger number of D22-P27-L36 molecules are provided by copolymers of equal mass and produce the interfacial monolayer with stronger electrostatic repulsion and steric hindrance compared to D22P27-L51, which better guarantees the monolayer strength and emulsion stability against flocculation and coalescence. This phenomenon was also observed by other researchers10,42 and suggested that interfacial tension reduction efficiency of the surfactant alone could not predict its emulsifying performance. An optical microscope was utilized to image the emulsion droplets (24 h after the emulsification) to check the influence of copolymer concentration on the size of dispersed droplets in emulsions formed with D22-P27-L36. As shown in Figure 7, a decrease in average diameter of droplets was clearly observed

tension. In contrast, since the water solubility of D22-P27-L51 is relatively low, deprotonation of the PDMA block at alkaline medium further reduces its solubility and causes weakened interaction of adsorbed copolymer with water. 3.4. Emulsification of Model Oil. The emulsification effects of the triblock copolymers were evaluated with 50/50 (v/v) mixed dodecane and water at pH 7, and the emulsification data was summarized in Table 3. In all cases, Table 3. Emulsification Performances of Triblock Copolymers and Emulsion Stabilities block copolymer D22-P27-L11

D22-P27-L23

D22-P27-L36 D22-P27-L51

concentration (mg·mL )

emulsion fractionb after 24 h (%)

coalescence fractionc (%)

0.3, 0.5 1.0 3.0 5.0 7.0, 10.0 0.3 0.5 1.0, 3.0 ≥5.0 0.3 ≥0.5 0.3 0.5 1.0−7.0 10.0

41 49 63 80 80 55 60 100, flocculating 100 88 100 44 77 100 100

2.4 6.1 4.8 2.5 stable 100.0 6.7 8.0 stable 6.8 stable 6.8 11.7 stable 18.0

a

−1

a

Copolymer concentration relative to the original aqueous phase. Volume proportion of emulsified oil after standing for 24 h at 20 °C. c Volume proportion of coalesced emulsion after standing for 3 months at 20 °C. b

Figure 7. Optical micrographs of O/W emulsions formed with different amounts of D22-P27-L36: (a) 0.3, (b) 1, (c) 5, and (d) 10 mg·mL−1. Scale bar = 20 μm. G


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Figure 8. Evolution of (a) apparent viscosity and (b) stability of crude oil-in-water emulsion versus concentration of different copolymers. Stability of the emulsion was appraised by resolved fraction of the water phase after 3-month-standing. The copolymer concentration was relative to the original volume of water.

Figure 9. Optical micrographs of crude oil emulsions with (top row) 5 mg·mL−1 D22-P27-L36 and (bottom row) 10 mg·mL−1 D22-P27-L23 after standing for (a, c) 24 h and (b, d) 3 months. Scale bar = 10 μm.

when the concentration of D22-P27-L36 was increased from 0.3 to 5 mg·mL−1. At a low concentration, the block copolymer was not sufficient to adsorb on the large surface area of small droplets,43 so the dispersed droplets exhibited large size with wide diameter distribution, and the emulsified oil phase occupied a low proportion in the emulsion. As the concentration of D22-P27-L36 increased to 5 mg·mL−1, an emulsion with compact oil droplets and a fairly narrow distributed droplet size was generated. The size of the dispersed droplets did not change obviously when the concentration of copolymer was further increased. This is supposed to be caused by the interfacial tension lowering capacity of this copolymer; the valley value of interfacial tension (3.1 mN/m) could not maintain the stability of emulsion with smaller dispersed droplets. The likelihood might also be that the ultrasonic mixer does not provide the large energy and intense agitation required to break the oil droplets into an even smaller size. 3.5. Emulsification of Crude Oil. In order to appraise the emulsification effects of the block copolymers on crude oil, apparent viscosities of the emulsions, which represents their flowability in pipeline, were measured at a shear rate of 100 s−1

and plotted in Figure 8a. As expected, the apparent viscosity of crude oil decreased after emulsification by any of the copolymers, indicating the formation of an O/W emulsion.44 Viscosity reduction could be very significant, 2 orders of magnitude with D22-P27-L23 or D22-P27-L36, and the largest value was achieved with 3 mg·mL−1 D22-P27-L36, leading to the decline from around 34 000 mPa·s to less than 350 mPa·s which conforms to the required value of pipeline specification (typically limited to 400 mPa·s at ambient temperature).3,45 Performance of the copolymer on viscosity reduction was in accordance with that on the model oil emulsification. D22-P27L36 and D22-P27-L51 exhibited more superior efficiencies than the other two copolymers, since much lower amounts of them, namely 3 mg·mL−1 in both cases, were required to reduce the viscosity to a valley value, which was a sign of the saturated adsorption of copolymers on oil droplets. Beyond this critical concentration, the apparent viscosity increased again presumably due to the formation of smaller oil droplets and dissolution of excess copolymers in the continuous phase. The viscosity of emulsion increasing as dispersed droplet size decreases is a general behavior reported for kinds of emulsions.46 However, H


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Scheme 2. Representation of Crude Oil-in-Water Emulsification by Block Copolymers in Association with Natural Surfactant Components

The optical micrographs of emulsions with 5 mg·mL−1 D22P27-L36 confirmed that the emulsion was quite stable against flocculation and coalescence, as the morphology and size of the dispersed phase barely changed after standing for 3 months (Figure 9a,b). It is noteworthy that emulsions formed with D22P27-L23, even at a high concentration of 10 mg·mL−1, were not supposed to contain adequate copolymers as a guarantee of their stability. Nevertheless, the stability of them turned out to be quite acceptable at the copolymer concentration of above 5 mg·mL−1 (Figure 8b). From the optical images in Figure 9c,d, a subtle increase in the size of dispersed oil droplets was observed, suggesting that a more complex mechanism might probably play an important role in the stabilization of the crude oil emulsion. The complexity of the crude oil emulsion comes from the oil composition in terms of the natural surfactant components in the crude, which fall into three main categories, asphaltenes, resins, and wax crystals.3,49 These components can interact and reorganize at oil/water interfaces and consequently affect the emulsion stability. Asphaltenes and resins, which contain the highest amounts of the surface-active species, are the most polar oil fractions. Asphaltenes are flat sheets of condensed polyaromatic hydrocarbons, which are interconnected by functional groups like sulfide, ether, aliphatic chain, and naphthenic rings, with alkyl tails surrounding the edges.49 Resins are believed to be molecular precursors of asphaltenes. Their polar heads surround the asphaltenes, while the aliphatic tails extend into oil. In crude oil emulsions, owing to the comparatively low molecular weight of resins, they adsorb quickly to the oil/water interfaces and slow down the coalescence of the dispersed phase, providing adequate time for asphaltenes to adsorb and form stacked aggregates.50 By this means, the interfacial layers become thick and turn into solidlike “skins” with a large aggregate trapped in them, which

given that the normal route to emulsion breakdown involves the droplet diameter increasing during coalescence, emulsions with smaller droplets are regarded as more stable and farther from the phase separation stage.45 Besides, excess emulsifiers, which are larger than required for a full coverage of the dispersed droplets, are always used for the generation of emulsions in order to ensure the equilibrium adsorption on the droplets surface.47 The micelles or nanometer-sized aggregates formed by the excess surfactants also induce a depletion of attraction between the droplets of the dispersed phase.48 Consequently, though excess emulsifiers might cause thickening of the emulsion, they are still advantageous since a relatively high viscosity slows most destabilization phenomena and contributes to the stability of the emulsion. The stability of emulsions formed with different copolymers was assessed by evaluating the fraction of resolved water after resting for 3 months, and the results were plotted in Figure 8b. Generally, emulsion stability was remarkably improved by increasing the concentration of copolymers, indicated by the reduced proportion of resolved water, and then kept relatively constant beyond the threshold amount of copolymers. Emulsions generated with D22-P27-L11 exhibited the highest fractions of water resolved, i.e., the lowest stability, within the whole concentration range, which should be presumably attributed to the weak adsorption of D22-P27-L11 on oil droplets caused by its short hydrophobic anchoring block. The stability of emulsions constituted of D22-P27-L36 and D22-P27-L51 proved to be the best as expected. The resolved water of those emulsions was less than 26% even at a rather low concentration of copolymers (1 mg·mL−1) and decreased to its plateau value below 10% when the amount of copolymer reached the threshold of approximately 7 mg·mL−1, which was an excess than that required for a full coverage of the oil droplets in both cases. I



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restrains the thinning process of the interfacial film.51,52 Several researchers53−57 have pointed out that strong films made of asphaltenes with resins are predominantly responsible for stabilizing crude oil emulsions against coalescence and significant stresses. In addition, it is well documented that solid particles could stabilize emulsions by sterically hindering the coalescence,47,58−60 namely the Pickering emulsion. Thereby, adsorption of wax crystals would also contribute to the interfacial film strength in the same manner. Mouraille et al.61 characterized the stability of the emulsion based on the crude oil with a high wax content with a small asphaltenic proportion and found that the stability of the emulsion shows a high temperature dependence. At temperatures well above the wax appearance temperature (WAT), the emulsion phase-separated immediately, while at temperatures below the WAT very stable emulsions were generated, suggesting that wax crystallization benefitted the stabilization of emulsion. They also mentioned that the presence of any kind of surfactant components alone was not sufficient to stabilize emulsions; it was the association of those fractions in the crude that played the dominating role. Given all that mentioned, in our case, resins would adsorb quickly to the oil/water interfaces and promote the adsorption of block copolymers through interactions of their polar sites with the tertiary amine and ester groups of copolymer and their aliphatic tails with the lauryl pendants of the copolymer. Asphaltenes adsorb on the interfaces in the form of physical cross-linked networks that aggregate through intermolecular forces62 or with resin and copolymer molecules on their interfaces. Meanwhile, coadsorption of waxes at the interface further enhance the strength of the interfacial film. Hence, the adsorbed copolymers, asphaltenes, resins, and waxes associate and create mechanically strong and viscoelastic films that act as a steric barrier against flocculation and coalescence.63 As a consequence, emulsion stability arises even with a relatively insufficient amount of copolymers. The possible mechanism is shown in Scheme 2.

Article

ASSOCIATED CONTENT

S Supporting Information *

Preparation of triblock copolymer micelles, kinetic study and evolutions of Mn,GPC and Đ with monomer conversion for PMA and LMA (target DP = 50), and images of model oil-in-water emulsions formed with different amounts of triblock copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (51003028, 21004021, and 51273063), 111 Project Grant (B08021), and the Fundamental Research Funds for the Central Universities are gratefully acknowledged. The authors also thank Petrochina Shengli Oilfield Company for affording oil samples and technological supports.

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REFERENCES

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Synthesis of Triblock Copolymers via RAFT Polymerization and

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