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carboxylic acid mixtures in molecular dynamics simulation
Journal of Molecular Liquids 240 (2017) 412–419
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Interfacial behaviors of betaine and binary betaine/carboxylic acid mixtures in molecular dynamics simulation Zi-Yu Liu a,⁎, Zhicheng Xu a, He Zhou a, Yanlei Wang b, Qi Liao c,⁎, Lu Zhang a, Sui Zhao a a b c
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Department of Engineering Mechanics, Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, PR China Technical Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 20 April 2017 Accepted 19 May 2017 Available online 23 May 2017 Keywords: Molecular dynamics simulation Betaine Carboxylic acid Interfacial property Interfacial tension
a b s t r a c t The interfacial properties of betaine surfactants and betaine/carboxylic acid mixtures at decane-water interface have been studied via molecular dynamics simulations. The effect of surfactant structure on the surfactant orientation is discussed firstly based on the pure betaine systems. Then Synergistic effect of mixed Betaine/carboxylic acid systems at the interface is explored using mass density profile, interfacial thickness and spatial distribution function. Based on the simulated results, one can find the hydrophilic tail of betaine has a strong tendency to flat on the interface, long alkyl main chain tends to stretch into oil body and dominates the orientation of hydrophobic chain while benzene ring can change and fix the order of alkyl chain to some extent. Carboxylic acid molecules locate in the crack of ASB18 hydrophobic chains and the addition of appropriate carboxylic acid molecules could induce an impressive decrease of interfacial tension. Our simulated results prove experimental conjecture and reveal the mechanism about the decrease of interfacial tension at molecule level, which is important for the enhanced oil recovery processes. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ultralow interfacial tension (IFT) between crude oil and driving phases is significant in many enhanced oil recovery processes. Surfactants, which can achieve ultralow IFT between oil and water, are widely applied in the oil industry [1]. Surfactants derive their interfacial behavior directly from their molecular structure of two opposing solubility in water. In recent years, effects of hydrophilic structure and hydrophobic structure on surfactant adsorption properties have been investigated extensively [2,3]. In comparison to the conventional ionic surfactants, zwitterionic betaine-type surfactant [4–6] has many advantages, such as its high interfacial activity at high-salt condition. Therefore, synthesis and performance studies of betaine-type surfactant have gained wide attentions nowadays [7,8]. Zhou designed and synthesised trimeric betaine-type surfactants and studied their surface-active properties, wetting ability of a felt chip, foaming properties, and lime-soap dispersing ability [9,10]. Shekhovtsov et al. [11] improved the synthesis of pyridinium-N-phenolate betaine, carried out an X-ray crystal structure analysis and its protonated form. Moreover, properties of betaine-type surfactant on lowing IFT are also investigated. Zhao and Dai studied the surface and interfacial properties of five sulfobetaines [12]. They ⁎ Corresponding authors. E-mail addresses: [email protected], [email protected] (Z.-Y. Liu), [email protected] (Q. Liao).
http://dx.doi.org/10.1016/j.molliq.2017.05.094 0167-7322/© 2017 Elsevier B.V. All rights reserved.
found the hydroxypropyl sulfobetaine surfactant can reduce IFT between oil and water at a very low concentration, and ultralow IFT phenomenon only occurs in a specific concentration. Not only that, betaine-type surfactant is also most used in mixed surfactant systems. Many reports indicate synergism in mixtures is better than the individual surfactant components [13–15]. Wang et al. [16] investigated the physicochemical properties of mixed novel betaine surfactant/SDS system using IFT and steady-state fluorescence measurements. They observed that the mixture exhibited a stronger synergism and formed longer and stronger wormlike micelles. As we all know, fatty acid is one of important components in crude oil. Some reports [17] showed that acidic fractions were dominant factors on affecting the assignment of surfactants between the oil phase and water phase. Zheng and Ren explored interactions between sulfobetaines and fatty acid alkanolamides in aqueous micellar solution from experiment [18]. Our previous work studied dynamic IFT in betaine-acidic model oil system by a spinning drop interfacial tensiometer and found fatty acid could help the system to achieve the ultralow IFT [19]. In addition to the experiments, molecular dynamics (MD) simulations are also used widely to study the structural, mechanical, electronical and interfacial properties of systems including surfactants [20–24], especially the betaine systems [25–28]. Qu and Xue studied the structure and interfacial properties of sulfobetaine at the decane/water interface by MD simulation [29]. Aranda-Bravo studied removal of alkanes from the solid surface by using sodium alpha olefin sulphate and betaine
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via MD simulation [21]. Although betaine surfactant behaviors at the interface and in the brine have widely studied, the surfactant configurations at interface and the effect of structure on IFT in betaine/acid mixed systems remain poorly understood. And it prohibits a continuously growing development of aforementioned applications. In this work, the betaine/acid structure on surfactant adsorption, assignment at the interface and the mechanisms to achieve ultralow IFT are explored by performing MD simulations. Firstly, we build five different betaine systems (molecular structures are shown in Scheme lA) and study the configuration of surfactants at the aqueous solution/decane interface for different interfacial coverages (surface area per molecule). Then we construct the betaine/acid systems consisting an above-mentioned surfactants ASB18 and four carboxylic acids with various concentrations (molecular structures are shown in Scheme 1B), and study the relation between carboxylic acid molecules and ultralow IFT in betaine/ acid systems. Finally, synergistic effect and mechanism on ultralow IFT in betaine/acid mixtures are discussed. 2. Models and methods All the MD simulations of water-betaine/acid-oil hybrid systems are performed using GROMACS 4.5 [30–32]. The GROMACS96 force field is used to describe the interaction between atoms in the betaine/acid-oil hybrid system and the united atom approach is employed for the oil and surfactant parameters, which has been used to study the interfacial property of water/surfactant/oil hybrid system successfully [31,33]. The water molecules are represented by the SPC model [34], and the van der
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Waals interaction is described using Lennard-Jones potential with a cutoff distance of 1 nm. The particle mesh Ewald (PME) method is used to calculate the long-range electrostatic interaction [35], where the cutoff is 1 nm in our simulations. The SHAKE algorithm is applied for the stretching terms between the hydrogen atoms and other atoms to reduce high-frequency vibrations that require a shorter time step for numerical integration [36]. Different systems can be performed after charges and potentials are assigned to each atom. To study surfactants adsorbed at an interface, we set up a system by placing two surfactant layers on opposite sides of a slab with 7000 water molecules, which is thick enough for two surfactant layers remaining effectively isolated from each other. 800 decane molecules are distributed equally in two thick boxes. The temperature is constantly set at 300 K using a Berendsen thermostat [37]. The snapshots of one interface about betaine ASB18 and ASB18/acid C8H16O2 systems are shown in Fig. 1A and B, respectively. The time step for integrating equations of motion is set to be 2 fs. The periodic boundary conditions are used in the three directions with a super cell of LX × LY × LZ = 5.0 × 5.0 × 15.0 nm3, which was large enough to give the correct bulk density for water and oil at 300 K. The self-assembly MD simulations of betaine and acid at oil/water interface is performed in the NPT ensemble with pressure of 1 atm via coupling to a Berendsen barostat [37]. All the simulations are at least 10 ns long, during which the potential energy, the dimensions of the simulation box, and interfacial tension remain stable. The last 2 ns long trajectory is used to analyze the properties of oil/surfactant/water interface, which has been proved that it is long enough to get valuable interfacial properties.
Scheme 1. Chemical structure of betaine surfactants and carboxylic acids.
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objective group. The quantity SZ stands for the extent to object groups stand up along the interface normal and has been used to represent the orders of surfactant at liquid-liquid interface successfully [33,38, 39]. A value of SZ = 1 can be interpreted as a perfect orientation along the interface normal, while SZ = −1/2 as an orientation fully parallel to the interface. Appropriate atom tags are marked with a blue in Scheme 1. Fig. 2A shows order of entire molecule for five surfactants. It is not difficult to find that orders increases when surfactant concentration increases, which stands for a surfactant upright process to accommodate more molecules at the interface. However, the upright extent for five surfactants is slight different. To see it, more details for betaine order are necessary to be explored.
Fig. 1. Snapshots for (A) betaine ASB18 system, (B) ASB18/C8H16O2 mixture at the interfacial coverages of ASB18 Ainital = 0.81 nm2, (C) ASB18 and C8H16O2 molecules. Coloring is as follows: red for water molecules, gray for decane molecules, violet for C8H16O2 molecules, yellow and red for Satoms and O atoms in ASB18 head group, and green for the hydrophobic tail of ASB18 molecules respectively. (Picture prepared with VMD [41]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion 3.1. Effect of surfactant structure and concentration on interfacial configuration To study the effects of surfactant structure and concentration on interfacial configuration, five different betaine surfactants placed at the water-decane interface will be considered in MD simulations. In the following discussion, monolayer coverage Ainitial (0.78–2.5 nm2) is used to represent the different surfactant concentration, which is the reciprocal value of the initial surfactant concentration in MD simulations. 3.1.1. Determine the application scope of order parameter Five different betaine surfactants adsorbed at the oil/water interface are modeled, where the chemical structures are shown in Scheme 1A. Surfactant concentrations range from 0.78 nm2 to 2.5 nm2. The main parameter considered for surfactant configuration is surfactant order, which is based on surfactant monomolecular adsorption at the interface. Therefore, a method about confirming the monomolecular scope is necessary to be established in this paper. In principle, the layer is monomolecular if all of its molecules are simultaneously in contact with at least a water and at least an oil molecule. To see this, the contact distance of all of the atoms of a surfactant from all of the water and decane atoms should be checked, which can be defined through the first minimum position of the respective RDFs. Because electrostatic repulsion of hydrophilic group is much stronger than steric hindrance of hydrophobic group, superabundant surfactants are inclined to assemble oil layer at the interface. Therefore, contact distance of water and surfactant hydrophilic group will change dramatically if surfactant multilayer occurs. Based on this, the RDF of hydrophilic SO3/ COO group and water is simulated. By calculating with above method, no obvious changes are found in our simulated concentration scope from 0.78 nm2 to 2.5 nm2 for five surfactant systems, which means monolayer exists at all simulated concentration. Therefore, order parameter used to analyze surfactant configuration at the interface is feasible. The conformational property of surfactant molecules can be used to represent the orders of surfactant at the oil/ water interface, which can be calculated as a function of the surface coverage [33,38,39]: SZ ¼
3 1 cos2 θ− 2 2
ð1Þ
where θ is the angle between the interface normal and the molecular axis defined as the united vector from the first to the last atom of
3.1.2. The order of surfactant hydrophilic group at the interface Hydrophilic group orders of five betaine surfactants at different concentrations, which includes S-N, S-CO and CO-N part, are shown in Fig. 2B–D to investigate the effect of hydrophilic structure on surfactant molecular orientation. It is obvious to find from Fig. 2B that all the S-N values maintain below − 0.3 although orders of hydrophilic groups for different betaines change slightly. In other words, surfactant hydrophilic tail has a stronger tendency to spread on the horizontal direction. Moreover, hydrophilic group of B2SB10-8-2 extends into water slightly as the longer chain, while BCB10-8-2 has similar tendency as the weaker charge distribution of N-R-COO than N-R-SO3. The whole hydrophilic part of a betaine molecule is almost flat on the interface, which is accordant with the experimental assumption before [19]. That is to say, the simulation has proved the previous experimental assumptions about hydrophilic part of a betaine molecule. Fig. 2C and D describe order of hydrophilic part S-CO and CO-N, respectively. It can be seen that orders of these two hydrophilic parts have not obvious change for different surfactant hydrophobic structures, which means the structures of hydrophobic group has little effect on for the order of hydrophilic group at the interface. Moreover, for the part S-CO, the average order is −0.45, and for CO-N, the average order is close to − 0.5. Order of S-CO part is higher than CO-N which means comparing with S-CO, CO-N is more prone to flat on the interface. This phenomenon implies, besides SO3/COO and amino, hydroxyl also can assist the whole hydrophilic group to keep an orientation fully parallel to the interface. 3.1.3. The order of surfactant hydrophobic part at the interface Orders of hydrophobic main chain N-C18 for five surfactants are shown in Fig. 3A. From the order of ASB18, one can find alkyl chain stretches to the oil phase with a little slop. Although hydrophobic structures of five betaines are different, no obvious differences are found from main chain order. That seems to mean the dominant effect of hydrophobic long alkyl chain on the hydrophobic orientation and the loose assignment of hydrophobic parts. Order parameters between hydrophobic head N and middle atom C10 connected aromatic group are depicted in Fig. 3B. All the orders have similar tendencies and ASB18 achieves lower value while B2SB10-8-2 reaches lowest order. This result implies the effect of both aromatic group and hydrophilic group on the first half of hydrophobic group. Nevertheless, this effect is not significant. Fig. 3C illustrates rest part C10-C18 order of hydrophobic group for different betaines. Orders for all betaines show an obvious decrease comparing with the first part. However, unlike ASB18 decreases as gravity, order of hydrophobic alkyl chain for betaines containing aromatic group appears a more dramatic decrease. For the first part, the order ranges from 0.1–0.4. For the last part, the order changes from −0.2–−0.5 in stark contrast. Comparing with ASB18, betaines containing aromatic hydrophobic chain, especially the part behind aromatic group, come to parallel to the interface instead of extending to the oil phase. Distinct orientation of two hydrophobic part and similar hydrophobic order values of different surfactant imply the different effect of
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Fig. 2. Order of entire molecule for five betaine surfactants at different concentrations (A). (B–D) shows order of hydrophilic part S-N, S-CO and CO-N respectively.
rigid benzene and flexible alkyl chain on surfactant orientation. Rigid benzene ring can change and fix the order of flexible alkyl chain which makes hydrophobic chain spread to the interface while individual alkyl chain has a tendency to stretch to the oil phase.
The order of benzene center axis CB-CB2 is depicted in Fig. 3D. Comparing with BSB10–8-0, benzene center axis of BSB10-8-2 appears to keep more upright. The sole interpretation is two methyls have a tendency to make surfactants stretch to the oil phase, which is consistent
Fig. 3. Orders of hydrophobic part N-C18, N-C10, C10-C18, CB-CB2 respectively (D–F).
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with alkyl group. Thus considering different aromatic structures of these two surfactants, we can deduce that benzene ring of surfactant hydrophobic group is inclined to parallel to the interface while alkyl group tends to stretch into oil body which is uniform with above assumptions from C10-C18. According to the order of hydrophobic group and hydrophilic groups, diagrams of two typical betaine surfactant molecule (ASB18 and SBS10-8-2) adsorbing at the interface with A = 0.78 nm2 concentrations are shown in Fig. 4. From the results in Fig. 4, one can find the configuration of a betaine molecule is similar to the rough supposition based on experiments in our previous work: hydrophilic part flats at the interface while hydrophobic part stretches into oil. Therefore, the simulation about betaine configurations at the oil-water interface proves experimental conjecture and reveals an essence: ultralow interfacial tension will not reach in pure betaine systems because of the mismatching size between hydrophobic group and hydrophilic groups, which results in a loose surfactant layer. Meanwhile, more details, such as hydroxyl, benzene ring and other small part of surfactant, are also analyzed though simulation, which elaborates the important effect of every functional group on surfactant interfacial configuration. 3.2. Synergistic effect of mixed betaine/carboxylic acid systems at interface Besides the surfactant structure and concentration, the carboxylic acid can also influence the oil-water interfacial configuration. In our previous experimental work [19], one can find that IFTs decrease as the addition of acid in the ASB18 systems, and further decrease with the increase of alkyl chain length of organic acids. There exists an optimal alkyl chain length of organic acid to reach a lowest IFT. To study the synergistic effect of mixed betaine/carboxylic acid on the interfacial property, ASB18 and carboxylic acid mixtures are performed in our simulations. To compare with experimental results effectively, we change chain lengths of carboxylic acids with 0.81 nm2 ASB18 concentration (saturated adsorption capacity) to study the synergistic effect of mixed surfactants. 3.2.1. Carboxylic acid distributions at the interface in ASB18/acid systems Z-dependent mass density profile for 0.81 nm2 ASB18/C8H16O2 systems against different surfactant concentrations is shown in Fig. 5A. A slight peak difference of coordinate axis x between ASB18 and C8H16O2 can be seen clearly, which means different location of betaine and acid at the interface. Comparing with ASB18, C8H16O2 inclines to locate in the position approaching oil phase, which implies the stronger hydrophobicity of carboxylic acid. Distance difference of density peak (d) for 0.81 nm2 ASB18 with different acids systems is shown in Fig. 5B. From the figure, one can find that distance difference ranges from 0.4 nm to 0.43 nm, which is significantly less than the hydrophobic chain length of ASB18 (1.77 nm). Thus, it can be considered that C8H16O2 locates in the crack of ASB18 hydrophobic chain, and that
makes matching surfactant size possible. Furthermore, with the number of carboxylic acid molecules and carboxylic acid chain increases, distance difference increases accordingly because of steric hindrance and increasing hydrophobicity. Interfacial thickness seems to be used to explore more mysteries. Fig. 6A shows interfacial thicknesses of water, oil and surfactant layer in ASB18/C12H24O2 system. Surfactant thickness keeps stable as the acid concentration increases. Similar tendencies are found in C8H16O2–C14H28O2, which means the interfacial monolayer still exists. However, because of the increase of steric hindrance, surfactant thickness changes dramatically when C16H32O2 is added to ASB18 in simulated scope. Therefore further studies will be deepened among ASB18/C 8H 16 O 2 – C14 H28 O 2 mixed systems. In addition, from our simulation, just like ASB18/C12H24O2 in Fig. 6A, no significant changes of water layer can be found in all ASB18/carboxylic acid systems which mean little effect of carboxylic acid on interfacial water layer. Nevertheless, interfacial oil layer appears distinct difference. Oil thickness not only increases followed increased carboxylic acid concentration, but also be consistent with the increased length of carboxylic acid, which is illustrated in Fig. 6B. As we know, the interfacial thickness is calculated by the ‘90–10’criterion, which is defined as the distance along the interface with the densities range from 90% to 10% of their bulk values. Therefore, the increase of oil thickness implies low - rangeability of oil density at the interface. That is to say, increased oil thickness stands for decreased oil molecules at the interface because of enhancive surfactant compactness, which can always decrease the interfacial tension. Visual insight can be achieved from spatial distribution functions (SDFs) [24,40] of decane molecules around ASB18, which is shown in Fig. 6C–F. SDFs stand for the three-dimensional probability distributions of ASB18 and decane molecules. Comparing with Fig. 6 C, D and F, one can find distribution range of decane at 0.7 kg/m3 decrease successively, which means decane molecules decrease when the number of carboxylic acid increases. Furthermore, effect of carboxylic acid chain length can also be observed through Fig. 6C, D and F. The number of oil molecules at the interface decreases with increased chain length of carboxylic acid, and this result is consistent with that from interfacial thickness. 3.2.2. Mechanisms about interfacial assignment of mixed betaine/carboxylic acid systems From the distribution of carboxylic acid around the betaine surfactants, we can see that if carboxylic acid chain is too short, the addition could not achieve the maximized compactness of surfactant layer and the matching size between hydrophobic group and hydrophilic groups, even acid concentration maintains a high level, just as C8H16O2, which may just result in a slight decrease of interfacial tension. And if carboxylic acid chain is long too much, such as C16H32O2, acid molecules will hard to insert the crack of betaine hydrophobic group, which still couldn't increase interfacial compactness effectively and immensely
Fig. 4. Arrangement of one betaine molecule at decane/water interface. (A) plots a ASB18 molecule adsorbing on the interface; (B) expresses the adsorption of a BSB10-8-2 molecule.
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Fig. 5. (A) Mass density profile for ASB18/carboxylic acid C8H16O2 systems. (B) the distance difference of density peak for ASB18/different acid systems.
decrease interfacial tension. So that, it can be summarized that addition of appropriate carboxylic acid molecules could most effectively decrease the number of oil molecules at the interface because of the matching size leading to maximized surfactant compactness, which will result in an impressive decrease of interfacial tension. The mechanism about interfacial assignment of ASB18/carboxylic acid is shown in Fig. 7. Above simulated tendency is consistent with experimental result, and this
mechanism also makes the reason of decreased interfacial tension become clearer. 4. Conclusions The configurations of five betaine surfactants at decane/water interface have been studied via MD simulations. Betaine ASB18 and
Fig. 6. (A, B) the illustration of interfacial thickness of surfactant, water and oil respectively in ASB18/C12H24O2 system; (C–F) the spatial distributions function (SDF) of decane molecules around ASB18 for different systems. (C) for pure ASB18 at the concentration of 0.81 nm2; (D) for mixture of 0.81 nm2 ASB18 and 2.08 nm2 C8H16O2; (E) for mixture of 0.81 nm2 ASB18 and 12.5 nm2 C14H28O2; (F) for mixture of 0.81 nm2 ASB18 and 12.5 nm2 C8H16O2. Coloring is as follows: yellow for decane molecules; green for the hydrophobic tail of ASB18 molecules and red for ASB18 hydrophilic group respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Mechanisms about interfacial assignment of ASB18 with different carboxylic acids. Plots on (A) illustrates pure betaine system; (B) for ASB18 and carboxylic acid with short chain system; (C) for ASB18 and appropriate carboxylic acid mixed system; (D) for ASB18 and carboxylic acid with long chain system.
carboxylic acids with different chains are also simulated to explore the synergistic mechanism. Based on the results from simulations, the following major findings can be concluded: A. Betaine hydrophilic tail has a stronger tendency to flat on the interface. The addition of hydroxyl slightly changes the order of hydrophilic part which ensures the whole hydrophilic group flatting on the interface. Hydrophobic long alkyl chain has a dominant effect on the hydrophobic orientation. Benzene ring can change and fix the order of alkyl chain to some extent, which makes hydrophobic chain spread to the interface while alkyl group tends to stretch into oil body. Our simulation not only proves experimental conjecture but also finds more configuration details. B. In Betaine/carboxylic acid mixed systems, carboxylic acid molecules locate in the crack of ASB18 hydrophobic chains. The addition of appropriate carboxylic acid molecules could decrease the number of oil molecules at the interface, and result in an impressive decrease of interfacial tension. C. The results from MD simulations also reveal a mechanism theoretically at the molecule level: ultralow interfacial tension will not reach in pure betaine systems as the mismatching size between hydrophobic group and hydrophilic groups, but the addition of appropriate carboxylic acid chain can makes the similar size of surfactant hydrophobic group and hydrophilic groups, which leads to the decrease of interfacial tension. Acknowledgment The authors thank financial supports from the National Natural Science Foundation of China through Grant No. 21606245 and No. 51308461, and the National Science and Technology Major Project through Grant No. 2016ZX05011-003. References [1] A.A. Olajire, Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: prospects and challenges, Energy 77 (2014) 963–982. [2] R.J.K.U. Ranatunga, R.J.B. Kalescky, C.C. Chiu, S.O. Nielsen, Molecular dynamics simulations of surfactant functionalized nanoparticles in the vicinity of an oil/water interface, J. Phys. Chem. C 114 (2010) 12151–12157. [3] M.J. Rosen, Surfactants and Interfacial Phenomena, 2004. [4] T. Ishida, P.J. Rossky, Consequences of strong coupling between solvation and electronic structure in the excited state of a betaine dye, J. Phys. Chem. B 112 (2008) 11353–11360. [5] S.J. Dong, Y.L. Li, Y.B. Song, L.F. Zhi, Synthesis, characterization and performance of unsaturated long-chain carboxybetaine and hydroxy sulfobetaine, J. Surfactant Deterg. 16 (2013) 523–529.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Interfacial behaviors of betaine and binary betaine/carboxylic acid mixtures in molecular dynamics simulation Zi-Yu Liu a,⁎, Zhicheng Xu a, He Zhou a, Yanlei Wang b, Qi Liao c,⁎, Lu Zhang a, Sui Zhao a a b c
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Department of Engineering Mechanics, Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, PR China Technical Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 20 April 2017 Accepted 19 May 2017 Available online 23 May 2017 Keywords: Molecular dynamics simulation Betaine Carboxylic acid Interfacial property Interfacial tension
a b s t r a c t The interfacial properties of betaine surfactants and betaine/carboxylic acid mixtures at decane-water interface have been studied via molecular dynamics simulations. The effect of surfactant structure on the surfactant orientation is discussed firstly based on the pure betaine systems. Then Synergistic effect of mixed Betaine/carboxylic acid systems at the interface is explored using mass density profile, interfacial thickness and spatial distribution function. Based on the simulated results, one can find the hydrophilic tail of betaine has a strong tendency to flat on the interface, long alkyl main chain tends to stretch into oil body and dominates the orientation of hydrophobic chain while benzene ring can change and fix the order of alkyl chain to some extent. Carboxylic acid molecules locate in the crack of ASB18 hydrophobic chains and the addition of appropriate carboxylic acid molecules could induce an impressive decrease of interfacial tension. Our simulated results prove experimental conjecture and reveal the mechanism about the decrease of interfacial tension at molecule level, which is important for the enhanced oil recovery processes. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Ultralow interfacial tension (IFT) between crude oil and driving phases is significant in many enhanced oil recovery processes. Surfactants, which can achieve ultralow IFT between oil and water, are widely applied in the oil industry [1]. Surfactants derive their interfacial behavior directly from their molecular structure of two opposing solubility in water. In recent years, effects of hydrophilic structure and hydrophobic structure on surfactant adsorption properties have been investigated extensively [2,3]. In comparison to the conventional ionic surfactants, zwitterionic betaine-type surfactant [4–6] has many advantages, such as its high interfacial activity at high-salt condition. Therefore, synthesis and performance studies of betaine-type surfactant have gained wide attentions nowadays [7,8]. Zhou designed and synthesised trimeric betaine-type surfactants and studied their surface-active properties, wetting ability of a felt chip, foaming properties, and lime-soap dispersing ability [9,10]. Shekhovtsov et al. [11] improved the synthesis of pyridinium-N-phenolate betaine, carried out an X-ray crystal structure analysis and its protonated form. Moreover, properties of betaine-type surfactant on lowing IFT are also investigated. Zhao and Dai studied the surface and interfacial properties of five sulfobetaines [12]. They ⁎ Corresponding authors. E-mail addresses: [email protected], [email protected] (Z.-Y. Liu), [email protected] (Q. Liao).
http://dx.doi.org/10.1016/j.molliq.2017.05.094 0167-7322/© 2017 Elsevier B.V. All rights reserved.
found the hydroxypropyl sulfobetaine surfactant can reduce IFT between oil and water at a very low concentration, and ultralow IFT phenomenon only occurs in a specific concentration. Not only that, betaine-type surfactant is also most used in mixed surfactant systems. Many reports indicate synergism in mixtures is better than the individual surfactant components [13–15]. Wang et al. [16] investigated the physicochemical properties of mixed novel betaine surfactant/SDS system using IFT and steady-state fluorescence measurements. They observed that the mixture exhibited a stronger synergism and formed longer and stronger wormlike micelles. As we all know, fatty acid is one of important components in crude oil. Some reports [17] showed that acidic fractions were dominant factors on affecting the assignment of surfactants between the oil phase and water phase. Zheng and Ren explored interactions between sulfobetaines and fatty acid alkanolamides in aqueous micellar solution from experiment [18]. Our previous work studied dynamic IFT in betaine-acidic model oil system by a spinning drop interfacial tensiometer and found fatty acid could help the system to achieve the ultralow IFT [19]. In addition to the experiments, molecular dynamics (MD) simulations are also used widely to study the structural, mechanical, electronical and interfacial properties of systems including surfactants [20–24], especially the betaine systems [25–28]. Qu and Xue studied the structure and interfacial properties of sulfobetaine at the decane/water interface by MD simulation [29]. Aranda-Bravo studied removal of alkanes from the solid surface by using sodium alpha olefin sulphate and betaine
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via MD simulation [21]. Although betaine surfactant behaviors at the interface and in the brine have widely studied, the surfactant configurations at interface and the effect of structure on IFT in betaine/acid mixed systems remain poorly understood. And it prohibits a continuously growing development of aforementioned applications. In this work, the betaine/acid structure on surfactant adsorption, assignment at the interface and the mechanisms to achieve ultralow IFT are explored by performing MD simulations. Firstly, we build five different betaine systems (molecular structures are shown in Scheme lA) and study the configuration of surfactants at the aqueous solution/decane interface for different interfacial coverages (surface area per molecule). Then we construct the betaine/acid systems consisting an above-mentioned surfactants ASB18 and four carboxylic acids with various concentrations (molecular structures are shown in Scheme 1B), and study the relation between carboxylic acid molecules and ultralow IFT in betaine/ acid systems. Finally, synergistic effect and mechanism on ultralow IFT in betaine/acid mixtures are discussed. 2. Models and methods All the MD simulations of water-betaine/acid-oil hybrid systems are performed using GROMACS 4.5 [30–32]. The GROMACS96 force field is used to describe the interaction between atoms in the betaine/acid-oil hybrid system and the united atom approach is employed for the oil and surfactant parameters, which has been used to study the interfacial property of water/surfactant/oil hybrid system successfully [31,33]. The water molecules are represented by the SPC model [34], and the van der
413
Waals interaction is described using Lennard-Jones potential with a cutoff distance of 1 nm. The particle mesh Ewald (PME) method is used to calculate the long-range electrostatic interaction [35], where the cutoff is 1 nm in our simulations. The SHAKE algorithm is applied for the stretching terms between the hydrogen atoms and other atoms to reduce high-frequency vibrations that require a shorter time step for numerical integration [36]. Different systems can be performed after charges and potentials are assigned to each atom. To study surfactants adsorbed at an interface, we set up a system by placing two surfactant layers on opposite sides of a slab with 7000 water molecules, which is thick enough for two surfactant layers remaining effectively isolated from each other. 800 decane molecules are distributed equally in two thick boxes. The temperature is constantly set at 300 K using a Berendsen thermostat [37]. The snapshots of one interface about betaine ASB18 and ASB18/acid C8H16O2 systems are shown in Fig. 1A and B, respectively. The time step for integrating equations of motion is set to be 2 fs. The periodic boundary conditions are used in the three directions with a super cell of LX × LY × LZ = 5.0 × 5.0 × 15.0 nm3, which was large enough to give the correct bulk density for water and oil at 300 K. The self-assembly MD simulations of betaine and acid at oil/water interface is performed in the NPT ensemble with pressure of 1 atm via coupling to a Berendsen barostat [37]. All the simulations are at least 10 ns long, during which the potential energy, the dimensions of the simulation box, and interfacial tension remain stable. The last 2 ns long trajectory is used to analyze the properties of oil/surfactant/water interface, which has been proved that it is long enough to get valuable interfacial properties.
Scheme 1. Chemical structure of betaine surfactants and carboxylic acids.
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objective group. The quantity SZ stands for the extent to object groups stand up along the interface normal and has been used to represent the orders of surfactant at liquid-liquid interface successfully [33,38, 39]. A value of SZ = 1 can be interpreted as a perfect orientation along the interface normal, while SZ = −1/2 as an orientation fully parallel to the interface. Appropriate atom tags are marked with a blue in Scheme 1. Fig. 2A shows order of entire molecule for five surfactants. It is not difficult to find that orders increases when surfactant concentration increases, which stands for a surfactant upright process to accommodate more molecules at the interface. However, the upright extent for five surfactants is slight different. To see it, more details for betaine order are necessary to be explored.
Fig. 1. Snapshots for (A) betaine ASB18 system, (B) ASB18/C8H16O2 mixture at the interfacial coverages of ASB18 Ainital = 0.81 nm2, (C) ASB18 and C8H16O2 molecules. Coloring is as follows: red for water molecules, gray for decane molecules, violet for C8H16O2 molecules, yellow and red for Satoms and O atoms in ASB18 head group, and green for the hydrophobic tail of ASB18 molecules respectively. (Picture prepared with VMD [41]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion 3.1. Effect of surfactant structure and concentration on interfacial configuration To study the effects of surfactant structure and concentration on interfacial configuration, five different betaine surfactants placed at the water-decane interface will be considered in MD simulations. In the following discussion, monolayer coverage Ainitial (0.78–2.5 nm2) is used to represent the different surfactant concentration, which is the reciprocal value of the initial surfactant concentration in MD simulations. 3.1.1. Determine the application scope of order parameter Five different betaine surfactants adsorbed at the oil/water interface are modeled, where the chemical structures are shown in Scheme 1A. Surfactant concentrations range from 0.78 nm2 to 2.5 nm2. The main parameter considered for surfactant configuration is surfactant order, which is based on surfactant monomolecular adsorption at the interface. Therefore, a method about confirming the monomolecular scope is necessary to be established in this paper. In principle, the layer is monomolecular if all of its molecules are simultaneously in contact with at least a water and at least an oil molecule. To see this, the contact distance of all of the atoms of a surfactant from all of the water and decane atoms should be checked, which can be defined through the first minimum position of the respective RDFs. Because electrostatic repulsion of hydrophilic group is much stronger than steric hindrance of hydrophobic group, superabundant surfactants are inclined to assemble oil layer at the interface. Therefore, contact distance of water and surfactant hydrophilic group will change dramatically if surfactant multilayer occurs. Based on this, the RDF of hydrophilic SO3/ COO group and water is simulated. By calculating with above method, no obvious changes are found in our simulated concentration scope from 0.78 nm2 to 2.5 nm2 for five surfactant systems, which means monolayer exists at all simulated concentration. Therefore, order parameter used to analyze surfactant configuration at the interface is feasible. The conformational property of surfactant molecules can be used to represent the orders of surfactant at the oil/ water interface, which can be calculated as a function of the surface coverage [33,38,39]: SZ ¼
3 1 cos2 θ− 2 2
ð1Þ
where θ is the angle between the interface normal and the molecular axis defined as the united vector from the first to the last atom of
3.1.2. The order of surfactant hydrophilic group at the interface Hydrophilic group orders of five betaine surfactants at different concentrations, which includes S-N, S-CO and CO-N part, are shown in Fig. 2B–D to investigate the effect of hydrophilic structure on surfactant molecular orientation. It is obvious to find from Fig. 2B that all the S-N values maintain below − 0.3 although orders of hydrophilic groups for different betaines change slightly. In other words, surfactant hydrophilic tail has a stronger tendency to spread on the horizontal direction. Moreover, hydrophilic group of B2SB10-8-2 extends into water slightly as the longer chain, while BCB10-8-2 has similar tendency as the weaker charge distribution of N-R-COO than N-R-SO3. The whole hydrophilic part of a betaine molecule is almost flat on the interface, which is accordant with the experimental assumption before [19]. That is to say, the simulation has proved the previous experimental assumptions about hydrophilic part of a betaine molecule. Fig. 2C and D describe order of hydrophilic part S-CO and CO-N, respectively. It can be seen that orders of these two hydrophilic parts have not obvious change for different surfactant hydrophobic structures, which means the structures of hydrophobic group has little effect on for the order of hydrophilic group at the interface. Moreover, for the part S-CO, the average order is −0.45, and for CO-N, the average order is close to − 0.5. Order of S-CO part is higher than CO-N which means comparing with S-CO, CO-N is more prone to flat on the interface. This phenomenon implies, besides SO3/COO and amino, hydroxyl also can assist the whole hydrophilic group to keep an orientation fully parallel to the interface. 3.1.3. The order of surfactant hydrophobic part at the interface Orders of hydrophobic main chain N-C18 for five surfactants are shown in Fig. 3A. From the order of ASB18, one can find alkyl chain stretches to the oil phase with a little slop. Although hydrophobic structures of five betaines are different, no obvious differences are found from main chain order. That seems to mean the dominant effect of hydrophobic long alkyl chain on the hydrophobic orientation and the loose assignment of hydrophobic parts. Order parameters between hydrophobic head N and middle atom C10 connected aromatic group are depicted in Fig. 3B. All the orders have similar tendencies and ASB18 achieves lower value while B2SB10-8-2 reaches lowest order. This result implies the effect of both aromatic group and hydrophilic group on the first half of hydrophobic group. Nevertheless, this effect is not significant. Fig. 3C illustrates rest part C10-C18 order of hydrophobic group for different betaines. Orders for all betaines show an obvious decrease comparing with the first part. However, unlike ASB18 decreases as gravity, order of hydrophobic alkyl chain for betaines containing aromatic group appears a more dramatic decrease. For the first part, the order ranges from 0.1–0.4. For the last part, the order changes from −0.2–−0.5 in stark contrast. Comparing with ASB18, betaines containing aromatic hydrophobic chain, especially the part behind aromatic group, come to parallel to the interface instead of extending to the oil phase. Distinct orientation of two hydrophobic part and similar hydrophobic order values of different surfactant imply the different effect of
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Fig. 2. Order of entire molecule for five betaine surfactants at different concentrations (A). (B–D) shows order of hydrophilic part S-N, S-CO and CO-N respectively.
rigid benzene and flexible alkyl chain on surfactant orientation. Rigid benzene ring can change and fix the order of flexible alkyl chain which makes hydrophobic chain spread to the interface while individual alkyl chain has a tendency to stretch to the oil phase.
The order of benzene center axis CB-CB2 is depicted in Fig. 3D. Comparing with BSB10–8-0, benzene center axis of BSB10-8-2 appears to keep more upright. The sole interpretation is two methyls have a tendency to make surfactants stretch to the oil phase, which is consistent
Fig. 3. Orders of hydrophobic part N-C18, N-C10, C10-C18, CB-CB2 respectively (D–F).
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with alkyl group. Thus considering different aromatic structures of these two surfactants, we can deduce that benzene ring of surfactant hydrophobic group is inclined to parallel to the interface while alkyl group tends to stretch into oil body which is uniform with above assumptions from C10-C18. According to the order of hydrophobic group and hydrophilic groups, diagrams of two typical betaine surfactant molecule (ASB18 and SBS10-8-2) adsorbing at the interface with A = 0.78 nm2 concentrations are shown in Fig. 4. From the results in Fig. 4, one can find the configuration of a betaine molecule is similar to the rough supposition based on experiments in our previous work: hydrophilic part flats at the interface while hydrophobic part stretches into oil. Therefore, the simulation about betaine configurations at the oil-water interface proves experimental conjecture and reveals an essence: ultralow interfacial tension will not reach in pure betaine systems because of the mismatching size between hydrophobic group and hydrophilic groups, which results in a loose surfactant layer. Meanwhile, more details, such as hydroxyl, benzene ring and other small part of surfactant, are also analyzed though simulation, which elaborates the important effect of every functional group on surfactant interfacial configuration. 3.2. Synergistic effect of mixed betaine/carboxylic acid systems at interface Besides the surfactant structure and concentration, the carboxylic acid can also influence the oil-water interfacial configuration. In our previous experimental work [19], one can find that IFTs decrease as the addition of acid in the ASB18 systems, and further decrease with the increase of alkyl chain length of organic acids. There exists an optimal alkyl chain length of organic acid to reach a lowest IFT. To study the synergistic effect of mixed betaine/carboxylic acid on the interfacial property, ASB18 and carboxylic acid mixtures are performed in our simulations. To compare with experimental results effectively, we change chain lengths of carboxylic acids with 0.81 nm2 ASB18 concentration (saturated adsorption capacity) to study the synergistic effect of mixed surfactants. 3.2.1. Carboxylic acid distributions at the interface in ASB18/acid systems Z-dependent mass density profile for 0.81 nm2 ASB18/C8H16O2 systems against different surfactant concentrations is shown in Fig. 5A. A slight peak difference of coordinate axis x between ASB18 and C8H16O2 can be seen clearly, which means different location of betaine and acid at the interface. Comparing with ASB18, C8H16O2 inclines to locate in the position approaching oil phase, which implies the stronger hydrophobicity of carboxylic acid. Distance difference of density peak (d) for 0.81 nm2 ASB18 with different acids systems is shown in Fig. 5B. From the figure, one can find that distance difference ranges from 0.4 nm to 0.43 nm, which is significantly less than the hydrophobic chain length of ASB18 (1.77 nm). Thus, it can be considered that C8H16O2 locates in the crack of ASB18 hydrophobic chain, and that
makes matching surfactant size possible. Furthermore, with the number of carboxylic acid molecules and carboxylic acid chain increases, distance difference increases accordingly because of steric hindrance and increasing hydrophobicity. Interfacial thickness seems to be used to explore more mysteries. Fig. 6A shows interfacial thicknesses of water, oil and surfactant layer in ASB18/C12H24O2 system. Surfactant thickness keeps stable as the acid concentration increases. Similar tendencies are found in C8H16O2–C14H28O2, which means the interfacial monolayer still exists. However, because of the increase of steric hindrance, surfactant thickness changes dramatically when C16H32O2 is added to ASB18 in simulated scope. Therefore further studies will be deepened among ASB18/C 8H 16 O 2 – C14 H28 O 2 mixed systems. In addition, from our simulation, just like ASB18/C12H24O2 in Fig. 6A, no significant changes of water layer can be found in all ASB18/carboxylic acid systems which mean little effect of carboxylic acid on interfacial water layer. Nevertheless, interfacial oil layer appears distinct difference. Oil thickness not only increases followed increased carboxylic acid concentration, but also be consistent with the increased length of carboxylic acid, which is illustrated in Fig. 6B. As we know, the interfacial thickness is calculated by the ‘90–10’criterion, which is defined as the distance along the interface with the densities range from 90% to 10% of their bulk values. Therefore, the increase of oil thickness implies low - rangeability of oil density at the interface. That is to say, increased oil thickness stands for decreased oil molecules at the interface because of enhancive surfactant compactness, which can always decrease the interfacial tension. Visual insight can be achieved from spatial distribution functions (SDFs) [24,40] of decane molecules around ASB18, which is shown in Fig. 6C–F. SDFs stand for the three-dimensional probability distributions of ASB18 and decane molecules. Comparing with Fig. 6 C, D and F, one can find distribution range of decane at 0.7 kg/m3 decrease successively, which means decane molecules decrease when the number of carboxylic acid increases. Furthermore, effect of carboxylic acid chain length can also be observed through Fig. 6C, D and F. The number of oil molecules at the interface decreases with increased chain length of carboxylic acid, and this result is consistent with that from interfacial thickness. 3.2.2. Mechanisms about interfacial assignment of mixed betaine/carboxylic acid systems From the distribution of carboxylic acid around the betaine surfactants, we can see that if carboxylic acid chain is too short, the addition could not achieve the maximized compactness of surfactant layer and the matching size between hydrophobic group and hydrophilic groups, even acid concentration maintains a high level, just as C8H16O2, which may just result in a slight decrease of interfacial tension. And if carboxylic acid chain is long too much, such as C16H32O2, acid molecules will hard to insert the crack of betaine hydrophobic group, which still couldn't increase interfacial compactness effectively and immensely
Fig. 4. Arrangement of one betaine molecule at decane/water interface. (A) plots a ASB18 molecule adsorbing on the interface; (B) expresses the adsorption of a BSB10-8-2 molecule.
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Fig. 5. (A) Mass density profile for ASB18/carboxylic acid C8H16O2 systems. (B) the distance difference of density peak for ASB18/different acid systems.
decrease interfacial tension. So that, it can be summarized that addition of appropriate carboxylic acid molecules could most effectively decrease the number of oil molecules at the interface because of the matching size leading to maximized surfactant compactness, which will result in an impressive decrease of interfacial tension. The mechanism about interfacial assignment of ASB18/carboxylic acid is shown in Fig. 7. Above simulated tendency is consistent with experimental result, and this
mechanism also makes the reason of decreased interfacial tension become clearer. 4. Conclusions The configurations of five betaine surfactants at decane/water interface have been studied via MD simulations. Betaine ASB18 and
Fig. 6. (A, B) the illustration of interfacial thickness of surfactant, water and oil respectively in ASB18/C12H24O2 system; (C–F) the spatial distributions function (SDF) of decane molecules around ASB18 for different systems. (C) for pure ASB18 at the concentration of 0.81 nm2; (D) for mixture of 0.81 nm2 ASB18 and 2.08 nm2 C8H16O2; (E) for mixture of 0.81 nm2 ASB18 and 12.5 nm2 C14H28O2; (F) for mixture of 0.81 nm2 ASB18 and 12.5 nm2 C8H16O2. Coloring is as follows: yellow for decane molecules; green for the hydrophobic tail of ASB18 molecules and red for ASB18 hydrophilic group respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Mechanisms about interfacial assignment of ASB18 with different carboxylic acids. Plots on (A) illustrates pure betaine system; (B) for ASB18 and carboxylic acid with short chain system; (C) for ASB18 and appropriate carboxylic acid mixed system; (D) for ASB18 and carboxylic acid with long chain system.
carboxylic acids with different chains are also simulated to explore the synergistic mechanism. Based on the results from simulations, the following major findings can be concluded: A. Betaine hydrophilic tail has a stronger tendency to flat on the interface. The addition of hydroxyl slightly changes the order of hydrophilic part which ensures the whole hydrophilic group flatting on the interface. Hydrophobic long alkyl chain has a dominant effect on the hydrophobic orientation. Benzene ring can change and fix the order of alkyl chain to some extent, which makes hydrophobic chain spread to the interface while alkyl group tends to stretch into oil body. Our simulation not only proves experimental conjecture but also finds more configuration details. B. In Betaine/carboxylic acid mixed systems, carboxylic acid molecules locate in the crack of ASB18 hydrophobic chains. The addition of appropriate carboxylic acid molecules could decrease the number of oil molecules at the interface, and result in an impressive decrease of interfacial tension. C. The results from MD simulations also reveal a mechanism theoretically at the molecule level: ultralow interfacial tension will not reach in pure betaine systems as the mismatching size between hydrophobic group and hydrophilic groups, but the addition of appropriate carboxylic acid chain can makes the similar size of surfactant hydrophobic group and hydrophilic groups, which leads to the decrease of interfacial tension. Acknowledgment The authors thank financial supports from the National Natural Science Foundation of China through Grant No. 21606245 and No. 51308461, and the National Science and Technology Major Project through Grant No. 2016ZX05011-003. References [1] A.A. Olajire, Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: prospects and challenges, Energy 77 (2014) 963–982. [2] R.J.K.U. Ranatunga, R.J.B. Kalescky, C.C. Chiu, S.O. Nielsen, Molecular dynamics simulations of surfactant functionalized nanoparticles in the vicinity of an oil/water interface, J. Phys. Chem. C 114 (2010) 12151–12157. [3] M.J. Rosen, Surfactants and Interfacial Phenomena, 2004. [4] T. Ishida, P.J. Rossky, Consequences of strong coupling between solvation and electronic structure in the excited state of a betaine dye, J. Phys. Chem. B 112 (2008) 11353–11360. [5] S.J. Dong, Y.L. Li, Y.B. Song, L.F. Zhi, Synthesis, characterization and performance of unsaturated long-chain carboxybetaine and hydroxy sulfobetaine, J. Surfactant Deterg. 16 (2013) 523–529.
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