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C12A surfactants: Experimental and molecular simulation studies
Chemical Engineering Science 209 (2019) 115218
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CO2/N2 switchable aqueous foam stabilized by SDS/C12A surfactants: Experimental and molecular simulation studies Shuangqing Sun a, Xiqiang Zhang a, Shengxiang Feng a, Hongbing Wang a, Yan Wang a, Jianhui Luo b, Chunling Li a,⇑, Songqing Hu a,⇑ a b
Schools of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, PR China Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CO2/N2 switchable foam can be
formed using a mixed solution containing C12A and SDS with a fixed molar ratio of 1:1. Foam can be destroyed quickly after introducing CO2 at room temperature. Molecular dynamics simulation is used to investigate the CO2/N2 switching mechanism. Aggregation of SDS and C12A molecules in the water phase leads to rapid defoaming.
a r t i c l e
i n f o
Article history: Received 27 June 2019 Received in revised form 4 September 2019 Accepted 8 September 2019 Available online 9 September 2019 Keywords: CO2/N2 switchable Aqueous foam CO2-responsive surfactant Molecular simulation Electrostatic attraction Hydrophilicity
a b s t r a c t A novel CO2/N2 switchable aqueous foam was developed using a CO2-responsive surfactant N,N-dimethylN-dodecyl amine (C12A) and a conventional surfactant sodium dodecyl sulfate (SDS). The performance and mechanism of this switchable foam were studied by both experiment and molecular simulation methods. Experimental results demonstrate that the foam stabilized by SDS/C12A surfactants has better stability and water carrying capacity than single C12A foam. The foam could be rapidly switched between stable and unstable states at ambient temperature with CO2 or N2 as the triggers. Simulation results shows that when CO2 was introduced, most of SDS and C12A molecules aggregated in the water phase, instead of adsorbing at the air-water interface to stabilize the foam films. The strong electrostatic attraction between protonated C12A and SDS molecules could account for the aggregation. Due to the presence of limited surfactants located at the air-water interface after introducing CO2, the foam bursts quickly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Foam has been widely used in many fields, including foam dewatering gas recovery (Wu et al., 2016), enhanced oil recovery (Yang et al., 2017; Guo and Aryana, 2016), mineral flotation ⇑ Corresponding authors at: Schools of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, PR China. E-mail addresses: [email protected] (C. Li), [email protected] (S. Hu). https://doi.org/10.1016/j.ces.2019.115218 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.
(Huangfu et al., 2018), food industry (Patino et al., 2008) and firefighting (Hill et al., 2018), etc. However, in many applications, stable foam is only needed in some processes, on-demand and rapid burst of the stable foam after application is also required. For example, in the process of foam dewatering gas recovery, the foam carries the fluid from the bottom of the well to the ground (Wu et al., 2016). Then, a large amount of residual foam results in difficulties in post-processing, such as emulsifying gas condensate, tripping of separation equipment in gas/water separating
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plants, and creating high pressure in the pipeline, etc. (Heeres et al., 2016). The common method to remove residual foam is to use defoaming agents to break the foam (Garrett, 2016, 2015; Denkov, 2004). Unfortunately, defoaming agents would not only fail to avoid the formation of emulsion but also prevent the recovery and reuse of the foaming agent (Miller, 2008). Therefore, the formation of stable foams and on-demand removal of foam without changing the system composition are both required. To remove foam effectively and avoid the disadvantages of defoaming agents, stimuli-responsive aqueous foam becomes a great alternative (Fameau et al., 2015). The stability of foam generated by stimuli-responsive foaming agents can be controlled by external triggers including temperature (Fameau et al., 2011; Jochum and Theato, 2013; Fameau et al., 2013), light (Schnurbus et al., 2018; Lei et al., 2017; Chevallier et al., 2012), pH (Fujii et al., 2015; Binks et al., 2007), magnetic field (Lam et al., 2011) and CO2 (Wang et al., 2019; Wang et al., 2018; Zhu et al., 2014). Compared with other triggers, CO2 is regarded as a non-toxic, inexpensive and readily removable trigger (Zhang et al., 2014). Jessop et al. (Liu et al., 2006) first reported a CO2-responsive surfactant which is a long chain alkyl amidine. In the presence of CO2 and water, long chain alkyl amidine is protonated into charged amidinium bicarbonate with excellent surface activity. The protonated amidinium bicarbonate could be converted back to neutral alkyl amidine with low surface activity by blowing inert gas (Ar) at 65 °C to remove CO2. According to the principle of CO2-responsive surfactant, Zhu et al. (Zhu et al., 2014) prepared a CO2/N2 switchable particle-stabilized aqueous foam by mixing a CO2-responsive surfactant N0 -dodecylN,N-dimethylacetamidinium bicarbonate with bare silica nanoparticles. The bare nanoparticles can be hydrophobized in situ by N0 -d odecyl-N,N-dimethylacetamidinium bicarbonate to become surface-active nanoparticles and then the particle-stabilized aqueous foam can be formed. When N2 is introduced into the foam system at 65 °C for 80 min, the surfactants desorb from the surface of nanoparticles and the foam bursts. Bubbling CO2 at 0–5 °C for 50 min, the foam can be formed again. However, it is noticed that the defoaming of this CO2/N2 switchable particle-stabilized aqueous foam requires heating the entire foam system and the re-foaming must require CO2, which limits the practical application of this switchable foam. Moreover, the CO2/N2 switching mechanism of the aqueous foam is only speculated by experimental phenomenon. The research and analyses at molecular level are needed. Molecular dynamics (MD) simulation can reveal the structural and dynamical properties of foam films at molecular level (Wu et al., 2017; Wang et al., 2016, 2017, 2018; Gao et al., 2017), however, the study using molecular simulation method on switchable foam system has not been reported. In this context, a novel CO2/N2 switchable aqueous foam was prepared by mixing a CO2-responsive surfactant N,N-dimethylN-dodecyl amine (C12A) and a conventional surfactant sodium dodecyl sulfate (SDS). First, the foam performance of this SDS/C12A solution including foam stability and switchability was studied by foam experiments. Additionally, the CO2/N2 switching mechanism of this aqueous foam was also investigated by molecular dynamics simulations. The results obtained in this work will be beneficial to the design of CO2/N2 switchable aqueous foams for the convenient application, and further the understanding on the switchability mechanism of these foams. 2. Experiment and simulation details 2.1. Experiment section 2.1.1. Chemicals N,N-dimethyl-N-dodecyl amine (C12A, 97%) was purchased from Beijing J&K Scientific Co., Ltd. Sodium dodecyl sulfate (SDS,
95%) was purchased from Shanghai Macklin Biochemical Co., Ltd. n-Propanol (C3H7OH, 99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Oil red O (C26H24N4O, 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Carbon dioxide gas (CO2, 99.8%) and Nitrogen gas (N2, 99.999%) was purchased from Qingdao Tianyuan Gas Manufacturing Co., Ltd. 2.1.2. Foam preparation and performance tests In this work, foams were generated using a foam analyzer (JPM2012, Shanghai Zhongchen digital technic apparatus Co., Ltd). Foaming, foam stability and instability are determined by measuring the height of foam or the volume of foam by optical lens. The drainage behavior of transparent samples can also be measured in this way. A typical liquid containing foam is used to record the drainage changes under gravity by measuring electrical conductivity. In addition, foam stability can be assessed from foam structure. A 0.02 mol/L C12A solution was first prepared by dissolving C12A in the solvent consisted of n-propanol and water with the equal volume to characterize the CO2 switchability of C12A by measuring the solution conductivity changes when CO2 and N2 were introduced alternatively. An aqueous solution containing both C12A and sodium dodecyl sulfate (SDS) with a fixed molar ratio of 1:1 was also prepared (SDS/C12A solution), as well as a 0.02 mol/L C12A aqueous solution (only C12A), to compare the foam performance of these two aqueous solutions. To further study the concentration of surfactants on the foam performance, a range of concentrations (0.001–0.024 mol/L) of SDS/C12A solution was prepared. After the preparation of C12A and SDS/C12A solutions, foams were generated by introducing CO2 or N2 with a flow rate of 0.6 L/ min into the foam solutions at room temperature. Defoaming of the SDS/C12A foam was carried out by introducing CO2 into the foam system from the bottom of the container with a flow rate of 0.4 L/min until the foam disappeared completely. Then N2 was introduced into the SDS/C12A solution after defoaming at 60 °C until the foam was generated again. The foam performances of two foam solutions were compared by analyzing their time-dependent foam conductivity, foam height, foam half-life, drainage half-life and foam drainage rate. The foam half-life is the time that the foam takes to decay to half of the initial value (Rudin, 1957). The drainage half-life is the time required for the liquid in the foam to decrease to the half of the initial value (Huang and Kinsella, 1987). The drainage rate is defined as the ratio of the real-time foam conductivity (r) to the initial foam conductivity (r0). In addition, we also analyzed the color and surface tension change of foam solution in the foaming and defoaming processes. In particular, the surface tension of the solution was measured by a JC2000C1 contact angle measuring instrument (Shanghai zhongchen digital technic apparatus Co., Ltd) using the pendant drop method (Berry et al., 2015; Fordham, 1948; Stauffer, 1965). Five-point method (Fig. S1) was used to calculate the surface tension. 2.2. Simulation section 2.2.1. Models construction In this work, model constructions, MD simulations and subsequent analyses were all performed using Materials Studio 8.0 software. Sandwich model with a water layer in the middle and two surfactant layers on both sides is widely used in molecular dynamics simulation studies of foams, especially in the studies on the airwater interface properties of foams (Li et al., 2016, 2013; Sun et al., 2015; Yan et al., 2011). To investigate the change of foam before and after introducing the external stimuli, i.e. CO2 in this work, two types of foam models were built. In the models before introducing CO2, the same number of SDS and C12A molecules were
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randomly distributed in the water layer to form a solution phase, and a vacuum layer was added to both sides of the solution phase (Fig. 1a). The size of the initial foam model is 5.0 5.0 13.2 nm3, the thickness of the solution phase and the vacuum layer is 3.2 nm and 5 nm, respectively. The number of water molecules in the solution phase is 2675. According to the occupied area of surfactant molecules at the air-water interface, the number of SDS and C12A molecules at the critical micelle concentration in the model of this work was 96, so five models with different surfactant concentrations were constructed and the total number of SDS and C12A molecules was 48, 72, 96, 120 and 144, respectively. In addition, the corresponding Na+ ions were added to the solution as counter ions. In the models after introducing CO2, C12A molecule were protonated with a positive charge. In addition to Na+ ions, HCO 3 ions were also added to the solution phase as counter ions. Five models with different surfactant concentrations were also constructed, and the sizes of the model were the same as the models before introducing CO2.
2.2.2. Simulation methods For initial configurations, energies of models were first minimized by running 10,000 steps of optimization using conjugate gradient method to avoid the possible molecule overlapping, then a 4 ns MD simulations were performed based on the optimized configurations with a time step of 1 fs. All simulations were equilibrated at a constant volume and temperature (298 K). The temperature was modulated using the Nose thermostat. The van der Waals (vdW) interactions were calculated by the atom-based summation method with a cutoff distance of 1.20 nm, while the longrange electrostatic interactions were treated by the Ewald algorithm (Ewald, 1921). The data were collected every 5000 steps to obtain meaningful statistics, and the last 1 ns of trajectory data was used to analyze and evaluate the foam properties. In this work, we recorded the final configurations and dynamics process of the models to show the structural changes of the foam films before and after introducing CO2. Then the van der Waals energy, electrostatic energy and radial distribution function between SDS and C12A molecules were analyzed to investigate the interactions between the two surfactants. Finally, the potential energy, radial distribution function and mean square displacement between the SDS/C12A and water molecules were analyzed to study the hydrophilicity of the two surfactants.
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3. Results and discussion 3.1. Performance of CO2/N2 switchable aqueous foam
3.1.1. CO2/N2 switchability of C12A solution In order to clarify the CO2/N2 switchability of N,N-dimethyl-Ndodecyl amine (C12A), we first studied the conductivity change of C12A solution when CO2 and N2 were introduced alternatively. Fig. 2 shows the time-dependent conductivity of C12A solution. As shown in Fig. 2, the initial conductivity of 0.02 mol/L C12A solution was very low, about 45 lS/cm. This indicates that C12A is a neutral compound and its solution hardly contains almost no free anion and cation. However, when CO2 was introduced, the conductivity increased rapidly and reached a maximum after about 5 min, about 1360 lS/cm. This indicates that the tertiary amine group was rapidly protonated to a hydrogencarbonate, and a large amount of anion and cation were present in the solution. After that, the solution was heated to 60 °C and N2 was introduced into the solution. The conductivity of the solution was found to drop significantly, but the declining rate was lower than the rising rate when CO2 was introduced. After about 25 min, the conductivity of the solution was no longer falling down. However, it should be noted that the conductivity could not return to the original level (i.e., 45 lS/cm), but only drop to 145 lS/cm. This indicates that the hydrogencarbonate cannot be completely decomposed into neutral compounds under the condition of heating and introducing N2. But the reversible degree is high than 90%, so it does not affect its utilization as a CO2-responsive surfactant. A consistent law emerged when CO2 and N2 were alternately introduced into the same solution three times. This shows that C12A has a good CO2/N2 switchability and reversibility. Therefore, it is feasible to design CO2/N2 switchable aqueous foam on the basis of C12A. 3.1.2. Foam performance of C12A and SDS/C12A solution Although we have found that C12A solution has good CO2/N2 switchability, it is still necessary to evaluate its foam performance if we aim to use C12A solution to generate CO2/N2 switchable foam. As shown in Fig. 3, the foam performance of C12A solution was studied by introducing CO2 into 0.02 mol/L C12A aqueous solution to generate 150 mm foam column and measuring the timedependent foam column height and drainage rate of C12A foam.
Fig. 1. Initial configuration of the foam film models (a) before and (b) after introducing CO2.
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Fig. 2. Time-dependent conductivity of C12A solution when CO2 and N2 were introduced alternatively.
We can know from Fig. 3a that the foam column could reach 150 mm in 200 s, but it was completely shattered in 20 min. This indicates that although protonated C12A molecules have a certain foaming ability, the foam stability was poor. Moreover, the foam conductivity dropped to 20% of the initial value within 1 min (Fig. 3b), which demonstrates that the water carrying capacity of C12A foam was insufficient. Considering the poor stability and water carrying capacity of C12A foam, we believe that it could not be used alone as a good foaming agent. As a comparison, the foam performance of SDS/C12A solution was also studied by introducing N2 into 0.02 mol/L SDS/C12A solution to generate 150 mm foam column. From Fig. 3, we can know that the defoaming time of SDS/C12A foam exceeded 6000 s and the time for the conductivity dropping to 20% of the initial value also exceeded 400 s. This demonstrates that the stability and water carrying capacity of SDS/C12A foam were significantly enhanced compared with C12A foam. Moreover, the stability of the SDS/ C12A foam is comparable to that of other foam systems, such as cetyltrimethylammonium bromide and polysorbate 80 foams (Kumar and Mandal, 2017; Jones et al., 2016). Therefore, we suppose that the SDS/C12A solution can be used as a foaming agent to generate CO2/N2 switchable foam. Additionally, we found that the decay and the drainage rate were steady when the SDS/C12A solution concentration higher than 0.012 mol/L (Fig. S2). Therefore, we fixed the concentration of SDS/C12A solution at 0.012 mol/L in the following studies on the CO2/N2 switchability of SDS/C12A aqueous foam.
3.1.3. CO2/N2 switchability of SDS/C12A aqueous foam After the generation of foam column by blowing N2 into 0.012 mol/L SDS/C12A solution, CO2 was then introduced into the solution to study the CO2/N2 switchability of SDS/C12A aqueous foam. The time-dependent foam height is shown in Fig. 4. It can be seen that when CO2 was introduced into the foam column, the foam height continued to increase slowly at first. This is because C12A was not fully protonated and the SDS/C12A solution still had good foaming ability. However, when the time was about 1350 s, the height of the foam column began to drop. Furthermore, when the time was about 1850 s, foam collapsed suddenly, and the foam height dramatically dropped to about 110 mm. After that, residual foam continued to coalesce until it completely disappeared. In the whole CO2 introducing process, the color of SDS/C12A solution was found to change from light blue to milky white. This change may be caused by the protonation of C12A and the formation of hydrogencarbonate. It is thought that hydrogencarbonate can react with SDS by electrostatic interaction and form a complex (Zhang et al., 2016; Li et al., 2016). In addition, it can be seen in Fig. 4 that the surface tension of the SDS/C12A solution was significantly increased from 27.36 mN/m to 36.67 mN/m after defoaming by introducing CO2. This means that the surface activity of the solution was greatly reduced after the introduction of CO2. The changes of foam height, solution color and surface tension consistently confirm the pretty good CO2/N2 switchability of SDS/C12A foam. Compared with previous reported work (Zhu et al., 2014), the CO2/N2 switchable foam prepared in this work has a lower defoaming temperature (at room temperature) and a shorter defoaming time. In order to study the cyclability of CO2/N2 switchable aqueous foam, the SDS/C12A solution after complete defoaming was heated to 60 °C and N2 was again introduced into the solution for about 30 min. First, we found that the solution color changed back to light blue, which indicates that the SDS/C12A solution may return to its original state. Based on this observation, we performed foaming and defoaming experiments by introducing N2 and CO2 alternatively for 5 such cycles on the same SDS/C12A solution. The timedependent foam height is shown in Fig. 4b. It is obviously seen that the SDS/C12A solution still had good foaming performance after defoaming by introducing CO2. After introducing CO2, the foam height increased slowly at first and then decreased until it defoamed completely in each cycle experiment, and the time used for each cycle was also generally similar. After 4-cycle experiments, the height of foam column could still reach 150 mm within 400 s when N2 was introduced. However, the maximum height that foam can reach gradually decreased after introducing CO2 with increasing the number of cycles. This might be due to the
Fig. 3. Time-dependent (a) foam height (mm) and (b) drainage rate of C12A and SDS/ C12A foam.
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Fig. 4. Time-dependent foam height when CO2 and N2 were alternatively introduced into the foam (a) 1 cycle and (b) 5 cycles.
inevitable loss of the SDS/C12A solution during multiple cycles of experiments. Therefore, the foam produced by the SDS/C12A solution had good CO2/N2 switchability and cyclability. 3.2. CO2/N2 switching mechanism of SDS/C12A aqueous foam 3.2.1. Structural changes of the foam films Our experimental work discussed in Section 3.1 and some previously published work has shown that C12A could be protonated and form hydrogencarbonate (Zhang et al., 2016a, 2016b). However, it is still not clear that how the protonated C12A molecules interact with SDS molecules in the foam film, and how the structure of foam film change after introducing CO2. So a series of foam film models with different concentrations before and after introducing CO2 were simulated to investigate the CO2/N2 switching mechanism of SDS/C12A aqueous foam. The final configurations of SDS/C12A foam models before and after introducing CO2 are shown in Fig. 5. In the models before introducing CO2, C12A molecules were not protonated, all SDS and C12A molecules were distributed at the air-water interface. In the models after introducing CO2, C12A molecules were protonated to form hydrogencarbonate. Generally, a lot of SDS and C12A molecules were found to agglomerate in the water phase in the form of micelles regardless of the concentration of SDS and C12A. Compared with the models before introducing CO2, there were much fewer SDS and C12A molecules distributed at the airwater interface. For the foam film models after introducing CO2, the number and the proportion of SDS and C12A molecules at the air-water interface of the total of surfactant molecules were further calculated. As shown in Table 1, surfactant molecules at the airwater interface was reduced from 100% in the models before introducing CO2 to around 40% in the models after introducing CO2. In addition, in the models before introducing CO2, SDS molecules were orderly arranged at the air-water interface, with the head groups extending into the water phase, and the tail chains extending to the air phase. Most of the sodium ions were distributed around the SDS head groups, and the others were free in the liquid phase. C12A molecules were disorderly distributed at the air phase and entangled with the tail chains of the SDS molecules. Such an arrangement of surfactant molecules can stabilize the foam films, so the SDS/C12A foam before introducing CO2 has good stability. However, in the models after introducing CO2, only a small number of SDS and C12A molecules were adsorbed at the air-water interface, which are not enough to stabilize the foam films. Therefore, the SDS/C12A foam can be quickly defoamed after introducing CO2.
As can be seen from Table 1, the proportion of molecules at the air-water interface was the lowest when the total of surfactant molecules was 72. In other words, the SDS/C12A foam with this surfactant concentration realized the maximum degree of defoaming when CO2 was introduced, exhibiting the best CO2/N2 switchability. Therefore, in the following work, we focused on the foam film models before and after introducing CO2 at this surfactant concentration (72 surfactant molecules) to clarify the switchability mechanism of SDS/C12A aqueous foam. The dynamic process of the foam model containing 72 surfactant molecules were studied. As shown in Fig. 6, in both initial models before and after introducing CO2, SDS and C12A molecules were disorderly distributed in the water phase. In the early stages of the dynamic process (300 ps), most SDS and C12A molecules in both models had a tendency to aggregate in the water phase. As the dynamic process continued (800 ps), SDS and C12A molecules that accumulated in the water phase gradually moved toward the air-water interface of the models before introducing CO2. All SDS and C12A molecules were distributed at the air-water interface finally (1200 ps). However, in the models after introducing CO2, the SDS and C12A molecules that accumulated in the water phase remained in the water phase until the end of the dynamic process.
3.2.2. Interaction between SDS and C12A From the structure changes of the foam film models (Fig. 6), it can be seen that the accumulation of SDS and C12A molecules in the water phase is the main cause of defoaming of the foam system after introducing CO2. Therefore, the van der Waals (vdW) energy and the electrostatic energy between SDS and C12A molecules were calculated by molecular simulation method to clarify the reason for the aggregation of the surfactant molecules. As shown in Fig. 7, in the models before and after introducing CO2, the value of vdW energy between the two types of molecules was generally similar. This proves that the protonation of C12A had little effect on their van der Waals interaction. However, the electrostatic energies between SDS and C12A molecules in the models before and after introducing CO2 were apparently different. The electrostatic energy was close to zero in the models before introducing CO2, which indicates that there was a weak electrostatic interaction between the non-protonated C12A and SDS. In the models after introducing CO2, the value of electrostatic energy became negative and large. This indicates that there was a strong electrostatic attraction between protonated C12A and SDS molecules, and this electrostatic attraction is thereby thought to be the most important reason that SDS and C12A molecules agglomerated in the models after introducing CO2.
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Fig. 5. Final configurations of the foam film models in different concentrations.
Table 1 Number and proportion of SDS and C12A molecules at the air-water interface in the models after introducing CO2. The total of the surfactant molecules
48
72
96
120
144
The number of surfactants at air-water interface The proportion of surfactants at air-water interface
22 45.83%
22 30.56%
46 47.92%
53 44.17%
66 45.83%
Fig. 6. Structural changes of foam film models during the dynamic process.
Moreover, we obtained the radial distribution function (RDF) between head groups of SDS and head groups of C12A, as well as that between head groups of SDS and sodium ions in the models before and after introducing CO2, as shown in Fig. 7c. Before introducing CO2, there was no distinct peaks in the RDF between head groups of SDS and C12A, but a high peak in the RDF between SDS and sodium ions. However, after introducing CO2, there were two distinct peaks in the RDF between head groups of SDS and C12A, while
the peak of RDF between SDS and sodium ions became lower. This shows that before introducing CO2, a large number of sodium ions rather than C12A molecules were distributed around SDS head groups. On the contrary, after introducing CO2, there were more C12A molecules distributed around the head groups of SDS. This further proves that there was a strong attractive interaction between protonated C12A and SDS molecules in the models after introducing CO2, and they tended to attract each other to aggregate.
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Fig. 7. Simulation time dependent the (a) van der Waals energy and (b) electrostatic energy between SDS and C12A and radial distribution function of (c) C12A head groups and (d) sodium ions around SDS head groups in the models before and after introducing CO2.
3.2.3. Hydrophilicity and diffusion behavior of SDS/C12A surfactants In the models after introducing CO2, the SDS and C12A molecules can be aggregated as micelles, and it can be stably present in the water phase. In order to investigate the reason why the micelles were difficult to reach the air-water interface, the interaction between surfactant molecules and water molecules (Fig. 8), as well as the diffusion behavior of surfactant molecules in water (Fig. 9), were also analyzed. Fig. 8a shows the potential energy between surfactant molecules and water molecules. We could know that the attraction between surfactants and water was
enhanced in the models after introducing CO2. Moreover, from the RDF between the head groups of surfactants and water (Fig. 8b), more water molecules were distributed around the head groups of surfactants in the models after introducing CO2. This confirms that the hydrophilicity of SDS/C12A surfactants was improved after introducing CO2. We also analyzed the diffusion behavior of surfactant molecules in the foam systems though plotting their time-dependent mean square displacement (MSD) curve. Fig. 9 shows the mean square displacement (MSD) of SDS and C12A molecules in the dynamic
Fig. 8. (a) The potential energy and (b) the radial distribution function between the SDS/C12A and water molecules.
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and the reduction of diffusion rate of the SDS and C12A molecules made it difficult for the micelles to reach the air-water interface, most surfactant molecules accumulate in the water phase after introducing CO2. In the present study, we reported a CO2/N2 switchable aqueous foam that has good performance and switchability, it provides an alternative for solving the defoaming and demulsification difficulties in the process of foaming drainage gas recovery and foam flooding. In a word, we believe the switchable foam has usage potentials in the field of oil and gas exploitations. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 9. Mean square displacement of the SDS/C12A compounding surfactants.
process. It is well known that the slope of MSD curves is proportional to the diffusion rate of the molecules (Patterson and Lynden-Bell, 1998), so a bigger slop of MSD curve means a higher diffusion rate. Compared with the models before introducing CO2, the diffusion rate of SDS/C12A surfactants was significantly reduced in the models after introducing CO2. This makes it difficult for surfactant molecules to move towards the air-water interface, so they could stay in the water phase. Therefore, we believe that the enhancement of hydrophilicity and the reduction of diffusion rate result in the stable presence of the micelles in the water phase rather than reaching the air-water interface. In all, when CO2 was introduced into the foam stabilized by SDS/C12A surfactants, complex, i.e. the micelles, were formed by electrostatic attraction between protonated C12A and SDS molecules. In addition, the hydrophilicity of C12A and SDS molecules was improved and the diffusion rate in water phase was reduced. This causes the micelles can be stably present in the water phase and most surfactants cannot be absorbed at the air-water interface to stabilize foam films, which leads to the collapse of foam in a short period of time. 4. Conclusions Our work demonstrates that the mixture of two common surfactants (SDS and C12A) can be used to develop CO2/N2 switchable aqueous foam. The performance and CO2/N2 switching mechanism of this SDS/C12A aqueous foam were investigated by both experiments and molecular simulations. We found that the foam stabilized by the mixture of SDS and C12A was significantly more stable than the foam stabilized only by C12A. When the concentration of SDS and C12A surfactants in the foam solution was higher than 0.012 mol/L, the surfactant concentration had insignificant effect on the stability of the foam. We also found that stable foam could be formed by introducing N2, and the foam could be destroyed quickly when CO2 was introduced at room temperature. The stable foam could be formed again by heating the defoamed SDS/C12A solution to 60 °C and introducing N2. Such foaming/ defoaming could be repeated many times. In molecular simulations, all SDS and C12A molecules were absorbed at the air-water interface before introducing CO2, so the foam was stable. While most of SDS and C12A molecules aggregated in the water phase after introducing CO2, so most surfactant molecules were unable to stabilize foam films leading to the collapse of foam quickly. The main reason for this results was the electrostatic attraction between the protonated C12A and SDS after introducing CO2. In addition, the enhancement of hydrophilicity
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CO2/N2 switchable aqueous foam stabilized by SDS/C12A surfactants: Experimental and molecular simulation studies Shuangqing Sun a, Xiqiang Zhang a, Shengxiang Feng a, Hongbing Wang a, Yan Wang a, Jianhui Luo b, Chunling Li a,⇑, Songqing Hu a,⇑ a b
Schools of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, PR China Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
CO2/N2 switchable foam can be
formed using a mixed solution containing C12A and SDS with a fixed molar ratio of 1:1. Foam can be destroyed quickly after introducing CO2 at room temperature. Molecular dynamics simulation is used to investigate the CO2/N2 switching mechanism. Aggregation of SDS and C12A molecules in the water phase leads to rapid defoaming.
a r t i c l e
i n f o
Article history: Received 27 June 2019 Received in revised form 4 September 2019 Accepted 8 September 2019 Available online 9 September 2019 Keywords: CO2/N2 switchable Aqueous foam CO2-responsive surfactant Molecular simulation Electrostatic attraction Hydrophilicity
a b s t r a c t A novel CO2/N2 switchable aqueous foam was developed using a CO2-responsive surfactant N,N-dimethylN-dodecyl amine (C12A) and a conventional surfactant sodium dodecyl sulfate (SDS). The performance and mechanism of this switchable foam were studied by both experiment and molecular simulation methods. Experimental results demonstrate that the foam stabilized by SDS/C12A surfactants has better stability and water carrying capacity than single C12A foam. The foam could be rapidly switched between stable and unstable states at ambient temperature with CO2 or N2 as the triggers. Simulation results shows that when CO2 was introduced, most of SDS and C12A molecules aggregated in the water phase, instead of adsorbing at the air-water interface to stabilize the foam films. The strong electrostatic attraction between protonated C12A and SDS molecules could account for the aggregation. Due to the presence of limited surfactants located at the air-water interface after introducing CO2, the foam bursts quickly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Foam has been widely used in many fields, including foam dewatering gas recovery (Wu et al., 2016), enhanced oil recovery (Yang et al., 2017; Guo and Aryana, 2016), mineral flotation ⇑ Corresponding authors at: Schools of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, PR China. E-mail addresses: [email protected] (C. Li), [email protected] (S. Hu). https://doi.org/10.1016/j.ces.2019.115218 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.
(Huangfu et al., 2018), food industry (Patino et al., 2008) and firefighting (Hill et al., 2018), etc. However, in many applications, stable foam is only needed in some processes, on-demand and rapid burst of the stable foam after application is also required. For example, in the process of foam dewatering gas recovery, the foam carries the fluid from the bottom of the well to the ground (Wu et al., 2016). Then, a large amount of residual foam results in difficulties in post-processing, such as emulsifying gas condensate, tripping of separation equipment in gas/water separating
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plants, and creating high pressure in the pipeline, etc. (Heeres et al., 2016). The common method to remove residual foam is to use defoaming agents to break the foam (Garrett, 2016, 2015; Denkov, 2004). Unfortunately, defoaming agents would not only fail to avoid the formation of emulsion but also prevent the recovery and reuse of the foaming agent (Miller, 2008). Therefore, the formation of stable foams and on-demand removal of foam without changing the system composition are both required. To remove foam effectively and avoid the disadvantages of defoaming agents, stimuli-responsive aqueous foam becomes a great alternative (Fameau et al., 2015). The stability of foam generated by stimuli-responsive foaming agents can be controlled by external triggers including temperature (Fameau et al., 2011; Jochum and Theato, 2013; Fameau et al., 2013), light (Schnurbus et al., 2018; Lei et al., 2017; Chevallier et al., 2012), pH (Fujii et al., 2015; Binks et al., 2007), magnetic field (Lam et al., 2011) and CO2 (Wang et al., 2019; Wang et al., 2018; Zhu et al., 2014). Compared with other triggers, CO2 is regarded as a non-toxic, inexpensive and readily removable trigger (Zhang et al., 2014). Jessop et al. (Liu et al., 2006) first reported a CO2-responsive surfactant which is a long chain alkyl amidine. In the presence of CO2 and water, long chain alkyl amidine is protonated into charged amidinium bicarbonate with excellent surface activity. The protonated amidinium bicarbonate could be converted back to neutral alkyl amidine with low surface activity by blowing inert gas (Ar) at 65 °C to remove CO2. According to the principle of CO2-responsive surfactant, Zhu et al. (Zhu et al., 2014) prepared a CO2/N2 switchable particle-stabilized aqueous foam by mixing a CO2-responsive surfactant N0 -dodecylN,N-dimethylacetamidinium bicarbonate with bare silica nanoparticles. The bare nanoparticles can be hydrophobized in situ by N0 -d odecyl-N,N-dimethylacetamidinium bicarbonate to become surface-active nanoparticles and then the particle-stabilized aqueous foam can be formed. When N2 is introduced into the foam system at 65 °C for 80 min, the surfactants desorb from the surface of nanoparticles and the foam bursts. Bubbling CO2 at 0–5 °C for 50 min, the foam can be formed again. However, it is noticed that the defoaming of this CO2/N2 switchable particle-stabilized aqueous foam requires heating the entire foam system and the re-foaming must require CO2, which limits the practical application of this switchable foam. Moreover, the CO2/N2 switching mechanism of the aqueous foam is only speculated by experimental phenomenon. The research and analyses at molecular level are needed. Molecular dynamics (MD) simulation can reveal the structural and dynamical properties of foam films at molecular level (Wu et al., 2017; Wang et al., 2016, 2017, 2018; Gao et al., 2017), however, the study using molecular simulation method on switchable foam system has not been reported. In this context, a novel CO2/N2 switchable aqueous foam was prepared by mixing a CO2-responsive surfactant N,N-dimethylN-dodecyl amine (C12A) and a conventional surfactant sodium dodecyl sulfate (SDS). First, the foam performance of this SDS/C12A solution including foam stability and switchability was studied by foam experiments. Additionally, the CO2/N2 switching mechanism of this aqueous foam was also investigated by molecular dynamics simulations. The results obtained in this work will be beneficial to the design of CO2/N2 switchable aqueous foams for the convenient application, and further the understanding on the switchability mechanism of these foams. 2. Experiment and simulation details 2.1. Experiment section 2.1.1. Chemicals N,N-dimethyl-N-dodecyl amine (C12A, 97%) was purchased from Beijing J&K Scientific Co., Ltd. Sodium dodecyl sulfate (SDS,
95%) was purchased from Shanghai Macklin Biochemical Co., Ltd. n-Propanol (C3H7OH, 99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Oil red O (C26H24N4O, 99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Carbon dioxide gas (CO2, 99.8%) and Nitrogen gas (N2, 99.999%) was purchased from Qingdao Tianyuan Gas Manufacturing Co., Ltd. 2.1.2. Foam preparation and performance tests In this work, foams were generated using a foam analyzer (JPM2012, Shanghai Zhongchen digital technic apparatus Co., Ltd). Foaming, foam stability and instability are determined by measuring the height of foam or the volume of foam by optical lens. The drainage behavior of transparent samples can also be measured in this way. A typical liquid containing foam is used to record the drainage changes under gravity by measuring electrical conductivity. In addition, foam stability can be assessed from foam structure. A 0.02 mol/L C12A solution was first prepared by dissolving C12A in the solvent consisted of n-propanol and water with the equal volume to characterize the CO2 switchability of C12A by measuring the solution conductivity changes when CO2 and N2 were introduced alternatively. An aqueous solution containing both C12A and sodium dodecyl sulfate (SDS) with a fixed molar ratio of 1:1 was also prepared (SDS/C12A solution), as well as a 0.02 mol/L C12A aqueous solution (only C12A), to compare the foam performance of these two aqueous solutions. To further study the concentration of surfactants on the foam performance, a range of concentrations (0.001–0.024 mol/L) of SDS/C12A solution was prepared. After the preparation of C12A and SDS/C12A solutions, foams were generated by introducing CO2 or N2 with a flow rate of 0.6 L/ min into the foam solutions at room temperature. Defoaming of the SDS/C12A foam was carried out by introducing CO2 into the foam system from the bottom of the container with a flow rate of 0.4 L/min until the foam disappeared completely. Then N2 was introduced into the SDS/C12A solution after defoaming at 60 °C until the foam was generated again. The foam performances of two foam solutions were compared by analyzing their time-dependent foam conductivity, foam height, foam half-life, drainage half-life and foam drainage rate. The foam half-life is the time that the foam takes to decay to half of the initial value (Rudin, 1957). The drainage half-life is the time required for the liquid in the foam to decrease to the half of the initial value (Huang and Kinsella, 1987). The drainage rate is defined as the ratio of the real-time foam conductivity (r) to the initial foam conductivity (r0). In addition, we also analyzed the color and surface tension change of foam solution in the foaming and defoaming processes. In particular, the surface tension of the solution was measured by a JC2000C1 contact angle measuring instrument (Shanghai zhongchen digital technic apparatus Co., Ltd) using the pendant drop method (Berry et al., 2015; Fordham, 1948; Stauffer, 1965). Five-point method (Fig. S1) was used to calculate the surface tension. 2.2. Simulation section 2.2.1. Models construction In this work, model constructions, MD simulations and subsequent analyses were all performed using Materials Studio 8.0 software. Sandwich model with a water layer in the middle and two surfactant layers on both sides is widely used in molecular dynamics simulation studies of foams, especially in the studies on the airwater interface properties of foams (Li et al., 2016, 2013; Sun et al., 2015; Yan et al., 2011). To investigate the change of foam before and after introducing the external stimuli, i.e. CO2 in this work, two types of foam models were built. In the models before introducing CO2, the same number of SDS and C12A molecules were
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randomly distributed in the water layer to form a solution phase, and a vacuum layer was added to both sides of the solution phase (Fig. 1a). The size of the initial foam model is 5.0 5.0 13.2 nm3, the thickness of the solution phase and the vacuum layer is 3.2 nm and 5 nm, respectively. The number of water molecules in the solution phase is 2675. According to the occupied area of surfactant molecules at the air-water interface, the number of SDS and C12A molecules at the critical micelle concentration in the model of this work was 96, so five models with different surfactant concentrations were constructed and the total number of SDS and C12A molecules was 48, 72, 96, 120 and 144, respectively. In addition, the corresponding Na+ ions were added to the solution as counter ions. In the models after introducing CO2, C12A molecule were protonated with a positive charge. In addition to Na+ ions, HCO 3 ions were also added to the solution phase as counter ions. Five models with different surfactant concentrations were also constructed, and the sizes of the model were the same as the models before introducing CO2.
2.2.2. Simulation methods For initial configurations, energies of models were first minimized by running 10,000 steps of optimization using conjugate gradient method to avoid the possible molecule overlapping, then a 4 ns MD simulations were performed based on the optimized configurations with a time step of 1 fs. All simulations were equilibrated at a constant volume and temperature (298 K). The temperature was modulated using the Nose thermostat. The van der Waals (vdW) interactions were calculated by the atom-based summation method with a cutoff distance of 1.20 nm, while the longrange electrostatic interactions were treated by the Ewald algorithm (Ewald, 1921). The data were collected every 5000 steps to obtain meaningful statistics, and the last 1 ns of trajectory data was used to analyze and evaluate the foam properties. In this work, we recorded the final configurations and dynamics process of the models to show the structural changes of the foam films before and after introducing CO2. Then the van der Waals energy, electrostatic energy and radial distribution function between SDS and C12A molecules were analyzed to investigate the interactions between the two surfactants. Finally, the potential energy, radial distribution function and mean square displacement between the SDS/C12A and water molecules were analyzed to study the hydrophilicity of the two surfactants.
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3. Results and discussion 3.1. Performance of CO2/N2 switchable aqueous foam
3.1.1. CO2/N2 switchability of C12A solution In order to clarify the CO2/N2 switchability of N,N-dimethyl-Ndodecyl amine (C12A), we first studied the conductivity change of C12A solution when CO2 and N2 were introduced alternatively. Fig. 2 shows the time-dependent conductivity of C12A solution. As shown in Fig. 2, the initial conductivity of 0.02 mol/L C12A solution was very low, about 45 lS/cm. This indicates that C12A is a neutral compound and its solution hardly contains almost no free anion and cation. However, when CO2 was introduced, the conductivity increased rapidly and reached a maximum after about 5 min, about 1360 lS/cm. This indicates that the tertiary amine group was rapidly protonated to a hydrogencarbonate, and a large amount of anion and cation were present in the solution. After that, the solution was heated to 60 °C and N2 was introduced into the solution. The conductivity of the solution was found to drop significantly, but the declining rate was lower than the rising rate when CO2 was introduced. After about 25 min, the conductivity of the solution was no longer falling down. However, it should be noted that the conductivity could not return to the original level (i.e., 45 lS/cm), but only drop to 145 lS/cm. This indicates that the hydrogencarbonate cannot be completely decomposed into neutral compounds under the condition of heating and introducing N2. But the reversible degree is high than 90%, so it does not affect its utilization as a CO2-responsive surfactant. A consistent law emerged when CO2 and N2 were alternately introduced into the same solution three times. This shows that C12A has a good CO2/N2 switchability and reversibility. Therefore, it is feasible to design CO2/N2 switchable aqueous foam on the basis of C12A. 3.1.2. Foam performance of C12A and SDS/C12A solution Although we have found that C12A solution has good CO2/N2 switchability, it is still necessary to evaluate its foam performance if we aim to use C12A solution to generate CO2/N2 switchable foam. As shown in Fig. 3, the foam performance of C12A solution was studied by introducing CO2 into 0.02 mol/L C12A aqueous solution to generate 150 mm foam column and measuring the timedependent foam column height and drainage rate of C12A foam.
Fig. 1. Initial configuration of the foam film models (a) before and (b) after introducing CO2.
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Fig. 2. Time-dependent conductivity of C12A solution when CO2 and N2 were introduced alternatively.
We can know from Fig. 3a that the foam column could reach 150 mm in 200 s, but it was completely shattered in 20 min. This indicates that although protonated C12A molecules have a certain foaming ability, the foam stability was poor. Moreover, the foam conductivity dropped to 20% of the initial value within 1 min (Fig. 3b), which demonstrates that the water carrying capacity of C12A foam was insufficient. Considering the poor stability and water carrying capacity of C12A foam, we believe that it could not be used alone as a good foaming agent. As a comparison, the foam performance of SDS/C12A solution was also studied by introducing N2 into 0.02 mol/L SDS/C12A solution to generate 150 mm foam column. From Fig. 3, we can know that the defoaming time of SDS/C12A foam exceeded 6000 s and the time for the conductivity dropping to 20% of the initial value also exceeded 400 s. This demonstrates that the stability and water carrying capacity of SDS/C12A foam were significantly enhanced compared with C12A foam. Moreover, the stability of the SDS/ C12A foam is comparable to that of other foam systems, such as cetyltrimethylammonium bromide and polysorbate 80 foams (Kumar and Mandal, 2017; Jones et al., 2016). Therefore, we suppose that the SDS/C12A solution can be used as a foaming agent to generate CO2/N2 switchable foam. Additionally, we found that the decay and the drainage rate were steady when the SDS/C12A solution concentration higher than 0.012 mol/L (Fig. S2). Therefore, we fixed the concentration of SDS/C12A solution at 0.012 mol/L in the following studies on the CO2/N2 switchability of SDS/C12A aqueous foam.
3.1.3. CO2/N2 switchability of SDS/C12A aqueous foam After the generation of foam column by blowing N2 into 0.012 mol/L SDS/C12A solution, CO2 was then introduced into the solution to study the CO2/N2 switchability of SDS/C12A aqueous foam. The time-dependent foam height is shown in Fig. 4. It can be seen that when CO2 was introduced into the foam column, the foam height continued to increase slowly at first. This is because C12A was not fully protonated and the SDS/C12A solution still had good foaming ability. However, when the time was about 1350 s, the height of the foam column began to drop. Furthermore, when the time was about 1850 s, foam collapsed suddenly, and the foam height dramatically dropped to about 110 mm. After that, residual foam continued to coalesce until it completely disappeared. In the whole CO2 introducing process, the color of SDS/C12A solution was found to change from light blue to milky white. This change may be caused by the protonation of C12A and the formation of hydrogencarbonate. It is thought that hydrogencarbonate can react with SDS by electrostatic interaction and form a complex (Zhang et al., 2016; Li et al., 2016). In addition, it can be seen in Fig. 4 that the surface tension of the SDS/C12A solution was significantly increased from 27.36 mN/m to 36.67 mN/m after defoaming by introducing CO2. This means that the surface activity of the solution was greatly reduced after the introduction of CO2. The changes of foam height, solution color and surface tension consistently confirm the pretty good CO2/N2 switchability of SDS/C12A foam. Compared with previous reported work (Zhu et al., 2014), the CO2/N2 switchable foam prepared in this work has a lower defoaming temperature (at room temperature) and a shorter defoaming time. In order to study the cyclability of CO2/N2 switchable aqueous foam, the SDS/C12A solution after complete defoaming was heated to 60 °C and N2 was again introduced into the solution for about 30 min. First, we found that the solution color changed back to light blue, which indicates that the SDS/C12A solution may return to its original state. Based on this observation, we performed foaming and defoaming experiments by introducing N2 and CO2 alternatively for 5 such cycles on the same SDS/C12A solution. The timedependent foam height is shown in Fig. 4b. It is obviously seen that the SDS/C12A solution still had good foaming performance after defoaming by introducing CO2. After introducing CO2, the foam height increased slowly at first and then decreased until it defoamed completely in each cycle experiment, and the time used for each cycle was also generally similar. After 4-cycle experiments, the height of foam column could still reach 150 mm within 400 s when N2 was introduced. However, the maximum height that foam can reach gradually decreased after introducing CO2 with increasing the number of cycles. This might be due to the
Fig. 3. Time-dependent (a) foam height (mm) and (b) drainage rate of C12A and SDS/ C12A foam.
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Fig. 4. Time-dependent foam height when CO2 and N2 were alternatively introduced into the foam (a) 1 cycle and (b) 5 cycles.
inevitable loss of the SDS/C12A solution during multiple cycles of experiments. Therefore, the foam produced by the SDS/C12A solution had good CO2/N2 switchability and cyclability. 3.2. CO2/N2 switching mechanism of SDS/C12A aqueous foam 3.2.1. Structural changes of the foam films Our experimental work discussed in Section 3.1 and some previously published work has shown that C12A could be protonated and form hydrogencarbonate (Zhang et al., 2016a, 2016b). However, it is still not clear that how the protonated C12A molecules interact with SDS molecules in the foam film, and how the structure of foam film change after introducing CO2. So a series of foam film models with different concentrations before and after introducing CO2 were simulated to investigate the CO2/N2 switching mechanism of SDS/C12A aqueous foam. The final configurations of SDS/C12A foam models before and after introducing CO2 are shown in Fig. 5. In the models before introducing CO2, C12A molecules were not protonated, all SDS and C12A molecules were distributed at the air-water interface. In the models after introducing CO2, C12A molecules were protonated to form hydrogencarbonate. Generally, a lot of SDS and C12A molecules were found to agglomerate in the water phase in the form of micelles regardless of the concentration of SDS and C12A. Compared with the models before introducing CO2, there were much fewer SDS and C12A molecules distributed at the airwater interface. For the foam film models after introducing CO2, the number and the proportion of SDS and C12A molecules at the air-water interface of the total of surfactant molecules were further calculated. As shown in Table 1, surfactant molecules at the airwater interface was reduced from 100% in the models before introducing CO2 to around 40% in the models after introducing CO2. In addition, in the models before introducing CO2, SDS molecules were orderly arranged at the air-water interface, with the head groups extending into the water phase, and the tail chains extending to the air phase. Most of the sodium ions were distributed around the SDS head groups, and the others were free in the liquid phase. C12A molecules were disorderly distributed at the air phase and entangled with the tail chains of the SDS molecules. Such an arrangement of surfactant molecules can stabilize the foam films, so the SDS/C12A foam before introducing CO2 has good stability. However, in the models after introducing CO2, only a small number of SDS and C12A molecules were adsorbed at the air-water interface, which are not enough to stabilize the foam films. Therefore, the SDS/C12A foam can be quickly defoamed after introducing CO2.
As can be seen from Table 1, the proportion of molecules at the air-water interface was the lowest when the total of surfactant molecules was 72. In other words, the SDS/C12A foam with this surfactant concentration realized the maximum degree of defoaming when CO2 was introduced, exhibiting the best CO2/N2 switchability. Therefore, in the following work, we focused on the foam film models before and after introducing CO2 at this surfactant concentration (72 surfactant molecules) to clarify the switchability mechanism of SDS/C12A aqueous foam. The dynamic process of the foam model containing 72 surfactant molecules were studied. As shown in Fig. 6, in both initial models before and after introducing CO2, SDS and C12A molecules were disorderly distributed in the water phase. In the early stages of the dynamic process (300 ps), most SDS and C12A molecules in both models had a tendency to aggregate in the water phase. As the dynamic process continued (800 ps), SDS and C12A molecules that accumulated in the water phase gradually moved toward the air-water interface of the models before introducing CO2. All SDS and C12A molecules were distributed at the air-water interface finally (1200 ps). However, in the models after introducing CO2, the SDS and C12A molecules that accumulated in the water phase remained in the water phase until the end of the dynamic process.
3.2.2. Interaction between SDS and C12A From the structure changes of the foam film models (Fig. 6), it can be seen that the accumulation of SDS and C12A molecules in the water phase is the main cause of defoaming of the foam system after introducing CO2. Therefore, the van der Waals (vdW) energy and the electrostatic energy between SDS and C12A molecules were calculated by molecular simulation method to clarify the reason for the aggregation of the surfactant molecules. As shown in Fig. 7, in the models before and after introducing CO2, the value of vdW energy between the two types of molecules was generally similar. This proves that the protonation of C12A had little effect on their van der Waals interaction. However, the electrostatic energies between SDS and C12A molecules in the models before and after introducing CO2 were apparently different. The electrostatic energy was close to zero in the models before introducing CO2, which indicates that there was a weak electrostatic interaction between the non-protonated C12A and SDS. In the models after introducing CO2, the value of electrostatic energy became negative and large. This indicates that there was a strong electrostatic attraction between protonated C12A and SDS molecules, and this electrostatic attraction is thereby thought to be the most important reason that SDS and C12A molecules agglomerated in the models after introducing CO2.
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Fig. 5. Final configurations of the foam film models in different concentrations.
Table 1 Number and proportion of SDS and C12A molecules at the air-water interface in the models after introducing CO2. The total of the surfactant molecules
48
72
96
120
144
The number of surfactants at air-water interface The proportion of surfactants at air-water interface
22 45.83%
22 30.56%
46 47.92%
53 44.17%
66 45.83%
Fig. 6. Structural changes of foam film models during the dynamic process.
Moreover, we obtained the radial distribution function (RDF) between head groups of SDS and head groups of C12A, as well as that between head groups of SDS and sodium ions in the models before and after introducing CO2, as shown in Fig. 7c. Before introducing CO2, there was no distinct peaks in the RDF between head groups of SDS and C12A, but a high peak in the RDF between SDS and sodium ions. However, after introducing CO2, there were two distinct peaks in the RDF between head groups of SDS and C12A, while
the peak of RDF between SDS and sodium ions became lower. This shows that before introducing CO2, a large number of sodium ions rather than C12A molecules were distributed around SDS head groups. On the contrary, after introducing CO2, there were more C12A molecules distributed around the head groups of SDS. This further proves that there was a strong attractive interaction between protonated C12A and SDS molecules in the models after introducing CO2, and they tended to attract each other to aggregate.
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Fig. 7. Simulation time dependent the (a) van der Waals energy and (b) electrostatic energy between SDS and C12A and radial distribution function of (c) C12A head groups and (d) sodium ions around SDS head groups in the models before and after introducing CO2.
3.2.3. Hydrophilicity and diffusion behavior of SDS/C12A surfactants In the models after introducing CO2, the SDS and C12A molecules can be aggregated as micelles, and it can be stably present in the water phase. In order to investigate the reason why the micelles were difficult to reach the air-water interface, the interaction between surfactant molecules and water molecules (Fig. 8), as well as the diffusion behavior of surfactant molecules in water (Fig. 9), were also analyzed. Fig. 8a shows the potential energy between surfactant molecules and water molecules. We could know that the attraction between surfactants and water was
enhanced in the models after introducing CO2. Moreover, from the RDF between the head groups of surfactants and water (Fig. 8b), more water molecules were distributed around the head groups of surfactants in the models after introducing CO2. This confirms that the hydrophilicity of SDS/C12A surfactants was improved after introducing CO2. We also analyzed the diffusion behavior of surfactant molecules in the foam systems though plotting their time-dependent mean square displacement (MSD) curve. Fig. 9 shows the mean square displacement (MSD) of SDS and C12A molecules in the dynamic
Fig. 8. (a) The potential energy and (b) the radial distribution function between the SDS/C12A and water molecules.
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and the reduction of diffusion rate of the SDS and C12A molecules made it difficult for the micelles to reach the air-water interface, most surfactant molecules accumulate in the water phase after introducing CO2. In the present study, we reported a CO2/N2 switchable aqueous foam that has good performance and switchability, it provides an alternative for solving the defoaming and demulsification difficulties in the process of foaming drainage gas recovery and foam flooding. In a word, we believe the switchable foam has usage potentials in the field of oil and gas exploitations. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 9. Mean square displacement of the SDS/C12A compounding surfactants.
process. It is well known that the slope of MSD curves is proportional to the diffusion rate of the molecules (Patterson and Lynden-Bell, 1998), so a bigger slop of MSD curve means a higher diffusion rate. Compared with the models before introducing CO2, the diffusion rate of SDS/C12A surfactants was significantly reduced in the models after introducing CO2. This makes it difficult for surfactant molecules to move towards the air-water interface, so they could stay in the water phase. Therefore, we believe that the enhancement of hydrophilicity and the reduction of diffusion rate result in the stable presence of the micelles in the water phase rather than reaching the air-water interface. In all, when CO2 was introduced into the foam stabilized by SDS/C12A surfactants, complex, i.e. the micelles, were formed by electrostatic attraction between protonated C12A and SDS molecules. In addition, the hydrophilicity of C12A and SDS molecules was improved and the diffusion rate in water phase was reduced. This causes the micelles can be stably present in the water phase and most surfactants cannot be absorbed at the air-water interface to stabilize foam films, which leads to the collapse of foam in a short period of time. 4. Conclusions Our work demonstrates that the mixture of two common surfactants (SDS and C12A) can be used to develop CO2/N2 switchable aqueous foam. The performance and CO2/N2 switching mechanism of this SDS/C12A aqueous foam were investigated by both experiments and molecular simulations. We found that the foam stabilized by the mixture of SDS and C12A was significantly more stable than the foam stabilized only by C12A. When the concentration of SDS and C12A surfactants in the foam solution was higher than 0.012 mol/L, the surfactant concentration had insignificant effect on the stability of the foam. We also found that stable foam could be formed by introducing N2, and the foam could be destroyed quickly when CO2 was introduced at room temperature. The stable foam could be formed again by heating the defoamed SDS/C12A solution to 60 °C and introducing N2. Such foaming/ defoaming could be repeated many times. In molecular simulations, all SDS and C12A molecules were absorbed at the air-water interface before introducing CO2, so the foam was stable. While most of SDS and C12A molecules aggregated in the water phase after introducing CO2, so most surfactant molecules were unable to stabilize foam films leading to the collapse of foam quickly. The main reason for this results was the electrostatic attraction between the protonated C12A and SDS after introducing CO2. In addition, the enhancement of hydrophilicity
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