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Mechanism of Foam Film Destruction Induced by Emulsified Oil: A Coarse- Grained Simulation Study Hongbing Wang, Zhikun Wang, Qiang Lv, Chunling Li, Zehong Du, Shuangqing Sun, and Songqing Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08151 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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The Journal of Physical Chemistry

Mechanism of Foam Film Destruction Induced by Emulsified Oil: A Coarse-Grained Simulation Study Hongbing Wang a, Zhikun Wang a, Qiang Lv a, Chunling Li a, Zehong Du c, Shuangqing Sun *ab, Songqing Hu *ab a

School of Materials Science and Engineering, China University of Petroleum(East China), Qingdao

266580, P.R. China b

Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong (China

University of Petroleum), Qingdao 266580, P.R. China cSulige

Exploration and Development Branch Company of North China Petroleum Administration

Bureau, Erdos, 017300, P.R. China E-mail: [email protected] (SQ Sun) Tel number: +86 13964223177; E-mail: [email protected] (SQ Hu) Tel number: +86 0532 86983170

ABSTRACT Hydrophobic oil additives have been shown to be a unique class of chemicals in solution that affecting the stability of foam films. While the oil-induced destruction of a foam film has been largely understood, the entry process of oil droplet to the foam film remains vague in experiments. Here we perform mesoscale simulations to explore the mechanism of oil droplet entering the gas/water interface of a foam film composing of sodium dodecylsulfate (SDS). The microscopic entry process of oil droplet is dynamically predicted, and the driving force for this process and relevant variation of foam stability are interpreted by 1

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measuring the structural and energetic evolutions of the foam film. We demonstrate that the oil droplet entry process depends significantly on the frother concentration and the emulsification degree. We classify the entry process at different frother concentrations and emulsification degrees into three types. Our simulations suggest that the three types of the entry process follows the proposed “hole” mechanism, “attraction” mechanism and “budding” mechanism, respectively. In this context, the concentration of SDS yields considerable control over the entry behavior of the oil droplet into the gas/water interface and the creation of foam films with tunable stability.

1. INTRODUCTION Aqueous foams have been widely applied to many practical applications1-4 such as food, oil recovery, fire-fighting and personal care. Foam systems are thermodynamically unstable due to their high gas/water interfacial area, and there are many additional factors can decrease the foam stability5-9. Oil additives have been shown to be a unique class of chemicals in solution that affecting the stability of foam films10-14. Oil additives usually present as undissolved drops in solution and have the ability to enter the gas/water interface of foam films. The entry of oil drop into the foam film separates the water phase from the gas phase and therefore reduces the stability of the foam film. Wasan et al.15 first proposed a pseudoemulsion film indicating the thin film between the gas/water interface and the oil/water interface. The stability of the so-called pseudoemulsion film is a key issue in determining antifoam effectiveness too. 2


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After entering the foam film, the oil drop can spread over the gas/water interface. There have been numerous attempts revealing the relation between the foam stability and the spreading behavior of oil16-21. Underlying much of this work has been the perspective that spreading oil drops at gas/water surfaces produce shear forces and therefore lead to the rupture of foam films. Recent research on the mode of action of antifoams has been concerned with demonstrating the importance of the stability of the pseudoemulsion films, which separate antifoam oil drops from gas/water surfaces, and the necessity of oil spreading at that surface. However, it is also reported that spreading is not a necessary behavior of the oil that leads to the rupture of the foam film22. Classic entry and spreading coefficients23 that determine the probability of oils entering the and spreading at the interface, respectively, were proposed to understand the defoaming mechanism. Dalland et al.24 observed a stable foam of with negative entry coefficient. Andrianov et al.13 found that only very small values of the entry coefficient could be considered as a good indication of stable foam. The critical analysis of the available experimental results reported by Denkov and coworkers12, 25 showed that positive initial spreading coefficient, and high spreading rate could enhance significantly the antifoam activity 13,19. According to Bergeron et al.26, the classic entry coefficient is best for predicting foam stability in porous media, because it neglects much of the relevant physics of the porous medium and thin films. Unfortunately, in most cases, previous experimental results do not agree with this stability criterion. Lee et al.27 concluded that the spreading and entry coefficients are 3



unable to explain the effects of the type of oil (aliphatic or aromatic) added or the effects of the surfactant concentration on the rate of destabilization of foams. Mannhardt et al.28 found that there is no one-to-one correlation between the spreading or entry coefficient and foam stability in porous media. Overall, people’s understanding about whether the classic entry and spreading coefficients can predict foam stability is ambiguous. Another deficiency of previous experiments is that the microscopic movement process of oil droplet entering or spreading in foam films has never been described, leaving a large space for the existing defoaming mechanisms to be developed. Molecular dynamics (MD) simulation has been proved to be efficient in studying the structural and dynamical evolutions of foam film systems in limited time and length scales. Previous MD simulations mainly focus on the properties of foam films containing no additives29-33, while very few studies focus on the mixture of foam film and oil additives. Yuan et al. proposed an oil bridge-stretching mechanism for the rupture of foam films through MD simulations in order to provide supplements to the experiments at the molecular level34. They found that the bound water connecting the head groups of surfactants through strong hydrogen bonding interactions plays a vital role in the stability of the pseudoemulsion film. Steered molecular dynamics (SMD) was also carried out to investigate the destruction of the oil bridge, which is shown to be largely affected by the concentration and properties of oil molecules. The destruction of the pseudoemulsion film and the stretch of the oil bridge were achieved by adding vertical and horizontal forces to 4


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the foam film, respectively. However, as the rupture of foam film is a spontaneous process caused by its thermodynamic instability, SMD simulation is inappropriate for studying the stability of foam films under a specific condition. Here we explore the stability of sodium dodecylsulfate (SDS) foam films containing oil droplet through coarse-grained (CG) MD simulations. The effect of SDS concentration (at both the gas/water and oil/water interfaces) on the entering and spreading behaviors of the oil droplet in the foam film is investigated. Equilibrium structures of the foam film are monitored to show the variation of foam stability. The dynamical evolution of foam films and the interaction energy between frothers (SDS at the gas/water interface) and emulsifiers (SDS at the oil/water interface) are analyzed to show the micro-process of oil entering the gas/water interface. This study will further our understanding on the control of foam stability in various technological processes.

2. SIMULATION DETAILS 2.1 Force field parameterization In the present study, model constructions, MD simulations and subsequent analyses were all performed using Materials Studio 8.0 software. Due to the limited approachable time and length scales of all-atom MD simulations, Martini35-36 CG MD simulations is used to study the stability of foam films containing oil droplet. In Martini force field, four main types of interaction sites, i.e., polar (P), nonpolar (N), apolar (C) and charged (Q), are 5



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considered. For more accurate representations of chemical structures, each interaction site has a number of subtypes, which are distinguished by a letter denoting the hydrogenbonding capabilities (d = donor, a = acceptor, da = both, 0 = none), or by a number indicating the degree of polarity (from 1, low polarity, to 5, high polarity). The mapping method and definition of bead types for the Martini CG model of SDS are verified by Jalili et al.37. The hydrophobic tail of SDS molecule is represented by three C1 beads (each representing four -CH3/-CH2 groups) and the hydrophilic head group (-SO4) is defined as one Qa bead (charge = -1). One hydrated sodium ion with approximately two water molecules in the first solvation shell are represented by a Qd bead (charge = -1). C12 molecule is represenjiangted by three C1 beads like the tail of the SDS molecule. Four water molecules are mapped into one P4 bead. Figure 1a illustrates the chemical structures and mapping method of SDS, C12 and water molecules.

(a)

(b)

Figure 1. (a) Mapping between atom structure and CG beads for SDS, C12, and water in Martini CG force field. (b) Initial configuration of the foam film model. 6


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The inter- and intra- molecular interactions of beads in Martini force field can be described as36: 1

[( ) ― ( ) ] +

1

V = 2𝑘𝑏(𝑙 ― 𝑙0)2 + 2𝑘𝜃[cos (𝜃) ― cos (𝜃0)]2 +4𝜀

𝜎 12

𝜎 6

𝑟𝑖𝑗

𝑟𝑖𝑗

𝑞𝑖𝑞𝑗 4𝜋𝜀0𝜀𝑟𝑟𝑖𝑗

(2.1)

where the four components indicates the bond stretching, angle bending, van der Waals and columbic interactions, respectively. The bonds are described by the weak harmonic potential with equilibrium bond length 𝑙0 = 0.47 nm and force constant 𝑘𝑏 = 1250 kJ ∙ mol-1 ∙ nm-2. The angles are described by the cosine harmonic potential with equilibrium angle 𝜃0 = 180° and force constant 𝑘𝜃 = 25 kJ ∙ mol-1. All bonds and angles in our systems have the same setups as mentioned above. The van der Waals interactions are described by the Lennard-Jones (L-J) 12-6 potential energy function, and the Columbic interactions for charged beads (Qd = +1 and Qa = -1) are represented by the shifted Columbic potential energy function with relative dielectric constant 𝜀𝑟 = 15 for explicit screening36. The L-J parameters for beads and interaction pairs involved in our systems are listed in Table 1 for convenience. Table 1. Non-bonded interaction parameters

7



The van der Waals interactions are calculated by the bead-based summation method with a cutoff distance of 12.5 Å, and the electrostatic interactions are treated by Ewald summation method38. The initial configurations (Figure 1b) is optimized for 10000 iterations by the steepest decent and the conjugate gradient method successively. After the optimization, MD simulations are operated at constant volume and temperature (NVT). The Nose thermostat is applied to control the simulation temperature at 298 K. A fixed time step of 10 fs is used, and data is collected every 5000 steps.

2.2 Simulation setup A series of foam film models were constructed to investigate the entry process of the emulsified oil droplet. The initial configuration of the SDS foam films (Figure 1b) consists of the vacuum layer, the SDS adsorption monolayer, and a water box containing an 8


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emulsified oil droplet and sodium ions. Periodic boundary conditions are applied to all three directions of the box. To avoid bead travelling across the box boundary in z direction, an energy barrier consisting of constrained water molecules is introduced. For the SDS adsorption monolayer in all of our simulation models, the sectional area is 57.76 nm2. Five numbers of frothers (NF, SDS at the gas/water interface) 64, 81, 100, 121 and 144 are considered in our work, corresponding to the occupied area of a single SDS molecule of 0.925, 0.713, 0.578, 0.478 and 0.401 nm2, respectively. The occupied area of a single SDS molecule is about 0.53 nm2 at the critical micelle concentration (CMC)39. Therefore, three levels of frother concentration are classified in our simulations: low (less than CMC, NF = 64 or 81), medium (close to CMC, NF = 100 or 121) and high (larger than CMC, NF = 144). The emulsified oil consists of two parts, one is the oil droplet which contains 30 dodecane molecules and the radius is about 1.5 nm, the other is a certain amount (0-50) of SDS around the oil droplet (emulsifiers). Finally, a corresponding number of sodium beads are introduced to keep the simulation system electrically neutral. For initial configurations, energies are first optimized by running 100,000 steps optimization to avoid possible molecule overlapping. Then, MD simulations are performed based on the optimized configuration. All simulations are equilibrated at a constant volume and temperature (298 K) for 100 ns.

3. RESULTS AND DISCUSSION

9



Figure 2. Equilibrium configurations of the foam film with various frother concentrations and emulsification degrees. NF and NE indicate the number of frothers and emulsifiers, respectively. Coloring scheme: water, blue; sodium ion, pink; oil, black; charged head of SDS, red; hydrophobic tail of SDS, tawny.

The equilibrium structures of the foam film with various number of frother (NF, SDS at the gas/water interface) and emulsifier (NE, SDS around the oil droplet) molecules are shown in Figure 2. According to its location and shape, the oil phase exhibits three typical states: (1) spreading at the air/water interface evenly (green frame line), (2) staying as a droplet in the water (black frame line), (3) partly spreading at the air/water interface and partly staying in a “bud-like” structure 40-41 (red frame line). It is assumed that the spreading of oil molecules at the air/water interface (states (1) and (3)) leads to large decrease of the 10


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integrity and stability of the foam film by squeezing the water molecules away42-43, while oil molecules staying in the water phase (state (2)) have no obviously negative influence on the foam film23. Therefore, it is clear that the stability of foam films contain emulsified oil depends to a great extent on the value of NF and NE. The general rule is foam film will be more stable with a larger NF or NE, except for the foam film with a high frother concentration and a relatively high emulsification degree (NF = 144 and NE ≥ 30, red frame line). This paper mainly focuses on the microcosmic entry process of oil droplet into the gas/water interface of foam film. Correspondingly, three entry mechanisms (the “hole”, “attraction” and “bud” mechanisms) are proposed in the present work to pursue a better understanding on the three different defoaming processes.

3.1 The “hole” mechanism

Figure 3. Morphological evolutions of the foam film and components as a function of time (NF = 64, NE = 20). (a) Side view of the whole foam film. (b) Top view of the surface density map of the frothers monolayer (only oil is displayed).

Figure 3a shows the morphological evolution of the foam film as a function of time at low frother concentration and relatively low emulsification degree (NF = 64, NE = 20). The oil 11



phase lie evenly at the gas/water interface after equilibrium, which means foam film is unstable. In order to obtain the intuitive structure and location of frother layer and oil droplet, the density distribution maps of frothers on the cross section as time changes are shown in Figure 3b. During the first 11 ns, oil droplet moves gradually from water to the gas/water interface and with no obvious change in shape. It is clear that a “hole” (the blank area without frother covered) is formed and the oil phase can be observed through the “hole” without any difficult before it enter the interface. Starting from 11.5 ns, dodecane molecules gradually enter and spread out at the gas/water interface, which is followed by the blend process of emulsifiers into frothers. Finally, the foam film configuration reaches the equilibrium state at about 15 ns, and the structure remains essentially the same even the simulation time is extended to 100 ns. It is considered a decisive factor of the entry process of oil droplet is the Coulomb repulsion between frothers and emulsifiers. Figure 4 shows the initial and final electrostatic energy (EI and EF) of the entry process between frothers and emulsifiers in foam film with various NF and NE. The positive value represent there is repulsive force between the frothers and emulsifiers. It can be seen from the diagram that the final electrostatic energy are greater than the initial one, which is mainly caused by the fusion of emulsifiers and frothers after the entry progress of emulsified oil (NF = 64, NE ≤ 50 and NF = 81, NE ≤ 40). However, the electrostatic energy makes no obvious difference as time changes when NF = 81 and NE = 50, which is because the oil droplet failed to enter the gas/water interface. And the initial electrostatic energy under this 12


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condition (1255.2 kcal·mol-1) is much higher than others (< 1050 kcal·mol-1). Thus, it is rational to consider that the initial electrostatic energy plays a decisive role on the entry process of oil droplet: a high initial elctrostatic energy which represents a strong rejection between emulsifiers and frothers will increase the difficulty of oil droplet to enter the gas/water interface. In addition, the initial electrostatic energy mainly influenced by the number of surfactants, it increase with the increase of frother (for example, EI (64, 10) = 257.5 kcal·mol-1 < EI (81, 10) = 585.7 kcal·mol-1) and emulsifier numbers (from 257.5 kcal·mol-1 to 1049.2 kcal·mol-1, when NF = 64 and 10 ≤ NE ≤ 50). Therefore, we can improve the oil resistance of foam by increase surfactant concentration in an appropriate range.

Figure 4. Initial electrostatic energy (EI) and final electrostatic energy (EF) of the entry process between frothers and emulsifiers in foam film with various NF and NE: (a) NF = 64, (b) NF = 81.

Figure 5. The “hole” mechanism of emulsified oil entry to the gas/water interface 13



The “hole” mechanism is proposed to illustrate the process of oil entry into the gas/water interface at a low frother concentration and a relatively low emulsification (NF = 64, NE ≤ 50 and NF = 81, NE ≤ 40), which is shown in Figure 5. At the beginning, frothers will be gathered together and leave a “hole” on the interface. It is assumed that the surfactant aggregation is most likely taken place in the saline systems44. We have also carried out another simulation work in which SDS are replaced by C12E5 (pentaethylene glycol monododecyl ether, a nonionic surfactant) equally and found that frothers dispersed homogeneously in the gas/water interface. It demonstrates that the presence or absence of ions is the deciding factor of surfactant aggregation. The configuration diagram of C12E5 foam film are shown in Figure S1 in the support information.

Thus, in the present work, we think the reason of aggregation is that sodium ions (counterion of SDS) has shielded the rejection between frothers (Figure 5a). After that, the emulsified oil droplet moves to the gas/water interface driven by the hydrophobic interaction (Figure 5b). And it prefer to enter the interface through the “hole” where emulsifiers suffer a weaker Coulomb repulsive force from frothers. In this process, emulsifiers enter the interface after the dodecane molecules due to its amphipathy (Figure 5c). Finally, the dodecane molecules spread out along the hydrophobic tail of surfactants and the emulsifiers mingled with frothers together (Figure 5d).

14


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3.2 The “attraction” mechanism

Figure 6. Morphological evolutions of the foam film and components as a function of time (NF = 100, NE =20). (a) Foam film. (b) Surface density map of the frothers monolayer (only oil droplet is displayed)

Figure 6a shows the morphological evolution of the foam film as a function of time at medium frother concentration and relatively low emulsification degree (NF = 100, NE = 20). Similar to the system with NF = 64 and NE = 20 (Figure 3), the oil droplet enters the gas/water interface and spreads out gradually with the increase of simulation time. However, there are two main differences before oil droplet enter the interface (0-1 ns): (1) frothers overlaid the whole surface, there is no “hole” formed; (2) some frothers sink into the water phase and contact with the emulsified oil (highlight in green). Begun at 1.5ns, oil phase come into contact with the interface, and then dodecane molecules gradually enter and spread out at the gas/water interface followed by the blend process of emulsifiers into frothers. At the medium concentration (NF = 100 or 121), the initial occupancy areas of per SDS molecule at the gas/water interface are 57.76 and 47.74 Å2 respectively. According to Milton J. et al38, they are very close to the 𝑎𝑠𝑚value (53 Å2, area per molecule at the interface at surface saturation) of SDS at room temperature. That is to say, the number of 15



SDS at the interface is almost saturated when NF = 100 or 121, even if considering the effect of counterions. Therefore, there is no “hole” formed before oil droplet enter the interface. The blank area shown in Figure 6b can be ignored because it is forced by the movement of oil. At 0.6 ns, we can observe an interesting oil droplet of which only the lower half is covered by emulsifiers. By contrast, the emulsifiers distribute evenly around the oil droplet before it reaches the interface (10 ns) in Figure 3a. In order to reveal the configurational variation of the oil droplet at different frother concentrations (NF = 64, 81, 100 and 121), the distance evolution between the centroid of oil molecules and the centroid of emulsifiers is monitored (Figure 7). It can be seen that the values of centroid distance at a smaller NE (10, 20) in Figure 7c and Figure 7d (the black and red curves) make great difference from others. It experienced a process that increased firstly and the then dropped, and the maximum values are 1.63 nm, 1.64 nm, 1.72 nm and 2.14 nm respectively. By contrast, the centroids distance variations in other curves can be ignored because almost all of the maximum values are less than 0.5 nm. Firstly, at low frother concentration, there is no apparent relative displacement between dodecane and emulsifier molecules, it indicates that the integrity of the monolayer affect the form of oil droplets. The emulsifiers suffer a far stronger repulsive force from an intact monolayer than the incomplete monolayers by the reason oil droplet will move to a position below the blank area. Secondly, at medium frother concentration (NF = 100 and 121) and a relatively high emulsification degree (NE ≥ 20), there is also no obvious relative displacement between dodecane and emulsifier 16


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molecules. The reason is the movement of emulsifiers were restricted by the strong repulsive interactions between themselves when there is a sufficient number of emulsifiers. At 1 ns, some frothers sink into the water phase drived by the attraction of bared oil droplet as shown in Figure 6a (highlight in green). The sunk frothers formed a bridge linking oil droplet and the gas/water interface, which is conducive to the entry process of oil droplet. An evidence is that the start times of the entry process at medium concentrations are much earlier than that at low concentrations, which is shown in Table 2. It can be seen that at a low frother concentration (NF = 64 and 81), the start time of the entry process are much bigger (about 10 times) than that at a medium concentration (NF = 100 and 121), except for when there is no emulsifiers. Although there is no inevitable connection between the stability of foam film and the start time of the entry process as a result of the uncertainty of initial configuration and the dynamic process itself, the large gaps between the start times at different frother concentrations can still prove that the entry difficulty is reduced at a medium frother concentration.

Figure 7. Evolutions of the centroid distance between oil droplet and emulsifiers during the entry process 17



Table 2. Start time of the entry process

Figure 8. The “attraction” mechanism of emulsified oil entry to the gas/water interface

Similarly, the “attraction” mechanism is proposed to illustrate the process of oil entry into the gas/water interface at a medium frother concentration and a relatively low emulsification (NF = 100 or 121, NE ≤ 20), which is shown in Figure 8. At the beginning, different from the low concentration, the frothers arrange neatly in the gas/water interface at the medium concentration. Some of the emulsifiers will suffer a strong repulsion force from the frothers monolayer when the oil droplet is closed to the gas/water interface driven by the hydrophobic interaction. As a result, the emulsifiers at the top of the droplet move down along the oil/water interface and leave a baldheaded oil droplet. Then, a small fraction of frothers sink into water driven by the attraction of the uncovered oil phase and a bridge connecting the frother monolayer and the emulsified oil droplet is formed. With the help of the bridge, dodecane molecules diffuse along the hydrophobic internal surface 18


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rapidly. Soon afterwards, all of the SDS molecules enter the gas/water interface. However, at a relatively high emulsification (NE ≥ 20), it's hard for the emulsifiers to move along the surface of the oil droplet because there is a limit to the capacity of SDS on the oil/water interface. Therefore, the emulsified oil droplet can’t enter the interface due to the strong repulsion between frothers and emulsifiers.

3.3 The “bud” mechanism

Figure 9. Morphological evolutions of the foam film and components as a function of time (NF = 144, NE =30). (a) Foam film. (b) Surface density map of the frothers monolayer (only oil droplet is displayed).

An abnormal phenomenon (Figure 2) is observed at high frother concentration (NF = 144). The emulsified oil droplet can’t enter the gas/water interface at a relatively low emulsification (NE ≤ 20) while part of the dodecane molecules enter and spread out on the interface at a relatively low emulsification (NE ≥ 30), which is inconsistent with the disciplines at low and medium frother concentrations. Figure 9a shows the morphological evolution of the foam film as a function of time (NF = 144, NE = 30). There are a large 19



amount of frothers sink into water phase and come into contact with the oil droplet before 0.4 ns. As shown in Figure 10a, another simulation work which has oil phase but has the same NF and NE (surfactants in the micelle) is carried out. We find that there are a large amount of frothers sink into water phase and come into contact with the micelle. It proves that, unlike the medium frother concentration, the submergence of frothers is mainly caused by the strong repulsive interaction between the frothers, although the upper part of the emulsified oil droplet is revealed in this system. After that, a bud-like structure that exist until the last frame in our work is formed by the sunk frothers and emulsifiers. And finally, some of the dodecane molecules spread out to the gas/water interface along the hydrophobic tail of SDS, and the others reserve inside the “bud”. The reason why the budlike structure has not disappeared after the dynamic equilibrium is considered to be influenced by two factors, surfactant concentration and oil phase. For convenience, we record the foam film system which is in discussing as System 1. As a contrast, at medium frother concentration (System 2), there are a diminutive “bud” is formed and disappears within just 2 ns (Figure 6a). Similarly, it can be seen from Figure 11 that for the foam film at high frother concentration but without oil droplet (System 3), there are also a short-life “bud” is formed (about 4ns). As shown in Figure 10c, for the three systems just mentioned, we calculated the mean square displacement (MSD) of SDS molecules at the shoulders of the “bud” for the reason the surface density of SDS in this region is quite high (Figure 9b). The slopes of the three MSD curves increase in the order System1, System2 and System3, 20


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and the diffusion coefficients are 1.58×10-10 m2·s-1, 2.83×10-10 m2·s-1 and 3.02×10-10 m2·s-1 respectively. It means that the movement of the SDS molecules in the shoulder of the “bud” is limited by the strong repulsive force between the charged headgroups at high frother concentration and the attraction between oil phase and the SDS tails caused by the hydrophobic interactions. While at a relatively low emulsification degree (NE < 30), as shown in Figure 10b (NF = 144, NE = 0), the oil droplet has not entered the gas/water interface and there are no bud-like structure is formed. The main reason is that the emulsifiers around the oil droplet is far from saturated so most of the sunk frothers are absorbed on the oil/water interface. In other words, a number of frothers turn into emulsifiers. In the example shown in Figure 10b, after the equilibrium, the NF and NE become 116 and 28 respectively. Thus, it has transformed into a system with medium frother concentration and relatively high emulsification degree from a high frother concentration and low emulsification degree system. The electrostatic interaction energy between the rest frothers and the new emulsifiers becomes 1315 kcal·mol-1, which large enough to prevent the oil droplet from approaching the gas/water interface.

Figure 10. (a) Evolution of the foam film without emulsified oil droplets (NF = 144, Ne = 28); (b) Evolution of the foam film when NF = 144 and NE = 0; (c) 21



Mean square displacement of SDS molecules at the shoulder of the “bud”.

Figure 11. The “bud” mechanism of emulsified oil entry to the gas/water interface

As shown in Figure 11, the “bud” mechanism is proposed in this section to illustrate the process of oil entry into the gas/water interface at a high frother concentration and a relatively high emulsification (NF = 144, NE ≥ 30). Initially, there is strong Coulomb repulsion between the charged headgroups of SDS as a result of the high frother concentration. It drives the excessive frothers to sink into the water phase and interact with the oil droplet. Under the influence of hydrophobicity, the sunk surfactants contact with the oil phase and make up a “bud” with emulsifiers. Part of the oil molecules can enter the gas/water interface by spreading out along the hydrophobic tails of SDS. However, the bud can exist continuously for the reason the strong repulsive force between SDS molecules in the shoulder of the “bud” and the attraction of them suffered from oil phase has restricted their movement. Thus, there are still a large amount oil is left in the “bud”.

4. CONCLUSIONS In the present study, the coarse-grained molecular dynamics simulation is employed to study the microscopic process of oil droplet entering the gas/water interface of SDS foam 22


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film. The frother concentrations and emulsification degrees make a great difference to the entry process of oil droplet. At low frother concentration and relatively low emulsification degree (NF = 64, NE ≤ 50 and NF = 81, NE ≤ 40), the oil droplet tends to enter the gas/water interface through the “hole” which is formed by the influence of counterions. In this process, the Coulomb repulsion between frothers and emulsifiers is considered to be a decisive factor. At medium frother concentration and relatively low emulsification degree (NF = 100 or 121, NE ≤ 20), the emulsifiers move down along the oil/water interface driven by the electrostatic interaction of frothers, then a spot of frothers sink into water due to the attraction of the uncovered oil droplet. After that, the emulsified oil droplet can enter the interface easily with the help of the sunk frothers. The attraction between frothers and the uncovered oil phase caused by the hydrophobic interaction has played an important role during this process. While at high frother concentration and relatively high emulsification degree (NF = 144, NE ≥ 30), lots of frothers sink into the water phase due to the strong repulsion, and a bud-like structure is formed by the sunk SDS and the oil droplet after they contact. Part of the oil molecules can enter the gas/water interface but the others tends to be left in the “bud” because this structure is quite longevous. In summary, we have concluded that there are three entry mechanisms of foam film induced by emulsified oil: the “hole”, the “attraction” and the “bud”, respectively. It shows that whether the oil droplet enter the gas/water interface or not and the difficulty of entry change a lot at different frother concentration and emulsification degree. Further more, there are no obviously 23



positive correlation between the stability of foam with emulsified oil and the surfactant concentration.

ASSOCIATED CONTENT Supporting Information Description Morphological evolutions of the C12E5 (pentaethylene glycol monododecyl ether) foam film models (Figure S1).

ACKNOWLEDGEMENTS This work is supported by the “National Natural Science Foundation of China” (51874331), the “Natural Science Foundation of Shandong Province” (ZR2017MEE028), the “Fundamental Research Funds for the Central Universities” (16CX05017A and 17CX05023), the “Petro China Innovation Foundation” (2016D-5007-0206 and 2018D5007-0213), the “China National Petroleum & gas Corporation science and technology development project” (2014A-1001) and the “Study on supporting technology for stable and stimulated production in Sulige gas field” (2016-HB-Z06).

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