*2.1. Dissipative Particle Dynamics Theory*

Dissipative particle dynamics (DPD) was firstly reported by Koelman and Hoogerbrugge as an efficient mesoscopic-level simulation method [36]. Several atoms or molecules are represented by beads that interact with each other via effective pair potentials. To simplify the calculations, the beads have the same mass, length, and time scales, in which the mass of the beads equals to 1 DPD unit. Every two beads *i* and *j* in a system interact with each other by the following formula from Groot [37]:

$$f\_{\rm ij} = F\_{\rm ij}^{\mathbb{C}}(r\_{\rm ij}) + F\_{\rm ij}^{R}(r\_{\rm ij}) + F\_{\rm ij}^{D}(r\_{\rm ij}) \tag{1}$$

where *Fij <sup>C</sup>*, *Fij <sup>R</sup>* and *Fij <sup>D</sup>* denote a conservative force, a random force and a dissipative force, respectively. In which, *Fij <sup>C</sup>* contains a harmonic spring force (*Fij Cr*) and a soft repulsion force (*Fij Cs*), which are given by

$$\begin{array}{l} F\_{i\bar{j}}^{\rm Cr} = \mathfrak{a}\_{i\bar{j}} \left( 1 - \frac{r\_{i\bar{j}}}{r\_c} \right) \stackrel{\scriptstyle r\_{i\bar{j}}}{r\_{i\bar{j}}} (r\_{i\bar{j}} < r\_c) \\ = \mathfrak{d}(r\_{i\bar{j}} > r\_c) \end{array} \tag{2}$$

and

$$F\_{ij}^{\mathbb{C}\_S} = -\mathbb{C} \cdot \overset{\wedge}{r}\_{ij} \tag{3}$$

Among them, *αij* is the maximum repulsion parameters between particles *i* and *j*, ∧ *rij* = ∧ *r<sup>i</sup>* − ∧ *rj* , *rij* = |<sup>∧</sup> *rij*|, *rij* is the distance between *i* and *j*, with the corresponding unit vector ∧ *rij*, *r<sup>c</sup>* is a cutoff radius which provides the extent of the interaction range and *C* is the spring constant. Moreover, the random force (*F R ij* ) and the dissipative force (*F D ij* ) can be shown by the following equations from Groot [37]:

$$F\_{\rm ij}^{\mathbb{R}} = \sigma \omega^{\mathbb{R}}(r\_{\rm ij}) \theta\_{\rm ij} \overset{\wedge}{r}\_{\rm ij} \tag{4}$$

$$F\_{\rm ij}^D = -\eta \omega^D \left( r\_{\rm ij} \right) \left( r\_{\rm ij} \cdot \nu\_{\rm ij} \right) \theta\_{\rm ij} \stackrel{\wedge}{r}\_{\rm ij} \tag{5}$$

Here, *θij* is the random fluctuation variable between 0 and 1, *vij* represents the relative velocities of the beads, and *ω* is the weight function. Furthermore, *h* is the friction coefficient and *s* is the noise amplitude, and *σ* <sup>2</sup> = 2*ηkBT*. To sample the canonical ensemble distribution, *s*, *h* and *αij* determine the amplitude of the dissipative, conservative and random forces [44].

*ω<sup>D</sup>* = (*ωR*) <sup>2</sup> was made to comply with the fluctuation-dissipation theorem, and the temperature follows from the relation between *h* and *s*. The same parameters, weight functions, and integration algorithm were used from Groot and Warren [37]:

$$
\omega^{\mathbb{C}}(r\_{i\bar{j}}) = \omega^{\mathbb{R}}(r\_{i\bar{j}}) = \sqrt{\omega^{D}(r\_{i\bar{j}})} = \omega(r\_{i\bar{j}}) \tag{6}
$$

where

$$\omega(r\_{ij}) = \begin{cases} 1 - \frac{r}{R\_{\mathbb{C}}}(r < R\_{\mathbb{C}}) \\ \quad 0(r \ge R\_{\mathbb{C}}) \end{cases} \tag{7}$$

A modified version of the velocity verlet algorithm is adopted in the Newton's equations of motion and the reduced units are used in our paper. Cutoff radius *Rc*, kB*T* and m of the particles are used as the unit of length, energy and mass, respectively. Here, kB*T* represents the micro temperature, in which k<sup>B</sup> is boltzmann constant and T the thermodynamic temperature. *h* = 4.5 and *s* = 3 are set in our research.

### *2.2. Models and Interaction Parameters*

Water, oil of n-hexane and surfactant of sodium lauryl sulfate (SDS) were included in our research system. The coarse-grained models and shorthand notation for each molecular are presented in Figure 1. SDS are separated into two groups of hydrophilic and hydrophobic parts with the beads H set in green and T designated in blue, respectively. The H and T connected by a harmonic spring can be denoted by the symbol H1T1. Water is bead W in red, n-hexane is bead O in rose.

To clearly show the simulation results, the periodic boundary condition of three directions was employed in the cubic simulation box, which was 15 × 15 × 15 *R<sup>c</sup>* 3 (L<sup>x</sup> × L<sup>y</sup> × Lz). There were approximately 10,125 beads in every simulation box, and the density of beads was set to *ρ* = 3.0. It is possible to convert the simulation surfactant concentration to the mole fraction with the isochoric property. Based on Groot's reports [37], the spring constant of every bead was set to 4.0.

The diffusivities of beads changed with an increasing simulation time, and are shown in Figure 2. A gradual decrease was observed with an increasing simulation time until 600 DPD units, and then remained unchanged after 800 DPD units. The simple modification was conducted following the velocity–varlet algorithm reported by Groot and Warren [37] and set ∆*t* = 0.05. Therefore, it is an equilibrium state can be reached by 20,000 timesteps simulations.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 12

**Figure 1.** A coarse‐grained model for surfactant, water and oil. **Figure 1.** A coarse-grained model for surfactant, water and oil. simulations.

**Figure 2.** The diffusivity of beads H, T, W and O with the simulation time. The oil‐water ratio was equal to 3:1. **Figure 2.** The diffusivity of beads H, T, W and O with the simulation time. The oil-water ratio was equal to 3:1.

<sup>3</sup> (Lx × Ly ×

<sup>3</sup> (Lx × Ly ×

Table 1 is the repulsive interaction parameters between different beads referred to the previous reports [40,41,45]. Table 1 is the repulsive interaction parameters between different beads referred to the previous reports [40,41,45].



**Figure 2.** The diffusivity of beads H, T, W and O with the simulation time. The oil‐water ratio was

Table 1 is the repulsive interaction parameters between different beads referred to

equal to 3:1.

the previous reports [40,41,45].

### **3. Results and Discussion**

**3. Results and Discussion**

*3.1. Transformation of W/O and O/W Microemulsion Systems* 3.1.1. Dynamics of the W/O and O/W Microemulsion Systems Formation

*3.1. Transformation of W/O and O/W Microemulsion Systems*

*Molecules* **2022**, *27*, x FOR PEER REVIEW 5 of 12

**Table 1.** The interaction parameters employed in this simulation.

3.1.1. Dynamics of the W/O and O/W Microemulsion Systems Formation A rigorous strategy for the formation of the W/O and O/W microemulsion systems is

A rigorous strategy for the formation of the W/O and O/W microemulsion systems is proposed, which provides the microstructure of formed microemulsions. We chose pure water and n-hexane as two incompatible systems, sodium lauryl sulfate as a surfactant to simulate the microemulsion by dissipative particle dynamics (DPD) simulation. The typical snapshots illustrating the evolution of the microemulsion structure with time as an example are presented in Figure 3, where the surfactant concentration is 0.1 and the value of oil-water is 1/3. To better observe the internal structure of the microemulsion, the oil beads were not exhibited in the simulation system. In the initial state (a), surfactants, oil and water were randomly dispersed in the simulation box. At a more extensive simulation time of 250 DPD units (b), the monolayer becomes gathered and the surfactant hydrophilic group associate with the water molecules to form a sheet aggregate. A further increase in the simulation time of 500 DPD units leads to the formation of the W/O microemulsion (c). Most surfactant molecules associate in the oil-water interfaces compactly and little surfactant molecules associate to form a little micelle in the system. Surprisingly, the little micelle disappears with increasing the simulation time to 800 DPD units (d), indicating that the interface of the microemulsion approaches the maximum interface concentration and forms a stable microemulsion. Therefore, the simulation time of 1000 DPD units is enough to create the microemulsion. proposed, which provides the microstructure of formed microemulsions. We chose pure water and n‐hexane as two incompatible systems, sodium lauryl sulfate as a surfactant to simulate the microemulsion by dissipative particle dynamics (DPD) simulation. The typ‐ ical snapshots illustrating the evolution of the microemulsion structure with time as an example are presented in Figure 3, where the surfactant concentration is 0.1 and the value of oil‐water is 1/3. To better observe the internal structure of the microemulsion, the oil beads were not exhibited in the simulation system. In the initial state (a), surfactants, oil and water were randomly dispersed in the simulation box. At a more extensive simulation time of 250 DPD units (b), the monolayer becomes gathered and the surfactant hydrophilic group associate with the water molecules to form a sheet aggregate. A further increase in the simulation time of 500 DPD units leads to the formation of the W/O microemulsion (c). Most surfactant molecules associate in the oil‐water interfaces compactly and little surfactant molecules associate to form a little micelle in the system. Surprisingly, the little micelle disappears with increasing the simulation time to 800 DPD units (d), indicating that the interface of the microemulsion approaches the maximum interface concentration and forms a stable microemulsion. Therefore, the simulation time of 1000 DPD units is enough to create the microemulsion.

**W H T O**

W 25 H 25.34 25 T 151.52 177.82 25 O 103.24 143.61 25.94 25

**Figure 3.** Snapshots of the evolution of the W/O microemulsion structure in DPD simulations with oil/water = 1/3. The hydrophilic group of surfactants are shown in green; the hydrophobic group is shown in blue. The water beads is shown in red. To clarity observe the internal structure, oil beads (pink) are not displayed. **Figure 3.** Snapshots of the evolution of the W/O microemulsion structure in DPD simulations with oil/water = 1/3. The hydrophilic group of surfactants are shown in green; the hydrophobic group is shown in blue. The water beads is shown in red. To clarity observe the internal structure, oil beads (pink) are not displayed.

3.1.2. Influence of Oil‐Water Ratio on the Transformation of W/O and O/W Microemul‐ sion Systems 3.1.2. Influence of Oil-Water Ratio on the Transformation of W/O and O/W Microemulsion Systems

The Oil‐Water ratio could influence the emulsion stability and the emulsion for‐ mation of O/W or inverse W/O under high‐energy input, which has extra stability with various affecting factors [23]. We set up the value of oil/water from 4:1 to 1:5 to study the microemulsion type affected by the oil/water ratio with the surfactant concentration of 0.05. Figure 4 shows the translucent three‐dimensional structure of the simulation system (a), the corresponding mean interfacial tension (b) and end to end distance of H1T1 (c). The Oil-Water ratio could influence the emulsion stability and the emulsion formation of O/W or inverse W/O under high-energy input, which has extra stability with various affecting factors [23]. We set up the value of oil/water from 4:1 to 1:5 to study the microemulsion type affected by the oil/water ratio with the surfactant concentration of 0.05. Figure 4 shows the translucent three-dimensional structure of the simulation system (a), the corresponding mean interfacial tension (b) and end to end distance of H1T1 (c). The Irving and Kirkwood (IK) method was adopted to analyze the mean interfacial tension by:

$$\mathcal{T}\_{\rm sim} = \frac{1}{2} \int\_{-L\_Z/2}^{L\_Z/2} [\mathcal{P}\_N(Z) - \mathcal{P}\_L(Z)] dz \tag{8}$$

Among them, *PN*(*Z*) was the pressure normal to the interface, the same as *Pzz*(*Z*). The lateral force was given by *PL*(*Z*) = 1/2[*Pxx*(*Z*) + *Pyy*(*Z*)] with the pressure tensor component in the *Z* direction. The reality units could be transformed from the mean surface tension satisfied using the simulations by *γ* = *γsim* × kB*T*/*R<sup>c</sup>* 2 , with *R<sup>c</sup>* = 0.711 nm and *T* = 298 K [39].

sion by:

The Irving and Kirkwood (IK) method was adopted to analyze the mean interfacial ten‐

<sup>1</sup> [ ( ) ( )]d <sup>2</sup>

Among them, *PN*(*Z*) was the pressure normal to the interface, the same as *Pzz*(*Z*). The lateral force was given by *PL*(*Z*) = 1/2[*Pxx*(*Z*) + *Pyy*(*Z*)] with the pressure tensor component in the *Z* direction. The reality units could be transformed from the mean surface tension

*r P Z PZ z* (8)

2, with *Rc* = 0.711 nm and *T* = 298 K [39].

/2 /2

*sim N L <sup>L</sup>*

*Z Z*

*L*

satisfied using the simulations by *γ* = *γsim* × kB*T*/*Rc*

**Figure 4.** The translucent three‐dimensional structure of the simulation system (**a**), the correspond‐ ing mean interfacial tension (**b**) and end to end distance of H1T1 (**c**). **Figure 4.** The translucent three-dimensional structure of the simulation system (**a**), the corresponding mean interfacial tension (**b**) and end to end distance of H1T1 (**c**).

As the same content of oil and water as the oil/water = 1/1, surfactants adsorb at a flat interface and form a layer‐like aggregate. The highest mean interfacial tension and end to end distance of H1T1 indicate that the surfactant molecular chain is most extended and has the weakest surfactant activity and emulsification capacity. With increasing oil con‐ tent to the oil/water = 2/1, the increased oil phase with small amount was not sufficient to change the layer oil‐water interface. The oil phase is sufficient to wrap the water phase and begin to form W/O microemulsion when the oil content increases to the oil/water = 3/1. Simultaneously, the decrease of the mean interfacial tension and end to end distance of H1T1 indicate the surfactant molecular chain owns a degree of bending and better emulsifying capacity. It is mainly due to the reduced oil and water interface with a larger oil phase and smaller water phase at the same surfactant concentration. As the oil phase increased to 3.5/1 and 4/1, the W/O microemulsion proved to be more stable, with a shrinking end to end distance of H1T1; however, the mean interfacial tension remained unchanged. Instead, O/W microemulsion was formed by increasing the water content to oil/water = 1:3. The mean interfacial tension reached its minimum value and did not change despite the addition of more water molecules. A gradual decrease in the end to As the same content of oil and water as the oil/water = 1/1, surfactants adsorb at a flat interface and form a layer-like aggregate. The highest mean interfacial tension and end to end distance of H1T1 indicate that the surfactant molecular chain is most extended and has the weakest surfactant activity and emulsification capacity. With increasing oil content to the oil/water = 2/1, the increased oil phase with small amount was not sufficient to change the layer oil-water interface. The oil phase is sufficient to wrap the water phase and begin to form W/O microemulsion when the oil content increases to the oil/water = 3/1. Simultaneously, the decrease of the mean interfacial tension and end to end distance of H1T1 indicate the surfactant molecular chain owns a degree of bending and better emulsifying capacity. It is mainly due to the reduced oil and water interface with a larger oil phase and smaller water phase at the same surfactant concentration. As the oil phase increased to 3.5/1 and 4/1, the W/O microemulsion proved to be more stable, with a shrinking end to end distance of H1T1; however, the mean interfacial tension remained unchanged. Instead, O/W microemulsion was formed by increasing the water content to oil/water = 1:3. The mean interfacial tension reached its minimum value and did not change despite the addition of more water molecules. A gradual decrease in the end to end distance of H1T1 shows that the surfactant molecules were more compact and orderly at the interface and the O/W microemulsion is more stable.

end distance of H1T1 shows that the surfactant molecules were more compact and orderly at the interface and the O/W microemulsion is more stable. Figure 5 shows the density distributions of beads (H, T, W, O) along the *x*‐axis in different oil‐water ratios (3/1 and 1/3) corresponding the translucent three‐dimensional structure used to observe the actual adsorption of surfactant molecules at the oil and water interface. W/O microemulsion is formed with the raised curve of W shown in Figure 5a, and the droplet size is around 12 DPD units (from 2 to 14 DPD units). Meanwhile, the internal water phase associated with the hydrophilic head makes the density of H slightly higher than T, which is caused by the smaller space of the internal water phase than the outside oil phase, with the same amounts of H and T. The translucent three‐dimensional Figure 5 shows the density distributions of beads (H, T, W, O) along the *x*-axis in different oil-water ratios (3/1 and 1/3) corresponding the translucent three-dimensional structure used to observe the actual adsorption of surfactant molecules at the oil and water interface. W/O microemulsion is formed with the raised curve of W shown in Figure 5a, and the droplet size is around 12 DPD units (from 2 to 14 DPD units). Meanwhile, the internal water phase associated with the hydrophilic head makes the density of H slightly higher than T, which is caused by the smaller space of the internal water phase than the outside oil phase, with the same amounts of H and T. The translucent three-dimensional structure (Figure 5b) could observe this structure more intuitively. In contrast, the raised curve of O and T was slightly higher than H (Figure 5c) indicate the formation of O/W microemulsion. Figure 5d shows the inner structure of the formed O/W microemulsion, of which the hydrophobic group associated with the surface of the oil phase and the hydrophilic group disperse in the water phase.

structure (Figure 5b) could observe this structure more intuitively. In contrast, the raised curve of O and T was slightly higher than H (Figure 5c) indicate the formation of O/W microemulsion. Figure 5d shows the inner structure of the formed O/W microemulsion, of which the hydrophobic group associated with the surface of the oil phase and the hy‐

**Figure 5.** The density distribution of beads (H, T, W, O) along the *x*‐axis in different oil‐ water ratios corresponding the translucent three‐dimensional structures: (**a**,**b**) O/W = 3/1, **Figure 5.** The density distribution of beads (H, T, W, O) along the *x*-axis in different oil-water ratios corresponding the translucent three-dimensional structures: (**a**,**b**) O/W = 3/1, (**c**,**d**) O/W = 1/3.

### (**c**,**d**) O/W = 1/3. *3.2. Stability of the W/O and O/W Microemulsion Systems*

drophilic group disperse in the water phase.

*3.2. Stability of the W/O and O/W Microemulsion Systems* 3.2.1. Influence of Temperature on the W/O and O/W Microemulsion Systems

3.2.1. Influence of Temperature on the W/O and O/W Microemulsion Systems For safety, some food products based on emulsion often require heating treatment such as cooking and pasteurization. However, the emulsion could transform from W/O emulsion to O/W emulsion by changing the temperature from that observed in previous studies [18,24,44]. Therefore, the stability of the emulsion will be affected by the tempera‐ ture. Figure 6 shows the impact of temperature on the W/O and O/W microemulsions. From the translucent three‐dimensional structure of O/W microemulsion (Figure 6a), we can observe the minimum value for the stability of O/W microemulsion at 0.8 kB*T*, but the maximum value at 1.0 kB*T* with the minimal mean interfacial tension (Figure 6c). How‐ ever, a greater increase is observed in the mean interfacial tension after heating over 1.0 kB*T* or cooling to 0.8 kB*T*. This may be due to the O/W microemulsion transformed into a rod structure with a more extensive oil‐water interface than microemulsion, which re‐ duces the ability to change the surface activity of H1T1. End to end distance of H1T1 (Fig‐ For safety, some food products based on emulsion often require heating treatment such as cooking and pasteurization. However, the emulsion could transform from W/O emulsion to O/W emulsion by changing the temperature from that observed in previous studies [18,24,44]. Therefore, the stability of the emulsion will be affected by the temperature. Figure 6 shows the impact of temperature on the W/O and O/W microemulsions. From the translucent three-dimensional structure of O/W microemulsion (Figure 6a), we can observe the minimum value for the stability of O/W microemulsion at 0.8 kB*T*, but the maximum value at 1.0 kB*T* with the minimal mean interfacial tension (Figure 6c). However, a greater increase is observed in the mean interfacial tension after heating over 1.0 kB*T* or cooling to 0.8 kB*T*. This may be due to the O/W microemulsion transformed into a rod structure with a more extensive oil-water interface than microemulsion, which reduces the ability to change the surface activity of H1T1. End to end distance of H1T1 (Figure 6e) gradually increases with increasing the temperature caused by more extension molecular chain consistent with the results obtained by Chen et al. [46].

ure 6e) gradually increases with increasing the temperature caused by more extension molecular chain consistent with the results obtained by Chen et al. [46]. By contrast, the W/O microemulsion demonstrated a better stability resistance to temperature as long as the temperature is below 1.55 kB*T*, as shown in Figure 6b. However, one big W/O droplet was separated into a number of tiny droplets when the temperature decreased to 0.1 kB*T*. Compared with O/W microemulsion, the better stability of W/O microemulsion may be due to the higher viscosity of oil molecules that were not easily spread in the decentralized system.

**Figure 6.** The translucent three‐dimensional structure of microemulsion—(**a**,**b**), the corresponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with increasing temperature. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O microemulsion. **Figure 6.** The translucent three-dimensional structure of microemulsion—(**a**,**b**), the corresponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with increasing temperature. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O microemulsion.

### By contrast, the W/O microemulsion demonstrated a better stability resistance to 3.2.2. Influence of Inorganic Salt on the W/O and O/W Microemulsion Systems

temperature as long as the temperature is below 1.55 kB*T*, as shown in Figure 6b. However, one big W/O droplet was separated into a number of tiny droplets when the temperature decreased to 0.1 kB*T*. Compared with O/W microemulsion, the better stability of W/O mi‐ croemulsion may be due to the higher viscosity of oil molecules that were not easily spread in the decentralized system. 3.2.2. Influence of Inorganic Salt on the W/O and O/W Microemulsion Systems Two salts (NaCl and CaCl2) affect the emulsion stability and were observed using experiment and simulation by Zhong et al. [20] and Zhang et al. [47]. Despite this, we investigated the influence of salt on the stability of W/O and O/W microemulsion by the DPD simulation shown in Figure 7. The decrease in head–head repulsion parameters (*αHH*) means adding the inorganic salt, and if no inorganic salt exists *αHH* = 25. The translucent three‐dimensional structure of O/W microemulsion (Figure 7a) exhibits that the droplet transform into the rod topology with decreasing *αHH* to 22; meanwhile, a significant in‐ crease occurred in the mean interfacial tension (Figure 7c). There are certain fluctuations in end to end distance of H1T1 with increasing simulation time whether the inorganic salt is added shown in Figure 7e. This is mainly due to the different degree of adsorption of surfactant molecules at the oil‐water interface. At the beginning of the simulation, the sur‐ factant molecules were scattered in an aqueous solution and showed a certain degree of Two salts (NaCl and CaCl2) affect the emulsion stability and were observed using experiment and simulation by Zhong et al. [20] and Zhang et al. [47]. Despite this, we investigated the influence of salt on the stability of W/O and O/W microemulsion by the DPD simulation shown in Figure 7. The decrease in head–head repulsion parameters (*αHH*) means adding the inorganic salt, and if no inorganic salt exists *αHH* = 25. The translucent three-dimensional structure of O/W microemulsion (Figure 7a) exhibits that the droplet transform into the rod topology with decreasing *αHH* to 22; meanwhile, a significant increase occurred in the mean interfacial tension (Figure 7c). There are certain fluctuations in end to end distance of H1T1 with increasing simulation time whether the inorganic salt is added shown in Figure 7e. This is mainly due to the different degree of adsorption of surfactant molecules at the oil-water interface. At the beginning of the simulation, the surfactant molecules were scattered in an aqueous solution and showed a certain degree of bending because of the intermolecular repulsion. With the increase in simulation time, small unstable microemulsion droplets gradually formed and finally reached the equilibrium state to form one stable and large microemulsion droplet and resulted in different degrees of bending. However, end to end distance of H1T1 gradually increases with increasing salt concentration (Figure 7e). It was probably caused by the opposite ion of inorganic salt could neutralize part of the charge of the hydrophilic group and reduce the electrostatic repulsion between the hydrophilic groups. It leads to looser surfactant molecules at the oil and water interface and the molecular chains are more extended.

bending because of the intermolecular repulsion. With the increase in simulation time, small unstable microemulsion droplets gradually formed and finally reached the equilib‐ rium state to form one stable and large microemulsion droplet and resulted in different degrees of bending. However, end to end distance of H1T1 gradually increases with in‐ creasing salt concentration (Figure 7e). It was probably caused by the opposite ion of in‐ organic salt could neutralize part of the charge of the hydrophilic group and reduce the electrostatic repulsion between the hydrophilic groups. It leads to looser surfactant mole‐

cules at the oil and water interface and the molecular chains are more extended.

**Figure 7.** The translucent three‐dimensional structure of microemulsion—(**a**,**b**), the corresponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with increasing inorganic salt. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O microemulsion. **Figure 7.** The translucent three-dimensional structure of microemulsion—(**a**,**b**), the corresponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with increasing inorganic salt. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O microemulsion.

However, W/O microemulsion begins to transform when *αHH* decreases to 14 (Figure 7b). The rod topology has the highest interface tension (Figure 7d) and most significant end‐to‐end distance (Figure 7f). W/O microemulsion has a better stability resistance to the inorganic salt compared with O/W microemulsion, which is consistent with the results observed by Huang et al. [24]. This may be due to the adsorption position of H1T1 at the oil and water interfaces. The hydrophilic group associated with the interface caused a more significant spatial block effect and hindered the electrostatic gravity of inorganic salt and hydrophilic ions in W/O microemulsion. However, W/O microemulsion begins to transform when *αHH* decreases to 14 (Figure 7b). The rod topology has the highest interface tension (Figure 7d) and most significant endto-end distance (Figure 7f). W/O microemulsion has a better stability resistance to the inorganic salt compared with O/W microemulsion, which is consistent with the results observed by Huang et al. [24]. This may be due to the adsorption position of H1T1 at the oil and water interfaces. The hydrophilic group associated with the interface caused a more significant spatial block effect and hindered the electrostatic gravity of inorganic salt and hydrophilic ions in W/O microemulsion.

### 3.2.3. Influence of Shear on the W/O and O/W Microemulsion Systems 3.2.3. Influence of Shear on the W/O and O/W Microemulsion Systems

Lee–Edwards sliding‐brick boundary conditions along *x*‐axis were applied to the simulation system representing the impact of shear flow on the structure and orientation of complex fluids. Figure 8 reveals the influence of shear on the O/W and W/O microemul‐ sion. The topology of the O/W microemulsion droplet is stable in the absence of shear and maintains this structure when increasing the shear rate to 0.008s−1. However, the oil cannot be wrapped by water after the shear rate is enhanced to 0.009s−<sup>1</sup> and transformed to a layer‐like aggregate (Figure 8a) with a significant increase in the mean interfacial tension (Figure 8c). This is mainly due to the shear rate inducing the aggregate spread out along the direction of the shear rate, similarly to an external force to the microemulsion. An increase in the end to end distance of H1T1 for O/W microemulsion indicates that the molecular chain is more extended with a larger shear rate (Figure 8e). However, for W/O microemulsion, the droplet emerges deformation until the shear rate to 0.031s−<sup>1</sup> (Figure 8b) with a more considerable mean interfacial tension (Figure 8d) and end to end distance of H1T1 (Figure 8f). In light of this, W/O microemulsion has better stability to resist the Lee–Edwards sliding-brick boundary conditions along *x*-axis were applied to the simulation system representing the impact of shear flow on the structure and orientation of complex fluids. Figure 8 reveals the influence of shear on the O/W and W/O microemulsion. The topology of the O/W microemulsion droplet is stable in the absence of shear and maintains this structure when increasing the shear rate to 0.008s−<sup>1</sup> . However, the oil cannot be wrapped by water after the shear rate is enhanced to 0.009s−<sup>1</sup> and transformed to a layer-like aggregate (Figure 8a) with a significant increase in the mean interfacial tension (Figure 8c). This is mainly due to the shear rate inducing the aggregate spread out along the direction of the shear rate, similarly to an external force to the microemulsion. An increase in the end to end distance of H1T1 for O/W microemulsion indicates that the molecular chain is more extended with a larger shear rate (Figure 8e). However, for W/O microemulsion, the droplet emerges deformation until the shear rate to 0.031s−<sup>1</sup> (Figure 8b) with a more considerable mean interfacial tension (Figure 8d) and end to end distance of H1T1 (Figure 8f). In light of this, W/O microemulsion has better stability to resist the shear rate owing to the higher viscosity of the oil phase as the continuous phase is not easy to affect by an external force.

shear rate owing to the higher viscosity of the oil phase as the continuous phase is not

**Figure 8.** The translucent three‐dimensional structure of microemulsion—(**a**,**b**), the corre‐ sponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with in‐ creasing shear rate. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O **Figure 8.** The translucent three-dimensional structure of microemulsion—(**a**,**b**), the corresponding mean interfacial tension—(**c**,**d**) and end to end distance of H1T1—(**e**,**f**) with increasing shear rate. In which, (**a**,**c**,**e**) represent O/W microemulsion, (**b**,**d**,**f**) represent W/O microemulsion.

### microemulsion. **4. Conclusions**

easy to affect by an external force.

**4. Conclusions** The topology and stability of O/W and inverse W/O microemulsion were studied using the dissipative particle‐dynamics simulation method. Coarse‐grained models were constructed for surfactant (H1T1), oil and water, respectively. The results show that the ratio of oil and water would change the topology of the microemulsion that transforms from W/O to O/W by decreasing the value of oil/water from 3:1 to 1:3. Meanwhile, the effects of the temperature, inorganic salt and shear rate on the stability of the formed mi‐ croemulsions were researched with a translucent three‐dimensional structure and corre‐ sponding parameters such as mean interfacial tension and end‐to‐end distance of H1T1. Inverse W/O microemulsion has better resistance to a higher temperature (1.5 kB*T*), inor‐ ganic salt (*αHH* = 14) and shear rate (0.03 s−1) than the O/W microemulsion of *T* = 1.0 kB*T*, *αHH* = 23 and s = 0.008 s−1. In the inverse W/O microemulsion of oil as the continuous phase with higher viscosity is not easy to affect the physical and chemical properties. The simu‐ lation provides a powerful tool to forecast the structure and the stability of various micro‐ The topology and stability of O/W and inverse W/O microemulsion were studied using the dissipative particle-dynamics simulation method. Coarse-grained models were constructed for surfactant (H1T1), oil and water, respectively. The results show that the ratio of oil and water would change the topology of the microemulsion that transforms from W/O to O/W by decreasing the value of oil/water from 3:1 to 1:3. Meanwhile, the effects of the temperature, inorganic salt and shear rate on the stability of the formed microemulsions were researched with a translucent three-dimensional structure and corresponding parameters such as mean interfacial tension and end-to-end distance of H1T1. Inverse W/O microemulsion has better resistance to a higher temperature (1.5 kB*T*), inorganic salt (*αHH* = 14) and shear rate (0.03 s−<sup>1</sup> ) than the O/W microemulsion of *T* = 1.0 kB*T*, *αHH* = 23 and s = 0.008 s−<sup>1</sup> . In the inverse W/O microemulsion of oil as the continuous phase with higher viscosity is not easy to affect the physical and chemical properties. The simulation provides a powerful tool to forecast the structure and the stability of various microemulsions, which is of great importance for developing new functional emulsions for many applications.

many applications. **Author Contributions:** Conceptualization, H.Z. and X.J.; methodology, H.Z.; software, H.Z.; validation, H.Z., Z.Z. and Z.W.; formal analysis, H.Z.; investigation, M.L.; resources, M.L.; data curation, M.L.; writing—original draft preparation, M.L.; writing—review and editing, **Author Contributions:** Conceptualization, H.Z. and X.J.; methodology, H.Z.; software, H.Z.; validation, H.Z., Z.Z. and Z.W.; formal analysis, H.Z.; investigation, M.L.; resources, M.L.; data curation, M.L.; writing—original draft preparation, M.L.; writing—review and editing, H.Z.; visualization, Z.Z.; supervision, Z.W.; project administration, H.Z.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript.

emulsions, which is of great importance for developing new functional emulsions for

H.Z.; visualization, Z.Z.; supervision, Z.W.; project administration, H.Z.; funding acquisi‐ tion, H.Z. All authors have read and agreed to the published version of the manuscript. **Funding:** The work was supported by the Research Foundation Project under Grant [BZXYLG2121]; the Key Technology Research and Development Program of Shandong Province under Grant [2019GSF109117].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of all the compounds are available from the authors.
