Microemulsions Based on Diverse Surfactant Molecular Structure: Comparative Analysis and Mechanistic Study

Abstract

Microemulsion flooding technology, known for significantly reducing interfacial tension, improving rock wettability, and providing strong driving forces at the microscopic level, has been widely applied in enhancing oil recovery in oil fields. This article summarizes the relevant literature and introduces the classification, formation mechanisms, research models, and factors affecting the performance of microemulsions. Particularly, it conducts a comparative analysis of microemulsion systems formed by surfactant molecules of different structures, aiming to provide new perspectives for the study of surfactant molecular structures and to further optimize the performance of microemulsion systems. The study finds that modifying surfactant molecules by adding benzene rings, increasing the length of hydrophobic tails, and enlarging hydrophilic heads can significantly increase the volume of the middle phase, exceeding 30%. These findings provide important guidance for optimizing microemulsion systems.

1. Introduction

The exploration and development of oil and gas primarily consist of three phases. In the first phase, the inherent energy of the oil reservoir is primarily harnessed, encompassing primary recovery techniques like natural water flooding, elastic energy propulsion, dissolved gas displacement, and gravity drainage. The oil recovery rate in this stage is only 15%. As the inherent energy diminishes, secondary oil extraction is facilitated in the second phase by the injection of gas or water to replenish the elastic energy of the reservoir rocks and fluids [1]. Due to the heterogeneity of geological formations, the injected fluids often exhibit a ‘fingering effect’, meaning they tend to move along the path of least resistance. This leads to suboptimal sweep efficiency and incomplete displacement in oil zones with greater resistance. At the same time, under the drive of the injected water/gas, crude oil cannot be fully extracted, leaving a significant amount of residual oil. The recovery rate in this stage is only 30%. The third phase encompasses enhanced oil recovery techniques like chemical displacement, thermal extraction, and gas flooding, aiming to elevate the recovery rate, extend the oil field’s longevity, and achieve optimal resource utilization.

Microemulsions, characterized by their small domain size and dynamic structural stability, are attracting increasing attention from researchers [2]. These are extensively employed in sectors like pharmaceuticals, cosmetics, petrochemicals, and nanoscale metallic materials [3,4,5,6,7]. Microemulsion flooding is a type of chemical flooding. It is formed through a system of surfactants/co-surfactants/salts, ultimately aiming to enhance the oil recovery rate [8]. Oil displacement via microemulsions can be bifurcated into the formation of microemulsions before injection and formation post-injection. Apart from their application in the displacement procedure, microemulsions frequently find use in the return flow of fracturing fluids, assisting in flow back, among other areas [9,10,11]. As exploration and development continue, the exploitation of conventional oil reservoirs is gradually reaching its limit. Consequently, the focus is shifting towards unconventional reservoirs, such as low-permeability and heavy oil reservoirs. Particularly, there is significant interest in forming stable microemulsions with a large volume at lower surfactant concentrations, which will be explored in detail in the following section [12].

Surfactants can be classified based on their structure into traditional surfactants (with one hydrophobic tail and one hydrophilic head), anionic–nonionic surfactants (with one hydrophobic tail, a chain of medium polarity groups, and one hydrophilic head), and Gemini surfactants (with multiple hydrophobic tails and multiple hydrophilic heads). Structure determines properties, and the molecular structure of surfactants has a significant impact on the formation of microemulsions and their stability. According to current research findings, increasing the volume of microemulsions primarily involves adjusting the combinations of different co-surfactants with various surfactants and improving the salinity of the formulation [13]. However, research on the targeted design of surfactant structures to form larger volume middle-phase microemulsions is relatively scarce. Therefore, this paper first elaborates on the classification, formation mechanisms, and models of microemulsions, the mechanism of enhancing recovery rates, and several common molecular structures of surfactants. Subsequently, drawing from the formation principles of microemulsions, an analysis is conducted on the molecular architecture of surfactants. A design approach is summarized that may yield surfactant molecules with better microemulsion performance.

2. Overview of Microemulsion

2.1. Types of Microemulsion

The concept of microemulsions first appeared in 1943 by Hoar and Schulman, who proposed another dispersion system based on emulsions and colloidal solutions [14]. Water and oil, when mixed with a large amount of surfactants and co-surfactants, can spontaneously form transparent or translucent systems [15]. Schulman provided the inaugural definition of microemulsions in 1959 [16]. Subsequently, as research on microemulsions deepened, as shown in Figure 1, microemulsions were classified into Winsor I, Winsor II, and Winsor III [17]. Winsor I type is an oil-in-water (O/W) microemulsion, where a small amount of oil dissolves in water under the solubilization of surfactants, with oil as the dispersed phase and water as the continuous phase, also known as the lower phase microemulsion. Winsor II type is a water-in-oil (W/O) microemulsion, where a small amount of water dissolves in oil under the solubilization of surfactants, with water as the dispersed phase and oil as the continuous phase, also known as the upper phase microemulsion. Winsor III type microemulsion has a unique structure, where a third phase appears between the excess oil phase and excess water phase, forming a “bicontinuous” network structure under the influence of surfactants, with both oil and water as continuous phases [18]. Within this context, an equivalent volume of oil and water is incorporated, often termed the middle-phase microemulsion.

2.2. Formation Mechanism and Research Modeling of Microemulsion

2.2.1. Instantaneous Negative Interfacial Tension Theory

Microemulsions, as a spontaneously formed thermodynamically stable system, have been extensively studied in terms of their formation mechanisms [20]. Schulman and Prince introduced a theory regarding transient negative interfacial tension [21]. This theory describes an equilibrium process. According to Equation (1) and grounded on the Gibbs equation, the introduction of surfactant molecules to an oil–water blend can drastically diminish the oil–water interfacial tension. Furthermore, the inclusion of a co-surfactant (another active agent) leads to mixed adsorption, amplifying the reduction in oil–water interfacial tension. This results in the interfacial tension reaching an ultra-low level (10⁻³ to 10⁻⁵ mN/m), and even negative interfacial tension can occur. At this point, since the system is in an unstable state, it will spontaneously undergo interfacial expansion to return to a stable state. During this process, the adsorption sites on the oil–water interface increase, and the active molecules in the bulk phase further adsorb, leading to a decrease in the concentration of the bulk phase, ultimately resulting in the interfacial tension returning to a slight positive value. If the structural integrity of the microemulsion is compromised, leading to domain enlargement, the specific interfacial area decreases, resulting in a reduction in effective interaction sites. The active molecules return to the bulk phase, and the concentration of the bulk phase increases, resulting in negative interfacial tension due to the effect of mixed adsorption [22].

$$-d\gamma =\sum {\mathsf{\Gamma }}_{i}d{\mu }_i=\sum {\mathsf{\Gamma }}_{i}RTd\mathrm{l}\mathrm{n}{c}_i$$

(1)

where γ is the interfacial tension between oil and water; ${\mathsf{\Gamma }}_i$ is the adsorption amount of component $i$; ${\mu }_i$ is the chemical potential of component $i$; and $c_i$ is the concentration of the bulk phase.

The theory explains the mechanism of microemulsion formation, but it has many shortcomings. Firstly, it has limited applicability, is only suitable for specific types of microemulsion systems, and cannot explain the phase changes of microemulsions caused by environmental variations, which are crucial. Secondly, the theory merely offers a simplified description of microemulsion stability. Microemulsions should be considered highly structured systems, with their stability influenced by various factors, such as the arrangement of surfactant molecules and solvent properties.

2.2.2. Double Layer Membrane Theory

To explain the formation of O/W and W/O types of emulsions, Schulman and Bowcott proposed the double film theory in 1955 [23]. The main point of the theory is that in microemulsion systems, besides the oil and water phases, there exists a “third phase” formed by the mixed adsorption of surfactants and co-surfactants. The “third phase” has two interface films at its contact position with oil/water, each interacting separately with the two phases. As shown in Figure 2, in the two interfacial films, the interactions between the “third phase” of surfactant molecules and the oil–water phases differ. A weaker interaction results in higher interfacial tension, and a higher tension prompts the interface area to contract to its smallest, ultimately leading to curvature of the interface. If γ_s/w (the tension at the interface of the “third phase” and water) is inferior to γ_s/o (the tension between the “third phase” and oil), it results in an O/W microemulsion. Conversely, a W/O microemulsion emerges. It is worth noting that when the “third phase” has similar interactions with both the oil and water phases, the interfacial tension at the two membranes is comparable, preventing the interface from bending and resulting in a Winsor III type microemulsion with a bicontinuous phase. Further research based on this theory revealed that if the third phase consists entirely of surfactants and co-surfactants, and if we assume the emulsion droplets are spherical, we can calculate the total amount of surfactants and co-surfactants required. This calculated amount is substantial, leading to the speculation that besides the active molecules in the third phase, a significant amount of oil and water molecules also permeate into it.

The double film theory oversimplifies the dynamic nature of microemulsions, overlooking the complex processes of continuous rearrangement and exchange among molecules. Additionally, it fails to adequately consider various factors affecting the stability and properties of microemulsions, such as temperature, pressure, oil–water ratio, and electrolyte concentration. This is particularly evident in its explanation of complex microemulsion types like Winsor III, which have a bicontinuous phase structure.

2.2.3. Packing Parameter Theory

The double film theory explains the preferential bending of the interface film under different conditions, leading to the formation of O/W and W/O type microemulsions. Expanding upon this foundation, Robbins et al. introduced the packing parameter theory, premised on the geometric alignment of molecules at the adsorption interface [24]. The essence of this theory is the introduction of the packing parameter P to consider the packing shape of molecules, thereby explaining the properties of microemulsions. Referring to Equation (2) [17], with P = 1, Figure 3 depicts an unbent interface, giving rise to either a liquid crystal or a bicontinuous phase microemulsion. When P > 1, the interface bends towards the oil phase, forming a W/O type microemulsion. When 1/3 < P < 1, the interface bends towards the water phase, forming an O/W type microemulsion. When P < 1/3, smaller micelles are formed [25].

$$P=\frac{V}{a_0{l}_c},$$

(2)

where P is the stacking parameter, $a_0$ is the area of the head group of the surfactant molecule; and $l_c$ and V represent the hydrophobic chain length and volume of the surfactant, respectively.

This theory simplifies the molecules adsorbed on the surface and the hydration layer near the molecules into a few basic geometric shapes. When P = 1, forming a liquid crystal or a bicontinuous phase microemulsion, the molecules can be viewed as a cylinder, hence the ratio of molecular volume to actual adsorption volume is one. When 1/3 < P < 1, forming an O/W type microemulsion, the geometric shape of the micelle–microemulsion transition boundary is viewed as a cone pointing towards the interior of the domain, hence P is one-third in critical situations. When P > 1, forming a W/O type microemulsion, the geometric shape of the adsorbed molecules gradually deviates from a simple cylinder.

The packing parameter theory simplifies the complexity of molecular arrangement and interfacial interactions by introducing the packing parameter P. Although this theory excels in predicting the types of microemulsions, it exhibits significant limitations when dealing with microemulsion systems in complex oil field environments. In particular, the theory’s simplified geometric assumptions may not accurately capture the interactions between molecules, and it lacks sensitivity to environmental factors such as temperature, pressure, and oil–water ratio. Therefore, it is recommended to combine it with other theories and experimental data for a more comprehensive and accurate prediction of microemulsion behavior in practical applications.

2.2.4. R-Ratio Theory

Unlike the aforementioned theories, Robbins first proposed the $R$-ratio theory in 1959, starting from the most basic intermolecular forces to study the formation mechanism of microemulsions. The concept of cohesive energy was introduced to represent the tendency of active molecules adsorbed on the surface to bond with oil and water molecules. This theory draws from the bilayer membrane theory where surfactant molecules adsorbed at the oil–water interface form a “third phase”, in which many oil and water molecules permeate. As shown in Figure 4, based on this, eight interaction forces are introduced. Based on this, the interaction of surfactants with oil molecules is $A_{so}$= $A_{io}$+ $A_{ho}$ and the interaction of surfactants with water molecules is $A_{sw}$= $A_{iw}$+$A_{hw}$. As per Equation (3) [17], the $R$-ratio is ultimately defined as:

$$R=\frac{A_{so}-A_{oo}-A_{ii}}{A_{sw}-A_{ww}-A_{hh}},$$

(3)

When R < 1, the interaction force between the surfactant molecules and the water phase is strong, causing the interface to spread in the water phase, forming an O/W microemulsion. When R > 1, the interaction force between the surfactant molecules and the oil phase is strong, leading the interface to spread in the oil phase, resulting in a W/O microemulsion. When R = 1, the interface does not bend. It is worth noting that under the premise of R = 1, due to environmental factors causing local concentration fluctuations, there are local changes in the R-ratio value of the adsorption layer. While the overall R-ratio value of the adsorption layer is 1, there are local fluctuations in the R-ratio value. This will result in a bicontinuous microemulsion; otherwise, a more orderly liquid crystal phase will form.

The R-ratio theory in microemulsion research considers the interactions between molecules, playing a significant role in explaining the formation and properties of microemulsions. The precise calculation of various interaction forces involved in the theory may be feasible in a laboratory setting, but it becomes overly complex and difficult to execute accurately in oil reservoir environments. Therefore, although the R-ratio theory provides an important foundation for theoretical research on microemulsions, it is difficult to apply effectively in actual research processes.

2.2.5. HLD Theory and the HLD-NAC Model

The design of microemulsion systems containing surfactants is typically a complex and labor-intensive process based on experimental trial and error. Without theoretical guidance, adjusting and optimizing these formulations for specific reservoir conditions can be both challenging and inefficient. After Salager et al. introduced the concept of Hydrophilic–Lipophilic Difference (HLD), Salager, Antón, and others further refined and presented a detailed HLD equation [27]. Subsequently, Skauge et al. revealed that pressure in the microemulsion system also affects its phase behavior [28]. Based on this discovery, Ghosh and Johns further elaborated on this theory in subsequent studies, taking into account the comprehensive impact of pressure and dissolved gases on phase behavior. In practical applications, particularly under optimal conditions, surfactants with long straight-chain tails tend to encounter issues of prolonged equilibration times, which significantly complicates the assessment of HLD parameters. To tackle this issue, Acosta and others utilized linear surfactant mixing rules and the reference surfactant sodium dihexyl sulfosuccinate (AMA), allowing for a more straightforward and precise assessment of the HLD parameters of the target surfactant [29]. The strength of HLD theory lies in its ability to predict and reproduce microemulsion phase behavior solely based on salinity, the head area of the surfactant, and characteristic length, even in the absence of oil hydrophobicity and surfactant characteristic curvature [30]. In summary, HLD theory provides a solid theoretical foundation for the field of emulsion science and engineering, making the processes of product development, emulsion preparation, and surfactant selection more efficient, predictive, and economical.

$$\mathrm{H}\mathrm{L}\mathrm{D}=\ln \left(S\right)-K\times \mathrm{E}\mathrm{A}\mathrm{C}\mathrm{N}-{\alpha }_T\mathsf{\Delta }T+\mathrm{C}\mathrm{c}+f\left(A\right),$$

(4)

where $\mathrm{C}\mathrm{c}$ is a characteristic parameter of the surfactant, mainly determined by its hydrophobicity; $S$ represents the salinity of the aqueous phase (NaCl g/mL); $K$ is a constant dependent on the head group of the surfactant and its hydrophilicity; EACN stands for Equivalent Alkane Carbon Number; $f\left(A\right)$ depends on the type of alcohol and is a function of concentration; ${\alpha }_T$ is a temperature-related constant; and $\mathsf{\Delta }T$ represents the difference from the reference temperature.

The HLD equation quantitatively describes the impact of factors such as salinity, EACN, and surfactant hydrophobicity on the transition of phase types. However, it cannot determine the amount of oil or water dissolved in the microemulsion phase. Therefore, Jin et al. [31], combining the HLD equation [32], introduced the net curvature state equation to establish the HLD-NAC (Hydrophilic–Lipophilic deviation–net-average curvature) model.

$$A_s={\sum }_i V_w\times C_{s_i}\times 6.023\times {10}^{23}\times a_s,$$

(5)

$$R_{\mathrm{c}}=\frac{3\times V_{\mathrm{c}}}{A_{\mathrm{s}}},$$

(6)

$$H_n=\left|\frac{1}{R_o}\right|-\left|\frac{1}{R_W}\right|,$$

(7)

$${\xi }^{\mathrm{*}}=\frac{6{\phi }_o{\phi }_w{V}_m}{A_s},$$

(8)

where $A_s$ is the total interfacial area of the surfactant; $V_w$ is the volume of water; $V_{\mathrm{c}}$ is the volume of the continuous phase; $C_{s_i}$ is the concentration of surfactant $i$ in water; $a_s$ is the surface area of the surfactant molecule; $R_{\mathrm{c}}$ is the assumed radius of the continuous phase; ${\phi }_o$ and ${\phi }_w$ are the volume fractions of oil and water in the middle phase microemulsion, respectively; and $V_m$ is the volume of the intermediate phase. For Winsor I and Winsor II type microemulsions, the radius of the continuous phase can be obtained from Equation (6). By combining it with Equation (7), the radius of the other phase can be determined. For the Winsor III type microemulsion with a double continuous phase, the characteristic length can be obtained from Equation (8), from which the radii of the two phases can be determined. This model takes into account the curvature effect of the interface, aiming to calculate the thickening rate of the microemulsion system and the interfacial tension between the oil phase and the water phase.

2.3. The Mechanisms of Microemulsions in Enhancing Oil Recovery

Enhancing oil recovery essentially involves expelling the residual oil trapped in geological formations. Residual oil refers to oil that cannot be extracted through conventional recovery methods or natural driving forces during the oil extraction process. Residual oil is primarily categorized into two types: oil unreached during the water flooding process and oil affected by water flooding but not effectively displaced due to factors like capillary forces and wettability. As illustrated in Figure 5, this specifically includes five forms: droplet, island, columnar, dead-end, and cluster formations.

The reason microemulsion systems can enhance oil recovery is due to their significant impact on sweep efficiency and displacement efficiency. On one hand, the formation of microemulsion systems can generate ultra-low interfacial tension, which facilitates the deformation of residual oil during the microemulsion displacement process. This low interfacial tension also promotes phase mixing, leading to substantial removal of crude oil [34]. Additionally, microemulsion systems have a strong capacity to increase the volume of oil and water, and they can increase the number of capillaries, reducing the saturation of residual oil. Microemulsions can also alter the wettability of rock formations, changing the wetting angle at the forefront of trapped oil, and thereby reducing the impact of capillary forces [35]. On the other hand, the formation of microemulsions is accompanied by the continuous growth of micelles, leading to the formation of long linear micelle structures that increase viscosity, thereby improving the mobility ratio. Hence, the volume of the microemulsion affects the viscosity of the displacement fluid, ultimately enhancing sweep efficiency.

3. Effects of Microemulsion

3.1. Salinity

Salinity plays a crucial role in microemulsion systems. It not only determines the surfactant concentration required to form a middle-phase microemulsion but also can alter the phase behavior of the microemulsion system. Moreover, due to the impact of salinity on the electrostatic interactions between surfactant molecules, it significantly regulates the domain size of the microemulsion. This salinity effect is essential for understanding and optimizing the performance of microemulsion systems in various applications. Chai et al. [36] conducted an in-depth study of microemulsion systems with sodium dodecylbenzene sulfonate (AS), sodium dodecyl sulfate (SDS), and sodium dodecylbenzene sulfonate (SDBS) as surfactants. To form a middle-phase microemulsion, the required surfactant concentration increases in the order of SDBS, SDS, and AS. When the surfactant concentration is consistent, the volume of the formed middle phase increases in the order of AS, SDS, and SDBS. Notably, when the NaCl concentration increases from 2.5 wt% to 5 wt%, the surfactant concentration required to form a middle-phase microemulsion decreases. Liu et al. [37] extensively explored the specific effects of cation valence and concentration on the microemulsion system. The research results show, as illustrated in Figure 6, that as the salt concentration gradually increases, the microemulsion system undergoes a transition from Winsor I to III to II type. Bera et al. [38] noted that this change is related to the increase in NaCl concentration. As its concentration rises, the interfacial tension (γ_s/o) between the excess oil phase and the surfactant decreases, while the interfacial tension (γ_s/w) between the excess water phase and the surfactant increases. Additionally, the critical micelle concentration of the surfactant SDS also decreases accordingly. Under consistent cation concentration, different types of salts have varying effects on the surfactant concentration required to form a middle-phase microemulsion. The specific order is CaCl₂ < KCl < NaCl < Na₂CO₃. This phenomenon can be explained by the effect produced by Ca²⁺ due to its higher charge density. K⁺ can more effectively compress the double layer due to its larger atomic radius and attract water molecules, thereby reducing the interaction between water molecules and surfactant molecules, causing the curvature of the interface membrane to change from positive to negative, leading to a phase transition. For NaCl and Na₂CO₃, although they both contain the same monovalent cations, due to the difference in their anions, their “Valence Activity Factor (VAF)” is different [39]. As in Equation (9):

$$\mathrm{V}\mathrm{A}\mathrm{F}=\frac{2}{1+Z},$$

(9)

where Z is the valence electron of the anion. The higher the VAF, the higher the activity of the corresponding cation. Bera et al. [40] also studied how the concentration of NaCl affects the domain size of microemulsions formed by SDS. They found that as the salinity increases, the average domain size of the microemulsion first decreases and then increases. When the concentration of NaCl is 4 wt%, it reaches the optimal salinity, and at this point, the average domain size of the microemulsion is minimized, approximately 3 nm.

3.2. Temperature

Temperature plays a significant role in the microemulsion system. It notably controls the solubility of hydrophobic groups and head groups in water solutions and their mutual interactions, subsequently influencing the phase transitions of the microemulsion. At the same time, temperature affects the thermodynamic behavior of the microemulsion, determining the kinetic properties of surfactant molecules when forming the microemulsion system. More crucially, the viscosity of the microemulsion system also undergoes significant adjustments with temperature changes. Therefore, in the research and application of microemulsions, controlling and understanding temperature is essential. John et al. [41] explored how temperature affects the microemulsion system formed by sodium dodecyl sulfate. The results showed that as the temperature rises, the optimal salinity of the system gradually decreases. When the temperature exceeds 70 °C, the optimal salinity remains relatively stable. This phenomenon can be attributed to the weakening of the hydrophilicity of the surfactant as the temperature increases. Nagao et al. [42] used bis(2-ethylhexyl) sulfosuccinate sodium (AOT) as a surfactant and conducted an in-depth study on the effect of temperature on the phase transition of the microemulsion system.

As the temperature rises, the spontaneous curvature of the surfactant adsorption layer changes, causing the system to transition from a W/O droplet structure to a lamellar structure, and then to an O/W droplet structure. The reason for this change is that as the temperature increases, the electrostatic repulsion between the head groups of the surfactant molecules strengthens, causing the am value of the AOT molecules to rise, leading to a decrease in the stacking parameter P. Tartaro et al. [43] studied the effect of temperature on the phase changes of alkyl poly-oxyethylene type nonionic surfactants relative to microemulsions. As shown in Figure 7, an increase in temperature favors the transition of the reverse curvature system from O/W droplets to a bicontinuous phase structure and then to W/O droplets. Das et al. [44] studied how temperature affects the performance of tetramethyl-1,4-bis(dimethyl tetradecyl ammonium bromide) during the formation of microemulsions. As the temperature increases, both the CMC and the degree of ionization of the surfactant show an upward trend. Temperature has two distinct effects on micellization. On one hand, as the system temperature rises, the water molecule structure near the hydrophobic chain is disrupted, which inhibits micellization. On the other hand, the increase in temperature leads to a decrease in the hydration level around the hydrophilic groups, which promotes micellization. In the temperature range of 298–323 K, the disruptive effect on the water structure near the hydrophobic chain is more pronounced. Additionally, as the temperature rises, both the Gibbs free energy (${\mathrm{\Delta }\mathrm{G}}_{\mathrm{m}}^0$) and entropy (${\mathrm{\Delta }\mathrm{S}}_{\mathrm{m}}^0$) of micellization decrease, while the enthalpy (${\mathrm{\Delta }\mathrm{H}}_{\mathrm{m}}^0$) of micellization increases. Gao et al. [45] conducted a detailed study on the microemulsion system composed of alkyl sulfate Gemini surfactants. As the temperature rises from 55 °C to 85 °C, the time for the surfactant to reach IFT equilibrium is reduced from 40 min to 15 min. Dustin et al. found that at a shear rate close to 10 s⁻¹, when the temperature rises from 55 °C to 85 °C, the viscosity of the microemulsion system drops to one-tenth of its original value. For anionic–nonionic surfactants, Velásquez et al. [46] found that as the temperature rises, the hydrophilicity of the anionic–nonionic surfactants decreases, exhibiting the properties of nonionic surfactants.

3.3. Types of Oil Phase

The oil phase of the microemulsion contains various organic compounds, including alkanes, alkenes, halogenated hydrocarbons, and aromatic hydrocarbons. Sripriya et al. [47] used various aliphatic hydrocarbons and aromatic hydrocarbons as the oil phase and studied the microemulsion systems formed by two surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), in these oil phases. As the number of carbon atoms in the oil molecules increases, the viscosity of the microemulsion system also increases. It is worth noting that this trend is more pronounced when using CTAB as a surfactant than when using SDS. Moreover, the viscosity of the microemulsion system with cyclic aliphatic hydrocarbons as the oil phase is significantly higher than that with linear aliphatic hydrocarbons as the oil phase. This is because cyclic oil molecules are more easily absorbed into the surfactant layer than linear oil molecules. Liu et al. [48] studied the effects of different components in crude oil (saturates, aromatics, asphaltenes, and resins) on the performance of different surfactants (branched alkyl tails and linear alkyl tails). When the non-polar components (saturates and aromatics) in the oil phase increase, the IFT of the system rises. However, when the polar components (asphaltenes and resins) in the oil phase increase, the IFT of the straight-chain surfactant system decreases, while the IFT of the branched-chain system rises. For oil phases with different numbers of carbon atoms, the initial surfactant concentration required for phase transitions in the microemulsion system varies. Chai et al. found that as the number of carbon atoms in the alkyl chain of the oil phase increases, such as from hexane to octane to dodecane, the surfactant concentration required to start forming a middle-phase microemulsion also increases [36]. The reason for this phenomenon is that oil molecules with shorter carbon chains are more likely to penetrate and embed between surfactant molecules, causing the average net curvature of the interface layer to shift in the direction of the oil phase.

4. Influence of Surfactant Molecular Structure on Microemulsions

Surfactants are an indispensable part of microemulsion systems. Their functions include reducing the interfacial tension between oil and water, stabilizing the dispersion of microemulsion droplets, and regulating the type and performance of microemulsions. Through the study of the effects of salinity, temperature, and oil phase composition on microemulsions, it was found that they are closely related to the molecular structure of surfactants. Therefore, a deeper study of the molecular structure of surfactants and their impact on the properties and efficacy of the microemulsion system is very important.

4.1. Conventional Surfactants

Conventional surfactants (also known as surface active agents) are a class of compounds with a unique molecular structure: a hydrophilic head group and a hydrophobic tail group. This amphiphilic structure allows surfactant molecules to interact with both water and oil, thereby reducing the interfacial tension between them. They can be classified into anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants [49].

By adjusting the salinity, Zeng Hongxia et al. [50] explored the phase transitions in the microemulsion system of sodium dodecyl sulfate. When the salt concentration is in the range of 18.9–19.6 wt%, a bicontinuous phase microemulsion appears, with a mid-phase volume of about 15%. Through core displacement tests, Kwan et al. [51] found that with a surfactant concentration of 2 wt%, a NaCl concentration of 3 wt%, and a core permeability of 114 mD, employing 1 PV of the surfactant solution can boost the recovery efficiency by 26.6%. Yu Tao et al. [52] have designed and synthesized six alkyl aryl sulfonates with different structures. They studied the influence of the change in the position of the phenyl ring and the length of the alkyl chain on the behavior of microemulsion phases. The results found that the phase volume in MФC_16-5S (the alkyl chain has a length of sixteen, the phenyl ring is connected to the 5th carbon atom of the long alkyl chain, and M represents toluene.) can reach 30.7%. The phase volume in ФC_14-7S can reach 22%. As the molecular alkyl chain lengthens or the aromatic group moves to the middle position of the long alkyl chain, the formation and disappearance of salt content in the microemulsion system, the width of the middle-phase salt, and the optimal salinity value decrease, while the optimal middle-phase volume and the optimal volume value increase. This phenomenon can be explained by the double membrane theory and the R-ratio theory. The theory suggests that in the third phase composed of surfactants, a large number of oil and water molecules will penetrate, leading to the expansion of the two interfaces. In this case, if the permeation amounts of oil and water phases are not significantly different, the more oil–water molecules that can permeate in the third phase, the easier it is to form a larger volume of the intermediate microemulsion. Meanwhile, the third phase needs to have an interaction force similar to that of oil and water molecules. That is, under conditions where $A_{so}$ and $A_{sw}$ are approximately equal, the larger their values, the greater the augmenting ability of the third phase, favoring the formation of intermediate microemulsion. The movement of the phenyl ring towards the middle position of the alkyl chain results in an increased intermolecular oil infiltration volume in surfactants. The penetration of water molecules in the third phase is related to the interaction between the polar head and the water molecules. Thus, at lower salinities, the interaction between the polar head and the water molecules can be shielded to a certain extent, making the oil and water in the third phase have similar penetration amounts and the same interactions. This ultimately manifests as a decrease in the optimal salinity value and an increase in the intermediate phase volume. Pal et al. [5] developed and synthesized a surfactant of the sulfonic acid methyl ester class. Research on the phase behavior of microemulsions revealed that with a surfactant concentration of 0.8 wt%, a NaCl concentration of 1.5 wt%, and a rock core permeability of 3240 mD, the recovery efficiency can be boosted by 29.81%. When using heptane as the oil phase, the relative volume of the microemulsion system can reach 23.19%. Compared to sodium dodecyl sulfate, the volume of the intermediate phase has risen by 8%. Relative to sodium dodecyl sulfate, this composition includes sulfonic and ester groups. The sulfonic group has a moderate polarity as a hydrophilic entity and can bolster its anti-salt precipitation properties. At the same time, the C-S bond boasts a high energy threshold, resisting degradation under elevated temperatures. Incorporating the ester group increases polarity while adding a methyl group offers hydrophobic properties. This augments the interaction of surfactant molecules with oil and water, thereby expanding the microemulsion’s volume.

4.2. Gemini Surfactants

Gemini surfactants are frequently viewed as oligomeric forms of conventional surfactants (monomers), generally comprising dimers, trimers, and tetramers, with a predominant focus on the study of dimers [53]. This article mainly focuses on discussing the structure of the dimers. As shown in Figure 8, unlike traditional surfactants, Gemini surfactants have two hydrophilic heads and two hydrophobic tails in their basic structure, bridged between the hydrophilic heads by a linker. Chen et al. [54] studied the phase behavior of microemulsion systems with secondary alkyl-α,ω-bis(dimethyl alkyl ammonium bromide) as the surfactant. They found that at the optimal salinity (NaCl = 0.8 wt%), the relative volume of the microemulsion phase was about 35%. The volume of the middle phase in the sodium dodecyl sulfate microemulsion system mentioned earlier has increased by more than double. The exceptionally low starting salinity in the formation of Winsor III microemulsions stems from the distinct structure of Gemini surfactants. In contrast with ionic surfactants, wherein elevated repulsion between the hydrophilic head and water dictates spontaneous curvature and consequently the curving direction of the surface film (necessitating salt addition to invoke a shielding effect that lessens the repulsion, rendering the spontaneous curvature null or even negative), the presence of the bridge bond in Gemini surfactants diminishes the influence of repulsive forces. Zhou et al. [55] studied a Gemini-type betaine surfactant. The results showed that when the surfactant concentration was 0.5 wt%, the interfacial tension could reach 9.1 × 10⁻⁴ mN/m. Through the study of microemulsion phase behavior, it was found that when the surfactant concentration was 1.5 wt%, the relative volume of the microemulsion phase could reach up to 31%. Regarding the performance of Gemini surfactants in enhancing oil recovery, Dabiri et al. [56] studied a naturally derived oil-based Gemini surfactant. When the surfactant concentration was 0.3 wt%, MgSO₄ was 0.1 wt%, and the core permeability was around 10 mD, using a 1 PV surfactant solution could increase the recovery rate by about 40% compared to formation water flooding. This is because Gemini surfactants have a unique dual-head structure, and their adsorption capacity at the oil–water interface is stronger than that of sodium dodecyl sulfate, as shown in Figure 9. Moreover, the presence of the bridge bond structure allows for a greater volume of oil infiltration and prevents the strong repulsion introduced by the enhanced hydrophilic head, leading to a tighter arrangement. This results in stronger interfacial activity, forming a more stable interfacial layer, and reducing oil droplet coalescence and separation, thereby improving crude oil recovery. Nguele et al. [57] conducted in-depth research into the microemulsion phase behavior formed by trimethyl-1,3-bis(dodecyl dimethyl ammonium bromide) and trimethyl-1,3-bis(hexadecyl dimethyl ammonium bromide), revealing a significant influence of increasing alkyl carbon number on the pre-micellization process. The study found that as the alkyl chain lengthened, the CMC value decreased. Moreover, by merely adding one carbon atom to the spacer of the dimeric surfactant, the CMC value could be reduced to 75%. In addition, increasing the polarity of bridge bonds (s) or extending the spacer can notably decrease the interfacial tension (IFT). The specific trend appears as trimer > dimer (s ≥ 2) > dimer (s with enhanced polarity) > monomer > non-ionic. Gao et al. [41,58] also extensively researched microemulsion systems made of Gemini surfactants with varying lengths of alkyl tails. For the Gemini surfactant with a tail containing eighteen carbon atoms, it requires 40 min to reach IFT equilibrium at 55 °C. In contrast, the Gemini surfactant with a tail containing sixteen carbon atoms reaches IFT equilibrium in a shorter time at 55 °C, taking only 10 min. Based on previous discussions regarding the temperature affecting Gemini surfactants, we can infer that reducing the number of carbon atoms in the alkyl tail in colder environments aids in achieving IFT equilibrium more quickly.

The performance of Gemini surfactants is largely influenced by their molecular structure, particularly regarding the molecules’ solubility and the process of micelle formation. Molecules of different structural variants demonstrate varying efficiencies and effects in micelle formation and in regulating micellar properties. To better reveal how structural differences impact the performance of Gemini surfactants in practical applications,

Table 1. Impact of different structures on the performance of Gemini surfactants.

Reference	Gemini Surfactants	Structure	Performance
Danino et al. [60]		Flexible bridge bond	Short spacer linear micelle, long spacer spherical micelle
Zhu et al. [61]		Flexible and rigid bridge bond	CMC decreases, and vesicles are easily formed.
Xie et al. [62]		Rigid bridge bond	Long linear micelle
Yang et al. [63]		C-C double bond	Lower the Krafft point, increase water solubility
Pei et al. [64]		Hydroxyl group	Long linear micelle
Taleb et al. [65]		Tail chain with a phenyl group	CMC decreases
Shaban et al. [66]		Increase tail chain length	CMC decreases

provides the structures of several different Gemini surfactants and the variations in their respective performances. It is evident that using short flexible spacers and rigid spacers is conducive to forming linear micelles, while long flexible spacers tend to form vesicles (the length of the spacers is relative, presumably related to the equilibrium distance between polar heads). Adding benzene rings to the tail chain or extending the tail chain length can lower the critical micelle concentration (CMC), facilitating the formation of microemulsions at low concentrations.

4.3. Anionic–Nonionic Surfactants

As shown in Figure 10, anionic–nonionic surfactants (extended-surfactants) refer to the insertion of a segment of moderately polar nonionic surfactant polar groups between a hydrophilic head and a hydrophobic alkyl tail into a conventional ionic surfactant [67]. The inserted group is typically polyethylene oxide (PEO) and polypropylene oxide (PPO).

Owing to the medium-polarity group (PPO), the central segment of the molecule undergoes hydration and consequently lies flat at the interface. This distinctive structure allows anionic–nonionic surfactants to effectively interact with oil molecules without the need for co-solvents, resulting in low-viscosity microemulsions and extremely low IFT, making them subjects of widespread research. Shi et al. [69] used molecular dynamics simulations to study the impact of the number of PO and EO chains on the performance of anionic–nonionic surfactants. They found that surfactant molecules containing three PO or EO groups form more stable interfacial layers due to stronger electrostatic interactions. Surfactant molecules with six PO or EO groups are more likely to produce low IFT and form microemulsions.

Phan et al. [70] investigated the influence of anionic–nonionic surfactants on the formation of microemulsions and their IFT values in polar oil phases. The results showed that the lowest IFT can reach 10⁻³ mN/m, and a Winsor III type microemulsion forms only when there are at least eight PO groups in the molecule. Furthermore, the IFT of the system is also influenced by the oil phase. Liu et al. [48] studied the effects of different components in crude oil (saturated hydrocarbons, aromatics, asphaltenes, and resins) on the performance of surfactants with different molecular structures (branched alkyl tails and straight-chain alkyl tails). When the non-polar components (saturated hydrocarbons and aromatics) in the oil phase increase, the IFT of the system rises. Conversely, an increase in the polar constituents (colloids and asphaltene) of the oil phase leads to a decrease in IFT for surfactant systems with a straight-chain structure, but for those with a branched structure, the IFT increases. The weak polarity of colloids and asphaltene allows for synergistic adsorption with straight-chain surfactants, leading to a reduction in IFT. On another note, due to the compact arrangement of branched surfactants at the oil–water boundary, the addition of colloids and asphaltene results in steric interference, leading to a looser arrangement of adsorbed molecules on the interface, which in turn causes an increase in IFT. Changes in interfacial tension directly affect the overall thermal resistance of the microemulsion system, thereby influencing the system’s heat transfer. To further investigate the microemulsion phase behavior of anionic–nonionic surfactants, Witthayapanyanon et al. [71] conducted studies on sodium alkyl sulfate surfactants with varying polypropylene (PO) chain lengths. With an increase in the quantity of PO chains, there was a reduction in both the optimal salinity and the interfacial tension (IFT). At the optimal salinity (NaCl = 8 wt%), the surfactant with eight PO groups exhibited an interfacial tension of 10⁻³ mN/m, and the relative volume of the microemulsion phase was 57%. There is a strong correlation between ultra-low interfacial tension and various aspects such as the formation and stability of microemulsions. Zhang Meijun et al. [72] developed four distinct surfactants to investigate how the end-capping of polyether chains influences the properties of microemulsions. The results indicated that when the polyether structure is PPO and is end-capped, the interfacial tension can be significantly reduced, achieving an ultra-low value of 10⁻³ mN/m. Al-Badi et al. [73] researched a microemulsion system using alkyl ether carboxylates as surfactants. It was found that the interfacial tension of the system could be reduced to a minimum of 10⁻⁴ mN/m. Moreover, it was observed that Winsor III type microemulsions could only be formed by surfactant molecules that possessed branched alkyl chains. This is attributed to the fact that surfactants with branched structures have a higher methyl content compared to methylene, resulting in a lower interfacial energy. Liyanage et al. [74] designed and synthesized a petroleum-based hydrophobic triethanolamine surfactant. By varying the quantity of PO and EO groups, we can precisely adjust the balance between their hydrophilic and lipophilic nature. This approach not only results in efficient surfactant activity and lowered interfacial tension but also guarantees optimal temperature responsiveness. Furthermore, by appending large hydrophobic groups at the molecular end, we can augment its oil-enhancing capacity while ensuring its solubility, resulting in the production of low-viscous microemulsions. In the end, core flooding tests indicated that at a surfactant concentration of 0.5 wt%, NaCl concentration around 0.9 wt%, and a core permeability of 3537 mD, the overall crude oil recovery efficiency could achieve 98%. This effectively boosts the recovery rate of crude oil.

5. Conclusions and Recommendations

This article critically summarizes the formation mechanism of microemulsions and the impact of different surfactant molecular structures on the structure–function relationship of microemulsions. From the perspective of the formation mechanism of microemulsions, the development has gone through instantaneous negative interfacial tension theory, double layer membrane theory, R-ratio theory, and packing parameter theory. Through continuous and detailed studies on microemulsion systems, the HLD theory was established, and subsequently, the HLD-NAC model was introduced. The aim is to simulate the phase behavior of microemulsion systems, shortening the screening process for surfactants, thereby aiding in the chemical design and optimization of enhanced oil recovery formulas and improving the predictability of surfactant flooding.

Conventional surfactants increase the intermolecular oil penetration volume by methods such as changing the position of the benzene ring, extending the alkyl tail, and choosing hydrophilic heads of different polarities to enhance interactions between surfactant molecules and oil–water molecules; anionic–nonionic surfactants, based on Conventional surfactants, introducing intermediate polar groups, enhancing the interaction between surfactant molecules and both oil and water phases; and Gemini surfactants enhancing the adsorption capacity of surfactant molecules by increasing the number of hydrophilic heads, with the linkage of bridge bonds significantly reducing the repulsion between the head groups, forming a denser adsorption layer with higher interfacial activity. Ultimately, this can result in the formation of larger volume Winsor Type III microemulsions, further enhancing the recovery rate.

The development of microemulsion theory should be integrated with the molecular design of surfactants. It is essential not only to guide molecular design with existing theories but also to explore through experimental molecular design to study the behavior of microemulsion systems under the combined factors of temperature, pressure, and pH value. This approach will more comprehensively describe the complexity and heterogeneity of microemulsions, which is a crucial direction for future research and vital for understanding the dynamic equilibrium and environmental adaptability of microemulsions.

Merely increasing the penetration volume of oil molecules through molecular structure is insufficient; the key lies in actually increasing the number of penetrating oil molecules. Given the intricate interplays among oil molecules, they generally fail to infiltrate the tertiary phase systematically, resulting in a diminished efficiency of the penetration volume. By designing molecules with multiple hydrophilic and hydrophobic groups and adding lipophilic linkers, the practical utilization rate of the permeation volume can be enhanced. Therefore, attention should be paid to the interaction between the alkyl tail and oil molecules in structural design.

The microemulsion oil displacement technique shows significant advantages in enhancing oilfield recovery rates. In contrast to conventional waterflood methods, it is adept at efficiently removing oil droplets from rock formations and infiltrating the minute pores of oil reservoirs, accomplishing microlevel displacement of oil by water. Moreover, by modifying the wettability of reservoir rocks and decreasing the oil’s viscosity, microemulsions amplify the flow characteristics of the oil and also display merits concerning environmental conservation and economic returns, given their generally lesser dependency on chemical additives. In summary, the microemulsion oil displacement technique integrates multiple mechanisms, significantly elevating oilfield recovery rates and emerging as a production enhancement technology with broad application prospects in oilfield development.