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Microemulsion

A microemulsion is a thermodynamically stable, isotropic liquid dispersion composed of water, oil, surfactant, and often a cosurfactant, forming nanometer-sized droplets typically ranging from 10 to 100 nm in diameter, which results in optical transparency and spontaneous formation without high-energy input.^[1]^[2] Unlike conventional emulsions, microemulsions exhibit low interfacial tension between the oil and water phases, enabling high solubilization capacity for both hydrophilic and hydrophobic substances.^[3]^[4] The term "microemulsion" was coined in 1959 by Jack H. Schulman and colleagues, building on earlier observations of stable oil-water mixtures stabilized by amphiphiles dating back to 1943.^[1] Microemulsions can adopt various microstructures, including oil-in-water (O/W), water-in-oil (W/O), and bicontinuous phases, where oil and water domains interpenetrate, depending on the composition ratios and the nature of the surfactants used.^[3]^[1] These systems are characterized by low viscosity, thermodynamic stability over time, and the ability to incorporate a wide range of active ingredients, making them distinct from macroemulsions that require mechanical agitation and are prone to phase separation.^[4] Microemulsions have found extensive applications in pharmaceuticals as drug delivery vehicles, particularly for enhancing the bioavailability of poorly soluble drugs through topical, oral, and parenteral routes, due to their ability to improve skin permeation and protect payloads from degradation.^[1]^[3] Beyond medicine, they are utilized in cosmetics for improved texture and stability, in the food industry for emulsification, and in chemical processes like enhanced oil recovery owing to their tunable interfacial properties.^[4] Recent advancements focus on formulating microemulsions with biocompatible components to address challenges in scalability and regulatory approval for clinical use.^[1]

Fundamentals

Definition

A microemulsion is defined as a dispersion made of water, oil, and surfactant(s) that forms an isotropic and thermodynamically stable system with dispersed domain diameters ranging from approximately 10 to 100 nm. These systems consist of two immiscible liquids, typically oil and water, mixed in comparable proportions to create low-viscosity, microheterogeneous fluids stabilized by amphiphilic surfactants that significantly reduce interfacial tension.^[5] The concept of such transparent dispersions was first described in 1943 by Hoar and Schulman, who observed the formation of clear water-in-oil systems upon addition of alcohols to emulsions. Microemulsions form spontaneously without requiring high-energy mechanical input, owing to the ultra-low interfacial tension (often near zero) enabled by the surfactant and optional cosurfactant components, which allows for easy self-assembly into nanoscale structures.^[5] This thermodynamic stability distinguishes them from kinetically stable emulsions, as microemulsions maintain their properties indefinitely under given conditions without phase separation.^[6] The resulting domain sizes, smaller than the wavelength of visible light (approximately 400–700 nm), impart optical transparency or translucency to the mixture. The typical composition includes an oil phase (hydrophobic solvent), an aqueous phase (water or ionic solution), surfactants (amphiphilic molecules that adsorb at the interface to minimize energy), and frequently a cosurfactant such as short-chain alcohols (e.g., n-butanol) to increase the interfacial flexibility and enlarge the stability region.^[5] Droplet diameters in these domains generally fall between 10 and 100 nm, enabling unique solubilization properties for both hydrophilic and lipophilic substances.

Properties

Microemulsions are characterized by ultralow interfacial tension between the oil and water phases, typically on the order of $10^{-3} mN/m or less, which facilitates the spontaneous formation of nanoscale domains without external energy input.^[7] This low tension, combined with the presence of surfactants and cosurfactants, enables high solubilization capacity for both hydrophobic and hydrophilic substances, allowing microemulsions to act as versatile carriers for poorly soluble compounds across a wide range of compositions.^[8] Physically, they exhibit Newtonian flow behavior, with viscosities generally ranging from 1 to 10 cP, akin to that of water, due to their dilute, isotropic nature at low concentrations.^[9] Optically, microemulsions appear transparent or translucent because their droplet sizes, typically 10–100 nm, are smaller than one-twentieth of the visible light wavelength (\lambda \approx 400–700 nm), preventing the Tyndall effect observed in larger emulsions.^[10] Electrical conductivity in microemulsions varies significantly with their type: oil-in-water (o/w) systems display high conductivity (on the order of mS/cm) due to the continuous aqueous phase, while water-in-oil (w/o) systems show low conductivity (on the order of \muS/cm) dominated by the non-conducting oil phase; bicontinuous structures fall between these extremes.^[11] Microemulsions demonstrate thermodynamic stability over broad temperature ranges, often from -10°C to 100°C depending on composition, without phase separation or coalescence under equilibrium conditions.^[12] This thermal resilience arises from the balanced intermolecular interactions that maintain the nanoscale structure across environmental variations.

Comparison with Other Systems

Microemulsions differ fundamentally from macroemulsions in their formation and stability. While microemulsions form spontaneously due to their thermodynamic stability, characterized by a negative Gibbs free energy of formation (ΔG < 0), macroemulsions require significant mechanical energy input for creation and are merely kinetically stable (ΔG > 0), leading to eventual creaming or phase separation over time.^[13] Additionally, microemulsion droplets typically range from 1 to 100 nm in diameter, rendering them transparent and low-viscosity, in contrast to the larger 0.1–100 μm droplets of macroemulsions, which appear opaque and milky.^[13]^[14] In comparison to nanoemulsions, microemulsions share nanoscale droplet sizes (typically <100 nm) but exhibit distinct stability profiles. Microemulsions achieve thermodynamic equilibrium without the need for high-energy processes, often relying on cosurfactants to lower interfacial tension, whereas nanoemulsions are kinetically stabilized and commonly produced via methods like high-pressure homogenization, potentially without cosurfactants, though they may destabilize over extended periods.^[15] This thermodynamic favorability (ΔG < 0) in microemulsions contrasts with the higher free energy state of nanoemulsions relative to phase-separated systems.^[15] Microemulsions also contrast with micelles and liposomes in their structural complexity and composition. Unlike micellar solutions, which consist of dilute surfactant aggregates forming simple spherical or cylindrical structures with limited oil incorporation and no tension gradient across the interface, microemulsions feature structured phases, including bicontinuous networks, that accommodate comparable volumes of oil and water through a duplex surfactant film with differing tensions on each side.^[16] Similarly, while liposomes are bilayer vesicles primarily composed of phospholipids for encapsulating hydrophilic or lipophilic drugs in an aqueous core, microemulsions employ surfactants and cosurfactants to create thermodynamically stable, isotropic dispersions without such vesicular architecture, enabling broader solubilization of both phases.^[17] A common misconception arises from the term "micro," which refers to the nanoscale domain sizes (1–100 nm) rather than visibility under a light microscope, as microemulsions appear clear and isotropic to the naked eye. Furthermore, unlike colloidal sols (solid particles dispersed in liquid) or gels (semi-solid networks), microemulsions remain fluid liquids with reversible phase behavior.^[14]

Components and Formation

Key Components

Microemulsions are formulated using four primary components: an oil phase, an aqueous phase, surfactants, and often cosurfactants, each selected to achieve thermodynamic stability and desired functionality.^[18] The oil phase typically consists of hydrocarbons such as decane or triglycerides like those found in vegetable oils (e.g., olive or sesame oil), or silicones, which provide hydrophobic domains for solubilizing lipophilic substances.^[19]^[3] Selection of the oil phase emphasizes compatibility with surfactants and the target application, as well as volatility for processes requiring evaporation.^[18] The aqueous phase is generally water or buffered solutions (e.g., phosphate-buffered saline at pH 7-9), forming hydrophilic regions for polar components.^[18] Its composition influences stability, with factors like pH and salinity playing key roles; for instance, high salinity concentrations favor water-in-oil microemulsions by altering ionic interactions at the interface.^[18] Surfactants, such as ionic types like sodium dodecyl sulfate (SDS), non-ionic ones like Tween 80 (polysorbate 80), or cationic variants, are essential for reducing interfacial tension between oil and water phases to near-zero values, enabling spontaneous formation.^[3] Selection criteria include biocompatibility, low toxicity, and the hydrophilic-lipophilic balance (HLB) value, typically 4-6 for water-in-oil systems and 8-18 for oil-in-water systems.^[18] Cosurfactants, often short-chain alcohols like butanol or pentanol, are not always required but enhance the flexibility of the surfactant film at the interface, lower the packing ratio of amphiphiles, and broaden the phase boundaries for microemulsion existence.^[20] They are chosen for their amphiphilic nature and synergy with surfactants, typically used in amounts less than the surfactant to avoid destabilization.^[18] Typical compositions involve 10-40% oil and 10-40% water by weight, with the balance consisting of combined surfactant and cosurfactant, though these ratios are optimized via phase diagrams to ensure clarity and stability.^[21]

Formation Mechanisms

Microemulsions form through spontaneous emulsification upon mixing oil, water, surfactants, and often cosurfactants at ambient temperature, leading to self-organization that minimizes the system's free energy without requiring external mechanical shear or energy input.^[22] This process relies on the surfactants' ability to rapidly adsorb at the oil-water interface, creating a stabilized dispersion that achieves thermodynamic equilibrium.^[23] Unlike macroemulsions, which demand high-energy homogenization, microemulsions emerge as clear, isotropic fluids due to the nanoscale domain sizes (typically 10–100 nm) that prevent light scattering.^[24] The formation is predominantly entropy-driven, where the entropy gain from dispersing the immiscible phases outweighs the enthalpic penalty of mixing, facilitated by ultralow interfacial tensions (on the order of 10⁻³ to 10⁻⁵ mN/m).^[23] Surfactants form flexible interfacial films with high bending elasticity, allowing thermal fluctuations to promote disorder and stabilize structures such as spherical droplets or bicontinuous networks.^[22] This entropy dominance arises from the large interfacial area per surfactant molecule, enabling efficient packing and curvature without significant energy barriers.^[24] Cosurfactants, such as short-chain alcohols (e.g., n-butanol), play a crucial role by increasing the curvature elasticity of the surfactant film, which accommodates diverse geometries ranging from spherical micelles to saddle-like bicontinuous phases.^[22] They enhance molecular mixing at the interface, further reducing interfacial tension and promoting phase inversion, such as through changes in temperature (phase inversion temperature, PIT) or composition, where the optimal PIT for nonionic surfactants marks the transition from oil-in-water to water-in-oil structures.^[23] Experimentally, microemulsion formation is assessed by titrating components—typically adding water or oil dropwise to a surfactant-cosurfactant-oil mixture—until optical clarity indicates a single-phase system.^[24] Pseudo-ternary phase diagrams are constructed to identify optimal compositional ratios, plotting the boundaries of microemulsion regions against variables like surfactant concentration.^[22] Factors such as salinity influence curvature via the hydrophilic-lipophilic deviation (HLD) framework, where increasing salt concentration (e.g., NaCl) shifts the spontaneous curvature from positive (favoring oil-in-water) to negative (water-in-oil), optimizing formation at HLD ≈ 0.^[23]

Classification

Oil-in-Water

Oil-in-water (o/w) microemulsions feature discrete nanoscale oil droplets, typically ranging from 10 to 100 nm in diameter, dispersed throughout a continuous aqueous phase and stabilized by a monolayer of surfactant molecules adsorbed at the oil-water interface.^[11] These systems are characterized by a high water content, generally exceeding 50% by volume, which ensures the water phase dominates the overall composition and provides a hydrophilic external environment. The formation of such structures relies on achieving ultralow interfacial tension between the oil and water phases, often facilitated by the presence of surfactants and cosurfactants.^[25] Key characteristics of o/w microemulsions include high electrical conductivity, attributable to the percolation of the continuous water phase that allows ion mobility.^[26] They exhibit a hydrophilic external feel due to the surrounding aqueous medium, making them distinct from oil-continuous systems.^[27] Additionally, the dispersed oil droplets enable effective solubilization of lipophilic compounds within the aqueous matrix.^[28] Representative examples include systems formulated with non-ionic surfactants such as Brij 30 (polyoxyethylene(4) lauryl ether) in water containing soybean oil as the dispersed phase, which form stable o/w microemulsions suitable for applications like creams or oral formulations.^[29]^[30] Stability in o/w microemulsions is optimized with surfactants having a hydrophilic-lipophilic balance (HLB) value greater than 10, which favors the curvature required for oil droplet formation in water.^[31] These systems are particularly sensitive to temperature increases, which can alter surfactant hydrophilicity and induce phase inversion to water-in-oil (w/o) structures via mechanisms like the phase inversion temperature (PIT).^[32]^[33]

Water-in-Oil

Water-in-oil (w/o) microemulsions feature discrete nanodroplets of water, typically ranging from 10 to 100 nm in diameter, dispersed within a continuous oil matrix, where the oil phase constitutes more than 50% of the total volume.^[22] These nanodroplets are stabilized by a monolayer of surfactant molecules at the water-oil interface, which curves toward the water phase to accommodate the reverse micellar structure, enabling the solubilization of polar substances in a nonpolar environment.^[22] Key characteristics of w/o microemulsions include their thermodynamic stability, low electrical conductivity due to the insulating oil continuous phase (often in the range of microsiemens per centimeter), and inherently hydrophobic nature, which makes them suitable for hosting water-sensitive chemical reactions by confining aqueous reactants within isolated nanodroplets.^[22] This structure protects hydrophilic solutes from the surrounding oil, facilitating applications such as microreactors for nanoparticle synthesis or enzymatic processes that require an aqueous microenvironment.^[34] Representative examples of w/o microemulsions include systems formulated with Span 80 (sorbitan monooleate) as the surfactant in isooctane as the oil phase, incorporating brine as the aqueous component, which forms stable dispersions used in extraction processes. Such systems are commonly employed in metal extraction, where heavy metals like copper or zinc achieve high extraction efficiencies through selective partitioning into the aqueous nanodroplets, followed by phase separation. In cosmetics, w/o microemulsions enhance the delivery of water-soluble actives, such as vitamins or peptides, into oil-based formulations for improved skin penetration and product stability. Stability in w/o microemulsions is primarily favored by surfactants with low hydrophilic-lipophile balance (HLB) values, typically below 8 (e.g., 3-6 for optimal curvature), which promote oil-continuous phases by enhancing lipophilicity.^[22] High salinity levels further stabilize these systems by screening electrostatic repulsions at the interface and altering the effective headgroup area of ionic surfactants, thereby reducing the spontaneous curvature toward the oil side.^[22] Phase inversion to or from w/o microemulsions can occur through Winsor progression, where changes in composition or conditions shift from Winsor Type I (oil-in-water) to Type II (water-in-oil) via intermediate bicontinuous states.^[35]

Bicontinuous

Bicontinuous microemulsions represent a distinct class within microemulsion systems, characterized by interpenetrating domains of oil and water that form infinite, interconnected channels rather than discrete droplets. The structure arises from saddle-shaped surfactant films that exhibit negative Gaussian curvature, enabling the creation of a sponge-like network where both oil and water phases percolate continuously throughout the system. These domains typically span 5-20 nm in size, allowing for a highly ordered yet fluid arrangement stabilized by the surfactant monolayer. This configuration is most prevalent at balanced oil-to-water volume ratios, approximately equal, which promotes the isometric nature of the phases without favoring one over the other.^[36]^[37]^[38] Key characteristics of bicontinuous microemulsions include moderate electrical conductivity, stemming from the percolation of both conductive (water) and non-conductive (oil) pathways, which results in a transitional regime between high-conductivity oil-in-water and low-conductivity water-in-oil systems. The interfaces maintain zero mean curvature on average, a feature that minimizes energetic penalties and supports the balanced curvature required for the networked topology. Additionally, these systems boast a high internal interfacial area, on the order of 100 m²/g, which dramatically enhances molecular interactions and facilitates rapid diffusion of solutes through the interconnected channels, enabling efficient transport across phases.^[39]^[40]^[41] Representative examples of bicontinuous microemulsions occur in middle-phase regions of ternary surfactant-oil-water systems, such as those formed with Aerosol OT (AOT), water, and decane at optimal salinity levels around 0.6% NaCl, where the structure emerges as a single isotropic phase. These are often observed during transitions in phase diagrams, including the appearance of fish-eye phases or shifts from lamellar structures, highlighting the dynamic equilibrium in such formulations.^[42] The stability of bicontinuous microemulsions hinges on the flexibility of the surfactant films, which must accommodate the saddle-like deformations without fracturing, a property enhanced by surfactants with tunable bending rigidity. They are commonly associated with Winsor III systems, where the middle phase coexists with excess oil and water, providing a thermodynamically stable environment under balanced formulation conditions.^[38]^[37]

Theoretical Foundations

Thermodynamic Stability

Microemulsions exhibit thermodynamic stability, meaning they form spontaneously and persist indefinitely without phase separation, as the Gibbs free energy change (ΔG) for their formation is negative. This stability arises from the balance in the equation ΔG = ΔH - TΔS, where ΔH represents the enthalpy change (primarily the positive interfacial energy cost), T is the absolute temperature, and ΔS is the entropy change (positive due to increased disorder from dispersing one phase into the other). In microemulsions, the entropy gain from the dispersed state dominates over the interfacial enthalpy penalty, ensuring ΔG < 0 and spontaneous formation under appropriate conditions.^[43]^[44] The free energy of mixing in microemulsions can be expressed as ΔG_mix = RT [x \ln x + (1-x) \ln (1-x)] + \gamma \Delta A, where R is the gas constant, T is temperature, x is the mole fraction of the dispersed phase, the first term captures the ideal entropy of mixing, and the second term accounts for the interfacial energy contribution with \gamma as interfacial tension and \Delta A as the increase in interfacial area. This spontaneity is enabled by ultra-low interfacial tension (\gamma < 0.1 , \mathrm{mN/m}), achieved through efficient surfactant packing at the interface, which minimizes the positive \gamma \Delta A term relative to the entropic contribution. Cosurfactants, such as medium-chain alcohols, further enhance stability by reducing \gamma and increasing configurational entropy through greater flexibility in the surfactant monolayer, allowing more dispersed configurations without energetic penalty.^[44]^[11] Unlike macroemulsions, which rely on kinetic stability against processes like creaming or coalescence, microemulsions achieve true equilibrium stability, rendering them reversible upon changes in composition or temperature without hysteresis. This equilibrium nature stems from the minimized free energy state, where the system resists separation due to the favorable balance of entropy and low interfacial energy.^[11]^[43]

Interfacial Tension and Packing

Microemulsions form due to the adsorption of surfactants at the oil-water interface, which dramatically lowers the interfacial tension (IFT) to ultralow values, typically ranging from 10^{-3} to 10^{-4} mN/m, enabling the creation of nanoscale droplets or bicontinuous structures with enormous interfacial areas on the order of 10^3 m^2 per gram of surfactant. This reduction occurs through the Gibbs adsorption isotherm, which relates changes in IFT to surfactant accumulation at the interface; for systems following Langmuir adsorption behavior, the IFT can be modeled as

\gamma = \gamma_0 - RT \Gamma \ln(1 + K c)

where \gamma_0 is the IFT without surfactant, R is the gas constant, T is temperature, \Gamma is the maximum surface excess concentration, K is the adsorption equilibrium constant, and c is the bulk surfactant concentration. Such near-zero IFT minimizes the energy penalty for interface formation, promoting thermodynamic stability without requiring external energy input, unlike conventional emulsions. The molecular geometry of surfactants dictates the curvature and topology of microemulsion interfaces through the packing parameter P = v / (a l), where v is the volume of the hydrophobic tail, a is the effective cross-sectional area of the hydrophilic headgroup, and l is the extended length of the tail. Values of P < 1/3 correspond to conical-shaped surfactants that favor oil-in-water (o/w) microemulsions with spherical micelles exhibiting positive mean curvature; $1/3 < P < 1 indicates wedge-shaped molecules suitable for bicontinuous phases with saddle-like curvatures; and P > 1 promotes cylindrical or inverted cone geometries for water-in-oil (w/o) microemulsions with negative curvature. This parameter provides a geometric rationale for how surfactant molecular design influences aggregate morphology, with cosurfactants like short-chain alcohols often tuning P by altering headgroup hydration or tail volume. The flexibility of surfactant monolayers in microemulsions arises from their low bending rigidity, allowing spontaneous adoption of both positive and negative curvatures essential for diverse structures. This elasticity is quantified by the Helfrich bending energy functional,

F_b = \int \left[ \frac{\kappa}{2} (2H - C_0)^2 + \bar{\kappa} K \right] dA,

where \kappa is the bending modulus (typically 0.5–2 k_B T for microemulsions), \bar{\kappa} is the Gaussian modulus, H is the mean curvature, C_0 is the spontaneous curvature (tunable by formulation), K is the Gaussian curvature, and the integral is over the monolayer surface. Low \kappa values facilitate the high interfacial fluctuations needed for microemulsion formation, contrasting with rigid bilayers in lamellar phases. The Winsor classification links these interfacial properties to phase behavior: Type I systems feature o/w microemulsions in equilibrium with excess oil, where the IFT between the microemulsion and oil phase is relatively higher (around 10^{-2} mN/m); Type III bicontinuous microemulsions coexist with both excess oil and water, exhibiting ultralow IFT (<10^{-3} mN/m) with both phases due to optimal packing and minimal bending energy; and Type II systems involve w/o microemulsions with excess water, mirroring Type I but with inverted curvature and IFT characteristics. This progression reflects how formulation variables like salinity or temperature shift C_0 and IFT, transitioning between types while maintaining near-zero effective tension for stability.

Phase Behavior

The phase behavior of microemulsions is characterized by reversible transitions between distinct structures, influenced primarily by temperature, salinity, and composition, which alter the interfacial curvature and stability of the dispersed phases.^[45] Temperature plays a pivotal role in nonionic surfactant-based microemulsions, where increasing temperature shifts the hydrophilic-lipophilic balance toward oil solubility, leading to a phase inversion from oil-in-water (o/w) to water-in-oil (w/o) structures. This transition occurs at the phase inversion temperature (PIT), a characteristic point where the spontaneous curvature of the surfactant film changes sign due to dehydration of the hydrophilic headgroups, enabling efficient emulsification when the system is heated through the PIT and then cooled. For nonionic surfactants like polyethoxylated alcohols (e.g., C12E4), the PIT correlates closely with the cloud point, above which the surfactant becomes less hydrophilic, promoting w/o microemulsions; typical PIT values range from 20–80°C depending on the ethoxylation degree.^[46] Salinity effects are prominent in ionic surfactant systems, where added electrolytes screen the electrostatic repulsion between charged headgroups, reducing the effective headgroup area and favoring negative curvature for w/o microemulsions. As salinity increases, the system progresses from Winsor I (o/w microemulsion coexisting with excess oil) through a three-phase region (Winsor III, with a bicontinuous microemulsion) to Winsor II (w/o microemulsion coexisting with excess water), with the optimal salinity marking the point of minimal interfacial tension in the middle-phase regime. Critical salinity for phase splitting occurs when the solubilization capacity is exceeded, leading to macroscopic separation; for anionic surfactants like sodium dodecyl sulfate, this threshold can be around 5–10 wt% NaCl, depending on cosurfactant presence.^[47]^[48] Composition sweeps, particularly varying surfactant concentration at fixed oil-to-water ratios, reveal characteristic "fish" diagrams that delineate the one-phase microemulsion channel flanked by two- and three-phase regions. In these diagrams, the "fish tail" at low surfactant concentrations shows a narrow channel of isotropic one-phase microemulsions, widening into a three-phase body as surfactant increases, before narrowing again at high concentrations where ordered phases dominate; this behavior arises from the balance of curvature and packing constraints. For systems like water/n-dodecane/C8E3, the one-phase region spans surfactant fractions of 5–20 wt% at optimal temperature or salinity. The packing parameter briefly influences these sweeps by dictating curvature preferences, but observable transitions reflect macroscopic equilibrium. Microemulsions often coexist with excess oil or water phases outside the one-phase channel, forming macroscopically separated layers while maintaining ultralow interfacial tensions at the boundaries. At high surfactant concentrations (typically >20 wt%), viscoelastic transitions to lamellar phases occur, driven by increased monolayer bending rigidity and entropy reduction, resulting in stacked bilayers that separate oil and water domains; this is evident in systems like SDS/brine/cyclohexane, where the lamellar phase stabilizes above the microemulsion channel.^[45]

Phase Diagrams

Construction Methods

Phase diagrams for microemulsions are constructed using a combination of experimental and computational approaches to map the compositional regions where stable, isotropic microemulsion phases form. For ternary systems involving surfactant, oil, and water, diagrams are plotted in equilateral triangular coordinates, with each vertex representing 100% of one component. The microemulsion region is identified as the area of low viscosity and optical transparency, often encompassing oil-in-water, water-in-oil, or bicontinuous structures.^[11] Experimental construction of ternary diagrams typically employs titration methods, where fixed ratios of surfactant and oil are progressively diluted with water (or vice versa) until the mixture achieves clarity, indicating the phase boundary. Visual inspection confirms the isotropic nature of the microemulsion by observing transparency against a light source, while centrifugation at 10,000–20,000 rpm for 30–60 minutes accelerates phase separation along tie-lines, allowing separation of coexisting phases for further analysis. Conductivity measurements, using a probe to scan along dilution lines, distinguish phase types: high conductivity (mS/cm) signals oil-in-water microemulsions due to ionic pathways in the aqueous phase, while low values (µS/cm) indicate water-in-oil structures; bicontinuous regions show intermediate, percolating conductivities. Turbidity scans, monitored via spectrophotometry at 600 nm, complement these by detecting cloudiness at phase boundaries where macroscopic separation occurs.^[11] When cosurfactants such as short-chain alcohols are included, pseudo-ternary diagrams simplify the quaternary system by fixing the cosurfactant-to-surfactant weight ratio (e.g., 1:1 or 2:1) and treating the mixture as a single "effective surfactant" vertex. These diagrams are constructed similarly via water titration, revealing expanded microemulsion regions due to reduced interfacial tension, with the largest isotropic areas often at optimal ratios determined iteratively. For full quaternary systems (surfactant, cosurfactant, oil, water), tetrahedral diagrams are challenging to visualize, so slice diagrams are used, projecting fixed-ratio cuts (e.g., constant cosurfactant concentration) onto ternary planes to map phase behavior. Computational modeling aids this, with software like COMSOL Multiphysics simulating phase equilibria through phase-field methods that solve coupled diffusion equations for component concentrations, predicting boundaries without exhaustive experiments. Fish-tail diagrams, particularly for nonionic surfactants, are generated by scanning temperature and salinity (or surfactant concentration) at fixed oil-to-water ratios (e.g., 1:1). These plot the lower boundary of the single-phase microemulsion region, forming a "fish tail" shape where the phase widens at optimal conditions and narrows with increasing salinity or temperature, reflecting curvature transitions; the tail tip marks the minimum surfactant concentration for stability. Anionic surfactants yield narrower tails, while nonionics show broader, temperature-sensitive regions.^[11]

Interpretation

Phase diagrams for microemulsions are graphical representations that map the equilibrium phase behavior of surfactant-oil-water systems, typically as functions of composition, temperature, or salinity, allowing prediction of stable microemulsion regions and phase transitions. The one-phase region, often shaded in these diagrams, denotes the area of thermodynamic stability where a single, isotropic microemulsion phase forms without phase separation, reflecting the system's ability to solubilize oil and water at ultralow interfacial tensions on the order of 10^{-4} mJ/m². The width of this region indicates surfactant efficiency, defined as the minimum surfactant concentration (φ_s^*) required to achieve solubilization of equal volumes of oil and water; for instance, non-ionic surfactants like C_{12}E_5 (pentaethylene glycol monododecyl ether) typically yield broader one-phase regions due to their temperature-tunable hydrophile-lipophile balance (HLB), whereas ionic surfactants often require co-surfactants or electrolytes to expand the region, resulting in narrower stability domains under similar conditions.^[11]^[14] In multiphase regions of the diagram, tie-lines connect the compositions of coexisting phases, such as a microemulsion in equilibrium with excess oil or water, enabling analysis of phase compositions and relative volumes via the lever rule: the position of a bulk composition along a tie-line determines the volume fractions of each phase, with the distance to each end inversely proportional to the phase amounts. This interpretation is central to classifying Winsor types of phase behavior—Winsor I (Type I), where an oil-in-water (o/w) microemulsion coexists with excess oil; Winsor II (Type II), featuring a water-in-oil (w/o) microemulsion with excess water; and Winsor III (Type III), characterized by a bicontinuous microemulsion in equilibrium with both excess oil and water, forming a three-phase tie-triangle with invariant phase compositions at the vertices. These types arise from curvature transitions at the oil-water interface, driven by formulation variables, and the diagrams predict transitions between them, such as from o/w to bicontinuous as conditions favor zero mean curvature.^[11]^[14] The "fish diagram," a common binary cut of the phase prism at equal oil-to-water ratios (φ_w = φ_o), provides a simplified view for identifying optimal formulations, where the midline or "fish head" at the balance temperature (T_0) corresponds to ultra-low interfacial tension and maximum solubilization capacity, ideal for applications requiring efficient oil recovery or drug delivery. Along this diagram, temperature dependence reveals inversion loci: for non-ionic surfactants, increasing temperature shifts from Winsor I (negative curvature, o/w) through Winsor III (zero curvature, bicontinuous) to Winsor II (positive curvature, w/o), with the three-phase body forming the "fish tail" at lower surfactant concentrations; this tunability stems from the dehydration of ethylene oxide headgroups, altering spontaneous curvature. In contrast, ionic systems often use salinity as the tuning parameter, with similar inversion patterns but potentially sharper transitions.^[11] Recent advances as of 2024 include the integration of machine learning to predict phase boundaries from limited experimental data and advanced in situ techniques like small-angle X-ray scattering under confinement to study nanoscale phase behavior in porous media.^[49]^[50] Despite their utility, phase diagrams have limitations in interpretation, as they depict thermodynamic equilibrium states and ignore kinetic factors such as phase separation times or metastable states that may persist outside the predicted regions. Additionally, most diagrams are 2D projections of higher-dimensional phase prisms (e.g., ternary composition with temperature or salinity), simplifying complex 4D behaviors and potentially overlooking nuances in cosurfactant effects or multicomponent interactions. These tools thus excel in predicting stable microemulsion formation but require complementary experimental validation for dynamic processes.^[11]^[14]

Applications

Industrial Uses

Microemulsions play a significant role in enhanced oil recovery (EOR) by forming low-interfacial-tension systems that mobilize residual oil trapped in reservoir pores. These formulations, often involving surfactants like sodium dodecyl sulfate combined with cosurfactants such as n-butanol, reduce oil-water interfacial tension to ultralow levels (10⁻²–10⁻³ mN/m), enabling efficient displacement through surfactant flooding. For instance, in low-permeability reservoirs, microemulsion systems with internal olefin sulfonates have achieved additional oil recovery of up to 30% over conventional water flooding, as demonstrated in laboratory core flooding experiments and field trials like those at the Jidong oilfield in China, where cumulative oil production increased by 3000 tons.^[51]^[52] In cosmetics and personal care products, microemulsions enable the creation of transparent formulations such as shampoos and creams that solubilize fragrances and oils without phase separation. Their nanoscale droplet sizes (10–100 nm) contribute to optical clarity and thermodynamic stability, while nonionic surfactants like polyoxyethylene (20) oleyl ether ensure low irritation potential for skin and hair applications. Examples include clear perfuming gels incorporating 33% perfume oil with glycerol for enhanced spreadability and long-term stability exceeding one year at room temperature, and hair conditioners using amino-functional silicones for improved conditioning without residue.^[53] The food industry utilizes food-grade microemulsions as emulsifiers to incorporate flavors and oils into aqueous-based products like beverages, achieving clear, stable dispersions. Lecithin-based systems, for example, solubilize hydrophobic bioactives such as essential oils, enhancing their bioavailability while maintaining low viscosity and transparency suitable for commercial drinks. These formulations leverage the isotropic nature of microemulsions to prevent creaming or sedimentation, supporting applications in flavor delivery without altering sensory attributes.^[54] Microemulsions enhance cleaning products, including detergents, by providing superior grease solubilization through their oil-in-water structures stabilized by surfactants like Pluronic block copolymers. In formulations with essential oils such as thyme or cinnamon, they effectively remove oily soils, reducing carbon content by up to 52.9% in cleaning tests on metal surfaces, while also serving in agrochemicals for controlled pesticide release via nanoscale encapsulation. This dual action of emulsification and dispersion improves efficacy in both household and industrial cleaners.^[55] In paints and coatings, microemulsions facilitate better pigment dispersion and enable low-volatile organic compound (VOC) formulations by stabilizing nanoscale domains of resins and solvents. Systems polymerized with surfactants like cetyltrimethylammonium bromide produce particles as small as 0.02–0.14 µm, leading to homogeneous, high-intensity colors and reduced environmental impact compared to traditional solvent-based paints. Acrylate lattices derived from such microemulsions exemplify their role in durable, eco-friendly coatings.^[56]

Pharmaceutical Applications

Microemulsions have emerged as versatile carriers in pharmaceutical formulations, particularly for enhancing the solubilization of poorly water-soluble drugs, which often belong to Biopharmaceutics Classification System (BCS) classes II and IV. Their large interfacial area and low interfacial tension enable the incorporation of lipophilic actives into the oil phase or at the interface, significantly improving aqueous solubility and bioavailability compared to conventional emulsions or suspensions. For instance, cyclosporine, a hydrophobic immunosuppressant, is solubilized in microemulsion-based formulations like Neoral, where it achieves effective oral absorption by forming a fine emulsion upon dilution in gastrointestinal fluids.^[57]^[58] In drug delivery systems, microemulsions facilitate various administration routes. For topical applications, oil-in-water (o/w) microemulsions promote skin penetration of actives like ibuprofen or estradiol by reducing the skin's barrier resistance and enhancing partitioning into the stratum corneum. Oral delivery benefits from self-microemulsifying drug delivery systems (SMEDDS), which spontaneously form microemulsions in the gut, promoting lymphatic absorption and bypassing first-pass metabolism; examples include formulations of saquinavir and ritonavir for antiretroviral therapy. Parenteral routes utilize stable microemulsions for intravenous administration, such as those loading paclitaxel at concentrations up to 2.5 mg/mL, offering Cremophor EL-based systems with improved tolerability over traditional solvents.^[58]^[59] Additionally, vitamin E (α-tocopherol) microemulsions serve as antioxidant carriers, enhancing mucosal delivery and stability in oral or topical contexts.^[60] Key advantages of microemulsions in pharmaceuticals include controlled release profiles, which sustain drug levels and reduce dosing frequency, and decreased toxicity through lower surfactant concentrations compared to macroemulsions. These systems also mitigate issues like precipitation in vivo, ensuring consistent pharmacokinetics. However, challenges such as potential surfactant-induced irritation, particularly from ionic types, can occur; this is often addressed by employing non-ionic surfactants like polysorbates, which exhibit better biocompatibility and lower cytotoxicity. Regulatory approval underscores their clinical viability, with FDA-cleared products like Neoral (cyclosporine microemulsion) demonstrating enhanced bioavailability over predecessors like Sandimmune, achieving up to 50% higher absorption in transplant patients.^[58]^[61]^[57]

Emerging Developments

Recent advancements in microemulsion technology have integrated nanotechnology to enhance targeted drug delivery systems, particularly for cancer therapies. Hybrid microemulsions incorporating gold nanoparticles (AuNPs) in oil-in-water (o/w) configurations have shown promise for simultaneous imaging and treatment, where AuNPs serve as contrast agents in magnetic resonance imaging (MRI) while enabling photothermal ablation of tumor cells. Studies have demonstrated improved tumor accumulation via the enhanced permeability and retention effect and higher cellular uptake compared to free AuNPs, with minimal toxicity to healthy tissues.^[58] Sustainable formulations of microemulsions are gaining traction through the use of bio-based surfactants derived from renewable sources like sugars, aiming to reduce environmental impact in applications such as enhanced oil recovery (EOR). These green surfactants, such as alkyl polyglucosides, lower interfacial tension to below 10^{-2} mN/m, facilitating oil mobilization while being biodegradable and non-toxic. In 2023 research, bio-based surfactant microemulsions achieved 15-20% additional oil recovery in core flooding experiments under reservoir conditions, outperforming synthetic counterparts in salinity tolerance.^[62] Enzyme encapsulation within microemulsions has emerged as a key strategy for biocatalysis, protecting enzymes from denaturation and enabling efficient substrate conversion in non-aqueous media. Reverse microemulsions have been used to immobilize lipases, maintaining over 90% activity after multiple cycles in biodiesel production. A 2023 report highlighted enzyme-loaded microemulsions that catalyzed esterification reactions with 95% yield, demonstrating reusability up to 10 times without significant loss in performance.^[63] In gene and vaccine delivery, bicontinuous microemulsions provide a protective matrix for sensitive nucleic acids like siRNA, shielding them from nuclease degradation while facilitating endosomal escape. These structures, with interconnected water and oil domains, enhance siRNA loading and achieve targeted silencing in hepatocytes. For COVID-19 vaccines, emulsion-based adjuvants have been explored to boost immune responses, with squalene-in-water systems eliciting higher antibody titers in preclinical models compared to non-adjuvanted formulations.^[58]^[64] Microemulsion-based approaches are advancing environmental remediation, particularly through soil washing to extract hydrophobic pollutants like petroleum hydrocarbons. Biodiesel-derived microemulsions reduce interfacial tension to ultralow levels (<10^{-3} mN/m), desorbing up to 70% of total petroleum hydrocarbons from contaminated soils in batch tests. Additionally, microemulsions enhance CO2 solubilization for carbon capture, with surfactant-stabilized systems increasing CO2 solubility by 25-30% in aqueous phases under mild pressures, aiding post-combustion capture processes.^[65]^[66] Despite these innovations, challenges in scalability persist, as transitioning from lab-scale to industrial production often results in phase instability and inconsistent droplet sizes due to mixing inefficiencies. As of 2025, AI-optimized phase diagrams have addressed this by predicting optimal surfactant ratios with high accuracy, reducing formulation trials and enabling custom designs for specific applications. Machine learning models trained on hydrophile-lipophile balance data have successfully mapped microemulsion regions, forecasting stability under varying temperatures and salinities.^[67]^[68]^[69]

History

Early Discoveries

In 1943, researchers T. P. Hoar and J. H. Schulman at the University of Cambridge observed clear, stable mixtures of oil, water, and soap, which formed spontaneously upon mixing without requiring high-shear emulsification.^[70] These transparent dispersions exhibited low interfacial tension at the oil-water interface, allowing nanoscale domains to persist indefinitely, marking the first documented recognition of such thermodynamically stable systems, though the term "microemulsion" was not yet applied. During the 1950s, J. W. McBain introduced the concept of "swollen micelles" to characterize similar oil-swollen surfactant aggregates in aqueous solutions, emphasizing their role in solubilizing hydrocarbons through expanded micellar structures. Early investigations by McBain and collaborators also examined soap films and low-tension interfaces, revealing how surfactants reduced interfacial energy to facilitate these isotropic, low-viscosity phases without external energy input. In the 1960s, L. M. Prince and colleagues advanced understanding through studies of ternary phase behavior in detergent systems, mapping regions where water, oil, and surfactants formed stable, isotropic liquids via electron microscopy and phase diagrams. This work highlighted spontaneous structuring in nonionic and anionic detergent mixtures, paving the way for practical insights. In 1968, W. R. Foster proposed using these low-tension surfactant systems for tertiary oil recovery, demonstrating their potential to mobilize residual oil by achieving ultralow interfacial tensions on the order of 10^{-3} mN/m. Key experiments in this era relied on conductivity and viscosity measurements to confirm the nanoscale domains in these mixtures; for instance, high electrical conductivity in oil-continuous systems indicated percolating water channels, while Newtonian viscosity profiles underscored the absence of large droplets, distinguishing them from macroemulsions that require intense mechanical shear for formation. Thermodynamic favorability of these systems, later formalized, stemmed from the entropy-driven mixing enabled by surfactants.

Terminology Evolution

The term "microemulsion" was first introduced in the late 1950s by J.H. Schulman and colleagues to describe transparent, thermodynamically stable dispersions of oil and water in the presence of surfactants and cosurfactants, distinguishing them from kinetically stable macroemulsions that scatter light and appear opaque.^[71] This terminology emerged from early studies on low-interfacial-tension systems, where Schulman et al. noted the spontaneous formation of these isotropic solutions without high-energy input, unlike conventional emulsions.^[37] By the early 1970s, researchers including I. Danielsson and P. Stenius further characterized these systems through colloidal and interfacial analyses, emphasizing their nanoscale domain sizes (typically 1–100 nm) and optical clarity. In the 1970s, P.A. Winsor's earlier classification framework from 1948—categorizing surfactant systems into Types I (oil-in-water microemulsion in equilibrium with excess oil), II (water-in-oil microemulsion with excess water), and III (bicontinuous middle-phase microemulsion coexisting with both excess oil and water)—gained widespread adoption for microemulsions, providing a basis for understanding phase behavior and stability.^[72] This classification helped standardize discussions at emerging international forums, highlighting the reversible phase transitions driven by variables like salinity, temperature, or cosurfactant concentration. In 1976, L.E. Scriven introduced the concept of "bicontinuous," describing interconnected oil and water domains in Type III systems without discrete droplets, a concept formalized through thermodynamic modeling of minimal surfaces and later validated by scattering techniques.^[73] Debates arose over nomenclature, particularly distinguishing microemulsions from "nanoemulsions," which are kinetically stabilized fine dispersions formed by high-energy methods, lacking thermodynamic stability; these discussions intensified in the 1980s as applications in enhanced oil recovery prompted clearer terminological boundaries.^[74] From the 1990s onward, the Hydrophilic-Lipophilic Difference (HLD) framework developed by J.L. Salager in the late 1970s and refined into the HLD-Net Average Curvature (NAC) model in the 2000s provided a quantitative tool for predicting and standardizing microemulsion formulation by balancing surfactant hydrophilicity and lipophilicity against oil type and salinity.^[75] This approach enabled optimal Winsor III conditions for applications like drug delivery and extraction processes. The International Union of Pure and Applied Chemistry (IUPAC) formalized the definition in the 1990s as "a dispersion made of water, oil, and surfactant(s) that is an isotropic and thermodynamically stable system with dispersed domain diameter generally in the 1–100 nm range," underscoring stability over kinetic aspects. Terminological controversies persisted, notably confusion with "miniemulsions" in polymerization contexts, where miniemulsions refer to metastable nano-sized droplets (20–500 nm) stabilized by surfactants and hydrophobes for controlled radical polymerization, unlike the equilibrium structures of microemulsions.^[76] Current consensus, as reflected in recent reviews, avoids interpreting "micro" as implying microscopic visibility, instead emphasizing the nanoscale, non-scattering nature and thermodynamic favorability to prevent conflation with larger, unstable emulsions.^[77]

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