Review on Some Confusion Produced by the Bicontinuous Microemulsion Terminology and Its Domains Microcurvature: A Simple Spatiotemporal Model at Optimum Formulation of Surfactant-Oil-Water Systems

Abstract

Fundamental studies have improved understanding of molecular-level properties and behavior in surfactant-oil-water (SOW) systems at equilibrium and under nonequilibrium conditions. However, confusion persists regarding the terms “microemulsion” and “curvature” in these systems. Microemulsion refers to a single-phase system that does not contain distinct oil or water droplets but at least four different structures with globular domains of nanometer size and sometimes arbitrary shape. The significance of “curvature” in such systems is unclear. At high surfactant concentrations (typically 30 wt % or more), a single phase zone has been identified in which complex molecular arrangements may result in light scattering. As surfactant concentration decreases, the single phase is referred to as a bicontinuous microemulsion, known as the middle phase in a Winsor III triphasic system. Its structure has been described as involving simple or multiple surfactant films surrounding more or less elongated excess oil and water phase globules. In cases where the system separates into two or three phases, known as Winsor I or II systems, one of the phases, containing most of the surfactant, is also confusedly referred to as the microemulsion. In this surfactant-rich phase, the only curved objects are micellar size structures that are soluble in the system and have no real interface but rather exchange surfactant molecules with the external liquid phase at an ultrafast pace. The use of the term “curvature” in the context of these complex microemulsion systems is confusing, particularly when applied to merged nanometer-size globular or percolating domains. In this work, we discuss the terms “microemulsion” and “curvature”, and the most simple four-dimensional spatiotemporal model is proposed concerning SOW equilibrated systems near the optimum formulation. This model explains the motion of surfactant molecules due to Brownian movement, which is a quick and arbitrary thermal fluctuation, and limited to a short distance. The resulting observation and behavior will be an average in time and in space, leading to a permanent change in the local microcurvature of the aggregate, thus changing the average from micelle-like to inverse micelle-like order over an extremely short time. The term “microcurvature” is used to explain the small variations of globule size and indicates a close-to-zero mean curvature of the surfactant-containing film surface shape.

Advantages and limitations of different methods to determine the optimum formulation in surfactant–oil–water systems: A review

“…Different cases of so‐called microemulsions continue to be the subject of academic and industrial research still with some semantics confusion (Salager et al, 2020; Salager, Graciaa, & Marquez, 2022; Salager, Marquez, Rondon, et al, 2023), extending beyond their initial recurring theme of enhanced oil recovery (EOR). They have become associated with the “optimum formulation” concept and are now being used in increasingly demanding applications that require more sustainable components and innovative formulations.…”

“…It is crucial to note that there is some correspondence between HLD, packing parameter and curvature, even if they are different concepts as noted by Kunz et al (2009). This is in controversy with some studies calling the surfactant parameter in the HLD equation a “characteristic curvature” (Acosta et al, 2008; Leng & Acosta, 2023), when in other publications the surfactant parameter is said to be different from a measure of curvature (Salager, Graciaa, et al, 2022; Salager, Marquez, Rondón, et al, 2023). Tartaro et al (2023) measured the curvature of a SOW system, clearly showing that the curvature not only depends on the surfactant but also on other variables such as the type of oil and temperature.…”

“…Winsor III systems, in which the interactions between surfactant and oil and water are equal, exhibit a three‐phase behavior with a middle phase (Bourrel & Schechter, 2010; Scriven, 1977), which have been represented by Scriven as a bi or tri‐continuous structure with very complex shapes containing oil and water fluctuating domains, looking as arbitrary “sponge” geometry or other unusual models (Scriven, 1976, 1977). While this phase has been commonly referred to as “microemulsion,” it is noteworthy that there are no emulsion droplets present (Reger et al, 2012; Scriven, 1976, 1977) nor micrometer size objects, and consequently, the term is probably misleading as discussed by several researchers (Salager, Marquez, Rondon, et al, 2023) including one of the coauthors of Schulman initial paper (Prince, 1977).…”

“…There is a body of knowledge with diverse explanations and models on the bicontinuous microemulsions subject, and there are uncertainties in relation to these models and interpretations. While not detailed herein, the different proposed models can be explored in the literature (Bourrel & Schechter, 2010; Hyde et al, 1997; Langevin, 2020; Salager, Marquez, Rondón, et al, 2023). In one of the models proposed by De Gennes, the degree of flexibility of the surfactant film can be described by a correlation length ξ (Tartaro et al, 2023).…”

Exploring the Versatility of Microemulsions in Cutaneous Drug Delivery: Opportunities and Challenges

“…These systems typically contain a hydrophobic or a hydrophilic compound, a surfactant, and a cosurfactant [ 31 ]. However, the term “microemulsion” has been incorrectly used to describe other systems, such as micelles, reverse micelles, mesophase or liquid crystalline systems, and even coarse emulsions that have undergone micronization, causing confusion [ 32 ]. To address this, Danielsson and Lindmann proposed a definition in 1981 that states, “A microemulsion is a thermodynamically stable isotropic liquid system comprising an aqueous phase, an oil phase, and an amphiphilic substance” [ 33 ].…”