A cryogenic-electron microscopy study of the one-phase corridor in the phase diagram of a nonionic surfactant-based microemulsion system

Cryo-electron tomography study of the evolution of wormlike micelles to saturated networks and perforated vesicles

HYPOTHESIS

The formation of micellar aggregates and the changes in their morphology are crucial for numerous practical applications of surfactants. However, a proper structural characterization of complicated micellar nanostructures remains a challenge. This paper demonstrates the advances of cryo-electron tomography (cryo-ET) in revealing the structural characteristics that accompany the evolution of surfactant aggregates.

EXPERIMENTS

By using cryo-ET in combination with cryo-transmission electron microscopy (cryo-TEM), small-angle neutron scattering (SANS), and rheometry, studies were carried out on a model system composed of zwitterionic and nonionic surfactants. In this system, the molecular packing parameter was increased gradually by increasing the molar fraction of nonionic surfactant.

FINDINGS

A series of structural transformations was observed: linear wormlike micelles (WLMs) → branched WLMs → saturated network of multiconnected WLMs → perforated vesicles (stomatosomes). The transformations occur through an increase in the number of branches at the expense of cylindrical subchains and semispherical endcaps. Exponential distribution of subchains length was confirmed experimentally for multiconnected saturated networks. The stomatosomes were formed when the length of subchains becomes much shorter than the persistence length, causing the three-dimensional (3D) structure to transform into a two-dimensional (2D) membrane. This work identifies the mechanism of the structural changes, which can be further used to design various surfactant self-assemblies.

Advancements in Characterization Techniques for Microemulsions: From Molecular Insights to Macroscopic Phenomena

Microemulsions are thermodynamically stable, optically isotropic, transparent, or semi-transparent mixed solutions composed of two immiscible solvents stabilized by amphiphilic solutes. This comprehensive review explores state-of-the-art techniques for characterizing microemulsions, which are versatile solutions essential across various industries, such as pharmaceuticals, food, and petroleum. This article delves into spectroscopic methods, nuclear magnetic resonance, small-angle scattering, dynamic light scattering, conductometry, zeta potential analysis, cryo-electron microscopy, refractive index measurement, and differential scanning calorimetry, examining each technique's strengths, limitations, and potential applications. Emphasizing the necessity of a multi-technique approach for a thorough understanding, it underscores the importance of integrating diverse analytical methods to unravel microemulsion structures from molecular to macroscopic scales. This synthesis provides a roadmap for researchers and practitioners, fostering advancements in microemulsion science and its wide-ranging industrial applications.

In situ micro-emulsification during surfactant enhanced oil recovery: A microfluidic study

HYPOTHESIS

It is generally believed that the improved efficiency of surfactant enhanced oil recovery (EOR) comes from ultra-low interfacial tension (IFT) between oil and surfactant solution owing to the formation of middle-phase microemulsion. However, hindered visibility in underground porous media prevents direct observation of in situ generation of middle-phase microemulsion during surfactant flooding. Thus, direct visualization of the process is vital, and could clarify its contribution to EOR.

EXPERIMENTS

Micro-emulsification of a displacing fluid containing sodium 4-dodecylbenzenesulfonate and alcohol propoxy sulfate with model oil was investigated. Phase diagrams were drawn using salinity scans, and the influence of polymer on emulsification was analyzed. Micro-emulsification was monitored through in situ fluorescent tagging via 2D-microfluidics and ex situ visualization via cryo-electron microscopy and small angle X-ray scattering. Its contribution to the oil recovery factor was quantified by measuring the volume of each phase in the eluates.

FINDINGS

On-chip experiments indicated that in situ micro-emulsification occurred when the prescreened surfactant solution flowed in contact with trapped oil. The aqueous phase initially invaded the residual oil, forming a low mobility microemulsion. This microemulsion was then diluted by subsequent displacing fluid, forming a new driving fluid that caused ultra-low IFT in the trapped oil downstream. Under the synergistic effect of micellar solubilization and trapped-oil mobilization, the recovery factor could be increased by up to 40% over waterflooding and 43% on polymer inclusion in the formulation.

Cryogenic Electron Microscopy Methodologies as Analytical Tools for the Study of Self-Assembled Pharmaceutics

Many pharmaceutics are aqueous dispersions of small or large molecules, often self-assembled in complexes from a few to hundreds of molecules. In many cases, the dispersing liquid is non-aqueous. Many pharmaceutical preparations are very viscous. The efficacy of those dispersions is in many cases a function of the nanostructure of those complexes or aggregates. To study the nanostructure of those systems, one needs electron microscopy, the only way to obtain nanostructural information by recording direct images whose interpretation is not model-dependent. However, these methodologies are complicated by the need to make liquid systems compatible with high vacuum in electron microscopes. There are also issues related to the interaction of the electron beam with the specimen such as micrograph contrast, electron beam radiation damage, and artifacts associated with specimen preparation. In this article, which is focused on the state of the art of imaging self-assembled complexes, we briefly describe cryogenic temperature transmission electron microscopy (cryo-TEM) and cryogenic temperature scanning electron microcopy (cryo-SEM). We present the principles of these methodologies, give examples of their applications as analytical tools for pharmaceutics, and list their limitations and ways to avoid pitfalls in their application.

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Extending Cryo-EM to Nonaqueous Liquid Systems

Cryogenic-temperature transmission electron microscopy (cryo-TEM) of aqueous systems has become a widely used methodology, especially in the study of biological systems and synthetic aqueous systems, such as amphiphile and polymer solutions. Cryogenic-temperature scanning electron microscopy (cryo-SEM), while not as widely used as cryo-TEM, is also found in many laboratories of basic and applied research. The application of these methodologies, referred to collectively as cryogenic-temperature electron microscopy (cryo-EM) for direct nanostructural studies of nonaqueous liquid systems is much more limited, although such systems are important in basic research and are found in a very large spectrum of commercial applications. The study of nonaqueous liquid systems by cryo-EM poses many technical challenges. Specimen preparation under controlled conditions of air saturation around the specimen cannot be performed by the currently available commercial system, and the most effective cryogen, freezing ethane, cannot be used for most such liquid systems. Imaging is often complicated by low micrograph contrast and high sensitivity of the specimens to the electron beam.At the beginning of this Account, we describe the basic principles of cryo-EM, emphasizing factors that are essential for successful direct imaging by cryo-TEM and cryo-SEM. We discuss the peculiarities of nonaqueous liquid nanostructured systems when studied with these methodologies and how the technical difficulties in imaging nonaqueous systems, from oil-based to strong acid-based liquids, have been overcome, and the applicability of cryo-TEM and cryo-SEM has been expanded in recent years. Modern cryo-EM has been advanced by a number of instrumental developments, which we describe. In the TEM, these include improved electron field emission guns (FEGs) and microscope optics, the Volta phase plate to enhance image contrast by converting phase differences to amplitude differences without the loss of resolution by an objective lens strong underfocus, and highly sensitive image cameras that allow the recording of TEM images with minimal electron exposure. In the SEM, we take advantage of improved FEGs that allow imaging at a low (around 1 kV) electron acceleration voltage that is essential for high-resolution imaging and for avoiding specimen charging of uncoated nonconductive specimens, better optics, and a variety of sensitive detectors that have considerably improved resolution and, under the proper conditions, give excellent contrast even between elements quite close on the periodic table of the elements, such as the most important oxygen and carbon atoms.Finally we present and analyze several examples from our recent studies, which illustrate the issues presented above, including the remarkable progress made in recent years in this field and the strength and applicability of cryo-EM methodologies.

Oxidation of organosolv lignin in a novel surfactant-free microemulsion reactor

Lignin is considered as a promising substitute for fossil resources, but its efficient conversion remains a huge challenge due to the structural complexity and immiscibility with typical solvents. Herein, a series of surfactant-free microemulsion reactors comprised of n-octane, water and n-propanol were designed and their corresponding phase behaviors alongside their ability to intensify oxidative depolymerization of lignin were explored. Experimental results show that the phenolic monomer yield improves substantially (40-500 wt%) by comparison with processes performed in a single solvent. Detailed characterizations also suggest that the above intensification is rationalized by the solubilization effect of microemulsion system and directional aggregation of lignin at the microemulsion interface.

Evaluating physicochemical properties of crude oil as indicators of low-salinity-induced wettability alteration in carbonate minerals

The injection of low-salinity brine enhances oil recovery by altering the mineral wettability in carbonate reservoirs. However, the reported effectiveness of low-salinity water varies significantly in the literature, and the underlying mechanism of wettability alteration is controversial. In this work, we investigate the relationships between characteristics of crude oils and the oils’ response to low-salinity water in a spontaneous imbibition test, aiming (1) to identify suitable indicators of the effectiveness of low-salinity water and (2) to evaluate possible mechanisms of low-salinity–induced wettability alteration, including rock/oil charge repulsion and microdispersion formation. Seven oils are tested by spontaneous imbibition and fully characterized in terms of their acidity, zeta potential, interfacial tension, microdispersion propensity, water-soluble organics content and saturate-aromatic-resin-asphaltene fractionation. For the first time, the effectiveness of low-salinity water is found to positively correlate with the oil interfacial tension in low-salinity water. Oils with higher interfacial activity are found to respond more positively to low-salinity water. Moreover, cryogenic transmission electron microscopy images suggest that microdispersion is essentially macroemulsion, and its formation is an effective indicator – but not the root cause – of wettability alteration. The repulsive zeta potential for the rock and the oil in low-salinity water is found to be an insufficient condition for wettability alteration in carbonate minerals.

Structure, interfacial film properties, and thermal fluctuations of microemulsions as seen by scattering experiments

The physics of microemulsions and in particular Dominique Langevin's contributions to the understanding of microemulsion structure and bending properties using scattering techniques are reviewed. Among the many methods used by her and her co-workers, we particularly emphasize optical techniques and small angle neutron scattering (SANS), but also neutron spin echo spectroscopy (NSE). The review is then extended to more recent studies of properties of microemulsions close to surfaces, using reflectometry and grazing-incidence small angle neutron scattering (GISANS).