Shape transitions in supercritical CO2 microemulsions induced by hydrotropes

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Shape transition of water-in-CO2 reverse micelles controlled by the surfactant midpiece

Designing CO₂-philic surfactants for generating wormlike reverse micelles (RMs) is an effective approach to enhance the viscosity of supercritical CO₂ (scCO₂), however this remains challenging.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO₂), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C₈Benz). It is found that adding the C₈Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C₈Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C₈Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C₈Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO₂.

Tuning Micellar Structures in Supercritical CO2 Using Surfactant and Amphiphile Mixtures

For equivalent micellar volume fraction (ϕ), systems containing anisotropic micelles are generally more viscous than those comprising spherical micelles. Many surfactants used in water-in-CO₂ (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO₂-philic surfactants with hydrotropes and cosurfactants may generate elongated micelles in w/c systems at high-pressures (e.g., 100-400 bar). A range of novel w/c microemulsions, stabilized by new custom-synthesized CO₂-phillic, partially fluorinated surfactants, were formulated with hydrotropes and cosurfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure, and hydrotrope tail length were all investigated. Dispersed water domains were probed using high pressure small-angle neutron scattering (HP-SANS), which provided evidence for elongated reversed micelles in supercritical CO₂. These new micelles have significantly lower fluorination levels than previously reported (6-29 wt % cf. 14-52 wt %), and furthermore, they support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio; from this value a relative viscosity value can be estimated through combination with the micellar volume fraction (ϕ). Combining these new results with those for all other reported systems, it has been possible to "map" predicted viscosity increases in CO₂ arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.

Hydrotrope induced structural modifications in CTAB/butanol/water/isooctane reverse micellar systems

SAXS and NMR studies to gain insight of structural alterations in reverse micellar nanotemplates in presence of hydrotropes.

Molecular dynamics study of di-CF4 based reverse micelles in supercritical CO2

Reverse micelles (RMs) in supercritical CO₂ (scCO₂) are promising alternatives for organic solvents, especially when both polar and non-polar components are involved. Fluorinated surfactants, particularly double-chain fluorocarbon surfactants, are able to form well-structured RMs in scCO₂. The inherent self-assembly mechanisms of surfactants in scCO₂ are still subject to discussion. In this study, molecular dynamics simulations are performed to investigate the self-aggregation behavior of di-CF4 based RMs in scCO₂, and stable and spherical RMs are formed. The dynamics process and the self-assembly structure in the RMs reveal a three-step mechanism to form the RMs, that is, small RMs, rod-like RMs and fusion of the rod-like RMs. Hydrogen-bonds between headgroups and water molecules, and salt bridges linking Na⁺ ions, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The results show that di-CF4 molecules have a high surfactant coverage at the RM interface, implying a high CO₂-philicity. This mainly results from bending of the short chain (C-COO-CH₂-(CF2)₃-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful in understanding surfactant self-assembly phenomena and designing new CO₂-philic surfactants.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.

Supercritical carbon dioxide: a solvent like no other

Supercritical carbon dioxide (scCO2) could be one aspect of a significant and necessary movement towards green chemistry, being a potential replacement for volatile organic compounds (VOCs). Unfortunately, carbon dioxide has a notoriously poor solubilising power and is famously difficult to handle. This review examines attempts and breakthroughs in enhancing the physicochemical properties of carbon dioxide, focusing primarily on factors that impact solubility of polar and ionic species and attempts to enhance scCO2 viscosity.