Anionic surfactant-stabilized hydrophobic ionic liquid-based bicontinuous microemulsion: Formulation, microstructure and laccase kinetics

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.

Electropolymerization of EDOT in an anionic surfactant-stabilized hydrophobic ionic liquid-based microemulsion

Here, an anionic surfactant [AOT]^- (bis-(2-ethylhexyl) sulfosuccinate)-stabilized H₂O/[Omim][PF₆] (1-octyl-3-methylimidazolium hexafluorophosphate) microemulsion has been tested for the first time as a medium for the electropolymerization of 3,4-ethylenedioxythiophene (EDOT). To formulate AOT-stabilized [Omim][PF₆]-based microemulsions of different water contents, the phase triangle was determined at 35 °C. Measurements of the conductivities of the microemulsions, their solubilization capacities toward EDOT and their catalytic effects on EDOT electrooxidation show that the present [AOT]^--stabilized ionic liquid microemulsion is a good medium for EDOT electropolymerization. Studies on the process of the electropolymerization of EDOT in this [Omim][PF₆]-based microemulsion indicate that the water content (i.e., microstructure) of the microemulsion medium is an important factor affecting the onset potential and the deposition rate of the PEDOT. The morphology and the doping level of the as-prepared PEDOT are also found to be correlated with the water content of the ionic liquid microemulsion. The microemulsion with higher water content results in a PEDOT with better electroactivity and higher doping levels. FTIR spectra and XPS analysis show that the PEDOT electrosynthesized in the microemulsion is co-doped by both [AOT]^- and [PF₆]^-. Compared with the neat [Omim][PF₆], the use of the ionic liquid microemulsions can reduce not only the consumption of the expensive ionic liquid, but also the onset potential for the electrooxidation of EDOT. Moreover, by tuning the water content of the medium, the electropolymerization of PEDOT and its electrochemical properties could be regulated accordingly. Under the identical deposited charge, the PEDOT originated from the high water content microemulsion (50% H₂O μE) has a higher specific capacitance (124 F g^-1) than that from neat [Omim][PF₆] (117 F g^-1). It follows that the present ionic liquid microemulsion is a good medium for EDOT electropolymerization. The present study opens up a new route for the green and low-cost electrochemical preparation of high-performance PEDOT.

Mixed micelles and bicontinuous microemulsions: Promising media for enzymatic reactions

Enzymes, the natural catalysts, replace catalysts of chemical origin in a wide spectrum of reactions and generally work under environment friendly conditions. Various strategies are adopted to modify catalytic activities of enzymes further, of which one is application of novel reaction medium. This work reviews applicability of novel media like mixed micelles and bicontinuous microemulsions in enzymatic reactions and points out their capability to play bigger roles in enzyme catalysis. Ionic reverse micelles reduced catalytic activities of enzymes through denaturation. Addition of nonionic surfactant to these reverse micelles led to corresponding mixed micelles and thus restored or sometimes enhanced catalytic abilities of enzymes. Mixed micelles comprising of two nonionic surfactants, bicontinuous microemulsion containing two anionic surfactants also acted as efficient reaction media for enzymes. Even a cationic/anionic/nonionic mixed micelle was found to increase activity of enzyme. Mixed micelles and bicontinuous microemulsions comprising of anionic and zwitterionic surfactants augmented enzyme catalysis. Mixed micelles and bicontinuous microemulsions containing ionic liquid and surfactant also had critical impact on enzyme catalysis. Catalytic abilities of enzymes altered significantly in substrate/surfactant and bile salt/surfactant mixed micelles. Concentrations of individual surfactant, molar ratio of surfactants, and molar ratio of water to total surfactants had notable impacts on enzyme catalysis in those media.

Good's buffer ionic liquid tunes the phase behavior of an anionic surfactant SDBS-stabilized n-octane-water microemulsion and the stability of the solubilized horseradish peroxidase

A Good's buffer ionic liquid (GB-IL) composed of quaternary ammonium cations and Good's buffer anions is first introduced into a microemulsion system as a self-buffering and biocompatible electrolyte. The effects of the constituting ions of a GB-IL and their concentrations on the phase behavior of the anionic surfactant SDBS stabilized n-octane-H₂O microemulsion system were studied for the first time using the ε-β fish-like phase diagram method. The result indicates that the phase behavior of the above microemulsion system is greatly affected by GB-IL cations with a longer alkyl chain on the cation being more favorable for phase inversion. Compared with NaCl, a GB-IL of the same concentration is more efficient for achieving phase inversion, due to the dual role of an electrolyte and a co-alcohol. In addition to the phase behavior, the stability of horseradish peroxidase (HRP) solubilized in an SDBS stabilized bicontinuous microemulsion is also affected by a GB-IL. It is found that the variation of the cationic alkyl chain has a negligible effect on the microemulsion microstructure, but has a significant influence on the stability of the solubilized HRP. At a fixed concentration of the GB-IL, the quaternary ammonium cation with a longer alkyl chain is better for the stabilization of the HRP activity. For a given GB-IL, a higher level of the GB-IL results in a better HRP stability. More importantly, the GB-IL-buffered microemulsion, at the same level of the buffering salt, is more advantageous than the phosphate-buffered one for the stabilization of the HRP activity. This advantage is more pronounced for higher concentrations of the GB-IL. This difference in the HRP stability, caused by the buffering salts, should be ascribed to the microemulsion microstructure effect as well as the Hofmeister effect. The present study provides a guideline for the construction of a bicontinuous microemulsion with a simplified composition and stabilizing effect on the solubilized enzyme.

Anionic-Surfactant-Stabilized Hydrophobic Ionic-Liquid-Based Bicontinuous Microemulsion as a Medium for Enzymatic Oxidative Polymerization of Aniline

The hydrophobic ionic liquid [C8mim][PF6] (1-octyl-3-methylimidazolium hexafluorophosphate)-based bicontinuous microemulsion stabilized by the anionic surfactant [C4mim][AOT] (1-butyl-3-methylimidazolium bis(2-ethylhexyl) sulfosuccinate) was first tried as a medium for horseradish peroxidase (HRP)-triggered oxidative polymerization of aniline. The effects of the mass ratio of [C8mim][PF6]-to-water (α), the mass fraction of [C4mim][AOT] in the total mixture (γ), and temperature (T) on the enzymatic polymerization were investigated using UV-vis-NIR absorption, electron spin resonance, and small-angle X-ray scattering spectroscopy techniques. The bicontinuous microemulsion is demonstrated to play a template role in the biosynthesis of polyaniline (PANI). The conductivity of the resulting PANI depends on the microemulsion microstructure and the microstructure- and T-dependent catalytic properties of the solubilized HRP. With the increase in α, the conductivity of the synthesized PANI decreases due to the increase in the template curvature (decrease of the microdomain size) and the decrease in the activity and stability of HRP. Compared with α, γ has little effect on the microdomain size of the template; so, the γ-dependent change in the conductivity of PANI is mainly caused by the changes of the microstructure-dependent activity and stability of HRP. Over the range of 20-35 °C, T has little effect on the microdomain size, but it greatly changes the activity and stability of HRP. With the increase in T, the activity of HRP increases steadily, but its stability decreases significantly, which should be one of the reasons why the conductivity of PANI decreases with increasing T. In conclusion, lower values of α, γ, and T are favorable for the biosynthesis of conductive PANI. The present study not only deepens the insight into the role of the template in the process of PANI synthesis, but also opens up a green new way for the biosynthesis of the conducting polymer.