CO₂ and Temperature Control over Nanoaggregates in Surfactant-Free Microemulsion

Abstract

Smart microemulsions (MEs) recently have attracted significant interests. However, MEs, especially surfactantfree MEs (SFMEs) that respond to more than one stimulus, are rarely reported to date. Here, we reported the first example of dualresponsive SFME in which a CO 2 -sensitive hydroxyethylamine was used as an amphisolvent. This SFME was investigated utilizing ternary phase diagram, dynamic light scattering, and UV−visible spectrum techniques. It was found that three hydroxyethylamines could stabilize the octanol−water mixture to form transparent and isotropic SFMEs including nanoaggregates-rich pre-ouzo zone, regardless of the number of the hydroxyl group. Among them, 2-(dimethyl amino) ethanol (DMEA)-based SFME possesses the largest single-phase region and most sensitive to CO 2 and the changes in temperature. With bubbling of CO 2 /N 2 or decreasing/increasing temperature, both the single-phase region and pre-ouzo zone reversibly shrink and expand, as well as with breathing. However, CO 2 /N 2 -induced change is more significant than that induced by temperature. The former is mainly ascribed to the reversible protonation and deprotonation of DMEA, while the latter is generally interpreted as the effects of temperature on hydrogen bond interaction. Note that CO 2 leads to a thorough demusification from Winsor IV ME to oil-rich and water-rich two phases without nanoaggregates, while cooling only causes to a particular phase separation, producing two new MEs phases, not typical Winsor I or II MEs. Such a unique dual-responsive SFME can not only be applied in the remediation of contaminated soil, drug delivery, and nanoparticles preparation but also opens a new door to switchable emulsion.

Integrated Thermoelectric Design Inspired by Ionic Liquid Microemulsion-Based Gel with Regulatable Dual-Temperature Responsiveness

“…40−43 However, due to the inherent thermodynamic stability, it will spontaneously return to the uniform and transparent state after removing the external stimuli. 44,45 Hence, the microemulsion exhibits dual-temperature sensitivity, which will take place noticeable transmittance changes of opacity−transparency−opacity with the increase of temperature. Also, this phenomenon has stable repeatability.…”

CO2-Responsive Hydrophobic Deep Eutectic Solvent Based on Surfactant-Free Microemulsion-Mediated Synthesis of BaF2 Nanoparticles

“…Nevertheless, SFMEs are confronted with the challenge of difficult SFME recovery. 13,14 With the development of green and sustainable concepts, SFMEs with stimulus-response are gradually being developed, such as pH, 15 CO 2 , 16,17 and temperature-responsive 18,19 SFMEs. These SFMEs can mod-ulate the microstructure or reversible transformation of SFME properties through environmental stimulus sources, showing excellent switching performance.…”

“…The SFME shares similar properties and structures in surfactant-based microemulsions (SBMEs). ,− Significantly, there are no surfactants in SFMEs, which directly avoids the issue of surfactant residues. Nevertheless, SFMEs are confronted with the challenge of difficult SFME recovery. , With the development of green and sustainable concepts, SFMEs with stimulus-response are gradually being developed, such as pH, CO 2 , , and temperature-responsive , SFMEs. These SFMEs can modulate the microstructure or reversible transformation of SFME properties through environmental stimulus sources, showing excellent switching performance.…”

pH-Responsive Regulation of a Surfactant-Free Microemulsion Based on Hydrophobic Deep Eutectic Solvents

“…In addition, the disruptive effect of water has a strong effect on the properties of the DESs. The oil phase in most SFMEs is an unsustainable and environmentally unfriendly solvent. , Therefore, it is necessary to construct inexpensive, hydrophobic, and environmentally friendly pH-responsive HDESs to replace organic solvents as an oil phase for a range of responsive SFME systems. − …”