CO2/pH-Controllable Viscoelastic Nanostructured Fluid Based on Stearic Acid Soap and Bola-Type Quaternary Ammonium Salt

Computational Insights into a CO2-Responsive Emulsion Prepared Using the Superamphiphile Assembled by Electrostatic Interactions

The superamphiphiles exhibit broad prospects for fabricating stimuli-responsive emulsions. Because the superamphiphiles are assembled via noncovalent interactions, they have the advantage of fast response and high efficiency. Recently, a series of switchable emulsions using CO2-responsive superamphiphiles have been developed, which extends the applications of CO2-responsive materials in widespread field. However, there is still a lack of fundamental understanding on the switching mechanism related to the assembled structure of superamphiphiles at the oil-water interface. We employed molecular dynamics (MD) simulations to investigate the reversible emulsification/demulsification process of a responsive emulsion system stabilized by a recently developed responsive superamphiphile (BTOA), which consists of oleic acid (OA) and cationic amine (named 1,3-bis(aminopropyl)tetramethyldisiloxane, BT). The simulation results present the morphologies in both the emulsion and demulsification states. It is found that the ionized OA- and the protonated BT+ together form an adsorption layer at the oil-water interface. The hydrophobic parts of BT+ are inserted into the adsorption layer, and the two amine groups contact the water phase. This adsorption layer reduces the interfacial tension and stabilizes the emulsion. After the bubbling of CO2, the surfactants were fully protonated to OA and BT2+. Because of the changes in the molecular polarity, OA and BT2+ entered the oil and water phases, respectively, resulting in demulsification. The structural and dynamical properties were analyzed to reveal the different intermolecular interactions that were responsible for the reversible reversibility of the emulsion. The observations are considered to be complementary to experimental studies and are expected to provide deeper insights into studies on developing responsive materials via supramolecular assemblies.

Viscoelastic Fluid Formed by Ultralong-Chain Erucic Acid-Base Ionic Liquid Surfactant Responds to Acid/Alkaline, CO2, and Light

As a leftover of grease processing, the efficient utilization of erucic acid is still a challenge. An alternative strategy is to develop erucic acid-derived surfactants. However, erucic acid-based ionic liquid surfactants were barely involved. Here, a novel ionic liquid surfactant, benzyltrimethylammonium erucate (ErBTA), was developed by a simple neutralization reaction, and its aggregations in the diluted and concentrated solution were systematically studied by surface tension, conductivity, rheology, and cryo-TEM techniques. The results showed that ErBTA has a very low metaling point (-7.03 °C) and possesses excellent water solubility (Krafft temperature <4 °C). ErBTA alone starts to form micelles at a very low concentration (0.028 mmol/L) and then to form worm-based viscoelastic fluid at 4.07 mmol/L without any additives, exhibiting excellent self-assembly ability and thickening ability. This viscoelastic fluid formed by ErBTA can simultaneously respond to three stimuli: common acid/alkaline, CO₂ gas, and light, accompanied by an interesting gel-sol conversion, reflecting microstructure transition from wormlike micelles to spherical micelles. Although in essence CO₂ and light also act as pH regulators in the current system, they provide more sophisticated approaches to tune pH. Such a viscoelastic fluid with the characteristics of easy availability, renewability of raw materials, the simplicity of fabrication, good water-solubility, and excellent thickening ability may be an attractive candidate for clean fracturing in oil/gas recovery and fluid drag reduction.

Oxidation-Induced Breakage of the Imine Bond and Aggregate Transition in a Se-Containing Dynamic Covalent Surfactant

Controlling the dynamic imine bonds upon a novel trigger except for pH and temperature is still a significant challenge. Here, a Se-containing imine-based dynamic covalent surfactant (HOBAB-BSeEA) was developed for the first time by mixing two precursors in situ: an asymmetric double-chain cationic surfactant bearing a formyl group at the terminal of one hydrophobic tail and a Se-containing amine (2-(benzylselanyl)ethan-1-amine) in order to confirm the effect of redox on the imine bonds. The imine bond in HOBAB-BSeEA can be regulated not only upon changing the pH as well as other common imine-based surfactants but also by oxidation. The conversion efficiency of imine bonds is closely related with the degree of oxidation and pH. Complete oxidation can decrease the conversion efficiency from ∼87 to 48%, which is comparable to the result of changing the pH from 10.0 to 7.0. With the formation and breaking of imine bonds, the surfactant can be reversibly switched between symmetric and asymmetric structures, accompanied by a morphological transition from vesicles to spherical micelles. Although oxidation cannot demolish all imine bonds, it can completely convert vesicles to spherical micelles, which is mainly ascribed to an increase in the polarity of the micellar microenvironment stemming from the oxidation of Se. However, this transition can only be achieved by reducing the pH to 5.0 instead of 7.0. Nile red loaded in HOBAB-BSeEA vesicles can be quickly, controllably, and step-by-step released upon oxidation stimulus but not pH. Understanding the mechanism of oxidation-induced breakage of imine bonds and disruption of vesicles would be useful in designing redox-responsive imine-based carriers that can unload cargoes according to the level of the local reactive oxygen species.

pH-Responsive Wormlike Micelles Formed by an Anionic Surfactant Derived from Rosin

A novel pH-responsive wormlike micellar viscoelastic solution was constructed by a rosin-based anionic surfactant (Na-MPA-AZO-Na) in the presence of cetyltrimethylammonium bromide (CTAB). The viscoelasticity, aggregate morphology, and pH-responsiveness of the pH-responsive wormlike micelles have been investigated through the method of rheology and cryogenic-transmission electron microscopy. Its corresponding mechanism has been studied using ¹H NMR and ¹H-¹H 2D NOESY HNMR. The zero-shear viscosity (η₀) of the wormlike micellar solution rapidly decreases by 3 orders of magnitude as the pH increases from 5.21 to 9.56. The viscoelastic fluids and water-like solutions can be converted by tuning the pH between 3.62 and 12.00, and the corresponding aggregates also transform between wormlike micelles and spherical micelles. In addition, the wormlike micellar cross-sectional diameter is approximately 10 nm, which is remarkably larger than that of the common wormlike micelles. The phenomenon can be attributed to the large steric volume of the rosin rigid skeleton. When the pH is 12.00, a "pseudo" Gemini surfactant is constructed by Na-MPA-AZO-Na and CTAB through the electrostatic interactions. Wormlike micelles also can be formed with the increasing concentrations. The η₀ of the wormlike micellar system shows strong dependence on concentration with an exponent of 9.6 (η₀ ∝ C^9.6). This work further promotes new applications of forest resources.

A CO2-responsive smart fluid based on supramolecular assembly structures varying reversibly from vesicles to wormlike micelles

CO2-responsive smart fluids have been widely investigated in the past decade. In this article, we reported a CO2-responsive smart fluid based on supramolecular assembly structures varying from vesicles to wormlike micelles. Firstly, oleic acid and 3-dimethylaminopropylamine reacted to form a single-chain weak cationic surfactant with a tertiary amine head group, N-[3-(dimethylamino)propyl]oleamide (NDPO). Then, 1,3-dibromopropane was used as the spacer to react with NDPO to form a gemini cationic surfactant, trimethylene α,ω-bis(oleate amide propyl dimethyl ammonium bromide) (GCS). By controlling the feed ratio of 1,3-dibromopropane and NDPO, we found that the mixtures of GCS and NDPO with the molar ratio of 7 : 3 approximately could form vesicles in aqueous solution by supramolecular self-assembly. After bubbling CO2, the tertiary amine of NDPO was protonated. The packing parameter of the mixed surfactants reduced accordingly, accompanied by the transition of aggregates from vesicles to wormlike micelles. As a result, the zero-shear viscosity of the solution increased by more than four orders in magnitude. When the solid content of GCS/NPDO mixtures was higher than 5 wt% in solution, the sample treated by CO2 behaved as a gel over a wide frequency range with shear-thinning and self-healing properties. In addition, the sol-gel transition could be repeatedly and reversibly switched by cyclically bubbling CO2 and N2. Our effort may provide a new strategy for the design of CO2-responsive smart fluids, fostering their use in a range of applications such as in enhanced oil recovery.

Photoresponsive Behavior of Wormlike Micelles Constructed by Gemini Surfactant 12-3-12·2Br- and Different Cinnamate Derivatives

The photoresponsive wormlike micelles constructed by Gemini surfactants and cinnamate derivatives play a great role in the field of smart materials. However, how the structure of cinnamate derivatives affects the photoresponsive behavior of micelles is still a hotspot for scientists to research. Here, three kinds of aromatic salts with different ortho-substituted groups including trans- o-methoxy cinnamate ( trans-OMCA), trans- o-hydroxy cinnamate ( trans-OHCA), and trans-cinnamate ( trans-CA) were introduced into Gemini surfactant 12-3-12·2Br^- aqueous solutions to construct photoresponsive wormlike micelles through their noncovalent interactions. Their properties were researched using the rheological method, cryo-transmission electron microscopy, and ¹H NMR and two-dimensional nuclear Overhauser effect spectra. The results show that these cinnamate derivatives could well construct wormlike micelles with 12-3-12·2Br^-. Furthermore, subtle differences in the ortho substituents' structure have a significant effect on the photoresponsive behavior of formed wormlike micelles. Specifically, the zero viscosity (η₀) of 40 mM 12-3-12·2Br^-/24 mM trans-OHCA mixed solution decreases from 26.72 to 2.6 Pa·s with the shortening of the length of wormlike micelles after UV irradiation. Correspondingly, the η₀ for the same ratio of 12-3-12·2Br^-/ trans-OMCA decreases from 2.42 to 0.06 Pa·s and the wormlike micelles are transited into rodlike micelles and even spherical micelles after the same UV irradiation time. However, the variation of wormlike micelles in the 12-3-12·2Br^-/ trans-CA system induced by UV light is not obvious with η₀ being maintained at around 2.89 Pa·s. This study will help us better understand the effects of chemical groups on macrophenomena and microinteraction for micellar systems. It provides a theoretical basis for the construction of photoresponsive micelles, thus widening their application in the field of soft materials.

Viscoelastic micellar solution formed by a Se-based ionic liquid surfactant and its response to redox changes

The incorporation of a Se atom endows the surfactants with redox-sensitivity, and that its site plays a crucial role both in single surfactant solution and viscoelastic micellar solution formed by surfactant and NaSal.

Controlling Foam Stability with the Ratio of Myristic Acid to Choline Hydroxide

The interfacial and foam properties of a model system based on the mixture between myristic acid and choline hydroxide have been investigated as a function of the molar ratio ( R) between these two components and temperature. The aim of this study was to obtain insight on the links between the self-assemblies in bulk and in the foam liquid channels, the surfactant packing at the interface, and the resulting foam properties and stability. A multiscale approach was used combining small angle neutron scattering, specular neutron reflectivity, surface tension measurements, and photography. We highlighted three regimes of foam stability in this system by modifying R: high foam stability for R < 1, intermediate at R ∼ 1, and low for R > 1. The different regimes come from the pH variations in bulk linked to R. The pH plays a crucial role at the molecular scale by setting the ionization state of the myristic acid molecules adsorbed at the gas-liquid interface, which in turn controls both the properties of the monolayer and the stability of the films separating the bubbles. The main requirement to obtain stable foams is to set the pH close to the p K_a in order to have a mixture of protonated and ionized molecules giving rise to intermolecular hydrogen bonds. As a result, a dense monolayer is formed at the interface with a low surface tension. R also modifies the structure of self-assembly in bulk and therefore within the foam, but such a morphological change has only a minor effect on the foam stability. This study confirms that foam stability in surfactant systems having a carboxylic acid as polar headgroup is mainly linked to the ionization state of the molecules at the interface.

A responsive anionic wormlike micelle using pH-directed release of stored sodium based on polybasic acids

Responsive wormlike micelles are very useful in a number of applications, whereas it is still challenging to create dramatic viscosity changes in anionic surfactant systems. Here a differential pH-responsive wormlike micelle based on sulfonic surfactants was developed, which is formed by mixing sodium dodecyl trioxyethylene sulphate (SDES) and ethylenediaminetetraacetic acid tetrasodium (EDTA4-·4Na+) at the molar ratio of 1 : 1. The phase behavior, aggregate microstructure and viscoelasticity of the SDES/EDTA4-·4Na+ solution were investigated via macroscopic observation, cryo-TEM and rheological measurements. It was found that the phase behavior of the SDES/EDTA4-·4Na+ solution undergoes transitions from a water-like fluid to viscoelastic upon decreasing the pH. On decreasing the pH from 12.01 to 3.27 by adding HCl, the viscosity of the transparent solutions with wormlike micelles was increased rapidly and reached ∼3100 mPa s. Furthermore, on increasing the pH by adding NaOH, the viscosity was slightly increased due to the addition of Na+. However, the increase in the concentration of Na+ is much smaller than the theoretical addition. The same phenomenon was noted in the sodium citrate solution, but does not exist in the sodium formate system. The viscosity of the micellar solution has a sensitive response to inorganic acids and tolerance to inorganic bases due to the characteristics of polybasic acids.

Reversibly Switching Wormlike Micelles Formed by a Selenium-Containing Surfactant and Benzyl Tertiary Amine Using CO2/N2 and Redox Reaction

Multiresponsive wormlike micelles (WLMs) remain a significant challenge in the construction of smart soft materials based on surfactants. Herein, we report the preparation of a viscoelastic wormlike micellar solution based on a new redox-responsive surfactant, sodium dodecylselanylpropyl sulfate (SDSePS), and commercially available benzyl tertiary amine (BTA) in the presence of CO₂. In this system, SDSePS can be reversibly switched on (selenide) and off (selenoxide) by a redox reaction, akin to that previously reported for benzylselanyl or phenylselanyl surfactants. By alternately adding H₂O₂ and N₂H₄·H₂O, WLMs can be reversibly broken and formed because of the transformation of the hydrophilic headgroup of SDSePS, originating from the reversible formation of selenoxide. Moreover, WLMs can also be switched on and off by cyclically bubbling CO₂ and N₂ because of the variation of the binding interaction between SDSePS and BTA, resulting from the reversible protonation of BTA. This interesting and unique multiresponsive behavior makes the current WLMs a potential candidate for smart control of the "sol-gel" transition or substantial thickening of solutions.

The mechanism difference between CO2 and pH stimuli for a dual responsive wormlike micellar system

To indicate the difference between pH and CO₂ stimuli mechanisms, a dual responsive system is constructed with the aid of N-[3-(dimethylamino)propyl]docosanamide (NDPD) and sodium salicylate (NaSal), which is responsive to pH and CO₂.

CO2-Triggered Pickering Emulsion Based on Silica Nanoparticles and Tertiary Amine with Long Hydrophobic Tails

We describe a strategy of fabricating CO₂-triggered oil-in-water Pickering emulsion based on silica nanoparticles functionalized in situ by a trace amount of conventional CO₂-switchable surfactant, N-(3-(dimethylamino)propyl)alkyl amide (C_nPMA). By alternately bubbling CO₂ and N₂ at a moderate conditions (30 °C, 80 mL min^-1), silica nanoparticles reversibly switch between amphipathic and hydrophilic as a result of the adsorption of ammonium (CO₂) and the desorption of tertiary amine (N₂). The emulsion can then be smart switched "on (stable)" and "off (unstable)", along with homogenization, without needing cooling and heating. The switching of the current tertiary-based system is simple, moderate, and environmentally friendly, without contamination and the restriction of rigorous conditions. The surfactant concentration window of the Pickering emulsion is closely related to the length of hydrophobic tail, and the upper limit is no more than 0.20 cmc of that of the corresponding ammonium surfactant. Such a strategy is also suitable for commercial alkyl tertiary amines, without needing complicated organic synthesis.

Redox-Controllable Interfacial Properties of Zwitterionic Surfactant Featuring Selenium Atoms

Control of interfacial properties (foaming and emulsification) plays an important role in industry. Here we developed a novel redox-responsive surfactant, 3-(11-benzylselanyl-undecyl)-dimethylammonium acetate (BSeUCB), using selenium atoms as an environmentally sensitive group. In a reduced state, BSeUCB aqueous solution showed good foaming and emulsification abilities as well as conventional betaine surfactants. After oxidization, BSeUCB transformed into a bola-type structure because of the presence of a new hydrophilic group (selenoxide), and thus the critical micellar concentration, equilibrium surface/interfacial tension, and molecular area at the interface correspondingly increase from 0.32 mM, 46.43 mN·m(-1), 5.30 mN·m(-1), and 0.61 nm(2) to 4.98 mM, 59.15 mN·m(-1), 18.29 mN·m(-1), and 1.22 nm(2), respectively, resulting in a greater amount of energy input required to produce foam or emulsion, and a less dense adsorption layer, i.e., poor foaming and emulsification ability. Such a conversion was reversibly controlled by simply adding a trace amount (<0.06 wt % of the dispersion) of oxidant (H2O2) and reductant (Na2SO3). The products of the redox reaction did not interfere in the switchability except at the first cycle. The oxidization was generally time-consuming, whereas the reduction was very fast.

Rheological characterizations and molecular dynamics simulations of self-assembly in an anionic/cationic surfactant mixture

The formation of self-assemblies in mixed amino acid-based anionic N-hexadecanoylglutamic acid (HGA) and cationic benzyldimethyl hexadecylammonium chloride (HDBAC) surfactants in aqueous solutions has been characterized. With rheological analysis, the viscoelastic properties of the mixed system are found to be completely dependent on the concentration of HDBAC. Molecular dynamics simulation results suggest that the morphology of self-assembly can be regulated from spherical micelles to wormlike micelles by the addition of HDBAC. The aromatic group of HDBAC adsorption provides a "charge-neutral" function to the micelle corona; the repulsive interactions within the head group of HGA are progressively screened and closely packed. In addition, the dynamic processes and formation mechanisms of self-assembly were analyzed in detail with molecular simulation techniques.

Tandem triggering of wormlike micelles using CO2 and redox

Wormlike micelles reversibly and tandemly respond to biocompatible CO₂ and redox, which in turn leads to a change in their rheological properties.