Preparation of CO2-responsive emulsions with switchable hydrophobic tertiary amine

Improvement in CO2 Capture of Polyamine with Micro-Interfacial System

Polyamines have emerged as a promising class of CO2 absorbents due to their remarkable sequestration capacity. However, their potential industrial application as aqueous absorbents is significantly hindered by a low regeneration efficiency and high energy consumption. To address these issues, this study investigates the use of triethylenetetramine (TETA) and ethylene glycol (EG) to develop a nonaqueous absorbent. The incorporation of EG enhances absorption performance and reduces the regeneration energy needed for TETA, whereas the high viscosity of the absorbent impedes absorption rate, amine efficiency, and regeneration efficiency. In order to enhance CO2 capture, micron-sized reaction units (SiO2@TETA-EG) were developed by encapsulating TETA solution with nanosilica. The SiO2@TETA-EG composite exhibits a large specific surface area (99 m2/g), with a porous shell structure and improved fluidity, which effectively counteracts the negative effects caused by high viscosity. Notably, SiO2@TETA-EG indicates a noticeably higher apparent rate constant of 4.29 min-1 at 323.2 K compared to the TETA-EG solution. Furthermore, SiO2@TETA-EG displays a 28.4% boost in regeneration efficiency while maintaining favorable stability in pore size and shape after regeneration.

CO2-responsive gels

CO₂-responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO₂. The use of CO₂ as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO₂-responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO₂-responsive polymers, the study of CO₂-responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO₂ such as swelling or a transformed morphology. In the past 15 years, CO₂-responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO₂-triggered aggregation and their capacity for reversible CO₂ absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO₂ sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO₂-responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO₂-induced self-assembled gels (including poly(ionic liquid) gels).

Reversible stability of colloids switched by CO2 based on polyhexamethylene guanidine

The stability of a colloid, including emulsion and polymer latex, can be destroyed irreversibly by the addition of salt. Using the CO₂ stimulus, amines can be converted into organic ammonium salts reversibly, which can access the switching of colloids. Polyhexamethylene guanidine (PHMG), was chosen as a switchable amine. The conductivity of PHMG aqueous solution switched by adding and removing CO₂. Surface tension measurements verified that, under CO₂, the critical micelle concentration of sodium dodecyl benzene sulfonate (SDBS) decreased from 1.0 × 10^-3 to 5.0 × 10^-4 M with the addition of PHMG. The crude oil emulsion containing SDBS and PHMG was destroyed and restored reversibly by the treatment with CO₂ and N₂. The polystyrene latex also occurred an obvious stratification after sparging with CO₂ and returned a homogeneous phase upon bubbling N₂. This study is intended to pave the way for colloids which has reversible stability in response to CO₂ stimulation.

pH-Induced reversible conversion between non-Pickering and Pickering high internal phase emulsion

Solid-stabilized high internal phase emulsions have received extensive attention. Many previous studies have confirmed that solid emulsifiers in high internal phase Pickering emulsions (HIPPEs) provide a great interface mechanical barrier. With the development of research, novel solid-stabilized emulsions have emerged. These emulsions are stabilized by the electrostatic repulsion between the surfactants and hydrophilic solid particles. They are distinct from Pickering emulsions in that the solid particles do not exist at the oil-water interface, but are dispersed in the continuous phase, so it is called a non-Pickering emulsion. However, high internal phase non-Pickering emulsions (HIPNPEs) are rarely reported. Herein, HIPNPEs that are synergistically stabilized by anionic surfactants with dynamic covalent bonds and negatively charged nano-SiO₂ particles were prepared. In the presence of dodecylamine, the acidity causes the dynamic covalent bonds to break and the surfactant to be inactivated. Additionally, the long-chain amine is protonated and adsorbed on nano-SiO₂ particles to form a new surfactant for stabilizing HIPPEs. However, alkalinity causes the HIPNPEs to form again. In addition, rheological tests confirmed that the HIPNPEs and HIPPEs had similar rheological behaviors, which were typical gel-like fluids. The emulsion can quickly respond to realize the conversion between the different types of high internal phase emulsion by simple stimulation, which provides a new direction for stimulus-responsive high internal phase emulsions.

Dual CO2 Responsiveness of an Oil-In-Water Emulsion by Using Sodium Oleate and Water-Soluble Tertiary Amines

Two kinds of water-soluble tertiary amines (TAs), triethylamine (TEA, monoamine), and tetramethyltrimethylenediamine (TMA, diamine) were introduced into a NaOA stable oil-water (O/W) emulsion, respectively, and their dual reactivity to carbon dioxide was studied. TA was converted into bicarbonate after bubbling of CO₂, which induced the increase of ionic strength of the aqueous phase, and formed ion pair with NaOA through electrostatic interaction. NaOA itself can also be protonated into oleic acid, which can be reverently deprotonated by alternating bubbles of CO₂ at 25 °C and N₂ at 50 °C, thus affecting the stability and demulsification process of the emulsion. In order to demonstrate TA's and NaOA's synergistic effect on CO₂ responsiveness, gas chromatography-mass spectrometry, ζ potential, electrical conductivity, pH value, ¹H nuclear magnetic resonance, morphological evolution, and interfacial tension were used to study the contributions of the single component and two components of NaOA, TEA, and TMA to emulsion stability and CO₂ responsiveness, respectively. Combined with the composition distribution under different pH conditions, it was further proved that TAs had an effect on the stability and CO₂ responsiveness of the NaOA emulsion.

Controllable CO2-Responsiveness of an Oil-in-Water Emulsion by Varying the Number of Tertiary Amine Groups or the Position of the Hydroxyl Group of Tertiary Amine

A series of water-soluble tertiary amines (TAs) are introduced into an oil-in-water (O/W) emulsion stabilized by sodium oleate (NaOA). TAs convert into bicarbonate salts upon bubbling of CO₂, which could induce the increase of ionic strength of the aqueous phase, form ion pairs with NaOA by electrostatic interaction, and finally result in demulsification. ζ-Potential, conductivity, pH value, ¹H NMR, separation rate, and interfacial tension are applied to figure out the effects of number of tertiary amine groups and different positions of the hydroxyl group. TA with an increasing number of tertiary amine groups can further stabilize the O/W emulsion and accelerate the process of demulsification by bubbling CO₂. More tertiary amine groups bring about a more stable emulsion and faster demulsification by bubbling CO₂. The position of the hydroxyl group is a key factor affecting the solubility of the corresponding ion pair formed with NaOA. The better the water solubility, the slower the demulsification. The worse the water solubility of the ion pair, the more perfect the demulsification is. More importantly, water-soluble TA, with proper structure, could bring about perfect demulsification.

Conversion of a surfactant-based microemulsion to a surfactant-free microemulsion by CO2

A microemulsion with a CO₂ response was prepared by mixing the surfactant sodium oleate (NaOA), the co-surfactant isopropyl alcohol (IPA), oil phase oleic acid (HOA) and water. This surfactant-based microemulsion (SBME) shows a CO₂ responsive behavior, and the introduction of CO₂ can breakdown the microemulsion. Through the research in this paper, it is found that the content of IPA has a direct impact on the CO₂ response behavior of SBME. It was found that the lower the IPA content (22.73 wt%), the more obvious the CO₂ response behavior of SBME. Conversely, when the concentration of IPA is high (54.05 wt% and 63.83 wt%), the introduction of CO₂ does not directly lead to the demulsification of the microemulsion. NaOA can be converted to HOA under the action of CO₂, which is why SBME shows CO₂ response behavior. By comparing the effects of CO₂ on the (pseudo-)ternary phase diagrams of SBME and surfactant-free microemulsion (SFME), we found evidence that SBME shows different CO₂ response behaviors. When CO₂ was bubbled into the SBME system with a low IPA content, IPA cannot stabilize the excessive HOA and water in the system and eventually break the microemulsion. The situation is different when CO₂ is applied to the SBME system with a high IPA content. IPA as an amphiphilic solvent can stabilize the HOA and water in the system to form SFME. In this process, SBME can be demulsified (low IPA content) or can be converted to SFME (high IPA content) in the presence of CO₂.

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.

Controllable CO2-responsiveness of O/W emulsions by varying the alkane carbon number of a tertiary amine

A series of CO₂-responsive oil-in-water (O/W) emulsions were prepared by introducing a hydrophobic tertiary amine (TA) with a varying alkane carbon number (ACN) into the emulsion stabilized by sodium dodecyl benzene sulfonate (SDBS).

Diamines as switchable-hydrophilicity solvents with improved phase behaviour

Removing solvents by distillation is not a sustainable process because it requires the use of volatile solvents and a high energy input. An alternative is to use a switchable-hydrophilicity solvent (SHS), which can be removed from products and recycled without any distillation step. SHSs are solvents that reversibly switch between hydrophilic and hydrophobic forms with the addition and removal of a trigger such as CO₂. Monoamine SHSs can be separated from dissolved products by extraction into carbonated water, but the solvent removal is limited by the distribution coefficient of the SHS between the carbonated water phase and the product phase. In this article, the use of diamines as SHSs with improved distribution coefficients is explored. Several diamine SHSs are identified and their properties compared to those of monoamine SHSs. Comparisons include the pK_aH (the pK_a of the conjugate acid of a base) and log K_ow (log of the octanol–water partition coefficient) requirements for amines to act as SHSs, distribution coefficients, removal from hydrophobic liquids, switching speeds, and risks to the environment and human health and safety.

Diamine switchable-hydrophilicity solvents can be removed from products, by carbonated water, with much greater efficiency than past switchable solvents.

The N-allyl substituted effect on wormlike micelles and salt tolerance of a C22-tailed cationic surfactant

Wormlike micelles (WLMs) have been observed in a wide variety of cationic surfactants. Here we developed WLMs based on an N-allyl substituted cationic surfactant with an unsaturated C₂₂-tail, N-erucamidopropyl-N,N-dimethyl-N-allyl-ammonium bromide (EDAA), and compared them with UC₂₂AMPM at the same concentration. The viscoelasticity, aggregate microstructure and salt tolerance of EDAA solutions were investigated by rheology, surface tension and Cryo-TEM measurements. It was found that EDAA exhibited a higher viscosity and a high salt tolerance. Upon increasing the concentration of NaCl, the viscosity of wormlike micelles in the solutions continuously increased and reached ∼1.10 × 10⁶ mP s at 200 mM. On further increasing the NaCl concentration to 2000 mM, the viscosity remained at ∼10⁶ mP s without any reduction. But the viscosity of UC₂₂AMPM solutions showed a drastic change with the increase of NaCl concentration. This drastic variation in rheological behavior is attributed to the presence of the N-allyl substituent. Besides, the EDAA also shows some advantages such as low overlapping concentration(∼2.2 mM) and stable viscosity over the whole pH range.