CO2 responsive emulsions stabilized with fatty acid soaps in NaCl brine

Advances in CO2-switchable surfactants towards the fabrication and application of responsive colloids

CO₂-switchable surfactants have selective surface-activity, which can be activated or deactivated either by adding or removing CO₂ from the solution. This feature enables us to use them in the fabrication of responsive colloids, a group of dispersed systems that can be controlled by changing the environmental conditions. In chemical processes, including extraction, reaction, or heterogeneous catalysis, colloids are required in some specific steps of the processes, in which maximum contact area between immiscible phases or reactants is desired. Afterward, the colloids must be broken for the postprocessing of products, solvents, and agents, which can be facilitated by using CO₂-switchable surfactants in surfactant-stabilized colloids. These surfactants are mainly cationic and can be activated by the protonation of a nitrogen-containing group upon sparging CO₂ gas. Also, CO₂-switchable superamphiphiles can be formed by non-covalent bonding between components at least one of which is CO₂-switchable. So far, CO₂-switchable surfactants have been used in CO₂-switchable spherical and wormlike micelles, vesicles, emulsions, foams, and Pickering emulsions. Here, we review the fabrication procedure, chemical structure, switching scheme, stability, environmental conditions, and design philosophy of such responsive colloids. Their fields of application are wide, including emulsion polymerization, catalysis, soil washing, drug delivery, extraction, viscosity control, and oil transportation. We also emphasize their application for the CO₂-assisted enhanced oil recovery (EOR) process as a promising approach for carbon capture, utilization, and storage to combat climate change.

The mediated rheological properties of emulsions stabilized by thread-like mesoporous silica nanoparticles in combination with CTAB

The combination of hydrophilic particles and surfactants provides a simple method to stabilize Pickering emulsions. The type and concentration of the particles and surfactants play important roles in the microstructure and rheological properties of the resulting emulsions. Herein, stable n-octane-in-water Pickering emulsions with tunable rheological properties were prepared using thread-like mesoporous silica nanoparticles (TMSNPs) and cetyltrimethylammonium bromide (CTAB) as emulsifiers. The CTAB concentration (C_CTAB) highly affected the properties of emulsions, which were divided into three regions according to the results of large-amplitude oscillatory shear responses. In the low C_CTAB range (0.03 mmol L^-1 ≤ C_CTAB ≤ 0.1 mmol L^-1), the emulsions gelled with a high storage modulus . With C_CTAB increasing, the value of emulsions, measured by the small-amplitude oscillatory shear, decreased from approximately 1000 Pa at 0.03 mmol L^-1 to 100 Pa at 0.3 mmol L^-1 and then to 40 Pa at 3 mmol L^-1. A three-dimensional percolation structure formed by cross-linking of TMSNPs in the emulsion continuous phase was observed via cryo-SEM in the low C_CTAB range but not in the intermediate and high C_CTAB ranges. The mechanisms showing the synergistic stability and rheological properties of these emulsions were investigated. It is attributed to the unique morphology of TMSNPs and the competitive adsorption of CTAB molecules at the oil-water interface and on the nanoparticle surface in different C_CTAB ranges. Moreover, owing to the porosity and hydrogen-bonding interactions between the TMSNPs and the confinement effect of the flocculated oil droplets, the viscoelasticity of the emulsions could be mediated by adding a trace amount of acid/base. This study provides a new strategy to regulate the rheological properties of emulsions. It also expands the Pickering emulsion systems with tunable rheological properties.

Salt-Resistant Switchable Pickering Emulsions Stabilized by Mesoporous Nanosilica Hydrophobized In Situ by pH-Insensitive Surfactants

Novel oil-in-water (O/W) Pickering emulsions (PEs) were prepared using mesoporous nanosilica in combination with a pH-insensitive cationic surfactant as a stabilizer and show an interesting sensitivity to acids and bases. Adding a suitable amount of NaOH (n_NaOH/n_{cationic surfactant} ≥ 1) led to prompt demulsification within 10 s. Upon further adding HCl solutions (n_HCl/n_NaOH = 1), stable PEs re-formed after homogenization. These emulsions remained stable for over 30 days after 60 cycles, switching from stable to unstable and back to stable states, and showed a high salt tolerance. A mechanism for the switching of the Pickering emulsion (PE) to unstable and back to stable states was derived and involved anionic and neutral forms of hydroxyl groups at the mesopores of the mesoporous silica nanoparticles (MSNPs). This work reveals a switchable PE system involving a pH-insensitive surfactant, in which the species of oils and cationic surfactants can be arbitrarily selected, a feature that greatly expands the applicability of PEs.

New Low-Energy Method for Nanoemulsion Formation: pH Regulation Based on Fatty Acid/Amine Complexes

Phase inversion composition methods and phase inversion temperature methods are the common methods for nanoemulsion formation. The mechanisms governing both PIC and PIT are the same: composition or temperature can trigger a change in the surfactant spontaneous curvature during the emulsification process. It is anticipated that pH may also induce a change in the spontaneous curvature of pH-responsive surfactants to prepare nanoemulsions. Therefore, fatty acid/amine complexes were synthesized through electrostatic interactions. Based on these complexes, nanoemulsions were successfully prepared by pH regulation. Electrical conductivity and pH measurements were employed to determine the phase inversion process. Dynamic light scattering, digital fluorescence microscopy, and transmission electron microscopy were employed to characterize the droplet size and morphology of the nanoemulsion. The effects of complex concentration, NaCl concentration, and pH of the system were investigated. The developed method, phase inversion pH (PIpH) method, is a moderate and easy-control method. Using this method, the size distributions of nanoemulsion are monomodal and narrow. Nanoemulsion prepared by PIpH has a unique pH-responsive behavior that can be controllably regulated among nanoemulsions, emulsions, and phase separation systems.