CO2-Switchable Nanoemulsion Based on N,N-dimethyl Oleoaminde-Propylamine (DOAPA) and Sodium Dodecyl Sulphate (SDS)

Influence of a CO2-switchable additive on the surface and foaming properties of a cationic non-switchable surfactant

Switchable materials in general and CO₂-switchable materials in particular are of great interest in environmental research. The replacement of common non-switchable materials (solutions, solvents, surfactants, etc.) with their switchable counterparts has a great potential to make processes more environmentally friendly by enhancing reusability and circularity and thus reducing energy costs and material consumption. Inspired by this, the present work deals with the surface and foaming properties of aqueous solutions of a non-switchable surfactant in presence of a CO₂-switchable additive. A 1 : 1 and a 1 : 5 (molar ratios) mixture of the non-switchable surfactant C₁₄TAB (tetradecyltrimethylammonium bromide) and the CO₂-switchable additive TMBDA (N,N,N,N-tetramethyl-1,4-butanediamine) were investigated. It was found that surface properties, foamability, and foam stability can be changed by switching the additive with CO₂ as a trigger. This observation can be explained by the fact that TMBDA is surface active in its unprotonated, i.e. neutral form, which disturbs the tight packing of the surfactant molecules on the surface. As a consequence, foams generated with surfactant solutions containing the neutral TMBDA are less stable than their TMBDA-free counterparts. On the other hand, the switched diprotonated additive is a 2 : 1 electrolyte with hardly any surface activity and thus does not affect surface and foam properties.

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.

Advanced Switchable Molecules and Materials for Oil Recovery and Oily Waste Cleanup

Advanced switchable molecules and materials have shown great potential in numerous applications. These novel materials can express different states of physicochemical properties as controlled by a designated stimulus, such that the processing condition can always be maintained in an optimized manner for improved efficiency and sustainability throughout the whole process. Herein, the recent advances in switchable molecules/materials in oil recovery and oily waste cleanup are reviewed. Oil recovery and oily waste cleanup are of critical importance to the industry and environment. Switchable materials can be designed with various types of switchable properties, including i) switchable interfacial activity, ii) switchable viscosity, iii) switchable solvent, and iv) switchable wettability. The materials can then be deployed into the most suitable applications according to the process requirements. An in-depth discussion about the fundamental basis of the design considerations is provided for each type of switchable material, followed by details about their performances and challenges in the applications. Finally, an outlook for the development of next-generation switchable molecules/materials is discussed.

pH-Responsive Non-Pickering Emulsion Stabilized by Dynamic Covalent Bond Surfactants and Nano-SiO2 Particles

A novel stimulus-responsive non-Pickering emulsion stabilized by nano-SiO₂ particles was prepared in our recent study. 4-formylbenzoic acid and hexylamine through a dynamic covalent bond form a surface-active substance, which was confirmed by Fourier transform infrared (FTIR) and ¹H NMR. Through optimization experiments, it was proved that a stable emulsion can be formed by low surfactant concentration (below cmc) and low nano-SiO₂ particle concentration (0.5 wt %). In this emulsion, nano-SiO₂ particles are not located at the interface of oil-water but dispersed in the continuous phase of the emulsion, which is different from the Pickering emulsion. The negatively charged nano-SiO₂ particles and anionic surfactants repel each other, thereby synergistically stabilizing the emulsion so that the concentrations of surfactants and nanoparticles required to stabilize the emulsion are reduced. In addition, the system can also control the formation and fracture of dynamic covalent bonds by changing pH, thereby controlling the stability and demulsification of the emulsion. At the same time, this non-Pickering emulsion could be used as a microreactor for chemical synthesis and still had a high yield after three cycles. This study provides a new application direction for this environmentally friendly emulsion.