Foaming Properties of CO2-Triggered Surfactants for Switchable Foam Control

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.

Molecular dynamics simulation of CO2-switchable surfactant regulated reversible emulsification/demulsification processes of a dodecane-saline system

CO2-Switchable surfactants are of great potential in a wide range of industrial applications related to their ability to stabilize and destabilize emulsions upon command. Molecular dynamics simulations have been performed to reveal the fundamental mechanism of the reversible emulsification/demulsification processes of a dodecane-saline system by a CO2-switchable surfactant that switches between active (i.e., N'-dodecyl-N,N-dimethylacetamidinium (DMAAH+)) and inactive (i.e., N'-dodecyl-N,N-dimethylacetamidine (DMAA)) forms. The density profiles indicate that DMAAH+ could increase the oil-water interfacial thickness to a greater extent compared to DMAA. DMAAH+ could sharply reduce the interfacial tension of the dodecane-saline system, while DMAA only exhibits a limited decrease, which is in accordance with the experimental observation that DMAAH+/DMAA can reversibly emulsify/demulsify alkane-water systems. Our simulations showed that both the number and lifetime of hydrogen bonds (HBs) between DMAA and water are almost equal to those between DMAAH+ and water. In DMAA, the N atom connecting with the alkyl tail acted as a HB acceptor, while the N atom attached by a proton in DMAAH+ acted as a HB donor. Furthermore, the HBs between DMAAH+ and HCO3- at the interfaces are relatively limited. Hence, it is deduced that the HBs are insufficient to achieve the CO2-switchability of DMAA/DMAAH+. The Lennard Jones and coulombic potentials between DMAA/DMAAH+ and other species show that the coulombic potentials between DMAAH+ and water or anions (i.e., Cl- and HCO3-) sharply decrease with the increase of DMAAH+ and are much lower than those in models with DMAA. The enhanced coulombic interactions between DMAAH+ and anions lead to a remarkable reduction in interfacial tension and the emulsification of the alkane-saline system. Therefore, coulombic interactions are of crucial importance to the reversible emulsification/demulsification processes regulated by CO2-switchable surfactants, namely DMAAH+/DMAA.

Photoresponsive Foams Generated by a Rigid Surfactant Derived from Dehydroabietic Acid

Innovation in the structure of surfactants is crucial to the construction of a surfactant-based system with intriguing properties. With dehydroabietic acid as a starting material, a nearly totally rigid azobenzene surfactant (R-azo-Na) was synthesized. The trans-R-azo-Na formed stable foams with half-lives of 636, 656, 976, and 872 min for 0.3, 1, 2, and 4 mmol·L^-1 aqueous solutions, respectively. Under UV light irradiation, a fast collapse of the foams was observed, showing an in situ response. The excellent foam stability of trans-R-azo-Na leads to the extremely high photoresponsive efficiency. As revealed by dynamic surface tension and pulsed-field gradient NMR methods, an obvious energy barrier existed in the adsorption/desorption process of trans-R-azo-Na on the air/water interface. The foams formed by trans-R-azo-Na are thus stable against coarsening processes. The results reveal the unique photoresponsive behavior of a surfactant with a rigid hydrophobic skeleton and provide new insights into the structure causing aggregation of surfactants.

Switchable foam control by a new surface-active ionic liquid

Switchable foam control was achieved for aqueous solution of new surface-active ionic liquid ([BAzoTMA][NTf₂]) by alternatively adding cucurbit[7]uril and spermine.

Synthesis and Properties of a Series of CO2 Switchable Gemini Imidazolium Surfactants

Novel switchable gemini imidazolium surfactants with different carbon atoms in hydrophobic group were successfully synthesized by condensation of fatty acid with triethylene tetramine, then the intermediates were reacted with CO2 to give the imidazolinium bicarbonates. The structures of intermediates and products were identified by IR and 1H-NMR spectra. As the results show, the structures of the products obtained correspond to the target compounds designed. By surface tension measurements, these Gemini surfactants have excellent surface activity with low cmc and surface tension. The conductivity and surface tension cycles show that these surfactants could be switched from imidazoline (neutral form) to imidazolium bicarbonate (charged form) reversibly and repeatedly.