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Similar to the responsive behaviors of emulsions under salt-free conditions, precise control of emulsification and demulsification is also required in some applications under saline conditions, especially in crude oil production and transport. So far, few studies have focused on the preparation of the responsive emulsions under saline conditions. In this study, CO2 responsive emulsions were prepared in the presence of NaCl using a fatty acid soap M-DA as stabilizer. M-DA complex was synthesized through electrostatic interactions between dimeric acid (DA) and Jeffamine (M1000). The droplet size and morphology of the oil–brine emulsions were characterized by dynamic light scattering and transmission electron microscopy, respectively. The oil–brine emulsion maintains high stability under air but completely separates into two phases upon exposure to CO2. NaCl plays a significant role in the CO2-induced demulsification, including salting-out effect and screening of electrostatic repulsion. It is worth noting that in the absence of NaCl, the emulsion remained stable after CO2 bubbling. CO2 responsive emulsions can also be prepared in the presence of other salts, or with other fatty acid soaps, which presents more interesting possibilities for preparation of CO2 responsive emulsions under saline conditions.

The application of hydrophobic associating polyacrylamide (HAPAM) was restricted due to the poor solubility. In order to enhance the solubility of HAPAM and avoid environmental pollution, aqueous two-phase polymerization was used to prepare HAPAM in ammonium sulfate aqueous solution and a milky dispersion was obtained. The HAPAM particles dispersed in the solution spherically and the average particle size was about 7.4 μm. The hydrophobic associating polyacrylamide prepared with aqueous two-phase polymerization exhibited superior solubility and it could dissolve completely in twenty seconds. HAPAM solution could aggregate to network through inter-molecular association, leading significant increase of viscosity. Moreover, the viscosity of HAPAM solution increased with addition of inorganic salt even in nearly-saturated solution, and the viscoelasticity of HAPAM solution also enhanced obviously in the presence of salt, exhibiting excellent salt resistance. Furthermore, anionic surfactant of sodium dodecyl sulfate (SDS) could aggregate with the cationic hydrophobic monomers on the polymer chains via electrostatic attraction to form mixed micelles, which could prompt the polymer from intra-molecular association to inter-molecular association, leading remarkable increase of viscosity and viscoelasticity of the solution. In addition, The HAPAM dispersion doesn’t contain any organic solvent and could become solution instantaneously once dilution into water, which provides a green and environmentally friendly strategy for preparation of hydrophobic associating polymers. The HAPAM dispersion was well fit for the hydraulic fracturing in shell gas due to the excellent solubility and environmentally friendly property. These features indicated that HAPAM dispersion would have a great potential application in hydraulic fracturing in shell gas.

A novel pseudogemini surfactant system, which has two types of pH-responsive groups in the connection of the alkyl chain and the spacer, has been developed and investigated by adjusting pH. These two types of pH-responsive groups both worked for the micelle transition because they had different ionization degrees and electrostatic attraction forces with the aid of pH variation. The system could be promptly switched between low viscosity fluid and high viscoelastic solution. Besides, the pH-responsive ability was studied by the steady and dynamic rheological measurements, which is effective after several cycles of pH adjusting with the almost invariable magnitude level of viscosity. The transition between spherical micelle and wormlike micelle for the surfactant system at different pH values could be further confirmed by dynamic light scattering and cryogenic transmission electron microscopy characterization.

The hydrophobically associating cationic polyacrylamides (C-HAPAM), containing a major part of hydrophilic polyacrylamide backbones and a minor part of ionic hydrophobic groups, i.e. N,N′-dimethyl octadeyl allyl ammonium chloride (DOAC), were synthesized. The self-assembling behaviors of the C-HAPAM molecules in water under different concentrations and pH values were investigated in detail and disclosed by rheological tests, dynamic light scattering (DLS), zeta potential, scanning electron microscope (SEM), and atomic force microscope (AFM). The critical aggregate concentration (CAC) for C-HAPAM was about 0.42 wt% and a solution-sol–gel appearance transition has been observed for C-HAPAM aqueous solutions with increasing concentration. Rheological tests demonstrated a pH reversible sol–gel transition when the pH was regulated from its original value of 7.45 to the weak alkaline value of 9.87 by adding NaOH and then switched back to 5.28 by adding HCl. The SEM, AFM and DLS results revealed that the sol–gel transition was actually attributed to the fiber-network aggregate structure transition. The mechanism of above pH responsive self-assembling behavior transition was explained by the remarkable counterion effect of OH−. Specifically, OH− was additionally capable of H-bonding with surrounding water molecules which promoted the solvation of C-HAPAM and induced thick hydration layer. Then, the hydrated layer led to the increment of zeta potential and volume expansion of C-HAPAM molecule which in turn strongly enhanced the viscosity of the solution. The strong counterion effect of OH− and the inter-molecular hydrophobic association interactions of C-HAPAM collectively triggered the formation of joints and hence the network structure transition, i.e. sol–gel transition.

Due to various industrial applications, aqueous foams stabilized by surfactants in combination without and with polymers, biopolymers or/and solid particles have been extensively investigated. In contrast, there are few reports involved in surfactant-free aqueous foams. Herein, a surfactant-free foam system containing xanthan-based amphiphilic polysaccharide (C8-g-XG) and nanosilica particle was presented and studied. With an attempt to address the framework, C8-g-XG was prepared via n-octylamine grafted onto xanthan gum (XG). C8-g-XG inherited good viscosification, salt tolerance and temperature resistance from XG, and lower values of γCMC (39.2 mN/m) appeared in C8-g-XG solutions. Additionally, the surface tension of C8-g-XG solutions (3000 mg/L) with 0.2 wt% nanosilica particles would not change significantly. However, C8-g-XG@nanosilica particle-stablized foams exhibited excellent thermal adaptability, thermal stability and proppant suspension ability due to synergy of C8-g-XG and nanosilica particles. As a result, this strategy may be used to develop eco-friendly low formation-injury foam fluids for hydraulic fracturing application because C8-g-XG could be degraded and its surface activity was disabled.

Due to the limitation of anhydrous conditions, the research on anhydrous microemulsion has been developed rapidly. In this paper, CO2/N2-responsive ILs was used as a polar phase instead of water to prepare anhydrous surfactant-free microemulsion (SFME) with ethanol and n-hexane. This research group has previously studied the preparation process and CO2/N2 responsiveness of the ILs. Then the boundary between multi-phase zone and single-phase zone of surfactant free microemulsion system was determined by drop addition. And the single-phase region was divided into three regions: O/W, B.C and W/O by conductivity and uv–vis. N2 was introduced into SFME system at 30℃. Interestingly, we found that there was no separation of oil and water. Through the study on DLS, pH and the mutual solubility of Dipropylamine, ethanol and n-hexane, it was found that demulsification occurred in the system, and the absence of oil-water separation was due to the formation of uniform and transparent mixed solution in the system. Finally, It is found that changing the temperature can make the boundary of the multi-phase zone and the single-phase zone in the three-phase diagram of SFME system move. In addition, temperature also had an effect on the particle size of SFME system.

Composite hydrogels have attracted great interest because of their superior performances and potential applications. In this work, microgel reinforced composite hydrogels (MC gels) with self-healing and pH-responsive properties were prepared via microgels as macro-cross-linkers. Compared to the general hydrogels cross-linked by borax (B gels), the MC gels demonstrate superior properties on the aspects of viscosity, viscoelastic and temperature resistance. The viscosities of MC gels increase with the rising of pH value and the MC gels can dynamically reconstruct after being destructed by external force. Furthermore, the microstructure of MC gels was characterized by SEM, which confirmed that the microgels indeed as cross-linkers and each macro-cross-linker can cross-link several chains. These features indicate that the microgels can enhance the strength of composite hydrogels significantly.

Responsive emulsions can be “switched off” by external environmental stimulus or trigger, allowing a simple and effective demulsification approach and thus attracting much attention. CO2 is a green trigger for the response process because it is non-toxic, abundant, benign, non-accumulating in the system and easily removed. This review focuses on the generation, response mechanism and potential industrial applications of CO2 responsive emulsions. The CO2 responsive emulsions can be prepared using either CO2 switchable emulsifiers adsorbed at the oil-water interface or CO2 switchable solvents in continuous or dispersed phase. The stability of emulsion can be reversibly controlled by alternate addition and removal of CO2, depending on the protonation/deprotonation degree of CO2 switchable materials. The CO2 responsive emulsions show great potential in application to heavy oil transport, soil washing, emulsion polymerization and chemical reactions.