The repulsive Coulomb interactions between surfactants and NPs with a similar charge will promote surfactant diffusion to the liquid–liquid interface. Generally, the higher the surfactant density at the water/oil interface, the lower the IFT. When 1,000 mg/L of negatively charged SiO₂ NPs was introduced into an anionic surfactant SDS solution, the kerosene/DI water IFT was reduced by another 30% on the basis of pure SDS (1,500 mg/L) (Zargartalebi et al., 2014). This mechanism also works for the nonionic surfactant dodecyl dimethyl phosphine oxide (C₁₂DMPO, )/silica mixture because of the fractional negative charge of the oxygen atom. When 2.5 wt% SiO₂ NPs were added into an 8.5 × 10⁻⁶ M C₁₂DMPO solution, a further ∼5 mN/m reduction in heptane/water IFT was reported (Vatanparast et al., 2019). In these cases, the concentration value derived from equilibrium IFT is denoted as equivalent concentration (EC), which increases with increasing NP concentration and decreasing NP size (with higher surface charge density).

NPs can concentrate surfactants on the interface and effectively reduce the interfacial area available to the surfactants. There are two action modes. 1) For NPs with no interfacial activity that originally stayed in the water or oil phase, surfactant molecules can adsorb on NPs through electrostatic attraction or hydrophobic interactions. NPs with modified hydrophilic-lipophilic balance can thereafter migrate to the interface. The competitive adsorption between the liquid and the NPs makes surfactant molecules partially desorb from NPs and redistribute on the interface to reduce the IFT. If the NPs can stay at the interface, the reduction in the interfacial area will promote IFT reduction. Otherwise, the impacts of NPs can be dismissed (Vu and Papavassiliou, 2018). Similar results were reported by Moghadam and Azizian (2014) and Vatanparast et al. (2019). In this case, NPs are good candidates as surfactant carriers, but the surfactant/NP concentration ratio should be carefully tailored. NP aggregation or precipitation may cause adverse impacts (Ravera et al., 2006). 2) For original NPs with interfacial activity (such as Janus NPs) that can stably stay at the interface, though there are negligible interactions between NPs and surfactants, this mechanism still works. According to Vu and Papavassiliou (2019), Janus NP alone barely had any impacts on heptadecane/water IFT, and surfactant SDS alone with an interfacial concentration of 0.91 molecule/nm² could reduce the IFT by ∼25%. When Janus NPs covered 37.5% of the interface, the local SDS concentration would increase from 0.65–0.95 to 1.8–2.4 molecule/nm², leading to a reduction of ∼75%. In this scenario, NPs that could occupy a larger interfacial area or preferentially be positioned in the middle of the interface are better choices when low IFTs are required at relatively low surfactant concentrations.

The interactions between surfactants and NPs can reduce surfactants’ solubility in the water or oil phase and, thereafter, facilitate their migration to the interface. For instance, driven by electrostatic attraction, oil-soluble cationic surfactant dodecylamine (DDA) could adsorb on negatively charged hydrophilic kaolinite NP surfaces. Therefore, its partition in the water phase and its adsorption at the oil–water interface would increase. According to Wang et al. (2004), 1 mM DDA alone could decrease the hexadecane/water IFT by around 10 mN/m from the original 49 mN/m. Introducing 2.0 wt% kaolinite into the system could induce another 10 mN/m reduction. While for palmitic acid (PA, oil-soluble-)–kaolinite system, the repulsion between particle and surfactant would hinder surfactant migration toward the interface and result in an IFT increase.

In real cases, surfactant adsorption loss is a worrying problem that increases the cost. Meanwhile, due to reduced effective concentration, a low IFT can hardly be ensured in middle and deep reservoirs. NPs can effectively reduce surfactant adsorption loss by forming NP shielding (Alonso et al., 2009; Zhong et al., 2019b), increasing the repulsion between surfactant and rocks (Zargartalebi et al., 2015), providing more fierce collision, and friction effect between NPs and sands (Wu et al., 2017), among others. Therefore, the validity of the surfactant in the presence of NPs can be longer.

Surfactant adsorption on NP surfaces through electrostatic attraction or hydrogen bonding can accelerate the co-adsorption of surfactant and NPs by inducing NP aggregation or deposition. Both the formation of nanostructures and the change in surface free energy are conducive to a more noticeable wettability change (Karimi et al., 2012; Songolzadeh and Moghadasi., 2017). In this case, higher salinity and temperature are preferred (Al-Anssari et al., 2018). However, the possible formation damage should be considered, and a suitable NP concentration should be selected.

NPs in an aqueous dispersion tend to self-assemble to form a wedge-like structure at the discontinuous phase. Under the drive of Brownian motion or electrostatic repulsion, dispersed NPs push the confined NPs forward and impart a huge force called disjoining pressure (Chengara et al., 2004). Generally, higher disjoining pressure corresponds to a more obvious wettability change. Either increasing NP concentration and stability or decreasing NP size and solution salinity can raise the disjoining pressure (Hendraningrat et al., 2013). The addition of surfactant into NP dispersion can increase NP dispersity and stability in the aqueous phase through hydrophobic interaction or imposing supercharging effect. By generating a more favorable wet–wet condition, a higher recovery can be obtained (Zhao et al., 2018; Zhong et al., 2020).

De-emulsion occurs in two successive steps: coagulation and coalescence. Adsorbed charged NPs can increase the electrostatic or steric repulsion between oil droplets and control coagulation. However, NPs can form an obstacle to restrict the coalescence by forming a rigid coating around the liquid droplets to prevent coalescence. Meanwhile, using surfactant and NPs together can also thicken the continuous phase by forming network structures (Ortiz et al., 2020). It is also worth noting that for Pickering emulsion preparation, a reduction in IFT is not an essential precondition. A lower IFT can reduce the required external energy input. To stabilize the emulsions, NPs should be able to go and stay at the oil/water interface. According to Lian et al. (2020), for mono-layer NP stabilized emulsions, NPs with contact angles of 15°–90° and 90°–165° should be selected for O/W and W/O emulsion, respectively. While for multiple-layer NP stabilized emulsions, the ranges are 15°–129.3° and 50.7°–165°. The adsorption of surfactants on NPs can in situ change NP wettability and favor their migration to the interface.

NPs have higher adhesion energy compared to surfactants. By accumulating at the interfaces, minimizing the contact area between air and water, increasing the film strength and lamella elasticity, and decreasing the gas diffusion, NPs are good foam stabilizers. However, the impacts of NP hydrophilicity, size, and surface charge are significant. NPs should be sufficiently hydrophilic to disperse in the aqueous phase and also sufficiently lipophilic to stay stably at the interface (Majeed et al., 2021). Properly taking full advantage of surfactant–NP interactions can partially alleviate this contradiction.