## 3 Results and Analysis
### Performance Evaluation of Modified Nano-SiO\({}_{2}\)
#### 3.2.2 Stability and Zeta Potential  
Modified nano-SiO\({}_{2}\) was dispersed in pure water, TX-100, ethanol and kerosene to explore the dispersibility of nano-SiO\({}_{2}\). As shown in Figure 6, due to the large specific surface area and high surface energy of modified nano-SiO\({}_{2}\), it was unable to disperse in water and agglomerate seriously. In ethanol solution, the dispersion effect was poor, and some modified nano-SiO\({}_{2}\) agglomerated and precipitated. In TX-100, the dispersion effect of modified nano-SiO\({}_{2}\) was better, but the solution was cloudy. The nano-SiO\({}_{2}\) had good dispersibility in the kerosene dispersion system, and the solution was clear and transparent.  
Figure 4: Thermogravimetric curve.  
To explore the dispersion stability, the dispersion liquid was stored for 30 days, and then the solution dispersion was observed. Figure 7 shows the dispersion situation of the above-mentioned solution. Compared with the dispersion solution not stored for 30 days, precipitation occurred in the surfactant aqueous dispersion system. The precipitation in the ethanol dispersion system increased significantly, but the kerosene dispersion system remained well dispersed.  
Figure 5: Contact angle of core before and after adsorption of modified nano-SiO\({}_{2}\).  
Figure 6: Modified nano-SiO\({}_{2}\) dispersion in water (**A**), Polyethylene glycol octylphenyl ether (TX-100) (**B**), ethanol (**C**) and kerosene (**D**).  
Figure 7: State of modified nano-SiO\({}_{2}\) after 30 days of dispersion in water (**A**), Polyethylene glycol octylphenyl ether (TX-100) (**B**), ethanol (**C**) and kerosene (**D**).  
A large amount of working fluid is generally used on site; so, using kerosene as a dispersant is costly and has high safety risks during storage and transportation. Therefore, the dispersion system of adding NaOH to the surfactant aqueous solution needed to be explored. As shown in Figure 8, the dispersion of the aqueous surfactant dispersion system without NaOH was turbid, whereas the dispersion of the aqueous surfactant dispersion system with NaOH was well dispersed, and the dispersion was clear and transparent. Then, the dispersion before and after NaOH adjustment was stored for 30 days. As shown in Figures 7 and 8, the precipitation formed in the dispersion system without NaOH, whereas the dispersion with NaOH remained clear and transparent.  
Nano-SiO\({}_{2}\) dispersion is a colloidal system, and zeta potential is also an important parameter affecting its stability. In addition to the stability experiments mentioned above, the zeta potentials of the surfactant aqueous dispersions before and after the introduction of NaOH were also measured. The larger absolute value of the zeta potential indicated that the system was more stable. As shown in Figures 9 and 10, after NaOH addition, the zeta potential of the system increased from \(-24.1\) mV to \(62.7\) mV. Analysis revealed that surfactant TX-100 can form a steric hindrance effect through adsorption onto the surface of modified nano-SiO\({}_{2}\), which can prevent mutual collision and coalescence between particles and improve the system stability. After adding NaOH, OH\({}^{-}\) adsorbed onto the surface of the modified nano-SiO\({}_{2}\) through hydrogen bonds, whereas Na\({}^{+}\) was arranged around the modified nano-SiO\({}_{2}\) via diffusion, forming a diffusion electric double layer. Consequently, the charge on the surface of nano-SiO\({}_{2}\) increased, which improved the repulsion between particles and the system stability.