Study on the mechanism of interaction between cations and the surface of nano-SiO2 particle in the salt solution by using molecular dynamics simulation

Nano-SiO2 solution has great potential for application in the field of oil and gas exploration and development. However, its application is restricted due to the aggregation of nano-SiO2 in stratum water. This paper studies the process of interaction between nano-SiO2 particles and salt solutions by using the molecular dynamics simulation experiments, including the characteristics of transportation and diffusion of cations around the nano SiO2, the effect of cations on the recombination and destruction of hydrogen bonds, the interaction energy between cations and nano SiO2. The main results are as follows. During the transportation and diffusion of cations in the solution, the hydrogen bonds of water molecules are broken, so that the hydrogen bonds can be rearranged, and a stable solvated layer can be formed. In the solvated layer, the repulsive force of solvation can prevent the charge exchange between cation and the charged surface of nano SiO2 particle. Compared with the monovalent salt solution, more divalent cations can break through the solvation layer to generate the charge exchange on the surface of nano-SiO2 particle. More divalent cations can conduct small amplitude oscillation movement near the surface of nano-SiO2 particle due to the strong electrostatic effect. When the concentration of salt solution is increased (3000 mg/L → 7000 mg/L) and the valence of cations is increased (monovalent → bivalent), the hydrogen bond is easier to break through the solvation layer to gather on the surface of nano SiO2 particles, which is unfavorable to the stability of nano SiO2 particles. When the cations transport to the position of 7 Å away from the surface of nano-SiO2 particles, the short-range force (including hydrogen bond force, van der Waals force, and electrostatic force) begins to affect the movement of cations. When the cations transport to the position of 2 ∼ 3 Å away from the surface of nano-SiO2 particles, the charge exchange occurs between cations and the surface of nano SiO2 particle, so that the local energy is increased and the cation continues to oscillate. In summary, with the increasing of concentration of salt solution and the valence state, more cations can break through the solvation layer to be adsorbed on the surface of nano SiO2 particle, showing that the ability of transportation and diffusion of cations is weakened, the interaction energy between nano SiO2 particle and salt solution molecules is enhanced. The electrostatic force is the main force to destroy the stability of nano SiO2 particle. The conclusions can provide a certain theoretical basis for the efficient application of nano-SiO2 materials in the field of oil and gas exploration and development and other interrelated industry.