Investigating the effect of hydrophilic SiO2 nanoparticles on foam stability using molecular dynamics simulation

Abstract

SiO₂ nanoparticles (SiO₂-NPs) enhance the foam stability primarily in two ways. Firstly, SiO₂-NPs located in the water channel can slow down the drainage process. Secondly, the high adsorption energy of SiO₂-NPs at the interface makes them difficult to desorb, forming an adsorption film with high mechanical strength. This paper investigates the mechanism by which hydrophilic SiO₂-NPs enhance foam stability using molecular dynamics (MD) simulations based on foam films formed by sodium dodecyl sulfate (SDS). The results show that the hydrophilic SiO₂-NPs can significantly impede the movement of water molecules around them and within the water channels, thereby inhibiting the drainage process of the foam film. Additionally, hydrophilic SiO₂-NPs and SDS form a solid bridge structure with strong mechanical strength at specific sites through hydrogen bond and van der Waals (vdW) interactions. The stability of the foam film is represented by its resistance to rupture, which involves both the drainage and rupture processes. The effect of hydrophilic SiO₂-NPs on the stability of various positions within the foam film was investigated using the steered molecular dynamics (SMD) method. External forces applied in the vertical direction simulate the foam film drainage process, while forces applied horizontally simulate the rupture process. The simulations demonstrate that hydrophilic SiO₂-NPs can significantly improve foam film stability compared to the pure surfactant system. This study elucidates the rupture process of foam film systems containing hydrophilic SiO₂-NPs at the molecular level, providing a deeper understanding of foam stability mechanism.