Mechanism of wetting by anionic surfactants with different polar groups on hydrophilic and hydrophobic nano-silica

Abstract

Context

With advancing technology, the hazards of hydrophilic and hydrophobic nano-silica dust have become increasingly apparent. Surfactants are widely used in dust control; however, their performance is primarily determined by their polar groups. To investigate the effect of various polar groups of anionic surfactants on the wettability of hydrophilic and hydrophobic nanosilica. The results indicate that the electronegativity of the electrostatic potential on the hydroxylated silica surface is relatively strong, the larger the electrostatic potential difference (ΔESP) between the surface binding sites and the polar groups of the surfactant, the less favorable the surface is for hydrophobic modification. Additionally, C and O atoms tend to form smaller negative electrostatic potentials compared to S and O atoms, with polar group activity ranked as carboxylate > sulfonate > benzene sulfonate > sulfate. The interaction between SiO2-OH surfaces and water molecules is approximately 3.4 times stronger than that of SiO2-CH3 surfaces. The interaction between water molecules and the SiO2-OH surface is primarily governed by van der Waals forces, whereas the interaction between water molecules and the SiO2-CH3 surface is mainly driven by electrostatic forces. The polar groups of the surfactant are distributed in the aqueous phase, while the nonpolar groups interact with the surface through electrostatic interactions. The hydration layer surrounding the polar groups of hydrophilic surfaces is primarily stabilized by strong hydrogen bonding with water molecules. In contrast, for hydrophobic nano-silica surfaces, the hydration layer is influenced by both van der Waals forces and weaker hydrogen bonding interactions. The SiO2-CH3 surface cannot form hydrogen bonds, while the SiO2-OH surface has a strong capacity to stably form hydrogen bonds with carboxylate and sulfate groups. Hydrogen bonding is an essential factor in wetting. The polar group COO- is suitable for controlling hydrophilic and hydrophobic nano-silica dust. These findings provide theoretical and technical references for the selection, application, and design of surfactants in nano-silica dust control.

Methods

To elucidate the effects of various polar groups of anionic surfactants on the wetting of hydrophilic and hydrophobic nano-silica, quantum chemical calculations and molecular dynamics simulations were used to investigate the interfacial adsorption and wetting behavior of anionic surfactants with identical chain lengths but different polar groups on these surfaces.