Mechanisms of N<sub>2</sub> Adsorption and Accumulation on the Hydrophilic Surface: Effect of Surface Wettability Regulation.

Abstract

The adsorption and accumulation of gas molecules at the solid-liquid interface are prerequisites for bubble nucleation and bubble-particle attachment in the flotation process. However, the wettability of solid surfaces on the adsorption and accumulation behavior of gas molecules on solid surfaces is still unclear. In this study, contact angle measurements and molecular dynamics simulations were combined to investigate how preadsorption of fatty alcohol polyoxyethylene ether-15 (C₁₂EO₁₅) on hydrophilic surface modulated the adsorption behavior of N₂ molecules. The results demonstrated that C₁₂EO₁₅ reduced surface tension on hydrophilic surfaces and modulated the solid-liquid interface tension through van der Waals forces, electrostatic interactions, and hydrogen bonding. This adsorption process resulted in the formation of a "mesh" layer on the hydrophilic surface. The layer masked hydrophilic sites and diminished the hydrophilic area and lowered the adsorption strength of H₂O molecules on the hydrophilic surface. Consequently, the weakened hydration layer facilitating gas enrichment at the hydrophilic surface. With increasing C₁₂EO₁₅ concentration, intermolecular interactions among C₁₂EO₁₅ molecules are enhanced, providing accumulation space and adsorption sites for N₂. Concurrently, the "mesh" layer restricted N₂ diffusion, promoting the adsorption of N₂ in the gas clusters on the substrate. Notably, N₂ exhibits preferential adsorption at carbon-containing sites of the substrate during this wettability modulation process. This work reveals the critical role of surfactant preadsorption in interfacial gas enrichment and bubble attachment, offering molecular insights for enhancing the flotation efficiency of hydrophilic coals.