Molecular Resolution Imaging of Adsorption of Dodecane on a Lignite Surface by Frequency Modulation AFM

The consumption of dodecane, a critical flotation collector, in the flotation of low-rank coal is particularly high; however, the adsorption mechanism of dodecane on coal still remains unclear. To shed light on this issue, the molecular-scale adsorption structure of dodecane on a lignite surface was investigated in situ by frequency modulation atomic force microscopy (FM-AFM) for the first time. The adsorption structure of dodecane on highly oriented pyrolytic graphite (HOPG) and mica has been detected in advance. Meanwhile, molecular dynamics (MD) simulations are utilized to elucidate and illustrate the adsorption structure on HOPG, mica, and lignite. Experimental results from FM-AFM show that a layered structure of dodecane molecules is formed on HOPG, while there is no stable adsorption structure on mica. Meanwhile, a few local monolayer structures of dodecane molecules are observed on the surface of lignite, and their thickness is approximately 1 nm, which is less than the molecular chain length of dodecane. It is speculated that the dodecane molecules are adsorbed on lignite selectively, with the capacity to form a stable adsorption structure. As indicated by MD simulations, the interaction energy between dodecane and the mentioned surfaces is in the order HOPG > lignite > mica. And the MD simulation results show a desirable match with direct observation by FM-AFM. In accordance with the characterization of the lignite sample surface, large amounts of polar oxygen-containing functional groups may hinder dodecane adsorption behavior. Through in situ molecular resolution imaging of the interface structure between dodecane and a raw lignite surface, we provide a new perspective for exploring the interaction mechanism between nonpolar collectors and a lignite surface.