Wettability Alteration Of Lignite Achieved By Nonionic Surfactants With Different Structures: A Molecular Dynamics Simulation Study

The strong hydrophilicity of lignite caused by the abundant oxygen containing groups and the developed pore structure is the key factor that affects the efficient utilization of lignite.An effective upgrade method to strengthen the hydrophobicity of lignite is covering the oxygen containing part of the lignite with the hydrophilic groups of surfactants,which relies mainly on the structure of the adsorbed layer formed by the surfactant on the lignite surface.The structure and composition of the surfactants have a significant influence on the structure of the adsorbed layer.Compared with experimental method,molecular dynamics simulation（MD）has the unparalleled advantages in safety,cost and exploring the microscopic mechanism.Therefore,in this paper,the effect of the structural properties of non-ionic surfactants on lignite’s wettability was investigated by MD.By studying the effect of the types of hydrophilic groups in nonionic surfactants on adsorption behavior and surface wettability of lignite,it could be known that the adsorption capacity of dodecyl maltoside C₁₂G₂ with polar hydroxyl groups on lignite surface is obviously stronger than that of dodecyl polyoxyethylene ether C₁₂E₇ with weak polar ether groups.However,the bulky nature of the maltose head group makes the OH groups in it expose as covering the oxygen-containing groups of lignite surface,resulting in weakening the ability to form an effective adsorption layer that inhibits the hydrophilicity of lignite.Thus,the C₁₂G₂ makes the lignite even more hydrophilicity.In contrast,the polarity of lignite surface is reduced by the coverage of oxygen content by ether groups in C₁₂E₇ and the exposure of hydrophobic chain of C₁₂E₇,and the interaction with water molecules is significantly decreased,i.e.the hydrophilicity of lignite surfaces is obviously decreased.The simulation results are consistent with the wetting heat results.Based on the above results,the effects of the number of ethoxy groups（EO）in dodecyl polyoxyethylene ether surfactants on the surface wettability of lignite were investigated.Molecular dynamics simulations were carried out using surfactants with ethoxylation numbers of 7,9 and 15(abbreviated as C₁₂E₇,C₁₂E₉ and C₁₂E₁₅ respectively).The results show that the increase of EO groups in the surfactant can not only make the adsorbed layer compactness but also facilitate its adsorption on lignite surface.The increase in the number of ethoxy groups also increases the degree of coverage of the oxygen-containing groups on the surface of lignite and enhances its ability to weaken the strong electrostatic interaction and hydrogen bonding between the lignite and water molecules.Ultimately,the hydrophilicity of lignite surface is further reduced with the increase of ethoxy groups in the surfactant.Also,the nonylphenol polyoxyethylene ether NPEO₁₅ was selected to compare with C₁₂E₁₅ aiming to investigate the influence of benzene ring structure in hydrophobic chain of surfactant on the wettability of lignite.The results of simulation show that the existence of benzene ring reduces the flexibility of the molecule,weakens the orderliness of the adsorbed layer structure and its adsorption capacity on lignite surface,and finally leads to the NPEO₁₅ being less effective in inhibiting the hydrophilicity of lignite.The same results were obtained by moisture re-adsorption.This study has certain theoretical significance for screening and designing suitable surfactants to inhibit the strong hydrophilicity of lignite surface.