Molecular Simulations On Adsorption Of Water Onto Lignite Surface And Its Wettability Modification

There is rich reserve of lignite in China and its application prospect is abroad.But the abundant pore structure and the large number of oxygen-containing groups cause the high water content of lignite,which limit its efficient utilization.How to decrease the hydrophilicity of lignite surface and inhibit moisture re-adsorption has become the key technology of the upgrading and utilization of lignite.The main drying technologies at present are evaporation drying and non-evaporation drying technology.These methods have disadvantages such as large energy consumption and serious environmental pollution.The hydrophilicity of lignite surface can be effectively reduced by the use of chemical agents adsorbed and covered on the strong polar oxygen-containing functional groups of lignite surface and the chemical agents could block the interaction between oxygen-containing functional groups and water molecules.In this paper,the behavior and capacity of water molecules adsorbing onto different oxygen-containing functional groups were studied by the method of quantum chemical density functional theory.The adsorption capacity and morphology of non-polar n-octane,n-dodecane and n-hexadecane onto lignite surface were investigated by molecular dynamics(MD)simulation.Due to the adsorption capacity of non-polar alkanes onto lignite surface is not strong,so the sodium dodecyl sulfate(SDS),sodium dodecyl benzene sulfate(SDBS)and dodecyl trimethyl ammonium bromide(DTAB)with amphiphilic structure were selected to study the adsorption behavior on the lignite surfaceand the influence on wettability of lignite by MD methods.The main conclusions of molecular simulations are as follows:(1)The water molecules could form two hydrogen bonds at the carboxyl sites and one hydrogen bond at other oxygen functional sites.The oxygen atoms in the oxygen-containing functional groups get electrons which lead to the change of the bond length of O-H,C=O and C-O.The bond angles of water molecules change because of the difference of the adsorption state.The order of adsorption strength on different sites is: carboxyl>phenolic hydroxyl>alcohol hydroxyl> carbonyl> ether bond.(2)It can be found that the ?LUMO-HOMO and interaction between single molecule and lignite become increase with the increase of chain length throngh the quantum chemical calculation of single molecule alkanes.The diffusion coefficient of alkane in the alkane-water system increased with the increase of alkane chain length.By means of MD calculation of alkane-lignite system,the results show that the shorter of alkane chain length,the larger of the diffusion coefficient and the interaction energy.(3)The surface area of lignite with adsorption of DTAB was the largest and the interaction between DTAB and lignite surface was the strongest.The radial distribution function showed its polar group had the smallest adsorption distance with the oxygen-containing groups of lignite surface.On the contrary,the adsorption of anionic surfactant on lignite surface is weak.(4)The adsorption morphology and relative concentration distribution of lignite-surfactant-water system were analysised,it could be found that the water molecules are adsorbed dispersedly on lignite surface with the adsorption of DTAB;the spatial arrangement of benzene ring in SDBS ions near the lignite bulk phase;polar groups of three kinds of surfactants adsorbed on the surface of lignite basically,the alkyl chain stretch to the water phase and the extension length of alkyl chains follows the order: SDS<SDBS<DTAB.The adsorption energy between water molecules and lignite surface with adsorption of surfactant and the diffusion coefficient of water molecules were calculated,the results showed that DTAB has the best effect on reducing the hydrophilicity of lignite surface and SDBS is next.