Investigation on the effect of functional groups on the wettability of coal dust: Experiments and theoretical validation

The interaction between the functional groups of pulverized coal and water molecules is essential to study the wettability of pulverized coal surfaces. Coal dust samples with the same degree of deterioration were modified in this study using appropriate concentrations of nitric acid and hydrogen peroxide. Based on macroscopic experiments such as Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), the functional group content and surface wettability changes of coal dust before and after modification were investigated, respectively. Experiments demonstrated that the increase in the percentage of oxygen-containing functional groups (OFGs) before and after the modification of coal samples improved the hydrophilicity of the coal surface, which was mainly dominated by the effect of O = C-O. Eight simulation systems were also constructed by grafting different functional groups with flaky graphene as the substrate in combination with molecular dynamics (MD) theory. The results of simulated contact angle, interaction energy, and radial distribution function were used to analyze the influence law of these groups on the wettability of pulverized coal in detail. Simulations indicate that the diffusion of water molecules on the coal surface is related to the number of hydrogen bonds formed between water and coal. In the simulated system of polar groups, the hydrophilicity of the interface is enhanced due to the strong hydrogen bonding between water molecules and functional groups. However, for the analysis of non-polar groups, the hydrophobic nature is improved due to the increase of carbon chains. The results of the experiments and the simulations correspond to each other, revealing the reasonableness of the simulations. It was finally confirmed that the wetting ability of water molecules for the eight functional groups was in the order of: -COOH > -OH > -CHO > -NH2 > -COC > -CH3 > -C2H5 > -C3H7.