Adsorption and wetting mechanisms of ionic liquids on long-flame coal: molecular-level structural insights

Abstract

This study explores the synergistic regulation mechanism of ILs’ anion type and alkyl chain length on the wetting performance of long-flame coal. C₁₆MImBr, C₁₄MImBr and C₁₆MImCl were selected for the experiment. Combining contact angle, surface tension, and microscopic characterization, it was found that C₁₆MImBr exhibited the optimal wetting performance, with a dynamic CA change constant increased to 3.812. This value was 12 times higher than pure water (0.3245), significantly exceeding the 2.271-fold increase of C₁₄MImBr and 1.623-fold increase of C₁₆MImCl relative to pure water. At 2 g/L, the ST reduction capability ranked as C₁₆MImBr > C₁₄MImBr > C₁₆MImCl, while C₁₆MImCl showed the highest critical micelle concentration. Microscopic characterization confirmed that ILs generate microporous structures via rearrangement of coal aromatic nuclei, which significantly increased C-O hydrophilic group proportion and decreased hydrophobic group content. Pearson correlation analysis showed a significant negative correlation between C-O and –CH content. Molecular simulation revealed that alkyl chain growth reduces the frontier orbital gap value from 3.50 eV in C₁₆MImCl to 3.12 eV in C₁₆MImBr, enhancing molecular activity. Weak interaction analysis confirmed that van der Waals forces dominated the adsorption process. Results indicated that anion type exerted higher control priority on wetting efficiency than alkyl chain length. Br⁻ promoted dynamic spreading of liquid film through weak hydrogen bonds, while long chains weaken the influence of Coulomb force, resulting in significantly better wetting performance of C₁₆MImBr than C₁₆MImCl. This research provides a theoretical basis for developing efficient coal dust suppressants by elucidating the structure–wetting performance relationship of ILs. It offers significant practical guidance for enhancing workplace wetting efficiency and mitigating dust pollution.