Effect and Mechanistic Study of Coal Wettability Regulation by Cationic and Zwitterionic Surfactants: Insights from...

Surfactants can significantly reduce the surface tension of water and hence effectively alter the water wettability of coal. Herein, macroscopic experiments and microscopic simulations were used to examine the wetting interactions between cationic cetyltrimethylammonium bromide (CTAB) and zwitterionic 3-sulfonatopropylhexadecyl betaine (SPHB) surfactants and a coal sample. Via low-field nuclear magnetic resonance (LF-NMR), the regulation effects were evaluated by relaxation time (T₂), magnitude area (M_a), and geometric mean distribution (T_2g). The results indicate that both CTAB and SPHB show obvious wetting sites on coal macropores and mesopores but almost no effect on micropores. Meanwhile, the kinetic response of CTAB (|k_1(CTAB)| = 8.33) was shown to be faster than that of SPHB (|k_1(SPHB)| = 2.44). Using insight from molecular dynamics (MD) simulations, SPHB was more stable than CTAB with interaction energies of E_CTAB...coal > E_SPHB...coal and ΔE_CTAB...coal < ΔE_SPHB...coal. Meanwhile, the results reveal changes in configuration and site distinctions for interactions of CTAB and SPHB with coal dust. Consequently, the model of wettability is inferred to be "single-molecule bridging water" and "single-hydrophobic force interaction" for CTAB but "semicelle bridging water" and "multiple-hydrophobic force interaction" for SPHB. The interactions between surfactants and coal dust were furthermore characterized and verified by SEM, ζ potential, XPS, and FTIR measurements. This work provides both macroscopic and microscopic investigations of coal wettability regulation by surfactants, which is of great significance during coal exploitation.