Wettability of the coal–water–Ar interface under coupled temperature and pressure conditions

Abstract

Coal wettability plays a critical role in a wide range of engineering applications, including mine disaster prevention, coalbed methane recovery, mineral flotation, and CO₂ geological sequestration. However, the mechanisms by which temperature and pressure affect wettability at the coal–water–gas interface remain inadequately understood. In this study, we systematically investigated the variations in surface tension of aqueous solutions and coal–water contact angles under an inert argon Ar atmosphere using a dedicated wettability testing system, in combination with molecular dynamics (MD) simulations to uncover the underlying microscopic mechanisms. Experimental results revealed that increasing temperature and Ar pressure led to a gradual decline in both water surface tension and coal–water contact angle. Despite this trend, the overall changes were relatively modest, indicating that coal wettability remains largely stable under such conditions. MD simulations further demonstrated that temperature and Ar pressure exert only minor influences on simulated contact angles, coal–water interaction energies, hydrogen bond numbers, diffusion coefficients of water molecules, and their relative concentration distributions. At the molecular level, elevated temperatures were found to promote water spreading on the coal surface, whereas increased Ar pressure slightly inhibited this behavior. Moreover, the contrasting impacts of adsorptive versus non-adsorptive gases on coal surface wettability were explored. Collectively, these findings provide critical insights into the macro- and microscopic wetting behavior at coal–water interfaces under complex thermodynamic conditions, offering theoretical guidance for the efficient and environmentally sustainable utilization of coal resources.