Study on the microstructural characteristics of coal and the mechanism of wettability of surfactant solutions at different pH levels

Abstract

The effect of acidity and alkalinity of mine water on the ability of surfactants to wet coal was studied with contact angle measurements to determine the optimal aqueous surfactant concentration, quantitative osmosis experiments to characterize the wettability of coal, and atomic force microscopy to analyze the surface microstructure of the coal. The contact angle measurements showed that the optimum concentrations of both selected surfactants, sodium dodecyl sulfate (SDS) and coconut oil diethanolamide (CDEA), were 1.0 wt%, with contact angles on the coal samples of 25.8° and 36.1°, respectively, which were lower than from other concentrations of the surfactants. The use of quantitative osmosis experiments revealed that the acid-base environment of the surfactant solutions impacted the effect of the surfactants to different extents depending on the type of surfactant. SDS at pH = 9 had the best wetting performance and its osmosis height was 17.2 × greater than the treatment with water at pH = 7. Analyses with atomic force microscopy determined the effect of the wetting mechanism of the surfactant in different acid-base environments on the microstructural characteristics of the coal. The experiments showed that the surface morphology and pore structure of the coal changed after surfactant treatment in different acid-base environments, with more pore structures appearing on the surface of the coal samples. Both acidic and alkaline conditions could enhance the surface roughness of coal. Coal treated with SDS at a pH of 9 had the highest surface porosity of 13.8%. These studies clarify the microscopic wetting mechanism of the surfactants SDS and CDEA at different pH levels of mine water in terms of the microscopic surface morphology and pore structure of the coal, and provide an experimental basis for improving the wetting of coal and the water injection process of a coal seam.