2.3.4. Measurements of the adsorption layer thickness formed by dispersants on coal surface

The X-ray photoelectron spectrum (XPS) of the coal samples were obtained by using an AXIS SUPRA spectrometer (Kratos, UK) with a monochromatic Al Kα source of 120 W. The binding energy was corrected based on the C1s peak at 284.6 eV.

Before the XPS measurements, blended coal sample was mixed with dispersant solution and vibrated at 25°C for 5 h. Then the suspension was filtered to remove water and dispersant that had not been adsorbed. After that, the coal sample was dried at 105°C to constant weight and then attached to the sample plate for XPS analysis. Coal is known to contain Si element, while the dispersant has no Si; thus the Si 2p can be used as the character element to estimate the thickness of the adsorption layer formed by the dispersant. The photoelectron intensity will decay after travelling through the adsorption layer. On the other words, the thickness of adsorption layer is negatively related to the photoelectron intensity. On the basis of the method from previous paper [24], the adsorption layer thickness of dispersant that formed on the coal surface was calculated by using the equations (2.2)–(2.4) as follows:

where Id is the intensity of the photoelectron transmitted through the adsorption layer, I0 is the incident photoelectron intensity, d is the thickness of adsorption layer (nm), λ(Ek) is the average depth (nm) at which the light electron escaped, Ek is the light electron kinetic energy and Eb is the atomic binding energy of Si. The value of I0 and Id can be obtained according to the area of the Si 2p photoelectron in the XPS spectrum.

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