The FTIR spectrum was measured by using a VECTOR-22 FT-IR instrument (Bruker, Germany). Approximately 5 mg copolymer sample was mixed with 500 mg KBr in a mortar, and then compressed into a disc. The scanning was performed from 4000 to 500 cm−1.

The 1H NMR spectrum was obtained on an AVANCE NMR spectrometer (400 MHz, Bruker, USA). D2O was used as the solvent and the measurement was conducted at 295.5 K.

The adsorption capacity of dispersant was analysed by using a Cary-60 ultraviolet spectrophotometer (Agilent, USA). A series of dispersant solutions were prepared and measured to build the calibration curve. Then, 20 ml dispersant solutions of different concentration were mixed with 0.5 g blend coal sample and vibrated in a water bath at 25°C for 5 h. The suspension was centrifuged at 8000 r.p.m. for 10 min, and the supernatant was filtered to remove coal particles. The concentration of residue dispersant in the supernatant was measured by ultraviolet spectrophotometer. Coal-water suspensions were used as the blank sample. The adsorption amount of dispersant was calculated by following equation (2.1).

where Г (mg g−1) is adsorbed dispersant on unit mass coal sample, Co (mg ml−1) is original mass concentration of dispersant solution, Cr (mg ml−1) is residue mass concentration of dispersant solution after adsorption, Cblank (mg ml−1) is the mass concentration of blank sample, V (ml) is the volume of dispersant solution, m (g) is the mass weight of coal sample.

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.

The zeta potentials of the coal particles were measured by using a NANO-ZS-90 dynamic light scattering instrument (Malvern Instruments Corp., UK). The coal suspension consists of 0.2 g blended coal and 50 ml dispersant solution (0–0.6 mg ml−1), which was vibrated in water bath at 25°C for 5 h and then centrifuged at 8000 r.p.m. for 10 min. The supernatant was isolated for zeta potential analysis. Each sample was measured three times and the mean value was used.

Before the contact angle measurements, 2.0 g blended coal was compressed to a disc with a diameter of 15 mm and a thickness of 2 mm at 60 MPa for 10 min. Then, distilled water and dispersant solution were dropped onto the surface of the discs respectively. The contact angles were measured by a DSA100 dynamic contact angle measuring instrument (Kruss, Germany).

The apparent viscosity of CWS samples was tested on a R/S-SST Plus rheometer (Brookfield Company, US). The measurements were carried out at 25°C. The average viscosity at a shear rate of 100 s−1 was counted as the apparent viscosity of CWS.

The static stability of CWS was analysed by using a TurbiscanLab stability analyser (Formulation Company, France). Before measurements, CWS samples were stirred for 10 min and then filled into the sample bottle to a certain height. Afterwards, the bottle was transferred to the analyser and scanned every 10 min over a period of 5 h. The coefficient Turbiscan stability index (TSI) was used to evaluate the stability of CWS [25,26]. A higher TSI value means poorer static stability.

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