Effects of surface tension and contact angle on pore structure development of

Surface tension and contact angle

Surface tension and contact angle of multicomponent composite solution containing different proportion of surfactants.

The surface tension and contact angle of multi-component composite solutions containing different proportions of SDBS and AEO-9 were measured. As shown in Fig. 2, under the condition of constant total concentration of surfactants, different proportions of SDBS and AEO-9 exhibited different surface properties. When only one surfactant was present, compared to AEO-9 (0/5), SDBS (5/0) reduced the surface tension of the solution. When both surfactants were mixed, the surface tension of the solution decreased with the increasing proportion of SDBS, indicating that SDBS had a more pronounced advantage in reducing the surface tension of the solution. Interestingly, when SDBS/AEO-9 was 4/1, the surface tension of the solution reached its minimum, which was 1.13% lower than SDBS (5/0) and 6.08% lower than AEO-9. This is because the presence of mixed micelles of surfactants in the solution weakened the electrostatic repulsion between anionic surfactants, and AEO-9 contained ether bonds that could form hydrogen bonds with water³⁸, thereby exhibiting a synergistic effect between the two. This indicates that mixing a small amount of nonionic surfactant in anionic surfactants helps to adjust the surface properties of the solution and further reduces the surface tension of the solution. This synergistic effect is still reflected in the contact angle between the solution and coal. Although the trend of the contact angle varies with the different proportions of the two surfactants, it is still evident that SDBS wets the coal more than AEO-9. When SDBS/AEO-9 was 4/1, the contact angle between the solution and coal reached its minimum, which was 0.27% lower than SDBS (5/0) and 7.43% lower than AEO-9, indicating that the combination of the two improved the wetting of the solution to coal. The combination of anionic and nonionic surfactants further reduced the surface tension and contact angle, which may have a positive effect on the chemical solution erosion of coal samples. Therefore, in the subsequent work, we chose 4/1 (SDBS/AEO-9) as the combination ratio for the follow-up experiments.

Images of contact angle between solution and coal.

Surface tension and contact angle of multicomponent composite solution containing different concentrations of surfactants.

Figure 3 shows the contact angle images between multi-component composite solutions containing different concentrations (0.05 wt% ~ 0.5 wt%) of surfactants (SDBS/AEO-9: 4/1) and coal, while Fig. 4 depicts the changes in surface tension and contact angle with the concentration of surfactants in the composite solution. Both surface tension and contact angle decrease with the concentration of surfactants, reaching the critical micelle concentration (CMC) at a concentration of 0.2 wt%, where the surface tension begins to stabilize, with a value of 31.57 mN/m. Preliminary experiments showed that the surface tension of pure water was 72.80 mN/m, while the surface tension of the solution without added surfactants was 40.99 mN/m. The addition of organic solvent and citric acid had already significantly reduced the surface tension of the solution, and the addition of surfactants further enhanced this reduction. Compared to the solution without added surfactants, the surface tension of the composite solution reaching CMC decreased by 22.98%, and this value remained relatively stable as the concentration continued to increase. The change in contact angle followed a similar pattern: before reaching CMC, the contact angle decreased rapidly with concentration, and after reaching CMC, the rate of decrease slowed significantly. The minimum value of the contact angle was 28.3°, which was 43.96% lower than the solution without added surfactants according to the preliminary test results.

NMR test

T₂ spectra of coal samples before and after treatment.

Prior to solution treatment, each coal sample was saturated with water and tested for its T₂ spectrum. To minimize the impact of differences between coal samples on the experimental results, coal samples with significantly different porosity were excluded. The selected coal samples had porosity ranging from 8.3599 to 11.3868%, with an average porosity of 9.7394%. The porosity of each coal sample is listed in Table 3. Figure 5 shows the T₂ spectra of coal samples before and after treatment with various solutions. The coal samples all exhibited a typical three-peak distribution, representing micropores, mesopores, and macropores, respectively. It can be observed that micropores and mesopores accounted for…