Study on the Compounding Optimization of Surfactants and Synergistic Effects on the Wettability of Anthracite Coal

Abstract

To enhance the wettability of surfactants on anthracite coal and to investigate their wettability mechanisms, a single-factor experiment was conducted using Zhulinshan smokeless coal as a model. By employing contact angle and sedimentation experiments, the optimal formulation method and concentration were established from four surfactants, which were mixed in pairs in equal proportions. Integrating the contact angle and sedimentation experiments of both single and compound surfactants with coal samples revealed that the compound solution of the anionic surfactant SDBS and the nonionic surfactant X-100 exhibited the most effective wetting performance on coal samples from Zhulinshan. The optimal compounding solution was identified as 0.05% SDBS and 0.05% X-100, yielding a contact angle of 16.910° and a sedimentation time of merely 5.79 s. A three-phase system consisting of water, surfactant, and coal was constructed by using the Build Layer tool. Subsequently, molecular dynamics simulations were performed with the Forcite module of Materials Studio software, where the interaction energy, relative concentration distribution, and diffusion coefficients of the water/surfactant/coal system were calculated and analyzed at the molecular level. The findings from contact angle experiments, sedimentation experiments, and optimal compounding schemes derived from molecular simulations regarding the impact of surfactant compounding on the wettability of coal samples were consistent.

Introduction

Coal mine is an important foundation for the national economy and social development, and in the process of coal mining, coal mine dust is the most serious occupational hazard and may also cause explosions, resulting in major casualties.¹⁻³ By the end of 2021, a total of 15,407 cases of various types of occupational diseases were reported, including 11,809 cases of occupational pneumoconiosis. The situation of occupational disease prevention and control in China is still severe;^4,5 therefore, how to efficiently reduce dust has been an urgent problem for Chinese coal mining enterprises.⁶⁻⁸ Currently, to prevent and control coal mine dust, spray dust reduction technology is widely utilized as the primary method of wet dust suppression, thanks to its advantages of simple equipment design, ease of operation, and high dust reduction efficiency.^9,10 Surfactant is an amphiphilic substance containing hydrophobic and hydrophilic parts; due to its special molecular structure, it can reduce the surface tension of the solution to a certain extent and change the hydrophobicity of the coal dust surface so as to improve the improvement of the wettability of the coal dust. In recent years, scholars at home and abroad have done a lot of research on the addition of surfactants to improve the wettability of coal dust and have made some progress.^11,12 Yan et al.¹³ added two surfactants, emulsifier OP-10 and penetrant JFC-S, to NaAlg-g-poly to improve the dust control efficiency of the dust suppressant. Jin et al.^14,15 synthesized highly efficient film-forming dust suppressants using graft copolymerization and other methods. Currently, parameters such as surface tension, static contact angle, settling time, capillary rise height, and permeability are used to evaluate the wettability of dust suppressants. When surface tension is used to evaluate the wetting ability of a dust suppressant, the smaller value indicates the stronger ability of the droplet to capture dust and vice versa¹⁶ Meng and co-workers^17,18 studied the wettability of four anionic surfactants by surface tension and contact angle experiments and found that SDBS had the lowest surface tension and superior wettability. Kilau and Pahlman¹⁹ investigated the wetting mechanism of anionic surfactants. It was found that the addition of sodium and potassium salts to the anionic surfactants increased the wetting substantially. Crawford and Mainwaring²⁰ chose three surfactants, SDS, CTAB, and G12A8, to modify three different coalification levels of coal to study the change in the wettability of coal dust after adsorption of surfactants. It was found that the surfactants adsorbed on the surface of low-rank coal did not improve the wettability of the coal dust significantly, but with the increase of the coal rank, the wettability changed significantly.

Molecular dynamics is an effective method to study microscopic phenomena, and the use of molecular dynamics to study the adsorption process of interfacial surfactants and the wettability of coal dust surfaces has been widely used^21,22 Chen²³ selected anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) using molecular dynamics simulation to investigate the microscopic mechanism of the anionic surfactants in coal mine dust removal; Han²⁴ used molecular dynamics simulation to investigate the wetting properties and wetting mechanism of lignite by the nonionic surfactant fatty alcohol polyoxyethylene ether (AEO3); Liu²⁵ investigated the effect of the anionic surfactant sodium aliphatic alcohol polyoxyethylene ether carboxylate (AEC) on the wettability of coal surfaces. Guo²⁶ investigated the mechanism of wetting modification of lignite surfaces by two nonionic surfactants with different hydrophilicity using molecular dynamics simulation. Zhang et al.,²⁷ using density functional theory and molecular dynamics simulation methods, constructed models of Wiser bituminous coal and examined the impact of different surfactants. In previous studies, scholars usually use molecular dynamics to investigate the adsorption process of a single surfactant on the surface of coal dust and its effect on the wettability of coal dust; the synergistic effect of surfactant combinations on the wettability of coal dust, especially the experimental study combining the macroscopic and microscopic levels, has been studied relatively little. Nie²⁸ and Zhao²⁹ compared the wettability and adsorption of single and composite surfactants from both macroscopic and microscopic perspectives to further explore the effects of key factors such as concentration, molecular weight, and ionic type of surfactants on coal modification. The combination of experiment and simulation can not only reduce the blindness of the experiment but also the simulation results can supplement the experimental results, explain the internal law from the molecular level, and study the mechanism of coal dust wetting from the microscopic level, which will be more conducive to the development of chemical dust suppression technology.

In summary, the wettability of coal dust is a pivotal factor affecting the efficacy of underground spray dust suppression with notable variations observed among different coal types. This study concentrated on Zhulinshan anthracite, systematically evaluating the influence of various surfactants on the wettability of bituminous coal through both contact angle and sedimentation experiments. Optimized monomer surfactants were compounded to identify the most effective formulation and surfactant concentrations tailored specifically for Zhulinshan anthracite. Both quantum chemical and molecular dynamics simulations were utilized to investigate the microscopic mechanisms underlying the wettability enhancement induced by these compounded surfactants. By constructing a water/composite surfactant/coal system, simulations were carried out to examine the interactions between the composite surfactant and anthracite coal. Key parameters, including interaction energy, relative concentration distribution, and diffusion coefficients, were meticulously calculated and analyzed. Comparisons were drawn with single surfactant systems to evaluate the effectiveness of surfactant combinations in enhancing the wettability of the Zhulinshan coal samples. The findings were corroborated by correlating simulation results with experimental data, thereby providing valuable insights for future investigations of surfactant-induced wettability in anthracite coal.

Experiments and Simulations

Preparation and Analysis of Experimental Coal Samples

The experimental coal samples were sourced from the Zhulinshan coal mine located in Yangcheng County, Jincheng City, Shanxi Province, China, which is known for its high-quality anthracite. The anthracite coal from this mine was first crushed using a coal mill, and the resulting coal dust was collected by passing the material through a 200-mesh sieve. For sample preparation, 200 mg of coal dust was placed into a tablet mold, which was then set on the table of a tablet press machine. A pressure of 20 MPa was applied for 3 min, producing a round coal flake with a diameter of 13 mm and a thickness of approximately 2 mm, featuring a smooth surface, as shown in Figure 1. The coal samples were analyzed according to the GB/T 30732-2014 standards for instrumental methods used in proximate analysis and ultimate analysis of coal. Afterward, ¹³C NMR, XPS, and FTIR analyses were conducted, and the resulting ¹³C NMR, XPS, and FTIR spectra were peak-fitted to obtain the structural parameters of the coal samples.

Figure 1.

Coal sheet pressing diagram. (a) Tablet press. (b) Tablet mold. (c) Circular coal flakes.

Preparation of Surfactant Compounding Solutions

In this study, the surfactants applied to the working face of the mine must meet several critical criteria: they should be nontoxic to humans, nonflammable, noncorrosive to metals, exhibit no deposition or salting-out phenomena in water, and offer low-cost and convenient transport. Based on these principles, four representative monomer surfactants have been selected: cationic surfactant dodecyltrimethylammonium bromide (DTAB), anionic surfactant sodium dodecylbenzenesulfonate (SDBS), amphoteric surfactant dodecyldimethylammonium oxide (OA-12), and nonionic surfactant emulsifier TX-100 (X-100). The four surfactants were prepared in solutions with mass concentrations of 0.01, 0.05, 0.07, 0.1, 0.3, and 0.5 wt %. They were mixed two by two in equal proportions to get a composite solution. The parameters for each surfactant are summarized in Table 1.

Table 1. Types of Surfactants.

species	abbreviation	chemical name of surfactants	chemical formula
anionic	SDBS	sodium dodecylbenzenesulfonate	C18H29NaO3S
amphoteric ion	OA-12	dodecyldimethylammonium oxide	C14H31NO
nonionic	X-100	TX-100	C34H62O11
cationic	DTAB	dodecyltrimethylammonium bromide	C15H34N·Br

Contact Angle Measurement

The experiments were conducted at a controlled temperature of 25 °C using a JY-JC2000C contact angle goniometer. The contact angles of four individual surfactant monomers and their binary solutions at different concentrations were measured by employing the goniometric method. For each experimental condition, three measurement repetitions were recorded, and the average value was calculated. Contact angle tests were performed on four surfactant monomers and their complex solutions at different concentrations.

Sedimentation Experiments

A platform was developed for the experimental determination of coal dust settling, allowing for the measurement of settling times of coal samples in four individual surfactant monomers and their binary solutions at different concentrations. In this process, coal dust samples were directed onto the surface of rapid characterization filter paper using a glass funnel, allowing them to naturally form a cone shape on the paper. The filter paper, carrying the coal dust, was then gradually lowered by adjusting the positioning ring and rotating the hook arm. When the filter paper made contact with the solution, a stopwatch was started, and timing continued until the entire coal sample had settled below the liquid surface, at which point the timing was stopped. Three measurements were recorded for each experimental setup, and the average settling time was calculated.

Molecular Dynamics Simulation

Based on the analysis of anthracite from the Zhulinshan coal mine, a planar model of the macromolecular structure was constructed using ChemDraw software. The chemical shifts of carbon atoms in the model were predicted by using the ACD/CNMR Predictor software. The predicted ¹³C NMR profiles were continuously adjusted by comparing them with the experimental ¹³C NMR data, refining the mode of connection and arrangement of functional groups within the molecular structure until a satisfactory match was achieved. The correction workflow for model construction is illustrated in Figure 2.

Figure 2.

Model construction revision flowchart.

The team continued to adjust the element positions and optimize the structure of the established model and finally obtained the planar structure model of anthracite molecules, as shown in Figure 3. To achieve the most accurate three-dimensional structural model, further geometry optimization through molecular mechanics is necessary. Geometry optimization was carried out using geometry optimization under the Forcite module, with the mass accuracy set to Medium, simulation step number was set to 5000, force field optimized COMPASSII, with selected charged charge assignment selected Force field assigned, to obtain the lowest energy configuration (see Figure 4).

Figure 3.

Plane model of the macromolecular structure of Zhulinshan anthracite.

Figure 4.

Lowest energy model of Zhulinshan anthracite.

Twenty optimized anthracite coal molecules were randomly stacked in a 40 Å × 40 Å × 40 Å cubic box by using the Monte Carlo principle in the Amorphous Cell module of Materials Studio software, and three-dimensional periodic boundary conditions were added. The density of the structural model was set to 1.6 g/cm³. After geometry optimization, the samples were annealed using high-temperature relaxation, followed by NVT system synthesis and the Nosé temperature-controlled method. The initial temperature was set to 298 K, the maximum temperature was set to 600 K, and the heating rate was 50 K/time. As the coal molecules are randomly stacked in the cubic box, there are many unreasonable contacts between them, and it is necessary to use the Forcite module to carry out an annealing simulation (Anneal) to reduce the energy of the system and eliminate the unreasonable contacts to obtain the aggregation state structure model of anthracite molecules, as shown in Figure 5. Subsequently, the Sketch tool in Materials Studio software was used to construct the macromolecular structure models of the monomer surfactants, and the obtained macromolecular models of the four surfactants are shown in Figure 6. After the model construction was completed, 5000 steps of geometry optimization were performed on the surfactant models using the Geometry optimization task of the Forcite module. The Amorphous Cell module was used to construct cubic boxes containing 10 DTAB and 6 SDBS surfactant molecules, 10 DTAB and 6 OA-12 surfactant molecules, 10 DTAB and 3 X-100 surfactant molecules, 6 SDBS and 6 OA-12 surfactant molecules, and 6 SDBS and 6 OA-12 surfactant molecules, respectively, according to the Monte Carlo principle: 10 OA-12 surfactant molecules; a cubic box containing 6 SDBS and 3 X-100 surfactant molecules; and a cubic box containing 10 OA-12 and 3 X-100 surfactant molecules, as shown in Figure 7.

Figure 5.

Molecular aggregate structure model of anthracite.

Figure 6.

Molecular structure of the surfactants: (a) DTAB, (b) SDBS, (c) OA-12, and (d) X-100.

Figure 7.

Different ionic surfactant compound box: (a) DTAB + SDBS, (b) DTAB + OA-12, (c) DTAB + X-100, (d) SDBS + OA-12, (e) SDBS + X-100, and (f) OA-12 + X-100.

Using the Build Layer tool, the coal box, surfactant box, and water box were combined in the order of coal, surfactant, and water, and finally, a vacuum layer of 50 Å was added above the water box to avoid the influence of the periodic boundary conditions, and the three-phase water/surfactant/coal system model of the single surfactant was constructed, as shown in Figure 8. The initial model was optimized using the Geometry optimization task of the Forcite module in the Materials Studio software, and molecular dynamics simulations were performed by the Dynamics task, and the structure of the system after reaching equilibrium is shown in Figure 9.

Figure 8.

Layer construction diagram of the water/surfactant/anthracite three-phase system.

Figure 9.

Equilibrium structure model of the water/mixed surfactant/coal three-phase system: (a) DTAB + SDBS, (b) DTAB + OA-12, (c) DTAB + X-100, (d) SDBS + OA-12, (e) SDBS + X-100, and (f) OA-12 + X-100.

Finally, molecular dynamics simulations were carried out by the Forcite module to calculate and analyze the interaction energy, relative concentration distribution, and diffusion coefficient of the water/complexed surfactant/coal system at the molecular level and to study and analyze the effect of surfactant compounding on the wettability of the coal samples from Zhulinshan.

Results and Discussion

Analysis of Experimental Results

The results of the proximate and ultimate analyses are shown in Table 2.

Table 2. Proximate Analysis and Ultimate Analysis of the Coal Sample^a.

sample	proximate analysis W/%				ultimate analysis Wdaf/%
	Mad	Ad	Vdaf	FCad	Cdaf	Odaf	Hdaf	Ndaf	Sdaf
ZLS	2.410	16.670	8.125	72.795	91.370	3.764	3.340	1.347	0.178

^aM_ad stands for moisture of air-dried coal; A_d stands for ash content of dry coal; V_daf stands for volatile matter content of dry anthracite; FC_ad stands for fixed carbon of air-dried coal; C_daf stands for elemental carbon content of dry anthracite; O_daf stands for elemental oxygen content of dry anthracite; H_daf stands for elemental hydrogen content of dry anthracite; N_daf stands for elemental nitrogen content of dry anthracite; and S_daf stands for elemental sulfur content of dry anthracite.

The results of the split-peak fitting of the ¹³C NMR spectra are presented in Table 3.

Table 3. Main Structural Parameters of Anthracite in the Zhulinshan Coal Mine^a.

sample	fa	fac	fa′	faN	faH	faB	fal	fal*	falH	falO
ZLS	84.20	3.18	81.02	26.01	55.01	26.01	15.80	3.04	5.88	6.88

^af_a, sp² hybridized carbon (f_a = f_a^c + f_a′); f_a^c, carboxy and carbonyl carbons; f_a′, total aromatic carbon (f_a′ = f_a^H+ f_a^N); f_a^H, protonated aromatic carbon; f_a^N, nonprotonated aromatic carbon (f_a^N = f_a^P + f_a^S + f_a^B); f_a^P, oxygenated aromatic carbon; f_a^S, side-branching aromatic carbon; f_a^B, bridging aromatic carbon; f_a^l, sp³ hybridized carbon (f_a^l = f_al* + f_al^H + f_al^O); f_al*, arylmethyl and aliphatic methyl groups; f_al^H, quaternary carbon, hypomethyl carbon; and f_al^O, oxygen-joined lipo carbon.

The results of peak fitting to the XPS profile are shown in Tables 4–6.

Table 4. Existing Forms and Relative Contents of Oxygen Elements in Zhulinshan Anthracite.

E/eV	oxygen form	content Wmol/%
533.4792	C–O	100

Table 6. Existing Forms and Relative Contents of Carbon Elements in Zhulinshan Anthracite.

E/eV	carbon	content Wmol/%
286.539	C–O	21.07
284.8	C–C/C–H	78.93

According to the data in Table 4, it can be seen that elemental oxygen exists mainly in the form of C–O. The existence of the oxygen element in coal is mostly in the form of a phenolic hydroxyl group.

According to the data in Table 5, nitrogen is present in the form of pyrrole and oxidized nitrogen.

Table 5. Existing Forms and Relative Contents of Nitrogen in Zhulinshan Anthracite.

E/eV	nitrogen	content Wmol/%
400.3949	pyrrole	82.77
403.8740	nitrogen oxides	17.23

Based on the data in Table 6, it can be seen that elemental carbon exists mainly in the form of C–O and C–C/C-H.

Based on the analysis presented in Figure 10a, four distinct absorption peaks are observed in the 700–900 cm^–1 region: specifically at 748.19, 793.40, 827.94, and 869.53 cm^–1. The peaks at 748.19, 793.40, and 827.94 cm^–1 are attributed to different types of aromatic ring substitutions: adjacent disubstitution, interdisubstitution, and pair disubstitution, respectively. Meanwhile, the peak at 869.53 cm^–1 corresponds to 1,2,4-substituted aromatic rings. Notably, the highest intensity peak at 748.19 cm^–1 indicates that adjacent substitution predominates among the various substitution forms in Zhulinshan anthracite. In Figure 10b, the ratio of phenols, alcohols, ethers, and lipids (C–O) to C–C groups is approximately 4:1. This suggests a significant abundance of oxygen-containing functional groups relative to the carbon–carbon linkages.

Figure 10.

Fitted plot of band peak division. (a) Fitting diagram of peak division at the 900–700 cm^–1 band. (b) Fitting diagram of peak division at the 1800–1000 cm^–1 band. (c) Fitting diagram of peak division at the 3000–2800 cm^–1 band. (d) Fitting diagram of peak division at the 3600–3000 cm^–1 band.

The analysis in Figure 10c reveals the proportion of aliphatic carbon groups: methyl, methylene, and hypomethyl groups are present in a ratio of 2:5:4. This indicates a diverse composition of aliphatic structures within the coal sample. Figure 10d illustrates that the anthracite from the Zhulinshan coal mine is primarily characterized by O–H or N–H stretching vibrations. This finding implies that hydroxyl groups in the coal’s molecular structure predominantly exist in the form of O–H or N–H.

Contact Angle Experiment

To optimize the formulation of the composite surfactants, contact angle tests were performed on compound solutions with varying compositions and concentrations. The contact angles corresponding to the optimal concentration ratios of the six composite solutions are presented in Figure 11 and Table 7.

Figure 11.

Contact angle between the coal sample tablet and the compound surfactant solution: (a) DTAB(0.10%) + SDBS(0.05%), (b) DTAB(0.10%) + OA-12(0.07%), (c) DTAB(0.10%) + X-100(0.05%), (d) SDBS(0.05%) + OA-12(0.07%), (e) SDBS(0.05%) + X-100(0.05%), and (f) OA-12(0.07%) + X-100(0.05%).

Table 7. Average Contact Angle between the Coal Sample Tablet and the Compound Surfactant Solution.

reagent name	contact angle /°
DTAB(0.10%) + SDBS(0.05%)	51.711
DTAB(0.10%) + OA-12(0.07%)	45.135
DTAB(0.10%) + X-100(0.05%)	79.917
SDBS(0.05%) + OA-12(0.07%)	32.092
SDBS(0.05%) + X-100(0.05%)	16.910
OA-12(0.07%) + X-100(0.05%)	34.681

It was observed that the contact angle between the compounded solution of anionic surfactant SDBS and nonionic surfactant X-100 and coal sample presses was the smallest, so the two compounds had the best effect on the wetting of anthracite in the Zhulinshan coal mine. The best compounding solutions were 0.05% X-100 and 0.05% SDBS.

Sedimentation Experiments

The settling times of coal samples in the six compounded surfactant solutions are shown in Table 8. As observed from Table 8, the settling time of coal dust in the solution of anionic surfactant SDBS and nonionic surfactant X-100 is only 5.79 s. It is shorter than the settling time of coal dust in other surfactant systems. The settling time of coal dust in the compound solution of cationic surfactant DTAB and nonionic surfactant X-100 is 517.84s, which is the longest settling time among all surfactant systems. It indicates that the compound solution of anionic surfactant SDBS and nonionic surfactant X-100 has the best effect in wetting anthracite coal from the Zhulinshan coal mine.

Table 8. Settling Time of Coal Dust in the Complex Surfactant Solution.

reagent name	sedimentation time/s
DTAB(0.10%) + SDBS(0.05%)	48.16
DTAB(0.10%) + OA-12(0.07%)	27.66
DTAB(0.10%) + X-100(0.05%)	517.84
OA-12(0.07%) + SDBS(0.05%)	12.79
SDBS(0.05%) + X-100(0.05%)	5.79
OA-12(0.07%) + X-100(0.05%)	18.37

Interaction Energy

Moncayo-Riascos³⁰ proposes that surfactant-induced differences in wettability can be explained by measuring the interaction energy of the system. The interaction energy is generally negative, indicating that the system can spontaneously undergo adsorption behavior, and the larger the absolute value of the interaction energy, the stronger the system interaction and the easier the adsorption of surfactant molecules with coal molecules. A positive interaction energy indicates that it is difficult for the system to interact. In order to study the difference in wettability improvement of different types of surfactants on anthracite coal from the Zhulinshan coal mine, the interaction energies in the water/coal system and water/surfactant/coal system were calculated as shown in eq 1.

1

In this equation, E_total is the total energy of the whole system, kcal·mol^–1; E_water is the energy of water molecules, kcal·mol^–1; E_surfactant is the energy of the surfactant, kcal·mol^–1; and E_coal is the energy of coal in the system, kcal·mol^–1.

Here, the energies of water molecules, surfactant, and coal were obtained by removing the remaining two components in the system and calculating them separately.

The interaction energy of the water/complex surfactant/coal three-phase system is shown in Table 9. From Table 9, it can be seen that van der Waals forces still dominate in the compounded surfactant system, as in the monomer surfactant system. The intermolecular interaction energies of the six compounding systems are all negative, indicating that the adsorption behaviors of the six complexed surfactants and coal samples can occur spontaneously. However, the interaction energy between the cationic surfactant DTAB and the nonionic surfactant X-100 was −784.38 kcal·mol^–1, which indicated that although the two surfactants could adsorb with the coal samples, the adsorption effect was weak, and it was not easy to be adsorbed on the surface of the coal samples, which was not conducive to the improvement of the wettability of the anthracite coal from Jhulin Mountain Coal.

Table 9. Interaction Energy, van der Waals Energy, and Electrostatic Energy in the Water/Mixed Surfactant/Coal System.

system	Evan/(kcal·mol–1)	Eele/(kcal·mol–1)	Eint/(kcal·mol–1)
water/DTAB + SDBS/anthracite	–1355.438	–796.05	–2151.49
water/DTAB + OA-12/anthracite	–1688.177	–937.292	–2625.47
water/DTAB + X-100/anthracite	–409.847	–274.553	–784.38
water/OA-12 + SDBS/anthracite	–2707.469	–854.990	–3562.46
water/SDBS + X-100/anthracite	–3465.870	–1136.879	–4602.75
water/OA-12 + X-100/anthracite	–2226.451	–758.068	–2984.52

The interaction energy between the anionic surfactant SDBS and the nonionic surfactant X-100 is the largest in the system after compounding, which is −4602.38 kcal·mol^–1. This is due to the fact that after combining the anionic surfactant and the nonionic surfactant, the electrical repulsion of the polar groups of the anionic surfactant SDBS is weakened, and there is no mutual electrical repulsion between the anion and nonion, so the synergistic effect between the two species is stronger. The synergistic effect between the two is stronger. Therefore, the strongest adsorption effect was observed on the surface of anthracite from the Zhulinshan Mine. Second, the amphoteric surfactant OA-12 and the anionic surfactant SDBS were compounded, and the interaction energy in the system was −3562.46 kcal·mol^–1, which was due to the strong interaction between positive and negative charges in the amphoteric surfactant and the electrostatic attraction between positive and negative charges in the polar groups of the two surfactants. The interaction energies in the system of the cationic surfactant DTAB complexed with other ionic surfactants are all small. According to the analysis of the interaction energy of the surfactant system, the interaction energy in the system after surfactant compounding is not necessarily stronger than that of the monomer surfactant system, which indicates that the adsorption strength of coal samples after surfactant compounding is not necessarily stronger than that of monomer surfactants. The interaction energy of the nonionic surfactant X-100 and anionic surfactant SDBS is the largest, and the adsorption effect on the surface of anthracite in the Zhulinshan coal mine is the strongest.

Relative Concentration Distribution

Based on the molecular dynamics simulation results of the water/complexed surfactant/coal system, the relative concentration analysis was performed on the equilibrium trajectories of the system to obtain the relative concentration distribution of each component along the Z-axis and analyze it in comparison with that of the monomer surfactant system to further study the effect of surfactant compounding on the wettability of Zhulinshan anthracite coal. The relative concentration distribution of each component along the Z-axis of the compounded surfactant system is shown in Figure 12, in which the peaks represent the positions where the molecules are more concentrated.

Figure 12.

Relative concentration distribution of surfactant complexes along the Z-axis. (a) DTAB + SDBS, (b) DTAB + OA-12, (c) DTAB + X-100, (d) OA-12 + SDBS, (e) SDBS + X-100, and (f) OA-12 + X-100.

As can be seen from Figure 12a–f, the relative concentration distributions of coal samples in the water/complexed surfactant/coal system are also not affected by the surrounding environment. The spatial distribution ranges of the complex surfactant system were different, which were 5.8–7.8, 5.2–7.7, 5.5–8.9, 5.7–8.1, 6.2–8.7, and 5.8–8.2 nm. The spatial distribution of surfactants in the complex surfactant system was slightly smaller, and the peaks were sharper than those in the monomer surfactant system, which was due to the fact that the different surfactant molecules were intertwined with each other and arranged more closely. The reaction between different surfactants may play a synergistic or antagonistic role in improving the wettability of coal samples.

It can also be seen from Figure 12a–f that the water molecules in DTAB + SDBS, DTAB + OA-12, DTAB + X-100, OA-12 + SDBS, SDBS + X-100, and OA-12 + X-100 complexed surfactant systems also started to appear after 6.0 nm. The position of the peak intensity of the emergence of the water molecules was located at 7.8, 7.2, 9.0, 8.0, 9.3, and 8.2 nm, respectively, compared with the system without surfactant and the individual monomer surfactant system for which it moved backward, but the concentration of water molecule peak intensity did not change much. This indicates that adding complex surfactants creates an adsorption layer on the surface of the coal samples, which separates the water molecules from the anthracite coal, similar to the action of monomer surfactants. However, the difference from the monomer surfactant system lies in the sharper peaks observed with the complex surfactant, indicating a more compact adsorption layer. Therefore, the effect of the complex surfactant on the wettability of anthracite depends on both the adsorption interactions between the surfactant and the coal surface and the diffusion coefficient of water molecules in the system.

Diffusion Coefficient

By conducting mean square displacement (MSD) analysis on the molecular dynamics simulation trajectory of the water/complex surfactant/coal three-phase system, the influence of the surfactant complex on the wettability of anthracite coal in the Zhulinshan coal mine was studied. The MSD curve of water molecules in the three-phase system of water/complex surfactant/coal is shown in Figure 13.

Figure 13.

Root-mean-square displacement of water molecules in the complex surfactant system.

The diffusion coefficients were calculated by linearly fitting the MSD curves of the complex surfactant systems, and the corresponding linear fitting equations for different systems are shown in eqs 2–7.

2

3

4

5

6

7

Y₁, Y₂, Y₃, Y₄, Y₅, and Y₆ denote the linear fitting equations of MSD curves for the water/DTAB + SDBS/coal system, water/DTAB + OA-12/coal system, water/DTAB + X-100/coal system, water/OA-12 + SDBS/coal system, water/SDBS + X-100/coal system, and water/OA-12 + X-100/coal system, respectively. The correlation coefficients (R²) of each equation were 0.99, and the fitting effect was good. The diffusion coefficient (D) of water molecules was calculated by using the diffusion coefficient formula, as shown in Table 10.

Table 10. Diffusion Coefficients of Water Molecules in Different Complex Surfactant Systems.

system	D/(10–5 cm2·s–1)
water/DTAB + SDBS/anthracite	5.89
water/DTAB + OA-12/anthracite	6.51
water/DTAB + X-100/anthracite	4.92
water/OA-12 + SDBS/anthracite	7.27
water/SDBS + X-100/anthracite	7.62
water/OA-12 + X-100/anthracite	6.76

According to Table 10, compared to the diffusion coefficient of water molecules in the monomer surfactant system, the diffusion coefficient of water molecules in the complexed surfactant system showed different degrees of growth and decline. This indicates that there are synergistic effects that make the movement of water molecules in the system become more active and antagonistic effects that make the movement of water molecules in the system become mild after the surfactants are compounded. Among them, the diffusion coefficient of water molecules in the system after the compounding of anionic surfactant SDBS and nonionic surfactant X-100 was the largest, 7.62 × 10^–5 cm²·s^–1, which was the easiest to enhance the attraction of coal samples to water molecules and make the movement state of water molecules become the most active and easy to wet coal samples. The diffusion coefficient of water molecules in the system after the compounding of cationic surfactant DTAB and nonionic surfactant X-100 was the smallest, 4.92 × 10^–5 cm²·s^–1, which had the worst effect on the enhancement of the attraction of water molecules to the coal samples.

The diffusion coefficient of water molecules in the system of anionic surfactant SDBS compounded with nonionic surfactant X-100 was larger than that of water molecules in other compounded systems and monomer surfactant systems. Therefore, it can be seen that the combination of the anionic surfactant SDBS and the nonionic surfactant X-100 has the best effect on improving the wetting of anthracite coal in the Zhulinshan Mine.

Conclusions

• 1.The contact angle experiment revealed that the contact angle between the compound solution of the anionic surfactant SDBS(0.05%) and the nonionic surfactant X-100(0.05%) and the coal sample tablet was 16.910°, which is lower than that of other surfactant solutions tested. Additionally, the sedimentation experiment indicated that the shortest sedimentation time for coal samples in the compound solution of SDBS(0.05%) and X-100(0.05%) was 5.79 s. These results suggest that the compound solution of SDBS(0.05%) and X-100(0.05%) is most effective in enhancing the wettability of anthracite from the Zhulinshan coal mine.
• 2.The joint analysis of the interaction energy of the system and the distribution of contributions along the Z-axis based on molecular dynamics simulations showed that van der Waals interaction energy remains predominant in the surfactant blend system. The maximum interaction energy observed in the blend of the anionic surfactant SDBS(0.05%) and the nonionic surfactant X-100(0.05%) was −4602.75 kcal·mol^–1, indicating the strongest adsorption effect on the coal sample.
• 3.The analysis of the diffusion coefficients of water molecules in the water/surfactant blend/coal three-phase system revealed that the combination of the nonionic surfactant X-100(0.05%) and the anionic surfactant SDBS(0.05%) yielded the highest diffusion coefficient of water molecules, measuring at 7.62 × 10^–5 cm²·s^–1. This suggests that water molecules are most dynamic in this system, resulting in the most effective wetting of anthracite from the Zhulinshan coal mine.

The authors declare no competing financial interest.