Research on Mechanism of Surfactant Improving Wettability of Coking Coal Based on Molecular Dynamics

Abstract

Coal dust is a major safety hazard in the process of coal mining and is of great importance to ensure production safety and maintain the health of operators. In order to understand the microscopic mechanism during coal seam water injection and reveal the mechanism of surfactants in improving the wettability of coal dust, coking coal was selected as the research object. Three surfactants, SDBS, AEO-9, and CAB-35, were chosen for molecular dynamics simulation research on the wetting and adsorption properties of water/coal/surfactants. The results show that surfactant molecules can cover the hydrophobic groups on the surface of coking coal, forming a hydrophilic adsorption layer, changing the coal surface from hydrophobic to hydrophilic, and enhancing the wettability. After adding surfactants, the thickness of the adsorption layer in the z-axis direction increases, expanding the contact area between coking coal and water molecules, thereby improving the wettability. When surfactants tightly cover the surface of coking coal, their binding strength increases, forming a more stable hydrophilic layer and further improving the wettability. At the same time, surfactants promote the diffusion of water molecules and enhance the interaction between hydrophobic alkyl chains and water molecules, further enhancing the wetting effect.

1. Introduction

Coal resources play an irreplaceable role in the Chinese economy, supporting social production and driving the development of the national economy [1,2,3]. However, safety hazards in coal mining, especially coal dust safety issues, involving occupational hazards [4,5,6], explosion hazards [7,8,9], environmental pollution [10,11,12], and mechanical equipment damage [13,14,15], have become a major challenge that urgently needs to be addressed. The “Healthy China” initiative emphasizes public safety systems and the development of occupational health [16,17]. In the coal mining industry, safety production is particularly crucial. Therefore, coal dust generation should be controlled at the source. Statistics as of 2022 show that the annual number of new patients with occupational pneumoconiosis in China has decreased to 7577, but the number of deaths caused by pneumoconiosis remains at up to 9613 each year. Therefore, in the current strategy for coal dust prevention and control, more emphasis must be placed on managing the dust concentration in coal mining faces and controlling the incidence of pneumoconiosis.

The wettability of coal is an important factor in improving the efficiency of dust reduction. Coking coal itself is highly hydrophobic, so in the process of coal mining, coal dust is a serious concern that seriously affects the operation of the staff and jeopardizes their health. Chemical modification can effectively alter the surface properties of coal, enhance its performance, and provide a new approach for the efficient utilization of coal resources. Hence, many scholars have studied the wettability of coal dust. With the increasing degree of metamorphism of the coal, its wettability shows a changing trend of increasing, then decreasing, and then increasing again. Among bituminous coals, long-flame coals show the strongest wettability, while coking coals show relatively stronger hydrophobicity, i.e., their wettability is weaker [18]. Referring to previous studies, the addition of surfactants to the dust removal system can substantially improve the dust removal efficiency [19]. Based on contact angle experiments and reverse osmosis experiments to study the effect of surfactant concentration on the contact angle and dust removal efficiency of coal dust, it was found that with an increase in surfactant concentration, the wettability of coal dust was enhanced [20]. When the surfactant concentration increases to the critical micelle concentration, the wetting effect of the solution on coal dust reaches its optimum state. Among the different types of surfactants, anionic surfactants had the most significant wetting effect on coal dust. While scanning electron microscopy experiments and infrared spectroscopy experiments were used to study the physical properties of coal [21], some scholars have investigated the modification effects of surfactants and ionic liquids on the physicochemical properties of acidified coal, showing that HNO₃ and SLS can improve the wettability of coal and increase its porosity [22]. SDS promoted the depressing effect of acidified coal bodies and reduced the strength of the treated coal samples [23]. Some scholars have mainly focused on the in-depth R&D and preparation of dust suppressants in order to be able to improve the wettability of coal more effectively [24,25,26]. Some scholars have used molecular simulation to reveal at depth the wetting mechanism of coal dust surface and concluded that an important factor affecting the wettability of coal surface is the ability of hydrophilic functional groups on the coal surface to form hydrogen bonds with water molecules [27,28,29].

Currently, adding surfactants to coal dust prevention and control has shown better wetting effects [30,31,32]. However, available studies mainly focus on the impact of water injection parameters and methods on the water injection effect, while research on how to enhance the water injection effect by changing coal quality characteristics is still insufficient. In this study, coking coal was selected as the research object, and we established the coking coal model based on elemental analysis, ¹³CNMR, and XPS analysis. In addition, complex systems, including water/coal and water/coal/surfactant, were built. The wetting adsorption performance of different systems was simulated at the molecular level, aiming to deeply analyze the wetting mechanism and reveal the microscopic mechanisms during coal seam water injection. The research results provide theoretical support for the application of surfactants in coal seam water injection.

2. Modeling

2.1. Experimental Materials

The coal samples selected for the experiment were coking coal from Wanfeng Coal Mine in Shanxi Province, China. The industrial analysis and elemental analysis of the coking coal are shown in

Table 1. Industrial and elemental analysis of coal sample.

Coal	Mad (%)	Aad (%)	Vad (%)	FCad (%)	Cd (%)	Hd (%)	Od (%)	Nd (%)	Sd (%)
Coking coal	2.56	12.72	14.94	69.78	89.38	5.52	2.64	1.88	0.58

Note: in the table, Mad is air-drying moisture; Aad is ash on air-drying basis; Vad is volatile matter on air-drying basis; FCad is the fixed carbon content.

. The coal samples were crushed for 1 min with a crusher and then sieved with a 150-mesh standard industrial sieve before being sealed in a container for storage. Figure 1 shows the particle size distribution of coking coal dust, with the particle size mainly within 0–200 μm. The particle size distribution is similar, so they can be used as experimental samples.

Based on relevant domestic and international studies, the surfactants selected in this study include sodium dodecylbenzene sulfonate (SDBS) (Wanhua (Guangzhou) Supply Chain Service Co., Ltd., Guangzhou, China), nonylphenol ethoxylate (AEO-9) (Wanhua (Guangzhou) Supply Chain Service Co., Ltd., Guangzhou, China), and cocoamidopropyl betaine (CAB-35) (Wanhua (Guangzhou) Supply Chain Service Co., Ltd., Guangzhou, China).

2.2. Coking Coal Modeling

2.2.1. Carbon Atom Analysis

The different forms of carbon in coal and their relative content can be effectively determined by ¹³CNMR technology. Figure 2 shows the ¹³CNMR spectrum of coking coal. The ¹³CNMR spectrum of coal samples includes three parts: the aliphatic carbon region from 0 to 75 ppm, the aromatic carbon region from 100 to 165 ppm, and the carbonyl carbon region from 165 to 230 ppm.

For the nuclear magnetic resonance (NMR) carbon spectrum of coking coal, data processing was performed by peak fitting to determine the carbon assignment and peak area, as shown in

Table 2. Peak position and peak area corresponding to ¹³CMRN of coking coal.

Coal Samples	Number of Peaks	Chemical Shift (ppm)	Relative Area (%)	Attribution
Coking Coal	1	18.59	7.93	Aromatic Methylene Carbon
	2	34.46	15.09	Methylene Carbon
	3	49.85	6.20	Quaternary and Hypomethyl Carbons
	4	73.02	5.56	Endocyclic Oxidized Fatty Carbons
	5	107.51	2.18	Protonated Aromatic Carbon
	6	124.18	48.59	Protonated Aromatic Carbon
	7	134.38	12.58	Bridged Aromatic Carbon
	8	154.52	2.63	Oxygen-Substituted Aromatic Carbons
	9	169.62	4.45	Carboxy Carbon
	10	221.48	2.89	Carboxy Carbon

. On this basis, the 12 main structural parameters of coking coal were calculated and determined. The specific results are listed in

Table 3. The 12 main structural parameters of coal samples.

Coal Samples	fa	fa′	fac	faH	faN	faP	faS	faB	fal	fal*	falH	falO
Coking Coal	73.32	65.98	7.34	50.77	15.21	2.63	0	12.58	34.78	7.93	21.29	5.56

Notes: f_a is SP² hybridized carbon; f_a^c is carbonyl carbon content; f_a′ is the aromatic carbon ratio of the coal sample; f_a^H is protonated carbon; f_a^N, is non-protonated carbon; f_a^P signifies phenolic hydroxyl or ether oxygen-linked carbon; f_a^S is alkyl-substituted carbon; f_a^B is aromatic bridging carbon; f_al is SP³ hybridized carbon content; f_al^H is methylene or methyl carbon; f_al* is methyl carbon; and f_al^O is oxygen-linked aliphatic carbon.

.

The ratio of aromatic bridging carbon to peripheral carbon (i.e., the bridging-to-peripheral ratio) X_BP is an important parameter for the molecular structure of coal. X_BP is calculated based on the formula in the table:

$$X_{BP}=\frac{{f_a}^B}{{f_a}^H+{f_a}^P+{f_a}^S}$$

(1)

From the distribution of different types of carbon atoms in the coal molecular carbon skeleton, it can be seen that in aliphatic carbon atoms, the content of methylene or methyl groups is much higher than that of methyl or methoxy groups. The f_a^H is above 44%, indicating that the large molecular structure of coal is almost entirely composed of protonated aromatic carbon. In this study, the X_BP of coking coal is 0.24.

2.2.2. Elemental Structural Analysis

In the complex structure of coal, various elements such as carbon, nitrogen, oxygen, and sulfur exist in the form of functional groups. Figure 3 shows the XPS spectrum after peak fitting. According to the results of peak fitting, the predominant form of carbon elements in the coking coal molecular structure is C-C/C-H, supplemented by C-O. Nitrogen mainly exists in pyridine-type nitrogen, supplemented by nitrogen oxides. Oxygen mainly exits in the form of C-O, supplemented by C=O. Sulfur mainly exists in the form of thiophene-type sulfur, supplemented by sulfur oxides.

2.2.3. Comprehensive Analysis

A comprehensive analysis was carried out on the aromatic structure, aliphatic carbon structure, and heteroatom structure of coal in combination with the above analyses.

(1)

Aromatic structure

Aromatic macromolecules are the main structures forming the backbone of coal molecules. The higher the proportion of aromatic macromolecules to cycloalkanes, the higher the degree of condensation.

Table 4. Forms of aromatic structures present in coking coal molecules.

Forms of Existence of Aromatic Structures	Coking Coal
Thiophene (C4H4S)	2
Pyridine (C5H5N)	6
Pyrrole (C4H5N)	2

shows the forms of aromatic structures in coking coal molecules. For aromatic structures, the X_BP of benzene, naphthalene, anthracene or phenanthrene, dibenzofuran, and dibenzothiophene is 0, 0.25, 0.4, 0.5, and 0.57, respectively. The X_BP of coking coal is 0.24, which is between benzene and naphthalene and closer to naphthalene, indicating that there is a structure with two bridging carbons, with benzene and naphthalene being the main components of the original coal. The types and quantities of aromatic structures are listed in Table 4. Actual coal molecular compositions can form aromatic structures with an X_BP of 0.25 when connected to benzene rings, i.e., aromatic structures with the same bridging carbon perimeter ratio as naphthalene.

The mass fraction of C in coking coal is 89.38%. The NMR data analysis reveals that the ratio of aromatic bridging carbon to peripheral carbon in coking coal X_BP is 0.24, indicating that the ratio of bridging carbons to peripheral carbons in aromatic compounds with degrees of condensation 1 and 2 is 0 and 0.25, respectively. From this, it can be inferred that the molecular model of coking coal is primarily composed of naphthalene, followed by benzene rings. The bridging-to-peripheral ratio is obtained through the combination calculation of different aromatic structural units. Moreover, the types and quantities of aromatic structural units are determined. At this point, the total number of aromatic ring carbons in the model is 246. Additionally, based on the results of ¹³CNMR, the proportion of aromatic carbons in coking coal reaches 65.98%, estimating a total of 373 carbon elements in the coal molecule.

(2)

Aliphatic carbon structure

Ethyl side chains, methyl groups, and cycloalkanes are the primary forms of aliphatic structures in coal. The coal quality is positively correlated with the carbon–hydrogen ratio; the higher the coal quality, the higher the carbon–hydrogen ratio, while the quantities of cycloalkanes and aliphatic side chains decrease accordingly. Aliphatic structures tend to exist in the form of cycloalkanes. With a carbon mass fraction of 89.38%, the average number of atoms in alkyl side chains ranges from 1 to 2, indicating that alkyl side chains in the coal structure should not be too long, with short chains being predominant. For coking coal, f_al^H = 21.29% and f_al^* = 7.93%, indicating that the relative percentage of methylene, methine, and quaternary carbons is greater than the relative percentage of methyl groups. Since the aromaticity of the coal is 65.98% and the number of aromatic atoms is 246, the number of aliphatic carbon atoms is tentatively fixed at 129.

(3)

Heteroatom structures

The heteroatom structures in coal are mainly composed of nitrogen and sulfur atoms. Nitrogen atoms exist in the form of pyrrole or pyridine, while sulfur atoms exist in the form of thiols, thioethers, or aromatic compounds.

Table 5. Forms of heteroatoms in coking coal molecules.

Element	Quantity	Form of Existence
N	8	C5H5N, C4H5N
O	11	C-O, C=O
S	2	C4H4S

shows the forms of aromatic structures in coking coal molecules. The number of carbon atoms in coking coal molecules is 373. The coal sample contains a certain amount of nitrogen, while the sulfur content is generally very low. Based on the atomic ratios of each element to carbon, the ratio of nitrogen atoms to oxygen atoms to sulfur atoms in coking coal is 8:11:2.

The specific forms and distribution of aromatic carbon, aliphatic carbon, and heteroatoms in coal are determined by calculation and analysis. Based on this and combined with the results of elemental analysis, an initial structural model of the coking coal macromolecule was built with the Chemdraw 19.0. The molecular formula of coking coal is determined to be C₃₅₂H₃₂₀N₈O₁₁S₂, as shown in Figure 4.

2.3. System Modeling

In this study, the Forcite module in Materials Studio 8.0 software was used, with the COMPASS II force field based on quantum mechanics selected. Layers of coal molecules and water molecules were constructed with the Amorphous Cell module and were merged into a system with a vacuum layer density of 20 Å. Four wetting adsorption system models were built. Figure 5 shows System B and simulates the reaction process between coking coal and SDBS solution. System A represents the water/coal system, System C is the water/coal/AEO-9 system, and System D is the water/coal/CAB-35 system. The Forcite module was chosen to geometrically optimize the different systems constructed, ensuring the molecular energy reaches its minimum and the structure becomes more stable. Dynamic tasks were executed after optimization. A nose thermostat was used in a 298K NVT system for molecular dynamics calculations for 300 ps with a time step of 1.0 fs and a cutoff set to 12.5 Å. The CPU time required for a system simulation was about 4.5 h.

3. Study on Wetting Adsorption Performance of Water/Coal/Surfactant

3.1. Spatial Distribution Characteristics

The states after the simulation of different systems are shown in Figure 6. In the initial configuration before simulation, the two molecular layers do not contact each other, and the surfactant molecules are randomly distributed in the aqueous solution. After simulation, there are significant changes in the positions of the two molecular layers. In Systems B, C, and D, there is a noticeable interaction between the surfactant solution layer and the coal surface. This interaction causes the solution layer to move towards the coal surface, finally reaching a new equilibrium state. The surfactant adsorbs onto the coal surface and aligns in a specific manner: the hydrophilic head group faces the water phase, while the hydrophobic tail chain points towards the coal molecules. This effectively covers the hydrophobic groups on the coal sample surface, forming an oriented adsorption layer with the hydrophilic end facing the water phase, leading to a significant change in the surface properties of the coal sample from hydrophobic to hydrophilic.

3.2. Relative Concentration Distribution Characteristics

In order to quantitatively describe the spatial distribution of this adsorption behavior and to delve into the adsorption characteristics of the water/coal and water/coal/surfactant systems, relative concentration distribution curves along the z-axis were plotted based on simulation data, as shown in Figure 7.

It can be observed from the figure that the thickness of the adsorption layer at the water/coal interface in System A is only 16.04 Å, indicating a certain repulsion between the aqueous solution and coking coal due to poor wettability. In System B, the thickness of the adsorption layer of the surfactant SDBS in the z-axis direction is the largest, at 20.34 Å; in System C, the thickness of the adsorption layer of the surfactant AEO-9 on water/coal is 17.57 Å; in System D, the thickness of the adsorption layer of the surfactant CAB-35 on water/coal is 16.13 Å. A greater thickness of the adsorption layer in the z-axis direction indicates a larger contact area between coal and water molecules, implying better wettability. The results of molecular dynamics simulations reflect the enhanced ability of the SDBS solution to promote water molecule movement and indicate its advantage in improving the wettability of the coking coal surface.

3.3. Analysis of Non-Bonding Interaction Energy

The energy in the adsorption system mainly comes from non-bonding interaction energy (E_int), where van der Waals energy (E_van) and electrostatic energy (E_ele) are the dominant factors during the interaction and adsorption processes. Non-bonding interaction energy between molecules is generally negative, and the magnitude of its absolute value reflects the strength of the interaction.

Table 6. Action energy analysis of different systems.

System	Eint/(kcal·mol−1)	Evan/(kcal·mol−1)	Eele/(kcal·mol−1)
System A	−9416.006	2427.363	−11891.369
System B	−9906.651	2395	−12252.468
System C	−9652.390	2353.543	−12005.933
System D	−9857.330	2375.737	−12282.388

shows the interaction energies for the four systems. It can be observed that E_in for the four systems is −9416.006, −9906.651, −9857.330 kcal/mol, and −9652.390 kcal/mol, all of which are negative. The absolute values of E_in follow the order: System B > System C > System D > System A. In all four systems, the process of water molecules adsorbing on the coal surface is spontaneous. When surfactant molecules cover the coal surface, their binding ability significantly increases, leading to an enhancement in the wettability on the surface of coking coal. Particularly, the effect of the SDBS solution on improving the wettability of the surface of coking coal is notably significant. The analysis of E_van and E_ele within E_in showed that in the water/coal/surfactant system, E_ele contributes more, indicating that the surfactant molecules’ head groups primarily adsorb to water molecules through electrostatic interactions.

3.4. Diffusion Analysis

The effect of surfactants on the aggregation behavior of water molecules was revealed by an in-depth analysis of mean square displacement (MSD) and its corresponding diffusion coefficient D; the mechanism of the effect of different surfactants on the wettability of coal was further clarified. The specific calculation formulas are as follows:

$$MSD=\frac{1}{N}{\displaystyle \sum _{i=1}^N\left[r_i(t)-r_i(0)\right]}{ }^2$$

(2)

$$D=\frac{1}{6N}\underset{t\to \infty }{\lim }\frac{d}{dt}{\displaystyle \sum _{i=1}^N\left[r_i(t)-r_i(0)\right]}{ }^2$$

(3)

where MSD is the mean square displacement; N is the number of diffusing molecules; r(t) and r(0) are the position vectors of the molecule at time t and t = 0, respectively; and D is the diffusion coefficient.

From the MSD curves of water molecules shown in Figure 8, it can be seen that the slope of the MSD curve in systems with surfactants is significantly greater than in systems without surfactants. This is attributed to surfactants enhancing the interaction between water and coal surfaces, promoting the vigorous movement of water molecules. The system with water/coal has the smallest MSD slope; adding surfactants increases the slope; and in the water/coal/surfactant SDBS system, the slope increase is the highest. This is due to the different hydrophobic alkyl chains in various surfactants exerting varying forces on water molecules, influencing their effect on coal wettability. The results indicate that surfactants have a promoting effect on the diffusion of water molecules. The diffusion coefficients are shown in

Table 7. Diffusion coefficients for the four systems.

System	Diffusion Coefficient (cm2/s)
System A	4.77383 × 10−5
System B	6.24917 × 10−5
System C	5.97467 × 10−5
System D	5.63033 × 10−5

. As the diffusion coefficient of water molecules increases, the interaction between hydrophobic alkyl chains and water molecules is also significantly strengthened. This enhanced effect further promotes the wetting effect between coal and water, thereby contributing to the improvement in coal’s wettability.

4. Conclusions

This study explores the mechanism by which surfactants improve the wettability of coking coal based on molecular dynamics and reveals the intrinsic mechanism of water injection into coal seams at a molecular level. The simulation results demonstrate that surfactant molecules effectively cover the hydrophobic groups on the surface of coking coal and form an oriented adsorption layer with hydrophilic ends facing the water phase, significantly enhancing the wettability on the coal surface. After adding surfactants, the thickness of the adsorption layer in the z-axis direction significantly increases, indicating an expanded contact area between coal and water molecules and an enhanced wettability. When surfactant molecules tightly cover the coal surface, their binding ability is significantly enhanced, which not only helps the formation of a more stable hydrophilic layer but also improves the wettability on the surface of the coking coal. Furthermore, surfactants have a promoting effect on the diffusion of water molecules. As the diffusion coefficient of water molecules increases, the interaction between hydrophobic alkyl chains and water molecules is also significantly strengthened. Such strengthened interaction further promotes the wetting effect between coal and water, helping improve the wettability of coal.