## 1 Introduction
[25] proposed that neutralizing the charge on coal particles can eliminate the electrostatic force between coal particles that have the same charges, which is beneficial for the agglomeration of coal fine particles. It has been demonstrated that VESFFs can influence the pore structure of coal by modifying the zeta potential on its surface. In this paper, the zeta potential of coal treated with VESFFs was measured. The surface of coal is composed of many HBGs, such as aromatic hydrocarbons and fatty hydrocarbons, making coal highly hydrophobic [26]. Oxygen-containing functional groups, such as hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), and Si-O-, on the surface of coal contribute to its hydrophilicity [27]. [PERSON] et al. [28] pointed out that VESs can alter the proportion of oxygen-containing functional groups on the coal surface. [PERSON] et al. [29] discovered that the COO/COOH group of coal exhibits a high adsorption capacity for hexadecyltrimethylammonium bromide. Additionally, VESs have been observed to cover the oxygenated functional groups on the surfaces of coal pores, thereby markedly reducing their hydrophilicity. [PERSON] et al. [30] found that the content of oxygenated functional groups (CO, C=O, O=C-O) on the coal surface decreased after VES adsorption, enhancing the hydrophobicity of the coal surface. Hence, it can be seen that VESFFs can change oxygen-containing functional groups on the coal surface, affecting the wettability of the FFs on the coal surface and in turn changing the effects of the FFs on the pore structure of coal. Fourier-transform infrared (FTIR) spectroscopy is a widely used and effective method for investigating functional groups on the surface of coal. This paper uses FTIR analysis to investigate the effect of VESFFs on oxygenated functional groups on coal surfaces. The degree of order and compactness of the macromolecular arrangement of coal can affect pores with diameters less than 500 nm [31]. The water molecules and VES molecules in the VESFFs adsorb on the coal surface, changing the aromatic size and orderliness of the microcrystalline structure (MS) of the coal, in turn leading to alterations in the nanoporous structure of the coal. The HLGs and HBGs of VESs have different influences on the MS of coal. For this reason, VESFFs with varying hydrocarbon chain lengths and those with a combination of different VES types have different effects on the nanopores of coal; however, there have been few studies in this area. Some carbon skeletons exhibit strong Raman spectral responses owing to their symmetrical vibrations and non-polar groups. Raman spectroscopy is a suitable way to identify the molecular skeleton of coal. Raman spectroscopy can identify the hybridization of carbon atoms forming the coal skeleton, and can non-destructively, simply, and quickly analyze the disorder and graphitization of carbon atoms qualitatively and quantitatively [32]. Since the 1970s, Raman spectroscopy has been used extensively in coal studies [33; 34]. [PERSON] et al. [27] used Raman spectroscopy to study the impact of the molecular structure of medium-to-high-rank coal on coalification. [PERSON] et al. [35] used Raman spectroscopy to quantitatively evaluate the reflectivity of vitrinite and O/C atoms in coal. This paper examines the impact of VESFFs on the MS of coal. In this study, VESs with different hydrocarbon chain lengths and VESs with different ionic types (such as cationic VESs, anionic VESs, and zwitterionic VESs) were selected as the main agents for fracturing fluids, and four different formulations of fracturing fluids were prepared. Two coal samples with different hardness coefficients were treated with deionized water and four VESFFs under the same experimental conditions. The treated coal samples were tested for zeta potential, functional groups, and MS using a zeta potential analyzer, FTIR spectrometer, and Raman spectrometer. The mechanism by which VESFFs act on coal nanopores is revealed from a molecular perspective.