Study of the Adsorption Behavior of Surfactants on Carbonate Surface by Experiment and Molecular Dynamics Simulation

Molecular Dynamics Simulations of the Short-Chain Fluorocarbon Surfactant PFHXA and the Anionic Surfactant SDS at the Air/Water Interface

The research and development of alternatives to long-chain fluorocarbon surfactants are desperately needed because they are extremely toxic, difficult to break down, seriously harm the environment, and limit the use of conventional aqueous film-forming foam fire extinguishing agents. In this study, mixed surfactant systems containing the short-chain fluorocarbon surfactant perfluorohexanoic acid (PFHXA) and the hydrocarbon surfactant sodium dodecyl sulfate (SDS) were investigated by molecular dynamics simulation to investigate the microscopic properties at the air/water interface at different molar ratios. Some representative parameters, such as surface tension, degree of order, density distribution, radial distribution function, number of hydrogen bonds, and solvent-accessible surface area, were calculated. Molecular dynamics simulations show that compared with a single type of surfactant, mixtures of surfactants provide superior performance in improving the interfacial properties of the gas-liquid interface. A dense monolayer film is formed by the strong synergistic impact of the two surfactants. Compared to the pure SDS system, the addition of PFHXA caused SDS to be more vertically oriented at the air/water interface with a reduced tilt angle, and a more ordered structure of the mixed surfactants was observed. Hydrogen bonding between SDS headgroups and water molecules is enhanced with the increasing PFHXA. The surface activity is arranged in the following order: PFHXA/SDS = 1:1 > PFHXA/SDS = 3:1 > PFHXA/SDS = 1:3. These results indicate that a degree of synergistic relationship exists between PFHXA and SDS at the air/water interface.

Mechanism of Low Chemical Agent Adsorption by High Pressure for Hydraulic Fracturing-Assisted Oil Displacement Technology: A Study of Molecular Dynamics Combined with Laboratory Experiments

This study investigates the influence of physical parameters such as porosity, permeability, pore-throat radius, and specific surface area/volume on the adsorption capacity of surfactants in the pore surface of reservoirs. In the meantime, the hydraulic fracturing-assisted oil displacement (HFAD) technique can effectively improve the permeability and porosity of pores in the reservoir, which may affect the adsorption capacity of surfactants in low-permeability reservoirs. This may help to reduce the adsorption loss of surfactants in low-permeability reservoirs. Based on physical simulation methods, dynamic adsorption experiments were conducted to clarify the dynamic saturation adsorption capacity effect of high-pressure and low-pressure displacement agents by the HFAD technique. In addition, the molecular dynamics simulation method was used to study the effect of high-pressure conditions of HFAD on the adsorption capacity of surfactants on weakly lipophilic silica walls. Under the condition of high injection pressure by the HFAD technique, the fluid flow velocity and the initial kinetic energy of molecules increase, while the absolute value of the electrostatic potential energy in the system decreases. In addition, the van der Waals potential energy increases. In other words, the smaller the gravitational attraction experienced by the surfactant molecules during adsorption, the greater the repulsive force. Under the dual action of electrostatic force and van der Waals forces, the absolute values of the adsorption energy and the free energy decrease. The adsorption capacity of the surfactant molecules is weakened. Moreover, the decrease in adsorption capacity has little effect on the improvement of wettability, indicating that the adsorption of the surfactant reduced by HFAD technology is mostly ineffective adsorption.

Study of the Micromechanism of the Effect of Fatty Alcohol Poly(oxyethylene) Ether-9 on the Wettability of Jincheng Anthracite

The influence mechanism of the adsorption of fatty alcohol poly(oxyethylene) ether (AEO₉) on the wettability of anthracite coal was studied by means of experiments and simulations. First, the contact angle and surface tension were measured. When the AEO₉ concentration was 0.5 wt %, the contact angle and surface tension were the smallest, which were 10.28° and 25.39 mN m^-1, respectively. X-ray photoelectron spectroscopy (XPS) indicated that the content of C-O functional groups on the anthracite surface increased by 20.76% after adsorption of AEO₉. The molecular orbital energy and electrostatic potential of AEO₉ and anthracite were calculated by density functional theory (DFT). There are two modes of electron transfer between the two orbitals: highest occupied molecular orbital (HOMO) transfer of AEO₉ to lowest unoccupied molecular orbital (LUMO) transfer of anthracite and HOMO transfer of anthracite to LUMO transfer of AEO₉. The dynamics simulation results show that the addition of AEO₉ increases the migration rate of water molecules, promotes the movement of a large number of water molecules toward the surface of anthracite, and enhances the thickness of the water molecular layer on the surface of anthracite. The analysis of the relative concentration shows that AEO₉ is distributed at the anthracite/water interface. AEO₉ molecules are intertwined and connected by hydrophobic chains to form a network structure, which covers the anthracite surface horizontally, thus promoting the strength of the anthracite/water interaction.