Theoretical Study On Interface Behavior And Salt Resistance Of Surfactants

Surfactants are a class of substances that can significantly reduce surface activity at lower concentrations.Because of their hydrophilic structure and hydrophobic groups,they have unique amphiphilic properties different from other compounds.These properties can make surface active.The agent forms a molecularly ordered assembly in the solution,and can also adsorb at the interface to change the interfacial tension and surface wetting properties.In the tertiary oil recovery in Daqing Oilfield,the main component of the surfactant as an additive plays an important role in oil displacement.In order to adapt to the current oil recovery situation,improve oil recovery,and change the supply and demand contradiction of petroleum,the research on the behavior and salt resistance of surfactants at the oil/water interface has been receiving much attention.In this paper,molecular dynamics simulation method was used to carry out systematic theoretical research work on the aggregation morphology,adsorption and interaction mechanism of surfactant at oil/water interface from the microscopic point of view,and the four types of oil displacement selected.Comparative analysis with surfactants gave a reasonable explanation for the results.The theoretical research results have important theoretical significance and practical application value for further guiding the screening of surfactants for oil displacement and the molecular design of surfactants for efficient oil displacement.In this paper,first,the microscopic behaviors of anionic,nonionic,zwitterionic and gemini surfactants at the oil/water interface were studied by molecular dynamics simulation.The interface of four surfactants at the oil/water interface was calculated.Tensile,interface thickness and interfacial formation energy were calculated.The radial distribution functions of the hydrophilic groups and water molecules of the four surfactants were calculated.The influence of different electric field strengths on the water diffusion coefficient in the surfactant system was calculated,which revealed from the microscopic point of view.The oil displacement mechanism of surfactants for oil displacement.Secondly,the interaction between different cations(Na⁺,Mg²⁺and Ca²⁺)and four different types of surfactant head groups was compared.Potential of Mean Force?PMF?was used to analyze the relationship between the cation and the surfactant head group.The binding energy and dissociation energy were formed,and the salt resistance of the surfactant was evaluated.The mechanism of surface active salt resistance for oil displacement was suggested.Finally,the thermodynamic parameters of four different types of surfactants in aqueous solution were calculated by free energy perturbation,and the ability of four kinds of surfactants to form micelles was compared.The results show that the low oil/surfactant/water interfacial tension is favorable for oil displacement,and the interface thickness is large and the interface formation energy is low,which is beneficial to reduce the interfacial tension.As the interfacial tension decreases,the surfactant is more stable at the oil/water interface.Among the four surfactants for oil displacement,the gemini surfactant has the strongest ability to lower the oil/water interfacial tension,and the formed structure is the most stable,and is more suitable as a surfactant for oil displacement.There is a chemical hydration layer and a physical hydration layer between the surfactant head group and the water molecule.For the oil displacement process involving different cations(Na⁺,Mg²⁺and Ca²⁺),the higher the peak value of the radial distribution function,the surfactant head base The stronger the interaction with the cation,the effect of the three cations on the hydration layer is Mg²⁺>Ca²⁺>Na⁺.The binding energy of three different cations to the four surfactants showed that the salt resistance of the zwitterionic and nonionic surfactants was higher than that of the anionic and gemini surfactants,and the dissociation energy showed that the nonionic surfactants were more more susceptible to loss of activity than the zwitterionic surfactants in the high salt state,and thus,in the case of high salt reservoirs,the zwitterionic surfactant is more suitable for oil displacement.The calculated thermodynamic data show that the four surfactants spontaneously form micelles in aqueous solution,and the micellization process is mainly entropy-driven.With the increase of temperature,the entropy-driven contribution is gradually reduced,while the contribution of enthalpy drive is gradually increased,there is a significant linear enthalpy-entropy compensation phenomenon.The Gibbs Free Energy data of the four surfactants showed that the zwitterionic and gemini surfactants have stronger ability to form micelles,have good stability,and have better oil displacement effects.