Theoretical Studies On The Interaction Between Anionic Surfactants And Inorganic Salts

Anionic surfactant can decompose into surfactant anion and counterion in aqueous solution, and therefore can be significantly affected by cations or inorganic salts. In the oil reservoir with high temperature and salinity, mineral salts such as Na⁺, Mg²⁺ and Ca²⁺ can significantly reduce surface activity and solubility of anionic surfactants, and even lead to “salt-out” or precipitation. The investigation on the salt-tolerant mechanism of anionic surfactants would be meaningful to understand the relative experimental phenomena and give a theoretical guide for screening and designing novel surfactants with great salt tolerance. In view of this, the present master thesis combines theoretical calculation and molecular simulation methods to research the microscopic interaction between anionic surfactants and inorganic salts systematically,for giving a deep insight on the effect of inorganic salts on the properties of anionic surfactants.The results of theoretic calculation show that the interaction between anionic surfactants and inorganic salts is not only related to the type of participating inorganic salt, but also related to the property of solvent medium. In the water phase, SDBS, SDSn and SDC bind stably with inorganic salts in a bidentate form. The binding between Ca²⁺ and anionic surfactants is the most stable, and then Mg²⁺, the worst is Na⁺. In the oil phase, the binding type between SDBS and inorganic salts is not changed, but the binding energies are significantly higher than that in the water phase, and Mg²⁺ binds stronger with SDBS than Ca²⁺. The salt tolerance of three common anionic surfactants is evaluated and the results follow the order: SDBS > SDSn > SDC. At the gas-water interface, SDBS anion would interact with six water molecules and form a stable hydrated complex DBS-·6H₂O. The impact degrees of inorganic salts on the structure of DBS-·6H₂O follow the order: Ca²⁺ > Mg²⁺ > Na⁺. The hydration shell is indicated plays an important role in preventing the electrostatic interaction between the polar headgroup and inorganic salts.The simulation results show that, Mg²⁺ with small ionic radius and high positive charge can enter into the interfacial area from the bulk phase, replace counterion Na⁺ and bind with the surfactant headgroup in surfactant systems. After entering into the interfacial area, Mg²⁺ compresses the thickness of water layer and the interface electric double layer and restrains the diffusion of Na⁺. Due to the strong hydration of Mg²⁺, the thickness of the first hydrated layer is increased. Compared with surfactant SDSn, A₁₂ESO₂ with ethyoxyl groups and therefore has a greater salt tolerance. The corresponding air-water interfacial properties of A₁₂ESO₂ system is less affected.