Interaction Between Anionic Surfactants And Inorganic Salts

Surfactants play an important role in enhanced oil recovery (EOR) and the salt tolerance and temperature resistance have been received much more attention. Consequently, it is important to investigate the salt tolerance and temperature resistance of EOR surfactants. But these investigations are mostly focused on the macroscopic properties, such as critical micelle concentration, aggregation number, rheological properties et al. There are few studies of the microscopic mechanism of interaction between surfactants and inorganic salts. Therefore it is of great meaningful to investigate the salt-tolerant mechanism between surfactants and inorganic salts. Molecular dynamics simulation study has become an effective method in researching the interaction between surfactants and inorganic salts in an atomic level.In this paper, molecular dynamics simulation has been used to investigate the interaction between anion surfactants and inorganic salts. The paper has been divided into five parts. In the first section, we have reviewed the effect of inorganic salts on the properties of surfactants and clarified the significance of salt effect in surfactant systems.The second section has described the calculation method of molecular dynamics simulation from the basic principle and calculation process.In the third section, the effect of NaCl, MgCl2 and CaCl2 on the properties of monolayer for two surfactant isomers of sodium hexadecane benzene sulfonate at the air/water interface have been investigated via molecular dynamics simulation method. The alkyl benzene sulfonates are two surfactant isomers in the family of sodium hexadecane benzene sulfonates defined by 1C16 and 5C16, indicating a benzene sulfonate group attached to the 1st and 5th carbon atom in hexadecane backbone. Alkyl benzene sulfonate is a kind of very popular surfactants which are widely used in surfactant flooding. However, the commercial products are the mixtures of compounds in which the benzene ring groups are attached to different carbon atoms on the alkyl chain. For detailed study it is necessary to know the exact location of the benzene ring groups, i.e. the alkyl benzene sulfonate should have definite structure and high purity. The MD simulation can solve this issue and give detailed information at an atomic level. The results reveal that arrangement of 1C16 monolayer is more out-of-order than that of 5C16 at the interface due to the stronger electrostatic repulsion. Inorganic counterions (Na+, Mg2+, or Ca2+) are distributed close to the air/water interface, screening the electrostatic repulsion among the surfactant ions, and the effect of the different counterions follows the series Ca2+>Mg2+>Na+. Smaller hydrated divalent ions (Ca2+) more easily penetrate into the hydration shell of surfactant head groups compare with larger hydrated ions (Mg2+). The translational motions of water molecules in first hydration shell are highly restricted by the introduced counterions. The salt tolerance of 5C16 with branched-chain is more superior to the 1C16 with linear-chain.In the fourth section, we have investigated the interaction between SDS and divalent salts by experimental and simulation method. The results showed that there is a significant precipitation-redissolution region in SDS/CaCl2 and SDS/MgCl2 systems. CaCl2 has a greater influence on the properties of SDS solutions for its smaller hydrated radius. NaCl and TX-100 can significantly improve the salt tolerance of SDS.In the fifth section, molecular dynamics simulations have been performed to investigate the effect of inorganic salts on the structural and dynamic properties of AOT monolayer formed at the air/water interface, including LiCl, NaCl, KCl, MgCl2 and CaCl2. The important notable feature is that a significant roughness developed near the surfactant head groups region of the interface during the nanosecond time scale of the simulation and the inorganic counterions are distributed close to the air/water interface, near the oppositely charged sulfonate head groups. There is a significant hydration layer formed by H-bonding near the surfactant head groups. The conterions can compress the electric double layer and reduce the electrostatic repulsion of AOT head groups. The effect of the different monovalent counterions follows the series K+>Na+>Li+. Furthermore, K+ may embed in the gap between the AOT head groups and decrease the interaction between water molecules and AOT head groups. Smaller hydrated divalent ions (Ca2+) have a greater influence on the AOT monolayer compare with larger hydrated ions (Mg2+).