| Surfactants are amphiphilic substances which have their hydrophilic and hydrophobic structural parts. Thus, surfactants exhibit many unusual physical properties compared with other compounds. That is, surfactants can be adsorbed as an orientated monolayer at air/vapor or oil/water interfaces and self-assemble aggregate in the solution. Therefore, surfactants reduce the surface/interfacial tension, change the wetting of the surface by adsorbing at the interface, and can also assemble in the bulk solution into a variety of aggregates. The property of these aggregates is essential in many biological processes and is used in many industrial and domestic applications.Surfactants play an important role in enhanced oil recovery (EOR). Recently, with the continuous development of the oil fields, the temperature and the salinity rise which reduces the efficiency of the EOR. The most widely used ionic surfactants such as sulfonate, sulfate, carboxylate and organic phosphate may bind with Ca2+, Mg2+ and the salt-out occurred with ions in the solution. Thus, the surfactants loose their surface activity. The non-ionic surfactants are also precipitated under high temperature conditions. Thus, it is meaningful to investigate the salt tolerance and temperature resistance of EOR surfactants.There are already many investigations on the solution of surfactants using many advanced experimental methods. However, these investigations are mostly focused on the macroscopic properties, such as critical micelle concentration, aggregation number, rheological properties et al. These studies usually inter the mechanism of the interaction between salt and surfactants by observing the changes of the physic-chemical properties of the surfactants. Thus, the precise description of the mechanism from the microscopic view is scare. Therefore, it is of great meaningful to investigate the salt-tolerant mechanism between surfactants and salts at molecular level.In this dissertation, a series of theoretical studies have been carried out for several EOR surfactants. On the one hand, by performing quantum mechanism calculations, we investigated the properties of the single molecule which decided the physic-chemical characteristics of the surfactants. On the other hand, we investigated the interaction between the surfactant and the divalent ions by performing density functional theory (DFT) calculations. By comparing the difference between the bind energies between surfactants and ions, we proposed the binding model between cations and surfactants. Then, we performed molecular dynamics on the surfactants aggregates to investigate the interior micro-environment of the aggregates. We focused on the influence of cations on the hydration shell of the surfactants. By calculating the potential of mean force between surfactants and ions, the binding energy between them are shown. It can reflect the difference of the salt tolerant among the EOR surfactants. The important and valuable results in this dissertation can be summarized as follows:1. We performed molecular dynamics to study the solubilization of pyrene in the SDS micelle. The distribution of the single pyrene can be observed at molecular level. By changing the concentration of pyrene, the structure of the excimer was obtained by the simulation. Meanwhile, the orientation and the distance between the pyrene in the excimer were investigated and the?-?conjugation structure was confirmed. Our simulation showed that free pyrene can be solubilized into the micelle spontaneously and prefers to be located in the hydrophobic core region, while two pyrene molecules are found to be distributed mainly in the palisade layer. These results are helpful and meaningful to utilize the surfactant aggregates better.2. Quantum mechanics (QM) method was used to calculate molecular properties of sodium dodecylbenzenesulfonate (SDBS) in vcuum and in solution. Moreover, molecular dynamics (MD) simulations have been performed to determine the dynamic behavior of SDBS moving from the bulk solution to the air/water interface.QM calculations suggest that two headgroup oxygen atoms on each surfactant molecule interact with a Na+ ion, despite the availability of three oxygen atoms in the headgroup. MD simulations showed that the Na+ ion must overcome the energy barrier between two solvent layers around the headgroup to form stable ion pair in solution, which is consistent with experimental results. In the simulation, in moving from the bulk to the interface, SDBS can aggregate in a short time, and the adsorption adopts a preferred orientation. The results indicate that formation of favorable hydrophobic interactions of the surfactant alkyl chains is the origin of interfacial adsorption of SDBS.3. The mechanics of the salt tolerance of EOR surfactants was investigated by QM and MM methods. These results reveal the interaction mechanism between surfactants and ions. (1) The structure of zwitterionic surfactant sulfobetaine, i.e. N-Dodecyl-N, N-dimethyl-3-ammonio-l-propanesulfonate, was optimized using density functional theory (DFT) and the interactions between the surfactant and Ca2+ or Cl- ions were studied at the molecular level. The results showed that:?) a 2:1 type pair between zwitterionic negative center (-SO3-) and Ca2+ was formed,?) the positive center (-N+(CH3)2-) bound with one Cl- through two methyl groups and one methylene which connect to N atom. Since there are some weak charges on the methylene nearest to the polar groups, the negative and positive centers in the polar group of surfactant should be re-divided. The calculation also showed that the tail chain has a weak charge resulting in the core of the micelle having polarity. This core polarity of the micelle is somewhere between the oil phase polarity and the water phase polarity, which favors surfactant aggregation in solution.(2) Molecular dynamics studies were performed to study surfactant sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in the solution with Ca2+ and Mg2+. We focused on the on the influence of cations on the hydration shell of the surfactants. Our results showed that the combination between the headgroup of surfactant and Ca2+ or Mg2+ is prevented not by the hydrate shells, but by a deep stabilizing minimum formed in the potential of mean force between the interacting ion pair. They can disturb the original H-bonding structure of water around the headgroup, leading to the decrease of the H-bonding number.The potential of mean force showed that the energy barriers of ion pair between the headgroup and Ca2+ and Mg2+ in SDSn system are more than those in SDS system, and the water coordinate numbers for Ca2+ or Mg2+ in SDS solution are the lowest. It indicates that SDS surfactant easily combines the ions compared with the SDSn surfactant, and it has strong effect on the original hydration structure. These results can be explained the reason that sulfonate surfactant (such as SDSn) has better efficient in salt solution with Ca2+ and Mg2+ in enhanced oil recovery (EOR) experiment.(3) The effect of Ca2+ ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. The simulations indicate that the adsorption structure of both surfactants not only depends on the surfactant surface coverage, but also on the Ca2+ ions circumstances. The PMFs show that the energy barrier of ion-pairs between the SDSn headgroup and Ca2+ is higher than that in SDC systems, which means sulfonate surfactants are more efficient in saline circumstance in EOR experiments. |
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