Theoretical Studies On The Self-assemble Of Mixed System Containing Surfactant

Nobel laureate Pierre-Gilles de Gennens once pointed out:"without surfactants, we whenever completely unable to confront90%industrial issues". Therefore, intensive efforts have been sent to investigate the property of surfactant. And it shows that the effect of single surfactant is often not as good as the mixtures. Under appropriate conditions, there have strong interactions between surfactant and polymer or different kinds of surfactants to form complexes. The new complexes in mixtures usually exhibit many fascinating and intriguing features which are different from any one component, such as rich phase, versatile aggregation morphology, lower surface tension, stronger viscosity and so on. Based on these special performances, the mixtures contained surfactant play an important role both in people’s daily life and industry.With the development of research, it is important to study the self-assembly rule of different mixtures containing surfatant as well as the interactions between them at molecular level by molecular dynamics simulation. Compared with the traditional experimental methods, molecular dynamics simulations could greatly reduce the blindness and revision test time-consuming of experiments. In additional, molecular simulation can not only provide a clear three-dimensional structure picture of the aggregates formed in studied systems, but also allow us to study the self-assembly process at molecular insight, and deepen in understanding the interaction mechanism between the mixtures, therefore it could be helpful and meaningful to design and explore new complex.In this dissertation, a series of molecular dynamics simulations have been carried out for several mixtures, including surfactant and polymer, catanionic surfactants, which were newly reported in experiments recently. On the one hand, molecular dynamics simulation were performed to investigated the self-assembly of surfactant and polymer complexes. We have studied the different adsorption behavior of surfactant on polymer at solution and interface. In addition, we also put forward the appropriate adsorption mechanism, which supplied some valuable theoretical information for designing and improving complexes of surfactant and polymer. On the other hand, we have studied the interaction between oppositely charged surfactants by coarse-grained simulation with the framework of Martini force field. We mainly forced on the self-assembly process of catanionic surfactants and the effect of mixing ratio on the structure and properties of aggregates.The important and valuable results in this dissertation can be summarized as follows:(1) We have performed coarse-grained molecular dynamics simulation to investigate the interaction between polyacrylamide (PAM) and sodium dodecylsulfate (SDS) in aqueous solution. The simulation results revealed that SDS micelle would be adsorbed on the polymer chain by the hydrophobic interaction between the surfactant hydrophobic tails and polymer backbone, and PAM was located at the interface of the hydrophobic and hydrophilic regions of the SDS micelle. The formation process of SDS-PAM complexes was divided into three stages by monitor the trajectory and the conformation of PAM,(i) PAM quickly curled with the adsorption of some SDS molecules until the radius of gyration (Rg) of polymer reaches a minimum;(ii) due to more and more SDS adsorbed, PAM stretched slowly with the increase of Rg of PAM;(iii) the commonly accepted "necklace" structure was formed when Rg just fluctuated around a value.According to the effect of surfactant on the polymer conformation, the interaction between SDS and polymer in very constrained environment (PEO, one end of a dense array of PEO chains attached to a solid surface) was studied by molecular dynamics simulations. The effects of adding SDS amount and grafting density on the adsorption behavior of SDS on the PEO brush were discussed. The simulation showed that the height of PEO brush was mainly affected by the adsorbed SDS at low grafting density; however, at high grafting density it was primarily controlled by the grafting density on solid surface. With the increase of grafting density, the adsorbed EO monomers were decrease due to the excluded volume interaction among polymer chains and the electrostatic repulsion among the negatively charged surfactant micelles. Our work about the adsorption of surfactant on polymer chains showed a clear microscopic picture of complex of surfactant and polymer, which are difficult to obtain from experiments, and also make a better understand and design the surfactant and polymer complex.(2) Molecular dynamic simulations were performed to study the adsorption of oppositely charged sodium poly(acrylic acid)(NaPAA) and dodecyltrimethylammonium bromide (DTAB) at the air/water interface. Our results have proven that the transition from monolayer adsorption of the complex of NaPAA and DTAB to a multilayer structure was observed with the increasing of DTAB concentration. And in the multilayer adsorption, two layers of DTAB which adopted tail-to-tail arrangement were used to link the two polyelectrolyte chains. It indicated that electrostatic interaction was the driving force which bound DTAB to NaPAA, while the hydrophobic interaction between surfactant tails was the main force in inducing a layer structure of the complex at high DTAB concentration. The dynamic properties of counterions implied that the formation of polyelectrolyte-surfactant complex was an ion-exchange process, which was in good accordance with experimental results. Our simulation work provided a microscopic perspective on the adsorption of oppositely charged surfactant and polyelectrolyte at interface that would be helpful and meaningful in studying the surface properties of surfactant and polymer complexes.(3) The self-assembly process and structure properties of aggregates formed by oppositely charged surfactant, SDS and DTAB, have been studied by coarse-grained simulation within the framework of the Martini force field. The simulation results indicated that the self-assembly processes can be divided into three stages:monomers firstly fast gather to form disorded oligomers with the continuous growth of the cluster size; and then collisions and merge between small aggregates to form the larger clusters, which caused the clusters growing like a ladder; the last stage, the size and structure of aggregates reached stable. Analysis of the size distribution and morphology of the clusters showed that the aggregates reached to the biggest when the mixing ratio of SDS and DTAB was2:1, and discal micelles with the hydrophilic headgroups of catanionic surfactants interdigitation and parallel in the surface was only formed in mixtures, which were are in excellent agreement with experiment. Therefore, our work not only obtained a clear microscopic picture to extract information about dynamic and structural properties, which are difficult to obtain from experiments, but also make a good guide for designing different self-assemble aggregates of oppositely charged surfactant.