Molecular Simulations On Inorganic/organic Hybrids Fabricated By Ag Nanoparticles And Lyotropic Liquid Crystals

The study of nanostructure materials has been one of the most active areas, owing to their unique size-dependent chemical and physical properties. Thereinto, lyotropic liquid crystal (LLC) can be used as a physical template to assemble nanoparticles, which has important significance in novel functional nanostructure materials design, device hybrid materials development, biocatalysis and analysis, drug nano-controlled release etc.Now, with the rapid development of computer science and technology, it obviously can't only depend on traditional experiment, and ignore molecular simulation method for the research of nanostructure materials. Molecular simulation can be used to obtain the system information about structures and properties at an atomic and molecular level with the help of computer numerical simulation.According to the time and length scale, we choose three computation methods: Molecular Mechanics (MM), Molecular Dynamics (MD) and Dissipative Particle Dynamics (DPD) to study the microstructures and behaviors of nanoparticles, LLC and their inorganic/organic hybrid systems. Molecular simulations provide us intuitionistic images and structures information, which have important guiding significance for the nanostructure materials assembly and experiments design. There are three main studies included in this dissertation.1. Molecular simulation studies of nanoparticles systems. In this part, two types of silver nanoparticles (negative and positive charged) are considered. Oleate anions and cetyltrimethyl ammonium bromide cations (CTA⁺) are respectively used as capping agents to fabricate negatively or positively charged nanoparticles. In the simulations of negative charged silver nanoparticles, our studies focus on the structures of hydrophilic and hydrophobic nanoparticles and the phase transfer process between them. First, molecular mechanics method is used to confirm the adsorption modes of oleate anions on the silver surface, then the silver nanoparticles microstructures are simulated by molecular dynamics method, at last dissipative particle dynamics is carried out to model the phase transfer process, the following effects are studied in detail, including the concentration of emulsifier, inorganic salts and shear. As for the simulations of positive charged silver nanoparticles, molecular dynamics method is mainly used to study the microstructures of silver nanoparticles capped by CTA⁺ double layers. These simulation results provide valuable theory support and micro-information to experiments.2. Molecular simulation studies of LLC systems. The phase behaviors of different surfactants, including anion, cation and non-ion amphiphilic molecules are studied by DPD simulations. Combine with experiment phase graph, we can obtain the variety aggregation structures of binary or ternary systems. In the phase behaviors studies for the surfactants and ionic liquid molecules, several effects such as temperature, shear are considered. As a supplement to the experiment, DPD results intuitively show us the mesostructures of these molecules in different concentration range, which provide a new approach to explain experiment phenomena.3. Molecular simulation studies of hybrid systems. The hydrophilic silver naoparticles capped by oleate molecules are doped into surfactant lamellar LLC. The distribution of nanoparticles in LLC, interaction between nanoparticles and LLC, and the stability of the hybrid systems are discussed from the DPD simulation results. Molecular simulation is used to study the relationship between structure and property at a molecular level, which provide a new idea for experiment.Based on the experiments, we set up reasonable molecule's structure model, choose suitable simulation method according to different study purpose and systems. We have obtained some innovative results about system's structures and properties after a serial of computations and analysis, which deep probe into system's physical and chemistry properties from atomic, molecular and mesoscale level.1. In the negative charged silver nanoparticles, oleate molecules' adsorption mode on the silver surface is changed with the solvent polarity. Molecular mechanics results show that the favorable adsorption mode of oleate is transformed from the double bond anchoring in the water to the carboxylate group anchoring in the isooctane solvent. A possible phase transfer mechanism is proposed based on the experiment and DPD results. It is analyzed that the addition of inorganic salts will decrease the critical micelle concentration and solubility of oleate in water, facilitating the adsorption of oleate molecules at the water/oil interface, there is also desorption of oleate molecules from silver nanoparticles surface. At the same time, there is some aggregation between nanoparticles at the interface. The adsorption of carboxylate group of sodium oleate molecules distributed at the interface makes the nanoparticles hydrophobic, and under high speed stirring, the silver particles can be transferred into isooctane eventually with a little increased diameter.2. Molecular dynamics simulations are carried out to investigate the microstructure of a positively charged silver nanoparticle capped by CTA⁺ bilayer on atomic scale. Three nanoparticle systems are constructed and calculated for structural comparison. The simulations show that in all three nanosystems CTA⁺ dense shell is coated on the surface of silver cluster to form a stable nanoparticle with a crystal-like local order within 5 A. The configurations of CTA⁺ molecular chains are also analyzed and it is found that the molecules are significantly curved. The distribution results of distance from N atoms to Ag cluster gravity center are in good agreement with the experiment deductive model.3. With a simple mesoscopic model, the different phase structures of AOT/water and C₁₂EO₄/water system are reproduced by the DPD simulations. The calculation results are in very good agreement with the experimental phase diagram. Water diffusivity results are used to illuminate the anomalous phase behavior in the AOT/water lamellar phase. It is proposed that in the Intermediate-Concentration Regime (ICR) at about 40% concentration, a defective structure, pseudo-reversed hexagonal phase, is formed to evidently decrease the water diffusivity, which might produce some mesophase property changes, such as reduction of the liquid crystalline long-range order. Water diffusivity curves of three lamellar systems as a function of temperature indicate that the pseudo-reversed hexagonal structure in ICR will be partly transformed to a normal lamellar phase structure and enhanced system lamellar ordering after increasing temperature. Therefore, the DPD simulation results could provide us a new insight for better understanding of AOT/water phase behaviors. 4. DPD simulations are carried out to predict the aggregation structures and phase graphs of alkylimidazolium chloride or hexafluorophosphate/decanol/water ternary systems. Although there are some errors, it is a significative try for us to predict phase behaviors using DPD methods.5. The DPD results of hydrophilic silver nanoparticles/LLC hybrids show that there is little influence of AOT and C12EO4 bilayers template on doped silver nanoparticles, which could keep spheral shape. There are also some aggregations of silver nanoparticles when the surfactants concentration is low. However, the doped silver nanoparticles bring much influence on the template structures. Almost in all hybrid systems, the template is curved and deformed, even the bilayer is destroyed, and the system order great decreased. It is found that the doped nanoparticles should be small as possible as can be to enter the water layer of LLC to form stable hybrids. When their sizes are not matched between nanoparticles and water layer, the silver nanoparticles would break or traverse through the surfactant bilayers. Therefore, the match between nanoparticles size and template mesoscale space is the basic factor for this inorganic/organic hybrids' order and stability.Molecular simulation provides us a new method to study nanostructure materials systems, which can obtain some information at atomic, molecular and mesoscale level. As a supplement to experiment, molecular simulation is becoming a new theoretical tool to give much valuable reference information for the study of novel functional nanomaterials. We believe that molecular simulation could accelerate the research and application of nanotechnique in various fields.