| A surfactant is a substance that contains both hydrophobic and hydrophilic moieties in a certain molecule.Because surfactants have the special properties of such parents(lipophilic and hydrophilic),such compounds have unique properties that are different from some other organics.Surfactants have a high ability to reduce surface tension and are active on the surface and interface.They can spontaneously aggregate into various ordered molecular assemblies at critical micelle concentrations,and this property is widely used.Used in industrial production and daily life.At present,a large number of experiments have been carried out to prove the properties of surfactants,including critical micelle concentration,number of aggregated molecules,size and rheological properties of surfactant aggregates,etc.,but most of these studies have remained in macroscopic experiments.Research,and these studies have focused on the macroscopic effects of salts on surfactant solutions.These studies speculate and explore the mechanism of interaction between inorganic salts and surfactant molecules through changes in the physicochemical properties of the system.If we use molecular simulation to study these macroscopic physical and chemical phenomena,and to simulate the theoretical mechanism of these processes at the molecular level,it will have great guiding significance for our understanding and application.In this thesis,the microstructural and microscopic properties of self-assembled systems were studied by molecular dynamics methods to modify the morphology of surfactants under the influence of p H and temperature changes.The work of this thesis mainly includes the following aspects:(1)Molecular dynamics simulation was used to study the structural transformation of cetyl ammonium bromide/salicylate(CTAB/Na Sal)under p H induction.It was found that when the p H was equal to 7,the p H was equal to 2 and the p H was equal to 0,the structure of the aggregate formed by the CTAB/Na Sal system changed under the three different p H conditions,and the micelle structure was rigid rod-shaped micelles.,spherical micelles and worm-like micelles.The changes of micelle structure were analyzed by analyzing the kinetic properties of the rms displacement and radial distribution function of the molecule.(2)The molecular dynamics simulation system was used to study the effect of temperature on the change of the state of cetyl ammonium bromide/salicylic acid(CTAB/Sal)aggregates in an acidic environment.The simulation results show that under high temperature and high acidity,the CTAB/Sal system will form spherical micelles.By simulating the molecular dynamics of the rms shift and radial distribution function of the molecule,we can find that the molecular motion can be significantly increased at elevated temperatures,leading to the interaction between CTA~+and salicylic acid molecules.The force is reduced.In particular,the anion diffusion around the micelles of the aggregates leads to an increase in the mutual repulsive force between the polar head groups,so that the structure of the aggregate micelles becomes spherical micelles at a temperature of 80℃in a strongly acidic environment.The simulation proves that the molecular dynamics of the micellar system can be studied by molecular dynamics method,which explains the influence of temperature and acidity and alkalinity on the aggregation properties of surfactant aggregates.(3)The oil-water interface self-assembly mechanism of SDBS/BMAB system was studied by molecular dynamics simulation.The self-assembly mechanism was explored by simulating the interface adsorption configuration of SDBS/BMAB system,and the adsorption process of SDBS and BMAB on the interface was given.The simulation results of surface tension agree well with the experimental data.According to the simulation results,the self-assembly structure of SDBS/BMAB system on the interface was proposed.At the same time,density curve,radial distribution function and monolayer film diffusion coefficient were calculated.Based on the calculation results,the effect of interfacial self-assembly between SDBS and BMAB on reducing interfacial tension was proposed and explored. |
