Molecular Dynamics Study On Tetraalkylammonium Interactions With Dodecyl Sulfate

Because of its special properties,TAADS surfactant system has important applications in industry and daily life.The main advantage of TAADS is that it has better solubility than the traditional SDS with small inorganic ions.As for TBADS with the relatively large counter ions,system will appear the cloud point phenomenon.The experimental work is devoted to the development of the micellar structure of the TAADS system to better explain the experiment and its application.At the same time,the bridged model between TBADS micelles is the basis for understanding the cloud point phenomenon of similar systems.TAADS has a great influence on the stability of foam.Therefore,it is of great significance to understand the microstructure of TAADS at the gas-liquid interface.Although these problems have been studied extensively experimentally,there is still no definite conclusion about the microstructure model.Molecular dynamics simulation is an effective assistant method with the experiment,which can be used to study the structure and dynamics of the system at the molecular level.Therefore,it is a useful tool to solve a series of problems in TAADS related system.In this thesis,we focus on the TAADS system by molecular dynamics method in the following aspects:(1)The properties of TAADS system are studied by molecular dynamics method.First,the thesis gives a study on the behavior of TAADS in aqueous solution.The micelle size and shape,distribution of surfactantpolar head,the distribution of theterminal methyl group,the binding patterns between the counterion and polar head were analyzed and studied.The results show that the radius of micelle has the order of TMADS<TEADS<SDS<TPADS<TBADS.Some methylene and terminal methyl groups are located in the polar shell.The polar head and the counterion are combined in four ways,in which the two TAA+ tails enter into the micelle surface formed by the DS-and thus form a mixed micelle is the most favorable one.The MD model of the system was obtained at the molecular level.In this paper,the model supportes the EPR/TRFQ model proposed in previous experiments,which is verified and refined further in this study.Then,the structural characteristics of TAADS at the gas-liquid interface are studied.The probability distribution of different atoms along z direction,the binding patterns between the counterion and polar head,the adsorption characteristics of the first hydration layer,the tail chain order parameters,and weak hydrogen bonds are analyzed.The results show that the TAA+ counterion has a great influence on the first hydration layer of the polar head of surfactant.The most important thing is that the number of water molecules in the first hydration layer becomes small.At the same time,weak hydrogen bonds are formed between the TAA+ counter ion and the polar head.The polar head and counter ions formed by four ways to form mixed adsorption.The pattern thattwo TAA+ tail chains penetrate into the layer formed by DS-so as to form a mixed adsorption layer isdominant.The characteristics of the adsorption layer on the gas-liquid interface of TAADS are obtained from the microscopic structure.Finally,the similarities and differences between TBADS and TBAC system are studied in detail.In this thsis,we study the degree of dissociation of the two compounds at different temperatures,the binding patterns between the polar head and the counterion,the adsorption free energy of the counterions,and the dynamical behavior and interaction between the micelles.It is found that the degree of dissociation of TBAC is higher than that of TBADS.The effect of temperature on the degree of dissociation is small.The configuration and charge of the polar head are affected the binding patterns.At the same time,the bridge model between TBADS micelles was studied and its structure was obtained from the microscopic level.The model of two layers of TBA+ counterions betweentwo DS-micelles is the most stable one.(2)The configurational entropy change of surfactantin the micellization process is studied by QH method.The detailed components include translational entropychange,rotational entropy change,vibrational entropy change are analyzed.The origin of entropy in the process of micellization is explained in detail.Thetranslational entropy change of surfactant molecules in the micellization process is very small.This phenomenon can explain the experimental findings that micelles exchange surfactant molecules with the surrounding environment.The increase of entropy in the process of micellization comes from the increase of the entropy of water molecules and the decrease of the molecular entropy of the surfactant.Among them,the increase of water molecular entropyincreasement is dominant.At the same time,the paper introduces the basic principle of HSMD and the programming of the the method.This makes HSMD method achieve with Gromacs trajectory.The results of SDS are calculated and compared with the results calculated by QH.This part of the study will give the foundation for the future research of complex systems and the exploration of new methods.