Using Molecular Mechanics Force Field Method To Study Surfactant Systems

In this thesis, molecular mechanics force field method as the core was used to studythe complex surfactant systems. The research topics involve the development ofTEAM force field, modification of TEAM force filed in surfactant systems, andapplication of simulation in surfactant systems. The main contents of this work are asfollows:1. To establish a transferable and accurate force field (TEAM force field), we startedoriginally from Quantum Mechanics (QM) calculation, studying differentfunctional groups’ effect to chemical environment. By fitting the QM data, wefixed on the valence parameters and atom partial charges for TEAM force field.To capture the Van der Waals parameters, we calculated pure liquid properties(Enthalpy of vaporization and Density) to fit to the experimental values. Thus theTEAM force field is characterized as transferability, extensibility, accuracy andmodularized. It behaves excenllent in describing simple pure liquid systems.2. Surfactant is one kind of molecule which has both hydrophilic and hydrophobicgroup and it plays a key role in daily life and industry such as cleaning, foodprocessing, cosmetics and petroleum abstraction. The surfactant behaviors at theair-water interface, expecially the ability to reduce the surface tension affects itsapplication, which draws a lot of attraction. Our goal is to build a series of forcefield to accurately predict the surfactant systems. Our force field was based onInitial TEAM force field because of its good performance in pure systems. Inorder to make up the additional interactions brought by polarized water, we adjustthe VDW parameters between certain atoms of surfactant and water. Uing themodified TEAM force field, we can predict the surface tension under equilibriumsurface concentration, which validates the accuracy of the force field. This part of work is also the fundermental base for further study of surfactant systems.3. Thin film consisting of surfactant/water/surfactant is also an important field bothin industrial application and scientific research. The use of experimentalmeasurements is not suitable for resolving all the structural details of thin film inmolecular level because of the requirement of high resolution measurementcapability and the instability of the film. However, molecular simulation isadvantageous as a powerful tool. The nanometer size of NBF is perfectly suitedfor common all-atom force field molecular simulations. In this work, we studiedthe thin films by performing simulated mechanical stretches based on MDsimulations. By analyzing structural and dynamic properties of the film, weproposed rupture mechanisms for different surfactant systems. Also, we simulatedmixed surfactant systems as an exploring part, in which we found beterperformance of mixed systems than pure ones. We compared our simulationresults with the measured suds volumes and found interesting correlationsbetween them.4. Predicting suds property is very popular in industrial application. We developpeda method to directly fit the experimental suds property form simulated data usingsimple function and got quite good coorelatin. This method can be utilized tojudge whether a new surfactant has the good potential quality using simulationbefore it is synthesized, which make it very helpful for industrial production.