Coarse-grained Force Field Simulations For Surfactant Systems

With the rapid advance of computer and molecular simulation,molecular simulation is developing to larger space and time scales.Normally, all-atom molecular dynamics simulation describes physicalphenomena occuring in nanoseconds or nanometers. As a result,coarse-grained(CG) force field has emerged and become a new researchfrontier. In this work, the application of CG force field in the surfactantsystem was studied based on molecular dynamics simulation. Two systemswere studied:1) surfactans on microemulsion of oil in water and2)surfactants and polymers on water solusion surfaces. These works providetheoretical guidance and support for in-depth study and understanding theproperties and structures of surfactants at the interfaces.Microemulsion is a thermodynamically stable system withpolydispersity which is widely used in daily life and industrial production.Molecular dynamics simulation was used to study the formation ofmicroemulsion particles and the effect of surfactant. It was conducive topromoting the application of microemulsion in various fields. Simulationsystem contained n-dodecane (C12H26), sodium dodecyl sulfate (SDS) andwater. The interactions between these molecules were described by MartiniCG force field. Twelve simulation boxes containing C12H26and SDS withdifferent concentrations were used in simulation running for100ns inisothermal-isobaric ensemble (NPT) simulation. Simulation results showedthat phase separation occured rapidly without surfactants, and wasaccompanied with significant decreases of potential energies. For the systems with surfactants, it tended to form stable microemulsion particles.Formation of microemulsion particles are that C12H26wrapped by SDSfirstly gathered into small droplets, then small droplets aggregated to bigdroplets. The big droplets were stable due to electrostatic repulsion. SDSleaded microemulsion particles with specific structure that C12H26werewrapped by SDS in the outer layer, lipophilic group of SDS was toward thecenter of the droplet, and hydrophilic group was far away from the center ofthe droplet. The relationship between the number of clusters and timeindicated that initial stage of the formation of microemulsion particles wasexpressed as the second order reaction approximately. According to thesecondary order reaction rate equation, rate constants was obtained. Basedon Arrhenius equation, we got the rate constants at different temperatures,and calculated the average activation energy14.6kJ/mol. This work showsthe great advantage of CG force field, but also provides a basis for furtheroptimization of the force field and in-depth study of microemulsificationprocess.Mixing polymer and surfactants is widely studied. The ratio and typeof polymer and surfactants are adjusted to stabilize different interfaces. It isparticularly important to study the properties and structures of surfactantsand polymer in interface. CG models and molecular dynamics simulationwere used for the research of structure of surfactants and polymer ingas-liquid surface and effect on surface tension. SDK force field providedthe parameters of SDS and water molecules. Based on the SDK force field,the parameters of polyallylamine hydrochloride (PAH) were obtained byfitting the radial distribution function (RDF), density, radius of gyrationfrom all-atom simulation which parameters were provided by the TEAMforce field. Dynamics simulation results indicated that the surface tensiondecreased with with the increasing number of SDS when PAH did not existon the surface. The surface tension remained constant when micelle formedin the system. The structure was similar to the system without PAH when aPAH existed on the surface, and increasing the number of SDS on the surface leaded to a slight increase in the remaining number of SDS on thesurface after100ns dynamics simulation, thereby reducing the surfacetension slightly. Strong electrostatic interaction between SDS and PAHcould cause increase of the number of SDS on the surface, and surfacetension was greatly reduced compared with the system without PAH whenthree PAH existed. The surface tension was greatly reduced, and rod-likemicelles formed. There were more SDS on the surface when six PAHexisted. But surface tension was slightly higher with less SDS due to PAHlack of surface activity. The remaining number of SDS on the surfaceincreased significantly with the increasing number of SDS, and surfacetension decreased rapidly. Different surface tension results and structurewere obtained by changing the number of surfactant and polymer in system,and this work provided theoretical guidance and support for theexperimental method.