Force Field Development And Application Of Coarse Grained Molecular Dynamics

Coarse-grained molecular dynamics simulation is a simulation method between atomic simulation andmesoscopic simulation. Comparing to all-atom simulation, coarse-grained molecular dynamics simulationcould undertake large-scale dynamic simulation. Comparing to mesoscopic simulation, coarse-grainedmolecular dynamics simulation reflect more microscopic information. Therefore, to a certain extent,coarse-grained molecular dynamics simulation could make up their shortcomings of both all-atom andmesoscopic simulations, and build a "bridge" between them. However, due to short developing history theincompletes force field largely limits it broad application. In this article, a new coarse-grained force fieldsadopting for "non-ionic surfactant-oil-water-solid" system is developed, and the aggregation and imbibitionbehavior of nonionic surfactants is investigated.The coarse grained force field is built as following. First, the bond length and bond angle are obtainedfrom average and standard deviation of corresponding bond and angle of all-atomic molecular dynamicssimulation. Second, the original non-bond interaction parameter applying to “non-ion surfactant-oil-water”system is calculated from every molecular segment via NAMD, these parameters are optimized byexperimental solution density and interfacial tension, and then the rationality of developed force field isfurther validated by bulk and interfacial properties of corresponding solution. Third, according to thewettability of solution on solid surface, the non-bond parameters between solid and “Non-ionsurfactant-oil-water” are obtained.In this article, some applications of developed coarse grained force field are conducted. First, theaggregation behavior of nonionic surfactants in aqueous is investigated, researched results are as follow:(1)the configuration of aggregations changes from sphere to hexagonal and lamellar structure with theincrease of concentration;(2) the length of hydrophobic and hydrophilic surfactant segments haveimportant influence on the aggregation numbers of surfactant, and the increase of hydrophobic tail anddecrease of hydrophilic head favor the increase of aggregation numbers. Second, the spontaneous imbibition is studied, calculated results are as follow:(1) the wettability of capillary have importantinfluence on imbibition behavior, and the increased water wettability will promote the imbibition of watersolution in capillary;(2) addition of surfactants in solutions would promote imbibition and displacement ofcrude oil in oil wettability capillary.