Dissipative Particle Dynamics Simulation On The Self-assembly Of Surfactants In Aqueous Solution

Surfactants because of its unique properties will assemble to different aggregates inaqueous solution, such as micelle, vesicle, liquid crystal and microemulsion. Thedifferent aggregates are widely used in petrochemical industry, cosmetic, biochemistry,foodstuff and drug field. Therefore, the interest in the self-assembly of surfactants isvery high for many years.The aggregation behaviors of surfactants are mainly affected by the hydrophilic andhydrophobic characteristics of surfactants, concentration and temperature. Thus, it isvery important to investigate these influencing factors to know the structural changeof the aggregates. In this paper, the self-assembly of surfactants with differentmolecular structures in aqueous solution are investigated by Dissipative ParticleDynamics (DPD) coarse-grained model simulation. The structures of modelsurfactants are liner chain with different hydrophobic or hydrophilic groups, thebranch structures and special construction. The effects of molecular structure on theself-assembly are obtained by comparing the parameter such as critical micelleconcentration, cluster size and the density distribution of the aggregate structure at themesoscopic level. The research contents and conclusions are as follows:(1) The effects of surfactants’ molecular structures on the formation of micelle wereinvestigated. The obvious effects on the aggregation properties of surfactants causedby the variation of the hydrophobic or hydrophilic groups and the branch structureswere investigated by corresponding structure charts in aqueous solution. Theproperties such as critical micelle concentration (CMC), cluster size and densitydistribution of surfactants were analyzed at the mesoscopic level. Simulation resultsare as follows: for liner surfactants, the micellar are larger and more compact withhydrophobic chain increasing at the same hydrophilic group; the micellar are smallerand there are more water in the micelle with hydrophilic chain increasing at the samehydrophobic group. It is also found that the surfactants with branch in tails are more efficient in forming micelles than linear ones, the micelle are larger and more compact;in contrary, the branch in heads restrain the surfactants aggregation and the micellesare smaller. Additionally, for special surfactants, the micelles of Gemini surfactantsare the largest and most compact; Bola surfactants are most difficult to assemble.(2) The effects of surfactants’ molecular structures on the formation of vesicle wereinvestigated. The properties such as the required concentration, time of vesicleformation and the inner space size of vesicle were investigated by comparing thecorresponding structure charts in aqueous solution. We obtained the followingconclusions: as to liner surfactants if it has same hydrophilic group, it is more apt toform vesicle and the inner space is smaller with hydrophobic chain increasing; if ithas same hydrophobic group, it is more difficult to form vesicle and the inner space islarger with hydrophilic chain increasing; if the hydrophilic interaction is strongest,there is small aggregate in aqueous solution and could not assemble vesicle. Besides,we also found that the surfactants with branch in tails are more efficient in formingvesicle than linear ones, but the branch in heads restrain the formation of vesicle;Gemini surfactant is easy to form vesicle, but Bola is incapable to assemble vesicle.(3) The effects of surfactant and polymer’ hydrophobic and hydrophilic feature onthe interaction between them were investigated. The properties such as the end-to-enddistance, density peak of the surfactant and polymer were investigated by comparingcorresponding structure charts in aqueous solution. Our simulation results show thatthe polymer chain is inclined to gather in the interior of surfactant micelles just likethe solubilization of polymer in a micelle when the polymer attracts with surfactanttail. However, when the polymer attracts with surfactant head, the polymer chaintends to gather in the surface of the aggregate, forming a wrapping structure. Whetherthe polymer attracts with the surfactant head or tail group, the polymer chain in thecomplex becomes more stretched and less compact with the increasing of hydrophilicchain and the decreasing of hydrophobic chain. However, when polymer attracts withsurfactant tail, the polymer chain is more curved. The presence of polymer also resultsin the conformation of surfactants in formed complexes more stretched and thecompactness of surfactant micelle depressed. Specially, the capacity of resisting shear for the two complex structures showed that the complex is difficult to be destroyed ifthe polymer chain locates in the interior of micelle and a solubilized structure isformed.