Molecular Dynamics Study on the Shape Transitions of Micelle Formation

Abstract

Surfactants are amphiphilic molecules having hydrophilic head and hy- drophobic tail groups. Due to the amphiphilc nature, surfactants self as- semble into a wide range of spectrum of mesoscopic structures which are stable and distinct. Understanding the factors which control the forma- tion of these mesoscopic structures is a great field of interests since these distinct structures are important for their applications in cosmetic formula- tions, detergents, food technology and so on. This thesis focus on under- standing how concentration of surfactants play a role in their phase change processes. Phase transformations of surfactants-water systems are stud- ied in the presence and absence of co-surfactants using all atom molec- ular dynamics simulations. Surfactants self-assemble into micelles which change their shapes at different concentrations of surfactants. Shape tran- sitions of micelles by changing the surfactant (behenyl trimethyl ammo- nium chloride, BTMAC) to co-surfactant (stearyl alcohol, SA) ratio. BTMAC and SA self-assemble into small patches of aggregates at lower BTMAC concentration, which transform into cylindrical micelles due to the increase in concentration of the surfactants. At the highest BTMAC concentration in our study, where no SA is present, cylindrical micelles transform into spherical ones. The total solvent accessible surface areas of BTMAC and SA are calculated which demonstrate the aggregation process by de- hydrating water molecules. The radial distribution functions of beads along the surfactants with respect to the center of mass of each micelles indicate respective symmetries of different micellar shapes. Other structural prop- erties such as head to tail distance and angle distributions between the head, the middle beads and the tail show that the single chains are more stretched in the small patches of aggregates and in the cylindrical micelles than in the spherical micelles where no SA is present. Our simulations show that concentration of the surfactants plays an important role in con- trolling the shape transitions of micelles.