(113d) Molecular Dynamics Simulations of Structure and Dynamics of Cylindrical Micelles and Micelle-Nanoparticle Complexes

Coarse-grained Non-equilibrium Molecular Dynamics (MD) simulations are performed to study the structure alignment effects of a cationic cylindrical micelle in shear flow. Micelles undergo cycles of flow alignment, elongation and tumbling when subjected to shear flow. The onset of flow alignment occurs at Weissenberg number (Wi) » 1 where Wi is the product of shear rate and angular relaxation time. The frequency of tumbling has a power law relationship with Wi with an exponent of 0.7. Simulations of relaxation of micelle from its elongated state provide a linear relationship between the micelle stretching energy calculated from pair potential energy and the length. This provides a constant force F needed to elongate the micelle to be about 653 kJ mol^-1 nm^-1. Micelle scission was observed at a very high shear rate corresponding to Wi = 76.2 where the micelle elongated by ~3.5nm and crossed an energy barrier of ~3700 kJ/mol.

Self-assembly of metal nanoparticles with surfactant micelles is known to produce stable nanogels with rich rheological and optical properties. Coarse Grained MD simulations are performed to explore the molecular mechanism underlying this self-assembly process. It is observed that charged nanoparticles prefer a vesicular structure with a double layer of surfactants whereas uncharged, hydrophobic particles prefer a monolayer with the tail groups adsorbing onto the surface of the particle. Interaction of cylindrical micelles with these supramolecular structures results in the formation of stable junctions primarily though the opening up of the end cap of the micelle. The energetics of the bridging process and the effect of flow shear on their stability will be discussed.

References

1)      A. V. Sangwai  & R. Sureshkumar, Langmuir, 27(11), 2011.

2)      A. V. Sangwai  & R. Sureshkumar, Langmuir, 28(2), 2012.

3)      T. Cong, S. N. Wani, P.A Paynter and R. Sureshkumar, Applied Physics letters, 99 043112 (2011)