Large scale molecular dynamics simulation of self-assembly processes in short and long chain cationic surfactants

Abstract

We report on an investigation of the structural and dynamical properties of n-nonyltrimethylammonium chloride (C₉TAC) and erucyl bis[2-hydroxyethyl]methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomistic description was used, and the time evolution was computed using molecular dynamics. The calculations were performed in collaboration with Silicon Graphics Inc. using the large-scale atomic/molecular massively parallel simulator (LAMMPS) code (version 5.0, CRADA Collaboration, Sandia National Laboratory, USA, 1997) on a range of massively parallel platforms. Simulations were carried out in the isothermal–isobaric (N, P, T) ensemble, and run for up to 3 ns. Simulated systems contained approximately 50 surfactant cations and chloride counterions, surrounded by 3000 water molecules. Starting from different initial configurations (spherical micelle, wormlike micelle) in the case of the C₉TAC molecule, we observe shape transformations on the timescale of nanoseconds, micelle fragmentations, and surfactant–monomer exchange with the surrounding medium. Starting from a random distribution of surfactant molecules in the solution, we observe the mechanism of micelle formation at the molecular level. The mechanism of self-assembly or fragmentation of a micelle is interpreted in terms of generalised classical nucleation theory. Our results indicate that, when these systems are far from equilibrium and at high surfactant concentration, the basic aggregation–fragmentation mechanism is of Smoluchowski type (cluster–cluster coalescence and break up); closer to equilibrium and at lower surfactant concentration, this mechanism appears to follow a Becker–Döring process (stepwise addition or removal of surfactant monomers). In the case of the EMAC molecule, we have characterised two different structures (spherical and cylindrical) of the micelle, and have found that water penetration is not important. We have also studied the effect of the introduction of co-surfactant (salicylate) molecules to the EMAC system; hydrogen bonds between surfactant head groups and co-surfactant molecules were observed to play an important role in stabilising wormlike micelles.

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