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

We report on an investigation of the structural and dynamical properties of n-nonyltrimethylammonium chloride (C(9)TAC) and erucyl bis[2-hydroxyethyl] methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomis tic description was used, and the time evolution was computed using molecul ar dynamics. The calculations were performed in collaboration with Silicon Graphics Inc. using the large-scale atomic/molecular massively parallel sim ulator (LAMMPS) code (version 5.0, CRADA Collaboration, Sandia National Lab oratory, USA, 1997) on a range of massively parallel platforms. Simulations were carried out in the isothermal-isobaric (N, P, T) ensemble, and run fo r up to 3 ns. Simulated systems contained approximately 50 surfactant catio ns and chloride counterions, surrounded by 3000 water molecules. Starting f rom different initial configurations (spherical micelle, wormlike micelle) in the case of the C(9)TAC molecule, we observe shape transformations on th e timescale of nanoseconds, micelle fragmentations, and surfactant-monomer exchange with the surrounding medium. Starting from a random distribution o f surfactant molecules in the solution, we observe the mechanism of micelle formation at the molecular level. The mechanism of self-assembly or fragme ntation of a micelle is interpreted in terms of generalised classical nucle ation theory. Our results indicate that, when these systems are far from eq uilibrium and at high surfactant concentration, the basic aggregation-fragm entation mechanism is of Smoluchowski type (cluster-cluster coalescence and break up); closer to equilibrium and at lower surfactant concentration, th is mechanism appears to follow a Becker-Doring process (stepwise addition o r removal of surfactant monomers). In the case of the EMAC molecule, we hav e characterised two different structures (spherical and cylindrical) of the micelle, and have found that water penetration is not important. We have a lso studied the effect of the introduction of co-surfactant (salicylate) mo lecules to the EMAC system; hydrogen bonds between surfactant head groups a nd co-surfactant molecules were observed to play an important role in stabi lising wormlike micelles.