A study of the micellar aggregation of aqueous N,N,N-decyltrimethyl ammonium chloride via extended microsecond-time atomistic molecular dynamics simulation and realistic potential models

Abstract We report on the micellization behavior of the cationic surfactant N,N,N-decyltrimethyl ammonium chloride in water, studied via an explicit-solvent, very long atomistic molecular dynamics simulation. We aim at systematically exploring the related properties of the surfactant solution, focusing at first on their accurate prediction via simulations long enough to allow their precise calculation based on phase space trajectories that fall within the “true thermodynamic equilibrium”. To accomplish this goal, explicit-solvent, atomistic molecular dynamics runs were performed attaining, for the first time, simulation time scales of ≈1.2 μs. According to the results obtained, these novel simulations led to a rigorous estimation and detailed analysis of the properties related to the aggregation process such as the average number of free monomers, the average size and shape of formed micelles, the critical micelle concentration of the solution, the temporal evolution of cluster size distribution and others. It turns out from the analysis of the principal moments of inertia of the micelles that their average shape is oblate ellipsoidal. Also, from the pair radial distribution functions the average cross-sectional structure of the micelles reveals a hydrophobic core composed of the surfactant tail groups, while their outer shell consists from the hydrophilic heads. Moreover, it is found that the chloride counterions form a Stern layer around the micelles and close to the hydrophilic heads, while water molecules were also identified very close to the micelles. The results obtained are in good agreement with quite recent experimental data (previously not available) but differ from those of previous atomistic simulations of similar systems, covering time scales between a few tens and hundreds of ns for the most recent of them. Our results clearly indicate that the required system equilibrium is achieved at about 800 ns and beyond. This key result is in agreement with previous considerations that, reaching true equilibrium of surfactant solutions with respect to their aggregate properties and specifically the cluster size distribution of micelles is extremely time-demanding. It is also shown that self-assembly takes place in three temporal phases, starting from aggregation of monomers into small oligomers, through micellar growth at the expense of monomers and smaller oligomers, to the final formation of stable micelles, with sizes fluctuating slightly around certain values. In this context, the present systematic study of the micellar formation properties aims at providing a stepping stone for a further rigorous computational investigation of several related and interesting molecular processes, such as the encapsulation of specific molecules within micelles; the desalination of water; and even usage of micelles as fundamental chemical reaction intermediates (e.g. micellar reactor catalysts) to mention but a few, all of them falling well within our theoretical-computational research interests.