Molecular Dynamics Study of Sodium Octanoate Self-assembly in Parallel-Wall Confinements

Abstract

The practical applications of surfactant solutions in confined geometries require a thorough understanding of the system properties. Coarse-grained simulation techniques are useful for studying the qualitative behaviour of these systems, whereas the atomistic molecular dynamics (MD) technique can be used to obtain a molecular-level description. In this work, canonical MD simulations were performed using GROMACS version 4.0 to investigate the self-assembling behaviour of sodium octanoate (SO) confined between two parallel walls. In particular, the effects of gap size, wall type, and surfactant concentrations on the morphology of the surfactant aggregates were studied to gain in-depth knowledge of the system. The simulation results reveal that the morphology of the micelles formed between two parallel walls are affected not only by the gap size and surfactant concentration, but also by the nature and characteristics of the confining walls. With the graphite walls, most octanoate molecules are adsorbed at lower concentrations, but they form micellar aggregates as the surfactant concentration increases. Spherical micelles were found in the larger gaps (4 nm and 5 nm) but not in the smaller gap (3 nm), and the micellar shape also changes with increasing surfactant concentration. SO forms bilayer structures instead of spherical micelles between two silica walls. Interestingly, in the hydrophilic silica confinement, the orientation of these bilayers changes with gap sizes, whereas in the hydrophobic silica confinement, these bilayers remain perpendicular to the wall in all cases. Potentials of mean force between different molecules and atomic groups were determined under different conditions in order to develop a better understanding of the simulation results. It reveals, the presence of the confinement can alter the intermolecular interactions among the surfactant molecules, which, in turn, directly affects the self-assembling process, particularly the size and shape of the aggregates. Indeed, the formation of bilayers in silica wall confinement, as opposed to spherical micelles in graphite confinement, is caused by the enhanced electrostatic interactions between the charged atoms in the solution. The results of this study are expected to provide further insight into the self-assembling behaviour of confined surfactant systems, and may ultimately lead to the development of novel nanomaterials.