The phase behaviour of cetyltrimethylammonium chloride surfactant aqueous solutions at high concentrations: an all-atom molecular dynamics simulation study

We explore the phase behaviour of aqueous solutions of the cetyltrimethyl ammonium chloride (CTAC) surfactant and in particular the transition from the micellar phase (L₁) to the hexagonal columnar phase (H₁) by employing all-atom (AA) molecular dynamics (MD) simulations for six CTAC concentrations in the range of 34.1 wt% to 70.5 wt%, at the temperature of 318 K and pressure of 1 atm. For the concentrations considered, we examine the spontaneous occurrence of the H₁ phase by testing a number of plausible values for the linear density (molecules per unit length) along the cylindrical columns. Using large simulation cells and starting from random initial configurations, the MD simulations demonstrate that the micellar phase occurs for concentrations up to 50.0 wt%, with CTAC molecules self-assembling into a mixture of spherical and rod-like micelles. At even higher concentrations, the system self-organizes into the H₁ phase in accordance with the available experimental data. For the analysis of the MD trajectories, we devise a clustering algorithm based on Voronoi tesselation which enables (a) the thorough characterization of the shape and structure of both molecules and assemblies, and (b) the investigation of the positional and orientational order in the system that are further scrutinised using radial pair correlation functions and X-ray diffraction patterns. Our work paves the way for the investigation of the phase behaviour at high concentrations of other surfactants.