Self-assembly of ionic surfactants and formation of mesostructures

Self-assembly of ionic surfactants resulting in micelle formation is studied using off-lattice Monte Carlo simulation in two dimensions (2D). We find that in addition to the Lennard-Jones (LJ) interaction, which acts among all of the particles, a screened Coulomb interaction, acting only among the surfactant heads, makes the geometric arrangement of micelles very different compared to that for neutral surfactants at the same density. At low concentrations, ionic surfactants produce spherical micelles and exhibit intermicellar ordering arising out of the longer-range repulsion of the surfactant head groups. For larger concentrations, they tend to form bi-layers. Motivated by recent experiments on preparation of mesoporous sieves through surfactant-templating routes, we then extend these studies to micellization of ionic surfactants in the presence of other neutral host particles. The morphologies of the surfactant–host composites are studied as functions of host particle densities, sizes, and their interaction strengths. Assuming the presence of micelles as quenched disorder, we obtain a qualitative understanding of the local ordering of the host particles in terms of a dimensionless density parameter from the known liquid–vapour phase diagram in 2D of a LJ fluid.