Lyotropic self-assembly mechanism of T-shaped polyphilic molecules

Abstract

We present coarse-grained molecular dynamics simulations of mixtures of a model of T-shaped polyphilic bolaamphiphile liquid crystal molecules with a solvent. Based on the premise that the most important features of the liquid structure stem from the balance between the close range repulsions and the strong directional forces typical of hydrogen bonding and association, we have employed a coarse-graining approach that simplifies and minimises the attractions present in the system. The model consists of six fused rigid spheres, where the two end spheres have a significant attraction amongst themselves while the rest are repulsive in nature. A weakly self-attracting lateral chain consisting of fully flexible tangently bonded spheres is attached to one of the central spheres. Thus, the T-shaped molecule is composed of three mutually repulsive segments which allow the pure system to self-assemble into a liquid crystalline honeycomb columnar phase. The stability of the columnar phase is probed by the sequential addition of a solvent that has affinity with only one of the segments of the molecule. Our coarse-graining technique allows us to observe dynamically not only the 1st level nanoscale segregation but also the 2nd level reorganization which leads to the formation of replicated periodic structures. It is seen how this latter structuring takes place at times which are an order of magnitude longer than the former, and by itself explains the practical limitations of studying self-assembly with more detailed atomistic models. Mobility coefficients (related to diffusion constants), order parameters and direct visualization of the configurations are used to present a phase diagram for the solvated system in the whole concentration range. At low solvent density, the solvent solvates the honeycomb structure, but does not alter the order significantly. At modest volume fractions of solvent, the solvent mostly segregates into a distinct phase, while the T-shaped molecules retain a phase with columnar structure. At very large solvent concentrations, the T-shaped molecules form structureless aggregates, while at the infinite dilution limit present themselves as dimers and monomers.