(189ah) The Influence of Pore Structure on Transport in Lyotropic Liquid Crystal Membranes

We study a lyotropic liquid crystal (LLC) system atomistically using molecular dynamics simulations. LLC's are a class of nanostructured materials that can be modified, assembled and crosslinked into stable membranes with uniform straight pores which can perform solute specific separations and offer the ability to control pore architecture at the atomic lengthscale. Pore diameters on the order of 1 nm make LLC membranes well-suited for aqueous separations such as desalination and biorefinement. A molecular model which is consistent with experimentally measured structural features and material properties can provide a link between monomer structure and macroscopic separation performance. We have used simulations with experimental data to create such a model. We have matched X-ray diffraction (XRD) patterns generated from equilibrated atomistic molecular dynamics simulation trajectories to experimental 2D wide angle X-ray scattering (WAXS) patterns in order to gain a detailed picture of the LLC membrane nanopores. We learned the density of monomers that pack around pore cylinders. We have evidence to support that aromatic monomer head groups π-π stack on top of each other in a sandwiched, rather than parallel displaced, configuration. We show that the long alkane tails of each monomer pack into hexagonal arrays with short range order. We see that the pores are not idealistic cylinders, but rather consist of a gradient in composition transitioning from hydrophilic cores to hydrophobic tail regions. Using our detailed model of the membrane pores we are able to study the transport of molecules within them with confidence that the chemical environment closely models the real system. We discuss the transport rates and mechanisms of a series of neutral solutes with various hydrodynamic radii. With a clear understanding of mechanisms of transport in these complex self-assembled systems, one can choose monomers to achieve specific separation goals. We can use this information to draw correlations between pore structure and selective preferences. These studies will help guide monomer choice for separation-specific objectives.