(728a) Understanding the Effect of Nanoscopic Pore Structure on Transport in Lyotropic Liquid Crystal Membranes

Lyotropic liquid crystals (LLC) are a class of nanostructured materials that can be modified, assembled and crosslinked into stable membranes with uniform straight pores that can perform solute-specific separations. Pore diameters on the order of 1 nm make LLC membranes well-suited for aqueous separations such as desalination and biorefinement. However, without a clear understanding of mechanisms of transport in these complex self-assembled systems, it is challenging to choose momoners to achieve specific separations goals. To advance our knowledge of transport in these systems, we have studied an experimentally characterized model system using molecular dynamics simulations in order to develop a predictive model for membrane design.

We have created a molecular model which is maximally consistent with experimentally measured structural features and material properties, and examined solute transport through the pores. We have developed methods to simulate the crosslinking mechanism, simulate X-ray diffraction (XRD) patterns and to measure ionic conductivity from atomistic simulations. By matching simulated and experimental 2D WAXS patterns, we have discovered an additional metastable state, mainly characterized by its pore structure, which may form under certain experimental conditions. The first state has a condensed, narrow pore with high radial disorder. Although stable out to 100's of nanoseconds, the XRD pattern does not match experimental results. The second, whose XRD pattern gives a good match to experiment, is noticeably more open with aromatic rings stacked in a parallel displaced conformation. Other theorized pore arrangements are not stable in simulation.

The difference in pore structure affects the mechanism by which solutes diffuse across the membrane, which are likely to significantly affect the performance of membranes. We examine transport of a range of solutes with various charges and 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.