(298b) Evaluation of Nanoporous Lyotropic Liquid Crystal Polymer Membranes for Reverse Osmosis

            Lyotropic liquid
crystal (LLC) assemblies provide an opportunity to make nanoporous polymer
membranes with precise control over chemical and structural features on the
nanometer scale, which is currently lacking in commercial reverse osmosis (RO)
and nanofiltration (NF) membranes available today. These LLC composite
membranes are prepared by casting ca. 10 wt% solution of a taper-shaped, sodium
carboxylate based LLC monomer over a polysulfone ultrafiltration membrane
support. The solvent is evaporated to afford an ordered surfactant assembly
composed of hexagonally packed monodisperse cylindrical nanochannels with a
diameter of ca. 1.1-1.2 nm.  The resulting submicron thick LLC coating is
radically photo-cross-linked under Argon. These LLC membranes have been found
to completely reject water-soluble solutes greater than 1.2 nm in size. The
present work focuses on the evaluation of these supported LLC membranes for
rejection of small salt ions from their aqueous solutions for potential RO
applications. A pure water flux of 0.026 ± 0.015 L m^-2h^-1
is obtained at a transmembrane pressure of 250 psi with these materials, and
the ionic rejection of Na⁺ is 15% and that of Mg²⁺ ions
is 35%. These low rejection values are not entirely unexpected considering the
LLC nanochannel diameter is ca. 1.2 nm. One of the ways to increase salt
rejection is to decrease the effective nanochannel diameter to the
sub-nanometer regime. The reduction in channel diameter can be achieved by the
use of larger multivalent cations such as Ca²⁺ or Eu³⁺
instead of monovalent Na⁺ as the carboxylate counterion. With Eu³⁺
as the LLC monomer counterion, we have been able to achieve a 15% reduction in
the channel diameter to ca. 0.9-1.0 nm. The ionic rejection capabilities of
these membranes will be presented.  Other methods for increasing the small salt
ion rejection capabilities of these LLC membranes and further optimizing their
membrane properties will also be presented.