Selective rejection of a water-soluble nerve agent stimulant using a nanoporous lyotropic liquid crystal–butyl rubber vapor barrier material: Evidence for a molecular size-discrimination mechanism

The selective vapor rejection performance of a “breathable”, cross-linked lyotropic liquid crystal (LLC)–butyl rubber (BR) composite membrane with a type I bicontinuous cubic (QI) morphology was studied for a water-soluble chemical nerve agent simulant, dimethyl methylphosphonate (DMMP). The thickness-normalized DMMP vapor flux of this nanoporous QI-phase LLC–BR membrane, which has a 3D interconnected water nanopore network, was found to be (2.5 ± 0.4) × 101 g m−2 day−1 μm with a driving force of 0.333 torr at 25 °C. This DMMP vapor penetration level is the same low level as that observed for dense, cross-linked BR films under the same testing conditions. This LLC–BR material also has a very high thickness-normalized water vapor flux ((5.9 ± 0.3) × 103 g m−2 day−1 μm at 16.87 torr, the normal partial pressure of water at 25 °C), whereas cross-linked BR has effectively no water vapor permeation. Consequently, the LLC–BR material was found to have two-orders of magnitude higher water/DMMP molar selectivity than cross-linked pure BR and commercial BR gloves. The QI-phase LLC–BR material also exhibited more than one order of magnitude higher water/DMMP molar selectivity than previously reported HII-phase LLC–BR composites with 1D water nanopores. The percent rejection of DMMP vapor was measured to be 99.91% in a cross-flow system for the QI-phase LLC–BR membrane. Since DMMP is highly water-soluble (614 g (L H2O)−1), the separation selectivity in this LLC–BR material cannot be due to low solubility of DMMP in the aqueous LLC nanopores (i.e., a solution-diffusion mechanism). Instead, the mechanism of DMMP rejection in this LLC–BR membrane appears to be molecular size-discrimination through the water nanopore network, which appears to have an effective gap size of ≤0.57 nm. This effective pore size is slightly smaller than that previously observed for the analogous BR-free QI-phase LLC material (0.75 nm) when used in high-pressure liquid water filtrations.