(617f) Lyotropic Liquid Crystal Templated Stimuli-Responsive, Super-Absorbent Hydrogel Draw Agents for Forward Osmosis

Forward osmosis (FO) shows great promise as an energy efficient method of purifying water or concentrating aqueous liquids. A high osmotic pressure draw agent is used to draw water across an FO membrane. Appropriate draw agents must both induce a high amount of water flux across the membrane and enable easy separation of the draw agent and water. The separation enables the water to be used and the draw agent to be reused. Lyotropic liquid crystal (LLC) templated copolymer hydrogels composed of poly(sodium acrylate) (PSA) and poly(n-isopropyl acrylamide) (PNIPAM)  exhibit both of these characteristics. PSA contributes high water absorbency, increasing the flux across the membrane and the overall swelling amount. The stimuli-responsive material, PNIPAM allows for the water to be released and the draw agent reused. PNIPAM is a well known thermoresponsive material that undergoes a coil-to-globule transition at 32˚C, causing absorbed water to be expelled from the material. LLC templates enhance the rate and degree of swelling and release. LLCs are a self-assembled mixture of surfactant and water that can form various concentration dependent nanoscale mesophases. The inherently rapid rate of photopolymerization in these ordered assemblies allows for development of specific nanostructures. These  nanostructures facilitate increased water absorption, higher flux rates across a membrane, and higher rates of water release than an isotropic material of identical chemical composition. The structure also increases the dynamic range of stimuli-responsive behavior, allowing the material to swell to greater degrees and to also release over eight times the water compared to an isotropic hydrogel. These structured materials are ideal FO draw agents due to their large water absorbencies as well as their rapid water release upon application of low (<50˚C) temperatures and could allow for dramatic improvements in forward osmosis processes.