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Membrane separation of fluid mixtures using nanoporous materials is currently a subject of great interest. The performance of membrane is strongly depended on the nanoscale chemical and structural properties of the membrane, which is essential to obtain atomic-level insights into the structure-property relation of membrane and its impact on the confined fluid behaviors under operation conditions.

In the course of our research, we have performed (1) grand canonical Monte Carlo (GCMC) simulations to study the adsorption behavior of pure water and methanol in a copolymer poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO) matrix and (2) grand canonical molecular dynamics (GCMD) simulations to study both transport and adsorption behaviors in the PEO-co-PPO polymers, with particular attention to the effects of PEO-to-PPO ratio of x_PEO = 0%, 30%, 50%, 70%, and 100% and temperature on transport and adsorption behaviors of liquids in the polymers. GCMC simulation results show that (i) methanol is preferentially adsorbed over water in the PEO-PPO polymer for a wide range of chemical potentials and temperatures, due to stronger dispersion interactions; (ii) the cooperative bonding mechanism results in the continuous pore filling of methanol in the PEO-PPO polymers, as compared to capillary condensation of water in the PEO-PPO polymers; (iii) the negative values of solvation force reflect the hydrophilic interactions of water and methanol with PEO dominated polymers, as compared to the positive values obtained in PPO dominated polymers.

GCMD simulations of water-methanol mixture (64mol%-36mol%, determined by separate GCMC simulations in the bulk) through block copolymers with different PEO-to-PPO ratios show that (i) methanol has larger flux than water when diffusing through different polymer matrixes; (ii) the transport of methanol achieves to the maximal flux value at the PEO-PPO (50%-50%) polymer, while the transport of water appears to be less dependent on the composition of polymers. Thus, it is expected that water-methanol mixture can be greatly separated at x_PEO = 50% and T=298 K. The transport of the liquid mixture through the copolymers is controlled by competitive interactions between liquid-liquid interactions and liquid-polymer interactions. When fluid molecules diffuse through nanoporous polymers, hydrophilic segments in the polymer attract fluids to enter the pore, but discourage fluids to diffuse inside the pore due to the energy penalty by reforming and breaking hydrogen bonds between fluids and polymers. Hydrophobic segments have opposite effects for liquids to enter the pore and to diffuse inside the pore. Thus, optimization of block polymers by rationalizing hydrophobic and hydrophilic ratio and by controlling their space distributions plays an important role in enhancing the selectivity and productivity of membrane separation.

This PRF Type G grant was my first grant. Two Ph.D students Xiang Yu (3-summer-month in 2008) and Chao Zhao (12-month in 2009-2010) have worked on this membrane separation project. The knowledge derived from this work have delivered 7 published papers, 10 talks at ACS conference in 2009-2010, and 1 talk at AIChE conference in 2008. Simulation methodology and preliminary results developed in this PRF-supported work help me to receive NSF CAREER Award 2010.