(2fk) High-Throughput Characterization of Transport Phenomena through Dynamic Membrane Systems

The growing demand for solute selective separations necessitates the development of materials and processes capable of separating species of similar charge and size. The development of membranes with solute tailored selectivity will require the study of model systems to elucidate the fundamental transport phenomena that govern separation processes. Fulfilling this goal requires the integration of both (1) new materials and (2) systems and processes capable of reducing the time, energy and resources needed to characterize polymer membranes.

Herein, we examine the performance of polymeric ion pumps which use cyclic changes in an external stimulus to promote the selective transport of solutes. Polymeric ion pumps are composite membranes composed of a gate layer situated on top of a sorbent layer. The gate and sorbent layers are designed to undergo changes in permeability and affinity, respectively, in response to an external stimulus. Experimentally, it has been demonstrated that polymeric ion pumps increase the diffusive flux of target solutes by undergoing conformational changes that result in time averaged concentration gradients that are larger than those that result from purely diffusive transport through noninteracting membranes. A mathematical model and numerical solver were developed to investigate the effect of the gate layer resistance on the performance of polymeric ion pumps. Notably, imperfect gate layers lead to solute diffusing back into the feed solution, reducing but not irrevocably hindering membrane performance. In the limit of high sorbent densities, the fraction of solute diffusing into the receiving solution is determined by the relative resistances of the gate and sorbent layers while the total flux of the target solute increases in proportion to the sorbent density.

These analyses are paired with rapid characterization techniques to understand the permeability of pH responsive gate layers. A diafiltration apparatus is developed to rapidly characterize membrane performance under a broad range of feed solution compositions. The apparatus doses a high concentration diafiltrate into a stirred cell to systematically increase the concentration of the retentate. The retentate concentration is continuously monitored by a conductivity probe within the head space of the stirred cell. The apparatus synchronizes the collection of mass, pressure and conductivity data to enable the calculation of the hydraulic permeability and solute permeability coefficient. In doing so, diafiltration experiments increase the amount of information generated and characterize membranes up to 10x faster than traditional filtration experiments. The methodology can replace standard approaches and identify concentration dependencies inherent to charged membrane systems. Additionally, the apparatus can detect nuanced changes in pore wall chemistries and guide the development of future materials.

Future Research Interests: Integration of materials and processes, transport phenomena in polymeric materials, stimuli responsive materials, unsteady state transport phenomena, rapid characterization techniques.