Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Douglas L. Gin

Second Advisor

Richard D. Noble

Abstract

The in situ cross-linking of reactive amphiphiles (i.e., surfactants) that self-organize in the presence of a solvent into a type I bicontinuous cubic (QI) lyotropic liquid crystal (LLC) phase have significant potential as a new type of membrane material. These QI-phase networks contain completely uniform pores on the nanometer scale and tunable pore chemistry that could offer significant improvements over conventional nanofiltration (NF) and reverse osmosis (RO) membranes. Despite their significant potential, cross-linked QI-phase networks have never been processed into thin-films necessary to demonstrate commercially relevant productivity and limited characterization has been performed on QI-phase membranes to understand their fundamental performance characteristics. The overall objective of this thesis research is to design and develop a method or approach to fabricate thin-film composite (TFC) QI membranes and perform additional fundamental filtration studies to better understand the transport characteristics of QI-phase membranes.

In this thesis research, a new fundamental processing approach is presented to fabricate thin-films of cross-linked QI-phase materials. In this approach, the water typically used for LLC phase formation is replaced with the low volatility, polar, organic solvent glycerol. Due to the low vapor pressure of glycerol, it does readily evaporate during thin-film processing. This allows LLC monomer and glycerol to be dissolved in a volatile casting solvent and applied to a porous support by solution-casting. Subsequent removal of the volatile casting solvent by gentle heating does not result in any appreciable evaporative loss of glycerol necessary to form the desired QI-phase. The applied thin-film can then be photo-cross-linked at the required temperature to create a TFC QI membrane.

Water filtration experiments on TFC QI membranes demonstrate they have similar rejection performance to previous QI-phase membranes, except the flux is ca. 10 times greater due to the much thinner active layer. Overall, TFC QI membranes have performance characteristics in-between conventional NF and RO membranes. They reject neutral solutes like a porous NF membrane and reject monovalent and divalent salts comparable to a RO membrane at brackish water feed concentrations. Additional water filtration experiments revealed that anion-exchange has a significant impact on the flux of TFC QI membranes. The flux of TFC QI membranes can be drastically tuned by exposing the membrane to feeds containing different anions with little to no change in the rejection performance.

The unexpected performance changes in TFC QI membrane with anion-exchange prompted the design and synthesis of new, cross-linkable, zwitterionic amphiphiles in which ionexchange is no longer possible. A new zwitterionic LLC monomer system has been developed that contains benzimidazolium cationic headgroups with covalently tethered anionic sulfonate groups. These new, cross-linkable, zwitterionic amphiphiles are capable of forming LLC phases in water and glycerol and may readily afford cross-linked LLC assemblies. Zwitterionic LLC assemblies could offer a number of advantages over conventional cationic or anionic LLC netwoks for a number of applications, including water filtration.

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