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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2015

Month Degree Awarded

February

First Advisor

Harry Bermudez

Subject Categories

Analytical Chemistry | Fluid Dynamics | Materials Chemistry | Physical Chemistry | Polymer Chemistry

Abstract

Room-temperature ionic liquids (ILs) exhibit a unique set of properties, leading to opportunities for numerous applications such as green solvents, batteries and lubricants. Their properties can be greatly tuned and controlled by addition of surfactants. It is therefore critical to obtain a better understanding of the aggregation and interfacial behavior of surfactants within ILs.

Firstly, the phase diagram and aggregation isotherms of surfactants in several distinct ILs were investigated by solubility and tensiometry. A connection between solubility of the surfactant and the physical properties of the underlying ionic liquid was established. We found that the interfacial energy was crucial in determining aggregation behavior while electrostatic interactions could be largely ignored. This finding could provide the general prediction of solubility and the first indication of how to choose ILs with desired properties. Secondly, this study was extended to include mixtures of cationic and anionic surfactants where our data further demonstrated near-complete charge screening. The critical micelle concentration (CMC) and mixed micelle composition were found to be close to ideal behavior. This so-called charge screening in IL is in sharp contrast to that of aqueous solution and can be explained by Debye theory. Moreover, our pulsed-field gradient spin-echo (PGSE)-NMR data confirmed the existence of micelle formation and showed evidence that the IL anion partially incorporates into surfactant micelles, resulting in slower diffusion when the surfactant concentration is above the CMC. Lastly, through use of X-ray photoelectron spectroscopy (XPS), the roles of surfactant alkyl chain length, concentration, and probing depth on interfacial properties were investigated. Depending on the chain length and concentration, surfactants can alter the IL interface to varying extents, highlighting a simple route to manipulate interfacial properties. XPS is further demonstrated to be a direct measurement of the surface activity and ion-exchange behavior in surfactant-ionic liquid system.

The results here give insight into the interaction between solutes and IL solvents and the nature of self-assembly of surfactants in ILs. This study could significantly broaden the potential application of ionic liquids such as novel solvents for protein storage and electrolytes for Li-ion batteries.