Molecular-level Mixing Behavior, Structure and Interactions of Mixed Hydrogenated-Fluorinated Surfactant Langmuir Monolayers

Abstract

This thesis constitutes findings from several projects designed with an ultimate goal of understanding the mixing behavior, molecular structures and interactions that play out in mixtures of perfluorotetradecanoic acid with hydrogenated surfactants at the micro and molecular-level. In particular, this thesis explores the importance of chemical structure of surfactants in controlling properties of surfactant mixtures. Notably, the difference in surfactant chain length and head group are key chemical identities which control mixing behavior. Therefore, mixtures of perfluorotetradecanoic acid (PF; CF3(CF2)12COOH) with nonadecanoic acid (ND; CH3(CH2)17COOH) were explored in the context of closely related mixed surfactant systems to understand how the difference in chain length controls miscibility, and to validate the miscibility rule proposed for these types of mixed systems. In addition, the impact of surfactant head group was explored by switching the carboxylic functional group on the nonadecanoic acid to an alcohol functional group (nonadecanol; NDOH; CH3(CH2)18OH). Both the fatty acid and fatty alcohol were immiscible with PF in monolayers, and formed phase-separated domains. These studies were all carried out using Langmuir trough compression isotherms, Brewster angle microscopy imaging and Atomic force microscopy imaging to characterize the monolayer films both at air-water and solid-air interfaces. However, during the course of these studies, it was noted that these characterization techniques do not provide direct information about film structure at the molecular length scale. In recognition of this shortcoming, a molecular-level study of several mixed surfactant systems at the air-water interface was performed using the synchrotron-based liquid surface X-ray scattering techniques Grazing incidence X-ray diffraction (GIXD), specular X-ray reflectivity (XR) and X-ray fluorescence near total reflection (XFNTR). A benchmark mixed film system comprised of arachidic acid (AA, C19H39COOH) and PF was adopted for this study. In all cases, the two components in the mixed film behaved entirely independently of film composition, which is exactly the expected result for a fully phase-separated, immiscible system. The mixed film systems explored here were monomeric fatty acids and were chosen to understand how the molecular-level interaction of the films contribute to phase-separation. However, with the emerging interest in dimeric compounds (gemini surfactants), the mixing behavior of anionic gemini surfactants (Ace(12)-2-Ace(12) and Ace(18)-2-Ace(18)) with PF was explored in comparison to the monomeric hydrogenated-PF mixed film systems. The gemini surfactant Ace(12)-2-Ace(12) was chosen to understand the effect and role dimeric hydrogenated surfactants play in miscibility with fluorinated surfactants (PF). The Ace(12)-2-Ace(12)-PF mixed film system was found to be miscible at the molecular-level with Ace(12)-2-Ace(12) showing an amorphous state while PF was highly crystalline. The mixing behavior of the longer chain variant of the gemini surfactant Ace(18)-2-Ace(18) with PF was also explored to understand the chain length effect in such systems. Similar miscibility trends in the short chain gemini surfactant with PF were observed. These studies were carried out at the molecular-level using GIXD and XR. Finally, the potential of the gemini surfactant as a chelating agent was explored by investigating its association with Fe3+ at the air-water interface using XFNTR.