Langmuir Monolayers of Minimal-linker Dimeric Surfactants

Surfactants are ubiquitous, useful compounds of immense practical significance in key industries ranging from drug-delivery to agrochemicals. The performance of these commercially valuable products depends largely on their chemical structure. The objective of the research presented in this thesis is to develop fundamental knowledge about the relationship that exists between the molecular structure of surfactants and the resulting properties of monolayer films formed from these surfactants. An ultimate goal of the project is to predict monolayer properties, including crystallographic and micron-scale structure, phase composition, miscibility and mechanical rigidity from the molecular structure of the surfactant(s) that comprise the monolayer. To achieve these goals, a minimal test system of anionic gemini surfactants referred to in this thesis as Cn-0-Cn compounds have been explored herein. The Cn-0-Cn molecules, which can be prepared with different tail-chain lengths using a tractable synthetic approach, have the shortest possible linker joining the two head-groups. Thus, these compounds are referred to as "minimal linker" compounds. The combination of a compact head-group and custom tail-chain length leads to a family of compounds that are unique amongst gemini surfactants. The shortest tail-chain homolog studied forms non-diffracting, amorphous films at the air-water (A/W) interface like other family of gemini surfacants with a bulkier head-group, whereas, the longer homologs (with n = 14, 18; Figure 0-1) present rare examples of a diffracting monolayer similar to simple fatty acids. The C14-0-C14 homolog also shows monolayer properties similar to a common phospholipid, with a defined phase transition in the classic surface pressure-area isotherms. Further, intermolecular dispersion forces play a major role in the interpretation of the observed effects in pure and binary films of Cn-0-Cn with a representative fluorosurfactant. Overall, the research presented in this thesis will be used to develop a simple, general structure-property relationship model for similar systems in future and expected to broaden the understanding of controlled variations in surfactant structures in order to obtain desirable monolayer properties.

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