Minimal Linker Gemini Surfactant Langmuir Monolayers: (Tail) Size Matters

Langmuir monolayers prepared from a new synthetic "minimal linker" variant of carboxylic acid-terminated Gemini surfactants (C₁₄-0-C₁₄) have been prepared, and the properties of the monolayers are investigated in the context of the surfactant's molecular structure. Pure C₁₄-0-C₁₄ monolayers exhibit interfacial features including phase-coexistence, micrometer-scale fractal domain morphology, and molecular-level packing and crystallographic film structures at ambient temperatures, which have parallels with common two-tailed phospholipid monolayers (e.g., dipalmitoylphosphatidylcholine) reported in the literature. Film characteristics are evaluated and compared with other tail-chain minimal-linker variants (C₁₂, C₁₈ alkyl tails) reported previously, which formed either liquid or fatty acid-like monolayers, and with this work, we have identified the full structural range of tail lengths in this system required to span the classically observed Langmuir monolayer forms under ambient conditions. Finally, the effects of the benchmark perfluorocarbon, perfluorotetradecanoic acid, on the interfacial properties of C₁₄-0-C₁₄ system are explored as a function of the composition of the mixed perfluorotetradecanoic acid-C₁₄-0-C₁₄ mixed monolayers. Different compositions of the mixed monolayer show phase-separation at the air-water interface using Brewster Angle Microscopy.