Influence of Structural Modifications on the Self-Assembly of Pyridine-Based Double-Tailed Surfactants in Thin Films

Amphiphilic molecules are attracting growing interest for diverse applications in drug delivery, advanced materials, and interfacial technologies. Their structural variations offer key insights into how molecular design influences self-assembly, interfacial behavior, and functional performance. This study examined the interfacial behavior of dihexadecylpyridine-2,6-dicarboxylate (EDTS)-based surfactants as a function of various structural modifications, including headgroup quaternization (EDTS⁺), zinc coordination to constrain the head-tail junction (Zn-EDTS), alteration of the head-tail linkage (ester vs imine, IDTS), along with a comparison with a conventional single-tail analogue (ECS). Gas-phase density functional theory calculations revealed V-shaped geometries with varying degrees of compactness for the investigated architectures. For Langmuir monolayer studies, surface pressure-area (π-A) isotherms demonstrated distinct differences in the interfacial behaviors of EDTS compared to their single-tailed analogues. These findings were supported by Brewster angle microscopy, surface potential-area (ΔV-A) isotherms and atomic force microscopy measurements of films fabricated at air-water and air-solid interfaces, showing that subtle architectural modifications significantly influence packing density and domain formation. Additionally, the effect of mixing with C16-chain amphiphiles, specifically their carboxylic acid, alcohol, and trimethylammonium variants, on EDTS monolayers was studied. Headgroup compatibility in this class of surfactants was evaluated by isolating tail-tail packing contributions from the excess Gibbs free energy of mixing (ΔG_mix^ex) for EDTS-ECS.