Exploration of Metallo-Gemini Surfactants in Thin Films

Engineering molecular architectures enables the systematic investigation of how metal coordination and ligand geometry influence the fundamental interfacial properties of surfactants at the aqueous surface. Here, we report hexadecylpyridine-n-carboxylate metallo-gemini surfactants (MGSs) in which the organic linker is replaced by a coordinated metal cation, allowing for controlled self-assembly and tuning of interfacial organization. The precise chemical structure of the pyridine-n-carboxylate isomers (n = 2, 3, and 4; PAR, NAR, and INAR), together with their metal coordination modes, strongly influences the packing efficiency, line tension, and domain morphology of the corresponding MGS monolayers (Zn-PAGS, Zn-/Zr-NAGS, and Zn-INAGS) at the air-water and air-solid interfaces. Metal coordination is further expected to control the effective headgroup area, leading to more expanded interfacial films for the MGSs compared to corresponding metal-free Py-n-CO₂R systems. Herein, metal ions Zn(II) and Zr(IV) with closed-shell electronic configuration are selected. The ability of PAR, NAR, and INAR to generate distinct coordination modes with Zn(II) arises from their constitutional isomerism, resulting in different metal-ligand binding geometries. For NAR, the use of Zr(IV), which adopts a higher coordination number than Zn(II), enables modulation of interligand separation. Density functional theory calculations show that MGSs adopt V-shaped structures, exhibiting wider interchain angles and larger methyl-to-methyl distances in the case of Zn-NAGS. The π-A isotherms of the MGSs reveal a larger lift-off area than that of hexadecylpyridine-n-carboxylate, a metal-free analogue. The observed approximately doubled molecular area at zero surface pressure indicates that the MGSs behave as dimers relative to their conventional counterparts. Brewster angle microscopy and atomic force microscopy analyses reveal domain morphologies that are strongly dependent on the chemical structure of the MGSs, where Zn-PAGS forms more compact domains with higher line tension whereas Zn-NAGS exhibits confined domain expansion relative to NAR, consistent with reduced cooperative van der Waals interactions in the 1,3-ligand configuration.