Role of chain length and spacers in the organization of long chain Schiff base compounds at air/water and solid/liquid interfaces

Langmuir films of long chain Schiff base compounds: N-(4-fluorobenzylidene)hexadecanamine (compound 1), N-(4-fluorobenzylidene)octadecanamine (compound 2) and 4-{[(4-fluorobenzylidene)amino]methyl}-N-hexadecylaniline (compound 3) have been studied at air/water interface using the characteristic surface pressure-molecular area (π-A) and surface potential-molecular area (ΔV-A) isotherms. Photoelastic modulated Fourier transform infrared spectroscopy has been used to analyze the films of the three compounds at air/water interface. The Langmuir-Blodgett films of these compounds have been characterized using X-ray diffraction that shows layered structures, with interlayer distances of up to 4.4 nm for compound 3. The results show that the layer-to-layer distance can be easily and rationally designed by choosing alkyl chains of different lengths and spacer. Compound 3 packs in an interdigitated bilayer structure with longer lamellar spacing whereas compounds 1 and 2 show a noninterdigitated monolayer structure. Atomistic model calculations have been carried out on the three compounds and the dipole moment values obtained agreed well with the experimental results of surface potential in Langmuir films. Among the three compounds, compound 3 forms stable vesicles spontaneously at room temperature.