Effects of tether length on the behavior of amphiphilic bent-core molecules at water surfaces

Alignment layers for bulk liquid crystalline phases can be created with monolayers formed by Langmuir-Schaefer techniques. Monolayer stability is a function of the propensity of the component molecule to effectively pack at a water interface; this propensity is enhanced when the molecule has an appropriate balance of hydrophilicity and hydrophobicity and the desired liquid crystalline order, as well as other structural factors. Our experiments show that molecules based on a bent-core with one hydrophilic and one hydrophobic end can form stable monolayers that act as effective alignment layers. However, the stable monolayers only form when the hydrophilic end has a sufficiently short chain. Molecular simulations carried out for both dilute concentrations (1 bent-core molecule) and high concentrations (25 bent-core molecules) on a water surface elucidate this behavior. The hydrophilic group acts to tether the molecule to the water surface, with a tether floppiness that depends on the tether length. At dilute concentrations, these molecules lay flat on the water surface (the molecular long axis approximately parallel to the surface), and the tether floppiness has little consequence. However, at high concentrations, the molecules pack with orientations approximately perpendicular to the surface; they stand upright on the tether, and the floppier tether leads to wobbly legs that cause large lateral fluctuations in the molecular positions and reduce monolayer stability.