Molecular films from the polymerizable lipid ethyl morpholine pentacosadiynoic amide

Abstract

A new polymerizable lipid, ethyl morpholine pentacosadiynoic amide (EMPDA), was synthesized. Monomeric and polymeric films formed from this lipid at the air–water interface were investigated by microfluorescence filmbalance techniques. Below 25 °C, monomeric EMPDA exhibited a pronounced fluid–solid coexistence with a transition enthalpy of ∼100 kJ/mol at room temperature. The coexistence pressure was found to increase with decreasing pH; below pH 4.5 no fluid–solid coexistence was measurable. Microfluorescence revealed that large achiral single‐crystalline domains grow in the coexistence regime. Upon ultraviolet exposure only the lipids in the crystalline domains were found to polymerize, indicating that the polymerization is a topochemical reaction. The polymerized domains exhibited a pronounced intrinsic fluorescence with a strong excitation anisotropy which is correlated with the macroscopic crystal axises. Polymerized films transferred onto wafers with thermally grown silicon oxide were investigated by atomic force microscopy. In the polymeric domains, stripelike corrugations were found with an orientation parallel to the fluorescence polarization. At high magnifications molecular resolution images were obtained. The lattice constants of the crystalline arrangement of the lipids are a=0.51±0.03 nm, b=0.48±0.06 nm, α=74±7°. Lateral force images revealed that the friction between tip and sample is dependent on the orientation of the polymer backbone with respect to the fast scan direction.