Atomic force microscopic observation of local supramolecular structure in phosphatidic acid Langmuir-Blodgett films

The Langmuir-Blodgett bilayer films of phosphatidic acid(DPPA) have been investigated by using atomic force microscopy in both large and nano-scale. Large scale AFM image shows that the bilayer of DPPA is a plane film with some defects which enable us to detect the thickness of bilayer by a trace of section analysis across the defects structure, Molecularly resolved images show that the polar head groups (phosphate groups) are packed in an ordered hexagonal organization with a long range orientational and positional ordering, Intermolecular hydrogen bonding interactions between adjacent phosphate groups are first visualized and local supramolecular structures are first observed in polar region of DPPA bilayer. Some phosphate groups in polar region linked with each other through intermolecular hydrogen bonding interaction to form local supramolecular structure with a diameter of 1.5 nm in size.