Orientation and organization of Langmuir-Blodgett mixed films consisting of a phospholipid and a viologen studied by infrared spectroscopy

Infrared transmission and reflection-absorption spectroscopies were used to determine the orientation of Langmuir-Blodgett (LB) mixed films consisting of a phospholipid (DMPA) and a viologen (TMV) in a ratio of TMV:DMPA = 1:2 , For comparison, the orientation of a DMPA monolayer was also studied. No important changes were observed for the organization of the DMPA monolayer when the TMV molecules were incorporated. The alkyl chains of both compound s adopted a hexagonal configuration where these alkyl tails were fully exte nded with all-trans conformation and an average tilt angle with respect to the support close to 20 degrees. Further information about the hydrated CO groups and the orientation of the phospholipid headgroups was obtained from the CO stretching vibration mode and from the 1300-1000 cm(-1) spectrum re gion, respectively. Finally, the average tilt angle for the plane of the bi pyridyl group with respect to the substrate surface was estimated close to 10 degrees from UV-visible absorption spectroscopy with plane polarized lig ht (s and p) under different angles of incidence. Moreover, the bipyridyl g roup was located encapsulated between the headgroups of the phospholipid an d the solid surface.