Molecular organization of LB multilayers of phospholipid and mixed phospholipid/viologen by FTIR spectroscopy

The molecular organization of Langmuir-Blodgett (LB) multilayers of a phospholipid (DMPA) and LB alternate multilayers consisting of mixed monolayers of a viologen (TDMV) and a phospholipid (DMPA), molar ratio of TDMV:DMPA= 1:2, and pure DMPA monolayers have been studied by infrared (IR) transmission and reflection-absorption (RA) spectroscopies. The infrared study shows that the DMPA monolayers are ordered like a crystalline phase where the alkyl chains are almost normally oriented to the substrate surface approaching an all-trans configuration. Also, the stretching mode of the carboxyl group lines up with a large component normal to the substrate, as seems to be the transition dipole moments of phosphate and COC groups. The presence of TDMV molecules in the mixed LB films does not significantly alter the structure of the DMPA films. The TDMV rings are tilted relative to the substrate surface, and located between the phosphate groups of the DMPA molecules of the mixed monolayer and of the subsequent DMPA layer. Thus, the influence of the TDMV molecules on the dehydration of the film is extended to the phospholipid in the mixed monolayer as well as to the one in the pure DMPA layer.