Dielectric loss spectroscopy at fatty acid salt multilayers

Dielectric loss spectroscopy (DLS) was performed at compact samples and lam ellary organized Langmuir-Blodgett (LB) films from various fatty acid salts . Previous thermoanalytical measurements at compact samples revealed the ap pearance of two different phase transition temperatures; the lower one is r elated to the acid the second one to the acid salt molecules. In spite of i ll defined electrical contacts with the film the characteristic DLS frequen cies obtained from about 100 nm thick multilayer films are similar to those recorded from bulk samples. No significant variations of frequencies were found changing the counter ions. Besides conductivity influence at low freq uencies we found two relaxations related to the mobility of the dipolar car boxylat-metal group at about 10(2) and 10(4) Hz. One of these frequencies i s related to the rotation around the chain axis. The strength of this relax ation increases significantly with increasing the sample temperature above 105 degrees C. This temperature is connected with a structural phase transi tion observed by X-ray reflectometry. In case of Pb-stearate the results of the dielectric measurements help to interprete this structural change as a transition from an orthorhombic into a free-rotator phase. The uncorrelate d rotation of molecules around their molecular axes initiates a much increa sed relaxation strength at the carboxylat-metal sites.