Structural and spectroscopic studies of DMPC/cationic surfactant system

Aqueous suspension of a mixture of 1,2-dimyrilstoyl-sn-glycero-3-phosphocholine (DMPC) and (dodecyloxymethyl)dodecydimethylammonium (DDMDDAC) has been investigated by Fourier transform infrared spectroscopy (FTIR), small angle scattering of synchrotron radiation (SAXS) and dielectric spectroscopy (DS). The introduction of surfactant to DMPC water solution probably induces formation of discoidal (bicellar) phase, as revealed by SAXS studies. The presence of the surfactant has been found to decrease the main phase transition temperature, i.e., from rippled gel to liquid crystalline phase, as indicated by the DS and FTIR results. The small steps in the real part of dielectric permittivity plot vs. temperature can be related to the: planar gel to rippled gel (Lβ′Lβ′ → Pβ′Pβ′) and rippled gel–liquid crystal (Pβ′Pβ′ → Lα) phase transitions, which is supported by the temperature dependencies of the symmetric and anti-symmetric CH2 band frequencies from FTIR measurements.