The dielectric permittivity spectrum of aqueous colloidal phospholipid solutions between 1 kHz and 60 GHz.

The dielectric permittivity spectrum between 1 kHz and 60 GHz of aqueous colloidal solutions of predominantly zwitterionic phospholipids is presented from results of previous and recent measurements. It shows three dispersion/loss regions around 22 GHz. 80 MHz and below 40 MHz (30 degrees C) which are attributed to rotational diffusion of the water molecules and of the zwitterionic phosphorylcholine groups, and to limited translational diffusion of ionic lipid molecules and/or its counterions. respectively. Merely a few mole percent of ionic lipids cause comparatively large dielectric dispersion. Ignoring the fact that such impurities may be present in zwitterionic phospholipid compounds, which have not been especially purified, this has led to misinterpretation of the dielectric spectrum in the past. An approximate quantitative description of the measured spectra is given for vesicle solutions with only very small additional low-molecular-weight salt content. It reproduces the sensitive dependence of the ionic lipid-induced dielectric dispersion (step height and frequency) on various parameters: phospholipid vesicle size. ionic lipid content, as well as the self-diffusion coefficient of the ionic lipid molecules and of its counterions, moving within the phospholipid bilayers or on their surface, respectively.