Broad frequency range dielectric spectroscopy of aqueous suspensions of phospholipid vesicles

The dielectric properties of aqueous suspensions of unilamelar phospholipid vesicles were measured over a broad frequency range extending from 1 kHz to 1 GHz. The low-frequency measurements (1 kHz to 10 MHz) were performed at Tucumán using a HP 4192 A Impedance Analyzer with a variable spacing cell and Short-Open-Load calibration. The high-frequency spectra (1 MHz to 1 GHz) were obtained at Göttingen using a HP 8753 A Network Analyzer with a reflection cut-off measuring cell. The vesicles were prepared using different concentration mixtures of non-ionic and ionic phospholipids: 1,2 Dimyristoyl-sn-Glycero-3 Phosphocholine and L-α-Phosphatidyl-DL-Glycerol-Dimyristoyl (Na-Salt). Their mean diameter had the value of 116 nm, obtained by means of multiple extrusions. They were suspended in either tridistilled water or an aqueous NaCl solution. The dielectric spectra were described and interpreted using existing theoretical models (for the counterion polarization and the membrane charging together with the Maxwell–Wagner dispersion processes) with the ζ potential as the only adjustable parameter.