Fatty acid contamination and dielectric relaxation in phospholipid vesicle suspensions

Aqueous vesicle or micelle suspensions from various synthetic lecithins or surfactants - most of them purified by a simple ion-exchange procedure in methanol - were investigated, some with ionic admixtures. The dielectric permittivity '(nu) between 5 kHz and 100 MHz was determined by different time-and frequency-domain methods, with attention given to electrode polarization below 1 MHz. Pure ether lecithins (used to reduce hydrolysis during preparation) as well as ester lecithins showed no dielectric dispersion below 10 MHz (Delta' 3). In contrast, even dilute colloidal solutions containing about 1 mol% (with respect to solute) ionic amphiphiles normally exhibited large dielectric dispersion (10 < Delta' < 700), especially with electrolyte present. This low-frequency dispersion is sensitive to vesicle coagulation or fusion. Underlying relaxation mechanisms are discussed, and the main relaxation is shown to be the same as for other charged colloids. This conclusion suggest a new interpretation of measurements, previously reported by other authors, who gave an interpretation in terms of correlated zwitterionic head group orientation in multilamellar lecithin liposomes. Possible effects from traces of impurities in lipids are discussed.