The partition of methylene blue between negatively charged phospholipid membrane and the bulk aqueous phase was measured by using visible spectroscopy in very dilute aqueous membrane suspensions, 0.05-0.3 mg of dried phospholipids in 1 mL of buffer solution. Under these experimental conditions, the turbidities of liposome systems and the overlapping of the electrical double layers of different liposomes were negligible. The positively charged probe, methylene blue, forms dimers in membrane phase, resulting in a reduction of the absorbance intensity. The surface potential of the membranes (liposomes) was calculated from the partition coefficient of the dye between the membrane and the bulk phase. The effects of charge density of the membrane and of the ionic strength on the surface potential were also studied.

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