Abstract

The cationic dye Methylene Blue included in the internal aqueous microphase of dioctadecyldimethylammonium chloride (DODAC) vesicle shows the typical monomer-dimer equilibrium observed in homogenous aqueous solutions, Using a simple spectrophotometric method, values of 54 000 +/- 1300 M-1 cm(-1) and 3860 +/- 130 M-1 have been obtained for the extinction coefficient of Methylene Blue monomer at 680 nm (epsilon(680)(M)) and the equilibrium constant (K), respectively, in homogeneous aqueous solution at 20 degrees C. At this wavelength dimer absorbance is negligible. Leakage of Methylene Blue from the inner aqueous pseudophase can be described in terms of Fick's law. The dependence of the release rate on monomer concentration, the value of the permeability constant (1.09 x 10(-9) cm s(-1) at 20.3 degrees C) and the dependence of the release rate on NaCl addition suggest that Methylene Blue diffuses across the lipidic bilayer of the vesicle as a monomer.