SOLUBILIZATION OF UNILAMELLAR PHOSPHOLIPID-BILAYERS BY NONIONIC SURFACTANTS

The mechanisms governing the solubilization of neutral or electrically charged unilamellar liposomes by a series of octylphenol polyethoxyla ted surfactants (average of ethylene oxide units between 8.5 and 20.0) were investigated. Solubilization was detected as a decrease in light -scattering of liposome suspensions. To this end, in accordance with t he nomenclature adopted by Lichtenberg, three parameters were consider ed as corresponding to the effective surfactant/lipid molar ratios (Re ) at which the surfactant (a) saturated the liposomes Re(sat); (b) res ulted in a 50% solubilization of vesicles Re50% and (c) led to a total solubilization of liposomes Re(sol). These parameters, corresponded t o the Re at which light scattering starts to decrease, reaches 50% of the original value and shows no further decrease. It is noteworthy tha t the Re(sat) parameter decreases as the EO contents or the surfactant critical micellar concentration (CMC) increases. However, the Re50% a nd the Re(sol) parameters show the lowest values for the surfactant wi th 12.5 EO units in its molecular structure regardless of the electric al charge of the lipid bilayers. As a consequence, these last paramete rs are not linearly correlated with the CMC of these surfactants. The CMC values of the surfactant/lipid systems at 0.5 mM lipid concentrati on corresponded in all cases to the surfactant concentration at which liposomes were saturated by surfactants (S(sat)).