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 polyethoxylated 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 the nomenclature adopted by Lichtenberg, three parameters were considered as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant (a) saturated the liposomes Re(sat); (b) resulted in a 50% solubilization of vesicles Re50% and (c) led to a total solubilization of liposomes Re(sol). These parameters, corresponded to the Re at which light scattering starts to decrease, reaches 50% of the original value and shows no further decrease. It is noteworthy that the Re(sat) parameter decreases as the EO contents or the surfactant critical micellar concentration (CMC) increases. However, the Re50% and the Re(sol) parameters show the lowest values for the surfactant with 12.5 EO units in its molecular structure regardless of the electrical charge of the lipid bilayers. As a consequence, these last parameters are not linearly correlated with the CMC of these surfactants. The CMC values of the surfactant/lipid systems at 0.5 mM lipid concentration corresponded in all cases to the surfactant concentration at which liposomes were saturated by surfactants (S(sat)).