SOLUBILIZATION OF UNILAMELLAR LIPOSOMES BY BETAINE-TYPE ZWITTERIONIC ANIONIC SURFACTANT SYSTEMS

The mechanisms governing the solubilizing interactions between zwitterionic/anionic mixed surfactant systems at different molar fractions of the zwitterionic surfactant (X(zwitter)) and neutral or electrically charged unilamellar liposomes were investigated. The mixed systems were formed by N-dodecyl-N,N-dimethylbetaine and sodium dodecyl sulfate in the presence of piperazine-1,4-bis-(2-ethanesulfonic acid) buffer at pH 7.20. Unilamellar liposomes formed by egg phosphatidylcholine, in some cases together with stearylamine or phosphatidic acid, were used. Solubilization was detected as a decrease in static light-scattering of liposomes. Two parameters were regarded as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant system (i) saturated the liposomes, Re-sat, and (ii) led to a total solubilization of liposomes, Re-sol. From these parameters the bilayer/aqueous medium surfactant partition coefficients for the saturation (K-sat) and complete bilayer solubilization (K-sol) were determined. When X(zwitter) was 0.40, The Re and K parameters showed a maximum, whereas the critical micellar concentration (CMC) of these systems exhibited a minimum, regardless of the electrical charge of bilayers. Given that the ability of the surfactant systems to saturate or solubilize liposomes is inversely related to the Re-sat and Re-sol parameters, these capacities appear to be directly correlated with the CMC of the mixed systems. The similarity of both K-sat and K-sol (particularly for X(zwitter) = 0.2-0.8) suggests that a similar partition equilibrium governs both the saturation adn the complete solubilization of bilayers, the free surfactant concentration (S-a,S-b), remaining almost constant with similar values to the CMC for each mixed system studied.