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

Abstract

The mechanisms governing the solubilizing interactions between zwitterionic/anionic mixed surfactant svstems at different molar fractions of the zwitterionic surfactant (X,,,tter) 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-l,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 surfactantllipid molar ratios (Re) at which the surfactant system (i) saturated the liposomes, Re,,,, and ( i i ) led to a total solubilization of liposomes, Re,,,. From these parameters the bilayer/aqueous medium surfactant partition coefficients for the saturation (K,,,) and complete bilayer solubilization (KSoI) were determined. When XZwitte, 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,,, and Re,,, parameters, these capacities appear to be directly correlated with the CMC of the mixed systems. The siniilarity of both K,,, and KSoI (particularly for X,,wi,,,, = 0.2-0.8) suggests that a similar partition equilibrium governs both the saturation and the complete solubilization of bilayers, the free surfactant concentration (Sa&), remaining almost constant with similar values to the CMC for each mixed system studied.