SINGLE-TAIL PHOSPHATES CONTAINING BRANCHED ALKYL CHAINS - SYNTHESIS AND AGGREGATION IN WATER OF A NOVEL CLASS OF VESICLE-FORMING SURFACTANTS

Four disodium monoalkyl phosphates containing branched undecyl and dodecyl groups have been synthesized. The aggregation properties of these surfactants were characterized by transmission electron microscopy of uranyl acetate-stained samples and freeze-fracture replicas. As predicted from a packing parameter analysis, the branching of the alkyl substituents induces aggregation into bilayers. Thermally stable unilamellar vesicles with diameters between 25 and 60 nm were prepared by vigorous stirring in water at room temperature. Lowering of the pH leads to partial protonation of the phosphate headgroup and to less electrostatic repulsion between headgroups and bilayers. This results in larger vesicle diameters (50-100 nm), a higher tendency to aggregate, and reduced thermal stability. Phase penetration experiments show a lamellar boundary phase upon dissolving the crystalline surfactant in water and growth of myelin structures upon increasing the temperature or decreasing the pH. Upon addition of Ca2+, the monoalkyl phosphate vesicles aggregate at [Ca2+] > 0.5 mM and undergo bilayer fusion, leading to stable vesicles (diameters between 100 and 250 nm) above a Ca2+ threshold concentration of 1.0 mM.