Study of the micelle-to-vesicle transition and smallest possible vesicle size by temperature-jumps

Formation of bilayer membrane and niosomes by double-tailed polyglyceryl-type nonionic surfactant

Vesicles with synthetic nonionic surfactants are called niosomes or NSVs, and these have been the focus of attention as an alternative to phospholipid liposomes as drug carriers. Especially it is demanded to discover novel niosomal systems with polyol-type nonionic surfactants from the viewpoint of environmental aspects. In this paper, a novel series of double-tailed nonionic surfactants, polyglyceryl dialkyl ethers, (C12)2Gn (n = 2.3, 5.4, 9.4, and 13.8), was synthesized, and its aqueous phase behavior and niosome formation were studied. Because of its double-tailed molecular structure, a lamellar liquid crystalline phase was dominant in the binary phase diagrams for different polyglyceryl chain lengths. The single lamellar liquid crystalline phase region was expanded as the polymerization degree in the hydrophilic moiety increased. Small-angle X-ray scattering spectra revealed the lamellar structure for the (C12)2G2.3 was extremely loose. Molecular packing in the lamellar phase was analyzed except for the (C12)2G2.3 system by using a geometrical model of the lamellar phase. The effective cross-sectional area per molecule at the interface increased extensively as dilution for the (C12)2G13.8 system but remained almost unchanged for the (C12)2G5.4 system. From the molecular parameters, water-holding ability in the lamellar phase was evaluated, and the results indicated strong hydration ability of the long polyglyceryl chain. In a dilute region, micron-sized giant niosomes and small niosomes of about 100 nm were formulated by vortex mixing and ultrasonication, respectively. The multilamellar structure of the small niosomes was confirmed by transmission electron microscopy. Cholesterol addition in the present surfactant lamellar phase induced the phase transition to the liquid ordered phase, which is the same phenomenon in a phospholipid-cholesterol mixture. The stability of niosomes with/without cholesterol was monitored by the niosome size change. In both cases, the niosomes were stable for at least 100 days.