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Sterically stabilized liposomes were produced by incorporation a poly (oxyethylene) poly (oxypropylene) block copolymer (Pluronic F-68) into the lipid bilayer using a high-pressure homogenizer. Mean vesicle size of liposomes incorporating Pluronic F-68 greatly decreased from 480 nm to 110 nm with minor change in marker entrapment after passing three times through the high-pressure homogenizer. Liposomes stabilized with Pluronic F-68 were characterized with respect to entrapment efficiency, vesicle size and surface charge. Entrapment efficiency of a marker increased with Pluronic F-68 content, and was maximum at 0.13 (% wt/vol) -Pluronic F-68. In particle size and surface charge, no change was noted with increase in Pluronic F-68 content. Surface charge decreased slightly for liposomes incorporating Pluronic F-68, compared to those coated with Pluronic F-68. Pluronic F-68 is thus distributed between the bilayer and/ or inner phase of liposomes as well as the liposome surface. Liposomes stabilized with Pluronic F-68 were significantly stable in calcium and fetal bovine serum (FBS) solution at 37°C compared with Pluronic F-68-free liposomes. The incorporation of Pluronic F-68 into the bilayer caused significant increase in 31P-NMR line width. Pluronic F-68 is thus shown to stabilize liposomes by restricting phospholipid motion in the bilayer.

Some interfacial chemical and physicochemical properties of various phospholipids which are used as a model biomembrane, as a emulsifier for intravenous fat emulsion and lipid emulsion, and as a main ingredient of liposomes (lipid vesicles) were reviewed. The hydrophile-lipophile balance (HLB) number of various phospholipids was calculated according to quantity of inorganic characters. The relation between the dynamic molecular shape and the HLB of phospholipid for stability of an emulsion was demonstrated.

The degree of lytic action due to interactions between nonionic surfactants and liposomes was determined in terms of affinity for lipid membranes. The solubilization potential of some surfactants toward liposome membranes was estimated based on physicochemical measurements such as fluorometry to detect the release of fluorescent markers (calcein) from liposomes, turbidimetry in suspensions and surface tensiometry. The lytic action of the alkyl poly (oxyethylene) ether (POE) toward lipid membranes was greatest when the alkyl and ethylene oxide chain lengths were 12 and 8, respectively. The lytic action of nonionic surfactants with various hydrophobic portions (hydrogenated caster oil POE ethers, cholesteryl POE ethers, nonylphenyl POE ethers and POE stearate) was found to depend on structure. Maximum activity in each homologous series was 1.2 I.O.B. (Inorganic Organic Balance).The critical micelle concentrations (cmc) of dodecyl POE (5) and POE (10) ether in the absence and presence of liposomes were determined using a surface tension meter. Addition of liposomes caused cmc of the surfactants to increase. Dodecyl POE (5) ether causes marker release at lower concentration than cmc without change in turbidity. Dodecyl POE (5) ether concentration increase further, marker release from liposomes greatly increase minor change in turbidity. For dodecyl POE (10) ether, marker release started from cmc with decrease in turbidity. The lytic action of nonionic surfactants on liposomes would thus appear related to their capacity formixed micelle formation with phospholipids.