Abstract
The formation of stable vesicles with a controlled size and high stability is an important matter due to their wide application in pharmaceutical and detergency formulations and as drug delivery vehicles. One can control the size of spontaneously formed vesicles in mixtures of zwitterionic and anionic surfactants by the admixture of small amounts of an amphiphilic copolymer of the PEO-PPO-PEO type. Of course, this effect should depend largely on the molecular architecture of the copolymer employed which was varied systematically in this work, and the temporal evolution of aggregate size and final structure was followed by means of DLS and three main effects could be observed. First the size of the formed vesicles is the larger the higher the molecular weight (MW) of the polymer and the higher the polymer concentration. Secondly the amount of copolymer required to induce long time stability is inversely proportional to the fraction of PEO in the polymer. Finally the architecture for a given MW and PEO/PPO ratio has no effect on the vesicle structure but their structure is directly controlled by the length of the PPO block of the copolymer. Thereby by appropriate choice of type and amount of PEO-PPO-PEO copolymer one can exert comprehensive control over size and stability of unilamellar vesicles.
Highlights
Key points in that context are the ways of preparation and the long-time stability of these unilamellar vesicles as frequently they are metastable structures that are kinetically stabilized and with their properties depending on the preparation history.[39]
As the amphiphilic copolymer is the key component for controlling structure and stability in this study we aimed at elucidating the effect of its architecture on this stabilizing property
In this work we studied the in uence of polymers of the Pluronic type (PEO-PPO surfactants) with different molecular architecture on the vesicle formation in a zwitanionic model system, where vesicle formation takes place via disc-like micelles a er mixing two micellar solutions
Summary
Vesicles have been studied extensively in the past due to their applications in pharmaceutical formulations[1,2,3] and as model systems for biological membranes.[4,5] They are interesting as delivery systems for active agents encapsulated either in their aqueous interior or in their hydrophobic bilayer,[6,7,8,9] and are suitable to interact with biological membranes and to introduce drugs into living cells.[10,11,12,13] Unilamellar vesicles can be prepared in a number of ways from lamellar phases, for instance by external forces like soni cation[14] or extrusion.[15,16,17,18] interesting are spontaneously forming vesicles as they are observed in catanionic[19,20,21,22,23,24,25,26,27,28] and zwitanionic[29,30,31,32,33,34] systems, and in mixtures of anionic surfactants with a cosurfactant,[35] in systems of anionic surfactants, where repulsion between the head groups is screened by the presence of salts,[36,37] or in mixtures of phospholipids with largely different lengths of the alkyl chains.[38]. When vesicles are prepared by mixing two micellar solutions, the vesicle formation takes place via a disc-like micellar state (while below the cmc may proceed via a torus-like state40), where small discs are formed shortly a er mixing stock solutions of both surfactants
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