Spontaneous Vesicles and other Solution Structures in Catanionic Mixtures
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SPONTANEOUS VESICLES AND OTHER SOLUTION STRUCTURES IN CATANIONIC MIXTURES
ERIC W. KALER*, KATHLEEN L. HERRINGTON*, AND JOSEPH A. N. ZASADZINSKI** *Department of Chemical Engineering, University of Delaware, Newark, DE 19716 **Department of Chemical and Nuclear Engineering and Materials Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106 ABSTRACT We have prepared spontaneous, single-walled, equilibrium vesicles of controlled size and surface charge from aqueous mixtures of simple, commercially available, single-tailed cationic and anionic surfactants. We believe vesicle formation results from the production of anion-cation surfactant pairs which then act as double-tailed zwitterionic surfactants. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time. These catanionic vesicles are stable for periods as long as several years and appear to be the equilibrium form of aggregation. Here we review the phase behavior and structural studies of several such mixtures, with particular focus on the effect of surfactant tail lengths on size and location of the vesicle phase in the appropriate phase diagram. The approach to equilibrium is also discussed.
INTRODUCTION More than a quarter of a century ago, Bangharn et al. [1] showed that phospholipids dispersed in water formed closed, multibilayer aggregates capable of separating an internal compartment from the bulk solution. As bilayers are relatively impermeable to many ions and non-electrolytes, it became simple to create small domains of different composition within such a bilayer aggregate or liposome, and mimic many of the properties that nature has designed into cells and organelles [2-5]. As a result, there has been great effort devoted to using liposomes as model membranes [2] and, more recently, as drug delivery systems [3] and microreactors for specialized chemistry [4]. Vesicles, [6] which are single-bilayer closed shells that encapsulate an aqueous interior, have become the preferred structure for use in most applications [2-7]. While vesicles often form spontaneously in vivo, they have only rarely been observed to form in vitro without the input of considerable mechanical energy or elaborate chemical treatments [7]. Hence, a variety of methods have been developed to create unilamellar vesicles, i.e. single bilayer compartments with sizes ranging from about 20 nm to more than 20 mim [6,7]. The stability of such vesicles is limited because the multilamellar liposome is the equilibrium form of aggregation under these conditions. Hence, unilamellar vesicles formed from multilamellar liposomes are metastable and eventually will revert to multilamellar liposomes. This reversion is invariably accompanied by a release of the vesicle contents and failure of the vesicle carriers. The stability of mechanically formed vesicles against aggregation can be enhanced in many of the same ways that other coll
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