Size and Stability of Catanionic Vesicles:  Effects of Formation Path, Sonication, and Aging

Abstract

The majority of vesicles prepared from dispersions of phospholipids in water are nonequilibrium structures. Thermodynamically stable vesicles must display distinctive physicochemical properties. To investigate the size and stability properties of the vesicles formed in the catanionic mixture of single-tailed (SDS) and double-tailed (DDAD) surfactants, we evaluate the influence of the formation path on size, polydispersity, and equilibration time, by means of microscopy methods and water NMR self-diffusion. The different paths used involve mixing of solutions, mixing of solids, or dilution of preequilibrated concentrated samples. Water self-diffusion and microscopy show that all of the methods are able to yield vesicle solutions, but their size and polydispersity present some differences. Under the effect of a short sonication time, all solutions gain identical macroscopic appearance and with time they apparently equilibrate to a common state (as probed by water self-diffusion data). This stable state coincides with one of the previous formation pathssmixing of solutions.

DNA-lipid systems: A physical chemistry study

“…Essentially all of these studies have concerned vesicles which are not thermodynamically stable. In our group, we have been investigating for some time vesicles in mixed cationic + anionic surfactant systems which form spontaneously, have an apparently unlimited life-time and are believed to be thermodynamically stable (21,35). The vesicles are polydisperse with sizes in the approximate range of 0.05 to 0.5 µm although there are also a few larger ones; the larger ones can be visualized directly by light microscopy while the smaller ones have been conveniently studied by cryogenic transmission electron microscopy (36,37).…”

Physicochemical and toxicological properties of novel amino acid-based amphiphiles and their spontaneously formed catanionic vesicles

“…see reviews [10,11]). In this context, mixtures of oppositely charged surfactants have emerged in the last 10-15 years as an exciting new class of vesicle-forming systems (catanionic vesicles), as described further [12][13][14][15][16].…”

“…In addition, results are also presented here for novel catanionic vesicles prepared from 12Lys12/DTAB [24] and 12Lys12/12Ser [25] mixtures. From a colloidal standpoint, catanionic vesicles bear several advantages as compared to conventional liposomes: (i) they form spontaneously upon mixing of individual co-solute micellar solutions, requiring no high-energy methods; (ii) they have long-term stability and in many cases exist as equilibrium structures (thus, indefinitely stable); (iii) they often present chain melting transitions well below room temperature; (iv) they offer the possibility of charge regulation, due to variation of the surfactant mixing ratio [9,[12][13][14][15]26,27].…”

Kinetic pathway of spontaneous vesicle formation

“…The first observation is consistent with the very low solubility of lecithin and the second with our finding that, among the disc-like intermediates, coalescence is rim-to-rim and not face-to-face. In contrast, relaxation to equilibrium may occur more rapidly for vesicles more prone to fusion or fission and/or formed by more soluble amphiphiles, such as some mixtures of anionic and cationic surfactants [9,25,26]. The ability to ripen or fuse thus plays an important role in determining whether vesicles reach equilibrium or represent kinetically trapped, metastable states of very long lifetime.…”

“…The ability to ripen or fuse thus plays an important role in determining whether vesicles reach equilibrium or represent kinetically trapped, metastable states of very long lifetime. This may give a new physicochemical basis for their classification [25]. (Note that a coarsening mechanism leading to partial equilibration could be mediated by mixed micelles in the coexistence region or simple bile salt micelles, if present.…”