(509a) The Temporal Evolution of Vesicle Dispersions

The stability of vesicle dispersions against phase separation in a gravitational field is a fundamental issue in the industry for a variety of surfactant-based formulations. The time required for phase separation appears to be strongly influenced by two main factors: the procedure of synthesis of the vesicle dispersion, and the time-scales of the various interactions between the microstructures. For example, if the vesicle size that results immediately after the synthesis is small, the settling or rise velocity of the vesicle is small, leading to long separation times. On the other hand, a shift of the microstructure towards larger-sized liposomes/aggregates would result in faster separation. In our preliminary effort to investigate the processes that lead to phase separation, we have studied the temporal evolution of a vesicle dispersion prepared by the ethanolic injection of lipids in an aqueous buffer, followed by extrusion. Various techniques were used to characterize the dispersion including dynamic light scattering, surface forces apparatus (SFA) measurements, cryo-TEM and optical microscopy. The key observations were: First, the d-spacing observed between bilayers in multilamellar vesicles is less than that expected from the SFA force measurements between the bilayers; an additional compressive pressure in the dispersions is required to resolve the spacing. Second, the cryo-TEM images indicate a preponderance of vesicles with two bilayers (doublets). Third, although the different observed shapes in the images, such as deflated unilamellar vesicles, curled up vesicles and stomatocytes, indicate that osmotic deflation may be taking place leading to these structures, no osmotic pressure gradients were intentionally imposed during the formation. Finally, over time, the number of deflated structures and stomatocytes decreases, while the number of multilamellar structures increases. In this paper, we attempt to explain some of these observations. In order to account for the observed d-spacing, we propose that it may be the encapsulation process of a closed vesicle by an open vesicle that leads to an additional ?hugging' pressure due to the line tension at the opening, thus bringing the bilayers closer than expected from the surface force measurements. It would also explain the formation of doublets, as well as multilayer liposomes. Alternatively, single vesicles could deflate due to a finite spontaneous curvature [e.g. Helfrich and Deuling, Biophys. J., 16, 861-868, 1976; Hubert et al., Langmuir, 16, 8973-8979, 2000] eventually closing up to form the observed doublets. These two mechanisms are compared with the help of cryo-TEM images of the time-evolution of vesicle dispersions under different conditions.