Swelling and dissolution of lamellar phases: Role of bilayer organization

The effect of surfactant bilayer arrangement on the swelling and dissolutio n of a bulk lamellar phase is studied by optical microscopy in penetration- scan experiments, on an AOT/brine system. Contact experiments with powder l amellar samples show classical myelin figures, as expected for systems with a wide L-alpha/L-1 miscibility gap, although a sponge phase (L-3) also for ms at the interface, whose role is discussed. By shearing the lamellar phas e prior to contact, we prepare a metastable state of close-packed multilame llar vesicles (the onion phase). Myelin features are strongly suppressed in contact experiments using the onion phase. The dissolution processes for o nions, which depend on the onion size, are interpreted in terms of the equi librium phase behavior of the system and the size-dependent stability of th e onion phase. When onions dissolve to join the L-3 phase, we observe two r emarkable features: (a) a "raspberry-like" texture at the surface of the LQ phase, suggesting that this is initially formed as an emulsion of discrete droplets; (b) the expulsion of small unswollen remnants or "onion cores" f rom the sponge phase as onions merge with it.