STRUCTURE AND DYNAMICS OF ION-INDUCED DOMAINS IN FREE AND SUPPORTED MONOLAYERS AND BILAYERS

Ion-induced domain formation was investigated in mixed lipid monolayer s using fluorescence microscopy techniques. Specifically, the mechanis ms and time scales of two-dimensional solid domain growth and disappea rance in fluid monolayers were investigated for mixed monolayers of an ionic and zwitterionic phospholipids, which were either transferred to a solid support or ''free'' at the air-water interface. Experimental observations showed differing behaviors for the two systems above some threshold pressure. Monolayers at the air-water interface showed imme diate formation of dendritic domains in the presence of calcium, with an average domain diameter of 20 mu m. These domains did not undergo f urther evolution with time, and their growth was reversible, as detect ed by reducing (reversing) the pressure to below the threshold value. In contrast, domains in films on a solid support exhibited a two-step growth mechanism; in the initial fast step, round, polydisperse domain s of average size 10 mu m formed within minutes of deposition of the m onolayer. These domains then grew via a much slower process, which app eared to be a diffusion-controlled two-dimensional Ostwald ripening, i n which large domains grow at the expense of smaller ones, and were st ill evolving after 45 h. Growth and disappearance did not follow a rev ersible path, as domain disappearance upon removal of calcium was a un iform shrinkage in diameter, of all the domains, regardless of size, a nd was complete in 20 h. These results show that long-term equilibrati on or ''aging'' effects can occur in motionally restricted monolayers and bilayers.