Complexity in a Simple Self-Assembling System: Lecithin-Water–Ethanol Mixtures Exhibit a Re-Entrant Phase Transition and a Vesicle–Micelle Transition (VMT) on Heating

We report surprising results for the self-assembly of lecithin (a common phospholipid) in water-ethanol mixtures. Lecithin forms vesicles (∼100 nm diameter) in water. These vesicles are transformed into small micelles (∼5 nm diameter) by a variety of destabilizing agents such as single-tailed surfactants and alcohols. In a surfactant-induced vesicle-micelle transition (VMT), vesicles steadily convert to micelles upon adding the surfactant─thereby, the turbidity of the solution drops monotonically. Instead, when an alcohol like ethanol is added to lecithin vesicles, we find a new, distinctive pattern in phase behavior as the ethanol fraction <i>f</i>_eth in water is increased. The turbidity first decreases (from <i>f</i>_eth = 0 to 37%), then rises sharply (<i>f</i>_eth = 37 to 50%), and then eventually decreases again (<i>f</i>_eth > 55%). Concomitant with the turbidity rise, the vesicles separate into two phases around <i>f</i>_eth = 50% before a single phase reappears at higher <i>f</i>_eth─in other words, there is a "re-entrant" phase transition from 1-phase to 2-phase and back to 1-phase with increasing <i>f</i>_eth. Vesicles near the phase boundary (∼<i>f</i>_eth = 45%) also show a VMT upon heating. Similar patterns are seen with other alcohols such as methanol and propanol. We ascribe these complex trends to the dual role played by alcohols: (a) first, alcohols reduce the propensity for flat lipid bilayers to bend and form closed spherical vesicles; and (b) second, alcohols diminish the tendency of lipids to self-assemble in the solvent mixture. At low alcohol fractions, (a) dominates, causing the initially unilamellar vesicles to grow into multilamellar vesicles (MLVs), which eventually phase-separate. Thereafter, (b) dominates, and the vesicles convert into micelles. Support for our hypothesis comes from scattering (SANS) and microscopy (cryo-TEM). Thus, we have uncovered a general paradigm for lipid self-assembly in solvent mixtures, and this may even have physiological relevance.