Self-Assembly Beyond Water—Forming Lipid Vesicles and Vesicle-Gels in Polar Organic Solvents

Surfactants added at low concentrations can thicken or gel water by two routes. In one case, they can self-assemble into long, flexible chains called wormlike micelles. In a second case, surfactants can form vesicles, which can close-pack and fill the volume. Laundry products such as fabric softeners are known to contain close-packed vesicles. Vesicles are nanoscale containers where a bilayer of surfactants encloses an aqueous core. They are extensively used in drug delivery. While vesicles in water are well-known, there are <i>no reports of vesicles in polar organic solvents</i> like glycerol, formamide, or ethylene glycol (EG). Here, we demonstrate the formation of nanoscale vesicles in the above solvents using the common phospholipid lecithin (derived from soy). Our samples are mixtures of lecithin and the solvent with no additional cosurfactants or salt. Lecithin spontaneously gives rise to viscous fluids at low concentrations (~ 3%), with structures ~ 200 nm detected by dynamic light scattering. At higher concentrations (&gt; 10%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology confirms the elastic response of the gels. Images from cryo-SEM indicate that the concentrated samples are <b>‘vesicle-gels’</b>, with close-packed multilamellar vesicles. The discovery of vesicles and vesicle-gels in polar solvents widens the scope of systems that can be created by self-assembly. Interestingly, vesicles in polar solvents are stable indefinitely whereas in water, vesicles tend to aggregate or coalesce over time. The stability is attributed to refractive-index-matching between lipid bilayers and the solvents, i.e., these vesicles are relatively <i>invisible</i> and thus experience only weak attractions. The ability to use lipids (which are ‘green’ or eco-friendly molecules derived from renewable natural sources) to thicken and form gels in polar solvents could also prove useful in a variety of areas, including cosmetics, pharmaceuticals, and lubricants.