Spontaneous Formation of Stable Vesicles and Vesicle Gels in Polar Organic Solvents

Abstract

The self-assembly of lipids into nanoscale vesicles (liposomes) is routinely accomplished in water. However, reports of similar vesicles in polar organic solvents like glycerol, formamide, and ethylene glycol (EG) are scarce. Here, we demonstrate the formation of nanoscale vesicles in glycerol, formamide, and EG using the common phospholipid lecithin (derived from soy). The samples we study are simple binary mixtures of lecithin and the solvent, with no additional cosurfactants or salt. Lecithin dissolves readily in the solvents and spontaneously gives rise to viscous fluids at low lipid concentrations (∼2–4%), with structures ∼200 nm detected by dynamic light scattering. At higher concentrations (>10%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology confirms the elastic response of gels, with their elastic modulus being ∼20 Pa at ∼10% lipid. Images from cryo-scanning electron microscopy (cryo-SEM) indicate that concentrated samples are “vesicle gels,” where multilamellar vesicles (MLVs, also called “onions”), with diameters between 50 and 600 nm, are close-packed across the sample volume. This structure can explain both the elastic rheology as well as the static birefringence of the samples. The discovery of vesicles and vesicle gels in polar solvents widens the scope of systems that can be created by self-assembly. Interestingly, it is much easier to form vesicles in polar solvents than in water, and the former are stable indefinitely, whereas the latter 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 “invisible” 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.

Iron doped BiOBr loaded on carbon spheres for improved visible-light-driven detoxification of 2-chloroethyl sulfide

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Edible oil‐in‐water emulsions stabilized by hydrophile–lipophile balanced sucrose ester

“…It is well known that surfactants tend to form aggregates in aqueous solution, such as micelles and vesicles (Ludwig et al, 2021; McCoy et al, 2020), depending on their structure, concentration, pH, temperature and presence of additives, which are significant for drug delivery systems. (Di Marzio et al, 2011; Marianecci et al, 2010; Valdés et al, 2014) On the other hand, the presence of aggregates like worm‐like micelles (Abdel‐Rahem & Hoffmann, 2006; Takahashi et al, 2013;) and vesicles (Abdel‐Rahem & Hoffmann, 2006) can significantly increase the viscosity of the aqueous phase causing it to be viscoelastic or gel‐like (Agrawal et al, 2021; Cao et al, 2021) which is beneficial to emulsion stabilization. (Friberg, 2007; Hu et al, 2021; Pichot et al, 2013; Rocchio et al, 2017; Yi et al, 2019).…”