Phytantriol phase behaviour in deep eutectic solvent-water mixtures

Deep eutectic solvents are highly tailorable non-aqueous solvents with potential applications ranging from energy catalysis to cryopreservation. Self-assembled lipid structures are already used in a variety of industries including cosmetics, drug delivery and as microreactors. However, most research into lipid self-assembly has been limited to aqueous solvents. This paper explores the self-assembly of a well-known lipid, phytantriol, in different deep eutectic solvents composed of choline chloride with urea, glycerol, or ethylene glycol, and one composed of betaine and glycerol. High-throughput small angle X-ray scattering was employed to examine self-assembly of this lipid in these deep eutectic solvents, and in mixtures with water from 25 to 66 °C. Choline chloride:urea and neat betain:glycerol supported complex phase formation including the Pn3m cubic phase, and an inverse hexagonal (H_II) phase, while choline chloride:glycerol and choline chloride:ethylene glycol favoured amorphous or unstructured lipid assemblies. In all cases, water contents above 50 wt% favoured the formation of highly structured phases. These results demonstrate that deep eutectic solvents can support lipid assembly, but also that small changes to the solvent can lead to significant changes in lipid behaviour. This provides an avenue for solvent-controlled tailoring of lipid structures as well as a mechanism for targeted release of cargo, such as through simple addition of water to trigger a phase change. These results provide significant new insight into solvent-controlled lipid self-assembly with far-reaching applications.