Phytantriol phase behaviour in choline chloride urea and water mixtures

Deep eutectic solvents (DES) are tailorable non-aqueous solvents with promising properties for a range of applications, from industrial dissolution of plant products to biomedicine. They are mixtures of hydrogen bond donors and acceptors with low melting points that can be tailored to specific applications, and many support the self-assembly of amphiphilic molecules into lyotropic liquid crystal phases. Self-assembled lipid structures have potential for numerous applications, including drug delivery. These ordered structures can act as carriers, slow-release vehicles, or microreactors. Lipid self-assembly in non-aqueous solvents, such as deep eutectic solvents, is important for applications at extreme temperatures, or involving water-insoluble or water sensitive components. However, lipid self-assembly in these solvents remains largely unexplored. In this paper, we have examined the self-assembly of phytantriol, a non-ionic lipid, at 10 and 30 wt% in the deep eutectic solvent choline chloride:urea, with and without water. Self-assembly was assessed using small angle X-ray scattering and cross polarised optical microscopy at temperatures from 25-66 °C. We found that pure choline chloride:urea supports a Pn3m cubic phase similar to that formed in water. However, mixtures of the DES with water resulted in phytantriol forming an inverse hexagonal phase and influenced the phase transition temperatures. These results demonstrate that choline chloride:urea can support diverse phase behaviour, and also provides a mechanism for tailoring the phase for particular applications simply by controlling the amount of water in the solvent. In the future this could lead to methods of triggered release of drugs and biomolecules by the simple addition of water which could be critical for drug delivery applications.