Phytantriol and monoolein in aqueous deep eutectic solvent and protic ionic liquid solutions

Abstract

Lyotropic liquid crystal gels of phytantriol and monoolein are well known examples of self-assembled systems in water, which have multiple applications across biomedical and materials science. However aqueous systems can be restricted by rapid solvent evaporation, and the limited solubility of some species in water. Here we explore the formation of liquid crystalline phases of phytantriol and monoolein in mixtures of water with two protic ionic liquids, ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN), and three deep eutectic solvents (DES) formed from mixtures of choline chloride with urea, fructose or citric acid. The structures of the gel phase in excess solvent were measured using small angle X-ray scattering for a fixed lipid concentration (5% w/w) as a function of temperature. The phase diagrams of both lipids in DES–water mixtures and the non-amphiphilic ionic liquid, EtAN, indicate that higher negative curvature inverse hexagonal structures are favoured by addition of water. However, the amphiphilic ionic liquid EAN swells and stabilises the cubic Pn3m structure. The interplay of solvent structure, polarity and molecular size are key to understanding the formation and stability of lyotropic liquid crystalline gels in these systems.