Evidence for an L3 phase in ternary deep eutectics: composition-induced L3-to-Lα transition of AOT

Pure and hydrated deep eutectic solvents (DES) are proposed to form self-assembled nanostructures within the fluid bulk, similar to the bicontinuous L(3 )phase common for ionic liquids (ILs). Labelled choline chloride : urea : water DES were measured using small-angle neutron scattering (SANS), showing no long-range nanostructure. However, solutions of the surfactant AOT in this DES yielded scattering consistent with theL(3) sponge phase, which was fitted using the Teubner-Strey model. A disclike model gave local structural information, namely, a linear increase in radius versus solvent water content (w = molar ratio of DES : water), eventually forming large, turbid lamellar phases at 10w; an L-3-to-L-alpha transition was observed. Simultaneous multi-contrast SANS fits show the surfactant headgroup region is dominated by interactions with poorly-soluble Na+ at low water contents, and numerically-superior [cholinium](+) as water content increases. The modified interfacial Gaussian curvature from cation : anion volume matching stabilizes the lamellar morphology, allowing the bilayer aggregation number to increase.

Automated summary

This study demonstrates that pure and hydrated deep eutectic solvents (DES) can form self-assembled nanostructures similar to the bicontinuous L(3) phase in ionic liquids (ILs). The addition of surfactant AOT in the DES results in the formation of the L(3) sponge phase, characterized using the Teubner-Strey model. The local structure of the DES changes with increasing solvent water content, forming large, turbid lamellar phases and an L-3-to-L-alpha transition. Multi-contrast SANS fittings reveal that the surfactant headgroup region interacts primarily with poorly-soluble Na+ at low water contents and numerically-superior [cholinium](+) at higher water contents. The modified interfacial Gaussian curvature stabilizes the lamellar morphology, allowing for an increase in the bilayer aggregation number.