Beyond water: Aquoline, a versatile wbDES for host-guest and self-assembly systems

Deep Eutectic Solvents (DESs) have gained increasing attention as sustainable and innovative media for a wide range of applications. These systems, typically formed by two or more components interacting through hydrogen bonding, exhibit strong negative deviations from thermodynamic ideality, resulting in a marked melting point (Tm) depression [1]. Water, traditionally regarded as an impurity or used as an additive to modulate DES properties, has recently been identified as an active component in their formation, leading to the development of water-based DESs (wbDESs) [2–4]. In this context, a choline chloride-water (ChCl-W) mixture has emerged as a promising wbDES, combining low viscosity and high conductivity; solid–liquid equilibrium analysis conducted on ChCl-W mixtures with molar ratios (n = W: ChCl) ranging from 2 to 10 identified the n = 4 composition as exhibiting deep eutectic behavior [4]. This mixture, henceforth referred to as aquoline, was investigated using Small and Wide-Angle X-ray Scattering (S-WAXS), which revealed the absence of large-scale aggregation [4]. At atomistic level, ab initio Molecular Dynamics (AIMD) simulations showed competitive solvation of the choline cation by water and chloride ions, forming a clathrate-like cage around its hydroxyl group. This structural motif was further supported by ¹H-NMR experiments, which confirmed the lack of proton exchange between water and the choline hydroxyl [4]. The applicative potential of aquoline was demonstrated in two key systems. Solubility tests on α-, β-, and γ-cyclodextrins (CDs), well-known host–guest molecules, were conducted across ChCl-W mixtures with n from 2 to 6. In all cases, maximum solubility was observed at n = 4, where it significantly exceeded that in pure water. Furthermore, aquoline enabled the control of micelle formation in ionic surfactant systems. Small-angle X-ray and neutron scattering revealed that anionic SDS (sodium dodecyl sulphate) formed cylindrical micelles whose size decreased with increasing concentration, while cationic DTAB (dodecyltrimethylammonium bromide) formed oblate ellipsoidal micelles with size independent of concentration. Overall, these results demonstrate the development of a novel and sustainable wbDES with appealing physicochemical characteristics, offering a versatile platform for applications ranging from CDs solubilization to the controlled self-assembly of amphiphilic systems.