Structural Behavior of Long-Chain Imidazolium-Based Ionic Liquid [C₁₀mim]Cl–Water Mixtures

The structural behavior of long-chain imidazolium-based ionic liquid [C10mim]­Cl–water mixtures has been investigated in detail by low-temperature and high-pressure crystallization methods, and the solid forms have been fully characterized by single-crystal X-ray diffraction. Form I, a monohydrate, crystallizes from solutions containing 6–8% (w/w) water; its structure exhibits bilayers, in which the cations alternate from layer to layer to create hydrophobic and hydrophilic regions. Form II, which is a Z′ > 1 structure, incorporates a variable amount of water content (from pseudo-hemihydrate to pseudo-monohydrate) depending on the sample treatment and environment; this form crystallizes from solutions containing < ca. 6% (w/w) water and exhibits a double bilayered structure characterized by ordered and disordered regions. Forms I and II have been obtained at both low-temperature and high-pressure conditions. Form III is a trihydrate that has been exclusively obtained under high-pressure conditions above 0.55 GPa from solutions containing >80% (w/w) water. This structure is closely related to that of form I. While homeotypic and isotypic structures of forms I and II have been previously reported, form III represents, to the best of our knowledge, the only example of a highly hydrated long-chain imidazolium-based ionic liquid isolated in the solid state. The formation of mesophases in the title compound has also been investigated by polarized optical microscopy, and the results were correlated with previous liquid-crystal studies on related compounds.