2022 journal article

Differentiating the Structure and Dynamics of Solute-Bound Vs. Bulk Water in Deep-Eutectic Aqueous AlCl3 Solutions

ECS Meeting Abstracts.

co-author countries: United States of America πŸ‡ΊπŸ‡Έ
Source: ORCID
Added: November 24, 2022

Aqueous solutions of aluminum chloride exhibit a deep eutectic (-52˚C) that is likely due to solute-directed formation of complex network structures in the solvent. It is well known that the solute-bound solvent must have a distinct chemical environment from that of the bulk solvent. Commonly used vibrational spectroscopic measurements of aqueous salt solutions are not selective enough to directly identify distinct water environments in solution. However, variable-temperature 1 H-NMR spectroscopy of aqueous solutions, specifically those with high charge density salts, can distinguish water directly bound to the cation from water of the bulk solution. This unique ability to resolve characteristic water peaks by NMR spectroscopy affords the opportunity to explore the hydration structure and dynamics of deep-eutectic aqueous solutions. The aqueous AlCl 3 system is an excellent model to study solution structure since we can exploit the high charge density of the ions for which strong ion-dipole interactions are structure-directing to the solvent waters, allowing resolution of distinct water chemical environments. We conducted a thorough investigation of the composition- and temperature-dependent solution structure of the AlCl 3 : water system using 1 H-NMR spectroscopy, and X-ray and neutron diffraction. The room-temperature proton chemical shift of water proceeds downfield with increasing salt concentration until AlCl 3 β€’22H 2 O, the eutectic composition, after which further increasing concentration results in an upfield chemical shift (see Figure.) Using VT-NMR, we can resolve the first-nearest neighbor water signals from the bulk signal upon cooling. 1 H-NMR T 1 and T 2 relaxation measurement of the distinct water environments reveal stark differences in proton mobility for compositions on the water-rich versus AlCl 3 -rich sides of the eutectic, implying significant changes in hydration structure. To evaluate the extent to which these changes are structural, a parallel set of composition- and temperature-dependent neutron and X-ray diffraction experiments demonstrate that both cations and anions exhibit significant structure-directing influences on the solution, which we hypothesize underlies deep eutectic behavior in aqueous solutions. Figure 1