Glassy dynamics of LiCl · 6H2O solution in nanoporous media

Abstract

Understanding how nanoconfinement alters the dynamics of glass-forming aqueous electrolytes is essential for clarifying the interplay among ionic hydration, hydrogen-bond structure, and interfacial effects. Here, LiCl · 6H2O was investigated in the bulk and under confinement in SBA-15 mesoporous silica with an average pore diameter of 8 nm. Differential scanning calorimetry, Raman spectroscopy, quasielastic neutron scattering, 1H spin–lattice relaxation, and pulsed-field-gradient nuclear magnetic resonance (NMR) were combined to probe thermal behavior, hydrogen-bond structure, local mobility, and translational transport over complementary time and length scales. The calorimetric results show that LiCl · 6H2O remains glass-forming under confinement, while its thermal signature of the glass transition becomes slightly broader and shifts upward relative to the bulk. Raman spectra in the O–H stretching region indicate that the concentrated LiCl solution possesses a weakened and less tetrahedrally connected hydrogen-bond network compared with bulk water. On the sub-nanosecond timescale, elastic fixed-window analysis reveals reduced mean-squared displacements under confinement, demonstrating suppressed motional amplitudes inside the pores. Inelastic fixed-window neutron scattering scans analyzed within a jump-diffusion framework yield lower effective translational diffusion coefficients and longer residence times for the confined liquid, indicating that confinement mainly hinders translational escape from transient local environments. 1H relaxometry further shows that confinement broadens the distribution of local proton fluctuation times, while PFG-NMR confirms that the measured long-range water mobility in bulk LiCl · 6H2O solution is reduced relative to bulk water. While the present data do not resolve distinct interfacial and pore-centered populations in confined LiCl · 6H2O, its dynamics are markedly altered across timescales, from the glassy to the liquid state, resulting in slower, spatially constrained, and more heterogeneous motions.