arXiv subjects

Armin Mozhdehei

Publications and source records attributed to Armin Mozhdehei.

4 recordsLinked to original sources

arXiv2026

Formation and Thermodynamic Behavior of THF-Water Hydrates in Confined Mesoporous Media

Tetrahydrofuran (THF) is a benchmark guest for probing clathrate hydrate thermodynamics because a stoichiometric aqueous solution (THF.17H2O) forms structure-II (sII) hydrate at ambient pressure with a well-defined dissociation temperature. Here, we combine differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) in bulk and confined media to resolve how composition, pore filling, and cooling rate govern hydrate formation in SBA-15 mesoporous silica. Bulk DSC establishes mass-balanced enthalpies for ice and sII hydrate and confirms reversible dissociation/melting temperatures. In confinement, the heating traces separate into a Gibbs-Thomson depressed ice melt (= -14.7 $\pm$ 0.2 {\textdegree}C), an in-pore hydrate dissociation (= -13.2 $\pm$ 0.2 {\textdegree}C). Confined hydrate appears only when two criteria are met: near-percolating filling ($ϕ$ = 1.0 -1.1 cm3/g) and sufficient THF ($\ge$ 1:16 mol:mol). Cooling-rate experiments (1.0 vs 0.5 {\textdegree}C/min) demonstrate that slower precooling increases the confined-hydrate fraction and reduces confined ice without shifting equilibrium temperatures: at $ϕ$ = 1.1, the hydrate enthalpy rises by ~60% at 1:11 and ~54% at 1:14, but by $\le$ 17% at 1:16. Temperature-cycling tests show invariant reheating peak positions, indicating that capillarity and composition, rather than kinetic history, fix the liquidus and dissociation temperatures. WAXS indicates that the phase formed in pores is crystallographically identical to bulk sII. Finally, the variation of melting points ($Δ$T___) plotted against inverse pore radius follows the Gibbs-Thomson law for both ice melting and hydrate dissociation, quantitatively linking the observed shifts to crystalline size and clarifying how confinement, cooling rate, and composition govern the competition between hydrate formation and water crystallization.

cond-mat.mtrl-sci↗

arXiv2026

Glassy Dynamics of LiCl.6H2O Solution in Nanoporous Media

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, 1 H spin-lattice relaxation, and pulsed-fieldgradient 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 shifted 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 subnanosecond 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. 1 H 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.

cond-mat.mtrl-sci↗

arXiv2025

Colossal Effect of Nanopore Surface Ionic Charge on the Dynamics of Confined Water

Interfacial interactions significantly alter the fundamental properties of water confined in mesoporous structures, with crucial implications for geological, physicochemical, and biological processes. Herein, we focused on the effect of changing the surface ionic charge of nanopores with comparable pore size (3.5-3.8 nm) on the dynamics of confined liquid water. The control of the pore surface ionicity was achieved by using two periodic mesoporous organosilicas (PMOs) containing either neutral or charged forms of a chemically similar bridging unit. The effect on the dynamics of water at the nanoscale was investigated in the temperature range of 245 -300 K, encompassing the glass transition by incoherent quasielastic neutron scattering (QENS), For both types of PMOs, the water dynamics revealed two distinct types of molecular motions: rapid local movements and translational jump diffusion. While the neutral PMO induces a moderate confinement effect, we show that the charged PMO drastically slows down water dynamics, reducing translational diffusion by a factor of four and increasing residence time by an order of magnitude. Notably, by changing the pore filling values, we demonstrate that for charged pore this effect extends beyond the interfacial layer of surface-bound water molecules to encompass the entire pore volume. Thus, our observation indicates a dramatic change in the long-range character of the interaction of water confined in nanopores with surface ionic charge compared to a simple change in hydrophilicity. This is relevant for the understanding of a broad variety of applications in (nano)technological phenomena and processes, such as nanofiltration and membrane design.

physics.chem-ph↗

arXiv2025

How special are the dynamics of deep eutectic solvents? A Look at the Prototypical Case of Ethaline

We investigated the molecular dynamics of the prototypical deep eutectic solvent (DES) ethaline. We disentangled the different motions of its two constituents, namely choline chloride and ethylene glycol on a spatio-temporal range that extends from sub-nanometer to micrometer distances and from picosecond to millisecond times. This was achieved by a combination of pulsed-field-gradient NMR, time-of-flight, and backscattering quasielastic neutron scattering experiments with isotopically labelled samples. On the micrometer scale, we observe that the translational motions of the two DES constituents obey classical hydrodynamics, with distinct diffusivities that reflect their different hydrodynamic radii. This is no longer valid at the nanometer-scale, where the two DES components present similar short-ranged diffusivities, which indicates a significant effect of their supramolecular association. The sub-nanometer scale motions include jumps that precede Fickian diffusion, and localized dynamics that precede the breaking of the transient cage formed by neighboring molecules. Therein, the spatial amplitude of the localized motions mirrors their different molecular sizes, while their respective correlation times contrast with observations made for other choline-based DES such as glyceline. This result underlines the importance of more subtle effects, such as the different H-bond propensities of the polyol donor, and demonstrate the difficulty to anticipate the nanoscale dynamic behavior of DES from the knowledge of their macroscopic properties.

physics.chem-ph↗