arXiv subjects

Markus Appel

Publications and source records attributed to Markus Appel.

7 recordsLinked to original sources

arXiv2026

Insight into the molecular dynamics of barocaloric molecular crystals using quasi-elastic neutron scattering

Barocaloric (BC) molecular crystals exhibit order-disorder phase transitions that o9er a promising basis for solid-state heating and cooling. However, rational design of these next-generation materials requires microscopic understanding of the molecular dynamics that drive the transitional entropy change. Quasi-elastic neutron scattering (QENS) techniques including inelastic fixed-window scans (IFWSs) enable the direct probing of such molecular motions throughout thermodynamic phase transitions. Here we present a QENS-IFWS global fitting methodology for systematically investigating the molecular dynamics underpinning BC performance in molecular crystals. We demonstrate this methodology on neopentyl glycol (NPG) and three derivatives spanning chemically distinct routes to dynamical perturbation. By fixing attempt frequencies across samples, we observe measurable differences in the relative activation energies and mobile fractions of hydroxymethyl and full-molecular rotations. The largest observed e9ect is on the hydroxymethyl rotation activation energies, which is closely related to hydrogen bond network stability, ranging from -3.6% to +11.1% in the derivatives compared to pure NPG. The capability of this global fitting strategy to extract subtle variations in molecular rotations highlights QENS as a versatile design tool for guiding BC molecular crystal design.

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↗

arXiv2025

Enhanced reversible barocaloric effect at low pressure in neopentyl plastic crystal solid solutions

The discovery of colossal barocaloric effects in neopentyl glycol (NPG) makes plastic crystals promising candidates for solid-state refrigerants with lower environmental impact than vapour compression fluids. Optimising operational temperatures and low-pressure operability remains challenging without compromising thermodynamic parameters. Here, we implement a strategy to improve the viability of NPG derivatives as barocaloric refrigerants. We blend pentaglycerine (PG) with NPG to lower the phase transition temperature, then dope the blend with 2% pentaerythritol (PE) to improve transition reversibility. In comparison with NPG under the same conditions, this ternary system has a seven-fold increase in reversible isothermal entropy change (|$\Delta S_{(it,rev)}$ | = 13.4 J kg$^{-1}$ K$^{-1}$) and twenty-fold increase in operational temperature span ($\Delta T_{span}$ = 18 K) at pressures of 1 kbar. Synchrotron x-ray diffraction and quasielastic neutron scattering reveal structural and dynamical effects that broaden the temperature range of the first-order phase transition due to intermolecular hydrogen bond network disruption by the molecular dopants. We propose that exploiting the compositional phase space of multi-component molecular blends is effective for designing practicable molecular BCs.

cond-mat.mtrl-sci↗

arXiv2024

Direct observation of thermal hysteresis in the molecular dynamics of barocaloric neopentyl glycol

Barocalorics (BCs) are emerging as promising alternatives to vapour-phase refrigerants, which are problematic as they exacerbate climate change when they inevitably leak into the atmosphere. However, the commercialisation of BC refrigerants is significantly hindered by hysteresis in the solid-solid phase transition that would be exploited in a refrigeration cycle. Here, we provide new insight into the hysteresis that is a critical step towards the rational design of viable BCs. By studying the benchmark BC plastic crystal, neopentyl glycol (NPG), we observe directly the liberation of the hydroxyl rotational modes that unlock the hydrogen bond network, distinguishing for the first time the molecular reorientation and hydroxymethyl rotational modes. We showcase the use high-resolution inelastic fixed-window scans in combination with quasielastic neutron scattering (QENS) measurements to build a comprehensive microscopic understanding of the NPG phase transition, directly tracking the molecular dynamics of the phase transition. Hysteresis previously observed in calorimetric studies of NPG is now observed directly as hysteresis in molecular rotational modes, and hence in the formation and disruption of hydrogen bonding. Furthermore, by tracking the thermal activation of three main reorientation modes, we suggest that their fractional excitations may resolve an outstanding discrepancy between measured and calculated entropy change. These results allow for direct study of the molecular dynamics that govern the thermal hysteresis of small molecule energy materials. They will be broadly applicable, as many promising BC material families possess first-order transitions involving molecular reorientations.

cond-mat.mtrl-sci↗

arXiv2023

Fractional matter coupled to the emergent gauge field in a quantum spin ice

Electronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. Their existence is conceptualized in models of two- or three-dimensional frustrated magnets that evade symmetry-breaking order down to zero temperature. Quantum spin ice (QSI) is a theoretically well-established example described by an emergent quantum electrodynamics, with excitations behaving like photon and matter quasiparticles. The latter are fractionally charged and equivalent to the `spinons' emerging from coherent phases of singlets in one dimension, where clear experimental proofs of fractionalization exist. However, in frustrated magnets it remains difficult to establish consensual evidence for quantum spin liquid ground states and their fractional excitations. Here, we use backscattering neutron spectroscopy to achieve extremely high resolution of the time-dependent magnetic response of the candidate QSI material Ce$_2$Sn$_2$O$_7$. We find a gapped spectrum featuring a threshold and peaks that match theories for pair production and propagation of fractional matter excitations (spinons) strongly coupled to a background gauge field. The multiple peaks are a specific signature of the $\pi$-flux phase of QSI, providing spectroscopic evidence for fractionalization in a three-dimensional quantum spin liquid.

cond-mat.str-el↗

arXiv2020

Dynamics of Water Confined in Mesopores with Variable Surface Interaction

We have investigated the dynamics of liquid water confined in mesostructured porous silica (MCM-41) and periodic mesoporous organosilicas (PMOs) by incoherent quasielastic neutron scattering experiments. The effect of tuning the water/surface interaction from hydrophilic to more hydrophobic on the water mobility, while keeping the pore size in the range 3.5-4.1 nm, was assessed from the comparative study of three PMOs comprising different organic bridging units and the purely siliceous MCM-41 case. An extended dynamical range was achieved by combining time-of-flight (IN5B) and backscattering (IN16B) quasielastic neutron spectrometers providing complementary energy resolutions. Liquid water was studied at regularly spaced temperatures ranging from 300 K to 243 K. In all systems, the molecular dynamics could be described consistently by the combination of two independent motions resulting from fast local motion around the average molecule position and the confined translational jump diffusion of its center of mass. All the molecules performed local relaxations, whereas the translational motion of a fraction of molecules was frozen on the experimental timescale. This study provides a comprehensive microscopic view on the dynamics of liquid water confined in mesopores, with distinct surface chemistries, in terms of non-mobile/mobile fraction, self-diffusion coefficient, residence time, confining radius, local relaxation time, and their temperature dependence. Importantly, it demonstrates that the strength of the water/surface interaction determines the long-time tail of the dynamics, which we attributed to the translational diffusion of interfacial molecules, while the water dynamics in the pore center is barely affected by the interface hydrophilicity.

physics.chem-ph↗