Supercritical water: Local order and molecular dynamics

This paper is a study of the structure and dynamics of near-critical and supercritical water for thermodynamic states above the critical temperature in a wide range of density by infrared absorption and quasi-elastic neutron scattering. The evolution of the shape of the infrared profiles associated with the internal vibrational modes of water has been investigated. In supercritical water, at T = 380 °C and low pressure (density), in the range 25-50 bar (0.01-0.05 g·cm-3), only monomers are detected. A progressive increase of the pressure (density) from 50 to 250 bar (from 0.05 to 0.4 g·cm-3) leads to the appearance of dimers and trimers. In order to obtain information on the dynamics, we have performed incoherent quasi-elastic neutron-scattering experiments on light water for several thermodynamic states (200 < T < 400 °C and 184 < P < 400 bar) corresponding to densities ranging from 0.2 to 0.9 g·cm-3. The results have been analyzed using a jump diffusion model and the two parameters of this model, namely, τ0, the residence time and D, the translational diffusion coefficient, have been determined as a function of the density.

Download Full-text

A quasi-elastic neutron scattering study of the dynamics of supercritical water

Chemical Physics ◽

10.1016/s0301-0104(03)00228-3 ◽

2003 ◽

Vol 292 (2-3) ◽

pp. 229-234 ◽

Cited By ~ 13

Author(s):

I.A. Beta ◽

J.-C. Li ◽

M.-C. Bellissent-Funel

Keyword(s):

Neutron Scattering ◽

Supercritical Water ◽

Elastic Neutron ◽

Scattering Study ◽

Elastic Neutron Scattering ◽

Neutron Scattering Study

Download Full-text

Methanol dynamics in H-ZSM-5 with Si/Al ratio of 25: a quasi-elastic neutron scattering (QENS) study

Topics in Catalysis ◽

10.1007/s11244-021-01450-z ◽

2021 ◽

Author(s):

Santhosh K. Matam ◽

C. Richard A. Catlow ◽

Ian P. Silverwood ◽

Alexander J. O’Malley

Keyword(s):

Neutron Scattering ◽

Rotational Diffusion ◽

Jump Diffusion ◽

Translational Diffusion ◽

Elastic Neutron ◽

Measurement Temperature ◽

Diffusion Dynamics ◽

Mobile Fraction ◽

Jump Diffusion Model ◽

Elastic Neutron Scattering

AbstractMethanol dynamics in zeolite H-ZSM-5 (Si/Al of 25) with a methanol loading of ~ 30 molecules per unit cell has been studied at 298, 323, 348 and 373 K by incoherent quasi-elastic neutron scattering (QENS). The elastic incoherent structure factor (EISF) reveals that the majority of methanol is immobile, in the range between 70 and 80%, depending on the measurement temperature. At 298 K, ≈ 20% methanol is mobile on the instrumental timescale, exhibiting isotropic rotational dynamics with a rotational diffusion coefficient (DR) of 4.75 × 1010 s−1. Upon increasing the measurement temperature from 298 to 323 K, the nature of the methanol dynamics changes from rotational to translational diffusion dynamics. Similar translational diffusion rates are measured at 348 and 373 K, though with a larger mobile fraction as temperature increases. The translational diffusion is characterised as jump diffusion confined to a sphere with a radius close to that of a ZSM-5 channel. The diffusion coefficients may be calculated using either the Volino–Dianoux (VD) model of diffusion confined to a sphere, or the Chudley–Elliot (CE) jump diffusion model. The VD model gives rise to a self-diffusion co-efficient (Ds) of methanol in the range of 7.8–8.4 × 10–10 m2 s−1. The CE model gives a Ds of around 1.2 (± 0.1) × 10–9 m2 s−1 with a jump distance of 2.8 (either + 0.15 or − 0.1) Å and a residence time (τ) of ~ 10.8 (either + 0.1 or − 0.2) ps. A correlation between the present and earlier studies that report methanol dynamics in H-ZSM-5 with Si/Al of 36 is made, suggesting that with increasing Si/Al ratio, the mobile fraction of methanol increases while DR decreases.

Download Full-text

Membranes studied using neutron scattering and NMR

Canadian Journal of Physics ◽

10.1139/p95-102 ◽

1995 ◽

Vol 73 (11-12) ◽

pp. 687-696 ◽

Cited By ~ 16

Author(s):

Myer Bloom ◽

Thomas M. Bayerl

Keyword(s):

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Neutron Scattering ◽

Spectral Characteristics ◽

Nmr Relaxation ◽

Elastic Neutron ◽

Fluid Membranes ◽

Dynamical Properties ◽

Elastic Neutron Scattering ◽

Complementary Techniques

After reviewing some of the basic measurements that characterize the study of physical properties of matter using neutron scattering and nuclear magnetic resonance (NMR), connections between information obtained in current research on fluid membranes using these two complementary techniques are explored in two major chapters. In the first, the type of information on the structure of fluid membranes obtained from coherent elastic neutron scattering is compared with that from NMR spectral characteristics. Then, the type of information obtained on dynamical properties from NMR relaxation (T1 and T2) measurements is compared with that from quasi-elastic neutron scattering. Examples of such connections are given with an emphasis on relationships between the time and distance scales intrinsic to neutron scattering and NMR.

Download Full-text