Articles written in Pramana – Journal of Physics

• Preface

• Quasi-elastic neutron scattering study of dynamics in condensed matter

Quasi-elastic neutron scattering (QENS) technique, known to study stochastic motions has been successfully used to elucidate the molecular motions and physical properties related to them, in a variety of systems. QENS is a unique technique that provides information on the time-scale of the motion as well as the geometry of the motions. In this paper, results of some of the systems studied using the facility available at Dhruva, Trombay and other mega-facilities are discussed. Emphasis is given on the results obtained from three different systems studied using QENS, namely, (1) alkyl chain motions in monolayer protected metal clusters, (2) molecular motions of propane in Na-Y zeolitic systems and (3) the study of reorientational motions of liquid crystal innO.m series in different mesophases.

• Excess water dynamics in hydrotalcite: QENS study

Results of the quasi-elastic neutron scattering (QENS) measurements on the dynamics of excess water in hydrotalcite sample with varied content of excess water are reported. Translational motion of excess water can be best described by random translational jump diffusion model. The observed increase in translational diffusivity with increase in the amount of excess water is attributed to the change in binding of the water molecules to the host layer.

• Dynamics of different molecules adsorbed in porous media

We present in this paper a comparative study on the dynamics of benzene, cyclohexane, and methanol molecules, confined in the pores of MCM-41 molecular sieve and HZSM-5 zeolite. The quasi-elastic neutron scattering (QENS) measurements revealed that the physical state of these adsorbed molecules depended not only on the structural characteristics of the host matrix but also on the chemical properties, such as dipole moment, of the guest molecules. Thus, while no motion was observed in the time-scale of 10−10−10−12 s in the case of methanol, the larger size benzene and cyclohexane molecules are found to perform six-fold and three-fold jump rotation, respectively, when adsorbed inside the cages of HZSM-5 at room temperature. At the same time, all the three molecules are found to undergo a translational motion inside the pores of MCM-41 molecular sieves, the value of diffusion constant being the lowest in case of methanol because of its higher polarity. Translationl motion of the guest molecules inside the pores of MCM-41 can be satisfactorily described by Chudley-Eliott fixed jump length diffusion and accordingly the residence time, jump length and diffusion constant are estimated.

• Acetylene diffusion in Na-Y zeolite

Study of diffusivity of acetylene adsorbed in Na-Y zeolite by quasi-elastic neutron scattering (QENS) measurements at temperatures of 300, 325 and 350 K is reported. A model in which the acetylene molecules undergo random-walk diffusion characterized by a Gaussian distribution of jump lengths inside zeolite cages describes the data consistently. The diffusion constant, residence time between jumps and root mean square jump length are determined.

• Preface

• Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations: An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.

• Preface

• Diffusion of propylene adsorbed in Na-Y and Na-ZSM5 zeolites: Neutron scattering and FTIR studies

Here we report the quasielastic neutron scattering and FTIR studies on the dynamics of propylene adsorbed in Na-Y and Na-ZSM5 zeolites. QENS data show that although the mechanism of translational motion of propylene is jump diffusion in both the cases of Na-Y and Na-ZSM5 zeolites, the diffusivity is affected by the host size and is hindered in the case of Na-ZSM5. FTIR studies showed that guest–host interaction in Na-ZSM5 is stronger than that in Na-Y zeolite corroborating the QENS results.

• Phase transitions in liquid crystal 6O.4 (p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline)

DSC measurements on p-𝑛-hexyloxybenzylidine-pˊ-𝑛-butylaniline (6O.4) showed that the crystalline to liquid crystalline ($K–S_{\text{H}}$) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the $K–S_{\text{H}}$ transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasi-elastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (𝐾 and $S_{\text{H}}$) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the $S_{\text{H}}$ phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.

• Rotational dynamics of propylene inside Na-Y zeolite cages

We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.

• # Pramana – Journal of Physics

Volume 97, 2023
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Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019