S. Mitra

Dynamics of water in confined space (porous alumina): QENS study

Dynamics of water contained in the pores of alumina gel as studied using a combination of the high and medium resolution quasi-elastic neutron scattering (QENS) technique at room temperature and extending to the supercooled region is reported. In the single particle picture of the dynamics of water molecules in confined geometry (Volino-Dianoux model), two types of water are found to be present in the pores of alumina gel. Some water molecules are attached to the surfaces (localized) and others undergo diffusion within the otherwise available space in the pores. The localization radius and diffusion constant (Dloc) characterizing the local dynamics and also the diffusion constant (Dt) and residence time (τ0) of the water molecules diffusing inside the alumina gel pores are obtained at different temperatures. Water molecules are found to undergo restricted diffusion in the pores at higher temperatures, which approach the bulk-like behaviour in the supercooled region.

QENS and FTIR studies on binding states of benzene molecules adsorbed in zeolite HZSM-5 at room temperature

Fourier-transform infrared (FTIR) spectroscopy and quasi-elastic neutron scattering (QENS) were employed for monitoring of the binding states of benzene molecules, adsorbed in HZSM-5 zeolite at 300 K and for loadings of 0.6 to 7 molecules per unit cell. While the in-plane combination C–C and C–H stretching bands of adsorbed benzene remained unaffected, a splitting was observed in the out-of-plane C–H bending vibrational bands, a feature reported for the transformation of benzene from liquid to solid phase. Also, the intensity ratio of the in-plane C–C stretching band (ν19) of adsorbed benzene at 1479 cm−1 and the bands in the region 3100–3035 cm−1 due to fundamentals and combination C–C and C–H stretching vibrations indicated a trend observed typically for a condensed phase of benzene. No shift was observed in the frequency of the above-mentioned IR bands when zeolite samples exchanged with Na+ or Ca2+ were employed. QENS results suggest that the benzene molecules occluded in zeolitic pores (∽3 molecules per unit cell) undergo a 6-fold rotation but their translation motion is too slow. Also, a high residence time of 16.5 ps was observed for the benzene entrapped in HZSM-5, compared to a time of ∽2.5 ps reported for the liquid and ∽19 ps for the solid state of benzene. These results reveal again the compression of the benzene molecules on adsorption in zeolitic pores. It is suggested that the benzene molecules confined in cavities experience a strong intermolecular interaction, giving rise eventually to their clustered state depending on the loading. In the clustered state, benzene molecules are packed with their plane parallel to zeolitic walls and interact with each other through π-electron clouds. No electronic bonding is envisaged between these clusters and the framework or the extra-framework zeolitic sites.

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.

Molecular motions in liquid crystal BBBA (4O.4): QENS study

Abstract Molecular rotational motions in butyloxybenzylidine butylanilene as studied by quasielastic neutron scattering technique in the smectic B and smectic A phases are reported. The end chain (butyl group) and the core of the liquid crystal are found to undergo rotational motions, independent of each other. Both of these are best described by the jump rotation, with N =12 equivalent sites on a circle or uniaxial rotational diffusion. Reorientation times, τ 1 , equal to 6.1×10 −11 ±1.6×10 −11 and 1.8×10 −11 ±0.6×10 −11 s are obtained for the core group and end chain, respectively, in the smectic B phase. The geometry of rotation has been found to be unchanged in the smectic A phase and the corresponding reorientation times equal to 4.1×10 −11 ±1.2×10 −11 and 1.6×10 −11 ±0.5×10 −11 s, respectively, are obtained.

Effect of pore characteristics on the dynamics of cyclohexane molecules confined in ZSM-5 and MCM-41 molecular sieves: FTIR and QENS study

Fourier-transform infrared and quasi-elastic neutron scattering investigations have revealed that the cyclohexane molecules undergo phase transformation on adsorption at room temperature in HZSM-5 and MCM-41 molecular sieves. A comparison with similar studies on polycrystalline silica and zeolite-Y confirmed that the physical state of occluded molecules depended upon the size and the structure of the pores in host material. The confinement in HZSM-5 resulted in the development of non-equilibrium and highly condensed phase of cyclohexane, which remained trapped in the zeolitic pores even after partial desorption. This compression of cyclohexane as a result of adsorption in HZSM-5 is in agreement with the reported phenomena of super-cooling on physical confinement of fluids inside the narrow pores. On the other hand, relatively smaller amounts of cyclohexane were held in the pores of MCM-41 during the adsorption under identical conditions; and for all the loadings below 5 mmol g−1 the guest molecules acquired a transition state the density of which lay between that of the vapor and the liquid phases of cyclohexane. The results of this study are consistent with our recent findings on the formation of a clustered state of benzene during its adsorption into microporous materials (A. Sahasrabudhe, V. S. Kamble, A. K. Tripathi and N. M. Gupta, J. Phys. Chem. B, 2001, 105, 4374; A. K. Tripathi, A. Sahasrabudhe, S. Mitra, R. Mukhopadhyay, N. M. Gupta and V. B. Kartha, Phys. Chem. Chem. Phys., 2001, 3, 4449), and are also in line with the current theories on capillary condensation of fluids in narrow pores.

Order–disorder transition in pyridinium iodide: quasi-elastic neutron scattering study

Abstract Reorientational motion of pyridinium cation in pyridinium iodide across the phase transition ( T c =247 K) temperature is studied using a high-resolution quasi-elastic neutron scattering (QENS) technique. A clear evidence of the order–disorder transition with respect to the pyridinium ion reorientation is observed. In the high-temperature phase the pyridinium ion is found to undergo a sixfold jump rotation around the axis perpendicular to its plane. Weak quasi-elastic broadening is observed below the phase transition temperature, at T =240 K, indicating presence of residual disorder. Reorientational time and activation energy of rotation are also obtained in the high-temperature phase.

Diffusion of water in molecular magnet Cu0.75Mn0.75[Fe(CN)6]?7H2O

Here we report the dynamical behaviour of water in Prussian blue analogue (PBA) Cu(0.75)Mn(0.75)[Fe(CN)(6)]·7H(2)O molecular magnet in the temperature range 260-360 K as studied using the quasielastic neutron scattering technique. While significant quasielastic broadening is observed in the hydrated sample, no broadening was observed in the dehydrated one. Data analysis showed that the observed quasielastic broadening in Cu(0.75)Mn(0.75)[Fe(CN)(6)]·7H(2)O corresponds to the dynamics of the non-coordinated water molecules at the 32f site and the coordinated water molecules at the 24e site, existing in the cavities created by the absence of Fe(CN)(6) units. The non-coordinated water molecules at 8c interstitial sites do not contribute to the broadening, suggesting that they are immobile at least within the time window of the spectrometer used. Behaviour of the elastic incoherent structure factor is consistent with the model where the water molecules undergo translational diffusion localized within the cavity of 5.1 Å. While all the non-coordinated water molecules at the 32f site are dynamic over the entire range of temperatures, the coordinated ones at the 24e site become progressively dynamic with temperature. The water molecules were found to undergo hindered (~1.16 × 10(-5) cm(2) s(-1) at 300 K) diffusion compared to bulk water and the diffusivity followed Arrhenius behaviour within the measured temperature range with an activation energy of 1.26 kcal mol(-1).

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 water in prussian blue analogues: Neutron scattering study

Dynamics of crystal water in Prussian blue (PB), Fe(III)4[Fe(II)(CN)6]3.14H2O and its analogue Prussian green (PG), ferriferricynaide, Fe(III)4[Fe(III)(CN)6]4.16H2O have been investigated using Quasielastic Neutron Scattering (QENS) technique. PB and its analogue compounds are important materials for their various interesting multifunctional properties. It is known that crystal water plays a crucial role towards the multifunctional properties of Prussian blue analogue compounds. Three structurally distinguishable water molecules: (i) coordinated water molecules at empty nitrogen sites, (ii) non-coordinated water molecules in the spherical cavities, and (iii) at interstitial sites exist in PB. Here spherical cavities are created due to the vacant sites of Fe(CN)6 units. However, PG does not have any such vacant N or Fe(CN)6 units, and only one kind of water molecules, exists only at interstitial sites. QENS experiments have been carried out on both the compounds in the temperature range of 260–360 K to elu...

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.