Status of experiments probing the dynamics of water in confinement

In many relevant situations, water is not in its bulk form but is instead attached to some substrates or filling some cavities. We shall call water in the latter environment confined or interfacial water as opposed to bulk water. This confined water is essential for the stability and function of biological macromolecules. In this review paper, we present the more recent up to date account of the dynamics of confined water as compared with that of bulk water. Various techniques are used to study the dynamics of confined water. Among them, quasi-elastic and inelastic neutron scattering is a powerful tool to study translational and rotational diffusion as well as vibrational density of states of confined water. Various examples involving water confined in porous media, adsorbed on surface of ionic crystals, in the presence of organic solutes and at the surface of biological molecules are presented. The combined effects of the hydration level and the temperature on the retardation of the water molecules motions are discussed on the basis of phenomenological models as well as of power law fits based on the Mode Coupling Theory.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.12.Ex Neutron scattering (including small-angle scattering) - 66.30.Pa Transport properties of condensed matter (nonelectronic): Diffusion in nanoscale solids     

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.     

Neutron scattering and compressibility measurements for the study of hydration effects on polymeric aqueous solutions

Summary The present paper reports the results of compressibility and incoherent quasi-elastic neutron scattering measurements performed on polymeric systems and on their aqueous solutions. From the compressibility data, the temperature evolution of the polymer hydration number can be derived. On the other hand, neutron data show that the translational diffusion coefficientD _T turns out to be higher in the case of ethylene glycol + water, with respect to that of pure, suggesting that the water molecules act as a ?structure breaker? of the intermolecular connectivity existing into the pure. Furthermore the dynamical properties of the H₂O molecules in the presence of poly(ethylene glycol) deeply differ from those in the bulk, and show that we are in the presence of entangled water. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Quasi-elastic neutron scattering of benzene in Na-mordenite

The rotational and translational dynamics of benzene adsorbed in Na-mordenite have been studied by incoherent quasi-elastic neutron scattering. The measurements were performed at two benzene coverages at 300, 400 and 450 K. The observed quasi-elastic broadenings are described by a uniaxial rotational model about the six-fold axis of benzene. The mean time between successive jumps, at 300 K, is τ=1.45 × 10^?12 s at low coverage and 2.05×10^?12 s at high coverage. The correlation times follow an Arrhenius law with E_A=4.51 kJ mol^?1, at both coverages. The translational diffusion coefficient has been measured at 300 K and was found to be 0.67 × 10^?6 cm²s^?1.     

The application of quasi-elastic neutron scattering techniques (QENS) in surface diffusion studies

Neutron scattering techniques such as quasi-elastic neutron scattering, QENS, have proven to be well-suited tools for studying structure and dynamics of surface adsorbed molecules. In contrast to many more widely used surface science techniques neutron scattering allows the microscopic characterization of samples under a wide range of thermodynamic conditions, as the samples are not constrained to ultra high vacuum environment. Moreover, neutron scattering allows the separation of coherent and incoherent scattering, giving access to different diffusive mechanisms such as single particle diffusion, mass transport, rotations, or vibrations. In this paper we will review recent progress and the state-of-the-art in neutron scattering experiments on surface adsorbed molecules in the sub-monolayer coverage range with a specific emphasis on studies of carbon and other high surface density substrates. We will also cover recent progress in theoretical modeling, since the usefulness of neutron scattering data on surface dynamics can be strongly enhanced by computational modeling, such as molecular dynamics (MD) simulations and the development of analytical models.     

Translational diffusion of water in compacted clay systems

The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.     

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.     

The dynamics of water in small volumes probed by incoherent quasi-elastic neutron scattering

Summary The dynamics of water molecules occupying very small volumes can be probed by incoherent quasi-elastic neutron scattering. We summarise some results obtained with wet samples of porous silica and hydrated proteins. The movements of the molecules in the vicinity of the substrate are extremely reduced as compared to those of bulk water at the same temperature. Almost only rotational movements subsist and the local diffusion is of the order of that observed in deeply supercooled water. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Dynamic crossover in hydration water of curing cement paste: the effect of superplasticizer

The influence of a new comb-shaped polycarboxylate-based superplasticizer (CSSP) on the hydration kinetics and transport properties of aged cement pastes has been investigated by high-resolution quasi-elastic neutron scattering (QENS) and low temperature differential scanning calorimetry (LT-DSC). A new method of analysis of QENS spectra is proposed. By applying the refined method we were able to access to four independent physical parameters including the self-diffusion coefficient of the hydration water confined in the cement paste. Mean squared displacement (MSD) of the hydrogen atom for mobile water molecules displays a dynamic crossover temperature in agreement with DSC data. The experimental results indicate that CSSP polymer added into cement paste moderates the hydration process and decreases the dynamic crossover temperature of the hydration water.     

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.      

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.     

Dynamical properties of water in living cells

With the aim of studying the effect of water dynamics on the properties of biological systems, in this paper, we present a quasi-elastic neutron scattering study on three different types of living cells, differing both in their morphological and tumor properties. The measured scattering signal, which essentially originates from hydrogen atoms present in the investigated systems, has been analyzed using a global fitting strategy using an optimized theoretical model that considers various classes of hydrogen atoms and allows disentangling diffusive and rotational motions. The approach has been carefully validated by checking the reliability of the calculation of parameters and their 99% confidence intervals. We demonstrate that quasi-elastic neutron scattering is a suitable experimental technique to characterize the dynamics of intracellular water in the angstrom/picosecond space/time scale and to investigate the effect of water dynamics on cellular biodiversity.     

Dipole moment and far infra-red absorption of quinuclidine (1-aza-bicyclo-[2-2-2]-octane)

Incoherent quasi-elastic neutron-scattering spectra have been measured on powder samples of D-BPBAC in its smectic E, B and A phases using the high-resolution backscattering technique. The data have been analysed in terms of translational diffusion and a localized rotational motion. It has been found that the apparent translational diffusion constant has a temperature dependent value of ～1 × 10^-6 cm² s^-1 in the smectic A phase but is smaller by at least an order of magnitude in the smectic B and E phases. Comparison of the widths of the rotational components of the spectra with other measurements suggests that simple rotational models are not adequate to explain the experimental data.     

Neutron quasi-elastic scattering of n-triacosane

A neutron quasi-elastic investigation on a well-orientated chain axis crystal of n-triacosane has been undertaken using the twin rotor spectrometer on the Pluto reactor at Harwell with an incident neutron wavelength of 2·64 Å. Data taken in the plastic phase show considerable quasi-elastic broadening, both perpendicular and parallel to the chain axis. Good agreement with experiment is obtained using a continuous rotation diffusion model with translational diffusion. On cooling below the plastic phase transition, slight quasi-elastic broadening isalso seen in both configurations.     

Dynamical molecular disorder and diffuse scattering in an alkane/urea incommensurate inclusion compound

The diffraction pattern of n-alkane/urea inclusion compounds is known to contain a broad diffuse scattering related to the molecular form factor of the alkane species, indicating individual rotational and/or translational disorder of these molecules. Inelastic coherent neutron scattering reveals for the first time the entirely dynamical character of the alkane molecular disorder around room temperature. This observation fully agrees with a recent model of the coherent scattering cross section of totally uncorrelated motions. In the ordered phase this diffuse scattering remains, but elastic, and very low energy molecular vibrational modes are evidenced. These observations are discussed in relation with previous incoherent neutron scattering results. Received 9 February 2001 and Received in final form 1st August 2001     

A theoretical study of rotational and translational diffusion dynamics of molecules with a six-fold point symmetry adsorbed on a hexagonal lattice by neutron scattering

A complete analytical model for the rotational and translational diffusion of molecules with a six-fold point symmetry on a hexagonal lattice is presented. It can be applied, in particular, to the diffusion of benzene molecules adsorbed flat on the basal plane of graphite in the case of incoherent scattering. Under the weak hindered approximation, the classical mechanics framework and making use of the van Hove formalism of correlation functions, the intermediate scattering function and its Fourier transform, the scattering law, are both obtained. They can be expressed as sums of exponential decays or Lorentzian functions, respectively, containing the contribution of each of the dynamical processes taking place. In the case of benzene lying flat on the substrate we expect translational diffusion, continuous rotations of isolated molecules and hindered rotations of molecules within clusters. Each particular diffusive mechanism can be recognized owing to its particular signature in the dependence of the quasi-elastic broadening on the momentum transfer.     

Comparison of the dynamics of MIL-53(Cr) and MIL-47(V) frameworks using neutron scattering and DFT methods

The low-frequency modes of MIL-47(V) and MIL-53(Cr) frameworks have been measured by inelastic neutron scattering. The librational modes of benzene are very close in frequency in the two structures. First-principles calculations have been performed to assign the measured spectra. The calculated values of the rotational barriers of the benzene rings are in reasonable agreement with experimental data. The relatively large barriers (413–588 meV) explain the absence of quasi-elastic broadening on the neutron time scale.     

Structure and dynamics of water confined in a nanoporous sol-gel silica glass: a neutron scattering study

During recent years, the understanding of the modification of the structure and dynamics of water confined in different environments has been the focus of much interest in scientific research. This topic is in fact of great relevance in a lot of technological areas and, in living systems, essential water-related phenomena occur in restricted geometries in cells, and active sites of proteins and membranes, or at their surface. In this paper we report on the most recent up to date account of structural and dynamical properties of confined water in comparison with the bulk state. In particular, as far as structure is concerned, we present new neutron diffraction results on heavy water confined in a fully hydrated sol-gel silica glass (GelSil) as a function of the temperature. At low T, the nucleation of cubic ice superimposed to liquid water, already observed for water within Vycor glasses, is discussed. As far as the dynamics is concerned, we report results of a detailed spectroscopic analysis of diffusive relaxation and vibrational properties of water confined in nanopores of Gelsil glass, at different temperatures and hydration percentages, performed by our research group during recent years by means of incoherent quasi-elastic (IQENS) and inelastic (IINS) neutron scattering. IQENS spectra are analysed in the framework of the relaxing cage model (RCM). IINS spectra show the evolution of the one-phonon-amplitude weighted proton vibrational density of states (VDOS), Z(ω), when water loses its peculiar bulk properties and originates new structural environments due to its surface interactions.     

Diffusion of water in nano-porous polyamide membranes: Quasielastic neutron scattering study

Dynamics of water sorbed in a reverse osmosis polyamide membrane (ROPM) as studied by quasielastic neutron scattering (QENS) is reported here. The trimesoylchloride-m-phenylene diamine based ROPM is synthesized by interfacial polymerization technique. QENS data indicates that translational motion of water confined in ROPM gets modified compared to bulk water whereas rotational motion remains unaltered. Translational motion of water in ROPM is found to follow random jump diffusion with lower diffusivity compared to bulk water. Translational diffusivity does not show the Arrhenius behaviour.     

Fluidity of hydration layers nanoconfined between mica surfaces

We perform molecular dynamics simulations to investigate the shear dynamics of hydration water nanoconfined between two mica surfaces at 1 bar pressure and 298 K. Newtonian plateaus of shear viscosity comparable to the bulk value for different hydration layers D=0.92-2.44 nm are obtained. The origin of this persistent fluidity of the confined aqueous system is found to be closely associated with the rotational dynamics of water molecules, accompanied by fast translational diffusion under this confinement.