P. Migliardo

Raman scattering and structure of normal and supercooled water

Raman spectra in the O–H stretching band in normal and supercooled water have been investigated. Spectra are taken as a function of temperature in the range +95 to −24°C. Absolute Raman cross sections corrected for instrumental and physical factors are obtained. It is shown that the isotropic O–H stretching bands in the range +20 to +95 °C are due to the contribution of ’’open’’ (or tetrahedrically bonded) and ’’closed’’ water that are temperature independent. The percentage of open water α(T) is connected and interpreted in the frame of the Stanley site percolation model. As a consequence, the isobestic point is obtained. In the supercooled region, a third spectral contribution arises, which is centered near the main peak of the isotropic O–H band of ice. Such a contribution is tentatively explained as the presence in supercooled water of true ice, as a heterophase fluctuation.

Anomalous cryoprotective effectiveness of trehalose: Raman scattering evidences

Results of Raman scattering measurements performed on aqueous solutions of the homologous disaccharides (trehalose, maltose, and sucrose) are reported. To get some insight into the effects of disaccharides on the hydrogen bond network of water, and to clarify the reasons that make trehalose the most effective in protecting organisms from dehydration and freezing, we investigate the intramolecular OH stretching mode. To carry out this study, two different approaches are employed: namely, a decomposition of the isotropic spectra into an “open” and a “closed” contribution, and a spectral stripping procedure to extract the “collective” contribution from the polarized spectra. Both procedures agree in suggesting that disaccharides promote, with a different strength, a destructuring effect on the tetrahedral H-bond network of pure water. This result makes plausible the hypothesis that disaccharides obstruct the crystallization process reducing the amount of freezable water, namely destroying the network of wate...

The puzzle of poly(ethylene oxide) aggregation in water: Experimental findings

Aqueous solutions of poly(ethylene oxide) were investigated using the ultrasonic technique, photon correlation spectroscopy (PCS) and nuclear magnetic resonance (NMR), in a wide range of molecular weight (from ethylene glycol to poly(ethylene oxide) 4 000 000 Da). Ultrasonic data reveal that the mixing process is not ideal and show that the polymer–water interaction strength increases with the polymerization degree. PCS and NMR, on the other hand, furnish a free particle diffusion coefficient which satisfies a unique scaling law from 8000 to 4 000 000 Da and demonstrates the good solvent nature of water. These experimental findings indicate that polymer–polymer aggregation processes are not an inherent property of these systems.

Rayleigh wing and Fourier transform infrared studies of intermolecular and intramolecular hydrogen bonds in liquid ethylene glycol

Depolarized low frequency light scattering and infrared data for ethylene glycol are presented. The measurements, performed in the temperature range 7–160 °C, help clarify the processes that characterize the dynamic response of the system. Rayleigh-wing data analysis suggests, through the width and intensity temperature evolution, the existence of a thermally activated process that can be rationalized in terms of a simple two-state model. The extracted activation (ΔH) and binding (ΔG) energies, have been attributed to the ‘transition’ from a gauche configuration, stabilized by an intramolecular H bond, to a simple gauche configuration. In addition, the study of the O-H stretching region, performed on ethylene glycol and on poly(ethylene glycol) by means of the Fourier transform infrared technique allows one to identify the intramolecular, H-bond imposed, sub-band, as well as to assign the various sub-bands originated by the existence of the H-bond potential. The observed dynamics are also discussed within...

Sound velocity and hydration phenomena in aqueous polymeric solutions

Sound velocity and density measurements were performed in polymeric aqueous solutions of poly(ethylene glycol) (PEG 600) and poly(ethylene glycol methyl ether) (PEGME 550) as a function of concentration and temperature. The experimental data unambiguously show that the presence of the polymer changes the water structure near the solute, building up a more compact arrangement in which up to two water molecules for each monomeric unity are allowed. Moreover we tested, at low concentration, a model to study the change induced in the polymer hydration by the addition of salt. No direct evidence of interaction between the hydration water of the polymer and salt is brought to light by the obtained results.

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.

Destructuring effect of trehalose on the tetrahedral network of water: a Raman and neutron diffraction comparison

Results on trehalose/water solutions by neutron diffraction are reported. The analysis of the partial structure factors obtained by neutron diffraction on trehalose/water solutions is interpreted in terms of a destructuring effect of trehalose on the tetrahedral H-bond network of water. This hypothesis, suggested by a previous analysis of the Raman intramolecular OH stretching band, can account for the greater cryoprotective action of trehalose on biological structures.

Experimental simulation of macromolecules in trehalose aqueous solutions: A photon correlation spectroscopy study

The protective effect of trehalose on biological membranes against freezing or dehydration has been the subject of many studies aimed to understand the reasons why some lower organisms, under stress conditions, synthesize trehalose. In this work we report the results of a study on Poly(Ethylene Oxide)/trehalose/water mixtures performed by Photon Correlation Spectroscopy. The chemical structure of the polymer, simpler than that of proteins and its helical conformation in water, constitute a useful starting point for understanding the more complex protein/trehalose/water interactions. In order to distinguish the different dynamics, trehalose and PEO have been studied separately in water, at different concentration and temperature values; then the ternary PEO/trehalose/water system has been investigated at different sugar amounts. The obtained findings support the “water-replacement” hypothesis, indicating that a direct polymer–trehalose interaction occurs. Furthermore, trehalose is shown to affect the swell...

DYNAMIC EVIDENCE OF CHEMICAL AND PHYSICAL TRAPS IN H-BONDED CONFINED LIQUIDS

Rayleigh wing, infrared (IR) absorption, and Raman-scattering measurements are employed to investigate the reorientational and vibrational dynamics of ethylene glycol and its homologous systems, namely ethylene glycol monomethyl ether and ethylene glycol dimethyl ether, in the bulk state and confined into 25 A pores of a sol-gel porous glass. As far as the Rayleigh wing data are concerned, the quantitative analysis of the different contributions in the spectra shows the clear influence of the physical and chemical confinement on the mobility of the studied liquids. In addition the IR and Raman measurements, performed in the O–H stretching region, allowed to identify the intramolecular, H-bond imposed, sub-band, as well as to connect the various sub-bands to different intermolecular environments originated by the existence of the H-bond potential. The observed dynamics is discussed on the basis of current theories for associated liquids.

Conformational distribution of poly(ethylene oxide) in molten phase and in aqueous solution by quasi-elastic and inelastic light scattering

We report on measurements performed by Raman scattering, PCS and ultrasonic velocity measurements on poly(ethylene oxide) both in the molten phase and in aqueous solution. Increasing the polymerization degree, m, the Raman analysis of the D-LAM (disordered longitudinal acoustic mode) spectral contribution to the pure polymers reveals a behaviour of the centre frequency and linewidth which has been connected with an oligomer-polymer transition occurring at . In aqueous solutions the frequency increase towards values corresponding to the crystal ones and the sharpening of the D-LAM spectral contribution indicate that the addition of water destroys the intermolecular interactions and stiffens the coil structure. In addition evidence of a more ordered conformation with respect to the melt phase is presented. The temperature analysis of the D-LAM band and of the hydrodynamic radius, evaluated by PCS, reveals that the solvent power of water increases up to , decreasing at higher temperature. Interpreted in conjunction with ultrasonic data, these apparently differing findings provide a coherent interpretative key capable of encompassing the structural properties of our systems. Finally the role played by inter- and intra-molecular interactions is discussed within the framework of current theories.