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By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature TL=223±2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190–280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1/D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time ≤sτT>, as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water [ABSTRACT FROM AUTHOR]

Small-angle neutron scattering (SANS) measurements have been performed in heavy water solutions of 2-butoxyethanol as a function of concentration and temperature. The results indicate that there exist some alcohol aggregation above a narrow concentration range (x=0.015–0.02, where x is the alcohol molar fraction) and the size of the aggregates increases with temperature taking a maximum value at a composition close to the critical concentration. The analysis of the size and of the absolute scattered intensity at q=0(I0) in terms of a microscopic model of aggregation, as well as the indications from other experiments, suggest that these aggregates are micelle-like structures. [ABSTRACT FROM AUTHOR]

Depolarized Rayleigh and low frequency Raman scattering data in bulk supercooled water are presented. The measurements, performed from the normal region down to -26.95 °C for the first time, allow a quantitative data analysis and reveal the existence of a fine structure of the central contribution. We have identified a ‘‘fast’’ and ‘‘slow’’ contribution whose intensities show an opposite behavior as a function of temperature. Such a behavior is understood as a decay of local order due to the H bond. The spectral density function, as derived from the depolarized Raman data, shows the presence of acoustical and intermolecular optical contributions characteristic of many structured disordered materials. A critical comparison with molecular dynamics (MD) and neutron scattering results is also presented. [ABSTRACT FROM AUTHOR]

In this work we study an attractive micellar system for which the percolation curve terminates near the critical point. We have studied such an intriguing situation by means of scattering (elastic and dynamical) and viscoelasticity experiments. Obtained data are accounted by considering in a proper way the fractal clustering processes typical of percolating systems and the related scaling concepts. We observe that the main role in the system structure and dynamics it is played by the cluster's partial screening of hydrodynamic interaction. This behaves on approaching the percolation threshold dramatic effects on the system rheological properties and on the density decay relaxations. The measured correlation functions assume a stretched exponential form and the system becomes strongly viscoelastic. The overall behavior of the measured dynamical and structural parameters indicates, that in the present micellar system, the clustering process originates dilute, polydisperse and swelling structures. Finally, this originates an interesting situation observed in the present experiment. As it has been previously, proposed by A. Coniglio et al., percolation clusters can be considered to be “Ising clusters” with the same properties as the Fisher's critical droplets. Therefore at the critical point the percolation connectedness length (ξ[sub p]) can be assumed as the diverging correlation length (ξ[sub p] ≡ ξ) and the mean cluster size diverges as the susceptibility. [ABSTRACT FROM AUTHOR]

Examines the scaling properties of porphyrins and dendritic polymer systems. Molecule size of porphyrins; Growth of large diffusion-limited fractal aggregates; Fractal nature of dendrimers.

All parameters of a spectrometer, signal detection, and normalization of the recorded Raman signal were controlled by a compatible IBM PC/XT computer which was connected to a Spex Ramalog 5 system. By the use of a motor-driven polarizer, we are able for each single step time of the instrument, to monitor simultaneously the contributions IVV and IVH of the spectrum. Moreover, the simultaneous normalization of the signal with the intensity of the exciting laser light allows one to avoid the effects of the unwanted laser instability. The program can be requested by electronic mail. [ABSTRACT FROM AUTHOR]

Background Gangliosides are biological glycolipids participating in rafts, structural and functional domains of cell membranes. Their headgroups are able to assume different conformations when packed on the surface of an aggregate, more lying or standing. Switching between different conformations is possible, and is a collective event. Switching can be induced, in model systems, by concentration or temperature increase, then possibly involving ganglioside–water interaction. In the present paper, the effect of GM1 ganglioside headgroup conformation on the water structuring and interactions is addressed. Methods Depolarized Rayleigh Scattering, Raman Scattering, Quasielastic Neutron Scattering and NMR measurements were performed on GM1 ganglioside solutions, focusing on solvent properties. Results All used techniques agree in evidencing differences in the structure and dynamics of solvent water on different time-and-length scales in the presence of either GM1 headgroup conformations. Conclusions In general, all results indicate that both the structural properties of solvent water and its interactions with the sugar headgroups of GM1 respond to surface remodelling. The extent of this modification is much higher than expected and, interestingly, ganglioside headgroups seem to turn from cosmotropes to chaotropes upon collective rearrangement from the standing- to the lying-conformation. Significance In a biological perspective, water structure modulation could be one of the physico-chemical elements contributing to the raft strategy, both for rafts formation and persistence and for their functional aspects. In particular, the interaction with approaching bodies could be favoured or inhibited or triggered by complex-sugar-sequence conformational switch. This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo. [ABSTRACT FROM AUTHOR]