The time dependence dynamics of hydration water changes upon crossing T∗

We carry out a Nuclear Magnetic Resonance (NMR) spectroscopy study on the dynamics of lysozyme hydration water. We consider a hydration level corresponding to a single water monolayer. We investigate the thermodynamical region from 295K to 355 K, at temperatures below and above the “magic” temperature T∗ ≈ 320 K. In particular, we focus our attention on hydration water mean-square displacement (MSD) as a function of the diffusion time at different temperatures. Our results suggest the occurrence of a smooth anomalous diffusion from a sub-diffusive state (T < T ∗) to a super-diffusive one (T > T∗). These conclusions confirm the importance of the temperature T∗ as the border for water behavior

The local order of supercooled water in solution with LiCl studied by NMR proton chemical shift

We study by means of Nuclear Magnetic Resonance (NMR) spectroscopy the local order of water molecules in solution with lithium chloride at eutectic concentration. In particular, by measuring the proton chemical shift as a function of the temperature in the interval 203K < T < 320K, we observe a net change at about 235 K. We ascribe this result to the increase of the hydrogen bond interaction that on decreasing the temperature favors the formation of the network that characterizes the low density liquid phase of water. Furthermore, the Gaussian deconvolution of the NMR peak allows the investigation of the mutual difference between the chemical shift of water solvating lithium and chlorine individually. The thermal behavior of this quantity confirms previous results about the role of the temperature in the solvation mechanisms down to about 225 K. This temperature coincides with that of the so-called Widom line for water supporting the liquid-liquid transition hypothesis

Some aspects of the liquid water thermodynamic behavior: From the stable to the deep supercooled regime

Liquid water is considered to be a peculiar example of glass forming materials because of the possibility of giving rise to amorphous phases with different densities and of the thermodynamic anomalies that characterize its supercooled liquid phase. In the present work, literature data on the density of bulk liquid water are analyzed in a wide temperature-pressure range, also including the glass phases. A careful data analysis, which was performed on different density isobars, made in terms of thermodynamic response functions, like the thermal expansion αP and the specific heat differences CP − CV, proves, exclusively from the experimental data, the thermodynamic consistence of the liquid-liquid transition hypothesis. The study confirms that supercooled bulk water is a mixture of two liquid “phases”, namely the high density (HDL) and the low density (LDL) liquids that characterize different regions of the water phase diagram. Furthermore, the CP − CV isobars behaviors clearly support the existence of both a liquid–liquid transition and of a liquid–liquid critical point

Tailoring chitosan/lta zeolite hybrid aerogels for anionic and cationic dye adsorption

Chitosan (CS) is largely employed in environmental applications as an adsorbent of anionic dyes, due to the presence in its chemical structure of amine groups that, if protonated, act as adsorbing sites for negatively charged molecules. Efficient adsorption of both cationic and anionic dyes is thus not achievable with a pristine chitosan adsorbent, but it requires the combination of two or more components. Here, we show that simultaneous adsorption of cationic and anionic dyes can be obtained by embedding Linde Type A (LTA) zeolite particles in a crosslinked CS‐based aerogel. In order to optimize dye removal ability of the hybrid aerogel, we target the crosslinker concentration so that crosslinking is mainly activated during the thermal treatment after the fast freezing of the CS/LTA mixture. The adsorption of isotherms is obtained for different CS/LTA weight ratios and for different types of anionic and cationic dyes. Irrespective of the formulation, the Langmuir model was found to accurately describe the adsorption isotherms. The optimal tradeoff in the adsorption behavior was obtained with the CS/LTA aerogel (1:1 weight ratio), for which the maximum uptake of indigo carmine (anionic dye) and rhodamine 6G (cationic dye) is 103 and 43 mg g−1, respectively. The behavior observed for the adsorption capacity and energy cannot be rationalized as a pure superposition of the two components, but suggests that reciprocal steric effects, chemical heterogeneity, and molecular interactions between CS and LTA zeolite particles play an important role

Complex viscosity behavior and cluster formation in attractive colloidal systems

The increase of the viscosity, which is observed in attractive colloidal systems by varying the temperature or the volume fraction, can be related to the formation of structures due to particle aggregation. In particular we have studied the non trivial dependence of the viscosity from the temperature and the volume fraction in the copolymer-micellar system L64. The comparison of the experimental data with the results of numerical simulations in a simple model for gelation phenomena suggests that this intriguing behavior can be explained in terms of cluster formation and that this picture can be quite generally extended to other attractive colloidal systems.Comment: 5 pages, 4 figure

A molecular interpretation of the dynamics of diffusive mass transport of water within a glassy polyetherimide

The diffusion process of water molecules within a polyetherimide (PEI) glassy matrix has been analyzed by combining the experimental analysis of water sorption kinetics performed by FTIR spectroscopy with theoretical information gathered from Molecular Dynamics simulations and with the expression of water chemical potential provided by a non‐equilibrium lattice fluid model able to describe the thermodynamics of glassy polymers. This approach allowed us to construct a convincing description of the diffusion mechanism of water in PEI providing molecular details of the process related to the effects of the cross‐ and self‐hydrogen bonding established in the system on the dynamics of water mass transport

The dynamic crossover in water does not require bulk water

Many of the anomalous properties of water may be explained by invoking a second critical point that terminates the coexistence line between the low- and high-density amorphous states in the liquid. Direct experimental evidence of this point, and the associated polyamorphic liquid–liquid transition, is elusive as it is necessary for liquid water to be cooled below its homogeneous-nucleation temperature. To avoid crystallization, water in the eutectic LiCl solution has been studied but then it is generally considered that “bulk” water cannot be present. However, recent computational and experimental studies observe cooperative hydration in which case it is possible that sufficient hydrogen-bonded water is present for the essential characteristics of water to be preserved. For femtosecond optical Kerr-effect and nuclear magnetic resonance measurements, we observe in each case a fractional Stokes–Einstein relation with evidence of the dynamic crossover appearing near 220 K and 250 K respectively. Spectra obtained in the glass state also confirm the complex nature of the hydrogen-bonding modes reported for neat room-temperature water and support predictions of anomalous diffusion due to “worm-hole” structure

Experimental Evidence For Self-Similar Structures In The Aggregation Of Porphyrins In Aqueous Solutions

We have studied the aggregation of the porphyrin t-H₂Pagg in aqueous solution by light scattering. The intensity profile of the elastically scattered light, studied in the exchanged wave-vector range 0.2 ≤ q ≤ 31.4 μ m⁻¹, indicates that the aggregation produces large monodisperse clusters having a fractal structure, and is driven by diffusion-limited aggregation kinetics. Additional measurements performed at different q values confirm such a picture giving a hydrodynamic radius R(H) consistent with the radius of gyration R(g) measured by elastic scattering. This is explained taking into account the q² dependence observed in the mean decay rate of the intensity-intensity correlation function and the effect on this latter dynamical quantity of anisotropies in the cluster structure

Static and dynamic heterogeneities in irreversible gels and colloidal gelation

We compare the slow dynamics of irreversible gels, colloidal gels, glasses and spin glasses by analyzing the behavior of the so called non-linear dynamical susceptibility, a quantity usually introduced to quantitatively characterize the dynamical heterogeneities. In glasses this quantity typically grows with the time, reaches a maximum and then decreases at large time, due to the transient nature of dynamical heterogeneities and to the absence of a diverging static correlation length. We have recently shown that in irreversible gels the dynamical susceptibility is instead an increasing function of the time, as in the case of spin glasses, and tends asymptotically to the mean cluster size. On the basis of molecular dynamics simulations, we here show that in colloidal gelation where clusters are not permanent, at very low temperature and volume fractions, i.e. when the lifetime of the bonds is much larger than the structural relaxation time, the non-linear susceptibility has a behavior similar to the one of the irreversible gel, followed, at higher volume fractions, by a crossover towards the behavior of glass forming liquids.Comment: 9 pages, 3 figure