The water bimodal inherent structure and the liquid-liquid transition as proposed by the experimental density data.

The bulk water density data are studied in a very large temperature-pressure range, from stable liquid to glass in the frame of water polymorphism. Because this thermodynamic variable evidences a crossover T*, above which the hydrogen bond (HB) is unable to arrange tetrahedral networks, the T-dependence of their isobars was considered. Such an analysis also shows pressure, P*, around which their thermal behaviors are completely different: concave below P* (with maxima and minima) and convex above (without extremes). Having ρ's measured values of the bimodal structures of the liquid phase, HDL (ρHDL), made of not-bonded monomers (ρNHB) and partially bonded dimers plus trimers (ρNHB), and LDL tetramers (ρLDL) the isobars of the relative distributions [W(P, T)] of the three species (WLDL, WPHB, and WNHB) have been evaluated. The results were studied by means of a logistic function (LF) that details the evolutions of the relative polpulations of the water LDL and HDL phases by decreasing T (for the isobars, in the range of 0.1-400 MPa). The LFs analysis obtained by proposing a full connection between liquid water from its supercooled metastable phase to the stable up to the boiling temperature identifies the Widom line quite satisfactorily and fully supports the presence of the liquid-liquid critical point in the deep supercooled region, located at about 190 K and in the range 200 > P > 180 MPa.

The Time-Temperature Superposition of Polymeric Rubber Gels Treated by Means of the Mode-Coupling Theory.

Viscoelastic relaxation measurements on styrene-butadiene rubbers (SBRs) doped with carbon nanotube (CNT) at different concentrations around the sol-gel transition show the time-temperature superposition (TTS). This process is described in terms of the mode coupling theory (MCT) approach to viscoelasticity by considering the frequency behavior of the loss modulus E″(ω) and showing that the corresponding TTS is linked to ω1/2 decay. From the analysis of the obtained data, we observe that the interaction between SBRs and CNT determines different levels of decay according to their concentration. Systems with the lowest CNT concentration are only characterized in the studied T-range by their fragile glass-forming behavior. However, at a specific temperature TL, those with the highest CNT concentration show a crossover towards pure Arrhenius that, according to the MCT, indicates the presence of kinetic glass transition (KGT), where system response functions are characterized by scaling behaviors.

The Hydrophobic Effect Studied by Using Interacting Colloidal Suspensions.

Interactions between nanoparticles (NPs) determine their self-organization and dynamic processes. In these systems, a quantitative description of the interparticle forces is complicated by the presence of the hydrophobic effect (HE), treatable only qualitatively, and due to the competition between the hydrophobic and hydrophilic forces. Recently, instead, a sort of crossover of HE from hydrophilic to hydrophobic has been experimentally observed on a local scale, by increasing the temperature, in pure confined water and studying the occurrence of this crossover in different water-methanol solutions. Starting from these results, we then considered the idea of studying this process in different nanoparticle solutions. By using photon correlation spectroscopy (PCS) experiments on dendrimer with OH terminal groups (dissolved in water and methanol, respectively), we show the existence of this hydrophobic-hydrophilic crossover with a well defined temperature and nanoparticle volume fraction dependence. In this frame, we have used the mode coupling theory extended model to evaluate the measured time-dependent density correlation functions (ISFs). In this context we will, therefore, show how the measured spectra are strongly dependent on the specificity of the interactions between the particles in solution. The observed transition demonstrates that just the HE, depending sensitively on the system thermodynamics, determines the hydrophobic and hydrophilic interaction properties of the studied nanostructures surface.

Survey on Adsorption of Low Molecular Weight Compounds in Cu-BTC Metal-Organic Framework: Experimental Results and Thermodynamic Modeling.

This contribution aims at providing a critical overview of experimental results for the sorption of low molecular weight compounds in the Cu-BTC Metal-Organic Framework (MOF) and of their interpretation using available and new, specifically developed, theoretical approaches. First, a literature review of experimental results for the sorption of gases and vapors is presented, with particular focus on the results obtained from vibrational spectroscopy techniques. Then, an overview of theoretical models available in the literature is presented starting from semiempirical theoretical approaches suitable to interpret the adsorption thermodynamics of gases and vapors in Cu-BTC. A more detailed description is provided of a recently proposed Lattice Fluid approach, the Rigid Adsorbent Lattice Fluid (RALF) model. In addition, to deal with the cases where specific self- and cross-interactions (e.g., H-bonding, Lewis acid/Lewis base interactions) play a role, a modification of the RALF model, i.e., the RALFHB model, is introduced here for the first time. An extension of both RALF and RALFHB is also presented to cope with the cases in which the heterogeneity of the rigid adsorbent displaying a different kind of adsorbent cages is of relevance, as it occurs for the adsorption of some low molecular weight substances in Cu-BTC MOF.

The Interplay between the Theories of Mode Coupling and of Percolation Transition in Attractive Colloidal Systems.

In the recent years a considerable effort has been devoted to foster the understanding of the basic mechanisms underlying the dynamical arrest that is involved in glass forming in supercooled liquids and in the sol-gel transition. The elucidation of the nature of such processes represents one of the most challenging unsolved problems in the field of material science. In this context, two important theories have contributed significantly to the interpretation of these phenomena: the Mode-Coupling theory (MCT) and the Percolation theory (PT). These theories are rooted on the two pillars of statistical physics, universality and scale laws, and their original formulations have been subsequently modified to account for the fundamental concepts of Energy Landscape (EL) and of the universality of the fragile to strong dynamical crossover (FSC). In this review, we discuss experimental and theoretical results, including Molecular Dynamics (MD) simulations, reported in the literature for colloidal and polymer systems displaying both glass and sol-gel transitions. Special focus is dedicated to the analysis of the interferences between these transitions and on the possible interplay between MCT and PT. By reviewing recent theoretical developments, we show that such interplay between sol-gel and glass transitions may be interpreted in terms of the extended F13 MCT model that describes these processes based on the presence of a glass-glass transition line terminating in an A3 cusp-like singularity (near which the logarithmic decay of the density correlator is observed). This transition line originates from the presence of two different amorphous structures, one generated by the inter-particle attraction and the other by the pure repulsion characteristic of hard spheres. We show here, combining literature results with some new results, that such a situation can be generated, and therefore experimentally studied, by considering colloidal-like particles interacting via a hard core plus an attractive square well potential. In the final part of this review, scaling laws associated both to MCT and PT are applied to describe, by means of these two theories, the specific viscoelastic properties of some systems.

Hydrophilic and Hydrophobic Effects on the Structure and Themodynamic Properties of Confined Water: Water in Solutions.

NMR spectroscopy is used in the temperature range 180-350 K to study the local order and transport properties of pure liquid water (bulk and confined) and its solutions with glycerol and methanol at different molar fractions. We focused our interest on the hydrophobic effects (HE), i.e., the competition between hydrophilic and hydrophobic interactions. Nowadays, compared to hydrophilicity, little is known about hydrophobicity. Therefore, the main purpose of this study is to gain new information about hydrophobicity. As the liquid water properties are dominated by polymorphism (two coexisting liquid phases of high and low density) due to hydrogen bond interactions (HB), creating (especially in the supercooled regime) the tetrahedral networking, we focused our interest to the HE of these structures. We measured the relaxation times (T1 and T2) and the self-diffusion (DS). From these times, we took advantage of the NMR property to follow the behaviors of each molecular component (the hydrophilic and hydrophobic groups) separately. In contrast, DS is studied in terms of the Adam-Gibbs model by obtaining the configurational entropy (Sconf) and the specific heat contributions (CP,conf). We find that, for the HE, all of the studied quantities behave differently. For water-glycerol, the HB interaction is dominant for all conditions; water-methanol, two different T-regions above and below 265 K are observable, dominated by hydrophobicity and hydrophilicity, respectively. Below this temperature, where the LDL phase and the HB network develops and grows, with the times and CP,conf change behaviors leading to maxima and minima. Above it, the HB becomes weak and less stable, the HDL dominates, and hydrophobicity determines the solution.

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.

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.

Comparing Molecular Mechanisms in Solar NH3 Production and Relations with CO2 Reduction.

Molecular mechanisms for N2 fixation (solar NH3) and CO2 conversion to C2+ products in enzymatic conversion (nitrogenase), electrocatalysis, metal complexes and plasma catalysis are analyzed and compared. It is evidenced that differently from what is present in thermal and plasma catalysis, the electrocatalytic path requires not only the direct coordination and hydrogenation of undissociated N2 molecules, but it is necessary to realize features present in the nitrogenase mechanism. There is the need for (i) a multi-electron and -proton simultaneous transfer, not as sequential steps, (ii) forming bridging metal hydride species, (iii) generating intermediates stabilized by bridging multiple metal atoms and (iv) the capability of the same sites to be effective both in N2 fixation and in COx reduction to C2+ products. Only iron oxide/hydroxide stabilized at defective sites of nanocarbons was found to have these features. This comparison of the molecular mechanisms in solar NH3 production and CO2 reduction is proposed to be a source of inspiration to develop the next generation electrocatalysts to address the challenging transition to future sustainable energy and chemistry beyond fossil fuels.

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.

A community-built calibration system: The case study of quantification of metabolites in grape juice by qNMR spectroscopy.

Nuclear Magnetic Resonance (NMR) is an analytical technique extensively used in almost every chemical laboratory for structural identification. This technique provides statistically equivalent signals in spite of using spectrometer with different hardware features and is successfully used for the traceability and quantification of analytes in food samples. Nevertheless, to date only a few internationally agreed guidelines have been reported on the use of NMR for quantitative analysis. The main goal of the present study is to provide a methodological pipeline to assess the reproducibility of NMR data produced for a given matrix by spectrometers from different manufacturers, with different magnetic field strengths, age and hardware configurations. The results have been analyzed through a sequence of chemometric tests to generate a community-built calibration system which was used to verify the performance of the spectrometers and the reproducibility of the predicted sample concentrations.

Specific Heat and Transport Functions ofWater.

Numerous water characteristics are essentially ascribed to its peculiarity to form stronghydrogen bonds that become progressively more stable on decreasing the temperature. However, thestructural and dynamical implications of the molecular rearrangement are still subject of debate andintense studies. In this work, we observe that the thermodynamic characteristics of liquid water arestrictly connected to its dynamic characteristics. In particular, we compare the thermal behaviourof the isobaric specific heat of water, measured in different confinement conditions at atmosphericpressure (and evaluated by means of theoretical studies) with its configurational contribution obtainedfrom the values of the measured self-diffusion coefficient through the use of the Adam-Gibbsapproach. Our results confirm the existence of a maximum in the specific heat of water at about 225K and indicate that especially at low temperature the configurational contributions to the entropy aredominant.

Paper aging and degradation monitoring by the non-destructive two-dimensional micro-Raman mapping.

The aim of this research is the identification of main and common physical-chemical parameters which induce paper degradation. A simple protocol to monitor the quality and molecular structure of ancient papers under natural degradation processes was proposed. Two-dimensional micro-Raman mapping were carried out. Then, point to point, the changes of both the cellulose Raman signature and fluorescence signal intensity were analysed. Modern papers were artificially aged to simulate the natural degradation and a comparison with the trend found for the ancient paper was presented and discussed. Cellulose degradation involves different mechanisms such as dehydration, cleaving of cellulose-glycosidic bonds and glucopyranose rings oxidation, that induce a reduction in the polymerization degree. As shown by the changes of Raman and fluorescence signals and NMR relaxation times, the rate of each processes depends on the initial amount of glycosidic bonds in the cellulose amorphous and crystalline regions, respectively. All these informations are useful both to define procedures to restore old documents and also to preserve the quality of modern papers for a long-term time.

The Proton Density of States in Confined Water (H2O).

The hydrogen density of states (DOS) in confined water has been probed by inelastic neutron scattering spectra in a wide range of its P-T phase diagram. The liquid-liquid transition and the dynamical crossover from the fragile (super-Arrhenius) to strong (Arrhenius) glass forming behavior have been studied, by taking into account the system polymorphism in both the liquid and amorphous solid phases. The interest is focused in the low energy region of the DOS ( E < 10 meV) and the data are discussed in terms of the energy landscape (local minima of the potential energy) approach. In this latest research, we consider a unit scale energy (EC) linked to the water local order governed by the hydrogen bonding (HB). All the measured spectra, scaled according to such energy, evidence a universal power law behavior with different exponents ( γ ) in the strong and fragile glass forming regions, respectively. In the first case, the DOS data obey the Debye squared-frequency law, whereas, in the second one, we obtain a value predicted in terms of the mode-coupling theory (MCT) ( γ ≃ 1.6 ).

Aggregation States of Aß1-40, Aß1-42 and Aßp3-42 Amyloid Beta Peptides: A SANS Study.

Aggregation states of amyloid beta peptides for amyloid beta A ß 1 - 40 to A ß 1 - 42 and A ß p 3 - 42 are investigated through small angle neutron scattering (SANS). The knowledge of these small peptides and their aggregation state are of key importance for the comprehension of neurodegenerative diseases (e.g., Alzheimer's disease). The SANS technique allows to study the size and fractal nature of the monomers, oligomers and fibrils of the three different peptides. Results show that all the investigated peptides have monomers with a radius of gyration of the order of 10 Å, while the oligomers and fibrils display differences in size and aggregation ability, with A ß p 3 - 42 showing larger oligomers. These properties are strictly related to the toxicity of the corresponding amyloid peptide and indeed to the development of the associated disease.

Some considerations on the water polymorphism and the liquid-liquid transition by the density behavior in the liquid phase.

The bulk liquid water density data (ρ) are studied in a very large temperature pressure range including also the glass phases. A thorough analysis of their isobars, together with the suggestions of recent thermodynamical studies, gives evidence of two crossovers at T* and P* above which the hydrogen bond interaction is unable to arrange the tetrahedral network that is at the basis of the liquid polymorphism giving rise to the low density liquid (LDL). The curvatures of these isobars, as a function of T, are completely different: concave below P* (where maxima are) and convex above. In both the cases, a continuity between liquid and glass is observed with P* as the border of the density evolution toward the two different polymorphic glasses (low and high density amorphous). The experimental data of the densities of these two glasses also show a markedly different pressure dependence. Here, on the basis of these observations in bulk water and by considering a recent study on the growth of the LDL phase, by decreasing temperature, we discuss the water liquid-liquid transition and evaluate the isothermal compressibility inside the deep supercooled regime. Such a quantity shows an additional maximum that is pressure dependent that under ambient conditions agrees with a recent X-ray experiment. In particular, the present analysis suggests the presence of a liquid-liquid critical point located at about 180 MPa and 197 K.

The Role of Hydrogen Bonding in the Folding/Unfolding Process of Hydrated Lysozyme: A Review of Recent NMR and FTIR Results.

The biological activity of proteins depends on their three-dimensional structure, known as the native state. The main force driving the correct folding mechanism is the hydrophobic effect and when this folding kinetics is altered, aggregation phenomena intervene causing the occurrence of illnesses such as Alzheimer and Parkinson's diseases. The other important effect is performed by water molecules and by their ability to form a complex network of hydrogen bonds whose dynamics influence the mobility of protein amino acids. In this work, we review the recent results obtained by means of spectroscopic techniques, such as Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopies, on hydrated lysozyme. In particular, we explore the Energy Landscape from the thermal region of configurational stability up to that of the irreversible denaturation. The importance of the coupling between the solute and the solvent will be highlighted as well as the different behaviors of hydrophilic and hydrophobic moieties of protein amino acid residues.

The evaluation of the hydrophilic-hydrophobic interactions and their effect in water-methanol solutions: A study in terms of the thermodynamic state functions in the frame of the transition state theory.

Aqueous solutions of amphiphilic molecules are characterized by the competition between hydrophilic and hydrophobic interactions. These interactions have a different energetic dependence with the temperature. Whereas hydrophilic interactions have been well characterized, a complete theory for the hydrophobic ones is still lacking as well as the comprehension of the effect that the solvent exerts on the solute and vice versa. In this paper from the measured relaxation time, we evaluated the thermodynamic state functions of water-methanol solutions in the frame of the transition state theory. In particular we study the behavior of the Gibbs free energy, enthalpy and entropy of water, methanol and some of their solutions as a function of both temperature and water molar fraction. Our results indicate that the temperature of about 280 K represents a crossover between two regions dominated by hydrophobicity (high T) and hydrophilicity (low T).

The role of water in the degradation process of paper using 1H HR-MAS NMR spectroscopy.

The thermodynamic properties of water are essential for determining the corresponding properties of every biosystem it interacts with. Indeed, the comprehension of hydration mechanisms is fundamental for the understanding and the control of paper degradation pathways induced by natural or artificial aging. In fact, the interactions between water and cellulose at the accessible sites within the fibres' complex structure are responsible for the rupture of hydrogen bonds and the consequent swelling of the cellulose fibres and consumption of the amorphous regions. In this paper we study the hydration process of cellulose in naturally and artificially aged paper samples by measuring the proton spin-lattice (T1) and spin-spin (T2) relaxation times of the macroscopic magnetization through nuclear magnetic resonance (NMR) experiments. The observed behaviour of T1 and T2 is quite complex and strictly dependent on the water content of paper samples. This has been interpreted as due to the occurrence of different mechanisms regulating the water-cellulose interaction within the fibres. Furthermore, we have measured T1 as a function of the artificial aging time comparing the results with those measured on three paper samples dated back to the 15th century. We found that the evolution of T1 in model papers artificially aged is correlated with that of ancient paper, providing therefore a way for estimating the degradation of cellulosic materials in terms of an equivalent time of artificial aging. These results provide fundamental information for industrial applications and for the preservation and restoration of cultural heritage materials based on cellulose such as ancient paper or textiles.

Energy landscape in protein folding and unfolding.

We use (1)H NMR to probe the energy landscape in the protein folding and unfolding process. Using the scheme ⇄ reversible unfolded (intermediate) → irreversible unfolded (denatured) state, we study the thermal denaturation of hydrated lysozyme that occurs when the temperature is increased. Using thermal cycles in the range 295 < T < 365 K and following different trajectories along the protein energy surface, we observe that the hydrophilic (the amide NH) and hydrophobic (methyl CH3 and methine CH) peptide groups evolve and exhibit different behaviors. We also discuss the role of water and hydrogen bonding in the protein configurational stability.