1,720,982 research outputs found
A singular thermodynamically consistent temperature at the origin of the anomalous behavior of liquid water
Full text linkThe density maximum of water dominates the thermodynamics of the system under ambient conditions, is strongly P-dependent, and disappears at a crossover pressure P[subscript cross] ~ 1.8 kbar. We study this variable across a wide area of the T–P phase diagram. We consider old and new data of both the isothermal compressibility K[subscript T](T, P) and the coefficient of thermal expansion αP(T, P). We observe that KT(T) shows a minimum at T* ~ 315±5 K for all the studied pressures. We find the behavior of αP to also be surprising: all the αP(T) curves measured at different P cross at T*. The experimental data show a “singular and universal expansivity point” at T* ~ 315 K and αP(T*) ≃ 0.44 10[superscript −3] K[superscript −1]. Unlike other water singularities, we find this temperature to be thermodynamically consistent in the relationship connecting the two response functions.Fulvio Frisone FondazioneNational Science Foundation (U.S.) (NSF Chemistry Division (grant CHE 0911389))National Science Foundation (U.S.) (NSF Chemistry Division CHE 1213217))Italy. Ministero dell'istruzione, dell'università e della ricerca (MIUR-PRIN2008
Erratum to: The fragile-to-strong dynamical crossover and the system viscoelasticity in attractive glass forming colloids
No full textErratum to: Colloid Polym Sci http://dx.doi.org/10.1007/s00396-015-3713-6 The original version of this article, unfortunately, contained errors
The fragile-to-strong dynamical crossover and the system viscoelasticity in attractive glass forming colloids
No full textThis paper is dedicated to Professor Heinz Hoffmann on the occasion of his 80th birthday in celebration of his long-time friendship with both of us (Professors Sow-Hsin Chen and Francesco Mallamace).The dynamical arrest phenomena of an adhesive hard-sphere (AHS) colloid, L64-D 2O system has been studied by using calorimetry and the complex shear modulus. This system is characterized by a rich temperature (T) and volume fraction (ϕ) phase diagram with a percolation line (PT). According to the mode-coupling theory (MCT), a cusp-like singularity and two glassy phases, one attractive (AG) and one repulsive (RG), are supposed to coexist in the phase diagram. The MCT scaling laws used to study the shear viscosity with ϕ and T as control parameters propose the existence of fragile-to-strong dynamic crossover (FSDC) analogous to that observed in molecular supercooled liquid glass formers. The measured critical values of the control parameters, coincident with the PT line, where the clustering process generates the AG phase, define the FSDC locus. This is in agreement with the extended mode-coupling theory that takes into account both cage and inter-cluster hopping effects. In this work, we demonstrate, by considering the frequency dependence of the complex moduli, that there is the onset of a system viscoelasticity as an effect of the clustering accompanying the FSDC. We will show as the measured frequency-dependent complex moduli satisfy the scaling relations predicted by the scalar elasticity percolation theory and well account for the system evolution toward the glass transition process.United States. Dept. of Energy (DOE Grant No. DE-FG02-90ER45429
On the ergodicity of supercooled molecular glass-forming liquids at the dynamical arrest: the o-terphenyl case
Full text linkThe dynamics of supercooled ortho-terphenyl has been studied using photon-correlation spectroscopy (PCS) in the depolarized scattering geometry. The obtained relaxation curves are analyzed according to the mode-coupling theory (MCT) for supercooled liquids. The main results are: i) the observation of the secondary Johari-Goldstein relaxation (β) that has its onset just at the dynamical crossover temperature T[subscript B] (T[subscript M] > T[subscript B] > T[subscript g]); ii) the confirmation, of the suggestion of a recent statistical mechanical study, that such a molecular system remains ergodic also below the calorimetric glass-transition temperature T[subscript g]. Our experimental data give evidence that the time scales of the primary (α) and this secondary relaxations are correlated. Finally a comparison with recent PCS experiments in a colloidal system confirms the primary role of the dynamical crossover in the physics of the dynamical arrest.United States. Dept. of Energy. Office of Basic Energy Sciences (Contract DE-FG02-90ER45429
The Role of Hydrogen Bonding in the Folding/Unfolding Process of Hydrated Lysozyme: A Review of Recent NMR and FTIR Results
No full textThe 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. Keywords: protein denaturation; FTIR; NMR; hydration water; hydrogen bonding; energy landscap
Reply to Elmatad: Supercooled viscous liquids display a fragile-to-strong dynamic crossover
No full textTheory predicts a fragile-to-strong (FS) dynamic crossover temperature Tx in supercooled liquids but, contrary to what is reported in ref. 1, Tx must be >Tg (2), where Tg is the glass transition temperature. Ref. 4 of ref. 1 hypothesizes that a parabolic form is valid in a range To > T > Tx, where To is defined as an onset temperature that marks the crossover from normal liquid behavior to supercooled liquid behavior. A second paper by the same authors (ref. 5 of ref. 1) proposes the range of the hypothesized parabolic behavior can be extended to cover T < T
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