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G Franzese

Publications and source records attributed to G Franzese.

9 recordsLinked to original sources
PubMed2004

Liquid-liquid phase transitions for soft-core attractive potentials.

Using event-driven molecular dynamics simulations, we study a three-dimensional one-component system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. In the case of a narrow attractive well, it has been shown that this potential has two metastable gas-liquid critical points. Here we systematically investigate how the changes of the parameters of this potential affect the phase diagram of the system. We find a broad range of potential parameters for which the system has both a gas-liquid critical point C1 and a liquid-liquid critical point C2. For the liquid-gas critical point we find that the derivatives of the critical temperature and pressure, with respect to the parameters of the potential, have the same signs: they are positive for increasing width of the attractive well and negative for increasing width and repulsive energy of the soft core. This result resembles the behavior of the liquid-gas critical point for standard liquids. In contrast, for the liquid-liquid critical point the critical pressure decreases as the critical temperature increases. As a consequence, the liquid-liquid critical point exists at positive pressures only in a finite range of parameters. We present a modified van der Waals equation which qualitatively reproduces the behavior of both critical points within some range of parameters, and gives us insight on the mechanisms ruling the dependence of the two critical points on the potential's parameters. The soft-core potential studied here resembles model potentials used for colloids, proteins, and potentials that have been related to liquid metals, raising an interesting possibility that a liquid-liquid phase transition may be present in some systems where it has not yet been observed.

Journal Article↗
PubMed2004

Alzheimer's disease and acetylcholinesterase inhibitor agents: a two-year longitudinal study.

Alzheimer's disease (AD) is the most common age-related neurodegenerative disease and an urgent public health problem in many areas of the world. We present the results of the activity of an ad hoc ward, called "Unità di Valutazione Alzheimer " (UVA) (Unit of Evaluation for Alzheimer), along the first two years of the "CRONOS" project (CP). We evaluated the efficacy and safety of acetylcholinesterase inhibitor (AChE-I) agents in the treatment of mild to moderate AD and the population served by the UVA. Three hundred fifty-four patients referred to our ward for cognitive deficits. According to the CP criteria, 66 patients (42 women and 24 men, mean age 74.1 +/- 5.8 years) were enrolled in the study. Evaluation was carried out at baseline and after 3, 9, 15, and 21 months. Our results showed a positive impact of these drugs on patients with mild to moderate AD during the first nine months of treatment. After this period, we observed a progressive lowering of the mini mental state examination (MMSE) scores, with a global impairment of 0.9-1 .0 points per year. The adverse events were generally not troublesome. Our data strongly support the usefulness of UVA in the AD case ascertainment during the early phase of the disease.Moreover, the CP provided a general better knowledge about this important and devastating disease.

Activities of Daily Living↗
PubMed2002

Metastable liquid-liquid phase transition in a single-component system with only one crystal phase and no density anomaly.

We investigate the phase behavior of a single-component system in three dimensions with spherically-symmetric, pairwise-additive, soft-core interactions with an attractive well at a long distance, a repulsive soft-core shoulder at an intermediate distance, and a hard-core repulsion at a short distance, similar to potentials used to describe liquid systems such as colloids, protein solutions, or liquid metals. We showed [Nature (London) 409, 692 (2001)] that, even with no evidence of the density anomaly, the phase diagram has two first-order fluid-fluid phase transitions, one ending in a gas-low-density-liquid (LDL) critical point, and the other in a gas-high-density-liquid (HDL) critical point, with a LDL-HDL phase transition at low temperatures. Here we use integral equation calculations to explore the three-parameter space of the soft-core potential and perform molecular dynamics simulations in the interesting region of parameters. For the equilibrium phase diagram, we analyze the structure of the crystal phase and find that, within the considered range of densities, the structure is independent of the density. Then, we analyze in detail the fluid metastable phases and, by explicit thermodynamic calculation in the supercooled phase, we show the absence of the density anomaly. We suggest that this absence is related to the presence of only one stable crystal structure.

Journal Article↗
PubMed2001

Generic mechanism for generating a liquid-liquid phase transition.

Recent experimental results indicate that phosphorus--a single-component system--can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order transition between two liquids of different densities is consistent with experimental data for a variety of materials, including single-component systems such as water, silica and carbon. Molecular dynamics simulations of very specific models for supercooled water, liquid carbon and supercooled silica predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL transition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part and two characteristic short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state (in particular, liquid metals), and such potentials are often used to describe systems that exhibit a density anomaly. However, our results show that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of a density anomaly.

Journal Article↗
PubMed2000

Potts fully frustrated model: thermodynamics, percolation, and dynamics in two dimensions

We consider a Potts model diluted by fully frustrated Ising spins. The model corresponds to a fully frustrated Potts model with variables having an integer absolute value and a sign. This model presents precursor phenomena of a glass transition in the high-temperature region. We show that the onset of these phenomena can be related to a thermodynamic transition. Furthermore, this transition can be mapped onto a percolation transition. We numerically study the phase diagram in two dimensions (2D) for this model with frustration and without disorder and we compare it to the phase diagram of (i) the model with frustration and disorder and (ii) the ferromagnetic model. Introducing a parameter that connects the three models, we generalize the exact expression of the ferromagnetic Potts transition temperature in 2D to the other cases. Finally, we estimate the dynamic critical exponents related to the Potts order parameter and to the energy.

Journal Article↗
PubMed1999

Precursor phenomena in frustrated systems.

To understand the origin of the dynamical transition, between high-temperature exponential relaxation and low-temperature nonexponential relaxation, that occurs well above the static transition in glassy systems, a frustrated spin model, with and without disorder, is considered. The model has two phase transitions, the lower being a standard spin glass transition (in the presence of disorder) or fully frustrated Ising (in the absence of disorder), and the higher being a Potts transition. Monte Carlo results clarify that in the model with (or without) disorder the precursor phenomena are related to the Griffiths (or Potts) transition. The Griffiths transition is a vanishing transition which occurs above the Potts transition and is present only when disorder is present, while the Potts transition which signals the effect due to frustration is always present. These results suggest that precursor phenomena in frustrated systems are due either to disorder and/or to frustration, giving a consistent interpretation also for the limiting cases of Ising spin glass and of Ising fully frustrated model, where also the Potts transition is vanishing. This interpretation could play a relevant role in glassy systems beyond the spin systems case.

Journal Article↗
PubMedDate not supplied

Photoparoxysmal response on eye closure in photosensitive patients.

The Authors studied the presence of photoparoxysmal response (PPR) during intermittent photic stimulation (IPS) in 2,888 consecutive EEG recordings. PPR was present in 2.3% of EEGs and 10% of all patients referred to EEG laboratory for epilepsy (45 patients, 18 males, 27 females (M:F ratio 1:1.5), mean age 12 yrs). In 24 (53.3%) of these patients PPR was evident only (24.4%) or strikingly (28.9%) on eye closure during IPS. In 7 patients no other epileptic abnormalities were found on basal EEG and during hyperventilation, nor during IPS with eyes closed and with eyes open. The Authors consider the eye closure during IPS the most useful method to reveal a PPR in photosensitive patients and believe that a good IPS technique must include this procedure in the routine EEG examination.

Child↗