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Dive into the research topics where G. Salvetti is active.

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Featured researches published by G. Salvetti.


Journal of Chemical Physics | 2005

Thermodynamic functions of water and ice confined to 2nm radius pores

Elpidio Tombari; G. Salvetti; Carlo Ferrari; G. P. Johari

The heat capacity C(p) of the liquid state of water confined to 2 nm radius pores in Vycor glass was measured by temperature modulation calorimetry in the temperature range of 253-360 K, with an accuracy of 0.5%. On nanoconfinement, C(p) of water increases, and the broad minimum in the C(p) against T plot shifts to higher temperature. The increase in the C(p) of water is attributed to an increase in the phonon and configurational contributions. The apparent heat capacity of the liquid and partially frozen state of confined water was measured by temperature scanning calorimetry in the range of 240-280 K with an accuracy of 2%, both on cooling or heating at 6 K h(-1) rate. The enthalpy, entropy, and free energy of nanoconfined liquid water have been determined. The apparent heat capacity remains higher than that of bulk ice at 240 K and it is concluded that freezing is incomplete at 240 K. This is attributed to the intergranular-water-ice equilibrium in the pores. The nanoconfined sample melts over a 240-268 K range. For 9.6 wt % nanoconfined water concentration ( approximately 50% of the maximum filling) at 280 K, the enthalpy of water is 81.6% of the bulk water value and the entropy is 88.5%. For 21.1 wt % (100% filling) the corresponding values are 90.7% and 95.0%. The enthalpy decrease on nanoconfinement is a reflection of the change in the H-bonded structure of water. The use of the Gibbs-Thomson equation for analyzing the data has been discussed and it is found that a distribution of pore size does not entirely explain our results.


Journal of Chemical Physics | 1995

Calorimetric effects of intergranular water in ice

G. Salvetti; Elpidio Tombari; G. P. Johari

The heat capacity, Cp, of microcrystalline and large or single crystal ice was measured from 270 to 272.6 K and the effect of the heat input (required before measuring the increase in temperature) on the measured Cp investigated. The measured Cp of microcrystalline solids was much higher than that of large and single crystal solids and increased further with increase in the heat input. At 271.16 K, the measured Cp was independent of the heat input, but for higher temperatures, it increased nonlinearly with the heat input. These effects are caused by the increase in the volume fraction of intergranular water in impurity free ice with increase in both its temperature and the heat input. A significant fraction of the heat used for Cp measurement is consumed in melting ice at grain junctions which does not raise the temperature. Thus, the measured Cp appears to be much greater than the true value. Quantitative analysis of the data yielded results which agree reasonably well with the prediction of a theory giv...


Journal of Chemical Physics | 2006

Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt

Elpidio Tombari; S. Presto; G. Salvetti; G. P. Johari

We report a thermodynamic study of the formation of tetrahydrofuran clathrate hydrate by explosive crystallization of water-deficient, near stoichiometric, and water-rich solutions, as well as of the heat capacity, C(p), of (i) supercooled tetrahydrofuran-H2O solutions and of the clathrate hydrate, (ii) tetrathydrofuran (THF) liquid, and (iii) supercooled water and the ice formed on its explosive crystallization. In explosive freezing of supercooled solutions at a temperature below 257 K, THF clathrate hydrate formed first. The nucleation temperature depends on the cooling rate, and excess water freezes on further cooling. The clathrate hydrate melts reversibly at 277 K and C(p) increases by 770 J/mol K on melting. The enthalpy of melting is 99.5 kJ/mol and entropy is 358 J/mol K. Molar C(p) of the empty host lattice is less than that of the ice, which is inconsistent with the known lower phonon frequency of H2O in the clathrate lattice. Analysis shows that C(p) of THF and ice are not additive in the clathrate. C(p) of the supercooled THF-H2O solutions is the same as that of water at 247 K, but less at lower temperatures and more at higher temperatures. The difference tends to become constant at 283 K. The results are discussed in terms of the hydrogen-bonding changes between THF and H2O.


Journal of Chemical Physics | 2002

Spontaneous decrease in the heat capacity of a glass

Elpidio Tombari; S. Presto; G. Salvetti; G. P. Johari

The real and imaginary components, Cp′ and Cp″, respectively, of the complex heat capacity, Cp*=Cp′−iCp″, of a molecular liquid have been measured in the temperature range of its vitrification and in the glassy state, and the effect of spontaneous structural relaxation has been determined in real time. Cp′ of the glassy state is found to decrease with time. Analysis shows that this is mainly due to the decrease of configurational entropy as the characteristic time of the Cp′ spectra increases and consequently the contribution from the unfrozen, faster modes of the α-relaxation process decreases. There may also be a significant decrease in the vibrational and anharmonic force contributions as the glass densifies. Interpretations in terms of the potential energy landscape model suggest that for each state of lower energy attained with time, the number of minima in the potential energy surface decreases, and the minima become deeper.


Thermochimica Acta | 2000

A modulated adiabatic scanning calorimeter (MASC)

G. Salvetti; C Cardelli; Carlo Ferrari; Elpidio Tombari

Abstract The modulated adiabatic scanning calorimeter (MASC) described here is designed to work in both the time-domain (adiabatic-like step-scanning) and in the frequency domain (modulated temperature/power-scanning) operational modes. The cylindrical form of the cell and the controlled thermal environment in which the sample is located make it possible: (i) to write detailed and reliable equations to describe the heat flow; (ii) to derive analytical relations to calculate the energy release from the sample and the real and imaginary parts of the complex heat capacity at the modulation frequency; (iii) to study narrow phase transitions using the power modulated scanning mode; and (iv) to generate adiabatic-like conditions. The thermal environment is controlled by means of an active thermal shield which is kept at a temperature close to that of the sample cell in order to minimise the thermal gradients to which the sample is exposed. The temperature difference between the shield and the cell is set at the value required to control or cancel heat leakage from the cell. MASC can be used to carry out measurements on a single sample in different operational modes that can be made operative under the control of its built-in software. The parameters of interest can be automatically calculated by means of a program based on a model of the cell+sample system that will be described in a second paper. To test the performance of the calorimeter, samples of materials which undergo a first-order phase transition or a glass transition (indium, ice, and polystyrene) were studied. The specific features of the calorimetric cell, the calibration procedure, and the experimental results are reported and discussed.


Journal of Chemical Physics | 2000

The temperature and polymerization effects on the relaxation time and conductivity, and the evolution of the localized motions

Elpidio Tombari; G. Salvetti; G. P. Johari

To examine the manner in which molecular dynamics of a polymerizing liquid (stoichiometric amounts of 4,4′-diaminodicyclohexylamine and diglycidyl ether of bisphenol-A) evolves during thermal cycling from its (molecular) vitreous state to its fully polymerized vitreous state, calorimetry, and dielectric spectrometry were performed simultaneously in real time. The half-width of the relaxation spectrum of the liquid was relatively narrow and became narrower on heating. This was followed by an increase in the characteristic relaxation time and the spectrum became broader as polymerization occurred and reached completion. The dc conductivity initially increased and then decreased. The faster dynamics of the Johari–Goldstein relaxation in the fully polymerized state evolved as polymerization reached completion and the temperature increased. The dielectric polarization associated with this relaxation had a broad spectrum, whose half-width increased with decrease in the temperature. Its relaxation rate followed ...


Thermochimica Acta | 1996

Excess energy of polymorphic states or glass over the crystal state by heat of solution measurement

G. Salvetti; E. Tognoni; Elpidio Tombari; G. P. Johari

The excess internal energy of one polymorph of a material over another may be determined from their heat of solution in a given solvent. (This energy is not to be seen as reversible heat of transformation of the polymorphs.) Thus the difference between the internal energy of amorphous and crystalline forms of sucrose, glucose and glucose monohydrate have been determined from measurement of their heat of solution in water. These differences are 21.2, 15.5 and 28.1 kJ mol−1, respectively. This difference in energy is caused by differences in van der Waals interaction energy, the extent of and the total energy associated with H-bonding in the two solids, and their vibrational frequencies. The implication of these studies and further use of the procedure are discussed in general terms, and it is proposed that this method is more accurate than the usual method of integrating heat capacity-temperature data. The method can be used for determining the excess energy at 0 K of materials which decompose or melt incongruently, and when neither the heat capacity of the high temperature phase nor the heat of phase transformation can be measured.


Journal of Chemical Physics | 2007

Vibrational and configurational heat capacity of poly(vinyl acetate) from dynamic measurements

Elpidio Tombari; C. Ziparo; G. Salvetti; G. P. Johari

The complex heat capacity C(p) (*) of poly(vinyl acetate) has been measured at 20.95 mrads modulation frequency during the cooling as well as on heating at 24, 8, and 2 Kh and during cooling at 0.5 Kh. The study is complemented with (the rate-dependent) C(p,app) measured during cooling and heating at 60, 24, and 8 Kh. At low temperatures, the real component of C(p) (*) yields the unrelaxed C(p) or C(p,vib), the vibrational part of C(p). It is found to be indistinguishable from C(p,glass) and lies on a line extrapolated to its equilibrium melts temperature. At T near T(g),DeltaC(p)(=C(p,melt)-C(p,glass)) shows no detectable contribution from C(p,vib). The finding conflicts with a modified entropy theory calculation [E. A. DiMarzio and F. Dowell, J. Appl. Phys. 50, 6061 (1979)], which had predicted that approximately 27% of DeltaC(p) of poly(vinyl acetate) at T near T(g) is vibrational in origin and the remainder configurational. At T


Journal of Chemical Physics | 2005

Endothermic freezing on heating and exothermic melting on cooling

Elpidio Tombari; Carlo Ferrari; G. Salvetti; G. P. Johari

Generally, a liquid freezes exothermally on cooling and a crystal melts endothermally on heating. Here we report an opposite occurrence--a liquids endothermic freezing on heating and the resulting crystals exothermic melting on cooling at ambient pressures. C(p) decreases on freezing and increases on melting, and the equilibrium temperature meets the thermodynamic requirement. Melting on cooling takes longer than freezing on heating. A rapidly cooled crystal state becomes kinetically frozen, evocative of a nonergodic state. Both C(p) and enthalpy relax like those of glasses, though the viscosity is only a few centipoise. The crystal state belongs to energy minima higher than those of the melt, which has consequences for the use of potential-energy landscape, or inherent structures, for a thermodynamic description of a material.


Journal of Chemical Physics | 2002

Experimental evidence for the heat capacity maximum during a melt’s polymerization

G. P. Johari; Elpidio Tombari; S. Presto; G. Salvetti

The equilibrium heat capacity Cp of a liquid whose molecules polymerized to form a linear chain structure at a fixed temperature has been measured at a fixed frequency of 3.33 mHz in real time simultaneously with the extent of polymerization α. The polymerization temperatures and this frequency were chosen such that the fully polymerized state remained a liquid and no heat capacity relaxation occurred. The plots of Cp against α show a maximum. This confirms the recent theoretical conclusions based on the polymer chain statistics [J. Wang and G. P. Johari, J. Chem. Phys. 116, 2310 (2002)] that during the course of a melt’s polymerization, the configurational heat capacity of the equilibrium liquid reaches a maximum value at a certain α, and then decreases to the finite value of the fully polymerized melt. The results are also discussed in terms of the potential energy landscape at a fixed temperature where each new covalent bond formed and/or a new molecular weight distribution causes the state of the samp...

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Elpidio Tombari

National Research Council

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Marco Angiuli

National Research Council

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Emilia Bramanti

National Research Council

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