The Raman frequency of a soft mode (238 cm -1 ) is analyzed as a function of pressure at 20 o C for NH 4 F using the experimental data from the literature. This analysis is performed for the pressure dependence of the Raman frequency shifts (1 / n )( ¶ n / ¶ P) T of the soft mode close to the I - III, III - V and V – VI transitions in NH 4 F. The frequency shifts increase as the pressure increases toward the phase transitions at T = 20 o C (293 K) in this ammonium structure. From the frequency shifts of the soft mode studied, the pressure dependence of the isothermal compressibility is predicted through the mode Grüneisen parameter. Our calculated isothermal compressibility can be compared with the experimental measurements.
The molar volume of solid and liquid benzene was calculated at various pressures (at constant temperatures), and the Pippard relations were examined close to the melting point in this organic molecule.The molar volume calculated is in good agreement with the observed data, which decreases as the pressure increases up to about 150 MPa. The Pippard relations are also valid within this pressure range at constant temperatures studied here for the solid and liquid phases of benzene.
We calculate the phase diagrams and predict the temperature dependence of the thermodynamic quantities at high pressures for the solid – liquid and solid – solid transitions in n-paraffins. This calculation is performed by fitting the phase line equations as obtainedfrom the Landau model to the experimental data from the literature. On thebasis of the Landau model, phase diagram of the solid – solid transition ispredicted at 0.1 MPa for the binary mixtures of n-tridecane + (n-hexane and cyclohexane).For the calculation of the heat capacity the extended mean field is employedfor the n-paraffins studied here. We find that our calculated phase diagrams explain the observed behaviour of the liquid – solid transitions at high pressures in those binary mixtures. Our phase diagrams predicted at 0.1 MPa and our predictions for the thermodynamic quantities can be compared with the experiments for those binary mixtures.
Abstract Phase transitions of kappa and iota carrageenan in KCl, NaCl, and CaCl, solutions were separately studied during heating and cooling processes by using fluorescence technique. Scattered light, I sc and fluorescence intensity, I was monitored against temperature to determine the critical transition temperatures and exponents. Kappa and iota carrageenans with monovalent (KCl, NaCl) salt system presented only coil to double helix (c-h) and double helix to coil (h-c) transitions upon cooling and heating processes, respectively. Two transition regions were observed in both heating and cooling of the iota carrageenan in CaCl 2 solution while in kappa carrageenan only (c-h) and (h-c) transitions was detected. During heating, dimers decompose into double helices by making (d-h) transition. At the high temperature region (h-c) transition takes place. During cooling, back transitions repeat themselves. A hysteresis was observed between (h-c) and (c-h) for the kappa and iota carrageenan salt system (except CaCl 2 ), respectively. Dimer to double helix (d-h) and double helix to dimer (h-d) transitions were observed for iota carrageenan in CaCl 2 salt. The gel fraction exponent b was measured and found to be in accord with the classical Flory-Stockmayer Model for both kappa and iota carrageenan salt systems
Abstract In this study using a mean field model we calculate the phase line equations for the β-δ, β-γ and γ-δ phase transitions in the NH4BrxCl1 x system. We then fit our phase line equations to the experimentally observed T-X Br phase diagram for this system. Our calculated phase diagram agrees well with the observed one for the NH4BrxCl1 x system.
The Pippard relations (C P vs. a P and a P vs. k T ) are examined at various temperatures up to 1200 K at zero pressure (P = 0) for the cubic gauche nitrogen. The specific heat (C P ) is related to the thermal expansion ( a P ) and the a P is also related to the isothermal compressibility ( k T ) as the Pippard relations for this solid structure. For those relations, experimental data are used from the literature. It is found that the variation of the C P with the a P and a P with the k T is linear. Using the linear relations dP/dT is predicted, which can be compared with the experimental measurements in the P – T phase diagram of cubic gauche nitrogen.
The fluorescence technique was employed to study thermal phase transitions of iota (ι-) carrageenan (IC) in CaCl2 solution. IC gels underwent coil to double helix (c-h) and double helix to dimer (h-d) transitions upon cooling. Upon heating IC gels presented dimer to double helix (d-h) and double helix to coil (h-c) transitions, showing hysteresis types of transition paths. Scattered light, Isc and fluorescence intensity, I, were monitored against temperature to determine phase transitions. Transition temperatures were determined from the derivative of the transition paths. The critical gel fraction exponent, β, was measured and found to be in accord with the classic Flory–Stockmayer model.
