Mercedes Cáceres

The temperature dependence of the equation of state at high pressures revisited: a universal model for solids

Abstract A general method to include temperature effects into the equation of state (EOS) of solids is discussed. A universal model based on a pseudo-spinodal approach is used to predict the pressure and temperature dependencies of the thermodynamic properties for a variety of solids: n -H 2 , Ar, Kr, Xe, NaCl, LiF, NaF, KCl, CsCl, Li, Na, K, Rb, Cs, Al, Fe, Cu, Zn, Ag, Cd, Pt, Au, and Pb. The predictive capabilities of the complete EOS are discussed and compared with available models.

Fluorescence studies of the polycomplexes formed by interaction of polyvinylpyridine with polyacids

Abstract Solutions containing both poly(4-vinylpyridine) (P4VP) and poly(acrylic acid) (PAA) in 50% v/v metanol-water have been studied at room temperature by means of fluorescence studies, spectrophotometry and potentiometry. The results show the formation of a polycomplex between the polymers. Its stoichiometry is 0.5–0.6 PAA:1 P4VP and only slightly depends on polymer concentration for a range of very dilute solutions (10−3–10−4 M in monomeric units). The polycomplex is largely stabilized through hydrogen bonds but there is some proton transfer and the resulting coulombic interactions also contribute to the stabilization. The role of hydrophobic interactions is also discussed.

The spinodal as a reference curve for the high-pressure volumetric behavior of liquids

Abstract Experimental and theoretical studies suggest that the thermal expansion coefficients α p of liquids follow a power law up to extremely high pressures along isothermal paths. Accurate experiments reveal that intersections between α p isotherms occur at high pressure for many liquids. It is shown how the high-pressure behavior of α p can be accurately predicted for real liquids from surface tension and low-pressure α p measurements. The method proposed here is based on the assumption that the spinodal curve does exist and can be estimated from surface tension measurements using Furths hole theory.

Pressure tuning of the Fermi resonance in liquid methanol: Implications for the analysis of high-pressure vibrational spectroscopy experiments

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.

A dynamic light scattering study of the hypersonic relaxation in liquid toluene

Brillouin spectra of liquid toluene have been measured between 288 and 358 K using a dynamic light scattering technique. The polarized spectra have been modeled according to Mountain’s theory. The most relevant quantity of the theory, namely the relaxation time, has been used to check the validity of several molecular models proposed to interpret vibrational–translational energy exchanges observed in nonassociated liquids.

Heat capacities and concentration fluctuations in mixtures of 1,2-dibromoethane with alkanes

Excess volumes, excess heat capacities and light scattering have been measured for several 1,2-dibromoethane–alkane (n-heptane, n-octane, n-tetradecane, cyclooctane and 2,2,4-trimethylpentane) systems. The results show that there is good agreement between values of the concentration–concentration correlation function Scc, as calculated from light-scattering data, and the predictions of the simple rigid lattice model of Flory and Huggins, which estimates the relative effects of the differences in molecular sizes and of differences in the interaction energies of the two components. The results also confirm the utility of Scc as predictor of the existence of W-shaped excess isobaric heat capacity (CEP) curves, although other effects, such as the destruction of the (partial) correlation of molecular orientation may complicate this issue.

Thermophysical properties of liquid m-xylene at high pressures

Experimental pρT measurements of m-xylene obtained with an expansion technique are reported from 230 to 298 K and up to 110 MPa, or freezing pressure when lower. The experimental results have been correlated in terms of a recently proposed equation of state which has been used to calculate the isothermal compressibility, κT, and the thermal expansion coefficient, αp, as functions of pressure and temperature. Experimental density results for liquid toluene, which was used to calibrate our experimental device, are also reported for the isotherms of 223 and 303 K. The ability of simple power functions recently proposed to correlate high-pressure results has been tested. The extrapolation capabilities of these functions have been confirmed using experimental results from the literature for the two compounds studied.

Equation of state of liquid o-xylene at low temperatures and high pressures

pρT values for liquid o-xylene have been measured with an expansion technique at temperatures between 258 and 298 K and pressures up to 108 MPa. Isothermal compressibility and thermal expansion coeffcient data are reported as functions of pressure and temperature. The pρT results have been used to test the performance of two recently developed equations of state and to study the physical significance of their characteristic parameters. The extrapolation capabilities of both equations have been checked with experimental results from the literature at higher pressures and temperatures.

High-pressure compressibility behavior of liquids referred to a pseudospinodal curve

Abstract Some experimental studies indicate that the isothermal compressibility, κT, of molecular liquids follows a power law up to very high pressures along isothermal paths. We show here that the high-pressure behavior of κT can be characterized from surface tension and κT data measured at low pressures. Our method is based on a pseudospinodal hypothesis. Comparison with experimental is satisfactory up to very high pressures for different molecular liquids. The results are also studied in terms of the reduced bulk modulus. B. Our method reproduces adequately the intersections experimentally found for the isotherms of B versus molar volume of molecular liquids.

Local, solvation pressures and conformational changes in ethylenediamine aqueous solutions probed using Raman spectroscopy

Raman spectra of 1,2-ethylenediamine (EDA) in aqueous solutions are used to demonstrate that EDA molecules experience an anti-gauche conformational change resulting from the interactions with water. The observed Raman shift reveals a compressive (hydrophobic) effect of water on both methylene and amino groups of EDA. Raman spectra of EDA at high pressures are used as reference to quantify the intermolecular EDA-H2O interactions in terms of local pressures. These results are compared with macroscopic solvation pressures calculated from the cohesive energy parameter. We compare and discuss all our observations with available computational and experimental studies.