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Calorimetric Data (calorimetric + data)
Selected AbstractsA General Approach to First Order Phase Transitions and the Anomalous Behavior of Coexisting Phases in the Magnetic CaseADVANCED FUNCTIONAL MATERIALS, Issue 6 2009Sergio Gama Abstract First order phase transitions for materials with exotic properties are usually believed to happen at fixed values of the intensive parameters (such as pressure, temperature, etc.) characterizing their properties. It is also considered that the extensive properties of the phases (such as entropy, volume, etc.) have discontinuities at the transition point, but that for each phase the intensive parameters remain constant during the transition. These features are a hallmark for systems described by two thermodynamic degrees of freedom. In this work it is shown that first order phase transitions must be understood in the broader framework of thermodynamic systems described by three or more degrees of freedom. This means that the transitions occur along intervals of the intensive parameters, that the properties of the phases coexisting during the transition may show peculiar behaviors characteristic of each system, and that a generalized Clausius,Clapeyron equation must be obeyed. These features for the magnetic case are confirmed, and it is shown that experimental calorimetric data agree well with the magnetic Clausius,Clapeyron equation for MnAs. An estimate for the point in the temperature-field plane where the first order magnetic transition turns to a second order one is obtained (the critical parameters) for MnAs and Gd5Ge2Si2 compounds. Anomalous behavior of the volumes of the coexisting phases during the magnetic first order transition is measured, and it is shown that the anomalies for the individual phases are hidden in the behavior of the global properties as the volume. [source] Dynamic Charge Equilibration-Morse stretch force field: Application to energetics of pure silica zeolitesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2002Jan Sefcik Abstract We present the Dynamic Charge Equilibration (DQEq) method for a self-consistent treatment of charge transfer in force field modeling, where atomic charges are designed to reproduce electrostatic potentials calculated quantum mechanically. Force fields coupled with DQEq allow charges to readjust as geometry changes in classical simulations, using appropriate algorithms for periodic boundary conditions. The full electrostatic energy functional is used to derive the corresponding forces and the second derivatives (hessian) for vibrational calculations. Using DQEq electrostatics, we develop a simple nonbond force field for simulation of silica molecular sieves, where nonelectrostatic interactions are described by two-body Morse stretch terms. Energy minimization calculations with the new force field yield accurate unit cell geometries for siliceous zeolites. Relative enthalpies with respect to quartz and third-law entropies calculated from harmonic vibrational analysis agree very well with available calorimetric data: calculated SiO2 enthalpies relative to ,-quartz are within 2 kJ/mol and entropies at 298 K are within 3 J/mol K of the respective experimental values. Contributions from the zero point energy and vibrational degrees of freedom were found to be only about 1 kJ/mol for the free energy of mutual transformations between microporous silica polymorphs. The approach presented here can be applied to interfaces and other oxides as well and it is suitable for development of force fields for accurate modeling of geometry and energetics of microporous and mesoporous materials, while providing a realistic description of electrostatic fields near surfaces and inside pores of adsorbents and catalysts. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1507,1514, 2002 [source] Bilayer to micelle transition of DMPC and alcohol ethoxylate surfactants as studied by isoperibol calorimetryJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 8 2005Leticia Barriocanal Abstract The interaction of dimyristoylphosphatidylcholine (DMPC) with non-ionic surfactants has been studied using isoperibol calorimetry. Phospholipid-surfactant systems were formed in the isoperibol calorimeter with varying amounts of surfactant and the change in enthalpy on formation was measured. Solubilization of the phospholipid lamellae was assessed as a decrease in the enthalpy of reaction of co-films containing DMPC and increasing amounts of three linear alcohol ethoxylate surfactants: C10H21(OCH2CH2)3OH, C10H21(OCH2CH2)5OH, or C12H25(OCH2CH2)7OH. The isoperibol calorimetry data for DMPC/surfactant/water systems were consistent with a theoretical three-stage model for the solubilization of phospholipids by surfactants, whereby phospholipid bilayers are transformed into mixed micelles with increasing amounts of surfactant. The results indicate that: (i) the interaction between phospholipid and surfactants results in a non-linear correlation between the enthalpy of reaction and the surfactant concentration; (ii) the structural stage of the lamellar to micelle transition (mixed bilayers, mixed micelles, or both) can be determined from calorimetric data; (iii) phase boundaries in the solubilization process (bilayer saturation, micelle saturation) can be identified as break points in the enthalpy-concentration curve; and (iv) increasing the hydrophilicity of the surfactant results in a decrease of the surfactant concentration producing the onset of solubilization. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:1747,1755, 2005 [source] A new method for the determination of cooperative hydrogen bonding enthalpy of proton acceptors with associated species of alcoholsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2006Boris N. Solomonov Abstract A calorimetric method for the determination of cooperative hydrogen bonding (HB) enthalpy of protonacceptors (B) with associated species of alcohols is proposed. The average enthalpy of cooperative HB ofpyridine with associated species of alcohols was found to be ,19.8,±,0.6,kJ,mol,1 for all alcohols investigated. Thisvalue exceeds the enthalpy of HB in the complex ROH,,,NC5H5 (the average for all alcohols is ,15.8,± 0.2,kJ,mol,1) by 20,30%. Cooperativity factors (Ab, AOx) of hydrogen bonds for (ROH)2,,,NC5H5 complexes were determined using the IR-spectroscopic method. The average values for the alcohols under consideration were found to be Ab,=,1.41,±,0.04 and AOx,=,1.54,±,0.05. On the basis of IR-spectroscopic and calorimetric data, the enthalpy of cooperative interactions of pyridine with the dimer (ROH)2 was estimated. This value for all the alcohols studied is, on average, ,20.9,±,0.1,kJ,mol,1. Copyright © 2006 John Wiley & Sons, Ltd. [source] Synthesis, characterization, and properties of novel ladderlike phosphorus-containing polysilsesquioxanesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2003Chin-Lung Chiang Abstract Novel ladderlike polysilsesquioxanes that contain phosphorus were successfully synthesized by the sol,gel method. The polysilsesquioxanes were characterized by Fourier transform infrared spectroscopy, 29Si NMR, and X-ray diffraction. The characterizations demonstrated that the polymer possesses a typical ladderlike structure. The thermogravimetric and differential scanning calorimetric data revealed that the polysilsesquioxanes possess excellent thermal stability. A kinetic analysis of thermal degradation showed that the activation energy of thermal degradation is 187 kJ/mol, according to Kissinger's method. The activation energy of thermal degradation normally increases with conversion (from 171 to 309 kJ/mol) according to Ozawa's method. The average activation energy, calculated by Ozawa's method, was 209 kJ/mol. The scanning electron microscopic photograph and Si and P mappings of ladderlike polysilsesquioxanes showed that the particles were uniformly dispersed at the molecular level and that the sizes of the polysilsesquioxane particles were less than 100 nm. The ultraviolet,visible spectra of the ladderlike polysilsesquioxanes revealed no absorbance in the range of 400,800 nm. Ladderlike polysilsesquioxanes possess excellent optical transparency and excellent flame retardance. This transmittance may be used as a criterion for identifying the formation of a homogeneous phase. These polymers have great potential in waveguide applications. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1371,1379, 2003 [source] | |||||||||||||