Calorimetric study of the energy on combustion of ammonium perchlorate in 11 atm helium

The combustion of pure ammonium perchlorate in helium at 11 atm was studied using a newly constructed twin-type combustion calorimeter. The calorimeter has an error of around ±3% for the measurements of the combustion energy of benzoic acid in 11 atm oxygen. The samples of ammonium perchlorate were ignited with external heating in the helium atmospheres, and the heat evolution on combustion has been measured to be 1280±55 cal g^?1.     

Calorimetric study of phase transitions in a liquid-crystal-based microemulsion

A lyotropic inverse micelle phase composed of water, thermotropic liquid-crystal octylcyanobiphenyl (8CB), and surfactant (DDAB) was studied by using high-resolution calorimetry on several mixtures with 3%, 8%, and 15% micelle concentration. Calorimetric results show strong depression of the isotropic to nematic (I-N) phase-transition temperature. Broad heat-capacity anomalies show the existence of a wide coexistence range of isotropic, nematic, and smectic-A phases, which mimics the behavior of a new nearly stable thermodynamic phase. An observation of the rather sharp almost bulklike nematic to smectic-A (N-A) transition at low-temperatures indicates that our heat capacity results are consistent with the phase separation scenario in which significant number of micelles is expelled during I-N conversion leaving almost pure nematic phase at lower temperatures. It was found that micelles get almost completely remixed on heating the mixture back to the isotropic phase.     

Calorimetric and diffractometric study of the lowest room-temperature transition of polytetrafluoroethylene

Through differential scanning calorimetry and wide angle x-ray scattering, the room-temperature transitions of native (never melted) polytetrafluoroethylene have been investigated. The independence of the transitions, and in particular of the lowest one, on the heating kinetics, seems to suggest that also this transition is a true crystal-crystal transition.     

Calorimetric study of phase transitions for octylphenylthiolpentyloxybenzoate in silica aerogels

An a.c. calorimetric study has been carried out on octylphenylthiolpentyloxybenzoate (8S5) in three silica aerogels with mass densities rho 0.08, 0.17 and 0.36g cm . Results for the least porous aerogel (rho 0.36) are completely consistent with those reported previously for 8CB in the same aerogel. Freezing of 8S5 damaged the more porous rho 0.08 and 0.17 aerogels by creating internal voids and cracks that are large compared with the intrinsic pore size. As a result, overlapping sharp bulk-like Cp features and rounded features due to 8S5 in residual aerogel pores were observed, and no quantitative separation of these features could be achieved. 3     

Gas phase electron diffraction study of the hafnium perchlorate molecule

The structure of the Hf(ClO₄)₄ molecule was investigated by electron diffractometry in the gas phase. The model with D₄ symmetry is in best agreement with experimental data; it contains ClO₄ fragments in the form of distorted tetrahedra bridged by two oxygen atoms to the central Hf atom. Ivanovo Chemical Technological Academy. Moscow State University, Chemical Faculty. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 487–493, May-June, 1996.     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Calorimetric study of phase transitions in the thiourea 1,1,2,2-tetrachloroethane clathrate compound

Heat capacities of the thiourea clathrate compound of 1,1,2,2-tetrachloroethane, {(NH₂)₂CS}₃(CHCl₂)₂, were measured at temperatures between 13 and 330 K. Two phase transitions were found. The enthalpy and entropy changes of the transition are 5940 J·mol^–1 and 28.1 JK^–1· mol^–1 for the one occurring at 224 K and 2756 J·mol^–1 and 11.3 JK^–1·mol^–1 for the other at 248 K. It is concluded from the transition entropy values that the guest molecules are orientionally disordered nearly to the same extent as in the neat liquid.Contribution No. 56 from the Microcalorimetry Research Center     

Calorimetric study of the effects of 2,4-dichlorophenol on the thermotropic phase behavior of DPPC liposomes

The effect of 2,4-dichlorophenol (DCP) on the main transition and pretransition of fully hydrated (20 mass%) dipalmitoylphosphatidylcholine (DPPC) multilamellar liposomes has been studied by differential scanning calorimetry (DSC). It was observed that an increase in the molar ratio of DCP/DPPC (from 4·10^-5 up to 2·10^-2) causes progressive reductions in the temperature and enthalpy of the pretransition. The higher concentration of DCP eliminates the pretransition. The influence of DCP on the main transition in this molar ratio range is not drastic, but a decrease in temperature and in the enthalpy values was observed. In the molar ratio range (from 2·10^-1 up to 4·10^-1) the DSC scans show multiple main transition peaks instead of the characteristic single peak of the main transition. Above a DCP/DPPC molar ratio of 0.6 a new peak appears at 25°C having about the same transition enthalpy as the main transition of the pure system.This revised version was published online in November 2005 with corrections to the Cover Date.     

Calorimetric study of water adsorption on saponite

The isotherms and differential heats of water vapor adsorption on Na- and Ca-saponite samples are measured. It is shown that the successive stages of hydration of the internal surface of this mineral appear as waves or inflections in the adsorption isotherms and maxima in the dependences of the differential heat on the adsorption value. The compositions of Na⁺ and Ca²⁺ aquacations in single- and double-layer saponite hydrates are determined. The heats of interaction between water molecules and interlayer Na⁺ cations in single-layer saponite hydrates and between Na⁺ cations and the ion exchanger matrix are estimated.     

Solid phase extraction of lanthanum on modified low-polar sorbents

The preconcentration of lanthanum has been studied on low-polar sorbents modified with 1-phenyl-3-methyl-4-benzoylpyrazol-5-one, including hexadecylsilica, hyper-crosslinked polystyrene, polyvinylidene difluoride F2M, and activated carbon. The coefficients of reagent and the lanthanum distribution have been calculated under dynamic conditions. The leaching of the attached reagent from the sorbent surface has been investigated. The recovery of lanthanum with modified and nonmodified sorbents has been compared. The efficiency of lanthanum desorption with nitric acid and ethanol has been studied.     

Calorimetric investigation of phase transitions in cholesteryl oleate

An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near phase transitions in the cholesteric liquid crystal cholesteryl oleate (CO). It is found that the blue phases in CO are thermodynamically stable and that the observed enthalpy differences between the phases are small.     

Calorimetric investigation of phase transitions in cholesteryl oleate

An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near phase transitions in the cholesteric liquid crystal cholesteryl oleate (CO). It is found that the blue phases in CO are thermodynamically stable and that the observed enthalpy differences between the phases are small.     

Solubility polytherms and eutectic concentrations of scandium,yttrium, and lanthanum perchlorate solutions

Freezing points of solutions M(ClO₄)₃-H₂O (M = Sc, Y, La) of different concentrations were determined. The solubility polytherms of the perchlorate systems studied, as well as aluminum, gallium, and indium perchlorates were compared with the concentration dependences of different properties of the corresponding solutions. Viscometry and IR spectroscopy allowed the special points on these dependences to be correlated with eutectic concentrations. Structural changes in the systems studied as a function of solute concentration were described.     

Raman scattering study of high-pressure phase transition in thiourea

A high-pressure Raman spectroscopic study of phase transitions in thiourea is reported. The changes in the Raman spectra with increasing and decreasing pressure have been followed to a maximum pressure of approximately 11 GPa. We observe several changes in the spectra including splitting of modes, appearance of new modes, and sudden change in the slope of the frequency-pressure curve at several pressures. On the basis of this study, we propose the existence of three more transitions in this system to phases VII, VIII, and IX at approximately 1, 3, and 6.1 GPa, respectively, in addition to the V-VI phase transition at 0.35 GPa reported earlier. All the transitions have been found to be completely reversible. We interpret these changes in terms of symmetry-lowering phase transitions.     

Calorimetric determination of equilibrium phase diagrams of inorganic systems

Calorimetric isobaric determination of the heat involved during mixing processes gives the concentration dependence of the enthalpy of mixing ΔH_M = F(x) and also, under particular conditions, some liquids concentrations of the phase diagram.For some simple typical diagrams (with a eutectic point or miscibility gaps, or a definite compound etc.) direct calorimetric experiments at many temperatures give the liquidus (e.g. NaBrNaNO₃, KBrKNO₃, NaBKNO₃, KBrNaNO₃, GaHg, GaSb).For more complicated or multicomponent systems, the setting up of the equilibrium phase diagram needs both experimental measurement and thermodynamic calculations (e.g. GaInSb).     

Calorimetric analysis of thermal phase transitions in functionalized microgels

Differential scanning calorimetry (DSC) is used to investigate the thermal phase transitions of a range of N-isopropylacrylamide (NIPAM)-based, carboxylic acid-functionalized microgels with well-defined radial and chain functional group distributions. The transition enthalpies of protonated microgels can be correlated with the hydrophobicity of the functional comonomer, while the transition enthalpies for ionized microgels can be correlated with the degree of microgel deswelling achieved across the thermal phase transition. The peak widths at half-height vary inversely with the average length of NIPAM blocks in each of the microgels, as calculated using a kinetic copolymerization model. Deconvolution of the asymmetric DSC thermograms is accomplished using a two-transition model, thought to relate to core-shell-type transitions induced by the significant local heterogeneities within the functionalized microgels. The ratio between the two transition temperatures of these deconvoluted peaks is a useful quantitative probe of the radial functional group distribution. An additional, low-temperature transition is also observed in the thermogram of the vinylacetic acid-functionalized microgel, indicative of the occurrence of local chain rearrangements prior to the macroscopic phase transition in this microgel. Complementary light scattering analysis suggests that microphase separation may account for this additional transition peak.     

The disproportionation reaction phase transition,mechanical, and lattice dynamical properties of the lanthanum dihydrides under high pressure: A first principles study

The pressure-induced disproportionation reaction phase transition, mechanical, and dynamical properties of LaH₂ with fluorite structure under high pressure are investigated by performing first-principles calculations using the projector augmented wave (PAW) method. The phase transition of 2LaH₂ → LaH + LaH₃ obtained from the usual condition of equal enthalpies occurs at the pressure of 10.38 GPa for Perdew–Wang (PW91) functional and 6.05 GPa for Ceperly–Adler (CA) functional, respectively. The result shows that the PW91 functional calculations agree excellently with the experimental finding of 11 GPa of synchrotron radiation (SR) X-ray diffraction (XRD) of Machida et al. and 10 GPa of their PBE functional theoretical result. Three independent single-crystal elastic constants, polycrystalline bulk modulus, shear modulus, Young's modulus, elastic anisotropy, Poisson's ratio, the brittle/ductile characteristics and elastic wave velocities over different directions dependences on pressure are also successfully obtained. Especially, the phonon dispersion curves and corresponding phonon density of states of LaH₂ under high pressure are determined systematically using a linear-response approach to density functional perturbation theory (DFPT). Our results demonstrate that LaH₂ in fluorite phase can be stable energetically up to 10.38 GPa, stabilized mechanically up to 17.98 GPa, and stabilized dynamically up to 29 GPa, so it may remain a metastable phase above 10.38 GPa up to 29 GPa, these calculated results accord with the recent X-Ray diffraction experimental finding and theoretical predictions of Machida et al.     

Calorimetric study of sodium-rich zirconium phosphate

The heat capacity investigation of crystalline pentasodium zirconium tris(phosphate) was carried out in a vacuum adiabatic calorimeter between 7 and 340 K and in a differential scanning calorimeter of the heat bridge type between 330 and 620 K. Between 389 and 424 K, an isostructural solid-to-solid phase transition of Na₅Zr(PO₄)₃, has been found, the nature of which is connected with a centering of off-centered zirconium atoms in octahedral sites and an occupation transfer between sodium sites in the structure. The results were used to calculate the characteristics of the phase transition and the thermodynamic functions of Na₅Zr(PO₄)₃: the transition temperature T°_trs, enthalpy of transition Δ_trsH°, entropy of transition Δ_trsS°; enthalpy H°(T)−H°(0), entropy S°(T) and Gibbs function G°(T)−H°(0) over the range from 0 to 620 K. From hydrofluoric acid solution microcalorimetry, the enthalpy of solution of Na₅Zr(PO₄)₃ at 298.15 K has been determined and the standard enthalpy of formation has been derived. By combining the data obtained by the two techniques, the Gibbs function of formation of Na₅Zr(PO₄)₃ at 298.15 K has been calculated.