Heat capacities and volumes of transfer of cholesteryl oleyl carbonate into various solvents

Heat capacities C _p and volumes V have been obtained at 25°C for the transfer of cholesteryl oleyl carbonate (ChOC) from a reference solvent, the highly-branched alkane 2,2,4,4,6,8,8-heptamethylnonane into various solvents. These include normal, branched, and cyclic alkanes as well as the pure cholesteric ChOC. V and most C _p are associated with the change of environmental free volume during the transfer process and are predicted by the Prigogine-Flory theory. However, for n-alkane solvents with more than twelve carbons, C _p is large, positive, and unattributable to free volume changes. It is consistent with a restriction of alkane segmental motion by the ChOC. The same mechanism has been proposed to explain an antiplasticization effect of ChOC on transitions in polymers.     

丁磺酸内酯对锂离子电池性能及负极界面的影响

用循环伏安(CV)、电化学阻抗谱(EIS)、扫描电镜(SEM)、能谱分析(EDS)及理论计算等方法研究了添加剂丁磺酸内酯(BS)对锂离子电池负极界面性质的影响. 研究表明, 在初次循环过程中, BS具有较低的最低空轨道能量, 优先于溶剂在石墨电极上还原分解, 并形成固体电解质相界面膜(SEI膜). 在含BS的电解液中形成的SEI膜的热稳定性高, 在70 ℃下储存24 h后, 膜电阻和电荷迁移电阻大小基本保持不变, 而在不含BS的电解液中形成的SEI膜的热稳定性较差, 在70 ℃下储存24 h后, 膜电阻和电荷迁移电阻大小有明显的增加. 从BS对锂离子电池电化学性能影响的研究表明, 加入少量的BS能够显著提高锂离子电池的室温放电容量、低温及高温储存放电性能.     

Heat capacity, enthalpy and entropy of bismuth niobate and bismuth tantalate

The heat capacity and the heat content of bismuth niobate BiNbO₄ and bismuth tantalate BiTaO₄ were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form C_pm=128.628+0.03340 T−1991055/T²+136273131/T³ (J K^-1 mol^-1) and 133.594+0.02539 T−2734386/T²+235597393/T³ (J K^-1 mol^-1) were derived for BiNbO₄ and BiTaO₄, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S_m(BiNbO₄)=147.86 J K^-1 mol^-1 and S_m(BiTaO₄)=149.11 J K^-1 mol^-1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.     

Thermodynamic properties of lithium mica: Lepidolite

Calorimetric measurements were made on natural sample of lepidolite having the composition (K_0.80Na_0.05Ca_0.07Rb_0.16Cs_0.03)(Li_1.34Al_1.40Fe³⁺_0.01)[Si_3.25Al_0.75O₁₀]F_1.80(OH)_0.20 from Na-Li-type rare-element-rich pegmatites of East Sayany, Russia. High-temperature enthalpy increments were measured with a Tian-Calvet calorimeter at 444-972 K using the drop method. The resultant (T) equation in the interval T = 298.15-972 K was calculated:  = 316.10 + 228.12 × 10⁻³ T − 50.10 × 10⁵ T⁻² (J K⁻¹ mol⁻¹) [± 0.4%] and the value of (298.15 K) = 327.8 J K⁻¹ mol⁻¹ was obtained. The standard molar enthalpy of formation from the elements was determined by high-temperature drop solution calorimetry in molten lead borate at T = 973 K. The value of Δ_f(298.15 K) for lepidolite was found to be −6201 ± 18 kJ mol⁻¹. The thermodynamic properties of lepidolite of idealized composition KLi_1.5Al_1.5[Si₃AlO₁₀]F₂ were estimated based on the experimental data obtained.     

Heat capacities of aqueous solutions of amino acid and dipeptide derivatives of fullerene

The concentration dependences of heat capacities of aqueous solutions of several amino acid and peptide derivatives of fullerene were measured by scanning differential calorimetry at 298 K. The heat capacities for the arginine, alanylalanine, and glycylvaline derivatives dissolved in water depend slightly on concentration. The concentration dependences of the heat capacities of aqueous solutions of the serine and alanine derivatives display extrema. The calculated contributions of hydration to the heat capacities of the dissolved fullerene derivatives have both positive and negative signs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2202–2204, November, 1998.     

Ionization of five aqueous fluorobenzoic acids: Conductance and thermodynamics

The three monofluorobenzoic acids together with 2,4-difluoro and 2,6-difluorobenzoic acids in aqueous solution are the subject of precision conductance measurements. The experimental data are analyzed to give ionization constants and limiting conductances at temperatures from 0 to 100°C. Walden products for the acid anions are derived from the limiting conductances while the ionization consatants are fitted by statistical methods to the function pK _a (m)=A+B/T+ C logT+DT. Only the 2,6- acid requires the fourth term of the function to fit the data to a precision of better than 0.03%. Mathematical analysis of the pK function gives the standard changes in enthalpy, entropy, and heat capacity. All the acids studied are more acidic than the parent, benzoic acid, as well as more acidic than the isoelectronic methylbenzoic acids. In general the increased acidity is tied to decreases in enthalpy while entropy changes on ionization differn little from those found for the parent acid.     

The thermal properties of poly(oxy-1,4-benzoyl), poly(oxy-2,6-naphthoyl), and its copolymers

The thermal properties, i.e., heat capacity, enthalpy, entropy, and Gibbs function, and the transition behavior of the copolymer system of 4-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid have been studied based on differential scanning calorimetry. The heat capacities of the glass, crystal, and anisotropic melt are shown to be largely additive on a molar basis. Additivity is lost in the two transition regions, glass transition and disordering transition. Isothermal crystallization experiments on the copolymers revealed the existence of two types of crystals which melt at high temperature (fast-grown crystals) and low temperature (slowly grown crystals). The ATHAS computation method is used to bring heat capacities of the solid state into agreement with approximate frequency spectra. The changes in heat capacity at the glass transitions occur at 434°K for the poly(oxy-1,4-benzoyl) [33.2 J/(K mol)] and at 420°K for poly(oxy-2,6-naphthoyl) [46.5 J/(K mol)]. The copolymers have a transition range of above 100°K. The anisotropic melt is linked to the well-known condis state of poly(oxy-1,4-benzoyl) by a continuous changes in disorder and mobility without an additional first-order transition.     

Preparation and characterization of sulfated zirconia (SO4/ZrO2)/Nafion composite membranes for PEMFC operation at high temperature/low humidity

Fine particle superacidic sulfated zirconia (SO₄²⁻/ZrO₂, S-ZrO₂) was synthesized by ameliorated method, and composite membranes with different S-ZrO₂ contents were prepared by a recasting procedure from a suspension of S-ZrO₂ powder and Nafion solution. The physico-chemical properties of the membranes were studied by ion exchange capacity (IEC) and liquid water uptake measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, thermogravimetry–mass spectrometry (TG–MS) and Fourier transform infrared (FT-IR) spectroscopy. The results showed that the IEC of composite membrane increased with the content of S-ZrO₂, S-ZrO₂ was compatible with the Nafion matrix, the incorporation of the S-ZrO₂ could increase the crystallinity and also improve the initial degradation temperature of the composite membrane. The performance of single cell was the best when the S-ZrO₂ content was 15 wt.%, and achieved 1.35 W/cm² at 80 °C and 0.99 W/cm² at 120 °C based on H₂/O₂ and at a pressure of 2 atm, the performance of the single cell with optimized S-ZrO₂ was far more than that of the Nafion at the same condition (e.g. 1.28 W/cm² at 80 °C, 0.75 W/cm² at 120 °C). The 15 wt.% S-ZrO₂/Nafion composite membrane showed lower fuel cell internal resistance than Nafion membranes at high temperature and low relative humidity (RH).     

Binary mixtures of cyclohexanone, 2-butanone, 1,4-dioxane and 1,2-dimethoxyethane

Densities and sound velocities of binary mixtures of cyclohexanone, 2-butanone, 1,4-dioxane and 1,2-dimethoxyethane were measured at 298.15 K and also the densities at 303.15 K. Excess volumes were determined from densities. Isentropic compressibilities were determined from densities and sound velocities, and excess thermal expansion factors were determined from excess volumes of two temperatures. Excess isothermal compressibilities and excess isochoric heat capacities were then estimated using excess isobaric heat capacities previously reported. Excess volumes and excess isentropic and isothermal compressibilities were negative except for cyclohexanone+1,4-dioxane system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solvation of small hydrophobic molecules in formamide: A calorimetric study

The solubility and enthalpy of solution of benzene, cyclohexane, hexane, and heptane in formanide have been determined from titration microcalorimetric experiments at 25°C. The solution enthalpies are significantly more endothermic than in water but still the solubility is much higher. The entropy changes in formamide are small and positive and do not vary significantly with size. The enthalpies of solution of some 1-alkanols, 1-chloro- and 1,5-dichloropentane and pentane-1,5-diol were measured as functions of concentration. The solution enthalpies for 1-alkanols from methanol to decanol increase linearly with chain length. The enthalpic interaction coefficients h_xx are small and negative in formamide while they are large and positive in water. The partial molar heat capacities C _p,2 ^o for 1-propanol, 1-pentanol, benzene and cyclohexane in formamide were determined at 25°C from drop heat capacity measurements. Values of C _p,2 ^o are only slightly larger than the molar heat capacities of the liquid solutes.