A view and a review of the melting of alkali metal halide crystals

Summary A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].     

A statistical model for the optimization of dsc performance in the evaluation of drugs for preformulation studies

Differential scanning calorimetry (DSC) is a thermal analytical tool for preformulation studies. Extrapolated melting temperature (T_P) and heat of fusion (ΔH_f) can be used as parameters for optimizing the DSC performance. Two model pharmaceuticals acetaminophen and nicotinamide are used in this study. Using a factorial design for the experimental model and matrix analysis the results, the effect of sample mass, heating rate and the nitrogen flow rate were evaluated on the ΔH_f values and T_P values. Two levels for each of the procedural variables were used as a balanced experimental design with two sample sizes, two heating rates and two nitrogen flow rates. It was found that the change in the heating rate caused significant changes in the ΔH_f values but not the T_p values for acetaminophen. However, no significant effect was found for the T_p value but ΔH_f value was affected to a certain extent for nicotinamide.     

Thermodynamic Study of the Three Isomers of Bromobenzoic Acid

The enthalpies of sublimation and fusion and triple-point temperatures of 2-bromo-. 3-bro-mo- and 4-bromobenzoic acids have been determined precisely by sublimation calorimetry, drop calorimetry and differential thermal analysis. The measurements of sublimation enthalpy of the three acids were made at 333, 348 and 363 K, respectively, using a Tian-Calvet microcalorimeter equipped with Knudsen effusion cells. The derived standard molar enthalpies of sublimation at 298.15 K are (95. 94±0. 41), (99. 20± 0.18), and (103. 08±0. 59) kJ · mol-1for the 2-bromo-, 3-bromo- and 4-bromobenzoic acids, respectively. In addition, the saturated vapour pressure of these compounds was also calculated on the basis of the sublimation experiments. The enthalpy of fusion, the triple-point temperatures and the mole fraction purities of the samples of the investigated substances were measured using the mean temperature version DTA apparatus developed by the CTM of the CNRS in Marseille. The triple-point temperature and the     

Microwave assisted low temperature synthesis of sodium zirconium phosphate (NaZr<Subscript>2</Subscript>P<Subscript>3</Subscript>O<Subscript>12</Subscript>)

Sodium zirconium phosphate [NaZr₂P₃O₁₂], a potential ceramic matrix for fixation of high level nuclear waste, was synthesized by heating the mixture of sodium carbonate [Na₂CO₃], zirconyl nitrate hydrate [ZrO(NO₃)₂·5H₂O] and ammonium dihydrogen phosphate [NH₄H₂PO₄] in air, in a resistance heated furnace and a microwave heating system respectively in the temperature range 450 to 650°C. The mixture heated for 1 h in a resistance furnace at 450°C yielded a poorly crystalline NaZr₂P₃O₁₂ [NZP]. Increasing the temperature to 650°C produced a highly crystalline product. The same mixture heated in a microwave oven at 450°C for 1 h however, yielded the most crystalline NZP.In an alternate method, the mixture of sodium dihydrogen phosphate (NaH₂PO₄), zirconium dioxide (ZrO₂) and diammonium hydrogen phosphate [(NH₄)₂HPO₄] heated in resistance furnace at 650°C for the same period did not react in air. It also did not yield the pure product at 450°C when heated in microwave assembly for 1 h.The authors thank the Board of Research in Nuclear Sciences (BRNS) of the Department of Atomic Energy (DAE) for the financial support for this work under the project No. 2000/37/19/BRNS/1959 dtd09-02-02.     

Electronic structure of BeH2

Nonorthogonal single‐configuration and multiconfiguration ab initio calculations are carried out on the BeH₂ molecule in its equilibrium (D_∞h) geometry, using a highly optimized even‐tempered Slater‐type orbital (STO) basis set. The results are used as a basis for a discussion of the electronic structure of the molecule in modern‐VB terms. A value of R_e = 1.329 Å is obtained. Values are also obtained for the symmetric‐stretch harmonic frequency (2053 cm^?1), electric quadrupole moment (?5.60 Buckingham), and nuclear quadrupole coupling constants (?3.59 MHz for ⁹Be and 0.0915 MHz for ²H). © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Self-consistent-field – Hartree–Fock method with finite nuclear mass corrections

We have upgraded a Self-consistent-field – Hartree–Fock routine to include a finite nuclear mass correction for molecules developed in our laboratory. The new routine can handle isotopomers without calculating any nuclear kinetic energy matrix element. Tests on H₂, LiH, HF, F₂, and H₂O isotopomers indicate the equivalence of our correction to the standard diagonal adiabatic correction. A further original application to C₂H₆ illustrates the usefulness of the method for polyatomic molecules. The resulting molecular orbitals carry the nuclear mass signature, exemplified with Koopmans ionization potentials.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002)     

Strategies to enhance the enzymatic hydrolysis of pretreated softwood with high residual lignin content

Pretreatment of Douglas-fir by steam explosion produces a substrate containing approx 43% lignin. Two strategies were investigated for reducing the effect of this residual lignin on enzymatic hydrolysis of cellulose: mild alkali extraction and protein addition. Extraction with cold 1% NaOH reduced the lignin content by only approx 7%, but cellulose to glucose conversion was enhanced by about 30%. Before alkali extraction, addition of exogenous protein resulted in a significant improvement in cellulose hydrolysis, but this protein effect was substantially diminished after alkali treatment. Lignin appears to reduce cellulose hydrolysis by two distinct mechanisms: by forming a physical barrier that prevents enzyme access and by non-productively binding cellulolytic enzymes. Cold alkali appears to selectively remove a fraction of lignin from steam-exploded Douglas-fir with high affinity for protein. Corresponding data for mixed softwood pretreated by organosolv extraction indicates that the relative importance of the two mechanisms by which residual lignin affects hydrolysis is different according to the pre- and post-treatment method used.     

The crystallinity of polyaryl ether ether ketone

Measurement of the degree of crystallinity of the polymer matrix in a composite is complicated by the presence of the reinforcing additive. This is particularly the case in APC-2 in which as much as 70% can be carbon fibre. A First Law procedure, developed for determining the degree of crystallinity of PEEK, which involves direct measurement of the enthalpy changes associated with melting, crystallization and heat capacity changes, has found to be an effective method for the determination of the crystallinity of the PEEK matrix. The procedure has been applied to carbon fibre and glass fibre PEEK composites.