Fundamental restrictions of the second-law and Arrhenius plot methods used in the determination of reaction enthalpies in decomposition kinetics

The apparent increase of the reaction enthalpy ΔH with temperature due to the self-cooling and condensation effects is responsible for the fundamental restrictions of the second-law and Arrhenius plot methods related to determination of this parameter. Theoretical analysis and a comparison with the experimental data indicate that the systematic underestimation of ΔH magnitudes determined by the second-law method equals 10–25% for the reactants decomposed to gaseous products and 15–50% for reactants decomposed to solids. Therefore, the use of these methods in decomposition kinetics is hardly acceptable. The replacement of the Arrhenius plot and second-law methods to the much more precise and accurate third-law method is desirable or even obligatory.     

Voltammetric Determination of Nifedipine on Carbon Nanotubes‐Modified Glassy Carbon Electrode: A new Application to Dissolution Test Studies

We report the adsorptive voltammetric determination of nifedipine on multiwalled carbon nanotubes (MWCNT)‐modified glassy carbon electrode (GCE). Nifedipine was adsorbed on the MWCNT and then reduced using linear sweep and cyclic voltammetry (LSV and CV). Parameters such as pH and accumulation time were tested. The MWCNT‐modified GCE showed enhanced currents and good signal‐to‐noise characteristics in comparison with the bare GCE. Consecutive measurements with the modified electrode were highly repeatable and reproducible. The MWCNT/GCE was used for the determination of nifedipine and is recommended for quantitation in dissolution test studies. In this study we have tested normal and extended‐release pharmaceutical formulations of nifedipine using USP apparatus 2 and tracking the released drug in solution by the proposed voltammetric method. The main advantage of the voltammetric determination is the feasibility to detect the drug in‐situ avoiding tedious intermediate steps such as filtration, collection and replenishment of sample solutions. This work seeks to demonstrate the feasibility of applying voltammetric techniques in dissolution test studies.     

Novel correlation of Schottky constants with lattice energies for II-VI and I-VII compounds

Correlations of computed Schottky constants () with structural and thermodynamic properties showed linear dependences of log K_S on the lattice energies for the Zn-, Cd-, Hg-, Mg-, and Sr-chalcogenides and for the Na- and K-halides. These findings suggest a basic relation between the Schottky constants and the lattice energies for these families of compounds from different parts of the Periodic Table, namely, =−(2.303nRT log K_S)+2.303nRm_b+2.303nRTi_b. is the experimental (Born-Haber) lattice energy (enthalpy), n is a constant approximately equal to the formal valence (charge) of the material, m_b and i_b are the slope and intercept, respectively, of the intercept b (of the log K_S versus linear relation) versus the reciprocal temperature. The results of this work also provide an empirical correlation between the Gibbs free energy of vacancy formation and the lattice energy.     

In vitro and in vivo degradation behaviors of thermosensitive poly(organophosphazene) hydrogels

Biodegradable and thermosensitive poly(organophosphazenes) with various substituents were synthesized and their hydrolytic degradation properties were investigated in vitro and in vivo. The aqueous solutions of all polymers showed a sol-gel phase transition behavior depending on temperature changes. The side groups of polymers significantly affected the polymer degradation and accelerated hydrolysis of polymers in the order of carboxylic acid > depsipeptide > without carboxylic acid and depsipeptide. The increased gel strength led to the decreased hydrolysis rate. The polymer hydrogels with 750 Da of α-amino-ω-methoxy poly(ethylene glycol) were rapidly decreased by dissolution. The polymer degradation was also influenced by pH and temperature. The in vivo behaviors of mass decrease of the polymer hydrogels were similar with the in vitro results. These results suggest that the biodegradable and thermosensitive poly(organophosphazenes) hold great potentials as an injectable and biodegradable hydrogel for biomedical applications with controllable degradation rate.     

三峡库区泥灰质岩石溶蚀作用的微观研究

泥灰质岩石的岩溶,特别是溶蚀作用研究是岩溶研究的一个薄弱环节,同时又是揭示泥灰质风化岩及残坡积类膨胀 土工程特性和成因的关键。通过对三峡库区三叠系巴东组(T2b)不同溶蚀程度泥灰质岩石样品的扫描电镜图象分析,发现泥 灰质岩石的溶蚀作用导致岩石微观结构变化,这种变化可因溶孔形成使岩石力学强度降低,或大量溶孔或残积土变形引起地 面不均匀沉降、地裂缝、滑坡、崩塌和岩溶塌陷等地质灾害的发生。