Netsu Sokutei

Four representative methods for the kinetic analysis of solid state reactions were examined using various theoretical data.
As a result, it was recognized that nine commonly used kinetic equations for solid state reactions are classified into a few groups, in which each equation is nearly indistinguishable from one another. In order to differentiate the equations belonging to the same group from each other, it was effective to observe the shift of the peak temperatures in the series of DTG curves under different heating rates. This procedure was examined for the dehydration of calcium oxalate monohydrate, yielding good results.

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Thermal studies on the phase transitions of the binary systems of water and surface active compounds such as (C₁₈H₃₇)N(CH₃)₃X(X=Cl, Br), (C₁₈H₃₇)₂N(CH₃)₂X(X=Cl, Br) and C₁₇H₃₅COOK were performed and revealed that all the so-called T_c curves go down stepwise with an increase of the water content. At a temperature below the T_c curve, new, two phase transitions of T_gel (stable coagel-to-stable gel) and T_c* (metastable gel-to-supercooled liquid crystal/micellar solution) were discovered with detailed annealing treatments. In connection with this, thermodynamic stability of the gel phases was discussed on the basis of the schematic diagram of Gibbs energy vs. temperature. Furthermore, the “intermediate water” named by the authors, which is newly incorporated between the polar-layers of the surface active molecules at the elevated T_gel transition, was discovered, referring to the ice-melting thermogram below 0°C, and the predominent role of the bulk free water coexisting with the coagel phase for the appearance of the T_gel transition was stressed. In the extremely dilute aqueous solution at the neighborhood of c. m. c., thermodynamical data obtained by the adiabatic calorimeter provided new informations regarding the dissolution process of the surface active compound in the “Krafft temperature region”, in the place of the so-called “Krafft point”.

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Isothermal gravimetric curves of the thermal decomposition of Mg(OH)₂ have been examined from the standpoint of kinetics and mechanisms. A new kinetic model is proposed; the decomposition occurs successively from the particles in which nuclei have been formed, and a single-particle decomposition once initiated proceeds according to a contracting disk model. The present model combining nucleation and two dimensional interface growth satisfactorily accounts for the thermal decomposition of Mg(OH)₂ prepared by different methods and thus having different crystallite sizes. Temperature dependences of the nucleation rate constant and the rate constant of interface growth is separately evaluated and related to the nature of sample.

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