Influence of Specific Absorbed Microwave Power on Activation Energy of Densification in Ceramic Materials

Correlation between the rate of densification in powder ceramic materials and specific absorbed microwave power is determined by the experimental method. The approach is based on a comparison of the densification curves obtained at different rates of heating. The changes in the ramping rate are provided by varying the microwave power fed into the microwave furnace. Using the energy balance for the microwave heated samples, the correlation between the apparent energy of activation at the initial stage of densification and the value of the specific microwave power absorbed in heated materials are found. The experiments with silicon nitride-based ceramics allowed to determine the reduction in the value of the activation energy resulted from an increase in the specific absorbed microwave power.

Identification of the product from the reduction of permanganate ion by trimethylamine in aqueous phosphate buffers

The product obtained from the reduction of potassium permanganate by trimethylamine in aqueous phosphate buffers has been identified as a soluble form of colloidal manganese dioxide which is stabilized in solution by adsorption of phosphate ions on its surface. The dependence of the rate of flocculation on several experimental variables has been studied. Phosphate ions have been found to increase the solubility of the colloid by increasing the apparent energy of activation of the flocculation process. This could explain the well-known capacity of those ions for retarding the precipitation of manganese dioxide.

Thermal fragmentation of 3-alkyl-2-phenyloxetanes, 3,3-dimethyl-2-aryloxetanes, and related compounds. A case study of 2-aryl-substituted oxetanes

Thermolyses of epimeric 3-alkyl-2-phenyloxetanes (1c, 1t, 2c, 2t, 3c, and 3t), 3,3-dimethyl-2-aryloxetanes (4, 5, 6, 7, and 8), 3,3,4,4-tetramethyl-2,2-diphenyloxetane (9), and 3,3-dimethyl-2,2-diphenyloxetane (10) were studied in degassed N,N,N′,N′-tetramethylethylenediamine at 270–350 °C. Although the fragmentation of 9 and 10 can be understandable on the basis of a diradical mechanism, there were several observations, in the reaction of certain other oxetanes, which could hardly be explained simply in terms of such a mechanism. Namely, (1) less strained 1t reacted faster than more strained 1c; (2) a major mode of the fragmentation for 1c, 2c, and 3c was "B" (forming an alkene and benzaldehyde), whereas that for 1t, 2t, and 3t was "A" (forming an alkenylbenzene and formaldehyde); (3) the apparent energy of activation for the "B" process seemed to be larger than that for the "A"; (4) a dramatic change of the major fragmentation mode from "B" to "A" was brought about by a substituent on the phenyl group, as was observed in 4–8. These results may be explained reasonably by assuming that the fragmentation proceeds, at least, in dual reaction courses. In competition with an anticipated diradical pathway, there will be another process, which is energetically little more favorable than the diradical fragmentation, rather specific to the "A" mode of fragmentation, and important particularly in the reaction of the trans isomers. Probable candidates for the second process are discussed.

Temperature Dependence of Vasopressin Action on the Toad Bladder

Toad bladders were challenged with vasopressin at one temperature, fixed on the mucosa with 1% glutaraldehyde, and then subjected to an osmotic gradient at another temperature. Thus, the temperature dependence of vasopressin action on membrane permeability was distinguished from the temperature dependence of osmotic water flux. As the temperature was raised from 20° to 38°C, there was a substantial increase in the velocity of vasopressin action, but osmotic flux was hardly affected. In this range of temperature the apparent energy of activation for net water movement across the bladder amounted to only 1.2 kcal/mole, a value well below the activation energy for bulk water viscosity. It is suggested that osmotic water flux takes place through narrow, nonpolar channels in the membrane. When the temperature was raised from 4° to 20°C, both vasopressin action as well as osmotic water flux were markedly enhanced. Activation energies for net water movement were now 8.5 kcal/mole (4°–9°C) and 4.1 kcal/mole (9°–20°C), indicating that the components of the aqueous channel undergo conformational changes as the temperature is lowered from 20°C. At 43°C bladder reactivity to vasopressin was lost, and irreversible changes in selective permeability were observed. The apparent energy of activation for net water movement across the denatured membrane was 6.6 kcal/mole. Approximately 1 µosmol of NaCl was exchanged for 1 µl of H2O across the denatured membrane.

THE MACROMOLECULAR NATURE OF THE FIRST COMPONENT OF HUMAN COMPLEMENT

Kinetic and ultracentrifugal experiments demonstrated that the previously described subcomponents of human C'1, designated C'1q, C'1r, and C'1s, interacted with each other in liquid phase to form a macromolecule which was then capable of converting sensitized erythrocytes (EA) to the state EAC'1. The apparent sedimentation constants of C'1q, C'1r, and C'1s and of the macromolecular product of their interaction were approximately 11S, 7S, 4S, and 18S respectively. Association of C'1 subcomponents was prevented and dissociation of macromolecular C'1 was effected by Na3HEDTA and Na2MgEDTA but not by Na2CaEDTA. The rate of formation of macromolecular C'1 was a function of concentration of subcomponents and temperature of interaction, with an apparent energy of activation of 21,000 calories per mol. Ultracentrifugal studies further indicated the macromolecular nature of C'1 in normal human serum. In the absence of EDTA, C'1 sedimented with the serum macroglobulins and C'1 subcomponents were not detected. Conversely, in the presence of EDTA, macromolecular C'1 was not demonstrable and individual C'1 subcomponents could be measured in lighter fractions. The significance of these observations in relation to previous studies on C'1 subcomponents, the role of Ca++ in C'1 function, and the subunit structure of Enzymes has been discussed.

The kinetics of decarboxylation in solution

In the study of chemical kinetics, great importance attaches to the manner in which the velocity coefficient, k , varies with the temperature, T . Their relation is generally expressed by the differential equation ( d ln k )/ dT = E A / RT 2 , (1) where E A is now by common consent referred to as the apparent energy of activation. Integrating on the assumption that E A is a constant, independent of temperature, we obtain the well-known equation of Arrhenius, In k = B - E A / RT . (2) Having regard to the accuracy usually ascribable to experimental velocity coefficients, the assumption is a valid one. In recent years, however, precise work on certain selected reactions has shown that the supposition is ill founded. The newer data are in better agreement with the differential equation ( d ln k )/ dT = E / RT 2 + J / RT . (3) Integrating on the assumption that E and J are independent of temperature, we have the more general relation In k = C + ( J/R )ln T - E/RT , (4) which reduces to the simpler formula when J is zero. This equation, though not wholly satisfactory, is to be regarded as an improvement on equation (2) because (1) it is in better accordance with experiment, and (2) it is not restricted to systems wherein the average energy of reactive molecules and the average energy of normal molecules respond identically to temperature. Very few reactions have been examined with sufficient precision to allow of an evaluation of the constants in equation (4), and it is with the object of increasing their number that the present investigation was undertaken.