Values ofD Research Articles

Effect of some anionic surfactants on the stability of oxytetracycline hydrochloride stabilized emulsion

The zeta potential of oxytetracycline hydrochloride stabilized emulsion flocculated by the addition of anionic surfactants has been calculated. The interaction energies of these emulsions have also been evaluated and the stability discussed in the light of D. L. V. O. Theory. The energy barriers, thus obtained, were very high, which prevented the occurrence of flocculation in primary minima. The flocculation could, however, occur in the secondary minima. The flocculation studies were carried out haemocytometrically and the degree of aggregation (D) calculated. In order to account for the observed values ofD, the effective Van der Waals constant of the system should be 2.3 x 10-13 erg. The binding parameters and free energy of adsorption have also been calculated.

The thermal stability of the fibrous copper-chromium eutectic

The elevated temperature stability of the fibrous copper-chromium eutectic was studied and found to depend strongly upon the extent of structural defects in the as-grown eutectic. Both highly branched and nearly “ideal,” or regular, fibrous structures were obtained by controlling the crystal growing conditions. The branched structure coarsens at a much faster rate than the regular structure. In the regular structure, the initial stage of coarsening is described by a simple two-dimensional Ostwald ripening mechanism modified to take into account the effects of fiber geometry and volume fraction. The coarsening rate is limited by volume diffusion of the chromium solute in the copper matrix. The activation energy for coarsening is approximately 298 kJ/mol. After long time annealing, the chromium-rich fibers begin to pinch off and three dimensional coarsening is initiated which leads to a rapid increase in the coarsening rate. Application of theoretical analyses to the observed coarsening rate yields the productDΣ as 1.3 × 10−13 J/s at 1000°C, whereD is the diffusivity of Cr in solid Cu and Σ the Cu-Cr interfacial energy. Using reported values ofD, Σ is bracketed between 0.27 and 2.5 J/m2. The latter is quite high for a solid-solid interfacial energy, but is consistent with the rapid coarsening observed in this system.

Axial parameter D of octahedral S-state ions in garnets

The values of the spin-hamiltonian parameters are reported for systems Fe3+:Ca3Sc2Ge3O12, Fe3+:Na3Sc2V3O12 and Mn2+:Na3Sc2V3O12. The values of the axial parameterD together with the values ofD compiled from literature are used in the discussion of the origin ofD. A close correlation ofD with the trigonal deformation of the octahedron of S-state ion indicates the importance of the covalency effects. On the other hand, it is shown thatD can be hardly understood on the basis of the relativistic model only.

Drift mobility and diffusion with an electric field of Ba2+ ions in NaCl crystals

Using a carrier-free tracer133Ba the temperature dependences of the diffusion coefficientD and mobilityμ of Ba2+ ions in an electric field have been measured on NaCl crystals in the temperature range from 500 to 750 °C. It is shown that the obtained values ofD are not affected by an electric field (∼30 V/cm) and that the temperature dependence of the mobility of Ba2+ ions is given byμ(T)=(12·3±1·9) exp [-(1·51±0·09) eV/kT]. Vacancy jump frequencies near a Ba2+ ion and the Gibbs free energy of association of this ion with a vacancy have been calculated in terms of a simple 5-frequency model for impurity diffusion by a vacancy mechanism.

Viscosity behaviour of solutions of Graham's salt in media of different dielectric constants

Reduced viscosity (ηsp/C) of an aqueous solution ofGrahams salt decreases if a water-miscible organic solvent like acetone is added to it. The reduced viscosity-concentration (ηsp/C-C) curve in water is concave upward but in 10% solution of acetone ethanol or dioxane a maximum is observed. The behaviour is reminiscent of what is observed on successive addition of a neutral salt. Just as after addition of a swamping excess of an electrolyte theηsp/C-C curve becomes a straight line, in presence of 25% solution of any of the three organic solvents studied, curves became straight lines in the case of low-molecular-weight samples ofGrahams salt. An expected increased interaction between polyanions probably does not take place because of formation of ion-pairs in solvents of low dielectric constant and coiling of the polyelectrolyte chain takes place. Whereas, apparently the effect of addition of salt is similar qualitatively to the effect of addition of a non-solvent, there is one important and significant difference. This is reflected in the plot of reduced viscosity versus concentration of the added nonsolvent. Theηsp/C of a 0.3% solution ofGrahams salt falls first slowly and then sharply with increase in the concentration of the non-solvent (acetone, ethanol or dioxane). In the case of addition of salt the reduced viscosity falls sharply firstand then slowly. Whereas in one case the curve is concave in shape it is convex in the other. A plot ofηsp/C vs.D, the dielectric constant of the medium, shows that the reduced viscosity increases sharply at first with increase in dielectricconstant but seems to stabilize at higher values ofD.

Further experiments on liquid Indium and Tin self-diffusion

Further experiments on the self-diffusion coefficientD of liquid Indium and Tin give evidence of an effect of the diameter size of the capillaries used. This effect is important for Indium and is almost negligible for Tin. The values ofD measured in the capillaries of the largest size (1.60 mm diameter) are taken to be the correct values for the self-diffusion coefficient. This is because using capillaries with diameter >1.6 mm the values ofD were constant in the experimental error. The dependence ofD from the absolute temperature is of Arrhenius type: for Indium: $$D = (33.4 \pm 2.1)10^{ - 5} \exp \left[ {\frac{{2554 \pm 79}}{{RT}}} \right]cm^2 /s,$$ for Tin: $$D = (32.4 \pm 1.2)10^{ - 5} \exp \left[ {\frac{{ - 2768 \pm 80}}{{RT}}} \right]cm^2 /s,$$

Measurements with a half-pitot tube

The use of a half-pitot tube for measuring the surface friction of a fluid flowing past a smooth surface was introduced by the late Sir Thomas Stanton (1920). The sketch, fig. 1, shows how the method works. In fig. 1, A is the half-pitot. This is a pitot tube set facing the fluid, one wall of which could, in Stanton’s apparatus, be raised or lowered. The other wall is the smooth surface over which friction measurements are to be made. The difference in pressure between the fluid in the half-pitot tube and fluid in a tube connected to a hole ( C , fig. 1) in the surface is measured by means of a manometer M . To calibrate the instrument it was set up in a pipe through which fluid was flowing at a speed below the critical velocity. Under these conditions the distribution of velocity is parabolic across the section of the tube, and the tangential stress at the fluid surface is known. Stanton expressed his results in terms of an “effective distance” d '. This is the distance from the wall at which the velocity, v , is such that the pressure in the half-pitot, p , is equal to ½ρ v 2 . When d , the opening of the pitot, was large it was found that d '/ d was approximately ½. On the other hand, it was found that when d is very small d '/ d increases. Stanton’s measurements of corresponding values of d ' and d seem to indicate that d ' tends to a finite limit as d tends to zero, so that d '/ d would, in that case, tend to become infinite when d is very small. This conclusion cannot, however, be regarded as an experimentally established fact because the highest value of d '/ d obtained in Stanton’s experiments was only 2·2.