Volume 220, Issue 3 - 3

The review provides an overview on the modelling concepts of first-order phase transition kinetics for Faradaic and capacitive systems. The basics of homogeneous and heterogeneous nucleation are considered. Special attention is paid to the role of a time-dependent supersaturation. The whole kinetics in a thermodynamic open system includes transport processes. They cannot be separated from nucleation and growth processes. Depending on the reaction pathways as well as on the rate-determining steps a line of models is evaluated, which reflect the actual kinetics in a suitable accuracy. The differentiation of the models can be made according to parallel and consecutive pathways. Parallel reaction pathways occur according to competitive and noncompetitive mechanism. Consecutive pathways distinguish between surface-diffusion controlled and adsorption controlled kinetics.

Quadrupole moment ( Q ) measurements of solvents are not possible due to their extremely fast molecular vibrations and tumbling motions which reduce the field gradient experienced by the nucleus to zero. Theoretical calculation of Q of solvents are also difficult. The method suggested by Buckingham for the calculation of Q H2O is defective and based on erroneous data. However, Q of solvents can be calculated by suitable modification of Buckingham’s method. A notable observation is that though Δ H 0 soln (MX) of an electrolyte differs from solvent to solvent but [Δ H 0 solv (MX) − Δ H 0 f,soln (MX)] is always a constant irrespective of solvents and is equal to the experimental quantities [Δ H 0 lattice (MX) − Δ H 0 f (MX) solid ] for any electrolyte MX.

A solid catalyst manganese pyrophosphate based on non-sieves to oxidize benzene to phenol with oxidant hydrogen peroxide has shown good conversion with good selectivity in CH 3 CN at 65°C investigating water contact angle data of three manganese salts, it is found manganese pyrophosphate has certain repulsive water character. It is further to be confirmed by benzene and phenol adsorption experiments onto catalyst surface by GC. With benzene/H 2 O 2 ratio of 1, the benzene conversion of 13.8% with phenol selectivity of 85.0% was achieved. It is noteworthy that no any products are obtained using manganese pyrophosphate as catalyst in the oxidation of phenol in CH 3 CN solvent.

The dielectric relaxation studies of different molar concentrations of ethanol (C 2 H 5 OH) in the binary mixtures of ethanol and tetramethylurea in benzene solutions have been studied at 9.883 GHz by using standard standing microwave techniques and Gopala Krishna’s single frequency concentration variation method at different temperatures (25°C, 30°C, 35°C and 40°C). It is found that dielectric relaxation time varies non-linearly with the variation in molar concentration of ethanol in the whole concentration range of the binary mixture. Based upon above results solute–solute and solute–solvent types of molecular association have been proposed. The dielectric relaxation process for 50% mole fraction of ethanol in the binary mixture is found to be an activated process. The energy parameters (Δ H ε , Δ F ε , Δ S ε ) for the dielectric relaxation process of binary mixtures containing 50% mole fraction of ethanol have been calculated at the respective given temperatures. Comparison has been made with the corresponding energy parameters for viscous flow process. It is found that the dielectric relaxation process can be treated as the rate process like the viscous flow process.

Cobalt(III) complexes of general formulae Co(L) 3 3+ , where L refers to (NH 3 ) 2 , en (1,2-diamino ethane) and pn (1,2-diamino propane) have been subjected to photolysis (excitation wavelength; λ = 254 nm) in water-methanol mixtures (0, 5, 10, 15, 20, 25 and 30% (v/v) methanol). In each mixture as the mole fraction of water surpassed, the observed quantum yield values were triggered. Therefore, Φ Co(II) measured in the binary solvents were subjected to Linear Solvation Energy Relationship (LSER) to model the Φ Co(II) data using the measured empirical solvent parameters in order to break open the solvation effects of excited state reactivities. The photo reduction is solvent dependent; has the form logΦ Co(II)  = a 0  + a 1 X 1 ( X 1  = x MeOH or ε r −1 ). Effect of solvent polarity in Grunwald–Winstein correlations ( r 2  = 0.950) is discussed by linear regression analysis incorporating solvent ionizing power ( Y GW ) scale and Φ Co(II) profile. Major causes for these phenomena are investigated in terms of long range solvation effects, in conjunction with short range solvation effects. Distinct border lines between the two are derived from photolysis yields in six solvent mixtures using the results of logΦ Co(II)  = a 0  + a 1 X 1  + a 2 X 2  + … plots, where X 1 , X 2 etc . are independent variables (like E T N , DN N , α, β, and π*) representing physicochemical characteristics of solvent. These plots show appreciable correlation of data, which were quantified to unravel solvation effects.

The structural and thermodynamic properties of liquid water and of the dilute solutions of methane and ethane in water were calculated by Monte Carlo simulations in the temperature range 298 K to 318 K and 298 K to 333 K, respectively. The nonpolar molecules were modeled as one- and two-center Lennard–Jones particles; for the interaction potential of water a modified TIP5P model was used. The results indicate that the nonpolar solutes tend to aggregate with increasing temperature. Methane molecules preferably form water-separated pairs, even at higher temperatures, whereas for ethane contact pairs are more likely. For the thermodynamic conditions studied here, the residual chemical potential of water is a linear function of temperature.

The polycondensation kinetics of silicic acid was measured cryoscopically in a temperature interval between 5°C and 35°C. Unlike polyamines such as polyallylamine, polylysine or polyarginine, catalitical amounts of Cytochrom c have a distinct retarding influence on the polycondensation kinetics. The Arrhenius analysis of the experimental reaction rates with and without Cytochrome c determines an invariant activation energy of E A CC/wo  = (69.9 ± 0.7) kJ/mol, while the pre-exponential factors differ by Δκ 0  = (1 × 10 8 ± 8 × 10 7 ) s −1 . The latter can be attributed to a three-times pejoration of a sterical factor, which then indicates, that the adsorption of reactive silicic acid oligomers on the protein is dominating the reaction rate.