Geschwindigkeit der Verdünnung flüssiger Filme

Colloid and Polymer Science - Die Verdünnung eines mit löslichen Tensiden stabilisierten freien flüssigen Filmes wurde mit hydrodynamischen Methoden unter Berücksichtigung der...     

Interpretation of negative values of the interaction parameter in the adsorption equation through the effects of surface layer heterogeneity

The problem of the negative values of the interaction parameter in the equation of Frumkin has been analyzed with respect to the adsorption of nonionic molecules on energetically homogeneous surface. For this purpose, the adsorption states of a homologue series of ethoxylated nonionic surfactants on air/water interface have been determined using four different models and literature data (surface tension isotherms). The results obtained with the Frumkin adsorption isotherm imply repulsion between the adsorbed species (corresponding to negative values of the interaction parameter), while the classical lattice theory for energetically homogeneous surface (e.g., water/air) admits attraction alone. It appears that this serious contradiction can be overcome by assuming heterogeneity in the adsorption layer, that is, effects of partial condensation (formation of aggregates) on the surface. Such a phenomenon is suggested in the Fainerman-Lucassen-Reynders-Miller (FLM) "Aggregation model". Despite the limitations of the latter model (e.g., monodispersity of the aggregates), we have been able to estimate the sign and the order of magnitude of Frumkin's interaction parameter and the range of the aggregation numbers of the surface species.     

STRATIFICATION IN EMULSION FILMS

Formation of films possessing a layered or stratified structure has been observed with foam films from liquid crystals, from concentrated surfactant solutions and in liquid layers on the surface of water. The stratifying films have a structure similar to that of the smectic phase which soaps are known to form in the bulk solution at high concentrations. The repeating units of which such films are built are the so-called black films: each unit consists of two surfactant layers interleaved by a thin aqueous core. In the study presented here we have observed that stratification can also take place in emulsion films from concentrated aqueous surfactant solutions. We have compared these results with those obtained for foam films using the same surfactant, i.e. sodium dodecylsulfate.     

EFFECT OF THICKNESS NON-HOMOGENEITY ON THE KINETIC BEHAVIOUR OF MICROSCOPIC FOAM FILM

The effect of thickness non-homogeneity on the drainage of microscopic horizontal foam films was studied experimentally and theoretically. Quasi-static and asymmetrical distribution of thickness irregularities over the film surface was established in the experiment. This type of dimpling was modelled theoretically and employed to derive a new equation for the film thinning. It differs from the theoretical expressions known from the literature and is in better agreement with the experimentally determined functional dependence of thinning rate on film size.     

An experimental test of the fractal model for drainage of foam films

Drainage of foam films with different radii (50-150 μm), stabilized by hexathylene glycol dodecyl ether C(12)E(6) and in a presence of 0.024 M NaCl, were analyzed in the light of a recent dynamic fractal classification of [1]. The latter accounts for the effect of film surface corrugations developed during the film drainage. For simplicity, the film surface mobility is neglected since the presence of surfactants reduces dramatically the film surface velocity. The magnitude of surface non-homogeneities, caused by the film drainage, is accounted via a dynamic fractal dimension parameter α being spanned between zero and two. Depending on the α-value the film drains by different kinetic laws. For example, if the thin film is planar α=2 and it drains according to the Reynolds law; if α=1 the film contains an axisymmetric dimple causing faster drainage; if α=1/2 the film exhibits number of asymmetric dimples and the film drains even faster; finally if α=0 the film contains spatially uncorrelated domains causing the fastest possible drainage. The present analysis of experimental data suggests that the parameter α is inversely proportional to the film radius R and it is independent of the type and concentration of surfactants. A semi-empirical model for α is proposed, thus completing the generic dynamic fractal classification.     

Palladium Release in Electrothermal Atomic Absorption Spectrometry

The absorbance signal in electrothermal atomic absorption spectrometry is a result of the processes of atom formation and dissipation and first of all depends on the initial generation of atoms from the surface. The rising part of the absorbance time profiles can be used for characterization of the release of the metals like Pd, Ag, Au etc. The method is applied to investigate the palladium release from a pyrocoated graphite support at different initial masses of the Pd. The kinetic parameters are evaluated and conclusions about the distribution of palladium on the graphite surface are made. The estimated values of apparent activation energies depend on the initial mass of analyte. The advantages and limitations of the present approach are discussed.     

Release of selenium and tin from the graphite support in the presence of tungsten and palladium modifiers

Electrothermal atomic absorption spectrometry is applied to investigate the release of the selenium and tin atoms from pyrocoated graphite support. The Coats and Redfern and Sturgeon approaches are used for estimation of the apparent activation energies of atom release. The addition of tungsten and palladium modifiers leads to enhancement of the activation energies and changes the kind of the vaporizing species for analytes.     

Optimization of the efficiency and cycling performance of Na0.1CrS2 electrodes in nonaqueous secondary lithium cells

The effect of the macrostructure (grain size) and the microstructure (crystal lattice parameters) factors on the coulombic efficiency of Na_0.1CrS₂ cathodes in secondary Li cells is studied. An attempt is made to employ Atlung's model in assessing the chemical diffusion coefficient of Li⁺ in the host structure leading to more realistic values with respect to effective interface area. The possibility of enhancing the mobility rate of Li⁺ by intercalation of water in the host structure is revealed. Using Atlung's model as a guideline it was possible to improve considerably the performance of the Na_0.1CrS₂ cathodes, while retaining their good cycling stability.     

Oxidation of carbon monoxide and hydrogen on the surface of crystalline and amorphous Cu60Zr40 alloys

The activity of amorphous and crystalline Cu₆₀Zr₄₀ alloys has been studied. Preoxidation of the alloys results in enhanced activity. The amorphous samples reveal a lower catalytic activity. An active catalyst for the oxidation of CO is obtained from the crystalline copper-zirconium alloy.     

Sequences generating permutations

Linear recurring sequences generating permutations of the elements of a finite ring are introduced and examined. A complete answer to the discussed problems is given for the second-order sequences over Z_M. The possibilities for applications are also discussed.