Michail Nedyalkov

Permeability of Newtonian black foam films to gas

The gas permeability of Newtonian black foam films, formed on the top of a small bubble at the solution surface, was studied experimentally. The aqueous solutions contained sodium dodecylsulphate with concentrations in the range 1.5×10−4 to 3×10−3 mol/dm3 and sodium chloride (constant concentration of 0.5 mol/dm3). A dependence of the gas permeability coefficient on the surfactant concentration was obtained. The experimental results are discussed on the basis of a theory assuming the presence of clusters of molecule vacancies (holes) in the bilayer foam film, their number and size depending on the surfactant concentration. The experimental results are in agreement with this film structure and confirm the existence of flow through both the hole-free bilayer film and the holes. It was found that the holes of three molecule vacancies make the main contribution to gas permeability at low surfactant concentration. The diffusion coefficients through the hole-free film and through the three-vacancy holes are calculated.

Method for direct measurement of the film tension of black foam films at various capillary pressures

A new experimental method to directly measure the film tension of black foam films is developed on the basis of the Laplace equation. The method allows the determination of the tension of curved (spherical) films with various radii and capillary pressures. Measurements with Newton black films from sodium dodecyl sulphate aqueous solution have been carried out. The results show that in the studied range of curvature radii (70 ÷ 360 μm) the film tension does not depend on the curvature and the capillary pressure.

PERMEATION OF GAS THROUGH NEWTON BLACK FILMS AT DIFFERENT CHAIN LENGTH OF THESURFACTANT

Abstract The gas permeability coefficients K (cm/s) of Newton black foam films stabilized by four alkyltrimefhylammonium chloride homologues have been measured. The experimental results demonstrate well pronounced dependence: K decreases with increasing surfactant alkyl chain length. The temperature dependence of K has been studied as well. The experimental data are discussed on the basis of the nucleation theory of fluctuation formation of holes in the foam bilayers, assuming that the gas permeation occurs both through holes and through hole-free part of the film (background permeability). The mechanism of permeation through the hole-free film is discussed using the theories of monolayer permeability. The results are in favour of the energy-barrier theory.

Measurement of the film tension of foam films in dynamic conditions

The method for direct measurement of the film tension of foam films has been developed with a view to measuring the film tension in dynamic conditions. The new method allows the determination of the dynamic film tension when the curvature radius, the contact line radius, and the area of the film increase or decrease with very different rates. Measurements with Newton black films from sodium dodecyl sulphate aqueous solution have been performed. The results show that in a wide range of variation rates of the film geometrical parameters the film tension remains constant.

High concentrated solutions of surfactants and nature of the black foam films. A study of electroconductivity

SummaryThe analogy between the properties of the high concentrated surfactant solutions and the black foam films is discussed. Measurements of the specific electric conductance ϰ of high concentrated (40–60 %) sodium lauryl sulphate solutions are carried out with the aid of a special conductance cell (fig. 1). The temperature dependence of ϰ for several surfactant concentrations is compared on fig. 3 with the temperature dependence of the specific electric conductance ϰf of the black foam films. The results show that at low temperatures a complete analogy between the electroconductivity properties of the second black film on one hand and the surfactant-water gel on the other is evident, which indicates an analogy between the structures of these two systems. A further analogy is available between the electroconductivities and respectively the structures of the first black film and the liquid crystal phase surfactant-water, both being stable at higher temperatures.ZusammenfassungDie Analogie zwischen den Eigenschaften der hochkonzentrierten Tensidlbsungen und der schwarzen Schaumlamellen wird diskutiert. Es wurden Messungen der elektrischen Leitfähigkeit ϰ von hochkonzentrierten (40–60%) Natriumdodecylsulphatlösungen mit der Hilfe einer speziellen Zelle (Fig. 1) durchgeführt. Die Temperaturabhängigkeit von ϰ für einige Tensidkonzentrationen wurde mit der Temperaturabhängigkeit der spezifischen elektrischen Leitfähigkeit ϰf von den schwarzen Schaumlamellen verglichen. Die Ergebnisse zeigen eine Analogie zwischen den elektroleitenden Eigenschaften des zweiten schwarzen Films und des Tensid-Wassergels bei niedrigen Temperaturen, was für eine Analogie zwischen den Strukturen dieser beiden Systeme spricht. Eine weitere Analogie wird bei höheren Temperaturen beobachtet, zwischen den elektrischen Leitfähigkeiten und beziehungsweise den Strukturen des ersten schwarzen Films und der lyotropen flüssigkristallinen Tensid-Wasserphase.

Freestanding black films of phospholipids and phospholipid with proteins

Abstract Free black films formed from aqueous solutions of phospholipids and phospholipids and proteins mixtures have been studied by a X-ray reflectivity method. From the reflectivity experiments one can extract the electron density profile perpendicular to the films. By an appropriate stratified model of the film we determined its structure (i.e. the thickness, the roughness and the densities of the different slabs). Additional surface tension measurements of all the solutions were performed in order to obtain complementary information about the state of the monolayers. The experiments were performed with mixtures of two types of lipids, dimyristoyl phosphatidyl choline (DMPC) which is a noncharged lipid and dimyristoyl phosphatidyl glicerol (DMPG) which is a negatively charged. Aqueous suspensions of unilamellar vesicles were prepared by sonication. Bovine serum albumin (BSA) which is a globular soluble protein was added to the above mixture. All the solutions were studied in the presence or absence of NaCl. The experiments were carried out at temperatures above the phase transition temperature of the lipids and of the mixture. The X-ray experiments show a very strong influence of DMPG on the thickness and on the structures of the phospholipid films. The addition of a protein in the DMPC/DMPG mixture results in the formation of thicker films due to the strong lipid/protein interactions. The film thickness increases with the BSA concentration. This result can be explained by the presence of protein molecules inserted between the two film layers.

A new method for direct measurement of the film tension of newton black films on a diminshing bubble

Abstract The problem of the measurement of the film tension of a Newton black film at the top of a diminishing bubble at the solution surface is discussed. It has been concluded that the shearing interference method cannot be used for this measurement. A new experimental method has been developed. It is based on the fact, that the film at the bubble top (a convex mirror) and the air/solution surface at the bubble bottom (a concave mirror) reflect, simultaneously, the incident light. They produce two images of the collimating aperture slit, which can be distinguished and photographed separately. The ratio of the lengths of both images is equal to the ratio of the curvature radii of the film and the bottom bubble surface, hence it is equal to the ratio of the film tension and the solution surface tension. Measurements with a sodium dodecyl sulfate aqueous solution show that the film tension of the Newton black film does not depend on the bubble size or the capillary pressure over a wide range (400 to 80 μm) of bubble radii. The measured film tension values coincide well with previous data.

Contact angles of protein black foam films under dynamic and equilibrium conditions

The contact angles of protein Newton black foam films from ALG (alpha-lactalbumin), BLG (beta-lactoglobulin) and BSA (bovine serum albumin) are measured here within. The measurements are carried out under dynamic and equilibrium conditions. For all proteins, a strong hystheresis effect of the contact angle is observed under dynamic conditions. An attempt is made to explain these results by the slow adsorption and desorption kinetics of the protein bilayers and by the dynamic structure and the rheology of the protein network forming the bubble walls.In addition, we propose a modification of the experimental device reported previously for contact angle measurements of large flat films in equilibrium. The advantages of this method are discussed in detail. Some shortcomings (precision, reproducibility) of this preliminary variant of the device in this initial stage of its application, do not allow one to draw reliable conclusions about the interactions of these films. Some improvements of the measurement quality are proposed.