Black Foam Films Research Articles

Pure and mixed lipid black foam films as models of membrane fusion

Black foam films (BFF) from water solutions of the phospholipid dilauroyl lecithin (DLL) with admixtures of palmitoyl lysolecithin (Lyso) were formed. Microscopic BFF were studied by the method of Scheludko and Exerowa. The formation probability for BFF and the BFF lifetime in a black state before film rupture were measured as functions of the film composition. At a fixed overal lipid concentration it was shown that an increased percentage of Lyso exponentially increased the lifetime of the film up to the CMC of Lyso. This stabilizing Lyso effect nicely corresponds with its stabilizing action on the waiting time for fusion of two contacting black lipid membranes (BLM), as found by Chernomordik et al. In contrast, Lyso is known to destabilize a single BLM. In this way we have found experimental proof of our earlier prediction that Lyso should have opposite effects on the lifetimes of BLM and BFF. In addition, we have shown for the first time that foam films made of lipids are a convenient model for monolayer membrane fusion studies. This model is characterized by its simplicity and experimental reliability and provides a means for quick screening of the fusogenic capacity of various amphiphilic and hydrophilic admixtures.

Direct measurement of disjoining pressure in black foam films II. Films from nonionic surfactants

Experiments are reported with foam films from two different polyoxyethylenic nonionic surfactants of relatively short and long polyoxyethylene chains. The direct measurement of the film disjoining pressure was combined with measurements of the film thickness dependence on the electrolyte concentration. The results obtained are discussed in connection with: (i) the type of black films observed, (ii) the influence of the POE chain length on the film properties, (iii) the agreement between the disjoining pressure isotherms obtained and the theoretical ones, (iv) the relationship between the charge and potential of the interacting diffuse double layers.

Effect of temperature on the rupture of newton black foam films

The effect of temperature on the rupture of foam bilayers is studied experimentally. The foam bilayers are microscopic horizontal Newton black foam films of radius 0.25 mm in contact with an aqueous solution of surfactant. The dependence of the mean time for rupture of the bilayers on the concentration of surfactant (sodium dodecyl sulfate, NaDoS) dissolved in the solution, in the presence of 0.5 mol dm −3 NaCl, is obtained at 10, 22 and 30°C. The following thermodynamic parameters: size of the nucleus hole, nucleation work and equilibrium surfactant concentration are determined for the above temperatures according to the theory of rupture of foam bilayers by hole nucleation. Values of 3.1·10 −11 J m −1 for the specific edge free energy of bilayer circular hole and 6.7·10 −20 J for the binding energy of NaDoS molecule in the foam bilayer are calculated.

Black foam and emulsion films from nonionic surfactant solutions: rupture by hole formation

The concept that rupture of bilayer films is caused by nucleation of holes was checked for foam and emulsion films stabilized by the same nonionic surfactant. Newtonian black (NB) films were formed in a ring-cell from a biconcave drop with air or nonane, respectively, as the ambient phase. The lifetime of the foam and emulsion films increases with surfactant concentration. This relation is analysed on the basis of above mentioned theory.

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.

On the curvature dependence of the film tension of Newton black films

The film tension of curved (spherical) Newton black foam films having radii of curvature between 70 and 700 μm has been studied experimentally. Measurements with films from sodium dodecyl sulfate aqueous solutions have been carried out using a method developed previously for direct measurement of the film tension at different capillary pressures. The results are discussed in comparison with the results of other authors for film tensions of curved and of plane Newton black films. The line tension values, determined with the different experimental film tension values, as well as their comparison with the theory are also discussed. It is concluded that there is no noticeable dependence of the film tension on the film curvature, or on the capillary pressure, in the whole range of film curvature radii covered by this study.

Rupture of newtonian black foam films by α- Particle irradiation: Hole mechanism

The stability of Newtonian black (NB) foam films with respect to rupture by α-particle irradiation is studied in order to establish how it is influenced by the concentration C of surfactant in the solution. An expression for the dependence of the irradiation-affected film mean lifetime on C is derived on the basis of the nucleation theory for formation of microscopic holes in the films by clustering of vacancies of surfactant molecules. The t (C) -dependence is determined experimentally for NB foam films obtained from aqueous solutions of sodium dodecyl sulfate and irradiated by α particles at a constant rate. A good agreement between theory and experiment is found and a value of 2 × 10 −8 mJ m −1 for the specific edge free energy of the holes in these films is calculated. It is concluded that the passage of one α-particle through a hole is responsible for the rupture of NB foam films by α-particle irradiation and that both the irradiation-stimulated and the irradiation-free rupture of the films are describable from a unified point of view by the fluctuation theory of nucleation.

Direct measurement of disjoining pressure in black foam films. I. Films from an ionic surfactant

Disjoining pressure isotherms for microscopic foam films were measured directly, using an apparatus where the film disjoining pressure was balanced by

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.

Stability of foam lipid bilayers of amniotic fluid

Black foam lipid films are studied, having been obtained by thinning of foam films stabilized with phospholipids of amniotic fluid (AF). Whilst in a suitable measuring cell, the black films are in contact with air (normally) and with a solution of AF (laterally). It is proved experimentally that the black films studied are bilayer ones. The probability W for observation of black foam film in a critically thick foam film is described on the basis of the theory of rupture of bilayers due to nucleation of holes in them. This probability can characterize the stability of the bilayer foam film, as it depends strongly on the concentration C of phospholipids in the AF solution. The experimentally determined W exhibits a steep dependence on C and shows the existence of a threshold concentration below which a black foam film is not observed. A correlation is established between the threshold concentration of phosphatidylcholines (PC) and that PC concentration which ensures functionally the human foetal lung maturity. On this basis is developed a diagnostic method for assessing the foetal lung maturity which is characterized by high reliability, rapidity and use of a small quantity of AF. It is proved that PC is the lipid which builds up the foam bilayer obtained from AF and it is found that a PC concentration of 46 μg cm −3 in the AF ensures foetal lung maturity.

Nucleation mechanism of rupture of newtonian black films: II. Experiment

A mechanism of rupture of Newtonian black films is considered on the basis of the classical nucleation theory (D. Kashchiev, and D. Exerowa, J. Colloid Interface Sci. 77, 501 (1980)). This mechanism is experimentally tested by studying the mean life time τ of a microscopic Newtonian black (NB) foam film as a function of the surfactant concentration C. Ionic surfactants and the nonionic decyl-4-ethylene oxide (C 10 (EO) 4) and nonylphenol-20-ethylene oxide (NP20) are used to stabilize the investigated films. The τ( C) dependence is confronted with the theoretical expression and the following parameters of the nucleation mechanism of film rupture are determined: nucleation work, number of vacancies in the nucleus, hole specific peripheral energy, lateral bond energy between nearest surfactant molecules in the film, supersaturation, critical surfactant concentration for NB film formation, and equilibrium surfactant concentration at which the NB films is infinitely stable. By studying the probability of NB film rupture, it is shown that from a certain moment on the nucleation rate is time-independent.

Electrostatic disjoining pressure in symmetrical films with adsorptive charge regulation

The case of a film containing k strong 1:1 electrolytes sandwiched between nonpolar phases is considered at low electric surface potentials. An analytical equation for the electrostatic disjoining pressure under adsorptive charge regulation is obtained. It gives the well-known limiting cases of constant surface charge or potential depending on the value of a dimensionless parameter Φ. The calculated balance between van der Waals' attraction and electrostatic repulsion is in agreement with experimental evidence for the dependence on electrolyte concentration of the transition between common black and Newton water foam films stabilized with nonionic surfactants.

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

The 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.

The similarity between black foam films and neat mesomorphic phase

The equilibrium thicknesses of black foam films and the interlayer spacings of the structurally similar lamellar mesomophic phase (neat phase) formed by the decyltrimethylammonium decylsulphate + NaBr + H 2O system have been measured at 298 K. A marked variation of film thickness and also neat-phase spacing occurs with changing system composition. This is attributed in both cases to the effect of inorganic ions on electrostatic interactions between surfactant layers, although the exact mechanism differs for each case.

Adsorption of inorganic ions in black foam films

The concentration of inorganic ions adsorbed in the thin aqueous core(∼20 Å) of black films at their limiting thickness has been measured directly by a radiotracer method and compared with the results obtained from film conductance measurements. The standard free energy of adsorption (2-3 RT/mole) is comparable to that observed for the adsorption of inorganic ions on monolayers at the bulk solution interface. It is suggested that the stability of these very thin films is determined by the work required to desorb water molecules from the aqueous core.

Extinction Coefficient of Water at 2.93µ and Water Content of Black Foam Films

Extinction Coefficient of Water at 2.93µ and Water Content of Black Foam Films

Structure of black foam films

Radiotracer measurements have shown that black foam films possess a sandwich structure, consisting of two monolayers of surface-active agent enclosing an aqueous core. The reflection coefficient and the angle at which reflected light becomes fully plane-polarized have been used together with the infra-red transmission at an absorption maximum for liquid water to calculate the total thickness of the film and the thickness of the surface monolayers. Total film thicknesses obtained using this three-layer optical model are in good agreement with those obtained by low-angle X-ray scattering. The aqueous core of a thin black film has a limiting thickness of ∼20 Å. Radiotracer methods have shown that the surface monolayers adsorb inorganic ions, and conductance measurements have indicated that these ions are mobile. It is suggested that the relative stability of these films is determined by the free energy required to dehydrate the hydrophilic head groups of the surface-active ions and the inorganic electrolyte in the film.

Determination of the optical parameters of thin foam films

Using the matrix technique for the evaluation of the optical properties of thin dielectric layers it is shown that optical measurements (with present accuracies) are unable to distinguish between a homogeneous model, a three-layer model or models with a higher number of layers for the secondary black foam films. This fact is used to develop a simplified method of extracting the film parameters corresponding to a symmetric 3-layer model from the optical data available. The technique is applied to the results of Corkill et al. on the equimolar sodium decyl sulphate+decyl trimethylammonium bromide system. Finally the effective extinction coefficients in very thin films can be very different from those of thick films because of optical interference effects related to the refractive index. For water films in air the errors involved may be as large as ∼33 %.

Thickness of black foam films

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The structure and stability of black foam films

The black film (50 to 100 A thick) has a simple lamellar structure in which two monolayers of surface-active molecules are spread on a thin aqueous core. The thickness of the film surface layers has been estimated from the simultaneous measurement of the total film thickness and the thickness of the aqueous core. From these measurements it has been concluded that the surface-active molecules are orientated perpendicularly to the film plane. Radiotracer experiments have shown th at the composition of the film surface is identical to the composition of the surface layer at the bulk solution/air interface. These surface concentrations are in excellent agreement with the limiting value calculated from the solution surface tensions by application of the Gibbs adsorption equation. The factors influencing the stability of the thin films have been discussed in terms of the evaporative equilibrium of the volatile film component. The equilibrium vapour pressure over a thin aqueous film should be greater than that over the bulk liquid phase. I t has been suggested that the stability of a black film can be maintained by an excess of solute, relative to the bulk solution, in the aqueous film core.