Black film structure

D. Exerowa, D. Kashiev, and D. Platrkanov Structural Properties of Soap Black Films Investigated by X-Ray Reflectivity. Adv. Colloid Interface Sci. 40, 201 (1992). 

It was already noted in previous studies, that added salt affects the micellar structure only above a certain value of the salt concentration cs. In studies of PtBS-PSSNa block copolymers a dependence of the brush dimension on the added salt concentration appeared above cs=0.01 mol/1. The thickness of a free-standing black films drawn from a diblock polyelectrolyte solution exhibited a steady drop above ionic strengths of 0.2 mol/1 [39]. 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

Corkill et al. [56] have used for the first time the infrared spectroscopy for foam films. The measurement of the adsorption of the infrared light provides information about the water content in the foam films which is of major significance for the black foam films. These studies involved the use of dispersion type instruments. In order to obtain measurable values of adsorption, the infrared light is passed through a series of vertical films (up to 10) formed in a cylindrical tube acting as a frame. Additional information about the film structure the authors derived from the correlation between the optical infrared transmission data and the film reflectance measurements. Here a three-layer model of the film structure consisting of an aqueous core sandwiched between two adsorption layers is assumed (see Section 2.1.3). 

Combined measurement techniques were successfully applied in the study of surface forces in microscopic foam films such as study of longitudinal electrical condictivity, study of black films with X-rays forced rupture of films by a-particles irradiation, etc. They permit to find the relation between surface forces and parameters of film structure. It is important also surface forces measurements to be performed at controlled state of the adsorption layer. As far as surface forces act normally to film surface, it is interesting to understand the role of... 

It is interesting to apply the method of n(h) isotherms at high capillary pressures to studying the multilayer structure of black films. The first results obtained refer to biostructures (see Chapter 11) and lamellar structures in solubilising solutions (see Section 3.4.2.5). 

The question about structure, properties and the factors determining the stability of black films, especially of NBF, is far from being thoroughly elucidated. The main problem remains the determination of the film type (CBF or NBF) and the conditions under which each of the two film types is stable. This provides an opportunity to explain quantitatively the reasons for their formation and stability as well as for understanding the origin of the surface forces acting in them. 

Additional data about the structure of black films are obtained by X-ray diffraction method. The first steps [336,338] have been performed with vertical foam films in a frame in a horizontal scanning diffractometer. Black films from decyltrimethyl ammonium decyl sulphate and NaBr solutions have been studied. The film thickness was calculated using a model of the mean electron density projection on the film normal. However, there was no indication whether the films were CBF or NBF. Platikanov et al. [339,340] used a new device for investigation of a horizontal black films from aqueous NaDoS solution (see Section 2.2.6). They found essentially different X-ray diffraction traces for the three types of black films CBF, NBF and stratified black films. This indicates their different structure. Precise X-ray reflectivity measurements with CBF and NBF films from NaDoS and NaCl aqueous solutions [341-343] provided more details about their structure. The data obtained for the thicknesses of the respective layers which detail the film structure are given below... 

The values of CBF differ slightly since they were obtained by different techniques vertical films in a frame put into a horizontal X-ray diffractometer [341,342] and horizontal film (Fig. 2.21) in a synchrotron ray diffractometer [343], The data presented in the above table confirm the concepts established from previous investigation about the structure of both types of black films a sandwich model of two adsorption layers of amphiphile molecules with an aqueous core in between (for CBF) and bilayer of amphiphile molecules in which molecules of the solvent are incorporated (for NBF). The bilayer structure of NBF has been confirmed [344] for films from aqueous solutions of C]2E6 obtained in the measuring cell shown in Fig. 2.21. Their thickness of 6 nm found is less than the double length of the amphiphile molecule. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Another configuration of the black film position, proposed by Scarpelli and Mautone [57] is also possible (Fig. 11.11) where it apposes the gas phase. According to Scarpelli and Mautone this configuration is consistent with the surfactant film structure and function in situ. 

In some cases, the growth of perturbations leads to the formation of spots of thinner metastable films (with thickness about 10 nm). The film at the spots is so thin that it appears black in reflected light. Such films are often referred to as black films. These objects are obliged by their origin to a sufficiently large value of the structural component of the disjoining pressure, which determines the existence of the second interval where dH/dh < 0 on the II(/i) curve. A rupture of the black films can also take place, but this mechanism is connected with a display of the vacancy instability [125]. 

For the static, trapped lamellae, the film reaches an equilibrium thickness set by the local capillary pressure and the film curvature in obedience to equation 1. The capillary pressure, in turn, depends, on the wetting-liquid saturation the film curvature depends on the particular location within the pore structure dictated by an approximately 90° contact angle with the pore wall. As the capillary pressure in the porous medium rises during drainage, the film thickness decreases along the common black branch until nmax is reached, and a Newton black film of 4-nm thickness emerges. 

Figure 12.12 Schematic representation of the structure of a Newton black film. The core (shaded) contains water and counter-ions.

It is importcuit, from a practical point of view, that extremely thin black films are ruptured under the condition of reduced humidity, since evaporation of the dispersion medium leads to their ultimate thinning and appearance of high de- wedging pressures. Here are two possibilities to produce stable (under the conditions of dry air) foams. On the one hand, this is reduction of permeability of adsorption layers for water vapours and thus inhibition of the evaporation rate. On the other hand, it is the use of surfactants for foam stabilisation leading to the formation of a structure-mechanical barrier, i.e. gel-like interlayer. 

Artificial lipid bilayer membranes can be made [22,23] either by coating an orifice separating two compartments with a thin layer of dissolved lipid (which afterwards drains to form a bilayered structure—the so-called black film ) or by merely shaking a suspension of phospholipid in water until an emulsion of submicroscopic particles is obtained—the so-called liposome . Treatment of such an emulsion by sonication can convert it from a collection of concentric multilayers to single-walled bilayers. Bilayers may also be blown at the end of a capillary tube. Such bilayer preparations have been very heavily studied as models for cell membranes. They have the advantage that their composition can be controlled and the effect of various phospholipid components and of cholesterol on membrane properties can be examined. Such preparations focus attention on the lipid components of the membrane for investigation, without the complication of protein carriers or pore-forming molecules. Finally, the solutions at the two membrane interfaces can readily be manipulated. Many, but not all, of the studies on artificial membranes support the view developed in the previous sections of this chapter that membranes behave in terms of their permeability properties as fairly structured and by no means extremely non-polar sheets of barrier molecules. 

The first optically black silicon structures were probably made by anodization as early as the 1950-1960s. Koltun studied films generated at lower current densities than Uhlir and Turner (Uhlir 1956 Turner 1958 Koltun 1964). He described their persistent black color being due to a high degree of dispersion of silicon. Interestingly, he also recorded reflectivity fi"om his photocells, but focused on the infrared rather than the visible range (Koltun 1964). 

Typical materials used as color or hiding pigments and to provide aesthetic value, retention of gloss and color, and to help with the film structure and impermeability are iron oxides, titanium dioxide, carbon or lamp black, and others. 

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