Soap film black films

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

J.A. De Feijter and A. Vrij Contact Angles in Thin Films, n. Contact Angle Measurements in Newton Black Soap Films. J. Colloid Interface Sci. 64, 269... 

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). 

One case, that of a black film, is particularly important and easy to understand. Soap films, which are thinner than 30 nm appear black. For this reason they are called black films. Why... 

The physicochemical properties of foam and foam films have attracted scientific interest as far back as a hundred years ago though some investigations of soap foams were carried out in the seventeen century. Some foam forming recipes must have been known even earlier. The foundations of the research on foam films and foams have been laid by such prominent scientists as Hook, Newton, Kelvin and Gibbs. Hook s and Newton s works contain original observations on black spots in soap films. 

The black spots on soap films, which are not more than 10 to 20 molecules thick, can remain for weeks in equilibrium with the thicker, coloured parts of the film,4 and hence it is assumed that they have the same vapour pressure as the normal liquid, and that Thomson s formula can be applied for a radius of curvature of 200 x 10 cm. or less. Bakker<5 gave reasons for supposing that the surface tension is independent of the radius of curvature of the capillary layer, although he recognised that in very thin films it has abnormal values, and he calculated that the maximum ascent of a liquid occurs in a tube of 2 5 m[jL radius. Woodland and Mack found no change of surface tension in a tube of 6 7 [I radius. 

Bingham s viscosity equation, 117 black spots on soap films, 373 blending factor, 119... 

The remarkable similarity between the sodium chloride concentration at which the mobility obtained a reasonable value and that at which a first black film was formed suggests that at this concentration chloride ions adsorb to the sulfoxide groups and thus produce a potential which can provide electrostatic repulsion between the monolayers in the soap film. In the case of potassium thiocyanate, adsorption appears to commence at a lower salt concentration, suggesting a stronger free energy of adsorption for the thiocyanate ion and consequently a greater extent of adsorption. Thus a higher potential would be obtained at the interface. 

The phenomena described above have been known for a long time indeed, Newton reported on the black spots in soap films. In the past 25 years, however, these thin, liquid structures have become a subject of intensive scientific studies. One of the main reasons is that the interaction forces between colloidal particles suspended in a liquid are of the same nature as those operating in soap films. Because the film geometry is well defined (i.e., a thin, flat liquid sheet, macroscopic in lateral extension), it is an attractive experimental subject for studying these forces, in particular with optical means. 

The transitions from an unstable thick film to black films arc easily demonstrated by allowing a vertical soap film supported on a frame to drain (Figure 12.11). Initially the whole film shows interference colours, then a dark boundary, separated from the... 

Figure 12.11 Draining soap film showing interference colours from thicker films and silver and black bands from thin films. The two types of black film are not distinguishable.

For decades, colloid and surface scientists have known that amphiphilic molecules such as phospholipids can self-assemble or self-organize themselves into supramolecular structures of bilayer lipid membranes (planar BLMs and spherical liposomes), emulsions, and micelles [2-4]. As a matter of fact, our current understanding of the structure and function of biomembranes can be traced to the studies of these experimental systems such as soap films and Langmuir monolayers, which have evolved as a direct consequence of applications of classical principles of colloid and interfacial chemistry. As already mentioned in Section I, the seminal work on the self-assembly of planar lipid bilayers and bilayer or black lipid membranes was carried out in 1959-1963. The idea started while one of the authors was reading a paperback edition of Soap Bubbles by C. 

It was found that up to 0.004% concentration of the iron soap in n-decane, which corresponds to the formation of a saturated adsorption layer at the interface, black spots are formed. However, these black spots do not produce a black film and interfacial film is broken quickly. Soap concentrations of up to 0.1% produce a film whose thinning-out stops when a thick stable grey film is created. Similar results have been observed for films stabilized by aluminum soap. During the formation of the film, monochromatic light showed alternative dark and bright bands corresponding to the interference maxima and minima. By measuring the parameters of the latter, the film thickness could be estimated. 

Estimation of hydrocarbon film thickness was carried out in relation to soap concentrations and the time from interface formation. Soap concentrations from 0,01 to 0.1% increased the film thickness from 600 to lOOOA, the time from interface formation influencing the film stability. Similar results were obtained when film thickness was determined by the ellipsometry. Thus, the study of emulsified hydrocarbon films stabilized with aluminum and iron soaps showed that the process of film formation did not lead to stable black films. 

The films are thinner at the top than at the bottom. After a while, the upper part often ends up becoming extremely thin, reducing to two soap monolayers sandwiching just a few molecules of water. Such films no longer produce optical interference effects instead, they look quite black. Newton studied these black films. Long before, the Assyrians had observed them and even used their unpredictable shapes (in horizontal geometries) to divine the future. 

Soap films that are rather thick (thickness is mainly due to the water) reflect red light and one observes blue-green colors. Lesser thin films cancel out yellow wavelength and blue color is observed. As the thickness approaches the wavelength of light, all colors are cancelled out and a black (or gray) film is observed. This corresponds to 25 mn (250 A). 

The film eventually reaches an equilibrium thickness in which both faces of the film are parallel (Fig. 1.18(c)). In this state there is no variation in intensity over the surface of the film. This is called the common black film. It occurs, typically, at a thickness of 300A (30nm). A further decrease in the film thickness, to another stable equilibrium state with a thickness of about 50A, is often possible and is known as the Newton black film. This film is darker than the first black film, the common black film, as the second of the two split rays, that is refracted into the film, travels through a thinner soap film than in the case of the common black film. Consequently the phase difference between the two split rays of the Newton black film is closer to tt than in the case of the common black film. Some films have only one equilibrium state while others have two or more equilibrium states. 

---