Soap film thickness

Describe the differences between the reflectivity of a soap film, thickness A/4, in air, compared with the same film on an oil surface. Assume that the refractive index of the soap film is very shghtly more than that of pure water (ra = 1.33), the refractive index of the oil is 1.44. 

In the case of solids, there is no doubt that a lateral tension (which may be anisotropic) can exist between molecules on the surface and can be related to actual stretching or compression of the surface region. This is possible because of the immobility of solid surfaces. Similarly, with thin soap films, whose thickness can be as little as 100 A, stretching or extension of the film may involve a corresponding variation in intermolecular distances and an actual tension between molecules. 

The repulsion between two double layers is important in determining the stability of colloidal particles against coagulation and in setting the thickness of a soap film (see Section VI-5B). The situation for two planar surfaces, separated by a distance 2d, is illustrated in Fig. V-4, where two versus x curves are shown along with the actual potential. 

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]. 

Fig. 2. Effective interface potential (left) and corresponding disjoining pressure (right) vs film thickness as predicted by DLVO theory for an aqueous soap film containing 1 mM of 1 1 electrolyte. The local minimum in H(f), marked by °, gives the equiHbrium film thickness in the absence of appHed pressure as 130 nm the disjoining pressure 11 = —(dV/di vanishes at this minimum. The minimum is extremely shallow compared with the stabilizing energy barrier.

Disjoining Pressure. A static pressure difference can be imposed between the interior and exterior of a soap film by several means including, for example, gravity. In such cases the equiHbrium film thickness depends on the imposed pressure difference as weU as on the effective interface potential. When the film thickness does not minimize lV(f), there arises a disjoining pressure II = —dV/(U which drives the system towards mechanical equiHbrium. 

We have noted that the adhesion of the polar groups to water and to one another is much greater than the weak adhesion of hydrocarbon chains either to water or to one another. It is thus reasonable to assume, an anticipation verified by Perrin Ann. Rhys. X. 160, 1918), that soap films may be made up of composite surfaces each of which consists of two layers of orientated molecules of soap the outer surface of each side consisting of hydrocarbon chains and the polar groups held together with water as a sandwich between the orientated hydrocarbon chains. These elementary leaflets which will possess but little adhesion for one another may be built up to form thick films similar in structure to the crystalline fatty acids examined by Shearer (see p. 73). The leaflets may slip over one another with great ease, thus providing the play of interference colours noticed in soap films. The elementary leaflet has in fact been shown by Perrin and others to be two molecules in thickness. 

The permeability of soap films has been examined in detail by Dewar who showed (Proo. R. I. xxil. 193, 1917) that the rate of penetration varied with both the film thickness and the excess pressure in the bubble, A few of the values obtained for hydrogen are cited overleaf. 

Estimate the concentration of Na ions at the centre layer of an aqueous soap film drawn from a mM NaCl solution, if the electrostatic potential at each surface is -20 mV and the film is lOnm thick. 

The equilibrium thickness of a (meta-)stable soap film will depend on the strength and range of the repulsive forces in the film. Electrostatic forces are long-range in water and hence give rise to thick (0.2 micron) films, which are highly coloured due to the interference of visible light... 

In this situation, the equilibrium thickness at any given height h is determined by the balance between the hydrostatic pressure in the liquid (hpg) and the repulsive pressure in the film, that is n = hpg. Cyril Isenberg gives many beautiful pictures of soap films of different geometries in his book The Science of Soap Films and Soap Bubbles (1992). Sir Isaac Newton published his observations of the colours of soap bubbles in Opticks (1730). This experimental set-up has been used to measure the interaction force between surfactant surfaces, as a function of separation distance or film thickness. These forces are important in stabilizing surfactant lamellar phases and in cell-cell interactions, as well as in colloidal interactions generally. 

The thicknesses of free soap films and liquid films adsorbed on surfaces (Figs. 1.26d and 1.26e), which can be measured using optical techniques such as reflected intensity, total internal reflection spectroscopy, or ellipsometry as functions of salt concentration or vapor pressure, can provide information on the long-range repulsive forces stabilizing thick wetting films. We see an example of this in Chapter 11. 

If a soap film is sufficiently thin, its equilibrium thickness is the result of the double-layer repulsion, given by Equation (82), and van der Waals attraction, given by... 

Influence of electrostatic repulsion on the thickness of soap films 523... 

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

Thickness is one of the main parameters of a foam film. The most widely employed technique for its determination is interferometry. It is based on the comparison between the intensities of the light falling on the thin film and that reflected from it. This technique permits to evaluate the thickness of equilibrium as well as thinning films. It has been used by Perrin [48] and Wells [49] with soap films. The intensity of the reflected light was measured with an interferometer comparing the two parts of the visual field (in a microscope). Thus the film thickness was determined with an accuracy of about 0.5 nm. 

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. 

Fisher Scientific) was emulsified into aqueous soap solutions of 0.25 w% each of sodium laurate and sodium oleate prepared from sodium hydroxide, lauric acid (Aldrich Gold Label) and oleic acid (Fisher Purified). Coarse emulsions were used for microscopy (as in Figure 2), but fine emulsions (with droplet sizes of about 0.2 microns), used for determination of middle phase film thicknesses, were made by ultrasonication with a cell disruptor. 

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