Secondary black films

Further thinning can cause an additional transformation into a thinner stable region (a stepwise transformation). This usually occurs at high electrolyte concentrations, which in turn leads to a second, very stable, thin black film that usually is referred to as Newton secondary black film, with a thickness in the region of 4nm. Under these conditions the short-range steric or hydration forces control the stability, and this provided the third contribution to the disjoining press, as described in Equation (16.9). 

The observed equilibrium thickness represents the film dimensions where the attractive and repulsive forces within the film are balanced. This parameter is very dependent upon the ionic composition of the solution as a major stabilizing force arizes from the ionic double layer interactions between any charged adsorbed layers confining the film. Increasing the ionic strength can reduce the repulsion between layers and at a critical concentration can induce a transition from the primary or common black film to a secondary or Newton black film. These latter films are very thin and contain little or no free interlamellar liquid. Such a transition is observed with SDS films in 0.5 M NaCl and results in a film that is only 5 nm thick. The drainage properties of these films follows that described above but the first black spot spreads instantly and almost explosively to occupy the whole film. This latter process occurs in the millisecond timescale. 

Figure 5.6 shows an example of a total interaction energy curve for a thin liquid film stabilized by the presence of ionic surfactant. It can be seen that either the attractive van der Waals forces or the repulsive electric double-layer forces can predominate at different film thicknesses. In the example shown, attractive forces dominate at large film thicknesses. As the thickness decreases the attraction increases but eventually the repulsive forces become significant so that a minimum in the curve may occur, this is called the secondary minimum and may be thought of as a thickness in which a meta-stable state exists, that of the common black film. As the... 

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. 

As already discussed in Section 3.4, these two states of black films are, respectively, common black (CBF) and Newton black (NBF) films. Initially, bilayer films were named Perrin films by Scheludko later Jones, Mysels and Scholten called them primary and "secondary films. It was not until the issuing of IUPAC nomenclature that they were termed CBF and NBF. It is rather arguable, however, whether it is fairer to name them after the scientist who observed them first or after the one that characterised them quantitatively. In many cases NBF are also called amphiphile bilayers . 

Several investigations were carried out to study the above transitions from CF to common black film, and finally to Newton black film. For sodium dodecyl sulphate, the common black films have thicknesses ranging from 200 nm in very dilute systems to about 5.4 nm. The thickness depends heavily on the electrolyte concentration, while the stability may be considered to be caused by the secondary minimum in the energy distance curve. In cases where the film thins further and overcomes the primary energy maximum, it will fall into the primary minimum potential energy sink where very thin Newton black films are produced. The transition from common black films to Newton black films occurs at a critical electrolyte concentration which depends on the type of surfactant... 

Black Film Fluid films yield interference colors in reflected white light that are characteristic of their thickness. At a thickness of about 0.1 /xm, the films appear white and are termed silver films. At reduced thicknesses, they first become grey and then black (black films). Among thin equilibrium (black) films, one may distinguish those that correspond to a primary minimum in interaction energy, typically at about 5-nm thickness (Newton black films) from those that correspond to a secondary minimum, typically at about 30-nm thickness (common black films). 

The corrosion resistance of copper is good, but in polluted, humid air, containing sulfur dioxide and/or chlorides, the copper surface is oxidized. A black film, mainly cuprite, Cu O, is formed. A green patina with varying formula is formed by secondary processes. Two common types are Cu.,(OH)gSO H2 Cu lOHjjCl. These compounds have a decorative effect but serve also as protecting layers, reducing the rate of further corrosion. 

Later, the relevance of the van der Waals forces in foam films was realized and it was suggested that the black films correspond to a secondary minimum. However, the DLVO theory could not adequately describe... 

A typical II(/i) isotherm is depicted in Fig. 17a. (The shape of the curve in Fig. 17a is discussed in Section VI.A.) One sees that the equilibrium condition, II = P can be satisfied at three points shown in Fig. 17a. Point 1 Corresponds to a film which is stabilized by the double-layer repulsion sometimes such a film is called the primary film or common black film. Point 3 corresponds to unstable equilibrium and caimot be observed experimentally. Point 2 corresponds to a very thin film which is stabilized by the short-range repulsion such a film is called the secondary film or Newton black film. Transitions from common to Newton black films are often observed with foam or emulsion films [237-240]. 

Fig. 7 Sequential micrographs of the evolution of the damage in a SiO 4.5 wt.% P film deposited on an Al substrate subjected to a tensile test (system C, Figure 6). The black arrows show the tensile direction, (a) Networks of primary and secondary cracks perpendicular to the tensile axis (e = 11%). The white arrows show a secondary crack which stops when getting close to primary cracks, (b) decohesion and buckling of the strips of film. Slip lines are observed on the Al surface under the buckled strips, and (c) transverse rupture of the buckled zones along the directions of maximum shear of the substrate (e = 19%).

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