The Black Film

The calculations presented in this section show that the behavior of the black films can be understood in terms of the interaction energy between planar films. However, they cannot explain why, for the same electrolyte concentration, the transition from the common to the Newton black film occurs at various pressures (for example, for Ce = 10 3 mol/dm3,p = (2.5 4- 9.8) x 104 N/m2).2 In addition, the thickness at the transition apparently does not depend on the electrolyte concentration (while the Debye length A2 does) and is larger than the upper bound 3A2 (which is obtained, when only the double layer and van der Waals interactions are present, using the approach employed to derive eq 21). In the next section, it will be shown that by accounting for the thermal fluctuations of the interfeces one can provide answers to these questions. 

It is clear that the stability of the films depends on the interaction between surfaces in addition, the domains of stability of the black films are strongly dependent on their rigidity. The calculations presented in Figure 8b differ... 

Expansion technique it allows accurate determination of large contact angles and is suitable to study Newton black films of thickness about 5 nm. It is based on the ratio between the parameters of the thick film (r( and i) and the black film, that results from the thick film at constant volume (V = const) of the meniscus [68,69]. 

Bubble of size of the order of tens of micrometers floating on the surfactant surface only little deviates from the spherical shape. This fact has been used in the method of diminishing bubble [128,129] which allows to measure the contact angle of the black film, the linear tension of the contact line film/meniscus and the coefficient of the gas permeability through the film. Fig. 2.24 presents the scheme of this device. 

A freely floating bubble of very small size is used in the diminishing bubble method [ 134] (Fig. 2.24) for the estimation of K by measuring the radii of the bubble R and the black film r with respect to time t. K is calculated from [135]... 

Other discrepancies between the black film behaviour and DLVO-theory are related to the difference in the critical electrolyte concentration, corresponding to the transition between the two black films types (see Section 3.4.2) the existence of a second minimum in the 11(A) isotherm the sharp rise in the disjoining pressure (after the second minimum). All this is evidenced by the measurements of contact angles between the film and bulk phase. 

Fig. 3.51 shows the disjoining pressure isotherms at 21 O 3 mol dm 3 CaCl2. Comparatively thick films were formed at low pressures and their thickness decreased with increase in n. The transition CBF/NBF occurred in the pressure interval 510 3 to 610 3 Pa. The black film thickness did not change with further increase in pressure up to 4-10 4 Pa (not shown in Fig. 3.51). 

The measurement of the parameters reflecting film properties which sharply change at the CBF/NBF transition lays at the basis of all experimental techniques for determination of the black film type. Microscopic black films render vast opportunities in the study of this transition by the dependences of film thickness, lifetime and contact angle on electrolyte concentration in the initial solutions. These dependences allow to estimate the critical electrolyte concentration Cei cr at which the CBF/NBF transition occurs. 

As it is known, the black film thickness h is a function of the electrolyte concentration (Fig. 3.62). Such a dependence has been studied in detail for microscopic foam films from sodium oleate solutions [14,73,96]. It has given the first quantitative evidence for the existence of the two types of black films. After a monotonous decrease in film thickness upon increasing electrolyte concentration up to 0.8 mol dm 3, a jump-like change in its thickness is observed (Fig. 3.62,a). An about twice thinner film is formed which does not change in thickness at a further increase in Cei. Thus Cei,cr was precisely determined and the concentration range in which the two types of black films are stable at a given temperature (21°C) was established. 

In order to apply the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa [27] involving quantitative interpretation of the W(C) dependence (probability for observation of black films vs. surfactant concentration), the black films from amniotic fluid should be bilayer films. This is proved experimentally by two dependences Y hw) (Fig. 11.1) and hw(Cei) (Fig. 11.2). As it can be seen in Fig. 11.1, the equivalent film thickness is 8 nm and does not change with the increase in IT (which is the difference between the pressures in the... 

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. 

The potentiostat is switched on and a black film is seen to form on the electrode. Electrolysis is then continued for about 30 mins. The black film is then washed and peeled from the electrode for subsequent analysis. 

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]. 

The presence of attractive colloidal forces becomes apparent in the black spots." The black film appears to be in a sort of metastable equilibrium state with a finite thickness in the colloidal size range. What are... 

The thickness measurements for the heptane-diluted bitumen films are presented in Fig. 16 (curve 1). Below the onset of asphaltene precipitation at a heptane itumen ratio of about 1 1, the film drained to an equilibrium gray film of about 27 nm thickness. Above the precipitation onset at a heptane/bitumen ratio of 2 1, the black film reached a thickness of about 28 nm. The film thickness then decreased with increasing diluent bitumen ratio to about lOnm at a ratio of 20 1. The thickness then remained constant, indicating that a bilayer film was probably reached. At lower diluent ratios (< 20 1), the greater thickness of heptane itumen films may be caused by the presence of unprecipitated asphaltenes. 

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