Black films, states

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... 

III.B. The Role of Thermal Fluctuations on the Transition from Common Black Films to Newton Black Films. The method described in the previous section will be now applied to thin films with fluctuating interfaces, with the interaction energy calculated as in section II.G. For low values ofthe external pressure, the enthalpy has two metastable minima at Zk and 2c, and a stable one at 2 - 0 (the former two correspond to the Newton and to the common black films, respectively, and the latter implies the rupture of the film), separated by two maxima located at Z and 22 (see Figure 7a). At metastable equilibrium the distances between the surfaces are distributed between 21 and 22 for the Newton black film and between z2 and 2 —°° for the common black film. The stability of the metastable states depends on the chance for a small area S of the interface to reach the... 

Both minima of the enthalpy, while metastable, might have a relatively long lifetime, and a system prepared in one state (common black film or Newton black film) might remain in that state during the time of the experiment, if the potential barriers are sufficiently high. However, because of the thermal fluctuations, it is possible to have, for the same experimental conditions, a transition in a range of pressures, and this explains one of the experimental results of Exerowa et al.2... 

In what follows, it will be considered that the lifetime of a metastable state exceeds the duration of experiment if the potential barrier for a small surface of area S, SAH, exceeds 3kT. For the first minimum in Figure 7a, the potential barrier is higher than 3kT for Kc > 20 x 10 19 J hence the Newton black films, once formed, remain stable in cases 4 and 5 during the experiment. In contrast, if the interfaces are more flexible (low Kc, cases 1,2, and 3), the probability for an individual small surface to reach the height which separates two minima is much higher. In this case, the metastable equilibrium of the Newton black film has a shorter lifetime while the film can either rupture or have a transition to a common black film, the second process has a higher chance. On the other hand, the common black films are more stable, because, while the minimum of the enthalpy is higher, the interfaces have more room to fluctuate. 

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... 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

Formation and stability studies of black foam films can be summarised as follows 1) surface forces in black foam films direct measurement of disjoining pressure isotherm DLVO- and non-DLVO-forces 2) thin foam film/black foam film transition establishing the conditions for the stability of both types of black films and CBF/NBF transition 3) formation of black foam films in relation to the state of the adsorption layers at the solution/air interface 4) stability of bilayer films (NBF) theory and experimental data. 

Quantitative studies performed by Bulgarian and Dutch scientists [e.g. 14,95,159,160] in the period of 1962 - 1964 proved that two different equilibrium states of black films exist which are realised under certain conditions, i.e. capillary pressure, electrolyte and surfactant concentration, film radius, etc. Studies with macroscopic film [e.g. 308] under a variety of conditions confirmed that fact. 

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 . 

Metastable states have been found not only for NaDoS black films. When the transition is characterised by a sharp jump in the d(Cej) dependence metastable films always form, i.e. their existence is a function of the course of the I I(/t) isotherm. 

The comparison of W(C) dependence with Ao(C) isotherm gives a relation between formation of black spots and films, and the adsorption layer state. It has been shown [332] that the W(Q dependences for black spot and black films of a very small radius (25 pm) coincide. The comparison of the W(C) curve of CBF from NaDoS (see Fig. 3.78) with the surface tension isotherm of the same surfactant (see Fig. 3.77) indicates that black spots begin to form when the state of adsorption layers deviates from the ideal one (Henry s region in Aa(Q isotherm). The probability for observation of a black film steeply increases with the increase in surfactant concentration to about 10 5 mol dm 3 where the adsorption layer saturation is... 

The beginning of the curves, indicating black film formation, is probably related to the state of the adsorption layers at the surfactant solution/air interface. However, the infinite stability of black films is not a function only of the adsorption layers. To find the reasons for... 

At large surfactant concentrations emulsion films as well as foam films exhibit a layer-by-layer thinning (stratification) and metastable black films are formed [31,347,512], Such a behaviour has been reported for hydrocarbon films obtained from solutions of lecithin in either benzene or a mixture of chloroform and decane at concentration higher than 0.6-0.8% as well as in films from oxidised cholesterol in decane [31,512]. Manev et. al. [347] have reported stratification of O/W type emulsion films, toluene being added as a disperse phase, occurring within a surfactant (NaDoS) concentration range of 0.017-0.14 mol dm 3. The number of metastable states was 5-6. Compared to foam films of analogous composition, the respective emulsion films were thicker, due to the weaker intermolecular attraction and the stratification occurred at lower surfactant concentrations. 

FIGURE 5.36 Main stages of formation and evolution of a thin liquid film between two bubbles or drops (a) mutual approach of slightly deformed surfaces (b) at a given separation, the curvature at the center inverts its sign and a dimple arises (c) the dimple disappears, and eventually an almost plane-parallel film forms (d) due to thermal fluctuations or other disturbances the film either ruptures or transforms into a thinner Newton black film (e), which expands until reaching the final equilibrium state (f). 

The films formed from potassium thiocyanate solutions behaved differently from those formed from sodium chloride solutions. At the lowest salt concentration examined, 4 X 10"4 mole/dm3, the film had a thickness of 975 A, clearly a first black film. With increasing salt concentration the film thickness decreased and reached 60 A at a thiocyanate concentration of 5 X 10 1 mole/dm3 it remained at this thickness as the salt concentration was increased to 1 mole/dm3. This corresponded to the second black film thickness obtained with the films in sodium chloride solutions, and in this state the films were quite stable. The considerably thicker films formed in the presence of the thiocyanate ion indicate a stronger double layer repulsion effect than with chloride and hence a stronger adsorption of the thiocyanate ion to the film surface. These re-... 

At high electrolyte concentrations the films become so thin that they loose ability to reflect light there are the so-called common black films. In addition to that, an increase in electrolyte concentration results in a decrease of the height of potential barrier which preserves the film in the state of this metastable equilibrium, i.e., film stability decreases. Thermal oscillations of interface, i.e., the Mandel shtam waves (See Chapter VI, 1), help the system to overcome a potential barrier. If other stabilizing factors are absent, such (local) overcoming of potential barrier results in film rupture. 

In the case of films with high stability the overcoming of potential barrier does not result in a rupture of film, but leads to another metastable state corresponding to the primary minimum (Fig. VII-10, point B). This results in the formation of a rather stable, very thin Newtonian black films [15]. The investigation of the nature of stability of black films is one of the central problems in colloid science nevertheless, at present there is no commonly accepted opinion concerning the nature of forces that are responsible for high stability of black films (see Chapter VIII). 

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 last effect to be described here is film elasticity In case of ionic surfactants the aqueous phase in the double layers contain dissolved counter ions of the surfactants. When the ionic density increases, the repulsive forces of equally charged ions become substantial, see Fig. 11. The repulsive forces are also responsible for a certain elasticity of double layers. The thickness of double layers in the well-known coloured air bubbles lies between 1,000 and 10,000 A. It can be determined by the order of interferential colours The process is very dynamic and fluctuates over the surface area. Under certain conditions the drainage reaches an end at a metastable state (so called black films ) giving the lamella or bubble a limited time of existence ... 

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