Black film spots

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

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. 

In spite of the overall resemblance between the results for the two copolymers, a marked difference in black film formation was observed during the experiments. In the thicker film of P85 small black spots emerged spontaneously and then grew rapidly over the whole... 

The results of the measurements equilibrium thickness of foam films from lyso PC as a function of NaCl concentration are shown in Fig. 3.49. At low electrolyte concentration thick equilibrium films that gradually decreased in thickness with increase in Cei were formed. When Cei exceed 10 3 mol dm 3, black spot formation occurred and spontaneous transition from silver to 7.6 nm thick black films was observed in some experiments. At 1.3-10 3 mol dm 3 NaCl predominantly black films were formed. 

As it can be seen from Fig. 3.49, the thickness where fluctuation appeared corresponds to the critical thickness of black spot formation [29,251], In the interval after Cei = 2-10 3 mol dm3 up to 0.5 mol dm3 only black films of constant thickness 7.6 nm were observed. 

The observed change at 2-1 O 2 mol dm 3 CaCl2 is accompanied by the appearance of black spots, leading to the formation of black films that decrease in thickness with the increase in Cei. This shows that CBF are obtained. Thus, a transition from silver to CBF is established. This process is usually observed in films stabilised with ionic surfactants [171]. Here it is possible to interpret the results as additional evidence for the increase in the diffuse electric layer potential as a consequence of Ca2+ ion binding. The next established transition is evidently from CBF to NBF. It occurs at a Cei close to the critical electrolyte concentration of transition to NBF observed for a typical ionic surfactant (NaDoS 1-1 valent electrolyte) [251] (see Section 3.4.1.3). The fact that NBF thickness at high Cei equals that of the NBF obtained... 

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... 

Black spot formation discussed here was carried out with foam films from soluble surfactants. The formation of foam films, especially of black films, from insoluble monolayers is also interesting. This will be considered in the next Section. 

As mentioned above, the appearance of black spots (black films) is observed in films from soluble surfactants. It is believed that the solubility of these substances is a necessary condition for formation of black foam films. That is why it is interesting to produce black films, especially NBF, from insoluble (or poorly soluble) surfactant monolayers. Bilayer lipid films formed in aqueous medium from insoluble in organic phase surfactants have been studied largely [e.g. 390]. 

The i(C) dependence for both types of black films has been studied in details with NaDoS films [332], presented in Fig. 3.81. Curves 1 and 2 begin at different surfactant concentration, corresponding to the formation of CBF and NBF. The arrows indicate Cm at which the respective black spots appear. 

The process of expansion of an emulsion film is also quite similar to that of black spots in a foam film at low electrolyte concentrations the spots in the emulsion film expand slowly, at high concentrations the process is very fast (within a second or less) and ends up with the formation of a black film with large contact angle with the bulk phase (meniscus). In the process of transformation of the black spots into a black film, the emulsion film is very sensitive to any external effects (vibrations, temperature variations, etc.) in contrast to the equilibrium black foam film. 

At low surfactant concentrations (for example, the concentrations that are reached at the waste water foam purification from surfactant pollutants) the foaming ability is usually limited by the minimum surfactant concentrations necessary for formation of stable foam films. This concentration is close to the concentration of black spot formation in microscopic films Cbi and black films Cfbu being an important characteristic of the foam stabilising ability of surfactants. The values of Cm for some surfactants are given in Table 3.1 and the dependence of Cm on various factors is considered in Section 3.4. 

The study of the VF IVg = f(C) dependence and the determination of cmin has been performed also for lysozyme solutions at different pH values and for NaDoBS solution with 0.1 mo dm 3 NaCl added [84,95]. Fig. 10.10 depicts the dependence of the probability for observation of black spots AN/N (curve 1) for films from lysozyme solutions with different concentrations at pH = 11.45 (isoelectric point). The techniques is described in Chapter 2. Izmailova and Yampolskaya [96] have investigated foam films from lysozyme solutions and found the concentration of black films formation c = 2.35-10"6 mol dm 3. As reported in [84], this concentration corresponds to a 100% probability for formation of stable black films. To the beginning of black spot formation in films from lysozyme solution with probability 5% corresponds the concentration of 1.82-1 O 6 mol dm 3. The expansion of black spots and formation of a black film occurs, as a rule, from the periphery to the centre, analogous to that for films from BSA solutions and its mixtures with lysozyme [97]. 

Fig. 11.12. Dependence of probability Ws for black spot formation on adsorption time at phospholipid concentration of (A) 65 pg cm 3, (B) 130 pg cm 3 and (C) 170 pg cm 3 black spot formation (W, = 1) by IN and SU required about 10 min at each concentration EX required adsorption times of about 40 min at the lowest concentration (A) and about 12 min at higher concentrations black films were formed only by IN and EX at the highest concentration below C, when adsorption times were increased to longer lhan 30 min for IN and longer than 40 min for EX (arrows in (C)) films of SU always ruptured t = 22°C.

FIGURE 7.1. The relative orientations of the reciprocal lattice of a crystal (expressed as a and b ), and its indexed X-ray diffraction pattern (expressed as h and k). In the diffraction pattern the intensities of the diffracted beams (/) (the blackness of spots on X-ray film, for example) and the directions of travel (sin 6) (positions of spots on the X-ray film) are measured. Note the relationship of a to h, and b to k. From the positions of spots on the photographic film it is possible to deduce the dimensions of the reciprocal lattice, hence of the crystal lattice, hence the indices hkl of each Bragg reflection. 

The critical thickness value at which the CF ruptures (due to thickness perturbations) fluctuates, and an average value may be defined. However, an alternative situation may occur as is reached and instead of mpturing a metastable film (high stabihty) may be formed with a thickness hobserved experimentally through the formation of islands of spots which appear black in light reflected from the surface consequently, this film is often referred to as first black or common black film. The surfactant concentration at which this first black film is produced may be one to two orders of magnitude lower than the cmc. 

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... 

Figure 15 Filrn-thimung process for bitumen in toluene (a) leads to a relatively thick gray film. For bitumen in heptane solution, thinning leads to formation of black spots, which eventually coalesce forming a thin black film (b).

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. 

Four categories of electrode models can be identified from Table 28.3 the spatially lumped model, the thin-film model, the agglomerate model, and the volume-averaged model. Schemes of the basic concepts of these four model categories are depicted in Figure 28.4. Each of the schemes shows an electrode pore, with the gas channels located at the top and the liquid electrolyte, depicted in gray, at the bottom. In some models, electrolyte is also present in the pore. The reaction zones are indicated by a black face (spot, line, or grid structure). Fluxes of mass and ions are indicated by arrows. 

The foam film c is formed in the middle of a biconcave drop b, situated in a glass tube of radius R, by withdrawing liquid from it (A and B) and in the hole of a porous plate g (C) (Figure 8.2). A suitable tube diameter in A and B is 0.2-0.6 mm and the film radius ranges from 100 to 500 nm. In C, the hole radius can considerably smaller, in the range of 120 pm and the film radius is 10 pm. When the film thins to form the so-called black film, black spots can be observed under the microscope. 

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