Black spot formation

Figure 13-16. Fvoluiion of black spot formation in air. ill an ITO/TI D/Ak i/Mg/Ag device. The size of the emissive area is 2 minx 1.6 nun. Reproduced with permission Irom I76. ...

Many countries do not allow use of S02 or sulfite salts on meats, fish, piocessed meat and fish products, 01 flesh fruits and vegetables. Where permitted, sulfite is helpful in eliminating black spot formation in shrimp. End-use application of sulfite related compounds as. for example, to prevent discoloration of leafy vegetables on salad bars, is now subject to regulation in the United States. 

Critical thickness of rupture and black spot formation in microscopic foam... 

Thus, the most probable value of hcr (hcr,bi) is determined with an accuracy of ca. 0.5 nm. The differently drawn experimental points in Fig. 3.13 reflect the smooth transition form the region of rupture to the region of black spot formation as well as the independence of hLr on the final state - rupture or black spot formation. 

Film rupture at hcr or black spot formation depends on the surfactant kind and concentration in the solution. Fig. 3.14 shows photographs of black spots at different stages of their formation. 

On that basis Exerowa and Scheludko [95] have introduced a new parameter bulk concentration Cm at which black spots begin to form in the microscopic foam film. It is also called concentration of black spot formation and has been studied in various aspects [e.g. 54,73,89,96-100]. This concentration is a very important quantitative characteristics of the surfactants. Its determination is done by observing microscopic films under a microscope in... 

Surfactant concentration in solution corresponding to black spot formation in a microscopic film... 

It is worth noting that the onset of a constant value of <(>o- and AV-potentials corresponds to the concentration Cm of black spot formation (see Section 3.2.2.2) which means that this is related to a definite saturation of the adsorption layer at the film surface (see Section 3.4.3). 

With the increase in surfactant concentration pH first decreases, then reaches a constant value, equal to 3.4. The curve pH (C) for the both surfactants studied reaches a plateau at different concentrations, corresponding to the concentration Cbi of black spot formation (see Section 3.4.3). This concentration for non-ionic surfactants is close to that necessary to saturate the adsorption layer. At C > Cbt the isoelectric points found for 6-... 

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. 

Theoretical analysis of sheeting in the drainage of thin liquid films has been conducted in [359]. Sheeting dynamics and hole formation (i.e. black spot formation) was described by non-linear hydrodynamic stability analysis based on the equilibrium oscillatory structural component of disjoining pressure. The effect of stepwise thinning, accompanied by formation of holes , was described qualitatively. It is rather arguable whether the term holes for a black spot is appropriate since in 1980 holes in NBF were described as lack of molecules. The use the same term for two different formations is at least confusing. Besides, to have a hole in a CBF is almost as to have a hole in the sea water . 

The process of black spot formation is studied by determining the probability IV for their observation in the thicker grey film. It is clear that black spot formation can be experimentally observed if its lifetime is longer than the reaction time of the observer or the resolving power of the device. 

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. 

The concentration of formation of black spots in emulsion films is close to the emulsifier concentration at which it is possible to disperse a small quantity of the organic phase in certain volume of the aqueous surfactant solution under definite conditions resulting in formation of stable emulsions. Kruglyakov et. al. [510] have compared the concentration of black spot formation in emulsion aqueous films and the minimum surfactant concentration Cmin needed to form stable heptane aqueous emulsion studying the NaDoS emulsifying ability vs. its concentration in the solution. They found that Cmin = 4.110 4 mol dm 3 in a solution containing 51 O 2 mol dm 3 NaCl and Cw = 3.5-4-10 4 mol dm 3, depending on the time of film formation. 

The study of a large number of various surfactants in aqueous and non-aqueous media has shown that a sharp transition towards films of high stability at increasing surfactant concentrations is always related to the appearance of black spots in the microscopic films [17,42,43]. It was established that the surfactant concentration corresponding to black spot formation lies in the range of sharp increase in the dependence foam lifetime x on surfactant concentration C. 

The results about spreading of antifoams as well as the inhibition of black spot formation in microscopic foam films, permit to draw a conclusion about the existence of a special mechanism of heterogeneous defoaming, based on the stability analysis [14,25,48]. The details of this mechanism are given later [18,55,56]. For the defoaming action of hydrophobic solid particles in heterogeneous system another (bridge) mechanism has been proposed [57]. Later it was applied on drops, mixtures of hydrocarbon oil and hydrophobic particles [19,20,53,54]. 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

The possibility to use Cm as a parameter characterising foam inhibition has been demonstrated for the first time in [60]. It was shown that the increase in the concentration of silicon oil Caf (antifoam) led to increase in Cm- That is why it was proposed to used the ratio Caf/Cm as a quantitative measure of the defoaming ability. However, it should be noted that the silicon oil concentrations at which inhibition of black spot formation was observed, were very low (10 5-10 9 %). For that reason it is difficult to conclude definitely whether the system was a real solution or represented a diluted emulsion of the antifoam in the surfactant solution. 

More convenient objects for the study of Cm(Caf) dependence have been found to be the low molecular compounds with relatively good solubility (compared to the silicon oil), for example, fatty alcohols, acids and hydrocarbons [48,55]. The experiments commented below were performed with these antifoams. First of all the Cm of films in the absence of an antifoam was determined by gradually increasing the surfactant concentration. Then small doses of the antifoam were introduced in a solution with surfactant concentration slightly above Cm, until the formation of black spots became impossible. At the same time the concentration of antifoam saturation in the solution was fixed. Table 9.2 presents the antifoam concentrations at which black spot formation in microscopic foam films is inhibited. 

Minimal antifoam concentration (%) for inhibition of black spot formation in foam films... 

The results from [48,55] indicate that the alcohol or acid molecules, entering the adsorption layer from the film bulk, are not sufficient to inhibit the black spot formation, and this is valid for all antifoam concentrations, even at saturation. To achieve a complete... 

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 dramatic increase in the foam lifetime is typical for the concentration regions corresponding to black spot formation in the microscopic foam films (see Section 3.2.2.2). 

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]. 

The lowest residual concentration of lysozyme found by extrapolation of the linear segment of the VFIVg = f(C) dependence gives the value of 2.1-10 6 mol dm 3 which is close to the concentration of the onset of black spot formation (Fig. 10.10, curve 2). At pH values less than the isoelectric point that corresponds to the formation of thick films from solutions of such surfactants [96], the VF/Vg = f(C) dependence has an S-shaped course. 

The studies performed reveal that the lower concentration limit of surfactant extraction using the foam separation technique is determined by the course of the surfactant stabilising ability versus surfactant concentration curves (lifetime dependences of films and foams, and probability for black spot formation on surfactant concentration). If there is a jump-like increase in the film (foam) lifetime with concentration, then VF IVg = f(C) and the accumulation ratio also undergoes a jump-like increase VF/Vg - from 0 to 1, and - from 1 to more than 1, corresponding to the lower concentration limit. 

Foam films and a foam from the aqueous and organic phases of an extraction system containing a 30% solution of tri-buthyl phosphate (TBP) in kerosene and nitric acid (1 mol dm 3) have been studied in a parallel mode [137]. The reasons for foaming and the effect of emulsion formation on foam stability were elucidated. Thus, a foam with a measurable lifetime was obtained when TBP was in concentration of 0.8 mol dm 3 which corresponded to the concentration of black spot formation. When the volume ratio of the organic to the aqueous phase was 1 5, the foam formed in the system was stabilised additionally by a highly disperse O/W emulsion. This was due to the reduced rate of drainage. These results are confirmed by the experimental data acquired with a specially constructed centrifugal extractor [136]. It makes it possible to perform an extraction process under conditions close to those in industry. 

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.

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