Black spot formation concentration

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. 

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 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.

In our work we also compared stabilities of model emulsions with those of thin hydrocarbon films. Soap concentrations corresponding to black spot formation did not lead to stable emulsions. To obtain them, concentrations much higher than those at which stable films are formed are necessary. This is due to the fact that during emulsification a sharp increase of the interfacial adsorption surface occurs, which results in considerably lower concentration of surfactant in solution. 

The transition from a disjoining pressure isotherm dominated by van der Waals forces, such as that shown in Eigure 1.10, to one with at least a region where d/i < 0 is present can be observed at a certain surfactant concentration. That is of course represented by a transition from rupture to black spot formation. The... 

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 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... 

For the case of film thickness measurements in the presence of CaCl2 (po and diffuse electric layer planes due to Ca2+ ion binding. The calculation in the electrolyte concentration range 10 3 - 5-10 2 mol dm3 indicates an increase in transition concentration from 10 3 to 2-1 O 3 mol dm 3 CaCl2, the potential is comparatively low (17 mV) and remains practically constant. Further on, it increases to reach values that are usually found for films from ionic surfactant solutions [e.g. 171,186,189] (see also Section 3.4.1.3). So, the formation of CBF through initial formation of common black spots can be interpreted as due to the specific interaction of Ca2+ ions with lyso PC. 

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... 

At higher electrolyte concentrations in the NaDoS solution, e.g. 0.35 mol dm-3 (curve 2, Fig. 3.77), formation of black spots is observed at higher surfactant concentrations which correspond to closer packing of the adsorption layer. Probably with the increase in electrolyte concentration the stabilizing ability of the electrostatic component of disjoining pressure decreases. 

The relation between the concentration Cbi of formation of black spots (see also Section 3.2.2.2) and the flexion in the Ao(C) isotherm has been found in 1964 by Scheludko and Exerowa [e.g. 73,95], They determined Cw as the minimum bulk concentration at which black sports were formed and did not employ the probability curve W(C). However, the values thus obtained (Table 3.1) are very close to the beginning of the W(Q dependences which are very steep in this concentration range. The early studies did not distinguish between Cbi for CBF and for NBF. The values measured were mainly for CBF (Cei = 0.1 mol dm 3 1-1 valent electrolyte for anionic surfactants). As already mentioned, this distinction is very important... 

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. 

Temperature dependence of the critical concentration Ce of a foam bilayer formation. The Cc concentration (see Eq. (3.129)) of formation of DMPC foam bilayer was determined on the basis of observations of the final state which the foam film reached during its drainage (see Section 3.2), i.e. either rupture at a definite critical thickness without formation of black spots occurs, or formation of foam bilayer via black spots is observed. Rupture at critical thickness occurred at lower DMPC concentrations in the solution (C < Cc) and black spots were formed at higher concentrations (C > Cc). These black spots encountered the film turning it into a foam bilayer of constant radius. At each temperature a series of observations were carried out at various DMPC concentrations for the determination of Cc (the minimum DMPC concentration at which a foam bilayer is formed). This concentration is... 

By many properties emulsion aqueous films are analogous to foam films. There are several review articles dedicated to properties of emulsion aqueous films [e.g. 320,503-506]. The properties of microscopic emulsion aqueous films (kinetics of thinning, determination of equilibrium thickness, etc.) are studied employing devices quite similar to those used for foam films [503]. Analogous to foam films, stable (metastable) emulsion films are formed only in the presence of surfactants (emulsifiers) at concentrations higher than the critical concentration of formation of black spots C or the concentration, corresponding to... 

Comparison of the concentrations corresponding to formation of black spots for emulsion and foam films, obtained from solutions of the same surfactants, indicate that Cbi for foam films are considerably lower than Cbi.f for emulsion films. This means that stable foam films (usually black) form at lower surfactant concentrations than emulsion films even from apolar organic phase. With the increase in the polarity of the molecules of the organic phase Cbi.f for emulsion aqueous films increases [507] which is analogous to the increase in Cbi for hydrocarbon emulsion films [509],... 

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. 

Such a relationship between Cm and the state of the adsorption layer is also found for CBFs (see Chapter 3). However, no quantitative link between Cm and the stability of these films has been found. As far as in the formation of black spots the adsorption layers at film surface plays a significant role, the clarification of the decrease in surface tension Act with the surfactant concentration is also important. Being a characteristic of surfactants Cm is in agreement with the commonly used quantity Act but is also related to the film properties. It is also important that Cm is in correlation with foam stability and, thus, is a more precise, suitable and physically better grounded characteristic. Such a correlation has been found for aqueous emulsion films [69,70]. 

Eqs. (10.45) and (10.46) are of little use for determining the limiting values of the purification coefficient, since at low surfactant concentrations foam stability dramatically decreases, and the foam layers containing the adsorbed product have no time to separate from the solution. For this reason determination of the minimum residual surfactant concentration, necessary to obtain a stable foam provides precise information on the purification coefficient. The minimum residual concentration is closed to that for the formation of black spots (cbi) in microscopic foam films, in case they form (cLR = cs,r = cbi) [24]... 

Thus it is possible to estimate the time for surfactant adsorption required for the formation of black spots. Table 11.2 presents the clinical and threshold concentrations for total phospholipids (PL) and for disaturated phosphatidylcholine (DSPC) in each preparation. The most abundant PL of the lung surfactant system is DSPC, principally the DPPC species, which is believed the essential determinant of surfactant function in vivo [2], While DPPC is the only PL in EX, both IN and SU contain other PLs and small quantities of hydrophobic surfactant-associated proteins that may add to the desired functional properties of the material in situ. 

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