Black foam films formation

Relationship between black foam film formation and the properties of the... 

In an interesting medical application, the formation of a stable black foam film from amniotic fluid can be used as an assessment of fetal lung maturity [206]. 

The measuring cell of Scheludko and Exerowa [e.g. 15-20] has proven to be a suitable and reliable tool for formation of microscopic horizontal foam films. It is presented in Fig. 2.2, variants A, B and C. The foam film c is formed in the middle of a biconcave drop b, situated in a glass tube a of radius R, by withdrawing liquid from it (variants A and B) and in the hole of porous plate g (variant C). Photographs of formation of black foam film via black spots taken under a microscope are presented in Fig. 2.3. 

Photographs of formation of black foam film via black spots... 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

Treatment of non-equilibrium properties of foam films requires consideration of the kinetics of expansion of a black spot in the grey film [102] as well as the formation of stratified black foam films (see Section 3.4). 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

Black foam films are one of the oldest objects of study (Boys, 1896 Rickenbacher, 1898, Johonnott, 1906, Perrin, 1916) because of their evident simplicity, easy formation, homogeneous surface which is not typical for solid surfaces, etc. 

The most suitable technique ensuring the formation of black films is the one that operates with horizontal microscopic films. It allows to work with the lowest possible surfactant concentration and to study in detail the very interesting stage of appearance of black films, including of foam bilayers (NBF). The microscopic foam films provide information about formation and stability of black foam films. On the other hand, as it will be demostrated, the microscopic film is a suitable model to measure several quantitative parameters characterising black film behaviour. 

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. 

Fig. 3.77 depicts Ao(C) isotherms of NaDoS solution containing an electrolyte at a level chosen to ensure formation of particular types of black foam films. The curves are drawn according to the regressive spline analysis [364]. 

Formation of black foam films from an insoluble surfactant monola yer... 

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]. 

Data on emulsion film formation from insoluble surfactant monolayer are rather poor. It is known, however, that such films can be obtained when a bubble is blown at the surface of insoluble monolayers on an aqueous substrate [391,392]. Richter, Platikanov and Kretzschmar [393] have developed a technique for formation of black foam films which involves blowing a bubble at the interface of controlled monolayer (see Chapter 2). Experiments performed with monolayers from DL-Py-dipalmitoyl-lecithin on 510 3 mol dm 3 NaCl aqueous solution at 22°C gave two important results. Firstly, it was established that foam films, including black films, with a sufficiently long lifetime, formed only when the monolayer of lecithin molecules had penetrated into the bubble surface as well, i.e. there are monolayers at both film surfaces on the contrary a monolayer, however dense, formed only at one of the film surfaces could not stabilize it alone and the film ruptured at the instant of its formation. Secondly, relatively stable black films formed at rather high surface pressures of the monolayer at area less than 53A2 per molecule, i.e. the monolayer should be close-packed, which corresponds to the situation in black films stabilized with soluble surfactants. 

In the references mentioned CWy corresponds to the critical concentration Cc for formation of microscopic black foam films (see Section 3.4) but was not determined from the W(C) dependence. 

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. 

The results on formation and stability of black foam films, on the first place those on bilayer foam films (NBF) (see Sections 3.4.1.2 and 3.4.4) have promoted the development of methods which enable lung maturity evaluation. The research on stability of amphiphile bilayers and probability for their observation in the grey foam films laid the grounds of the method for assessment of foetal lung maturity created by Exerowa et al. [20,24]. Cordova et al. [25] named it Exerowa Black Film Method. It involves formation of films from amniotic fluid to which 47% ethanol and 7-10 2 mol dm 3 NaCl are added [20,24]. In the presence of alcohol the surface tension of the solution is 29 mN m 1 and the adsorption of proteins from the amniotic fluid at the solution/air interface is suppressed, while that of phospholipids predominates. On introducing alcohol, the CMC increases [26], so that the phospholipids are present also as monomers in the solution. The electrolyte reduces the electrostatic disjoining pressure thus providing formation of black foam lipid films (see Sections 3.4.1.2 and 3.4.4). 

The linear dependence between the threshold dilution and the initial total phospholipid concentration (respectively, DPPC) found allows to determine the threshold dilution for a 100% probability for formation of NBF instead of Ct. Fig. 11.5 shows that if a sample dilution of 3.1 times is applied, then it is possible to detect almost all cases with a developed RDS. Therefore, the threshold dilution of 3.1 times allows to distinguish the mature from immature AF samples which gives a good reason to employ it in diagnosing of RDS, and respectively, to estimate the lung surfactant deficiency. Hence, the formation of black foam films from AF samples taken at different gestation weeks and diluted 3.1 times, indicates that there is no risk of RDS, while film rupture predicts an eventual RDS development. 

Let us summarise the conditions of formation of a microscopic foam film in order to serve the in vivo situation. These are film radius r from 100 to 400 pm capillary pressure pa = 0.3 - 2.5-102 Pa electrolyte (NaCl) concentration Ce 0.1 mol dm 3, ensuring formation of black films (see Section 3.4) and close to the physiological electrolyte concentration sufficient time for surfactant adsorption at both film surfaces. Under such conditions it is possible also to study the suitable dependences for foam films and to use parameters related to formation and stability of black foam films, including bilayer films (see Section 3.4.4). For example, the threshold concentration C, is a very important parameter to characterise stability and is based on the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa. As discussed in Section 3.4.4, the main reason for the stability of amphiphile bilayers are the short-range interactions between the first neighbour molecules in lateral and normal direction with respect to the film plane. The binding energy Q of a lipid molecule in the foam bilayer has been estimated in Section 11.2. 

The thinnest-black films have been found to play a particularly important role in the formation of highly stable foams. They are used as models in the study of surface phenomena at various interfaces, molecular interactions between two contacting phases at short distances, including at bilayer contact. This fact in itself is of the utmost importance in studying the formation and stability of concentrated disperse systems and in modelling the contact between the two biomembranes. For this reason the book discusses different aspects of black foam films and some intriguing perspectives for future development, for instance, as a self-organising nanomolecular system, have been pointed out. 

In order to determine the infants lung maturity and the necessity of surfactant therapy it is of great importance to substantiate the functionality of the alveolar surfactant, derived via invasive techniques [13], Several techniques and models have been largely used to investigate inteifacial physicochemical properties in vitro and to assess clinical efficiency of ES in vivo the Langmuir monolayer technique in combination with Wilhelmy plate method for surface tension measurements and black foam film method for determination of the ability of ES for stable film formation [14]. The pendant drop method combined with the Axisym-metric Drop Shape Analysis (ADSA) has been also used for similar purposes [4,15-18]. 

If the short-range repulsive disjoining pressure is large enough, the black foam films are stable. There are two types of black foam films common and Newtonian. While the common black films are the thicker type of black films (from about 5 to 20 nm in thickness), the Newtonian black (NB) films are bimolecular thin films (less than 5 mn in thickness). A mechanism of rupture of NB films is considered as a process of new phase nucleation in a two-dimensional system [105 108]. There exist in the film elementary vacancies (unoccupied positions of surfactant molecules) moving randomly, which associate to form clusters of vacancies called holes. A hole can grow up by fluctuations to a critical size and become a nucleus of a hypothetical two-dimensional phase of vacancies. Further spontaneous growth of the nucleus leads irreversibly to the rupture of the film. When the rupture of NB film is due to formation of holes in it by a nucleation mechanism, it has been shown that the mean film lifetime r depends on the monomer surfactant concentration C as ... 

Exerowa and co-workers [201] suggest that surfactant association initiates black film formation the growth of a black film is discussed theoretically by de Gennes [202]. A characteristic of thin films important for foam stability, their permeability to gas, has been studied in some depth by Platikanov and co-workers [203, 204]. A review of the stability and permeability of amphiphile films is available [205]. 

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... 

Another approach to explain foam film rupture has been developed by de Vries [101] who proposed to consider film rupture as a result of fluctuational formation of holes (black spots) in it - nuclei of critical size (see Section 3.4). This idea was used in the analysis of the... 

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. 

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. 

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. 

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],... 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

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]. 

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