Black foam films

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 direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

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... 

Fig. 2.10. Scheme of the measuring cell for study of black foam film at a-particle irradiation 1 - film... 

The technique for forced destruction by a-particle irradiation has been developed also for foams under strictly defined conditions (pressure, water content, etc.) in order to establish a correlation with the rupture of single black foam films (see Section 7.2). 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

The measuring cell given in Fig. 2.17 is used by Yamanaka et al. [112] to study the lateral electrical conductivity of black foam films formed by lysophospholipids. 

Eugelsen and Frens [113] have combined the ellipsometry and refractometry for the study of NaDoS black foam films. Thus, experimental information concerning the structure and the anisotropy of surface layers in CBF and NBF was provided. 

Corkill et al. [56] have used for the first time the infrared spectroscopy for foam films. The measurement of the adsorption of the infrared light provides information about the water content in the foam films which is of major significance for the black foam films. These studies involved the use of dispersion type instruments. In order to obtain measurable values of adsorption, the infrared light is passed through a series of vertical films (up to 10) formed in a cylindrical tube acting as a frame. Additional information about the film structure the authors derived from the correlation between the optical infrared transmission data and the film reflectance measurements. Here a three-layer model of the film structure consisting of an aqueous core sandwiched between two adsorption layers is assumed (see Section 2.1.3). 

This method gained a significant improvement with the introduction of the contemporary infrared technique with a Fourier transformer (FT-IR), permitting to obtain measurable values of adsorption of the infrared light even from single black foam films. The thickness of the aqueous core is derived from the adsorption at 3400 cm 1 which is related to the OH stretching vibration of the water molecules. Umemura et al. [114] have employed the polarised Fourier transformed infrared spectra for the study of the water content of NaDoS black films. The cell used to form films of ca. 2 cm2 area is illustrated in Fig. 2.19. By fitting the calculated curved of polarised FT-IR spectra to the respective experimentally obtained... 

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. 

Details about the theory of stability of thin liquid films, including foam films, can be found in some monographs [3-6]. However, the literature reflecting the theory of black foam films is rather poor. For this reason it will be granted special attention here. The new theoretical and experimental results accumulated during the recent years have brought nearer... 

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 (Nano Foam Films)... 

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. 

Chapter 7 will treat the role of black foam films on foam stability and the stabilising ability of surfactants. 

In Section 3.3.1 it was shown that the state of thin foam films is described by the Fl(/ ) isotherm of disjoining pressure. For relatively thick films, stabilised by surfactants, this isotherm is consistent with the DLVO-theory. However, black foam films exhibit a diversion from the DLVO-theory which is expressed in the specific course of the disjoining pressure isotherm. 

Fig. 3.42 depicts a H(/j) isotherm (in arbitrary scale) of an aqueous film from a surfactant solution containing an electrolyte. The two states of black foam films are clearly distinguished. Such a presentation of the 11(6) isotherm can explain the thermodynamic state... 

This foam film with a smaller equilibrium thickness hi is called Newton black film (NBF). Its point of equilibrium is situated on the rising left hand side of the isotherm and, alike the preceding minimum, is not described by the DLVO-theory. In Section 3.3 it was shown that the departure from the DVLO-theory begins to be expressed in the experimentally obtained fl(/i) isotherms at film thickness below 20 nm [254]. There are many other experimental data on black foam films [e.g. 18,96,201,202,253,254] which also indicate a deviation of the 1T(/i) isotherm from DLVO-theory that cannot be explained even if the various corrections reflecting the theory refinements are accounted for [e.g. 148,166,171,172,221,255-259]. One of the divergences from the DVLO-theory is the discrepancy between experimental and theoretical data about the interaction energy in black films. 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

IT(/i) isotherms of black foam films from C o(EO)4 and NP20 are shown in Fig. 3.44. The surfactant and electrolyte concentrations are chosen so that equilibrium films within a large range of thicknesses are obtained, including the CBF/NBF transition region [172],... 

Fig. 3.57. Thickness dependence of disjoining pressure of black foam films from NaDoS solution (103...

The h(Cei) dependences (at pG = const and t = const) are studied also for other surfactants, phospholipids and polymers, and they have proved to be very informative with respect to not only the CBF/NBF transition and Cei,cr values, but also surface forces acting in black foam films (see Sections 3.3. and 3.4). 

Analogous results have been reported from the systematic measurements of electrical conductivity and transference numbers of ions (// and tf) in black foam films [336] and parallel measurements of these quantities in highly concentrated surfactant/water system [337], Furthermore, it has been found that while the electrical conductivity of CBF depends on the electrolyte concentration in the initial solution, that of NBF does not. The transference numbers of the ions measured for films and a gel obtained from NaDoS-NaCl-HCl system are given below... 

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

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]. 

Probability for observation of black foam films depending on the adsorption... 

Black foam films appear in thermodynamically non-equilibrium films in the form of black spots (see Section 3.2.2.2). These clearly expressed thin regions (but not holes as named in [e.g. 35,381]) expand, fuse and occupy the whole film area. Thus, CBF and NBF reach an equilibrium state. This process can be most distinctly observed in microscopic foam films (see Fig. 3.14). 

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. 

Rupture of foam bilayers by a-particle irradiation. By means of a-particle irradiation a controlled external influence can be exerted on the rupture of black foam films [331,415,416]. The measuring cell in which the studied microscopic foam bilayer is formed is shown in Fig. 2.10. The a-source is placed at a distance of 3.5 cm away from the bilayer the Bragg distance at which the particle energy is almost constant. The statistical character of bilayer rupture is evidenced in experiments at different irradiation rates [416]. The bilayer mean lifetime ra is therefore an appropriate parameter for assessing the destructive action of the a-particles. 

Linear energ y of the contact une black foam film/bulk uquid... 

Lateral diffusion in phospholipid black foam films... 

A very suitable method for measurement of the lateral diffusion of molecules adsorbed at the foam film surfaces is Fluorescence Recovery after Photobleaching (FRAP) ([491-496], see also Chapter 2). Measurements of the lateral diffusion in phospholipid microscopic foam films, including black foam films, are of particular interest as they provide an alternative model system for the study of molecular mobility in biological membranes in addition to phospholipid monolayers at the air/water interface, BLMs, single unilamellar vesicles, and multilamellar vesicles. 

Fig. 3.112. Temperature dependence of diffusion coefficient D of surface-adsorbed 5-N-(octadecanoyl)aminofluorescein in black foam films stabilised by DLPE ( ) DMPE (A) DPPE ( ) and DOPE (+) [492],...

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