Bilayer black foam films

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. 

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 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. 

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]. 

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. 

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). 

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. 

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. 

Effect of temperature on the stability of foam bilayers. The effect of temperature on the rupture of foam bilayers has also been studied [414] with the help of microscopic NaDoS NB films with a radius of 250 pm. The dependence of the bilayer mean lifetime ton the surfactant concentration C in the presence of 0.5 mol dm 3 electrolyte (NaCI) at 10, 22 and 30°C has been obtained, the temperature being kept constant within 0.05°C. As in the above mentioned case, the NB foam films formed via black spots and the measurements were carried out after a sufficiently long time in order to allow equilibration of the system. At each of the NaDoS concentrations used and at the corresponding temperature, x was determined statistically and the comparison of the experimental with the theoretical x Q dependences was done by means of non-linear optimisation of the constants A, B and Ce. 

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... 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

The mechanism of the equilibrium elasticity acts until it is possible to provide a surfactant re-partition between the exterior and interior of the film. In a NBF such a repartition is not possible and this mechanism of elasticity ceases to act. The elasticity properties of bilayer films, in which the hydrodynamic and adsorption processes are characterised with normal time of relaxation, are due to Marangoni effect in the insoluble adsorption layers. That is why stable foams with black films are very sensitive to different local disturbances (heating, vibration, etc.). 

Fig. 11.4 shows separately curve 1 from Fig. 11.3 which is the dependence of W on the DPPC concentrations in the AF. The W(C) curves allow to determine the threshold concentration C i.e. the minimum phospholipid concentration at which there is a 100% probability of observation of black films (see Eq. (3.130)). At concentrations lower than C, NBFs are no more observed, since W sharply decreases to zero (films rupture). At concentrations higher than C, (W = 1), NBFs always form. Special studies with phospholipid analysis of amniotic fluid indicate that of all phospholipids in the AF, it is the DPPC that stabilises the foam bilayers. This analysis gives grounds to conclude that the concentration of each phospholipid (except DPPC) in the native AF is of an order lower than the corresponding... 

In some cases the interdrop film becomes very thin, i.e., only a few micellar or molecular sizes across. It decreases not continuously but stepwise, layer by layer, and could end up in a surfactant bilayer with no solvent content (sometimes referred to as black film because of its color). Such extremely thin films could exhibit a high resistance to rupture as it occurs in so-called foam emulsions (2.3.24). 

However, certain kinds of foams are known to persist for very long periods of time and many attempts have been made to explain their metastability. The TLF may be regarded as a kind of condenser. The repulsion between the two surfactant layers. Figure 1.17, will be determined by the EDL. The effect of added ions to the solution is to make the EDL contract, and this leads to thin films. It looks black-grey and the thickness is around 50 A (5 nm), which is almost the size of the bilayer structure of the detergent (i.e., twice the length [ca. 25 A] of a typical detergent molecule plus water). Actually, this is a remarkable fact that one can see two molecule... 

All three types of thin liquid films from both ABA and AB polymeric surfactants are stabilized by DLVO-forces at low electrolyte concentrations and by non-DLVO-forces at higher electrolyte concentrations. The latter are steric surface forces of the type brush-to-brush and loop-to-loop interactions (according to de Gennes). These steric forces act in 0/W emulsion films as well, but there transitions to Newton black films (NBF) have also been established. A difference between foam and O/W emulsion films has been observed. The barrier in the ri(h) isotherm for an emulsion film is much lower and the transition to NBF can occur. The NBFs from polymeric surfactants are very stable, as are the emulsions obtained from the same solutions. Actually, two types of bilayer emulsion films are obtained, those stabilized by brush-to-brush or loop-to-loop steric interactions and the others - by short-range interactions, also steric, in a two-dimensional ordered system. The minor difference in the experimentally measured thickness (about 2 nm) is not sufficient to characterize the state of these films. 

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