Foam bilayers stability

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. 

The foam bilayer is the main model system used to obtain experimental results for the stability of bilayers. The proof that the studied foam films are bilayers is based on the experimentally measured h(Cei) dependences and I"I(/i) isotherms. In both cases films with the same thickness are obtained, which corresponds to that of bilayers and does not change with further increase in Cei or IT (e.g. Figs. 3.44, 3.57, 3.62). This leads to the conclusion that the NB foam films do not contain a free aqueous core between its two monolayer of surfactant molecules. A similar conclusion is drawn from the investigatigations of NB foam films by infrared spectra [320,417] and by measuring longitudinal electric conductivity of CB and NB foam films [328,333,418]. 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

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. 

Adsorption isotherm of surfactant vacancies in foam bilayers. As discussed above, the investigation of the stability of foam bilayers at different temperatures allow determination of the binding energy Q of a surfactant molecule in the bilayer. At the highest temperatures of 30°C the Q value for a NaDoS molecule in the foam bilayer (Q 6kT) is high enough to ensure the occurrence of 2D first-order phase transition in the bilayer. Theoretically Q > 8kT is known to be the condition for such a transition in the most frequently encountered 2D lattices [423],... 

Since t(Q data for the emulsion bilayers are rather scattered, only Ce and % could be estimated Ce = 0.5 - 3-103 mol dm 3, % = 610"12 J m1. From the comparison of the t(C) dependences for the foam and emulsion bilayers in Fig. 3.92 it is seen that the stability of the foam bilayers is greater than that of the emulsion bilayers and that Ce is much lower for the... 

It seems quite possible to use some theoretical parameters, e.g. Cc or C for detecting transitions between the different phase states of the foam bilayers. Comparing these transitions with the corresponding phase transitions in bulk surfactant solutions would allow a deeper insight into the molecular interactions in biostructures and into the role of the surface forces in biomembrane formation, stability and fusion. Foam bilayers could also be used as a model for the investigation of reverse micelles and enzyme-substrate interactions which are top problems in biology. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

The values of Q obtained from the best fit of Eq. (3.115) (the solid lines in Fig. 3.95) to the experimental data (the circles in Fig. 3.95) assuming Cc = Ce are (1.93 0.04)-10 9 J for temperatures lower than 23°C and (8.03 0.19)-10 2° J for temperatures higher than 23°C. The possible error arising from the assumption that Cc = Ce is analysed elsewhere [384] it can raise the Q value by up to 20%. The good fit of the experimental results to the theoretical dependence and the high stability of the foam bilayers with respect to their rupture even under a-particle irradiation, show that in the case of DMPC bilayers the assumption Cc = Ce is probably accurate. 

Muller et. al. [421] have studied the behaviour of emulsion Newton bilayer films and compared it to that of foam films. They determined the dependence of the lifetime on surfactant concentration of emulsion films stabilised with 22-oxythylated dodecyl alcohol (see Section 3.4.1). Experimental data for both kinds of films proved to be in conformity with the theory of bilayer stability (see Section 3.4). The values of the equilibrium concentrations Ce calculated for emulsion films were higher (Ce 10 3 mol dm 3) than those for foam films (Ce 3 1 O 5 mol dm 3). It is worth noting that Ce value of foam films from certain surfactants is lower than CMC (C < CMC) while for emulsion films - Ce > CMC. That is why it is impossible to obtain thermodynamically stable films in the latter case. This result is of particular importance for the estimation of stability of aqueous emulsions with bilayer films between the drops of the organic liquid. 

The microscopic foam bilayer proved to be an appropriate model for investigation of alveolar surface and alveolar stability as well [21]. This approach is in agreement with the findings of Scarpelli that at birth the lung surfactant takes the form of intraalveolar bubbles with formation of foam films [e.g. 2,22,23],... 

The systematic study of foam bilayers from phospholipids [28,38-40] reveals that they do not rupture spontaneously at any concentration allowing their formation. That is why in the case of phospholipid foam bilayer the dependence of their mean lifetime on the bulk amphiphile concentration cannot be measured in contrast to foam bilayer from common surfactants [41,42], This infinite stability of phospholipid foam bilayers is the cause for the steep W(d) and W(C) dependences. In the case of AF foam bilayers this high stability was confirmed by a very sensitive method [19,43] consisting of a-particle irradiation of foam bilayers. As discussed in Sections 2.1.6 and 3.4.2.2, the a-particle irradiation substantially shortens the mean lifetime of foam bilayers. The experiments showed that at all temperatures and dilutions studied (even at d,), the foam bilayers from AF did not rupture even at the highest intensity of irradiation applied, 700 (iCi. 

This extreme stability of AF foam bilayers allowed to assume that C, = Ce and to use the data for the temperature dependence of threshold dilution (shown in Fig. 11.7) for determination of Q (binding energy) for each sample of amniotic fluid. For this reason Eq. (11.2) resulting from Eq. (11.1) and Eq. (3.115) can be used... 

The above mentioned threshold dilution d, and critical concentration for formation of a bilayer are used as measures for bilayer stability [19] being determined by the first neighbour lateral and normal interactions in the foam bilayer. This is the difference of the parameter d, from the change in the free surface energy which is usually used as a measure of the surface activity. Thus, the parameters d, and C, are proposed as new characteristics of the surface activity of an amphiphile molecule, evaluated with high accuracy from the sharp W(d) and W(C) dependences, respectively. 

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. 

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. 

A premiss for such quantification is the theory of a foam-bilayer lifetime (43). The main notions of this theory are similar to the theory of Deijaguin and coworkers (44, 45). However, the theory (43) is specified for amphiphile foam films, it is elaborated in detail, and is proven by experiment with water-soluble amphiphiles, such as sodium dodecyl sulfate (47). As the dependence of the mpture of the emulsion film on surfactant concentration is similar to that for a foam film, the modification of theory with respect to emulsions may be possible. Although this modification is desirable the specification of a theory for a given surfactant will not be trivial, since the parameters in the equation for the lifetime (45) are unknown and their determination is not easy. As the theory (43, 47) is proposed for amphiphiles and since a wider class of chemical compounds can stabilize, emulsions, the fihn-mpture mechanism (44) is not universal regarding emulsions. 

Regarding the drainage of a foam film stabilized by sole whole casein, this has been discussed in the previous section. The thickness of the whole casein film diminishes in a continuous manner until reaching a finite final thickness that corresponds to a bilayer of caseins with somehow screened electrostatic repulsion between layers and a higher intermolecular interaction between the different monomers composing the whole casein (Maldonado-Valderrama and Langevin, 2008). 

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

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 already noted, the NB foam films, the bilayer emulsion films and the BLMs, are amphiphile bilayers, and their stability in respect to rupture and their permeability can be considered from a unified point of view. 

In conclusion, let us outline some more important aspects of the hole-nucleation theory for stability of amphiphile bilayers of Kashchiev-Exerowa and its experimental support. The outlined theoretical and experimental investigations of the stability and permeability of foam, emulsion and membrane bilayers represent a new approach towards... 

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). 

As it was shown the stability of bilayer foam films (NBFs) and, respectively, the probability W for their observation of the thicker (grey) films depends considerably on the concentration of the surface active molecules (see Sections 3.4.3.2 and 3.4.4.3). Fig. 11.3 plots such dependences for various individual phospholipids such as phosphatidylglycerol, egg lecithin, DPPC, phosphatidylinosytol and their mixture (amniotic fluid). 

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. 

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