Foam bilayers

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. 

The molecular model of amphiphile bilayers with holes in them is a good basis also for the description of the rupture of NB foam films by a-particle irradiation [331,415,416]. The mean lifetime ra of the foam bilayer shortens dramatically under irradiation, and probability considerations [416] show that only a small area Sh S of the bilayer is active for the passage of the a-particles. Assuming that 5 is the overall area of those holes in the foam bilayer which are large enough to be irradiation-active makes it possible to represent Sh as... 

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]. 

Thus, the foam bilayer can indeed be regarded as a system of two amphiphile monolayers adsorbed onto each other. In view of the strong effect of the concentration C of surfactant in the solution on the bilayer lifetime T, it is very convenient to use the t(C) dependence for experimental verification of the theory [399,402,403] of hole-mediated rupture of bilayers. 

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. 

The z(C) dependence has been investigated with the help of microscopic foam bilayers of both ionics and nonionics [419,420]. Due to the fluctuation character of the film rupture, the film lifetime is a random parameter. Experimentally, the film mean lifetime r has been determined by averaging from a great number of measurements. Because of the assumption that the monomer and the total surfactant concentrations are practically equal, in all t(C) dependences given below, C refers to the total concentration. Using Eq. (3.120) to analyse the experimentally obtained time dependence of the probability P(t) of film rupture it was found... 

Calculated values of the fitting constants A, B and Ce and of some parameters of hole nuclealion in microscopic foam bilayer of NaDoS (r = 250 pm ) and NP20 (r = 300 pm) (419,420)... 

Parameters of hole nucleation and molecular characteristics of bilayers. Theoretically, the fitting constants A, B and Ce in Table 3.12 contain important information. Using their values in Eqs. (3.107), (3.110), (3.111), (3.126) and (3.129), the following characteristic parameters of the process of hole nucleation in the foam bilayers can be evaluated nucleation work W, number C of surfactant vacancies in the nucleus hole, specific... 

The lateral interaction energy between two nearest neighbour surfactant molecules in the foam bilayer can be estimated with the aid of a relation e yLd / 2 (d = 2(A0 / jc)U2) is the mean intermolecular distance in the film plain). As it can be seen in Table 3.12, for both surfactants is greater than kTand is twice largeer for the ionic NaDoS. 

The concentrations Ce and Cc of the monomer surfactant in the solution are important parameters for a given bilayer/solution system, as they determine the ability of the foam bilayer to exist in metastable equilibrium in the range Cc < C < Ce. For C > Ce the bilayer is... 

Because of the strong t(C) dependence at and below a given surfactant concentration Cc the foam bilayer cannot be observed experimentally, as it ruptures instantaneously. Hence, Cc is the experimental limit of bilayer metastability and is determined by the resolution of the measuring equipment. For NaDoS foam bilayers Cc - 1.2-10 4 mol dm 3, a value which coincides with that of the lowest bulk surfactant concentration at which maximum packing of the adsorption monolayer at the solution/air interface is attained [332,366]. 

The considered experimental tfC) dependences cannot be explained satisfactorily by the Derjaguin-Gutop-Prokhorov theory [405-407]. For instance the weak change in the specific surface energy a of the NP20 foam bilayers (less than 0.5% in the C range studied) cannot account for the steep increase in t with C when reasonable values of A and yi are used. 

Fig. 3.87. Probability of observing a foam bilayer vs. total surfactant concentration for NaDoS foam films...

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. 

To verify whether the rupture of a-particle-irradiated foam bilayers can also occur by the hole mechanism the experimental TjC) dependence for NB foam films of NaDoS has been analysed using Eq. (3.132) [415]. The circles in Fig. 3.88 show the experimental data, and the solid line is drawn according to Eq. (3.132) as a result of the best fit in the range C = 3 to 6T0 4 mol dm 3. The % value of 2-10 11 J m 1 obtained is in good agreement with the % value for spontaneous rupture. The size of the smallest irradiation-active hole is i = 3, and nucleus hole consists of = 12 to 83 NaDoS vacancies. The abrupt rise of the slope in the... 

The conclusion is, therefore, that both spontaneous and forced rupture of foam bilayer by a-particles are mediated by microscopic holes of surfactant vacancies and can be described from a unified point of view with the aid of the nucleation theory of bilayer rupture [399,402,403]. However, studying the effect of a-particle irradiation of the bilayer lifetime is an independent way of proving the applicability of the hole mechanism of bilayer rupture. 

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. 

Table 3.13 shows that the kinetic factor A increases with temperature. This fact cannot be explained without knowing the concrete mechanism of formation of the holes in the foam bilayer. A possible explanation is the occurrence of hole nucleation on preferred sites, for example along line defects (such as domain boundaries) in the bilayer. A similar preferential nucleation of voids on structural defects is known to occurs in crystals [422], If the density N0 of these preferred sites in the bilayer decreases with increasing temperature, according to Eq. (3.125). A will increase with Tprovided oKT) dependence is not significant. 

The analysis of the effect of temperature on the mean lifetime of foam bilayers provides further evidence for the applicability of the theory of bilayer rupture by hole nucleation [399,402,403]. The experiments show that the foam bilayers become less stable with increasing temperature, due both to the Boltzmann-type thermal activation of the hole nucleation and to the decreasing work of a nucleus hole formation. 

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],... 

The adsorption isotherms of NaDoS vacancies calculated in [424], from Eq. (3.13) with the aid of the values of Q and Co given previously are shown in Fig. 3.90 for 10°C (curve 1), 22°C (curve 2) and 30°C (curve 3). The equilibrium NaDoS concentrations Ce above which NaDoS foam bilayers of infinitely long lifetime are to be obtained are marked by arrows. In practice, this cannot be achieved because CMC < Ce. The hatched area shows the investigated concentration interval in which the gas of NaDoS vacancies in the foam bilayer undergoes a 2D first-order phase transition from a dilute phase (existing metastable bilayer) into a condensed phase (ruptured bilayer). 

Fig. 3.90. Calculated dependence of the degree of filling of the foam bilayer by surfactant vacancies on...

Fig. 3.91 displays the dependence of the density n of NaDoS vacancies in the foam bilayer on the concentration C of NaDoS in the solution for the three temperatures investigated 10°C (curve 1), 22°C (curve 2) and 30°C (curve 3). This dependence is calculated [424] using the formula... 

Fig. 3.91. Calculated dependence of the density of surfactant vacancies in the foam bilayer on the...

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... 

The experimental results discussed pertain to foam and emulsion bilayers formed of surfactants of different kinds and provide information about quantities and effects measurable in different ways. It is worth noting that analysing the observed effect of temperature on the rupture of foam bilayers enables the adsorption isotherm of the surfactant vacancies in them to be calculated. This isotherm shows a first-order phase transition of the vacancy gas into a condensed phase of vacancies, which substantiates the basic prerequisites of the theory of bilayer rupture by hole nucleation. 

One of the most important theoretical predictions is the existence of truly (i.e. infinitely) stable bilayers for C > Ce provided Ce < CMC. By fitting theoretical to experimental rfC) dependences it is possible to determine the equilibrium amphiphile concentration Ce and thus to judge whether in a given C range a bilayer, and in some cases, the corresponding disperse system, can be infinitely stable. BLMs, for example, are known to live for months and years. Thermodynamically, there is no difference between foam bilayers and BLMs so that the long lifetime of BLMs is apparently due to their existence in contact with amphiphile solutions of concentrations C either slightly bellow or above Cr. 

The good agreement between theoretical and experimental results of hole-mediated permeability of foam bilayers to air allows the determination of the permeability coefficient of bilayers of both ionics and nonionics. Though the mechanism of hole-mediated permeation of foam bilayers is not entirely clarified, its efficiency for lower surfactants concentrations in a wide range of temperatures is firmly established. This finding is in strong support of the basic idea of the existence of randomly nucleated microholes in the amphiphile bilayer. 

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. 

Thermal transition in phospholipid foam bilayer thickness. The equivalent thickness h of DMPC foam bilayers was determined by the microinterferometric method (see Section 2.1.1) in the range from 10°C to 30°C [384,386,439]. The mean values of the results obtained for h are presented as circles in Fig. 3.93. Three temperature ranges are clearly... 

Fig. 3.93 Temperature dependence of the equivalent thickness hw of foam bilayers formed from solution...

Values of the equivalent water-alcohol thickness kw, the thickness of the hydrocarbon layers hly the thickness of the polar core h2 and the thickness h (according to three-layer model) of DMPC foam bilayers at different temperatures. 

Note The results for the thickness of foam films according to the three-layer model depend on the place of the conditional boundary between the hydrophilic and hydrophobic parts of foam bilayer (see Section 3.4.1.2). The above values are calculated under the assumption that these boundaries are situated between the polar head groups and hydrocarbon chains of DMPC molecules. 

An important result is the coincidence of the temperature of the main phase transition determined for the water-ethanol dispersion by DSC (see below) with the temperature of the steep change in the foam bilayer thickness (23°C). Within the range from 22 to 23°C the foam bilayer thickness variation is similar to that of the interlamellar distance in water dispersions of DMPC [443]. These facts show that both in the bulk phase and in the foam bilayer a chainmelting phase transition occurs which is characterised by a sharp shift in the number of gauche conformations of carbon-carbon bonds [430,444]. 

The thickness transition at 13°C is an interesting experimental observation. It is in contrast to the pretransition in water dispersions of DMPC, for which the interlamellar spacing decreased with the fall of temperature [443], the foam bilayer thickness increases in the range of this transition upon cooling. 

Additional experiments at various electrolyte contents showed that an increase in NaCl concentration did not influence the foam bilayer thickness, hence, these transitions are not of electrostatic origin and are probably due to the occurrence of phase transitions in foam bilayers. The data in Fig. 3.93 suggest the existence of different types of DMPC foam bilayers, which are distinguished by their phase state. 

---