Transition foam films

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

Surface forces measurements with microscopic foam films permitted to study in details the long-range/short-range interaction transition, including the reversal transition occurring in some cases. A fluctuation zone of existence of metastable films is found, governed by the two types of forces. 

Surface forces in foam films from amphiphilic block copolymers 3.3.3.1. Transition from electrostatic to steric stabilisation in foam films from... 

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. 

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. 

At equilibrium film thickness hi the disjoining pressure equals the external (capillary) pressure, n = p This is a common thin film and its equilibrium is described by the DLVO-theory. If h < hcr, at which the film ruptures (see Section 3.2.2), the film is common black (schematically presented in Fig. 3.42). Such a film forms via black spots (local thinnings in the initially thicker non-equilibrium film). The pressure difference nmax - pa is the barrier which hinders the transition to a film of smaller thickness. According to DLVO-theory after nmax the disjoining pressure should decrease infinitely. Results from measurements of some thermodynamic parameters of foam films [e.g. 251,252] show the existence of a second minimum in the 17(6) isotherm (in direction of thickness decrease) after which the disjoining pressure sharply ascends. 

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... 

For the study of surface forces acting in foam films, including in black films, another type of isotherm proves to be most informative, i.e. the dependence of film thickness h on electrolyte concentration Cei at Cs = const, pa = const and f = const. This h(Cei) dependence allows to distinguish clearly the action of electrostatic disjoining pressure and to find the electrolyte concentration at which the CBF/NBF transition occurs. 

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],... 

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. 

The mechanism of Ca2+ binding is not clear yet. However, increase in repulsive double layer forces between neutral diacylphosphatidylcholine bilayer in aqueous media in the presence of divalent ions has been identified by other methods as well [293-296]. These systems differ from the foam- film model by virtue of their interface ordered lipid phase/water in place of the air/water interface of foam films. Nevertheless, the CaCb concentration where the transition from NBF to silver films is observed in experiments with foam films is very close to the concentrations where increase in the distance between the bilayers was found [293,294,296]. Results with microscopic films are also in good agreement with the established increase in the free energy of formation of macroscopic films stabilised with lysolecithin in the presence of CaCl2 [287]. 

Capillary (disjoining pressure) and foam film thickness at rupture or at CBF/NBF transition [171)... 

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). 

Therefore, the study of foam film behaviour under a-irradiation is an excellent detector for the CBF/NBF transition. Curve 2 in Fig. 3.65,b indicates Ta for CBF. The dashed line refers to ra for NBF up to the transition. These curves distingish the region in which metastable CBF exist. The values of Cei.Cr found under a-particle irradiation are presented in Table 3.9. They are in good correlation with the ones obtained by other techniques [253,323]. 

Critical electrolyte concentrations, corresponding to foam Film transitions (derived from a-parlicle irradiation) [325]... 

The CBF/NBF transition can also be found by studying the properties of macroscopic foam films with respect to their dependence on certain parameters. For instance, h(Cei) and h(T) dependences of NaDoS macroscopic films have been plotted in [308]. The ranges of electrolyte concentration and temperature where CBF and NBF are stable were determined. 

Some thickness transitions occurring in the foam films, such as CBF/NBF were considered so far and estimated from the h(Cei), /i(pH) and TT(/i) dependences. These are transitions in the equilibrium thickness from the thicker CBF to the thinner NBF. The reverse thickness transitions were also realised experimentally, for instance NBF/CBF (see Fig. 3.57) in the Tl(/x) isotherm of NaDoS films at Cei = 0.165 - 0.18 mol dm 3. Similar reverse transition was found in the h(Cei) dependence of lyso PC films in the presence of CaCl2. In this case there occurs a specific adsorption of the Ca2+ ion and the films transfer from CBF to NBF (Fig. 3.50). Along with transition from one equilibrium state into another, non-equilibrium thickness transitions also exist. This is the phenomenon known as stratification, i.e. a consecutive stepwise film thinning. During this process the initially formed films thin to... 

As already mentioned, stepwise transitions in foam films are observed, as a rule, at thicknesses less then 60 - 70 nm. The number of transitions increases with the increase in surfactant concentration. Manev et al. [351] have observed up to 10 transitions when the NaDoS concentration in the initial aqueous solution was raised to 0.5 mol dm 3 (in the absence of additional electrolyte). Upon increasing the ionic strength (addition of electrolyte or ionic surfactants) the differences in the transition thicknesses decrease. In some cases [351-353] electrolyte inhibits stratification. 

Fig. 3.76 presents an analogous P(h) isotherm of foam films obtained from system n. Here stratified foam films were also observed. At constant p0 (measuring cell A), seven metastable states of the films (in the various experiments) with thicknesses ranging from 82.1 to 45.2 nm were distinguished. The latter thickness was the lowest that could be realised by a spontaneous stepwise thinning. Spontaneous and forced transitions followed upon pressure increase, similar to those shown in Fig. 3.75. The final thickness reached was about 5.6 nm, i.e. a bilayer film. Therefore, on imposing a definite pressure on the films of both systems,...

The analysis of these P(h) isotherms emphasises that stratified foam films are formed from both systems (I and II). A phenomenon not revealed so far is that spontaneous (under constant capillary pressure) and forced (under various capillary pressures) stepwise thinning can occur in the same single foam film. A question arises as to whether the film that acquired such a thickness is in thermodynamic equilibrium or is kinetically stabilised. It should be noted that these transitions occur only in the direction of increasing pressure, i.e. the process... 

A comparison of the properties of the bulk micellar systems with those of the films in metastable equilibrium, in particular NBF, is of special interest. The existence of a correlation between the temperature dependent phase transition in NBF stabilised with phospholipids and the analogous phase transition taking place in the bulk phase is to be further discussed (see Section 3.4.4). Undoubtedly, for the systems considered the establishment of a similar correlation between the foam films and the bulk solubilising phases is worth studying. 

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 two thickness transitions may be regarded as reliable because the accuracy of the microinterferometric method is 0.2 nm [171] and because of the good reproducibility of experimental results. The absence of data for equivalent thickness at temperatures 21-22°C is due to existence of heterogeneities in the thickness of the foam films resulting in a significant error in the thickness determination. 

Analysis of the results and comparison with the lipid phase transition observed iq the bulk lipid/water systems allows to conclude that the lowest temperature during heating at which measurable diffusion occurred correlates with the onset of formation of the lamellar Ln liquid crystalline phase of the given phospholipid. Therefore, the data support a correlation between the surface and bulk phase transitions. This was confirmed in recent studies where the lipid surface phase transition was successfully measured for the first time in foam film by independent means involving the detailed investigations of the temperature dependences of the W(C) curve for the foam film and its thickness. 

It is known [41] that NBF/CBF transition occurs in a foam from NaDoS solution with constant electrolyte concentration (more than 0.3 mol dm 3 NaCl) within the temperature range from 30 to 35°C (see Section 3.4.2). To confirm that the difference in flow rates is related to the foam film type studies were performed to establish the temperature dependence of the foaming solution flow rate through NaDoS foam with NBF (in the presence of 0.4 mol dm 3 electrolyte). The results from this series of experiments show that with the change in temperature from 20 to 25°C, vexp increases in accord with the decrease in solution viscosity. Further temperature rise (30-35°C) leads to a jump-like increase in flow rate (Fig. 5.2. - (x) points lay on curve 2 which is for CBF). Obviously, this coincides with the NBF/CBF transition. 

The described above principle has served as a basis for the development of a method for foam destruction which is particularly useful for highly stable foams [20]. This method has also been employed for establishing the CBF/NBF transition in a foam. The critical electrolyte concentrations thus obtained proved to be close to those determined with free foam films (see Chapter 3). 

The study of a large number of various surfactants in aqueous and non-aqueous media has shown that a sharp transition towards films of high stability at increasing surfactant concentrations is always related to the appearance of black spots in the microscopic films [17,42,43]. It was established that the surfactant concentration corresponding to black spot formation lies in the range of sharp increase in the dependence foam lifetime x on surfactant concentration C. 

The potential and the charge of the diffuse electric layer is another important surfactant characteristics. Though these parameters are not directly related to the foam stability, they determine the type of foam films which affect foam stability. Another parameter directly connected to (p0 is the critical electrolyte concentration at which a CBF/NBF transition occurs at a given temperature. It allows to distinguish two equilibrium states of black foam films. The role of (p0 in the CBF/NBF transition permitted to find the value of the critical potential surfactant characteristic [71] (see Section 3.4.2). 

The difference found in the behaviour of steady-state foams from NaDoS solutions in the presence of various electrolyte concentration reflects the importance of foam films, which can be formed also in such systems. The existence of different types of foam films in the steady-state foams is proved by their destruction by a-particle irradiation [121], Fig. 7.23 shows the dependence of the foam column height on the electrolyte concentration. It is seen that at NaCl concentration higher than 0.35 mol dm 3, H does not change. This concentration is very close to the electrolyte concentration at which there occurs a transition from one foam film type to another. 

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. 

For C AOS there was found only one transition of the foam film both for the systems with and without electrolyte. 

For AOS without electrolyte, again there were two thickness transitions although the stepwise transitions seemed somewhat greater than those for the foam film. The final thickness of the emulsion film was also greater than that of the foam film (about 20nm). 

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