Foam films microscopic

Microscopic are considered to be only those foam films the radius of which is within the range of 10 - 500 p.m. Their application as models in the study of foam films has been proved most successful and that is why the experimental technique for their study is... 

The techniques involving microscopic foam films can be applied to the study of other types of thin films, such as emulsion films [e.g. 4-9], thin films on a substrate [e.g. 10-12], as long as a suitable measuring cell is utilised. 

Measuring cells for formation and study of microscopic foam films... 

One of the first devices employed in the study of microscopic foam films has been proposed by Derjaguin and Titijevskaya (Fig. 2.1). 

Fig. 2.2. Scheme of the measuring cell for the study of microscopic foam films A in a glass tube B ... 

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. 

The measuring cells for the study of microscopic horizontal foam films (especially variant A, Fig. 2.2 and variant D, Fig. 2.4) have been widely used in [e.g. 26-47]. 

Thickness is one of the main parameters of a foam film. The most widely employed technique for its determination is interferometry. It is based on the comparison between the intensities of the light falling on the thin film and that reflected from it. This technique permits to evaluate the thickness of equilibrium as well as thinning films. It has been used by Perrin [48] and Wells [49] with soap films. The intensity of the reflected light was measured with an interferometer comparing the two parts of the visual field (in a microscope). Thus the film thickness was determined with an accuracy of about 0.5 nm. 

A new way to form a microscopic foam film in the middle of a biconcave drop (Fig. 2.2) has marked the further improvement of the microinterferometric technique. The increase in accuracy and reliability of the photometric and registering devices contributed to this improvement. The experimental details and the metrological characteristics of the microinterferometric technique for determination of foam film thickness has been the object of numerous studies [e.g. 16,23,39-43,58],... 

When circular microscopic foam films (equilibrium or thinning) are studied it is necessary to know the pressure in the meniscus of the liquid being in contact with the film (see Fig. 2.2 A, B, C). In some cases it is very important to know the precise value of the capillary pressure, for example, in the calculation of low disjoining pressures n and the potential of the diffuse electric layer [Pg.50]

Fig. 2.8 depicts pa (C) dependence for a microscopic foam film formed from surfactant solutions. As anticipated that pa decreases with the increase in the surfactant concentration up to CMC and remains constant further on. 

A block-scheme of the apparatus for the study of foam films under applied pressure is shown in Fig. 2.11. The films are formed in the porous plate of the measuring cell (Fig. 2.4, variant D and E). The hydrodynamic resistance in the porous plate is sufficiently small and the maximum capillary pressure which can be applied to the film is determined by the pore material. The porous plate measuring cell (Fig. 2.4, variants D and E) permits to increase the capillary pressure up to 105 Pa, depending on the pore size and the surface tension of the solution. When the maximum pore size is 0.5 pm, the capillary pressure is 310s Pa (at cr = 70 mN/m). The cell is placed in a thermostating device, mounted on the microscopic table. 

A spherical foam film can be formed at the top of a foam bubble floating freely at the solution surface during the process of film thinning. Under gravity such a microscopic bubble,... 

A necessary prerequisite for the formation of microscopic foam films is the adsorption of surfactants at the solution/air interface. Different ways have been sought in order to obtain adsorption layers from insoluble surfactants at such interface. The easiest way to form a foam film is to blow a freely floating gas bubble at a liquid surface covered with a monolayer of insoluble amphiphile molecules. This approach has been used by various authors [138-142]. 

In the method developed by Richter, Platikanov and Kretzschmar [143] the insoluble monomolecular layer is formed by a conventional technique at the aqueous surface in a Teflon trough. Its density is defined by a barrier and its surface pressure is measured by Wilhelmy plate (Fig. 2.27). The microscopic foam film is formed when the gas semi-bubble approaches... 

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. 

Dynamics and stability of thin foam films have been and continue to be an object of intensive research [e.g. 28-35]. Model studies with vertical large macroscopic films with linear sizes of the order of centimeters as well as with horizontal circular microscopic films with radius of the order of millimeters were performed. The kinetics of thinning of vertical macroscopic films in described in detail in [33]. Some of the results presenting an interpretation of the dynamic properties of films and foam are considered in Chapter 7. Microscopic foam films offer certain advantages with respect to treatment of stability of foams and foam films, since the systems studied behave under strictly defined conditions. 

Reynolds relation requires liquid drainage from the film to follow strictly the axial symmetry between parallel walls. Rigid surfaces ensure such drainage through their non-deformability, while non-equilibrium foam films are in fact never plane-parallel. This is determined by the balance between hydrodynamic and capillary pressure. Experimental studies have shown that only microscopic films of radii less than 0.1 mm retain their quasiparallel surfaces during thinning, which makes them particularly suitable for model... 

Langevin et al. [35,71] have proposed a simplified hydrodynamic model of thinning of microscopic foam films that accounts for the influence of surface elasticity on the rate of thinning in a large range of thicknesses and Ap. However, as noted by the authors, in view of the rapid loss of surfactant molecules at the surface during film drainage, the elasticity would not correspond to the actual bulk surfactant concentration but to lower values since the system is very far from equilibrium. Frequency dependence of surface elasticity has been considered by Tambe and Sharma [72]. 

Fig. 3.11. depicts the dependence of drainage time on shear viscosity for NaDoS microscopic foam films in the presence of C12H25OH. From a certain value on the drainage time steeply increases in accordance with the increased surface shear viscosity and there occurs a symmetric drainage. 

Early studies of rupture of unstable thin films have been performed with macroscopic emulsion films [94] and foam films [53]. Very high values for hcr were obtained (of the order of 10 pm). Systematic investigations with microscopic films [e.g. 29,64,73] have shown that their critical thickness is considerably smaller. The probability character of rupture is illustrated by the curves in Fig. 3.12. As it is seen the most probable critical thickness increases with the increase in film radius. The most probable critical thickness of rupture is 30 nm (r = 0.1 mm). Usually such a thickness is reached by films from aqueous solutions of low molecular fatty alcohols at which the surfactant concentration is chosen so that the surface tension is equal in all cases [29,73]. Aniline films exhibit a higher hcr 42 nm. 

On that basis Exerowa and Scheludko [95] have introduced a new parameter bulk concentration Cm at which black spots begin to form in the microscopic foam film. It is also called concentration of black spot formation and has been studied in various aspects [e.g. 54,73,89,96-100]. This concentration is a very important quantitative characteristics of the surfactants. Its determination is done by observing microscopic films under a microscope in... 

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. 

Combined measurement techniques were successfully applied in the study of surface forces in microscopic foam films such as study of longitudinal electrical condictivity, study of black films with X-rays forced rupture of films by a-particles irradiation, etc. They permit to find the relation between surface forces and parameters of film structure. It is important also surface forces measurements to be performed at controlled state of the adsorption layer. As far as surface forces act normally to film surface, it is interesting to understand the role of... 

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. 

More precise verification of the theory was achieved with films studied by the microinterferometric technique. Though performed a long time ago (1960), these experiments deserve attention, since they represent the first quantitative proof of the DLVO-theory conducted on a model system (foam film) which still hold true. Independent studies were performed of the X ei(k) and Vlvw(h) as well as of their joint action at various electrolyte concentrations. At very low Cei equilibrium films of large thickness formed in which the electrostatic interaction was prevailing and their behaviour could be described completely with this interaction. At such film thickness Y vw was still very low so that the equilibrium film state was reached at equal electrostatic disjoining and capillary pressure (n, = p ). Fig. 3.15 depicts the equilibrium thickness dependence on electrolyte concentration for saponin microscopic foam films. 

Under certain conditions aqueous electrolyte solutions form foam films of equilibrium thickness. For a microscopic horizontal film this thickness is determined by the positive component of disjoining pressure (FU) which depends on the potential of the diffuse electric layer at the foam film/air(gas) interface. 

Fig. 3.22 depicts the p (Cs) dependence for Ci2(EO)n in a large range of electrolyte concentration. The capillary pressure in the measuring cell for microscopic foam films was measured for each solution employing the method of capillary rising in a tube, identical to that of the film holder (see Chapter 2, Section 2.1.4)... 

A comparison between (po(Cs), T(Cj) and Ao(Ct) dependences has been conducted on the basis of studies with microscopic foam films from Ci2(EO)s [190], It was shown (Fig. 3.23) that the plateau of the ( -potential precedes the beginning of saturation of the adsorption layer. For ionic surfactants such as NaDoS, the saturation of the surface charge is established earlier (210 5 mol dm 3) than the saturation of the adsorption layer ( 5-104 mol dm 3). The increase in % at low surfactant concentration is related to its adsorption at the solution/air interface [189,199],... 

The question of the ( -potential value at the electrolyte solution/air interface in the absence of a surfactant in the solution is very important. It can be considered a priori that it is not possible to obtain a foam film without a surfactant. In the consideration of the kinetics of thinning of microscopic horizontal foam films (Section 3.2) a necessary condition, according to Reynolds relation, is the adsorption of a surfactant at both film surfaces. A unique experiment has been performed [186] in which an equilibrium microscopic horizontal foam film (r = 100 pm) was obtained under very special conditions. A quartz measuring cell was employed. The solutions were prepared in quartz vessels which were purified from surface impurities by a specially developed technique. The strong effect of the surfactant on the rate of thinning and the initial film thickness permitted to control the solution purity with respect to surfactant traces. Hence, an equilibrium thick film with initial thickness of about 120 nm was produced (in the ideal case such a film should be obtained right away). Due to the small film size it was possible to produce thick (100 - 80 nm) equilibrium films without a surfactant. In many cases it ruptured when both surfaces of the biconcave drop contacted. Only very precise procedure led to formation of an equilibrium film. 

Fig. 3.29. Equilibrium thickness of microscopic foam films from 610-6 mol dm 3 lyso PC solution as a...

Microscopic foam films have been used to study the steric interaction between two liquid/gas interfaces [130]. Two ABA triblock copolymers of the Synperonic PE series were employed P85 and F108. These commercial non-ionic surfactant were used as obtained from ICI Surfactants, Witton, UK. Blocks A are hydrophilic polyethylene oxide (PEO) chains, while block B is a hydrophobic polypropylene oxide (PPO) chain. The molecular masses and average EO contents are known from the manufacturer and yield approximate chemical formulae (Table 3.3). Data about the surface tension of electrolyte-free aqueous copolymer solutions can be seen in Fig. 3.31 [130]. It was additionally checked that NaCl (up to 510 2 mol dm 3) had no influence on these values. 

The thickness of microscopic foam films from F108 was measured by the microinterferometric method (see Section 2.1.1). Fig. 3.32 depicts the dependence of the equivalent film thickness hw on the electrolyte concentration of the higher copolymer F108 (at two concentrations). 

Microscopic foam films from amphiphilic ABA triblock copolymers have been used to assess steric interactions. Most of the work on copolymers [128,129] has been carried out with the Thin Liquid Film-Pressure Balance Technique (see Chapter 2, Section 2.1.8). Nevertheless, some intriguing results have been obtained with the dynamic method for surface force measurement [127]. 

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. 

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. 

DMPC films. Direct measurement of interaction forces in films stabilised by neutral phospholipids has been first done with microscopic foam films from suspensions of small unilamellar DMPC vesicles [286]. Foam films formation from phospholipids is a difficult task since they are insoluble in water. Sonicated dispersions of insoluble phospholipids is an option, employed by Yamanaka et al. [287],... 

Fig. 3.47 depicts the disjoining pressure vs. thickness isotherm of microscopic foam films obtained from DMPC vesicle suspension. The vesicles were examined by electron microscopy after negative staining with 1% uranyl acetate solution as previously described [288]. More than 95% of the vesicles were of diameters between 15 and 35 nm. 

Fig. 3.47. Disjoining pressure thickness isotherm of microscopic foam films from DMPC vesicles...

Lyso PC and Lyso PE films. The knowledge in the field of interaction forces in foam films stabilised with soluble zwitterionic phospholipids lyso PC (lysophosphatidylcholine) and lyso PE (lysophosphatidylethanolamine) has improved due to the studies of microscopic foam films [e.g. 191,192,292], The main dependences studied were of film thickness vs. electrolyte concentration and disjoining pressure vs. thickness, under specially chosen conditions in the presence of Na+ and Ca2+. The /i(pH) dependence proved to be very informative for understanding the charge origin in films from the neutral phosopholipids lyso PC and lyso PE (see Section 3.3.2). 

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