Foam films disjoining pressure isotherms

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. 

FIGURE 1.12 Schematic diagram of disjoining pressnre isotherm exhibiting two stable regions where common black foam films and Newton black films can be formed, respectively (see text for full explanation). IIala air-Uquid-air (ALA) foam film disjoining pressure and h is film thickness. 

Figure 5.8 Hypothetical disjoining pressure isotherm for a foam film illustrating the primary and secondary minima. From Nguyen and Schulze [53]. Copyright 2004, Dekker.

Figure 5.15 shows an example of a disjoining pressure isotherm in which the steric force contributions have been superimposed on the classical DLVO force contributions. It can be seen that this creates two regions for meta-stable foam films. One region is the thick, common black film region, with film thicknesses of approximately 50 nm or so. The other region is the thin, Newton black film region, with film thicknesses of approximately 4 nm. While the common black films are mostly stabilized by electrostatic forces, the Newton black films are at least partly stabilized by the steric forces. 

The determination of the ( -potential from the directly measured disjoining pressure isotherms will be treated in Section 3.4. Thus, the (po(h) dependence can be followed along with understanding the charge-potential relationship of interacting diffuse electric layers in foam films. 

From a practical point of view the dynamic method is fast and relatively simple. It has the intrinsic advantage over any equilibrium technique that disjoining pressure isotherms with dYl/dh > 0 can be monitored. It has been successfully applied to measure van der Waals attraction and retardation effects in foam films [80,235], The dynamic method has been applied to foam films of liposomal suspensions [234] and quite recently surface forces of oscillating nature were monitored in foam [235] and pseudoemulsion [236] films. 

The most detailed information about the interaction of two interfaces can be obtained from the disjoining pressure vs. thickness isotherm. Disjoining pressure isotherms were obtained for foam films from 0.7-1.410 5 mol dm 3 F108 aqueous solutions. A disjoining pressure range encompassing 4 orders of magnitude (1 -104 Pa) has been monitored by two complementary techniques the dynamic method and the Thin Liquid Film-Pressure Balance Technique [128,129] (see Section 2.1.8). 

It is well documented that in many respects PEO-PPO-PEO triblock copolymers behave like non-ionic surfactants [e.g. 225], This is also true for the interactions in foam films. The disjoining pressure isotherm in Fig. 3.39 is very much like that obtained earlier with foam films from nonylphenol eicosaoxyethylene ether (NP(EO)2o) [172], In both cases the isotherm is reversible, monotonously increasing (the barrier mechanism typical for low molecular weight surfactants is not observed) and its slope increases with decreasing film thickness. These features seem to be characteristic of surfactants having long PEO chains as already suggested in [172],... 

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. 

In Section 3.3.1 it was shown that the state of thin foam films is described by the Fl(/ ) isotherm of disjoining pressure. For relatively thick films, stabilised by surfactants, this isotherm is consistent with the DLVO-theory. However, black foam films exhibit a diversion from the DLVO-theory which is expressed in the specific course of the disjoining pressure isotherm. 

Fig. 3.42. General schematic presentation of disjoining pressure isotherm of a thin foam film 1 - region...

Bergeron and Radke [235] have performed precise measurement of TT(/x) isotherms of NaDoS foam films at high surfactant concentrations employing both the Pressure Balance Technique and the dynamic method of Scheludko-Exerowa [73]. The disjoining pressure isotherms were established down to pressures of 10 Pa with specially constructed film holders and careful pressure isolation and control. 

The effect of foam film type on foam stability can be studied from the tp(Apo) dependences in a wide range of pressure drops, as mentioned above, as well as from the Yl(h) dependences (disjoining pressure isotherms) for single foam films having radii close to those of films in the foam [45,46]. Fig. 7.6 depicts the Tp(Ap0) dependence of foams obtained from NaDoS aqueous solutions with different electrolyte (NaCl) concentrations, i.e. the foams are built up of different types of foam films. The surfactant concentration used in all experiments ensured maximum saturation of absorption layer. All three curves have different courses, corresponding to different film types thin films (curve 1), CBF (curve 2) and NBF (curve 3). On increasing Ap0, the foam lifetime strongly decreases compared with the time for decay in... 

Figure 5.8 Illustration of a disjoining pressure isotherm for a foam film showing the total disjoining pressure (rt) and its electrical (rtj) and dispersion (rt ) components.

Measurement of Disjoining Pressure Isotherms of Air-Liquid-Air Foam Films... 

This type of cell can also be used to measure disjoining pressure isotherms in foam films. In this case, however, the films must be much thinner for the disjoining pressure to be non-zero (i.e., <0.1 micron). Near total destructive interference of reflected light will mean that they appear black in reflected light and do not exhibit interference fringes—film thicknesses must therefore be inferred from accurate measurements of reflected light intensities in comparison with the first-order fringe maximum and minimum intensity values in the adjacent meniscus [38, 39]. 

FIGURE 2.9 Measurement of disjoining pressure isotherms of air-liquid-air foam films, (a) Porous frit film holder located inside hermetically sealed cell subject to applied gas pressure, Pg. Bulk liquid pressure is Pl (= Pi + PhS hyd text for definition of terms)... 

FIGURE 3.9 Disjoining pressure isotherms for stratifying films., 0.1 M SDS foam film , 0.1 M SDS-dodecane pseudoemulsion film (both at 24°C). At each discontinuity in isotherm, film thins by a step of 10 nm, presumably as a result of loss of layer of micelles. (With kind permission from Springer Science+Business Media Colloid Polym. Sci., 273,1995, 165, Bergeron, V., Radke, C.J.)... 

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