Foams disjoining pressure

We can conclude that the stability of static foam in porous media depends on the medium permeability and wetting-phase saturation (i.e., through the capillary pressure) in addition to the surfactant formulation. More importantly, these effects can be quantified once the conjoining/disjoining pressure isotherm is known either experimentally (8) or theoretically (9). Our focus... 

V. Bergeron and C.J. Radke Equilibrium Measurements of Oscillatory Disjoining Pressures in Aqueous Foam Films. Langmuir 8, 3020 (1992). 

V. Bergeron Disjoining Pressures and Eihn Stabflity of Alkyltrimethylammonium Bromide Foam Films. Langmuir 13, 3474 (1997). 

Interfacial Forces. Neighboring bubbles in a foam interact through a variely of forces which depend on the composition and thickness of liquid between them, and on the physical chemistry of their liquid—vapor interfaces. For a foam to be relatively stable, Ihe net interaction must be sufficiently repulsive at short distances to maintain a significant layer of liquid in between neighboring bubbles, Interfacial forces include ihe van der Waals inieracliun. the electrostatic double layer imeruclion. and disjoining pressure. 

New experimental techniques and several of their applications were presented which help in the understanding of structure, texture and stability of food systems. For future research, the mechanism of film stability by the microlayering of colloid particles seems to be the most promising - especially in food emulsions and foams. Work is in progress in our laboratory to calculate the oscillatory disjoining pressure inside liquid films containing microlayers [30],... 

All techniques mentioned so far are mainly used to study the force between solid surfaces. In many applications one is interested in the disjoining pressure between liquid-liquid or liquid-gas interfaces, such as those found in foams and emulsions. One such technique is described in Section 12.5.3. 

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. 

Foaming capability relates to both foam formation and foam persistence. Surface tension lowering is necessary, but not sufficient. Other important factors include surface elasticity, surface viscosity, and disjoining pressure [303], Considering stabil-... 

The stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

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. 

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]

At equilibrium the capillary pressure in the flat horizontal foam film is equal to the disjoining pressure in it... 

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... 

The simplest explanation of film rupture involves reaching a thermodynamically unstable state [20]. A typical example of thermodynamically unstable systems are foam films in which the disjoining pressure obeys Hamaker s relation. Such are films from some aqueous surfactant solutions containing sufficient amount of an electrolyte to suppress the electrostatic component of disjoining pressure as well as films from non-aqueous solutions (aniline, chlorobenzene) [e.g. 80],... 

The DLVO-theory considers only the molecular van der Waals and electrostatic interactions. A complete analysis of the theory can be found in several monographs [e.g. 3-6] where original and summarised data about the different components of disjoining pressure in thin liquid films, including in foam films are compiled. 

A number of works are dedicated to the experimental verification of DVLO theory to foam films. As shown above, the disjoining pressure is given as a sum of ne/ and nvw, i.e. 

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. 

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. 

Disjoining pressure in foam films from ABA triblock copolymers... 

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...

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. 

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. 

To illustrate the above approach, a set of values for Ym and Xd derived from the experimental results of a direct measurement of film disjoining pressure on the film thickness dependence was used (see Fig. 3.44,a). These values are relevant to foam films from aqueous solutions of a non-ionic surfactant and an electrolyte (KC1) of concentration 51 O 3 and 31 O 3 mol dm 3, respectively. 

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