Foams film thickness

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. 

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],... 

The definition of Cei given here is somewhat different from that previously stated (see Section 3.4.2). Here Ce/,cr is the electrolyte concentration up to which the foam film thickness decreases. Above it the film thickness is constant, but greatly exceeds the thickness of a bilayer. A rough estimate of the electrical double layer (l/ cr) thickness may be obtained... 

Optical techniques for measurement of foam film thickness involve different models and plotting the optical parameters it is possible to find the real film structure. In this sense it is interesting to compare h(Cei) dependence of lyso PC films in the presence of CaCl2 depicted by two independent optical techniques microinteferometric (Fig. 3.50) and FT-IR spectroscopy [193,292]. In this case the thickness of the aqueous core d2 is determined from... 

The comparison between the results described with neutral phospholipids and those obtained by other authors in studies of intermembrane separation [293-296] demonstrate that foam film thickness studies can be very useful for investigation of interaction forces and... 

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

Fig. 3.111. Dependence of lateral diffusion coefficient (D, cm2 s 1) of surface adsorbed fluorophore molecules on phospholipid foam film thickness (h, nm) r = 100 - 500 pm t = 24°C [493].

The details of the influence that electrostatic surface forces on the stability of foam films is discussed in Section 3.3. As already mentioned, the electrostatic disjoining pressure is determined (at constant electrolyte concentration) by the potential of the diffuse electric layer at the solution/air interface. This potential can be evaluated by the method of the equilibrium foam film (Section 3.3.2) which allows to study the nature of the charge, respectively, the potential. Most reliable results are derived from the dependence foam film thickness on pH of the surfactant solution at constant ionic strength. The effect of the solution pH is clearly pronounced the potential of the diffuse electric layer drops to zero at certain critical pH value. We have named it pH isoelectric (pH ). As already mentioned pH is an intrinsic parameter for each surfactant and is related to its electrochemical behaviour at the solution/air interface. Furthermore, it is possible to find conditions under which the electrostatic interactions in foam films could be eliminated when the ionic strength is not very high. 

The foam behaviour at low surfactant concentrations is the same as the described above. However, the formation of polyhedral foam at the upper parts of the foam column occurs at much lower surfactant concentrations. It should be noted that these concentrations are considerably lower than those at which form CBF and NBF. This is related to the effect of surfactant concentration in the foam and depends mainly on the surface activity of the substances and on the foam film thickness [53,54,121]. The higher surface activity of... 

Equation 6 clearly shows that the foam-dilatational viscosity is directly proportional to surface viscosity and inversely to foam film thickness. The terms in the parenthesis (in eq 6) represent the disjoining pressure contribution and the Plateau border curvature radius. 

Foam is a disperse system in which the dispersed phase is a gas (most commonly air) and the dispersion medium is a liquid (for aqueous foams, it is water). Foam structure and foam properties have been a subject of a number of comprehensive reviews [6, 17, 18]. From the viewpoint of practical applications, aqueous foams can be, provisionally, divided into two big classes dynamic (bubble) foams which are stable only when gas is constantly being dispersed in the liquid 2) medium and high-expansion foams capable of maintaining the volume during several hours or even days. In general, the basic surface science rules are established in foam models foam films, monodisperse foams in which the dispersed phase is in the form of spheres (bubble foams) or polyhedral (high-expansion foams). Meanwhile, real foams are considerably different from these models. First of all, the main foam structure parameters (dispersity, expansion, foam film thickness, pressure in the Plateau-Gibbs boarders) depend... 

Schick and Schmolka (15) have summarized the influence of electrolytes on the foam film thickness and stability for nonionic surfactant systems. Specific effects of such anions on the film blackening times have been reported. The order of the magnitude of the effect follows the lyotropic series, where SCN and Cl act as... 

It is well known that impact of inherent liquid drops can also cause air entrainment (see, e.g., reference [50]), a process that is in fact the basis of the well-known Ross-Miles pour test [51] (see Section 2.2.3). That drop impact can cause both the formation and destruction of foam would seem to be contradictory. It is, however, clearly a real effect that probably concerns the relative magnitude of variables such as the gas phase volume fraction in the foam, foam film thickness, and drop properties such as size, velocity, and frequency. Such a vague statement suggests the need for improved understanding of the phenomena associated with inherent liquid drop impact on foam and foam films. 

In order to look for the origin of this large difference, it is interesting to evaluate some surface properties of these proteins. Table 10.1 shows the final surface tension, the surface elasticity ( in Equation 10.3), the surface viscosity ( "7co in Equation 10.3), and the final foam film thickness of both proteins under similar conditions to the foam stability experiments. Let us evaluate in detail each of them and their implications for foam stability. 

Regarding the systems used in this study, we use the same proteins as in the previous section (whole casein and P -casein) and they are mixed with Tween 20, respectively. This is a low molecular weight surfactant used in the food industry, which is water soluble and nonionic. The different behavior of these two mixed systems is again discussed on the basis of fundamental magnitudes such as surface tension and foam film thickness. 

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