Method of equilibrium foam film

The method of equilibrium foam film employs the experimental measurement of the equilibrium thickness and from the DVLO theory it is possible to determine (po and, respectively, the surface charge at the solution/air interface. This is a very valuable possibility since an equilibrium potential can be evaluated and all complications occurring at kinetic measurements, are avoided. The equilibrium values of (fo are important in the interpretation of electrostatic forces in thin liquid films, along with the other surface forces, acting in them. 

The method of equilibrium foam film allows to study the ( -potential at various aspects by means of the microinterferometric technique (see Chapter 2). For instance, to determine cpo at electrolyte solution/air interface (no surfactant) which is very hard to realise experimentally to find the origin of the surface charge in this case [186,187] to find the isoelectric points at the solution/air interface [173,188] to study the effect of the concentration of various kinds of surfactants [95,100,189,190] ionic effects influence of Na+... 

Dependence of -potential on surfactant kind and concentration. Detailed study with the method of equilibrium foam film of h(Cej) and A(pH) dependences in the absence of a surfactant, as well as h(C) at very low surfactant concentrations, gave (po 30 mV at the interface aqueous electrolyte solution/air [169,170,197]. It is important to note that this value of (po could be reconsidered in view of some recent results on numerical calculation of dispersion interactions in foam films [106,166,198]. For example, as shown by Kolarov, the (po value of 30 mV is reduced to about 15 mV when using the data on dispersion interactions reported in [166],... 

In conclusion it is worth noting that the method of equilibrium foam film proved to be very appropriate for the determination of the equilibrium diffuse electric layer potential at the solution/air interface. Though it is an indirect experimental technique, it provides reliable results about the appearance of a negative surface charge in the case of surfactant-free solutions as well as in the case of non-ionic surfactant solutions. The existence of an isoeletric point and the re-charging of the interface can be considered as a direct evidence. 

Another possibility to calculate the ( -potential and charge Oo for films from lyso PC is by the method of equilibrium foam film . In the presence of Cei = 3-10 4 - 10 3 mol dm 3... 

As already stated in Section 3.3.2, the precise values of the potential of the diffuse electric layer (po can be obtained by the method of equilibrium foam film . The results correlate well with the values of the electrokinetic potential, measured by the method of the rotating bubble [65], Table 8.1 presents the -potential values and the surface charge density Oo for foams from various surfactant kinds [65]. 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

It should not be forgotten that the values of ( -potential determined by the method of the equilibrium foam film are not directly measured. They depend on the way of calculation, i.e. which equations of the DLVO-theory are employed, what model of the film is used for thinner films, etc. Surely, the aim is to obtain the most reliable value of ( -potential at the given experimental conditions. In all cases, the primary experimental data about h, Cei, and equilibrium pressure, are presented so that the values of ( -potential could be corrected upon the further enrichment of knowledge in this field. 

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. 

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. 

It is well know that the direct comparison between the methods of estimation of foam stability (in most of the cases it is determined by the foam lifetime) is not possible. Each of the existing methods involves different parameters, for example, time for destruction of a foam column of a definite height (or part of it), rate of decrease in the specific foam surface, etc. The main reason for the impossibility to make such a comparison is that foam stability is determined at different pressures in the foam liquid phase. This means that the rate of drainage as well as the time of reaching an equilibrium state of the films in the foam is different. Another reason could be attributed to the possibility both foam formation (i.e. foam volume... 

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