Microinterferometric method

Since the microinterferometric method is largely employed in the study of thin liquid films, it would be considered in detail in the following sections. 

The experimental determination of the surface diffusion in foam films with FRAP technique is combined most successfully with the microinterferometric method of Scheludko-Exerowa (see Section 3.5) [39,40]. A systematic study showed the dependences of D on some film parameter such as film composition, type and film thickness, influence of proteins and lipid phase state, molecular length and charge, etc. (see Section 3.5.3). 

The existence of channels in microscopic circular horizontal films can be registered with the microinterferometric method of Scheludko-Exerowa (see Chapter 2) that plots the photocurrent/time curve. Each time when a channel passes through the place of photometry there appears a kink in this curve. 

The microinterferometric method employed in the study of kinetics of foam film thinning allows to establish experimentally the liquids that form or do not form foam films. If a liquid possesses even small affinity to produce a foam, a circular film with clearly pronounced Newton rings is formed when it is drawn out of the biconcave drop. Films from aqueous surfactant solution can be obtained even at very small decrease in the surface tension (Act < 10 4 N m 1). It is sufficient to ensure a tension gradient between the film center and periphery. 

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). 

Thermal transition in phospholipid foam bilayer thickness. The equivalent thickness h of DMPC foam bilayers was determined by the microinterferometric method (see Section 2.1.1) in the range from 10°C to 30°C [384,386,439]. The mean values of the results obtained for h are presented as circles in Fig. 3.93. Three temperature ranges are clearly... 

The two thickness transitions may be regarded as reliable because the accuracy of the microinterferometric method is 0.2 nm [171] and because of the good reproducibility of experimental results. The absence of data for equivalent thickness at temperatures 21-22°C is due to existence of heterogeneities in the thickness of the foam films resulting in a significant error in the thickness determination. 

Contrary to the case of P-casein/Tween 20 systans, although the surface pressure isotherms shed some light on the surface structure of whole casein/Tween 20 systems, the information extracted does not provide a full explanation to the behavior of foams stabilized by this system. Taking into account the key information provided by the confinement in thin liquid films as regards the foam stability of whole casein and P-casein solutions, which was examined in the previous section, let us evaluate the properties of the foam films stabilized with whole casein/Tween 20 mixtures. Table 10.2 shows the thickness of foam films stabilized by pure whole casein, pure Tween 20 and two mixed systems under similar conditions to the foam stability, and the surface pressure experiments. The film thickness is measured by using Scheludko s microinterferometric method (Maldonado-Valderrama and Langevin, 2008). 

Microinterferometric Method for Investigation of Thin Liquid Films... 

The microinterferometric method has widely been used by many authors to investigate both symmetric thin liquid films (foam and emulsion films) and asymmetric ones (wetting films). This method has been described in several papers, for example. Refs. [2, 3, 32, 33, 37] as well as in a book [1]. Here we give only a brief outline. 

Since, however, each model involves some assumptions, the calculation of h2 always renders certain inaccuracy. The most important problem in the three-layer model concerns the position of the plane that divides the hydrophobic and hydrophilic parts of the adsorbed surfactant molecule. In some cases it seems reasonable to have this plane passing through the middle of the hydrophilic head of the molecule, in others the head does not enter into the aqueous core. That is why it is worth comparing film thicknesses determined by the interferometric technique using the three-layer model, to those estimated by other methods. An attempt for such a comparison is presented in [63]. Discussed are phospholipid foam films the thickness of which was determined by two optical techniques the microinterferometric and FT-IR (see Section 2.2.5). The comparison could be proceeded with the results from the X-ray Reflectivity technique that deals not only with the foam film itself but also with the lamellar structures in the solution bulk, the latter being much better studied. Undoubtedly, this would contribute to a more detailed understanding of the foam film structure. 

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+... 

---