Foam films thickness transition

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

The foam stability of /3-cas foams progressively decreased with added Tween 20. In contrast, there was a very sharp transition in equilibrium film thickness at R = 0.5. Surprisingly, surface diffusion of /3-cas was not detected at any R value in these films. This was unexpected since it has been reported that adsorbed layers of /3-cas are characterized by a very low surface viscosity [3], signifying that protein-protein interactions in /3-cas films are very weak. We had expected to observe surface diffusion either in the films stabilized by... 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

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. 

IT(/i) isotherms of black foam films from C o(EO)4 and NP20 are shown in Fig. 3.44. The surfactant and electrolyte concentrations are chosen so that equilibrium films within a large range of thicknesses are obtained, including the CBF/NBF transition region [172],... 

The results of the measurements equilibrium thickness of foam films from lyso PC as a function of NaCl concentration are shown in Fig. 3.49. At low electrolyte concentration thick equilibrium films that gradually decreased in thickness with increase in Cei were formed. When Cei exceed 10 3 mol dm 3, black spot formation occurred and spontaneous transition from silver to 7.6 nm thick black films was observed in some experiments. At 1.3-10 3 mol dm 3 NaCl predominantly black films were formed. 

Some thickness transitions occurring in the foam films, such as CBF/NBF were considered so far and estimated from the h(Cei), /i(pH) and TT(/i) dependences. These are transitions in the equilibrium thickness from the thicker CBF to the thinner NBF. The reverse thickness transitions were also realised experimentally, for instance NBF/CBF (see Fig. 3.57) in the Tl(/x) isotherm of NaDoS films at Cei = 0.165 - 0.18 mol dm 3. Similar reverse transition was found in the h(Cei) dependence of lyso PC films in the presence of CaCl2. In this case there occurs a specific adsorption of the Ca2+ ion and the films transfer from CBF to NBF (Fig. 3.50). Along with transition from one equilibrium state into another, non-equilibrium thickness transitions also exist. This is the phenomenon known as stratification, i.e. a consecutive stepwise film thinning. During this process the initially formed films thin to... 

As already mentioned, stepwise transitions in foam films are observed, as a rule, at thicknesses less then 60 - 70 nm. The number of transitions increases with the increase in surfactant concentration. Manev et al. [351] have observed up to 10 transitions when the NaDoS concentration in the initial aqueous solution was raised to 0.5 mol dm 3 (in the absence of additional electrolyte). Upon increasing the ionic strength (addition of electrolyte or ionic surfactants) the differences in the transition thicknesses decrease. In some cases [351-353] electrolyte inhibits stratification. 

Fig. 3.76 presents an analogous P(h) isotherm of foam films obtained from system n. Here stratified foam films were also observed. At constant p0 (measuring cell A), seven metastable states of the films (in the various experiments) with thicknesses ranging from 82.1 to 45.2 nm were distinguished. The latter thickness was the lowest that could be realised by a spontaneous stepwise thinning. Spontaneous and forced transitions followed upon pressure increase, similar to those shown in Fig. 3.75. The final thickness reached was about 5.6 nm, i.e. a bilayer film. Therefore, on imposing a definite pressure on the films of both systems,...

The analysis of these P(h) isotherms emphasises that stratified foam films are formed from both systems (I and II). A phenomenon not revealed so far is that spontaneous (under constant capillary pressure) and forced (under various capillary pressures) stepwise thinning can occur in the same single foam film. A question arises as to whether the film that acquired such a thickness is in thermodynamic equilibrium or is kinetically stabilised. It should be noted that these transitions occur only in the direction of increasing pressure, i.e. the process... 

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. 

Analysis of the results and comparison with the lipid phase transition observed iq the bulk lipid/water systems allows to conclude that the lowest temperature during heating at which measurable diffusion occurred correlates with the onset of formation of the lamellar Ln liquid crystalline phase of the given phospholipid. Therefore, the data support a correlation between the surface and bulk phase transitions. This was confirmed in recent studies where the lipid surface phase transition was successfully measured for the first time in foam film by independent means involving the detailed investigations of the temperature dependences of the W(C) curve for the foam film and its thickness. 

For AOS without electrolyte, again there were two thickness transitions although the stepwise transitions seemed somewhat greater than those for the foam film. The final thickness of the emulsion film was also greater than that of the foam film (about 20nm). 

In a simple foam film the thickness of the interface is similar to the length of a surfactant molecule. The thickness of the so-called common black film (CBF) is determined by the DLVO forces, and the thinner Newton black film (NBF) is stabilized by steric repulsion and does not contain any free solvent molecules. A transition from a CBF to a NBF can be induced by the addition of salt leading to a screening of the surface potential. This confirms the electrostatic nature of the repulsive force stabilizing the CBF. The transition from a CBF to a NBF corresponds to an oscillation of the disjoining pressure because of the attractive van-der-Waals forces. This attractive part of the isotherm is mechanically unstable, and it cannot be measured by a TFPB. But a step in film thickness from the thicker CBF to a thinner NBF is detected. 

Figures 5.37 and 5.38 show the critical thicknesses of rupture, Rp for foam and emulsion films, respectively, plotted vs. the film radius." In both cases the film phase is the aqueous phase, which contains 4.3 x 10 M SDS + added NaCl. The emulsion film is formed between two toluene drops. Curve 1 is the prediction of a simpler theory, which identifies the critical thickness with the transitional one." Curve 2 is the theoretical prediction of Equations 5.270 to 5.272 (no adjustable parameters) in Equation 5.171 for the Hamaker constant the electromagnetic retardation effect has also been taken into account. In addition, Eigure 5.39 shows the experimental dependence of the critical thickness vs. the concentration of surfactant (dodecanol) for aniline films. Figures 5.37 to 5.39 demonstrate that when the film area increases and/or fhe electrolyte concentration decreases the critical film thickness becomes larger.

Lobo and Wasan (81) observed the drainage and stability of pseudoemulsion films from nonionic surfactant solutions (Enordet AE1215-30 ethoxylated alcohol) at concentrations much above the CMC. They observed that, for a 4 wt% surfactant system, the film thinned stepwise by stratification (Figure 27), in a fashion similar to the foam films from micellar solutions (Figure 14). Three thickness transitions were observed (81) at 4 wt% concentration with n-octane as oil, which was the same number of steps as observed by Nikolov et al. (54) in foam films at the same concentration. This observation on the micellar layering in the pseudoemulsion film confirms, again, the universality of the stratification phenomenon. 

The addition of electrolyte into a foamer solution reduces its molecular solubility and leads, as a result, to a sharp increase in the Wp value. On the other hand, in the concentration range studied, there is a transition of foam films from a higher thickness to common black films, and then to Newton black films [17]. Such a transition corresponds to an increase of the lifetime of the films as investigated by the Scheludko technique. 

Here, is the so called foam parameter, and t is the viscosity m the surfactant-containing phase (Liquid 1 in Fig. 15) the influence of the transition zone film - bulk liquid is not accounted for in Eq. (76). Note that the bulk and surface diffusion fluxes (see the terms with and Z) in the latter equation), which tend to damp the surface tension gradients and to restore the uniformity of the adsorption monolayers, accelerate the film thinning (Fig. 1). Moreover, since Din Eq. (76) is divided by the film thickness h, the effect of surface diflhsion dominates that of bulk diffusion for small values of the film thickness. On the other hand, the Gibbs elasticity Eq (the Marangoni effect) decelerates the thinning. Equation (76) predicts that the rate of... 

In the beginning of the twentieth century, Johonnott [476] and Perrin [477] observed that foam films decrease their thickness by several stepwise transitions. The phenomenon was called stratification. Bruil and Lyklema [478] and Friberg et al. [479] studied systematically the effect of ionic surfactants and electrolytes on the occurrence of the stepwise transitions. Keuskamp and Lyklema [480] anticipated that some oscillatory interaction between the film surfaces must be responsible for the observed phenomenon. Kruglyakov et al. [481,482] reported the existence of stratification with emulsion films. 

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