Stabilization common black films

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

The observed equilibrium thickness represents the film dimensions where the attractive and repulsive forces within the film are balanced. This parameter is very dependent upon the ionic composition of the solution as a major stabilizing force arizes from the ionic double layer interactions between any charged adsorbed layers confining the film. Increasing the ionic strength can reduce the repulsion between layers and at a critical concentration can induce a transition from the primary or common black film to a secondary or Newton black film. These latter films are very thin and contain little or no free interlamellar liquid. Such a transition is observed with SDS films in 0.5 M NaCl and results in a film that is only 5 nm thick. The drainage properties of these films follows that described above but the first black spot spreads instantly and almost explosively to occupy the whole film. This latter process occurs in the millisecond timescale. 

Figure 5.6 shows an example of a total interaction energy curve for a thin liquid film stabilized by the presence of ionic surfactant. It can be seen that either the attractive van der Waals forces or the repulsive electric double-layer forces can predominate at different film thicknesses. In the example shown, attractive forces dominate at large film thicknesses. As the thickness decreases the attraction increases but eventually the repulsive forces become significant so that a minimum in the curve may occur, this is called the secondary minimum and may be thought of as a thickness in which a meta-stable state exists, that of the common black film. As the... 

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. 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

III.B. The Role of Thermal Fluctuations on the Transition from Common Black Films to Newton Black Films. The method described in the previous section will be now applied to thin films with fluctuating interfaces, with the interaction energy calculated as in section II.G. For low values ofthe external pressure, the enthalpy has two metastable minima at Zk and 2c, and a stable one at 2 - 0 (the former two correspond to the Newton and to the common black films, respectively, and the latter implies the rupture of the film), separated by two maxima located at Z and 22 (see Figure 7a). At metastable equilibrium the distances between the surfaces are distributed between 21 and 22 for the Newton black film and between z2 and 2 —°° for the common black film. The stability of the metastable states depends on the chance for a small area S of the interface to reach the... 

The increase of the applied pressure decreases the stability of the common black film and the probability for a transition from a Newton to a common black film but also increases the probability of the rupture of the Newton black films. Figure 7b plots the enthalpy and the ratios pn(2)/pn(2n) and pdz)lpdzc), for p = 6.2 x 104 N/m2 (Kc = 20 x 10 19 J). The common black film becomes unstable (and a transition to a Newton black films occurs), while the Newton black film has a longer lifetime. At even a higher pressure (Figure 7c, p = 1.6 x 105 N/m2, Kc = 20 x 1(T19 J) the Newton black film ruptures. 

The stability of the undulating films depends not only on the difference between the corresponding local minimum of the enthalpy and the adjacent maximum but also on the shape of the enthalpy in the vicinity of the minimum. The thermal fluctuations affect especially the stability of the Newton black films, because their spatial confinement is stronger. The confinement of the fluctuating interface can drive the Newton black films either to the common black film or to rupture. The present analysis indicates that the fluctuations of the interfaces, while decreasing the stability of the film, lead to larger thicknesses, in agreement with experiment. 

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. 

Several investigations were carried out to study the above transitions from CF to common black film, and finally to Newton black film. For sodium dodecyl sulphate, the common black films have thicknesses ranging from 200 nm in very dilute systems to about 5.4 nm. The thickness depends heavily on the electrolyte concentration, while the stability may be considered to be caused by the secondary minimum in the energy distance curve. In cases where the film thins further and overcomes the primary energy maximum, it will fall into the primary minimum potential energy sink where very thin Newton black films are produced. The transition from common black films to Newton black films occurs at a critical electrolyte concentration which depends on the type of surfactant... 

At high electrolyte concentrations the films become so thin that they loose ability to reflect light there are the so-called common black films. In addition to that, an increase in electrolyte concentration results in a decrease of the height of potential barrier which preserves the film in the state of this metastable equilibrium, i.e., film stability decreases. Thermal oscillations of interface, i.e., the Mandel shtam waves (See Chapter VI, 1), help the system to overcome a potential barrier. If other stabilizing factors are absent, such (local) overcoming of potential barrier results in film rupture. 

In the case of films with high stability the overcoming of potential barrier does not result in a rupture of film, but leads to another metastable state corresponding to the primary minimum (Fig. VII-10, point B). This results in the formation of a rather stable, very thin Newtonian black films [15]. The investigation of the nature of stability of black films is one of the central problems in colloid science nevertheless, at present there is no commonly accepted opinion concerning the nature of forces that are responsible for high stability of black films (see Chapter VIII). 

Fig. 2D.3. demonstrates that an equilibrium film, the so-called common black film can reach a critical thickness at which it ruptures due to surface disturbances. Vrij (1966) studied surface fluctuations theoretically on the basis of Mandelstam s theory and computer simulations. Newton black film rupture was studied experimentally and theoretically by Exerowa et al. (1982) and Exerowa Kachiev (1986). They assume the existence of vacancies in the film. The mobility of these vacancies is the mechanism which controls the film stability (Fig. 2D.7),... 

A recently measured disjoining-pressure isotherm of an isolated lamella is shown in Figure 8 for the surfactant sodium dodecyl sulfate (SDS) at 10-3 kmol/m3 in aqueous 0.01 kmol/m3 sodium chloride brine (65). A solid line connects the data points for three independent experimental runs, shown by various symbols. The negative, attractive portion of the isotherm between thicknesses of about 4 and 5 nm is not sketched because equilibrium measurements are not possible there. The measured isotherm indeed obeys the classic S-shape. Film meta-stability demands that the slope of the isotherm be negative (2<5, 72). For positive slopes, even the slightest, infinitesimal disturbance ruptures the film. Thus, the lamella in Figure 8 can exist only along the two repulsive branches near 4 nm and above 7 nm. The thicker branch or common black film arises from electrostatic overlap forces, and the inner branch or Newton black... 

This surface interaction causes an excess pressure normal to the film interfaces, called disjoining pressure, which is the sum of repulsive electrostatic (Tiei), attractive van der Waals (7ivdw)> 2nd repulsive steric pressures (Tist)-Adapted from these interactions, two different types of thin films can be distinguished common black films (CBFs), stabilized by Jtei and Newton black films (NBFs), stabilized by Jtst. 

The critical thickness at which the CF ruptures (due to thickness perturbations) fluctuates and an average value h may be defined. However, an alternative situation may occur as ha is reached and, instead of rupturing, a metastable film (high stability) may be formed with a thickness h < ha- The formation of this metastable fihn can be experimentally observed through the formation of islands of spots , which appear black in light reflected from the surface. This film is often referred to a first black or common black film. The surfactant concentration at which this first black film is produced can be 1-2 orders of magnitude lower than the c.m.c. 

A typical II(/i) isotherm is depicted in Fig. 17a. (The shape of the curve in Fig. 17a is discussed in Section VI.A.) One sees that the equilibrium condition, II = P can be satisfied at three points shown in Fig. 17a. Point 1 Corresponds to a film which is stabilized by the double-layer repulsion sometimes such a film is called the primary film or common black film. Point 3 corresponds to unstable equilibrium and caimot be observed experimentally. Point 2 corresponds to a very thin film which is stabilized by the short-range repulsion such a film is called the secondary film or Newton black film. Transitions from common to Newton black films are often observed with foam or emulsion films [237-240]. 

The schematic of a soap film disjoining pressure isotherm is shown in Figure 2.12. It is important to note that thermodynamically metastable films can exist only in regions with negative slopes. Hence, the region with positive slope separates metastable regions between the common black films ( 50 nm) and the Newton black films ( 4 nm). Basically, common black films stability is due to the electric double-layer forces, while the Newton black films should be the results of entropic forces. 

Two types of black films can be distinguished [754, 755] common black films, which are stabilized by electrostatic double-layer repulsion, and Newton black films, which are stabilized by short-range forces. Common black films are 6-30 nm thick. Newton black films basically consist of a bilayer of surfactant. They have a defined thickness of 4—5 nm. For lipids, it is slightly larger [756]. If we consider a Newton black film formed from an oil phase in water, we are left with a lipid bUayer that still contains some oil molecules. Such films are extremely good electric insulators and are used in electrophysiological studies of membrane proteins [757]. 

The stages of foam film thinning just described are typical for aqueous solutions of most low-molecular-mass surfactants, and the time scale of the process is approximately the same the film thickness becomes on the order of 1 (Jim in a few seconds, about 1 min is needed for thinning of the film down to about 100 mn, and 2 to 4 min are needed until the final equilibrium film thickness is established. The main difference between the various systems is in the number of the stepwise transitions, which depends strongly on the smfactant concentration. Close to the cmc, when the volume fraction of the micelles is low, either there is only one transition from a common black film to a very thin Newton black film or there are no transitions at all because the equilibrium film thickness corresponds to a common black film, stabilized by electrostatic or steric forces. However, when the surfactant concentration is well above the cmc, up to five to seven transitions are... 

The recombination of surface charges with ions and the corresponding increase in repulsion also has consequences regarding the stability of common and Newton black films. However, in the latter case, the repulsive thermal undulations of the films also play a role [7.3]. As revealed by experiment and intuitively expected, an increase in temperature (which in turn increases the thermal undulations) decreases the stability of the films. In contrast, the insight provided by the traditional approach suggests that the increase in thermal undulations should lead to a larger Hel-frich repulsion, hence to a more stable system. However, the treatment of this repulsion by considering the membrane as composed of pieces of a suitable area implies that an increase in temperature decreases the area of the pieces, hence increases the probability of film rupture [7.3]. 

The effect of electrolyte concentration on the transition from common to Newton black films and the stability of both types of films are explained using a model in which the interaction energy for films with planar interfaces is obtained by adding to the classical DLVO forces the hydration force. The theory takes into account the reassociation of the charges of the interface with the counterions as the electrolyte concentration increases and their replacements by ion pairs. This affects both the double layer repulsion, because the charge on the interface is decreased, and the hydration repulsion, because the ion pair density is increased by increasing the ionic strength. The theory also accounts for the thermal fluctuations of the two interfaces. Each of the two interfaces is considered as formed of small planar surfaces with a Boltzmannian distribution of the interdistances across the liquid film. The area of the small planar surfaces is calculated on the basis of a harmonic approximation of the interaction potential. It is shown that the fluctuations decrease the stability of both kinds of black films. 

Such a relationship between Cm and the state of the adsorption layer is also found for CBFs (see Chapter 3). However, no quantitative link between Cm and the stability of these films has been found. As far as in the formation of black spots the adsorption layers at film surface plays a significant role, the clarification of the decrease in surface tension Act with the surfactant concentration is also important. Being a characteristic of surfactants Cm is in agreement with the commonly used quantity Act but is also related to the film properties. It is also important that Cm is in correlation with foam stability and, thus, is a more precise, suitable and physically better grounded characteristic. Such a correlation has been found for aqueous emulsion films [69,70]. 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

Although the two black films are common to many soap films it is possible to have only one stable black region. The stability of the black film will depend on the molecular forces present in the film. This will differ for different surfactant molecules and hence for different soaps. It is also possible to... 

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