Thin films common black

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. 

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 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]. 

This device was used in the study of the kinetics of common thin film thinning [16,106], in the determination of the critical thickness of rupture of macroscopic films having an area of about 1 cm2 as well as in the measurement of black film thickness [107]. In equilibrium films this technique does not give reliable results, since there are difficulties in the evaluation of the capillary pressure in the menisci. 

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. 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

Analogous tp(Ap) and H(x) relations were obtained for foams from non-ionic surfactants (Fig. 7.14). Here the Ross-Miles test cannot be used to distinguish foams with common thin and black films by H (the non-ionic surfactants form only one type of black films see Section 3.4). xp(Ap) dependence indicates that at low pressures (< 3103 Pa) the foam with common thin films has longer lifetime (curve 2) while in the range of higher pressures the foam with black films (curve 1) lives longer. The differences in Tp at Ap = 5-... 

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. 

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. 

Black Film Fluid films yield interference colors in reflected white light that are characteristic of their thickness. At a thickness of about 0.1 /xm, the films appear white and are termed silver films. At reduced thicknesses, they first become grey and then black (black films). Among thin equilibrium (black) films, one may distinguish those that correspond to a primary minimum in interaction energy, typically at about 5-nm thickness (Newton black films) from those that correspond to a secondary minimum, typically at about 30-nm thickness (common black films). 

Metal-plated articles are common in our society. Jewelry and tableware are often plated with silver. Gold is plated onto jewelry and electrical contacts. Copper is plated onto many objects for decorative purposes (Figure 21-5). Automobiles formerly had steel bumpers plated with thin films of chromium. A chrome bumper required approximately 3 seconds of electroplating to produce a smooth, shiny surface only 0.0002 mm thick. When the metal atoms are deposited too rapidly, they are not able to form extended lattices. Rapid plating of metal results in rough, grainy, black surfaces. Slower plating produces smooth surfaces. Tin cans are steel cans plated electrolytically with tin these are sometimes replaced by cans plated in a fraction of a second with an extremely thin chromium film. 

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. 

If the short-range repulsive disjoining pressure is large enough, the black foam films are stable. There are two types of black foam films common and Newtonian. While the common black films are the thicker type of black films (from about 5 to 20 nm in thickness), the Newtonian black (NB) films are bimolecular thin films (less than 5 mn in thickness). A mechanism of rupture of NB films is considered as a process of new phase nucleation in a two-dimensional system [105 108]. There exist in the film elementary vacancies (unoccupied positions of surfactant molecules) moving randomly, which associate to form clusters of vacancies called holes. A hole can grow up by fluctuations to a critical size and become a nucleus of a hypothetical two-dimensional phase of vacancies. Further spontaneous growth of the nucleus leads irreversibly to the rupture of the film. When the rupture of NB film is due to formation of holes in it by a nucleation mechanism, it has been shown that the mean film lifetime r depends on the monomer surfactant concentration C as ... 

---