Thin Newton black

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. 

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... 

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... 

J.A. De Feijter and A. Vrij Contact Angles in Thin Films, n. Contact Angle Measurements in Newton Black Soap Films. J. Colloid Interface Sci. 64, 269... 

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. 

In foam stability, gas bubbles and the liquid films between them, would be stabilized by the repulsive forces created when two charged interfaces approach each other and their electric double layers overlap. The repulsive energy VR for the double layers at each interface in the thin film is still given by Eq. (5.1) where H is the film thickness. Here also, for extremely thin films, such as the Newton black films, Bom repulsion becomes important as an additional repulsive force. 

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... 

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. 

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. 

FIGURE 5.36 Main stages of formation and evolution of a thin liquid film between two bubbles or drops (a) mutual approach of slightly deformed surfaces (b) at a given separation, the curvature at the center inverts its sign and a dimple arises (c) the dimple disappears, and eventually an almost plane-parallel film forms (d) due to thermal fluctuations or other disturbances the film either ruptures or transforms into a thinner Newton black film (e), which expands until reaching the final equilibrium state (f). 

After the entire film area is occupied by the Newton black film, the film radius increases until it reaches its equilibrium value, R = (Figure 5.36f). Finally, the equilibrium contact angle is established. For more details about this last stage of film thinning, see part IV.C of Reference 164. 

A third important force at very small separation distances where the atomic electron clouds overlap causes a strong repulsion, called Born repulsion. Therefore, in extremely thin films such as the Newton black films, Born repulsion becomes important as an additional repulsive force. 

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). 

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. 

Figure 2.5. The general form of the force curves (pressure J [ versus film thickness h) for a thin film containing fairly low concentrations of surfactant NBF, Newton black film CBF, common black film CF, common film...

If now the capillary pressure increases to p so that it exceeds the maximum at C in the isotherm then the film will drain to another unstable region and will jump from C to form another stable film at D. Equilibrium will again be established when the film thins to thickness where pf = II la ( 2) Such films are extremely thin, being little more than bilayer leaflets of thickness in the region of 5 nm. They are usually termed Newton black films. Further increases in capillary pressure to pf > n j (/ 3) should simply lead to film rupture. However, thickness fluctuations cannot exist in such thin films. It has therefore been argued by Kashchiev and Exerowa [56] that rupture occurs by nucleation of holes caused by thermal fluctuations in these films. 

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