Film thinning stepwise

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. 

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... 

Wasan et al. (27-28) explained the process of stratification on the basis of a micelle-latticing structure model. In Figure 8 a schematic of the latticing model for film thinning is provided. By fluctuations in the structure of the micellar lamellae (i.e. the individual rows of micelles in Figure 8), the film can change its thickness by stepwise transitions, each of which are equal to the micellar-lamellae thickness. According to this model the number of transitions will depend upon the micelle concentration. 

Oil Configurations in Foams. In the presence of oil, the mechanisms of foam stability are more complex than without oil. Solubilized oil decreases the stability by accelerating the stepwise foam film thinning, as shown in the previous section. The effect of emulsified oil on foams is closely connected with the configuration of oil relative to the aqueous and gas phases. This configuration can be, in most cases, one of the following (Figure 22) ... 

Thin films spin-coated from diluted BMBF solution onto precleaned Si wafer were prepared with different thickness and different 5/molar ratio, as well. The films were stepwise annealed, fast cooled, and examined offline. Figure 11.20 shows the XR-curves obtained on copolymer samples with different sfMA concentration and film thickness. It gives an impression about the influences on the structure formation in thin films, that is, in confined space dimensions. Most important influences are the kind of substrate and surface, preparation, annealing and cooling conditions, as well as the film thickness. 

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... 

For a very thin liquid film, the value of 3 cannot be evaluated, and it should be replaced by a new parameter 3, using the generalized turbulent boundary-layer profile in an adiabatic flow as in Reference (Levy and Healzer, 1980). GF can be solved stepwise along the pipe until the G value goes to zero, where dryout occurs. This analysis was performed to compare the calculated g"rit with Wurtz data (Wurtz, 1978) and also to compare with the predictions by the well-known Biasi et al. correlation (1968), as shown in Figure 5.90. For the limited data points compared, the agreement was good. 

The polymer/SWCNT composites can be used as Scaffolds in tissue engineering. The donor-acceptor interactions can be used to assemble thin polymer/SWCNT films stepwise. This method also can be expended to more thermally and oxidatively stable polymer systems. For example, the P4VP/SWCNT films can be used as scaffolds for the synthesis of novel hybrid structures (Correa-Duaite et al., 2004). The polyethyl-enimine (PEI)-SWCNTs composites were used as a substrate for cultured neurons, and promoted neurite outgrowth and branching (Rouse et al., 2004). Correa-Duarte et al. (2004 Landi et al., 2005) reported that 3D-MWCNT-based networks are ideal candidates for scaffolds/matrices in tissue engineering. 

The 67-fold amplification obtained for polymer 3 is restricted by an inherent limitation of the wired in series design. As the exciton travels in a one-dimensional random walk process down the polymer chain, it has equal opportunity to visit a preceding or an ensuing receptor. This represents 134 random stepwise movements for 134 phenylene ethynylene units, and so much of the receptor sampling by the exciton is redundant. Increasing the efficiency of receptor sampling requires maximization of the number of different receptors that an exciton can visit throughout its lifetime. To achieve this end we extended the polymer sensor into two dimensions by use of a thin film and thereby increased the sensitivity. 

Displacement of the protein from the adsorbed layer in o/w thin films shows very different behavior from its a/w counterpart. Although displacement of protein from the o/w interfaces initiates at approximately the same solution composition (i.e., R = 0.1), there is little evidence for the stepwise displacement observed in the a/w thin films. This observation is further confirmation of the monolayer versus multilayer structure at the o/w and a/w thin films. The displacement of /3-lg has also been investigated in oil-in-water emulsions of n-tetradecane [46,47], In these reports it was shown that the protein was not completely displaced until R = 10, which was considerably higher than R = 1 - 2 in Figure 22. This will be discussed further below. 

The stepwise construction of self-assembled organic/inorganic multilayers based on multivalent supramolecular interactions between guest-functionalized dendrimers and nanoparticles and host-modified gold nanoparticles has been developed, yielding supramolecular LbL assembly (Fig. 13.8).12 Multilayer thin films composed of... 

The CdS/ZnO interface is of particular importance in Cu(In,Ga)Se2 thin film solar cells because it is used in the standard cell configuration (Fig. 4.2). A first experimental determination of the band alignment at the ZnO/CdS interface has been performed by Ruckh et al. [102]. The authors have used ex-situ sputter-deposited ZnO films as substrates. The interface formation was investigated by stepwise evaporation of the CdS compound from an effusion cell. Photoelectron spectroscopy revealed a valence band offset of A Vb = 1.2eV. An identical value of 1.18eV has been derived using first-principles calculations [103]. With the bulk band gaps of CdS and ZnO of 2.4 and... 

Preparation of metal oxide thin film by means of stepwise absorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts [13]. For example, Asakura et al. prepared one-atomic layer of niobium oxide by repeating chemisorption of Nb(OEt)5 on silica beads. The catalyst obtained by immobilizing platinum particles on a niobum oxide layer showed improved reactivity for hydrogenation of ethylene in comparison with... 

A stratification phenomenon in foam films from sodium oleate aqueous solutions has been described by Johonnott [314] and Perrin [318] in the beginning of the century. The stepwise thinning has been studied by many authors [e.g. 235,345], It became evident that this phenomenon is universal and has been observed also in emulsion films [346,347], asymmetric films of the air-water/oil type [348], films from latex suspensions [349] and liquid crystalline films [350],... 

Theoretical analysis of sheeting in the drainage of thin liquid films has been conducted in [359]. Sheeting dynamics and hole formation (i.e. black spot formation) was described by non-linear hydrodynamic stability analysis based on the equilibrium oscillatory structural component of disjoining pressure. The effect of stepwise thinning, accompanied by formation of holes , was described qualitatively. It is rather arguable whether the term holes for a black spot is appropriate since in 1980 holes in NBF were described as lack of molecules. The use the same term for two different formations is at least confusing. Besides, to have a hole in a CBF is almost as to have a hole in the sea water . 

The results depicted in curve 2, Fig. 3.75 and obtained from the experiments carried out at an imposed constant pressure of P = 5-103 Pa, higher than the pressure for a transition to a NBF (Fig. 3.75, curve 1), confirmed the above considered. Curve 2 clearly demonstrates that the film subjected to a pressure of 5103 Pa shows a typical stepwise spontaneous thinning up to NBF (bilayer). 

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... 

We have seen evidence of a similar structuring of micelles in thin foam and emulsion films containing C AOS, in the form of stepwise transitions or stratification phenomena (see Figure 5). 

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