Duplex films

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

Based on this low surface tension feature and the commonly observed insolubiUty of defoamers, two related antifoam mechanisms have been introduced (29) (/) The agent dispersed in the form of fine drops enters the Hquid film between bubbles and spreads as a duplex film. The tensions created by this Spreading lead to the mpture of the original Hquid film. (2) A droplet of the agent enters the Hquid film between bubbles, but rather than spreading produces a mixed monolayer on the surface. This monolayer, if of less coherence than the original film-stabilizing monolayer, causes destabilization of the film. 

The information given above should make it possible in general to predict the usefulness of x-ray methods in problems involving films. In principle, these methods should be useful occasionally when more than one film is present. The problems in such cases are complex rather than complicated. If these methods are applicable to a duplex film, for example, there will be three characteristic lines to be counted, and absorption effects in three regions to be considered. The three counts should, however, contain enough information in many cases to permit the drawing of valid conclusions. 

In the above considerations, the O/S interface was taken to be a clear-cut boundary between the oxide and the electrolyte. In reality, however, the outer part of the oxide is likely to be hydrated and penetrated by the electrolyte. Hence, the true O/S interface is likely to be withdrawn from the surface to a sufficient depth such that some oxide is left without any electric field imposed across it. This is especially true of thick porous oxide layers, but it can occur with compact layers as well. For example, Hurlen and Haug35 found a duplex film in acetate solution (pH 7-10), composed of a dry barrier-type part and a thicker hydrated part consisting of A1203 H20. Although the hydrated part becomes thinner with decreasing pH and seems to practically vanish at low pH, even a thickness of less than a nanometer is sufficient for the surface oxide to stay outside the electrochemical double layer. 

In conclusion, one can say that most anodic oxide films are of a duplex, or even triplex, character, with only the inner portion being composed of a pure anhydrous oxide. In the duplex films, the outer layer contains anions and often a degree of hydration. There could exist a third thin oxide layer at the surface, again with somewhat different properties, which may have a role in the kinetics of oxide growth. 

Mixed film theories (4-8) The essential feature of the mixed film theories is to consider the film as a liquid, two dimensional third phase in equilibrium with both oil and water, implying that such a monolayer could be a duplex film, i.e., one giving different properties on the water side than on the oil side (4). According to these theories, the interfacial tension Y is given by the expression,... 

The above concept of duplex film can be used to explain both the stability of microemulsions and the bending of the interface. Considering that initially the flat duplex film has different tensions (i.e., different values) on either side of it, then the deriving force for film curvature is the stress of the tension gradient which tends to make the pressure or tension in both sides of the curved film the same. This is schematically shown in Figure 1. For example if ir > ir on the flat... 

A thin (1 or 2 m thick) duplex film in actual contact with the reinforcement. 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Spread as a thin film, which may show interference colours, until it is uniformly distributed over the surface as a duplex film. (A duplex film is a film which is thick enough for the two interfaces -i.e. liquid-film and film-air - to be independent and possess characteristic surface tensions.)... 

Photocurrent spectra of the CU2O / CuO,Cu(OH)2 duplex film suggest a smaller band gap of the CuO layer with respect to the value of Cu20 [97], However, the investigation of a simple anodic Cu(II) oxide layer was not possible. All trials to prepare this layer without a Q12O contribution were not successful. 

Fig. 45. Band structure model for passive layers on Cu for increasing potential explaining quantitatively the formation of the duplex film and the transpassive behavior [86],...

Reduction of thin CU2O layers reproduces smooth terraces of the Cu surface, however, with multi-atomic steps in between. This result is further proof for the high surface mobility of Cu atoms on pure Cu(lll) terraces. Reduction of the thicker duplex film yields a rougher metal surface. During the rearrangement of several atomic layers smooth large terraces cannot be achieved. 

A similar situation arises for the Cu20 / CuO, Cu(OH)2 duplex film at more positive potentials. In this case the sub-band extends through both parts of the passive layer. At sufficiently positive potentials, even an electron transfer to the empty states of the conduction band becomes possible (Figs. 60c and d). 

The surface pressure, or outward spreading force per centimetre, of the interfacial gaseous film at the lower interface of the duplex film would be... 

Equation (6) does not take account of any lateral adhesive forces which may be present between the water-soluble groups in the lower interface of the duplex film, but does allow, approximately, for the area occupied by these groups. 

The vapour-expanded films have pressure-area curves with small values of — F0. At low values of the pressure the area becomes so large that the molecules have to lie nearly flat on the surface the thickness of the hydrocarbon part of the film thus becomes so small that there is much less opportunity for the chains to oscillate about the vertical direction than with the smaller areas of the liquid-expanded films. The hydrocarbon part of the film cannot therefore be considered as a liquid layer and the theory of duplex films tends to become inapplicable owing to the molecules being nearly fiat, the expanded state merges into the gaseous without abrupt change of slope. 

Figure 28 also shows data from a duplex film consisting of an inner barrier layer and an unsealed outer porous layer. Superposition of this data set with a data set from a sample with the barrier layer only suggests that the resistance of... 

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