Adsorbed film /layer close-packed

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Thin films of materials play an important role in modern electronics. Langmuir-Blodgett films (Mort, 1980) are prepared from surface-active molecules that adsorb at the surface of an aqueous solution. The surface is gently compressed to produce a close-packed monolayer, which can be coated on appropriate substrates by dipping at constant surface pressure. The layers so produced are of a precise thickness, and repeated dipping gives multilayers. For example, diacetylenes and other polymerizable... 

Pt surfaces tend to restructure into overlayers with an even higher density of Pt atoms than the close-packed (111) surface [21]. The Pt atoms are closer to each other on the reconstructed surfaces than in the (111) surface. The overlap matrix elements and hence the bandwidth are therefore larger, the d bands are lower and consequently these reconstructed surfaces bind CO even weaker than the (111) surface. The reconstructed Pt surfaces are examples of strained overlayers. The effect of strain can be studied theoretically by simply straining a slab. Examples of continuous changes in the d band center and in the stability of adsorbed CO due to strain are included in Figure 4.10. The effect due to variations in the number of layers of a thin film of one metal on another can also be described in the d band model [22,23]. 

The formation and stabilization of 0/W emulsions prepared with mixed emulsifier systems has been extensively investigated. However, the mechanisms proposed differ greatly. One of the primary hypotheses attributes the enhanced stability to the formation of a molecular "complex" or layer at the oil/water interface (8-11). The mixture of emulsifier types increases the packing density of the adsorbed interfacial film. Several investigators have shown that more closely packed complexes produce more stable emulsions (9,12-14). Friberg, et al. (15-17) have attributed the enhanced stability of mixed emulsifier emulsions to the formation of liquid crystals at the oil/water interface, which reduce the van der Waals attractive forces. 

Gibbs equation. The adsorption process involves the transport of molecules from the bulk solution to the interface, where they form a specially oriented monomolecular layer according to the nature of the two phases. When a Gibbs monolayer forms, it does not necessarily mean that the molecules are touching each other in this monolayer. Instead, if the anchoring from the sub-phase molecules is weak, the molecules may move freely in the two-dimensional interfacial area. Thus, the physically measurable monolayer area is sometimes much larger than the close-packed area where all the molecules touch each other. When any monolayer is fairly well populated with adsorbed molecules, it exerts a lateral spreading (film) pressure, it, which is equal to the depression of the surface tension (see Section 5.5.2). 

The presence of a closely packed layer of adsorbed molecules at a polymer surface will have a marked effect on many of its surface properties, such as friction, adhesiveness, and wettability. These properties of an adsorbed film have already foimd some application in industry. Allan [1] has demonstrated that small amounts of oleylamide incorporated in polyethylene foil will diffuse to the surface of the foil and greatly reduce the friction and adhesion between sheets of the plastic. A committee from the Piedmont Section of the American... 

Isolated hydrophobic particles resting at one surface of a thin aqueous film can cause the film to rupture (depending on 6) when the second surface engages the particle, causing the particle to bridge the film. For this reason, small hydrophobic particles can act as antifoams or foam breakers. Commercially, antifoams are often formulations in which hydrophobic particles are dispersed in a mineral or a silicone oil. Particles adsorbed in close-packed layers on droplet interfaces in an emulsion can stabilize the emulsion even in the absence of surfactant. Indeed, in some respects monolayers of particles behave similarly to adsorbed smfactants. Thus, hydrophobic particles [which are akin to surfactants with low hydrophile-Upophile balance (HLB)] can stabilize water-in-oil emulsions, whereas hydrophilic particles can be expected to stabilize oil-in-water emulsions (just as high-HLB surfactants do). In the first part of the chapter aspects of the current state of imderstanding of the effects that adsorbed particles have on foam and emulsion stabihty are reviewed. 

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