Multilayers layer mixing

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). 

The irradiation effect on the PL spectrum of PVK-C60 form nanocomposite films was also studied [281]. The physical jet deposition technique [282] was applied to fabricate the PVK-C60 form composite films, including multilayer and mixed films. Multilayer PVK-C60 form films were fabricated by alternatively depositing PVK and C60 materials on the fused silica substrate with nearly equal layer thickness. It was found that lengthy irradiation with a 532 nm picoseconds laser beam steadily increased the intensity of PL emission of PVK at 630 nm in the mixed film, reflecting that the population of the triplet state of the C60 molecule has a great effect on the excitation transfer process in the nanocomposite films. 

The inner structure of polyelectrolyte multilayer films has been studied by neutron and X-ray reflectivity experiments by intercalating deuterated PSS into a nondeut-erated PSS/PAH assembly [94, 99]. An important lesson from these experiments is that polyelectrolytes in PEMs do not present well-defined layers but are rather interpenetrated or fussy systems. As a consequence, polyelectrolyte chains deposited in an adsorption step are intertwined with those deposited in the three or four previous adsorption cycles. When polyelectrolyte mobility is increased by immersion in NaCl 0.8 M, the interpenetration increases with time as the system evolves towards a fully mixed state in order to maximize its entropy ]100]. From the point of view of redox PEMs, polyelectrolyte interpenetration is advantageous in the sense that two layers of a redox polyelectrolyte can be in electrochemical contact even if they are separated by one or more layers of an electroinactive poly ion. For example, electrical connectivity between a layer of a redox polymer and the electrode is maintained even when separated by up to 2.5 insulating bUayers [67, 101-103]. 

Additional improvements can be achieved through the use of multilayers (based on different overlaid films). Such combination of the properties of different films has been documented with bilayers of Nation/CA (14) and Nafion/collagen (29). The former allows selective measurements of the neurotransmitter dopamine in the presence of the slightly larger epinephrine and the anionic ascorbic acid (Figure 5). In addition to bilayers, mixed (composite) films, such as PVP/CA (75) or polypyrrole/Eastman Kodak AQ (30) layers can offer additional permselectivity advantages, such composites exhibit properties superior to those of their individual components. Also promising are sensor arrays, based on electrodes coated with... 

For a solution-processed active interface, in which either the gate dielectric material is deposited from solution on to a solution-processible semiconducting material or vice versa, it is critical to avoid dissolution or swelling effects during deposition of the upper layer, which can lead to interfacial mixing and increased interface roughness. The preferred approach to achieve this is to choose orthogonal solvents for the deposition of the multilayer structure [23]. 

This approach is a purely thermodynamic approach in the sense that locally, the polymer is at thermal equilibrium. The density inside the layer is homogeneous and is equal to the complex density cC0mp- One of the important experimental results is that, in the multilayers, each layer strongly intermixes with its neighbors but that it keeps its identity in the sense that it does not mix with layers far apart. This freezing of the structure must be due to an extremely slow diffusion between the layers that our model is not able to study. 

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