Surfactants multilayer formation

Fig. 20. Multilayer formation by adsorption, using bifunctional silane surfactants as monolayer building units [193]...

The initial step of any multilayer formation model is the surface overcharge as a result of one polyelectrolyte layer adsorption. Only in this case the adsorption of the second oppositely charged polyion/surfactant ion layer is possible [9], In our work the negatively charged surface... 

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

From surfactant molecules it is known that the repeated vertical dipping of a substrate through a floating monolayer of these molecules leads to the formation of an LB multilayer on the substrate. In principle, the same procedure should also allow the preparation of multilayers of latex particles. In Figure 8b, the preparation of a particle bilayer is schematically indicated multiple repetition should result in the formation of an LB multilayer of particles. However, if one tries to realize this concept, one immediately gets into difficulties, because the contact of the particles with the underlying substrate is very poor, and the already deposited particle layer tends to detach from the surface when the substrate is dipped into... 

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

SA monolayers spontaneously form, by definition, upon the immersion of a substrate into an organic solution of a suitable surfactant. The method is attractive since it avoids the complex mechanical manipulation required for making LB films and it is amenable, at least in principle, to scale-up. Conditions for the reproducible formation of highly ordered, well-packed, and stable monolayers and multilayers have only been established during the last decade. 

The first report on the construction of a SA multilayer appeared in 1983 [193]. A terminally bifunctional surfactant, 15-hexadecenyltrichlorosilane, was the initial building block for the formation of a SA monolayer. The trichloro-silane functionality reacted, as did OTS, with both of the hydroxyl groups on the substrate surface and with adsorbed water to form a network of Si -O-Si bonds in the SA monolayer. Conversion of the terminal double bonds to hydroxyl group functionality allowed the chemisorption of a new layer of surfactants to produce a SA bilayer (Fig. 20) [193]. The process could be repeated to form subsequent multilayers. 

Composite cast multibilayers provided a route to the formation of multilayer, two-dimensional polymer networks [443, 445]. This method utilized the following steps (i) ultrasonic dispersal of 15 mM of the dialkylammonium surfactant, 35, and 15 mM of the bisacrylate monomer. 36 (ii) addition of... 

Our understanding of the influence of competitive adsorption on emulsion stability is less secure. Recent work has identified several marked differences between the adsorbed layer properties atair/water and oil/water interfaces (e.g., multilayer versus monolayer formation). Advancing our knowledge of the stabilization of emulsions by protein merits further investigation, since emulsions comprise a major sector of processed foods. If competitive adsorption of surfactants influences the stability of protein emulsions in a similar manner to foams, use of the strategies outlined above may be appropriate for controlling destabilization. 

Synthetic surfactants and polymers are probably most often used to modify the characteristics of a solid surface, i.e., they function at the solid - liquid interface, such as in the processes of detergency, lubrication, or the formation of adhesive bonds. The performance of modem FT - IR spectrometers is such that many new applications to the characterization of the solid - liquid interface, particularly in kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated total reflectance (ATR) techniques have been applied to "wet" interfaces, even the air - water interface, and have figured prominently in recent studies of "self -assembled" mono - and multilayers. 

The formation of multilayer structure can be carried out by several ways 1) adsorption on liquid/liquid interface - Langmuir-Blodgett films [5,6] 2) adsorption on solid/liquid interface - alternate adsorption of oppositely charged polyelectrolytes (PE) and surfactants on flat surfaces or spherical particles [7,8], To control the process of multilayer systems formation, it is necessary to under-... 

Because of the stronger screening In condensed media, monolayer adsorption is much more common than in gas adsorption, where the formation of multilayers is the rule, except at very low pressures. Notable exceptions are strongly associating adsorbates (like surfactants, but even with these adsorption usually remains limited to at most a bilayer) and liquid mixtures close to demlxlng conditions. 

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

Formation of lamellar liquid crystalline phases at the O/W interface This mechanism, as suggested by Friberg and coworkers [37], proposed that surfactant or mixed surfactant film can produce several bilayers that wrap the droplets. As a result of these multilayer structures, the potential drop is shifted to longer distances, thus reducing the van der Waals attractions. A schematic representation of the role of Hquid crystals is shown in Figure 10.32, which illustrates the difference between having a monomolecular layer and a multilayer, as is the case with hquid crystals. 

Cationic surfactants adsorb strongly on clay surfaces by cation exchange. The fatty tails of these adsorbed surfactants impart oil-wetness to the clay surfaces and shield the clays from direct contact with water. This shielding has an obvious stabilizing effect however, this change in wettability often results in undesirable side effects, such as a decrease in oil relative permeability. Moreover, because of the possibility of multilayered adsorption (formation of surface micelles), a high surfactant concentration is required to satisfy the cation exchange capacity of the clays, which can make such treatments rather expensive. 

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