Multilayer adsorption, polymer monolayers

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

In adsorption from solution, physisorption is far more common than chemisorption, although the latter is sometimes possible. Solute adsorption is usually restricted to a monomolecular layer, since the solid-solute interactions, although strong enough to compete successfully with solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers, because the interaction is screened by the solvent molecules. Thus, multilayer adsorption has only rarely been observed in a number of cases, and identified, when the number of adsorbate molecules exceeds the number of mono-layer molecules possible on the total adsorbent surface area. However, this analysis cannot be applied to polymer adsorption, because it is generally impossible to determine the surface area of a monomolecular layer of a polymer adsorbed flat on the solid surface. This is because the adsorbed polymer can only be anchored to the surface at a few points, with the remainder of the polymer in the form of loops and ends moving more or less freely in the liquid phase. 

An adsorption isotherm for suspensions of 20 vol% silica at pH = 3.7, wherein the zeta potential for the chosen silica is zero, is shown in Figure 4.6. It can be seen that initial additions of PVA are almost completely adsorbed on the silica particles and very little polymer remains in solution. When overall PVA concentration is increased, the particle surface tends to become saturated with adsorbed polymer and then higher amounts of residual polymer are left in solution. The initial plateau region of the adsorption isotherm is associated with approximately monolayer coverage of the particle surface. The second plateau is probably an indication of either the development of denser packing of polymer molecules in the adsorbed monolayer or the development of multilayer adsorption. 

Adsorbed surfactant layers present a different problem for theoretical analysis, especially if there is multilayer adsorption. Configurational entropy would not seem to be high in monolayers of molecules such as C12E6 which are vertically adsorbed and close-packed at the interface. Further problems arise when considering additives in the bulk phase which are incompatible with the stabilizing molecules under some conditions. As far as we are aware no one has considered this problem with surfactants as stabilizers. However, the effect of free polymer on the stability of sterically stabilized dispersions in which a polymer is used as stabilizer has been considered [41]. When two dilute polymers in the same solvent are mixed, they are, as a rule incompatible and will exhibit phase separation. Addition of free polymer (type i) to a polymer (type ii)-stabilized dispersion is likely to lead to instability [42]. However, it has been found, for... 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Ruths, J., Essler, F., Decher, G., Riegler, H. (2000). Polyelectrolytes I polyanion/polycation multilayers at the air/ monolayer/water interface as elements for quantitative polymer adsorption studies and preparation of hetero-superlattices on solid surfaces. Langmuir 16 8871-8. 

The consecutive layer-by-layer adsorption of polyanions and polycations built on glass or silicon wafers first (by chemisorption) with a monolayer of positively charged poly(ethyleneimine) (PEI) was recently reported [47]. Analogous hybrid multilayer architectures were prepared in which polycationic polymers... 

A key application for these materials involves the creation of electrostatic superlattices by layer-by-layer assembly [135-138]. The technique involves the sequential adsorption of polycationic and polyanionic monolayers from aqueous solutions to form nanoscale multilayer polymer films of controlled thickness. Such film architectures can be manipulated to achieve unique physical properties, and orchestrated for the construction of a range of devices. For example, well-characterized organic/organo-metallic polymer electrostatic superlattices have been prepared by alternate adsorp-... 

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