Surface excess, polymer adsorption layers

According to [15], the adsorption of PS on aerosil surface OH groups involves participation of benzene ring n electrons. Only each third aromatic ring undergoes interactions with the silica surface, therefore the adsorption layer has a bulk structure. 0.95mg/m2 of polystyrene is required for the formation of a monolayer on the surface of Si02 particles [15]. For the PS concentrations used in the present work, this corresponds approximately to C=1.5 g/100 mL, whereas the PS concentration of C=5,0 g/100 mL corresponds to about fourfold excess of polymer, and the PS concentration of C=0.25 g/100 mL is insufficient for the formation of a monolayer on the surface of SiC>2 particles. 

The surface excess layer for a polymer solution is closely related to the situation of polymer adsorption at a polymer/solid interface and, as described in more detail in section 5.2, both mean-field and scaling descriptions of the organisation of such layers have been put forward. A mean-field description gives... 

The same principles of total internal reflection can also be used to excite fluorescent chromophores attached to the polymer chain (9). The emitted fluorescence can be used to estimate the surface excess. In addition to subtracting the background signal, fluorescence quenching in the adsorbed layers may also have to be considered while estimating the adsorbed amount. The disadvantage of this technique is that the polymer chains have to be labeled with fluorescence molecules and that might influence the adsorption process. 

In analogy to the adsorption problem, one can characterize the wetting transition by the surface excess of polymer. If the polymer only partially wets the surface, O <0< 180 , there is only a miaoscopically thin enrichment layer of polymer at the surface and the surface excess... 

Most spraying processes work under dynamic conditions and improvement of their efficiency requires the use of surfactants that lower the liquid surface tension yLv under these dynamic conditions. The interfaces involved (e.g. droplets formed in a spray or impacting on a surface) are freshly formed and have only a small effective age of some seconds or even less than a millisecond. The most frequently used parameter to characterize the dynamic properties of liquid adsorption layers is the dynamic surface tension (that is a time dependent quantity). Techniques should be available to measure yLv as a function of time (ranging firom a fraction of a millisecond to minutes and hours or days). To optimize the use of surfactants, polymers and mixtures of them specific knowledge of their dynamic adsorption behavior rather than equilibrium properties is of great interest [28]. It is, therefore, necessary to describe the dynamics of surfeictant adsorption at a fundamental level. The first physically sound model for adsorption kinetics was derived by Ward and Tordai [29]. It is based on the assumption that the time dependence of surface or interfacial tension, which is directly proportional to the surface excess F (moles m ), is caused by diffusion and transport of surfeictant molecules to the interface. This is referred to as the diffusion controlled adsorption kinetics model . This diffusion controlled model assumes transport by diffusion of the surface active molecules to be the rate controlled step. The so called kinetic controlled model is based on the transfer mechanism of molecules from solution to the adsorbed state and vice versa [28]. 

Evanescent wave induced fluorescence (EWIF) and small angle neutron scattering (SANS) experiments have been performed at the polymer solution-solid interface, in the case of adsorption. The interfacial layer is characterized by its total surface excess T and by the monomer concentration profile (2) in the direction normal to the interface. 

The adsorption from solution of polymers has been studied extensively. The amount of polymer adsorbed usually reaches a limiting value as the concentration of polymer in solution is increased, but this value is usually well in excess of that which would be expected for a monomolecular layer of polymer adsorbed flat on the solid surface. This suggests that the adsorbed polymer is anchored to the surface only 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 phase179. 

The simplest method for fabricating PMEs from preformed polymers is based on adsorption from solution by dipping the electrode in a polymer solution. The reproducibility of this method is questionable however because of the need to shake or wipe excess solution from the surface before the solvent evaporates. More reproducible films can be prepared by dropping a measured aliquot of a polymer solution onto the electrode surface, followed by the slow evaporation, preferably in a solvent-saturated chamber, of the solvent. For thin polymer films, the technique of spin coating can produce more homogeneous and reproducible layers. This technique involves the drop coating and solvent evaporation of the polymer solution on a rapidly rotating electrode. 

A broad review of the effects of macromolecular compounds in disperse systems has been presented by Heller (311). He discusses the adsorption characteristics of polymer. molecules which on the one hand, are adsorbed at the surface with a terminal segment only, with the remainder of the chain protruding from the surface radially, and on the other hand, are adsorbed lying flat with all segments attached to the surface. Both situations can occur, of course, depending on the nature of the polymer and how much of an excess over the amount required to form a flat monomolecular layer is present. 

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