Adsorbed layer, equilibrium surface aggregation

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

Some consequences which result from the proposed models of equilibrium surface layers are of special practical importance for rheological and dynamic surface phenomena. For example, the rate of surface tension decrease for the diffusion-controlled adsorption mechanism depends on whether the molecules imdergo reorientation or aggregation processes in the surface layer. This will be explained in detail in Chapter 4. It is shown that the elasticity modulus of surfactant layers is very sensitive to the reorientation of adsorbed molecules. For protein surface layers there are restructuring processes at the surface that determine adsorption/desorption rates and a number of other dynamic and mechanical properties of interfacial layers. 

The adsorbed layer is the scene of various interesting phenomena, like re-orientation of the adsorbate molecules, co-adsorption, polylayer formation, surface aggregation, adsorption of oligomers and, finally, surface phase transformations. All these phenomena can be treated within the frames of either the STE model or the models based on the LBS approach in precisely the same way We express the equilibrium equations first in terms of chemical potentials and next we introduce into these equations the expressions of the chemical potentials given by Eqs. (13) and (14). In some cases certain modifications are needed, as discussed below. 

Note that in the inset we may observe that the capacity in the region between the two peaks is not symmetrical indicating transformations of the surface aggregates. Models for transformations of the surface aggregates have been developed and certain experimental features have been explained. However, we should keep in mind that the surface aggregation is usually governed by slow kinetics and this makes it difficult to determine precisely the equilibrium properties of the adsorbed layer and compare them with model predictions. 

Until recently, the fast rate at which a surfactant layer forms at the solid-liquid interface has prevented accurate investigation of the adsorption process. As a result, the mechanism of surfactant adsorption has been inferred from thermodynamic data. Such explanations have been further confused by misinterpretation of the equilibrium morphology of the adsorbed surfactant as either monolayers or bilayers, rather than the discrete surface aggregates that form in many surfactant-substrate systems.2 However, the recent development of techniques with high temporal resolution has made possible studies of the adsorption, desorption,25>38,4i,48-6o exchange rates of surfactants. In this section, we describe the adsorption kinetics of C ,TAB surfactants at the silica-aqueous solution interface, elucidated by optical reflectometry in a wall-jet flow cell. The adsorption of C jTAB surfactants to silica is the most widely studied system - and hence the adsorption kinetics can be related to the adsorption process with great clarity. For a more thorough review of adsorptions isotherms, the t5q)es of surfactant structures that form at the solid-liquid interface, and the influence of these factors on adsorption, the reader is directed to Reference 24. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

The adsorption of surface active molecules at an interface is a dynamic process. In equilibrium the adsorption flux to and desorption flux from the interface are in balance. If the actual surface concentration is smaller than the equilibrium one, T < To, the adsorption flux predominates, if r > To, the desorption flux prevails. If processes happen in the adsorption layer, such as changes in the orientation or conformation of adsorbed molecules, or the formation of aggregates due to strong intermolecular interaction, additional fluxes within the adsorption layer have to be considered. 

FIGURE 16.2 Surfactant molecules at a polar surface self-assemble into aggregates, even at very low surfactant concentration. A monolayer of surfactants, where hydrophobic layers with the surfactant head group facing the surface and the hydrophobic tail facing the aqueous phase, is not formed at the surface, as was earlier believed. The system is cetyl trimethyl ammonium bromide (CTAB), adsorbed on mica, in equilibrium with the bulk concentrations of 10 M, 10" M, and 10 M. (From Sharma, B. G., Basu, S., and Sharma, M. M. 1996. Langmuir 12 6506. With permission.)... 

The fraction of a surface that is covered by surface micelles depends on the shape of the micelle, so wettability should influence the shape of the adsorbed aggregate. When one considers a series of adsorbed aggregates in order of decreasing curvature (e.g., spherical micelle, cylindrical micelle, bilayer) the least ciuved aggregate (the flat layer) will generally cover the most solid surface [40]. Thus, adsorbed micelles should be perturbed from their equilibrium solution geometry to a less cmved geometry at a hydro-phobic surface. 

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