Aggregation of adsorbed molecules

For the description of mixed monolayers, the choice of the dividing surface proposed by Lucassen-Reynders (see Eqs. 2.18, 2.19) is superior [58, 59]. The results obtained using the Butler equation (2.7) and Lucassen-Reynders dividing surface model for the description of mixed monolayers of non-ionic or ionic surfactants, and proteins assuming reorientation or aggregation of adsorbed molecules were presented and discussed in overviews [58, 59]. In this chapter, these concepts are discussed and further developed. 

In this paragraph we give examples for each of the mentioned cases, starting with a simple surfactant system that follows essentially the classical diffusion model. Then the effect of reorientation and aggregation of adsorbed molecules will be discussed by demonstrating experimental dynamic surface tension data. The adsorption dynamics of ionic surfactants has not been studied systematically so that these systems cannot be presented here extensively. Also the dynamics of adsorption at the interface between two liquids is at the beginning and we present here an impressive example. 

When the composite-matrix is formed with a polystyrene solution as a dispersion medium, the self-assembly of silica particles is influenced by the adsorption of macromolecules on their surface. During adsorption, both solitary macromolecules and their aggregates transfer simultaneously onto the adsorbent surface. Depending on solution concentration, not only the conformation of adsorbed molecules but also the number and size of macromolecular aggregates in the solution change on adsorption. This leads to the formation of complex-shaped structures, which are linked by a system of nonvalent interactions and consist of polymeric-inorganic blocks[8,14] this is of interest in the preparation of a nanostructured medium (polystyrene-silica gel) as a precomposite for the fabrication of carbon structures in a matrix of silica particles. 

Another unique and specific feature of the interfacial reaction is the formation of aggregate of dye molecules, metal complexes, and other solvophobic molecules. As reported in many interfacial adsorption systems, the saturated interfacial concentration of usual molecules is of the order of 10 10mol/cm2, which can be attained even under an extremely low bulk phase concentration. This means that the liquid-liquid interface is ready to be saturated to form a two-dimensionally condensed state for the adsorbate. In solvent extraction process of metal ions, we used to find formation of some precipitate at the interface, which is called crud. The study of the interfacial aggregate is therefore important to know the real interfacial reaction as met in the industrial solvent extraction where rather concentrated solutes have to be treated. 

The rate of reduction of interfacial tension by proteins is determined by three consecutive or concurrent processes the diffusion of whole protein molecules or aggregates to and attachment at the interface spreading or unfolding of already adsorbed molecules on the interface molecular rearrangements and reconformations of adsorbed molecules. 

What is the specific feature in the reaction at the liquid/liquid interface The catalytic role of the interface is of primary importance in solvent extraction and other two-phase reaction kinetics. In solvent extraction kinetics, the adsorption of the extractant or an intermediate complex at the liquid/liquid interface significantly increased the extraction rate. Secondly, interfacial accumulation or concentration of adsorbed molecules, which very often results in interfacial aggregation, is an important role played by the interface. This is because the interface is available to be saturated by an extractant or mehd complex, even if the concentration of the extractant or metal complex in the bulk phase is very low. Molecular recognition or separation by the interfacial aggregation is the third specific feature of the interfacial reaction and is thought to be closely related to the biological functions of cell membranes. In addition, molecular diffusion of solute and solvent molecules at the liquid/liquid interface has to be elucidated in order to understand the molecular mobility at the interface. In this chapter, some examples of specific... 

Irregularities in dynamic surface tensions of adsorption layers of soluble surfactants were discussed by Lucassen-Reynders (1987) in terms of aggregation phenomena of adsorbed molecules. She gave a theoretical model for the frequency spectrum of surface dilational properties. 

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. 

As in the case of protein denaturation in solution, interfacial unfolding can trigger aggregation of adsorbed protein molecules. When it occurs, adsorption becomes really irreversible, in the same way as aggregation caused irreversibility of protein denaturation in solution. 

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 changes in the state of adsorbed molecules in the surface layer (reorientation, aggregation or cluster formation) have also a minor effect on the form of the equation which relates the surface pressure of mixed solutions with the corresponding values for the individual solutions. This will be demonstrated below for mixtures, where one component is able to form surface clusters. 

This chapter will also present a selected number of experiments representative for the various models discussed. This refers especially to surfactant systems where recently new phenomena have been observed and explained, i.e. the possibility of changes in the orientation of adsorbed molecules are alternatively the formation of aggregates at the interface. Adsorption kinetics as well as interfacial relaxation experiments will be reported and the results discussed in terms of the specific parameters of these new theories. This will include also some data on proteins as particular type of surface active molecules able to change their conformation at an interface and hence changing the molar area at the interface. 

In contrast to surface aggregation, changes in the molar area of adsorbed molecules can lead to an apparent enhancement of the adsorption rate. Thus, observed super-diffusion phenomena can be understood by considering changes in the molar surface area with changing surface coverage (cf. Fig. 4.6). Again, these systems are then quantitatively understood by a purely diffusion controlled model. 

The morphological features of nanostructured films have been correlated with the chain conformations in solution [13], for which a dummy atom model (DAM) was used. The AFM images in Figure 9.5 represented an off print of the solution conformation when molecules of emeraldine base POEA (POEA-EB) were adsorbed on the substrate. The size of the globules is comparable to the aggregates of polymer molecules in solution. Significantly, the adsorption mechanisms and film properties for polyanilines are altered by a mere change in pH [14], which has been exploited in sensors and other applications [15]. 

The fact that many surfactant systems cannot be adequately described by the Frumkin mode was the reason that other models have been derived. A comprehensive overview of such models was given recently elsewhere (Fainerman et al. 1998). We want to discuss two of the most recent models considering changes in orientation of adsorbed molecules and formation of two-dimensional aggregates (Fainerman et al. 2002). These new models are suitable to describe quite a number of surfactant adsorption layers much better than classical models do. 

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. 

As shown above, various such equations exist, such as the classical ones named Langmuir or Frumkin isotherm. However, it was also shown that peculiarities of surfactants in adsorption layers can be described quantitatively only if special models are used. Their impact on adsorption kinetics was reviewed recently (Fainerman et al. 1998) and found to be significant. While the reorientation of adsorbed molecules mimics an acceleration of the adsorption process, surface aggregation on the contrary apparently decelerates it. 

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