Adsorbed layer charged surfactants

The fits of experimental data to a Langmuir (or another) adsorption isotherm does not constitute evidence that adsorption is the actual mechanism that accounts for the loss of the sorbate from the solution. Very frequently adsorption to a surface is followed by additional interactions at the surface, e.g., a surfactant undergoes two-dimensional association subsequent of becoming adsorbed or charged ions tend to repel each other within the adsorbed layer. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

Fundamental investigation of the system at the molecular level. This requires investigations of the structure of the solid/liquid interface, namely the structure of the electrical double layer (for charge-stabiUsed suspensions), adsorption of surfactants, polymers and polyelectrolytes and conformation of the adsorbed layers (e.g., the adsorbed layer thickness). It is important to know how each of these parameters changes with the conditions, such as temperature, solvency of the medium for the adsorbed layers, and the effect of addition of electrolytes. 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

Nature of the interactions between the surface and the adsorbed solute The structure and orientation of the adsorbed layer depend on the relative strength of the interaction between the surfactant and the surface and the surfactant association behavior in the bulk solution. An example of such an interaction in aqueous systems is the electrostatic interaction between the charged surface sites and the charged surfactant molecules. 

The polyelectrolyte AM-MAPTAC-1 has only 1% of the segments charged. In this case no swelling of the preadsorbed layer on the mica surface is observed when SDS is added to the solution. The data obtained for an SDS concentration of 0.1 cmc are essentially the same as observed prior to the addition of SDS (Figure 23) and are dominated by a steric repulsion. We conclude that hardly any SDS is present in the adsorbed layer. However, when the SDS concentration is increased to 0.3 cmc, the range of the steric force is less than at the lower SDS concentration. A further increase in surfactant concentration results in a progressive decrease in the range of the... 

It has been established that the order in which polyelectrolyte and salt is added affects the state of the adsorbed polyelectrolyte layer for prolonged times [74,86]. It is thus no surprise that the experimental pathway also influences the character of adsorbed layers consisting of oppositely charged polyelectrolyte and surfactant [69]. For instance, let us compare the surface forces generated by adsorbed PCMA-SDS complexes at identical final bulk solution composition, 1 cmc SDS and 1 X 10 4 M KBr see Figure 38. In one case the polyelectrolyte was first adsorbed onto the mica surfaces from... 

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