Surfactants at interfaces

Based on the underlying physical chemistry of surfactants at interfaces, important features of foam stmcture, stabiHty, rheology, and their interrelationships can be considered as ultimately originating in the molecular composition of the base Hquid. 

The many remarkable physico-chemical properties of aqueous surfactant systems, as well as their numerous practical applications, can be referred to the tendency of the nonpolar groups to avoid contact with water at the same time as the polar part tends to be strongly hydrated. The adsorption of surfactants at interfaces between aqueous solutions and air, another liquid phase or a solid is one consequence of this, the extensive aggregation into various types of large aggregates termed micelles — from lat. micella meaning small bit — and liquid crystalline phases is another. 

Surfactants at Interfaces. Somewhat surprisingly, the successes described above in the in-situ studies of protein adsorption have not inspired extensive applications to the study of the adsorption of surfactants. The common materials used in the fabrication of IREs, thalliumbromoiodide, zinc selenide, germanium and silicon do, in fact, offer quite a range in adsorption substrate properties, and the potential of employing a thin layer of a substance as a modifier of the IRE surface which is presented to a surfactant solution has also been examined in the studies of proteins. Based on the appearance of the studies described below, and recent concerns about the kinetics of formation of self-assembled layers, (108) it seems likely that in-situ ATR studies of small molecules at solid - liquid interfaces ("wet" solids), will continue to expand in scope. 

The adsorption mechanisms of surfactant at interfaces have been extensively studied in order to understand their performance in many processes such as dispersion, coating, emulsification, foaming and detergency. These interfaces are liquid-gas (foaming), liquid-liquid (emulsification) and liquid-solid (dispersion, coating and detergency). 

As discussed in Section 2.2, surfactant has a tendency to adsorb at interfaces since the polar head group has a strong preference for remaining in water while the hydrocarbon tail prefers to avoid water. The surfactant concentration affects the adsorption of surfactants at interfaces. Surfactant molecules lie flat on the surface at very low concentration. Surfactant molecules on the surface increase with increasing surfactant concentration in the bulk and surfactant tails start to orient towards gas or non-polar liquid since there is not enough space for the surfactant molecules to lie flat on the surface. Surfactant molecules adsorb at the interface and form monolayer until the surface is occupied at which point surfactant molecules start forming self-assembled structures in the liquid (Section 2.3). 

Before going further into the adsorption of surfactants at interfaces, it is advisable to discuss the so-called electrical double layer at interfaces, since this is necessary for an understanding of the electrical aspects of adsorption. 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to understanding how these molecules act as stabilizers for suspensions and emulsions. Most basic theories on polymer adsorption and conformation have been developed for the solid/liquid interface (9). The same concepts may be applied for the liquid/liquid interface, with some modifications whereby some part of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modifications do not alter the basic concepts, particularly when one deals with the stabilization by these molecules. 

The adsorption of surfactants at interfaces is a time process. After the creation of a new surface the adsorption is zero and increases with time until reaching the equilibrium state. The main mechanism controlling this process is the diffusion of surfactants in the solution bulk. In this lesson the basic models will be discussed and the main physical parameters analysed. In particular, the type of adsorption isotherm plays an important role. On the basis of dynamic surface tensions the application of the theoretical models will be demonstrated in the subsequent paragraph. Besides complete solutions of the diffusion model, also approximate solutions exist. These models... 

The adsorption of surfactants at interfaces has been, and will be, discussed in specific contexts. However, surfactants also have a life of their own within... 

Proteins, naturally occurring macromolecular surfactants with amphiphilic nature, are adsorbed onto interfaces, thereby affecting the physical states of interfaces. Many enzymes are involved in catalytic reaction at interfaces. For enzymatic reaction at interfaces, different from the reaction in homogeneous systems, interfacial contact and subsequent conformational change of enzymes are important events determining their catalytic activity. In this chapter, I will describe the conformation of proteins and their interaction (protein-protein and protein-surfactant) at interfaces (mainly liquid-liquid interfaces). The characteristics of enzymatic reaction at liquid-liquid and solid-liquid interfaces, especially lipase reaction, wiU also be described. 

The interactions between proteins and low-molecular-weight surfactants at interfaces have crucial effects on physical states of interfaces, such as interfacial energy, interfacial rheological properties, C potential, and thickness of adsorbed layer. The competitive displacement of globular proteins by surfactants at liquid interfaces (normally, oil-water interfaces) has been extensively... 

Po = volume fraction of surfactant at interface (surface coverage)... 

Most commonly used surfactants are aqueous soluble, and unless otherwise needed, the terms hydrophobic and hydrophilic will be used in the following sections with an understanding that in general it could be any solvent. The adsorption of surfactants at interfaces is evidently a function of the interactions at the interface and in the bulk. The efficacy of the surfactant thus depends on the following three important interactions ... 

FIGURE 2.3 A schematic diagram showing the factors governing adsorption of surfactants at interfaces. 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to knowing how these molecules act as stabilizers. Most basic ideas on adsorption and conformation of polymers have been developed for the solid/liquid... 

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