Behavior at interfaces

MacRitchie and Dickinson and Stainsby have reviewed the behavior of proteins at a variety of interfaces. There have also been a number of recent research papers which have taken a theoretical or computational approach examples include the computation of the electrostatic interaction energy between a protein and a charged surface the computation of electrostatic and van der Waals contributions to protein adsorption Monte Carlo simulation of the conformational behavior of a polypeptide chain near a charged surface protein structure prediction based on statistical potential development of a model system for the interaction 

Examination of the literature supports the observation that adsorption at solid/liquid interfaces has been traditionally more difficult to study than that at liquid/liquid interfaces. All the features described in the previous section play an important role in the behavior of proteins at interfaces. Proteins tend to concentrate at surfaces as a result of their amphipathic nature arising ftom the mixture of polar and nonpolar side chains.  

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There has been a recent revival of interest in zwitterionic surfactants (L 4) because of certain useful properties shown by these molecules, including 1) mild behavior on the skin, 2) compatability with both anionics and cationics, 3) adsorption onto skin and hair, and 4) lime soap dispersing ability. Although this type of surfactant has been produced and used industrially for the last few decades, there have been few studies of the properties of well purified surfactants of this type (5-11) and almost all of these have been concerned with the micellar properties of these compounds rather than with their behavior at interfaces. 

Successfully developing a surface engineering strategy based on surfactant behavior at interfaces requires surface characterization techniques that can validate and quantify surface chemistry changes. This review describes the role of two surface chemistry analysis techniques that have proven highly successful in surfactant analysis x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS). In Section II, the methods by which these techniques analyze surface chemistry are described. In Section III, recent examples of their application in surfactant-based surface engineering are described. 

Magerle, Krausch and coworkers showed based on AFM data and simulations that the phase behavior at interfaces [127] and in thin films (interphases) [128] can be understood in terms of analogous to those of classic, inorganic crystals. In Fig. 3.55 thin triblock copolymer films of SBS on silicon are shown, which display a rich variety of morphologies depending on the local film thickness [129]. 

It is likely that experimentally found values of molecular cross-sectional areas do not correspond to the equilibrium states of the protein layers but reflect only the transition state configuration, as assumed by MacRitchie [16] and depicted in Fig. 1. The problem of adsorption reversibility is basic for understanding the protein behavior at interfaces. The belief in the protein adsorption irreversibility is mainly based on drastic conformational changes in interfacial film and the great difficulty of desorbing a protein from this film [15], However, these criteria are not always a proof of irreversibility. It was shown in many cases [3,24,39-41] that proteins can be desorbed... 

Since the question of whether protein adsorption can be considered to be a reversible process is basic to an understanding of protein behavior at interfaces, it is proposed to discuss the problem in some detail. Apart from the loss of solubility on adsorption, other observations that have been interpreted as evidence for irreversibility are ... 

For these reasons the basic understanding of crude oil and bitumen emulsions and demulsification has received considerable attention. Advances in the knowledge of the physicochemical-mechanical structure of the stable emulsions and their films are being made in our laboratory and elsewhere. The films response to demulsifiers are studied in order to understand the detailed mechanisms of demulsification. This knowledge will provide a more scientific basis for formulating products. The knowledge of surfactant chemistry, the behavior in solutions, the behavior at interfaces, and interactions with the crude-oil solvent base involve a wide interplay of complex processes in designing formulations. 

As mentioned above, a couple physical-chemical properties of ILs can influence the physics of surfaces significantly. For the behavior at the interface between a solid-state material and an IL the wettability, the surface energy, and the contact angle are fundamental properties. Facing the enormous possible combinations of materials with ILs, a fundamental understanding of the behavior at interfaces is necessary. 

Traube in 1891" observed that the regularities that are seen in the simple physical properties of a homologous series of organic compounds also extend to their behavior at interfaces. He found that the surface activity of organic solutes in aqueous solutions increased strongly and regularly as in a homologous series. As surface... 

Along with the classification by chemical nature, one can also classify surfactants with respect to the mechanism of their behavior at interfaces. Two main mechanisms of surfactant action at S/L interfaces are of importance in their relevance to contact interactions between particles that we will address broadly throughout this book. These are (1) the control of the wetting of a solid surface by a liquid (see Figures 2.10 and 2.11) and (2) the dispersion action, which facilitates the fracture of solids, which we will discuss in detail in the second part of this book. 

Influence of cell-surface chemical composition on microbial behavior at interfaces (adhesion, aggregation, flotation)... 

Influence of Cell-Surface Chemical Composition on Microbial Behavior at Interfaces... 

The next section discusses chemical structures, synthesis, and purification of gemini surfactants. Section III reviews the behavior of gemini surfactants in solutions below the CMC. Section IV deals with their behavior at interfaces. The fifth section reviews micelle formation and solubilization. Section VI deals with micelle properties. Microstructure of aqueous solution of gemini surfactants, rheology of these solutions, and mixed micellization are considered in the following three sections. Section X deals with the phase behavior of... 

The real difficulty when dealing with gemini surfactants lies in the purification of the raw surfactants. The purification of the crude gemini surfactants is essential, particularly in studies of adsorption and behavior at interfaces. The purification procedures are somewhat easier for the quaternary ammonium gemini surfactants. Sophisticated procedures must often be used. Indeed, one or more reaction steps that leads to gemini surfactants usually involves the two ends of some intermediate compound. This reaction rarely reaches full completion. It results in the formation of a mixture of mono- and difunctionalized compounds. Their separation is usually achieved through chromatography. 

The term amphiphile implies an affinity to two different media. Familiar amphiphilic molecules incorporate two incompatible components that give rise to this behavior. Similarly, in AB and ABA block copolymers there are two incompatible blocks of different solubility. However, ABC triblock copolymers incorporate three chemically different blocks. When the three blocks are mutually incompatible and of different solubilities the ABC surfactants can exhibit affinity to three different media rather than two. The consequences of this higher functionality have not been explored in detail. For example, little attention has been given to their behavior at interfaces. Linear ABC triblock copolymers or the corresponding star copolymers may be able to form two-dimensional mesophases [56], This can occur at the interface between two fluids I and II such that the B block is selectively solubilized in I while A and C are only soluble in II. In this situation, the A and C blocks are constrained to the surface and bound to each other. A two-dimensional amphiphile is obtained when the A and C blocks are incompatible. A dense monolayer of this type should undergo microphase separation leading to the formation of circular and striped mesophases. Note that cylindrical and lamellar mesophases are indistinguishable in this case. A mixed monolayer comprised of BC, BA, and ABC block copolymers will mimic the behavior of amphiphiles in the presence of two two-dimensional and incompatible fluids. When the ABC copolymers are a minority component, they should straddle the boundary line between the two-dimensional A and C phases. 

H. Mitsuyasu, Y. Nonaka, K. Eguchi, Analysis on Solid State Reaction at the Interface of Yttria-Doped Ceria/Yttria-Stabilized Zirconia, Solid State Ionics 113-115,279-284 (1998) N. Sakai, H. Kishimoto, K. Yamaji, T. Horita, M.E. Brito, H. Yokokawa, Degradation Behavior at Interface of LSCF Cathodes and Rare Earth Doped Ceria, SOFC X, ECS Transactions, 7(1) 389-398 (2007)... 

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