Association colloids vesicles

Interaction between double layers, one of the building bricks of colloid stability, is an important theme planned for Volume IV. It has a large number of spin-offs, in, for instance ion exchange, thin wetting films, free films, membranes, association colloids, vesicles, polyelectroljdes, emulsions and rheology. The dramatic influence of electroljrtes on these phenomena finds its origin in the changes in the double layer, discussed in this chapter. 

Association colloids (including mlcro-emulslons, vesicles, bllayers)... 

Small-molecule surfactants can give rise to a series of association colloids, including micelles, mesomorphic structures, vesicles, and microemulsions. 

There is, however, a large and important class of colloids in which nuclcation is absent. Growth is spontaneous, but the structures so formed are usually limited by geometric and energy factors to a finite size often towards the lower end of the colloid size range. This class comprises association colloids, or in more general terms self-assembly systems, it includes not only micelles but many more complex forms, e.g. vesicles with, as extreme examples, biological structures such as cell membranes. 

Reactivity in aqueous association colloids is generally analyzed with the assumption that colloids provide a discrete reaction medium distinct from bulk solvents. The colloidal microdroplets and the surrounding bulk solvent are treated as separate phases or, more correctly, pseudophases, and distributions of reactants are described by transfer equilibria between the aqueous and aggregate pseudophases [10,46-48]. This approach was originally developed to account for rates of reaction in aqueous micelles, but it has been extended to other colloidal systems, e.g., vesicles and microemulsions, and, because of similarities in the interfacial regions, to alcohol or other cosurfactant-modified micelles [1,10,11,28]. 

The combination of the pseudophase assumption with mass action binding constants of substrates and ion exchange of reactive and nonreactive counterions is called the pseudophase ion-exchange (PIE) model [10,48,66]. It successfully fits the kinetics of many bimolecular reactions and also shifts in apparent indicator equilibria in a variety of association colloids, especially reactions between organic substrates and inorganic ions in normal micelles over a range of surfactant and salt concentrations and types (up to about 0.2 M). It has also been successfully applied to cosurfactant-modified micelles [77,78], O/W microemulsions [79-81], and vesicles [82]. 

The use of surfactants or amphiphilic molecules in electrochemistry dates back over four decades [1,2]. Extensive research on electrochemistry in surfactant systems has been reported primarily in the last 20 years. Surfactant systems are ubiquitous. The aggregation of surfactant molecules may produce a variety of systems including micelles, monolayers and bilayers, vesicles, lipid films, emulsions, foams, and microemulsions. Developments in the area of electrochemistry in such association colloids and dispersions have been documented by Mackay and Texter [3]. Mackay [4] reviewed the developments in association colloids, particularly micelles and microemulsions. Rusling [5,6] also reviewed electrochemistry in micelles, microemulsions, and related organized media. This chapter focuses on microemulsions and does not deal with micelles, monolayers, emulsions, and other surfactant systems per se. 

Although the impact of this molecular-level effect is quite small (negligible in most cases), it can, under some circumstances, produce an appreciable energetic effect—repulsive in the case of two approaching surfaces—that is, it will be difficult to displace the last molecular layers separating the surfaces—or attractive in the separation of two contacting (adhesive) surfaces. These topics will appear again in Chapters 10 (on colloids and colloidal stability), 15 on association colloids micelles, vesicles and membranes), and 19 on adhesion). 

This class of association colloids can be further divided into several subgroups, which include micelles, vesicles, microemulsions, and bilayer membranes. Each subgroup of association colloids plays an important role in many aspects of colloid and surface science, both as theoretical probes that help us to understand the basic principles of molecular interactions, and in many practical applications of those principles, including biological systems, medicine, detergency, crude-oil recovery, foods, pharmaceuticals, and cosmetics. Before undertaking a discussion of the various types of association colloids, it is important to understand the energetic and structural factors that lead to their formation. 

Although the vast majority of surfactants form micelles of some kind in aqueous solution, some materials, because of their special structure or composition, will not associate in the normal way described above. They will, however, take part in other association processes to form equally interesting and important association colloids, including especially vesicles and bilayer membranes. 

The binding of concanavalin A (tetrameric) and succinylated concanavalin A (dimeric) to a yeast Schizosaccharomyces pombe) mannan gel has been studied. o-Mannans on thin sections of S. cerevisiae and C. utilis cells can be located with either the homologous antibodies or concanavalin A labelled with colloidal gold. Fully synthesized D-mannan was detected in the cell walls, on the plasmalemma, and in the cytoplasm, sometimes associated with vesicles and vacuoles. 

The primary aim is to introduce the current concepts used to interpret the properties of homogeneous, optically transparent, self-assembling aqueous solutions of small molecule surfactants that form into association colloids composed of charged or uncharged surfactants into micelles, miaoemul-sions, vesicles, or other mesophases. Pseudophase models are used to interpret chemical reactivity in surfactant solutions. Large surface-active molecules such as proteins, starches, and polymers are not considered. Much of the information is on surfactant solutions at room temperature and atmospheric pressure because most of the important properties, concepts, and unanswered questions can be developed at ambient conditions. Effects of additives such as salts, alcohols, and oils, and temperature are introduced briefly. Many introductory books include substantial sections on surfactant self-assembly. " Current research on a variety of topics is periodically reviewed in Current Opinion in Colloid and Interface Science. 

Pseudophase models work for several reasons (i) Reactions in association coUoids can be carried out under conditions of dynamic equilibrium. Thus the totality of the interfacial regions of all the aggregates in micelle, microemulsion, or vesicle solutions, and the totalities of their oil and water regions can be modeled as single interfacial, oil, and water reaction volumes of uniform properties with a separate rate constant for the reaction in each volume. Scheme 4. (ii) The requirement of dynamic equilibrium is met because the rate constants for diffusion of ions and molecules in association colloid solutions are near the diffusion-controlled limit. For example, the entrance and exit rate constants in micellar solutions in Table 1 are orders of magnitude faster than the example rate constants for thermal bimolecular reactions in micellar solutions in Table 4. Many additional examples are compiled in reviews. (iii) Measured rate constants for spontaneous reactions and... 

The pseudophase kinetic models for speeded or inhibited bimolecular, second-order, reactions are more complex. Here the focus is on reaction between a neutral organic substrate and a reactive counterion in micellar solutions in the absence of oil (d>o = 0, Scheme 4). Micellar effects on reactions of substrates with reactive counterions are important because they illustrate the general differences of micellar effects on spontaneous and bimolecular reactions and also how specific counterion effects influence the results. Pseudophase models also work for bimolecular reactions between two uncharged organic substrates and third-order reactions, reactions in vesicles and microemulsions, which may include partitioning into and reaction in the oil region, reactions of substrates with an ionizable (e.g., deprotonatable) second reactant, and the effect of association colloids on indicator equilibria. ... 

Association colloids Homogeneous, thermodynamically stable solutions of spontaneous self-assembled surfactant aggregates micelles, typically composed of single-tailed surfactants reversed micelles in oil with water pools, vesicles typically composed of twin-tailed surfactants and microemulsions composed of at least oil, water, and surfactant and also with alcohols, either aqueous (oil-in-water droplets), bicontinuous (no droplets), or reverse (water-inoil droplets). 

Pseudophase model The properties of the totality of the aggregates in homogeneous solutions are treated as a separate phase, a pseudophase. The totality of the association colloid present (micelle, vesicle, microemulsion) is modeled as three regions oil, interfacial, and water. 

Abstract Screened electrostatic interactions are commonly employed in colloid and polymer science for stabilization in aqueous solutions to avoid macroscopic phase separation, but these are equally versatile as driving force for complexation (or microscopic phase separation) into micelles, vesicles, multilayers and other nanostructured materials. In this introductory chapter, we present an overview of the field of electrostatically driven assembly of polyelectrolytes into nanometresized association colloids focusing in particular on the fundamentals followed by a discussion of selected application areas. 

Around the turn of the last century, chemists were reluctant to accept the idea of rubber, starch, and cotton as long, linear chains connected by covalent bonds. A popular alternative was the idea of an associated colloidal structure. As a matter of fact, some small molecules do exhibit such behavior. Soap molecules will associate into complex liquid crystalline structures and are used as the basis for the formation of mesoscopic solids. Other surfactant molecules such as the phospholipids present in the wall of many living cells will form micelles and vesicles. However, the effective molecular weight of such structures varies with concentration and temperature, whereas the molecular weights of true polymers with covalent links do not. 

Scheme 3 shows the reactions of a number of different weakly basic nucleophiles with z-AtN2 that have been studied in association colloids (published work is indicated by reference numbers in brackets and unpublished work by a bold asterisk in Scheme 3). In addition to the work described here, chemical trapping with Ifi-ArNj has already been used to determine the ion exchange constant between Cl and Br counterions in cationic micelles [26], Cl concentrations at the surfaces of zwitterionic micelles [27] and phospholipid micelles and vesicles [28], hydration numbers and terminal OH...

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