Surfactants, Micelles and Vesicles

8 TT Hoffmann, H. and Ebert, G., Surfactants, Micelles and Fascinating Phenomena , Angew. Chem., Int. Ed. Engl., 1988, 27, 902-912. 

In 1774 Benjamin Franklin recounted his first experiences with surfactants to the British Royal Society. 

Phase transitions of the system such as chain ordering transitions of lipids, appear in the isotherm as regions of constant pressnre in the case of first order phase transitions involving the coexistence of two phases, or as a kink in the isotherm corresponding to a second order phase transition. These kinds of surface measurements are highly sensitive to impnrities and mnst be carried out using very pure water and sample materials. 

There exists a plethora of books concerning surfactants (on the average of 60 per decade, for example (Adamson 1990) and more recently (Wendt and Hoysted 2010 Neumann et al. 2011 Tadros 2011), and review articles (increasing from 100 per year in 1982-1991 to 300 per year in 2002-2011), so it is counter-productive to 

Typical ionic surfactants are sodium dodecylsulfonate, sodium dodecylsulfate and cetyltrimethylammonium bromide. Typical non-ionic surfactants are short copolymers H(C2H4) (OC2H4)mOH with 6 n 12 and 6 m 10 as well as certain bio-molecules such as diacylglycerides. Such molecules or electrolytes are soluble in water at low concentrations (and are the so-called monomers). They may form monolayers at the surface with the heads pointing towards the bulk aqueous phase and the tails pointing towards the vapor phase. 

Abramzon AA, Gaukbeig RD (1993) Surface tension of salt solutions. J ApplChem 66 1139-1147, 1315-1320 

Adamson AW (1968) An adsorption model for contact angle and spreading. J Coll Interf, Sci 27 180-181 

Adamson AW (1990) Physical chemistry of surfaces, 5th ed. Wiley, New York, p 101 ff Alejandro 1, Tddesley DJ, Chapela GA (1995) Molecular dynamics simulation of the orthobaric densities and surface tension of water. J Chem Phys 102 4574—4583 Allen HC, Gregson DE, Richmond DL (1999) Molecular structure and adsorption of dimethyl sulfoxide at the surface of aqueous solutions. J Phys Chem B 103 660-666 Aveyard R, Saleem SM (1976) Interfacial tensions at alkane-aqueous electrolyte interfaces. J Chem Soc, Faraday Trans 1(72) 1609-1617 

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Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

The driving forces of the blackberry self-assembly process are unique and differ from many types of traditional systems that have been well studied. Unlike surfactant micelles and vesicles, hydrophobic interactions do not contribute to the blackberry formation because the POMs have no hydro-phobic moieties. In colloidal systems, self-assembly occurs by van der Waals forces. As previously mentioned, van der Waals forces do not contribute significantly to POM self-assembly. 

Self-aggregating amphiphiles can broadly be divided into hydrotropes and surfactants. The main difference between hydrotropes and surfactants lies in the fact that hydrotropes are typically not sufficiently hydrophobic to cooperatively self-aggregate and form organized structures, whereas surfactants form distinct aggregates such as micelles and vesicles above their critical aggregation concentrations. 

Depending on the shape of the surfactant, different highly dynamic aggregates can be formed. The morphologies of different micelles (and vesicles - vide infra) are... 

In the remainder of this article, discussion of surfactant dissolution mechanisms and rates proceeds from the simplest case of pure nonionic surfactants to nonionic surfactant mixtures, mixtures of nonionics with anionics, and finally to development of myehnic figures during dissolution, with emphasis on studies in one anionic surfactant/water system. Not considered here are studies of rates of transformation between individual phases or aggregate structures in surfactant systems, e.g., between micelles and vesicles. Reviews of these phenomena, which include some of the information summarized below, have been given elsewhere [7,15,29]. 

The emergence of novel properties due to self-assembly is also present in much simpler systems. Consider, for example, the formation of micelles and vesicles from surfactants, as already seen (Figure 5.3). 

A and B, which could react inside the boundary (but not outside) to yield as a product the very surfactant that builds the boundary (Luisi and Varela, 1990). In Chapter 7 it was also indicated how this theoretical paper led to the experimental implementation of self-reproducing reverse micelles, aqueous micelles, and vesicles (Bachmann et al., 1990, 1991, 1992 Luisi, 1994 Walde et al., 1994b). 

The basic common denominator for all these applications is qualitatively well understood surfactants and their aggregates permit mixing, or at least close interaction, between phases or substances that are per se immiscible with each other -mostly oil and water. This is how grease is washed off from our hands when we use soap, the removal being mediated by micelles. In turn, micelles and vesicles permit the formation of an extraordinarily efficient interfacial system. Figure 9.3 gives a dramatic demonstration of this, showing that the total surface of a concentrated soap solution in your sink may well correspond to the surface of a stadium ... 

Of all mentioned prebiotic membranogenic molecules, the ones that have gained more attention in the literature are long-chain fatty acids. In addition to their prebiotic relevance, these compounds are relatively simple from the structural point of view, and most of them are easily available. We will see in the next chapter that these vesicles have acquired a particular importance in the held of the origin of life. In fact, the hrst inveshgations on self-reproducing aqueous micelles and vesicles were carried out with caprylate (Bachmann et al, 1992) and most of the recent studies on vesicles involve vesicles from oleic acid/oleate (for simplicity we will refer to them as oleate vesicles). In this section, I would like to illustrate some of the basic properties of these surfactant aggregates. 

Other interesting examples of the organized molecular structures used to increase the quantum yield of charge photoseparation are micelles and vesicles. Micelles represent aggregates of surfactant molecules, one end of which is hydrophobic and the other hydrophilic. On reaching a certain critical concentration in a solution, these molecules group into spherical formations in which either the hydrophilic ends of the molecules are turned towards the micelle centre while their hydrophobic ends form its surface or vice versa. Micelles of the former type are usually formed in non-polar solvents and those of the latter type in polar solvents. The micelle is schematically represented in Fig. 1(d). 

All three methods give similar values of interfacial potentials typical results for some of micelles and vesicles are listed in Table 3. Also listed are estimates of interfacial dielectric constants (e), determined by comparing the position of absorption bands of solvatochromic indicators in the surfactant assemblies with that of reference 1,4-dioxane water mixtures with known e values. More generally, luminescence probe analysis [49], thermal leasing [50] and absorption spectroscopy [47, 51] are techniques that have all been utilized to measure local polarities in micelles and vesicles. It is important to note that these methods presume knowledge of the loca-... 

The concentration at which surfactants form rodlike micelles is called CMCII. Critical micelle concentration is almost independent of temperature while CMCII increases with temperature. Spherical micelles, rod-like or thread-like micelles, and vesicles are the three most common microstructures seen in dilute DR surfactant solutions. 

What is missing in this scheme are two elements which are essential for a molecular Darwinist approach to the origin of life, namely evolution of the replicators and their competition to produce the survival of the best fit. We have shown here that in principle it is possible to implement these mechanisms with vesicles. It is now needed to show that this is indeed the case. This is one fruitful direction for the work in the field of supramolecular surfactant aggregates, so as to give a dynamic aspect to the chemistry of micelles and vesicles. 

Figure 8.1 Micelles — surfactant aggregates In a single solvent microemulsions — domains of water in oil (or oil in water) surrounded by a surfactant monolayer and vesicles — domains of water in water (or oil In oil) surrounded by a surfactant bilayer.

Enzymes Characteristics and Mechanisms, p. 554 Micelles and Vesicles, p. 861 Surfactants, Part I Fundamentals, p. 1458 Vitamin Bj2 and Heme Models, p. 1569 Zeolites Catalysis, p. 1610... 

Robinson, B.H. Bucak. S. Fontana, A. On the concept of driving force applied to micelle and vesicle self-assembly. Langmuir 2000, 16. 8231-8237, and references therein. Shioi. A. Hatton, T.A. Model for formation and growth of vesicles in mixed anionic/cationic surfactant systems. Langmuir 2002. 18, 7341-7348. 

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