Micelle Formation in Aqueous Media

The surfactant aqueous solutions manifest two major forces that determine their behavior. The alkyl part, being hydrophobic, tends to separate out as a distinct phase, whereas the polar part tends to stay in solution. The difference between these two opposing forces thus determines the properties of the solution. The factors to be 

Below CMC, the detergent molecules are present as single monomers. Above CMC, they are present as monomers, Cmono, in equilibrium with micelles, Cmice. The micelle with aggregation number, Nag, is formed from monomers  

Nag monomers, which were surrounded by water, aggregate together above CMC and form a micelle. In this process, the alkyl chains have transferred themselves from the water phase to an alkane-like micelle interior. This occurs because the alkyl part has a lower energy in micelle than in the water phase  

Water phase Alkyl chain in water, surrounded by water 

Micelle Alkyl chain in contact with neighboring alkyl chains 

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Kitahara, A., Kon-No, R. Micelle formation in non aqueous media, in Colloidal dispersions and micellar behaviour. ACS-Symposia Series 9, 225 (1975)... 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

Introduction to the variety of types of surfactants, effect of surfactants on aqueous solution properties. Law of mass action applied to the self-assembly of surfactant molecules in water. Spontaneous self-assembly of surfactants in aqueous media. Formation of micelles, vesicles and lamellar structures. Critical packing parameter. Detergency. Laboratory project on determining the charge of a micelle. 

Another type of self-assembly mode is based on looser molecular interactions, where one of the main binding forces comes from hydrophobic interactions in aqueous media. Amphiphihc molecules (amphiphiles) that have a hydrophihc part and a hydrophobic part form various assembhes in water and on water. The simplest example of this kind of assembly is a micelle, where amphiphiles seh-assemble in order to expose their hydrophilic part to water and shield the other part from water due to hydrophobic interactions. A similar mechanism also leads to the formation of other assembhes, such as hpid bilayers. These molecules form spherical assembhes and/or two-dimensional membranes that are composed of countless numbers of molecules. These assembhes are usually very flexible. When external signals are applied to them, they respond flexibly while maintaining their fundamental organization and shape. This research held was initiated by the work of Bangham in 1964. It was found that dispersions of hpid molecules extracted from cells in water spontaneously form cell-like assembhes (liposomes). In 1977, Kunitake and Okahata demonstrated the formation of similar assembhes from various arti-flcial amphiphiles. The latter finding showed that natural lipids and artificial amphiphiles are not fundamentally different. 

The favored structure for most phospholipids and glycolipids in aqueous media is a bimolecular sheet rather than a micelle. The reason is that the two fatty acyl chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle. In contrast, salts of fatty acids (such as sodium palmitate, a constituent of soap), which contain only one chain, readily form micelles. The formation of bilayers instead of micelles by phospholipids is of critical biological importance. A micelle is a limited structure, usually less than 20 nm (200 A) in diameter. In contrast, a bimolecular sheet can have macroscopic dimensions, such as a millimeter (10 nm, or 10 A). Phospholipids and related molecules are important membrane constituents because they readily form extensive bimolecular sheets (Figure 1211). 

The mechanism of formation of uranyl selenate tubules in aqueous media is probably controlled for compound 2 by the presence of protonated butylamine (C4Hi2N) cations. In this regard, it can be similar to the process that leads to the formation of highly undulated uranyl selenate sheets in the structure of (H30)2[Ci2H3oN2]3[(U02)4(Se04)s](H20)5 (see below). The flexible uranyl selenate complexes crystallize around cylindrical micelles to produce a tubular inorganic substructure. 

Enthalpy of Micelle Formation of Mixed Sodium Dodecyl Sulfate and Sodium Deoxycholate Systems in Aqueous Media... 

The thermodynamic understanding of the aggregation phenomena of surfactant molecules in aqueous media have been investigated by using a wide variety of physico-chemical methods. In recent years, due to the advent of sensitive calorimeters, some enthalpy data on micelle formation have been reported in the literature (1-11). 

This study is a continuation of our previous investigations, in which the aggregation phenomena of surfactant molecules (amphiphiles) in aqueous media to form micelles above the critical micelle concentration (c.m.c.) has been described based on different physical methods (11-15). In the current literature, the number of studies where mixed micelles have been investigated is scarcer than for pure micelles (i.e., mono-component). Further, in this study we report various themodynamlc data on the mixed micelle system, e.g., ci H25soi4Na (NaDDS) and sodium deoxycholate (NaDOC), enthalpy of micelle formation (by calorimetry), and aggregation number and second virial coefficient (by membrane osmometry) (1 6). 

Single polymer micelles were observed at low polymer concentrations in aqueous media whereas at higher concentrations both inter and intrapolymeric aggregation took place. Above the critical solution temperature excimer formation decreased due to disruption of the pyrene aggregates. 

In contrast to aqueous systems, micelle formation in non-polar media is driven by the benefit in energy rather than by an increase in entropy. The replacement of polar group - hydrocarbon interaction (as in the case of dissolution) with the interaction between polar groups upon their association into micellar core is thermodynamically beneficial. The benefit in energy upon association of polar groups is so large, that even at low concentrations true surfactant solutions contain small pre-micellar associates rather than individual surfactant molecules. 

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