Aggregation, amphiphilic molecules

Fainerman, V.B. Vollhardt, D. Melzer, V. Equation of state for insoluble monolayers of aggregating amphiphilic molecules. J. Phys." Chem. 1996, 100, 15478. 

In the case of amphiphilic molecules, characterized by the coexistence of spatially separated apolar (alkyl chains) and polar moieties, both parts cooperate to drive the intermolecular aggregation. This simple but pivotal peculiarity makes amphiphilic molecules soluble in both polar and apolar solvents and able to realize, in suitable conditions, an impressive variety of molecular aggregates characterized by spatially separated apolar and polar domains, local order at short times and fluidity at long times, and differences in size, shape (linear or branched chains, cyclic or globular aggregates, extended fractal-like molecular networks), and lifetime. 

An enormous literature has been produced in recent decades in the field of molecular aggregation of amphiphilic molecules in liquid systems, emphasizing the extremely wide variety of accessible structures and dynamics. Among these molecular aggregates, in this chapter our attention will be restricted to those formed by some amphiphilic molecules (surfactants) in apolar solvents called reversed micelles [1]. 

The concept of micelles consists of aggregation of amphiphilic molecules that contain polar and non-polar moieties, which associate in a manner that minimizes hydrophobic and lipophilic interactions. However, a cascade molecule consisting of an internal lipophilic framework and a external hydrophilic surface would effectively be a unimolecular micelle [59] capable of hosting molecular guest(s). 

COOPERATIVE AGGREGATION OF AMPHIPHILIC MOLECULES AT AN AIR/WATER INTERFACE... 

The purpose of this study is to follow the aggregation process from the small amphiphile molecule to the infinite network of Figure 3. For a critical density of fibers, viscosity diverges while an elastic modulus appears (15,16). 

Note 2 Under suitable conditions of temperature and concentration, the similar parts of amphiphilic molecules cluster together to form aggregates or micelles. 

This is based on the fact that amphiphilic molecules tend to aggregate in order to decrease unfavourable contacts with water molecules - thus, upon aggregation, water molecules are set free and this brings about an increase of entropy. This is qualitatively illustrated in Figure 5.1. The example illustrates beautifully, in its simplicity, that formation of local order and increase of the overall entropy can take place simultaneously. The organized, ordered structures are thus a kind of by-product of the overall increase of entropy ( disorder ). 

We have already seen in Chapter 5, on self-organization, how and why amphiphilic molecules tend to form aggregates such as micelles, vesicles, and other organized structures. 

Figure 1.4. Schematic structures of aggregates of amphiphilic molecules in a polar solvent (the hydrophilic regions of each molecule are shaded).

In polar solvents amphiphilic molecules, that is molecules with a polar head and hydrophobic tail , tend to form various aggregates. The structure of micelles is usually much more complicated than that schematically shown in Figure 1.4 (see the pertaining discussion in Section 2.3). Nevertheless, in water they can include nonpolar molecules into their voids acting like surfactants applied in toiletry [15]. Similarly to cyclodextrins such as 11 [6, 16] and liquid crystals [7] discussed in Section 2.6, surfactants are examples of few supramolecular systems which have found numerous practical applications. 

A micelle is an assembly of amphiphilic molecules dispersed in water. Such molecules are made of two parts, a polar head group and a non-polar tail . The polar head is for example a carboxylic acid which can dissociate into ions (—COO- and H+) the non-polar tail is a saturated hydrocarbon chain. Since the non-polar parts are insoluble in a polar solvent, these molecules aggregate in water to form micelles which are microscopic droplets with a non-polar interior and polar groups at the water interface. This picture of micelles is probably an oversimplification, because water penetrates to some extent between the molecules it is however sufficient for an understanding of the special properties of micellar suspensions in photochemistry. 

In aqueous solution, amphiphilic molecules aggregate into micelles above the critical micelle concentration. Such solutions have been the object of research for many years, with special interest in shape and size of these micellar aggregates [37]. Size and shape (spherical, wormlike, or disklike micelles) depend strongly on the molecular structure of the amphiphilic molecule. 

Amphiphilic molecules, composed of a hydrophilic head group and hydrophobic chain (hydrocarton or fluorocarbon), have a tendency to aggregate at the air/water interface and, when compressed, form monomolecular Langmuir films. These films can be... 

Dispersants Keep contaminants of the lubricant in suspension and avoid their aggregation. Dispersants are amphiphilic molecules. Their long hydrocarbon tail helps to solubilize polar molecules in the base oil. The polar head group interacts with contaminants and facilitates the formation of (inverse) micelles around them. 

Amphiphilic molecules (surfactants) can assemble into nanoscopic supramolecular structures with a hydrophobic core and a hydrophilic shell micellar arrangement. As surfactant concentration is increased in aqueous solutions, the separated molecules aggregate into micelles upon reaching a concentration interval known as the critical micellar concentration (CMC). 

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