Amphiphilic-water aggregation concentration

The development of amphiphilic and aggregation behaviour as a fiinction of dendrimer generation of PS-t/enstudied with several diflferent techniques the amphiphilic character at a toluene-water interface was investigated with conductivity measurements, and at a water-air interface with monolayer experiments. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to examine the aggregates formed by the different generations in aqueous solutions, while the critical association concentrations were determined with the pyrene-probe luminescence technique. 

Table 1 Some amphiphilic molecules, the aggregates they form in water and their critical aggregate concentrations...

Aggregation of amphiphilic molecules occurs specifically in the presence of water (see Figure 1) (recently, though, aggregation has been observed in other solvents [12]) above the critical aggregate concentration (cac), dubbed the critical micelle concentration (cmc) in the case of the most simple of these aggregates the micelle. 

When amphiphilic molecules dissolve in water, their polar heads tend to be close to each other while the non-polar hydrophobic tails are as far away from water as possible. Depending on the concentration, the amphiphilic molecules aggregate to form spheres, or columns, or a laminar structure, see Figure 1.14. 

Consider a dilute solution of a mixture of an ionic surfactant and a cosurfactant in salt water. Figure 5.5 shows the typical phase behaviour for fixed temperature and salinity. The horizontal axis represents the total volume fraction (f) of amphiphile molecules and the vertical axis gives the ratio (pA/(f>s of cosurfactant concentration (f>A to surfactant concentration 0s- Almost all the amphiphile molecules aggregate together into micelles. 0 therefore represents the volume fraction of these objects, whilst 0a/0s represents their chemical composition with respect to the two chemical species which make them up. 

Fig. 3.1 Building blocks of thermotropic and lyotropic liquid crystalline phases. The upper part of the figure shows two examples of typical thermotropic mesogens. Calamitic mesogens, such as terephthal-bis-(p-butylaniline) (TBBA) [2], can be represented by prolate ellipsoids or rigid rods, while discotic mesogens, such as benzene-hexa-n-octanoate (BH8) [4], are usually described by oblate ellipsoids or discs. The lower part of the figure shows the typical surfactant molecule sodium dodecyl sulfate (SDS), which forms lyotropic phases with water [5], Such a surfactant molecule is basically composed of a polar head group and a flexible hydrophobic tail. These amphiphilic molecules aggregate into different types of micelles, which are the actual mesogens of lyotropic liquid crystals. The shape of the micelles depends mainly on the solvent concentration...

Adsorption refers to a tendency of water-soluble amphiphilic molecules (surfactants) to preferentially gather at the interface, and thus minimize their free energy by adopting a specific molecular orientation, i.e. hydrophilic parts of the molecule will be anchored in water, while hydrophobic parts will be exposed to air. The number of molecules that adsorb to the interface (and thus the layer lateral density) will depend on the chemical constitution, amphiphilicity, and solubility of the material. It is known that at a specific concentration called the critical aggregation concentration (cac), aggregates will start forming in solution and remain in equilibrium with the adsorbed (Gibbs) layer. Therefore, from that point on, no further... 

The phosphate end of the phospholipids is relatively polar, making it water soluble or hydrophilic. In comparison, the fatty acid ends are relatively insoluble or hydrophobic. Molecules with hydrophilic heads and hydrophobic tails are termed amphiphiles. When their aqueous concentrations reach a critical level, they spontaneously aggregate to isolate their hydrophobic ends from water. 

---