Amphiphilic-water aggregation

The structures that amphiphiles are able to build depend on their internal structures, on their concentration and the temperature. For the sake of simplicity, in the following we assume that the amphiphilic molecules are dissolved in water, so the molecules will be arranging themselves with the polar heads in contact with the water. However, it is very straightforward to apply the arguments for nonpolar solvents. 

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The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

The preceding discussion has been confined to two-component systems, amphiphile-water. In a large number of cases of practical importance one adds one (or more) additional component(s). Depending on the nature of the additive one can recognize different effects. If it is an amphiphile it is usually found that the micelles which form in the solution are of mixed composition. Under the assumption that the amphiphiles mix ideally in the micellar aggregate, Shinoda177 has derived expressions for the effective CMC of the amphiphile mixture. For nonionics... 

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. 

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. 

Tubular shaped superstructures formed by certain amphiphiles in water aggregate through intermediate helical ribbon structures.Since these amphiphiles feature a polar head in combination with a chiral hydrophobic group, the sol-gel... 

These amphiphilic molecules aggregate and form micelles when mixed with water. Figure 28.4 shows Pluronic surfactant molecules at the interface between the oil core and water, with the hydrophilic PEO termini extending into the water phase. Figure 28.7 shows the HPLC results of bupi-vacaine extraction from saline or blood plasma by microemulsions using the same ratios of reagents and the same reaction conditions, but different Pluronic surfactants. Clearly, the Pluronic... 

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...

In forming a micelle, amphiphile chains aggregate in the micelle s core to reduce expensive hydrocarbon-water contact. It is possible to imagine that the chains could pack in a frozen array of parallel dXL-trans chains, and indeed this is the packing found in solid bulk R-alkane. However, without considerable expensive hydrocarbon-water contact, it is impossible to see how such an array could exist in a globular micelle. We should expect, therefore, that the interior of a micelle is liquid, and that the alkyl chains are conforma-tionally disordered. This expectation is borne out by experiments, among the most definitive of which are... 

GA is well recognized as emulsifier used in essential oil and flavor industries. Randall et al., 1998, reported that the AGP complex is the main component responsible for GA ability to stabilize emulsions, by the association of the AGP amphiphilic protein component with the surface of oil droplets, while the hydrophilic carbohydrate fraction is oriented toward the aqueous phase, preventing aggregation of the droplets by electrostatic repulsion. However, only 1-2% of the gum is absorbed into the oil-water interface and participates in the emulsification thus, over 12% of GA content is required to stabilize emulsions with 20%... 

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