Self-Assembly of Amphiphilic Molecules

Self-assembled structures or, as they are otherwise called, association colloids are spontaneously formed supramolecular entities. In most cases, the constituting molecules are amphiphilic, that is, they contain a lyophobic and a lyophilic part. In a given solvent and beyond a critical concentration, the amphiphilic molecules spontaneously organize themselves to give structures of colloidal dimensions. 

Supramolecular assemblies may also be spontaneously formed because of electrostatic interaction between oppositely charged compounds, for example, polycations and polyanions. This is referred to as co-assanbly it is discussed in Section 12.6. 

Self-assembling molecules may form a variety of structures. These structures are widely studied, both for academic and application reasons. In Section 11.9, we address a few applications in the areas of bio(nano)technology and biomedicine. 

The reason for the self-assembly tendency of amphiphilic molecules is that their lyophobic parts are poorly soluble and tend to separate from the solvent, whereas the lyophilic parts prefer to be solvated. For water as the solvent, hydrophobic interaction is the major cause of aggregation of apolar molecules and molecular fragments. A more detailed discussion of the hydrophobic effect is given in Section 4.3.2. 

Amphiphilic molecules supplied to a (aqueous) solution adsorb at available interfaces. For that reason, amphiphilic molecules are also called surface active agents or, for short, surfactants. The decrease in interfacial tension y resulting from adsorption is, for a reversible process, given by the Gibbs equation (Equation 3.88) or Equation 7.2. Adsorbed layers of surfactants at fluid interfaces are extensively discussed in Chapter 7, and Chapter 14 deals with generic theoretical aspects of the adsorption process. 

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It is general considered that the driving force for the self-assembly of amphiphilic molecules is a solvophobic effect, more specific in an aqueous environment, this is referred to as the hydrophobic effect. The type of aggregate morphology formed can be predicted... 

Liposomes or vesicles are cell-like spherical aggregates formed by self-assembling of amphiphilic molecules such as phospholipids. Liposomes were first discovered... 

The self-assembly of amphiphilic molecules in non-aqueous polar solvents is usually attenuated compared to that in water. The CMCs increase significantly upon the substitution of water by polar solvents [2, 57, 58]. For example, the CMC of ionic surfactants in ethylene glycol are two orders of magnitude larger than that in water [57], while the monomeric solubility of sodium dodecyl sulphate in formamide is so high that micelles do not form at all [58]. Attenuation of self-assembly in non-aqueous polar solvents is the result of the reduced free energy of repulsion between polar solvents and the solvent-phobic parts of amphiphiles compared to that in water. 

Another caveat for the modelling from the atomistic level up to the macroscopic level is the requirement of sufficiently accurate interaction potentials. Minor inaccuracies in calculations on small length scales can give rise to pronounced effects on the mesoscopic scale. Consider, for instance the self-assembly of amphiphilic molecules (see section B3.6.3I into a spatially ordered structure. The free-energy difference between the different morphologies can be as small as lO T per molecule. The ah initio prediction of such a small free-... 

The general principles of self-assembly of amphiphilic molecules into finite-sized aggregates (micelles) are described in a number of classic books [34-36]. In our analysis of micelle formation we apply the equilibrium close association model. That is, we assume first that only one population of micelles, with an aggregation number p (number of copolymers in one aggregate), is present in the system at any given concentration of amphiphiles in the solution, or that there are no micelles at all and second, that the free energy per molecule in a micelle, Fp, exhibits a minimum at a certain value of the aggregation number, p = po. 

Self-assembly of amphiphilic molecules, such as surfactants and polar lipids, leads to a range of different structures, of which a few are shown in Figure 19.1. Systems containing amphiphiles are best classified into homogeneous, or single-phase, systems and heterogeneous systems of two or more phases. The single-phase systems can in turn be divided into isotropic solutions. 

In this chapter, the siliceous framework of the diatoms has been introduced as an example of a system that has micropores and channels that vary over many dimensions. Synthetic design approaches that allow for the self assembly of amphiphilic molecules into large units and phase separation provide a way to make such multi-scaled ordered porous materials. 

Supramolecular amphiphilic assemblies Micelles and liposomes are supramolecular aggregates that are formed under aqueous conditions by spontaneous self-assembly of amphiphilic molecules that contain both hydrophilic and hydrophobic ends (Figure 11). Large payloads of contrast agent can be incorporated into liposome and micelle supramolecular structures for signal amplification. Often, the lipid shell can be further modified for targeting and accumulation at the site of interest. The self-assembly of amphiphilic molecules into liposomes and... 

One way to manufacture nano-size particles is to create conditions which promote the self assembly of amphiphilic molecules in aqueous solutions. Amphiphilic molecules contain hydrophilic heads and hydrophobic tails. Two... 

Self-assembly of amphiphilic molecules in RTILs has been recently reviewed [71-73]. The ability to form micelles, vesicles, liquid crystal, and microemulsions opens a wide range of applications. The particular advantage of using ILs in the formulation of microemulsions is their versatile use as a polar phase (substituent of water), as an organic solvent (substituent of oil), as a cosurfactant, and/or as an amphiphilic component. In many cases, the microemulsions formulated with ILs show a continuous transition from water-in-oil to oil-in-water types. 

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