Surfactants typical micelle-forming

Table 1. Typical micelle-forming surfactants and their CMC values. Surfactant...

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

Micelles are spherical supramolecular assemblies of amphiphilic compounds with surface-active properties (surfactants, tensides) in a colloidal dimension [1] (see also Section 2.3.1). Typical micelle-forming molecules carry a hydrophilic headgroup and a hydrophobic tail. This is shown schematically in Figure 1. 

Micelle-forming surfactants typically have structures which are constituted from a polar head group and a straight chain of the alkyl group, usually 8-18 carbons in number. 

Contrary to hydrotropes, micelle-forming surfactants spontaneously self-aggregate cooperatively above the critical micelle concentration (cmc) even in the absence of solubilizate. Typical examples of micelle-forming surfactants include sodium dode-cylsulfate (SDS), dodecyltrimethylammonium bromide (DTAB), cetyltrimethyl-ammonium bromide (CTAB), and heptaoxyethylene dodecyl ether (C12E7) (Scheme 2). 

Figure 3.3 Typical graph of surface energy vs concentration for a micelle forming surfactant.

Except for some anionic/cationic surfactant mixtures which form ion pairs, in a typical surfactant solution, the concentration of the surfactant components as monomeric species is so dilute that no significant interactions between surfactant monomers occur. Therefore, the monomer—mi celle equilibria is dictated by the tendency of the surfactant components to form micelles and the interaction between surfactants in the micelle. Prediction of monomer—micelle equilibria reduces to modeling of the thermodynamics of mixed micelle formation. 

Reverse micelles form in aprotic organic solvents, such as hydrocarbons or CCI4, and can be seen as a core containing water (the water pool) solubilized in an oily environment (for example hydrocarbons) by the hydrophobic tails. Figure 9.9 also shows the structure of AOT (from aerosol octyl), which is the most popular surfactant for reverse micelles. A typical reverse micellar system appears as a clear... 

A typical spherical micelle contains 30-100 surfactants and has diameter of 3-6 nm. Micelles form because of two competing factors transfer of hydrocarbon chains out of water into an oil-like interior and repulsion between the head groups. 

Enhancement of the aqueous solubility by surfactants occurs as a result of the dual nature of the surfactant molecule. The term surfactant is derived from the concept of a surface-active agent. Surfactants typically contain discrete hydrophobic and hydrophilic regions, which allow them to orient at polar-nonpolar interfaces, such as water/air interfaces. Once the interface is saturated, th surfactants self-associate to form micelles and other aggregates, whereby their hydrophobic region are minimized and shielded from aqueous contact by their hydrophilic regions. This creates a discrete hydrophobic environment suitable forsolubilization of many hydrophobic compounds (Attwood and Florence, 1983 Li et al., 1999 Zhao et al., 1999). 

Aqueous micelles are 40-80 A diameter spherical aggregates which are dynamically formed from surfactants in water above a characteristic concentration, the CMC (9). Depending on the chemical structure of their hydrophilic headgroups, surfactants can be neutral or charged (positively or negatively). The alkyl chain of the surfactants typically contains between 5-20 carbon atoms. Micelles rapidly break up and reform by two known processes. The first process occurs on the microsecond time scale and is due to the release and subsequent reincorporation of a single surfactant from and back to the micelle. The second process occurs on the millisecond time scale and is ascribed to the dissolution of the... 

As mentioned, a surfactant typically has a hydrophobic and a hydrophilic part, and therefore it is difficult to dissolve it in just one phase. Only a small amount of the surfactant will dissolve molecularly in the liquid. When a larger concentration is applied, the surfactant wiU aggregate into micelles in an aqueous phase, the hydrophobic chains of a number of the surfactant molecules cluster together, and stick out their hydrophilic parts towards the surrounding water. A surfactant that in total has more affinity with water will be much more soluble in an aqueous phase and will form micelles in that phase a siafactant that has better interaction with an oily phase will dissolve in the oily phase and form micelles in that phase. In the latter case, one often speaks of reverse micelles. 

Polysoaps belong to a class of compounds which incorporate both of the features of polyelectrolytes and micelles into a single structure, and forms intramolecular micelles having a structural organization related to micelles formed from simple surfactants. Because of the high charge density at the surface and a compact hydrophobic core, they provide interesting microenvironments. A typical polysoap 22 can be formed throu treatment of polyvinylpyridine with linear alkyl halides (33). 

A typical emulsion polymerization recipe includes specific proportions of the added ingredients, e.g. (in wt%) monomer, 100 water, 150 initiator, 0.5 surfactant, 5. Because the monomer has low water solubility, it is clear that there will be two separate phases referred to as the monomer phase and the aqueous phase. The aqueous phase, containing the surfactant in the form of micelles, can be considered as consisting of two phases, the micellar phase and the true aqueous phase. The emulsifier helps disperse the monomer in the aqueous phase with droplets in the order of a few micrometers in size. The hydrophobic interior of the micelles contains solubilized monomer, which is apportioned by diffusion out of the emulsified monomer droplets and through the aqueous phase. 

The Fe-Au nanoparticles were reported to consist of metallic cores, having an average diameter of 6.1 nm, surrounded by an oxide shell, averaging 2.7 nm in thickness, for a total average particle diameter of 11.5 nm [101]. A surfactant solution is prepared with nonylphenol poly(ethoxylate) ethers. Au-coated Fe nanoparticles were also prepared in a reverse micelle formed by cetyltrimethylammonium bromide (CTAB), 1-butanol and octane as the surfactant, the co-surfactant and the oil phase, respectively [100]. The nanoparticles were prepared in aqueous solutions of micelles by reduction of Fe(II) and Au precursors with NaBH4. The typical size of the nanoparticles is about 20 nm. The existence of Fe and Au is again confirmed by energy dispersive X-ray microanalysis. 

The formation of lyophilic colloidal systems in broad temperature and concentration ranges is typical if molecules of one of the components are of a strong diphilic nature. Such are the surfactant molecules with either ionic or large non-ionic polar group and long hydrocarbon chain. The ability of these surfactants to spontaneously form equilibrium colloid systems, referred to as micelles, stipulates their broad use in various applications [1]. 

High HLB numbers are characteristic of hydrophilic surfactants which stabilize direct emulsions the highest numbers correspond to micelle-forming surfactants. Oppositely, low HLB numbers are typical for oleophilic surfactants which act as stabilizers of inverse emulsions. 

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