Surfactant bonding, micelle structure

The micellization of nonionic surfactants in RTILs has been reported in the literature [4—24]. The combinations of RTILs and nonionic surfactants employed in these papers are summarized in Table 3.1. In this table, the RTILs are classified by their protic/aprotic nature that is, protic RTILs can form a hydrogen-bonding network structure between ion pairs [25] leading to a sponge-like phase in the bulk as a result of their self-assembly. In contrast, aprotic RTILs do not form such a network structure, and hence, their self-assembly in the bulk is absent or weak. One of the key conclusions regarding the micellization of nonionic surfactants in RTILs is that the micellization is only observed when surfactant molecules are associated with each other in appropriate RTILs. In general, nonionic surfactants with excellent solubility in RTILs cannot form micelles because of their insufficient intermolecular solvophobic interaction and those with poor solubility in RTILs cannot do that either [6]. 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

It may be expected that other, highly structured solvents with a tri-dimensional network of strong hydrogen bonds, would also permit micelle formation by surfactants, but little evidence of such occurrences has been reported. On the other hand, surfactants in non-polar solvents, aliphatic or aromatic hydrocarbons and halocarbons tend to form so-called inverted micelles, but these aggregate in a stepwise manner rather than all at once to a definite average size. In these inverted micelles, formed, e.g., by long-chain alkylammonium salts or dinonyl-naphthalene sulfonates, the hydrophilic heads are oriented towards the interior, the alkyl chains, tails, towards the exterior of the micelles (Shinoda 1978). Water and hydrophilic solutes may be solubilized in these inverted micelles in nonpolar solvents, such as hydrocarbons. 

This process leads to a stable hydrogen bond network and consequently to the micelles. This micelle formation is accompanied by a partial compensation of the individual dipole moments of the soap molecules. Also in the case of DAP a dipole compensation interaction of the ammonium compared to the sodium ion and the correspondingly inferior bond formation the tendency towards structure formation of these aggregates is expected to be less pronounced. Moreover, structural variation, should be considered comparing these two surfactants. 

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