Aggregations in apolar solvents

TABLE V. Summary of Some Surfactants which Aggregate in Apolar Solvents... 

Much of the work has been done using alkylammonium carboxylates or Aerosol OT (bis-2-ethylhexylsulfosuccinate). These types of surfactants readily form aggregates in apolar solvents. 

The formation of normal micelles in water, or in diols or triols or sulfuric acid is well established, as is the formation of aggregates in apolar solvents containing traces of water. But there are examples of aggregation in dipolar aprotic solvents or in aqueous-organic solvents of relatively low water content. In some cases reaction rates have been studied in these media, but generally only physical studies have been reported. 

An enormous literature has been produced in recent decades in the field of molecular aggregation of amphiphilic molecules in liquid systems, emphasizing the extremely wide variety of accessible structures and dynamics. Among these molecular aggregates, in this chapter our attention will be restricted to those formed by some amphiphilic molecules (surfactants) in apolar solvents called reversed micelles [1]. 

A structure formed by the reversible association of am-phiphiles in apolar solvents. In inverted micelles, the polar portion of the amphiphile is concentrated in the interior of the macrostructure. Such association usually occurs with aggregation and is not typically characterized by a definite nucleation stage. Thus, inverted micelles (also referred to as inverse or reverse micelles) often fail to exhibit critical micelle concentration behavior. See Micelle... 

Similar arrangements of hydrogen bonds in diaminobipyridines result in the formation of C3-symmetrical discs. In the case of the systems such as the chiral 61, aggregation into a dynamic helix in apolar solvents is observed <2005JA5490>. 

Helicenes have an intrinsic, rigid helical shape and can be separated into enantiomers. Katz et al. found that the nonracemic helicene 6 with quinone residues self-assembles into one-handed helical columns in apolar solvents, where the molecules are stacked along their helix axes as shown in Fig. 4 [40-42], In this case, the 7r-donor-acceptor interactions appear to induce aggregations and stabilize the columnar stacks. Interestingly, unlike the nonracemic 6, the corresponding racemic 6 produced no such aggregation. 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

H NM R spectra in apolar solvents are less informative. Broad unresolved signals indicate the presence of undefined species. Obviously, the multicydic structure does not allow the formation of regular dimers, which would lead to an unfavorable overlap of the loops, but the interaction via the urea functions creates irregular aggregates. This inability to form hydrogen-bonded homodimers can be exploited in further reactions, e.g., for the construction of multiple catenanes (see Section 5.5). 

In fact, as expected, the interaction of these calixarene derivatives with barbiturates e.g. 136 (or cyanurates 138) in the ratio 1 2 results in the formation of box-like assemblies consisting of nine particles (3 x 135 and 6 X 136) held together by 36 hydrogen bonds. These aggregates are stable on the NMR timescale at ambient temperature in apolar solvents such as CDC13 or toluene-d6. However,... 

Reverse micelles are self-organized aggregates of amphiphilic molecules that provide a hydrophilic nano-scale droplet in apolar solvents. This polar core accommodates some hydrophilic biomolecules stabilized by a surfactant shell layer. Furthermore, reverse micellar solutions can extract proteins from aqueous bulk solutions through a water-oil interface. Such a liquid-liquid extraction technique is easy to scale up without a loss in resolution capability, complex equipment design, economic limitations and the impossibility of a continuous mode of operation. Therefore, reverse micellar protein extraction has great potential in facilitating large-scale protein recovery processes from fermentation broths for effective protein production. 

Due to the amphiphilic nature of the complex, inverted aggregation in apolar organic solvents was realized, leading to the formation or either inverted micelles or inverted vesicles. 

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