Aggregation, cationic surfactant

Higher order aUphatic quaternary compounds, where one of the alkyl groups contains - 10 carbon atoms, exhibit surface-active properties (167). These compounds compose a subclass of a more general class of compounds known as cationic surfactants (qv). These have physical properties such as substantivity and aggregation ia polar media (168) that give rise to many practical appHcations. In some cases the ammonium compounds are referred to as iaverse soaps because the charge on the organic portion of the molecule is cationic rather than anionic. 

Rate constants of bimolecular, micelle-assisted, reactions typically go through maxima with increasing concentration of inert surfactant (Section 3). But a second rate maximum is observed in very dilute cationic surfactant for aromatic nucleophilic substitution on hydrophobic substrates. This maximum seems to be related to interactions between planar aromatic molecules and monomeric surfactant or submicellar aggregates. These second maxima are not observed with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

Surfactants that form micelles have also been shown to accelerate the formation of nitrosamlnes from amines and nitrite (33.) A rate enhancement of up to 80 0-fold was observed for the nitrosation of dihexylamine by nitrite in the presence of the cationic surfactant decyltrimethylammonium bromide (DTAB) at pH 3.5. A critical micelle concentration (CMC) of 0.08% of DTAB was required to cause this effect, which was attributed to a micelle with the hydrocarbon chains buried in the interior of the micelle. The positively-charged ends of the micelle would then cause an aggregation of free nitrosatable amine relative to protonated amine and thus lead to rate enhancements. Since surfactants are commonly used in water-based fluids (25-50% lubricating agent or 10-2 0% emulsifier in concentrates), concentrations above the CMC of a micelle-forming surfactant could enhance the formation of nitrosamines. 

Nile Blue is used as a 0.01 to 0.1 %W/V aqueous solution and is simply added to or mixed with the substrate. The active component of the dye is actually a minor contaminant of the solution, not the blue-colored material [31]. The preparations are viewed with 450-490 nm excitation (an FTTC filter set. Figure 6). Emulsion stability is sometimes an issue in the presence of the cationic blue component of Nile Blue. In this case we use Nile Red, the pure form of this colorant. Nile Red solution is made fresh from a stock solution (0.1%W/V in acetone). This stock is added dropwise to water until a moderate blue color is seen and the solution is used immediately (it deteriorates quickly). For either colorant, the active molecule is fluorescent only when it is in a suitably hydrophobic environment. This usually means neutral lipid droplets [31] but other sites (aggregates of surfactants, the center of casein micelles, cutin plates in some seeds) are possibilities. 

Kitahara115,116,119,121 arrives at similar conclusions with fatty acid salts of higher aliphatic primary amines in benzene. Large amounts of data on cationic surfactants, particularly, their temperature dependent aggregation were collected by Kertes and coworkers109 n0, 11 141. In a number of cases thermodynamic data were calculated from this temperature dependence119. However, frequently the dependence of the aggregation number on the temperature was not duly considered which makes the derived quantities less useful. 

Summarizing, as a practical rule cationic surfactants show in general a more pronounced temperature (and concentration) dependent aggregation number. 

Variation of v. The last mixed micelle case studied were mixtures in which the volume of the hydrophobic tails was varied by mixing monoalkyl and dialkyl cationic surfactants (DTAB/DDAB). The aggregate structures found as a function of composition are detailed in Table IV. TTiey range from lamellar packed liquid... 

Bhattacharya and Snehalatha reported the utility of a cationic surfactant aggregate-based formulation of monoperoxyphthalate (MPP) as an inexpensive, off-the-shelf reagent for catalytic decontamination [11], They also presented evidence of the formation of the acylated or phosphorylated monoperoxyphthalate intermediates during the phosphorolysis and esterolysis... 

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