Cationic monomeric surfactant molecules

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with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

Polymeric quaternary ammonium surfactants made from w-dodecyl bromide and poly(2-vinylpyridine) are better solubilizers for oil-soluble azo dyes and for n-decanol than monomeric quaternary cationics with similar (monomeric) structures (Tokiwa, 1963 Inoue, 1964). Solubilization of n-decanol in the polycationics increased as the alkyl chain content increased to a maximum at 24% alkyl content and resulted, at high decanol content, in intermolecular aggregation of the poly cationic molecules (Inoue, 1964). 

Surfactants are those molecules that comprise both hydrophilic and hydrophobic groups simultaneously. The hydrophilic groups can be either ionic (e.g., -SOj, -SO3, -COOH, and -N(CHsIj) or nonionic (e.g., -0-(CH2-CH2-0) -H). As to the hydrophobic groups, the most widely used ones are the alkyl chains (-C H2 +i) and arakyl chains (-C H2 +i-C6H4-). Representative surfactants include anionic sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium bis(2-ethylhexyl) sulfosuccinate, sodium stearate, cationic hexadecyltrimethylammonium bromide, and nonionic i-octophenol polyethoxylate with an average of 40 monomeric units of ethylene oxide per molecule. Such amphoteric species tend to diffuse toward the interface between the oily and aqueous phases and reside therein. 

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