Cationic-anionic surfactant systems examined

Reference has already been made to the interesting finding by Laurent and Scott (65) that precipitation of various polyanion/cationic surfactant systems can be totally inhibited by the addition of a sufficient amount of simple salt. This work allowed the definition of a critical electrolyte concentration (c.e.c.), which was found to vary from system to system. Clearly, electrostatic screening effects are again involved. This phenomenon has been confirmed and examined in some detail by Lindman and co-workers (see next section). Less work has been carried out in this respect on polycation/anionic surfactant systems and, at least in some systems involving cationic cellulosic polymer/SDS combinations, resolubilization by salt addition was found not to be facile (59,103). 

Most systems examined to date have employed the AOT anionic reversed micellar system (366-370). In one case, amylase was extracted using trioctylmethylammonium chloride (cationic surfactant) in isooctane (375) while in another, catalase was extracted using a cationic DTAB/octane/hexanol reversed micelle (377). In our own research, we have successfully employed nonionic Igepal CO-530 -CCl, cationic CTAB - hexanol, and zwitterionic lecithin - CC1, reversed micellar systems in the extraction of some amino acids and proteins (379). The availability of such a pool of different charge-type micellar systems allows one flexibility in the development of such extraction schemes. In fact, preliminary results seem to indicate that better extractions are obtainable in some instances via use of zwitterionic reversed micellar media (379). 

Specific roles of the so-called co-surfactants (commonly, but not necessarily alcohols) have been examined by various workers [122, 126, 136] some points are discussed here. For example, a critical thermodynamic analysis in conjunction with experimentations led Eicke [ 136] to the conclusion that a co-surfactant should decrease the interfacial free energy under isothermal conditions, while causing an uptake of water into the microemulsion and extension of its domain. The anionic surfactant AOT assists the formation of large reverse microemulsion domains (high water uptake) in different ternary systems without help from a co-surfactant (Section 2.2), but cationic surfactants do generally need this fourth component. In spite of this, enhanced solubilization by the addition of (small quantities of) a co-surfactant has been observed by various workers in AOT systems. Eicke [136] used cyclohexane, benzene, carbon tetrachloride and nitrobenzene in the system AOT/ isooctane/water and found considerable water uptake (the fraction of the oil phase, i.e. isooctane was 0.8 or more). With increasing polarizability or polarity of the CO-surfactant, the water uptake decreased. 

Structural and conductivity studies of mixed surfactants of anionic-nonanionic, cationic-nonionic, and cationic-anionic types were also examined by Seedher and Manik [60] and Li et al, [61,62]. The former authors observed that the threshold volume fraction of percolation for water/AOT/TX 100/n-heptane and water/CPB/ TX 100/heptane depends on the mixing ratio it increases with increasing proportion of the ionic component. The formation of bicontinuous phases for AOT/ TXlOO, CTAB/TX 100, and Na oleate/CTAB combinations together with lower alkanols (n-butanol, n-pentanol, and n-hexanol) was reported by Li et al. [61,62] with diesel oil and heptane as the continuous media. The systems showed conspicuous conductance behavior. 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

Using steady-state absorption studies, several other authors examined the micropolarity of confined IL in microemulsions stabilized by ionic surfactants [64,85,87], For example, Sarkar and coworkers examined [bmim][BF ]/benzene mixtures stabilized by the anionic SAIL surfactant [bmim][AOT] and observed that, within the studied range, the A for solubilized MO continued to undergo redshift with increasing R [85, 87], In another work, Falcone and coworkers compared the micropolarities of [bmim][BF4]/benzene mixtures stabilized by cationic BHDC and nonionic TX-lOO surfactants using l-methyl-8-oxyquinolinium betaine (QB), a dye that locates mainly at the surfactant interfacial layer [64]. When [bmim][BF ] was added to both BHDC/benzene and TX-lOO/benzene systems, a larger hypsochromic shift was sensed by the probe in the former. This implies that the local environments in BHDC/benzene system are more polar. The authors ascribed this phenomenon to the strong electrostatic interactions between the [BFJ anion and the BHD moiety of the cationic surfactant. 

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