CTABr

The members of Wolfbeis team constructed an optical sensor for ammonia-based on ion pairing76. They immobilized pH-sensitive dye (bromophenol blue) as an ion pair with cetyltrimethylammonium bromide (CTABr) in a silicone polymer matrix. Bromophenol blue, while contact the ammonia (both in water as well as in gaseous form) changes its color reversibly from yellow to blue. The immobilized dye shows long wave absorption with a good photostability. 

CTABr(OH) n-Hexadecyltrimethylammonium bromide (hydroxide) or cetyltrimethylammonium bromide (hydroxide)... 

Ci6H33NMe3 Br /. V Hexadecyltrimethylammonium bromide, HTABr Cetyltrimethylammonium bromide, CTABr 9 x 10 4... 

Figure 2 illustrates the variation of the first-order rate constants, kv, with [CTABr] for reactions of benzoic anhydride with 0.01 M NaOH and with 0.02 M sodium formate. The lines are calculated using the ion-exchange treatment with the following parameters Ks = 650 M (3 = 0.75, Kgr = 10 for X = OH- and HCO 2 and kM = 200 and 0.06 s 1 for OH" and HCO 2 respectively. Similar values of the rate constants were used in fitting the data... 

Fig. 2 Reactions of benzoic anhydride in CTABr , 0.01 M NaOH , 0.02 M HC02Na. The lines are calculated using the ion-exchange model. (Reprinted with permission of the American Chemical Society)...

The application of this mass-law treatment to the reaction of benzoic anhydride in CTAOH is illustrated in Fig. 4 (Al-Lohedan and Bunton, 1982). The lines are calculated taking Ks = 650M 1, fCOH = 55 M 1 and, ku = 180 s 1. The value of Ks is that estimated in CTABr and values of kM are almost the same in CTAOH and CTABr + NaOH. 

Despite these uncertainties values of kM for reactions of hydroxide ion in CTAOH and mixtures of CTABr or CTAC1 with NaOH calculated using the ion-exchange or mass-action models agree reasonably well, and some examples are given in Table 3. 

Examples of this behaviour are shown in Table 7 where k+ is related to reaction of substrate fully bound to a CTAX micelle and k to reaction in an anionic micelle of SDS. The ratio k+/k is consistently larger than unity for hydrolyses of open chain anhydrides, diaryl carbonates and aryl chloroformates. In addition hydrolysis of 4-nitrophenyl chloroformate is slightly faster in cationic micelles than in water. Spontaneous hydrolyses of N-acyl triazoles are also inhibited less by cationic micelles of CTABr than anionic micelles of SDS (Fadnavis and Engberts, 1982). 

The similarity for many reactions of second-order rate constants in aqueous and micellar pseudophases, and the observation that substrate hydrophobicity usually affects binding and not inherent reactivity in the micelle, suggests that substrate location or orientation is relatively unimportant. This conclusion is strongly supported by a quantitative analysis of the effects of CTABr micelles on the reaction of OH- and arylsulfonylalkyl arenesulfonates (16) (van der Langkruis and Engberts, 1984). 

Deprotonation of benzimidazole can be studied onlyiin dilute NaOH, but deprotonation of 5-nitroindole and 5-nitroindole-2-carboxylate ion can be followed in 0.01-0.1 M NaOH in both CTABr and CTAOH. For CTAOH the concentration of micellar bound OH- was estimated from (19), and values of K] were similar in both CTABr and CTAOH micelles (Cipiciani et al., 1983b, 1985). 

Microemulsions are potentially useful reaction media because of their solubilizing ability (Mackay, 1981). Reductions of ketones and enones by borohydride go readily in microemulsions of CTABr, hexane and 1-butanol, and there is more 1,4-reduction than in 2-propanol (Jaeger et ah, 1984). 

An especially interesting example of micelle-mediated specificity is the observation that formation of rrans-dibromide or mwis-bromohydrin from cyclohexene and Br2 (27) is affected not only by CTABr, but also by the... 

Hexane, aq.KBr, CTABr, 1-butanol. Rate decreases with increasing hexane. Products PhCH2Br with small amounts of alcohol and ether... 

Hexadecane, CTABr, 1-butanol + aq.OH" or F. Rates in microemulsions compared with those in HzO, aq.dioxan and aq.CTABr... 

CTABr, n-C6H13NH2, octane. Second-order rate constants in the microemulsion droplets calculated... 

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