Solubilization constant, benzene

The primary results in Table I may also be processed to yield values of the benzene activity coefficient in the Intramicellar solution, Tg, defined as fg/(fg Xg), where fg is the fugacity of benzene in equilibrium with the aqueous surfactant solution, and fg is the fugacity of pure benzene at the given temperature. Figures 1-4 are plots of the solubilization constant (K) and the benzene activity coefficient (Tg) against the intramicellar mole fraction of benzene (Xg) for the surfactants CPC and SDS at the Indicated temperatures. 

It seems likely that the cationic CPC micelles, which have a large positive charge at or near the micellar surface, interact attractively with the n-molecular orbital system of benzene, and that this interaction contributes to the fact that the solubilization constant for benzene in CPC is approximately twice as large as that in SDS micelles. A preferential interaction between cationic surfactants and aromatic solutes has been reported by several groups of investigators (25-27), and recent work in our laboratory shows that 1-hexadecyltrimethylammonium bromide micelles also solubilize benzene more effectively than do the anionic alkylsulfate surfactant micelles (28). Thus, the tendency of benzene molecules to solubilize near the surface of the cationic micelles, at low XB values, may lead to a partial saturation of surface "sites" by benzene, diminishing the ability of additional benzene molecules to bind near the surface. Such an effect could be responsible for the initial increase in activity coefficient that occurs, particularly in the CPC solutions, as Xg increases. 

Figure 1. Dependence of the solubilization constant for benzene in 1-Hexadecylpyridinium Chloride [CPC] on the composition of the micelle at temperatures varying from 25 to 45° C.

Calculations usirig this value afford a partition coefficient for 5.2 of 96 and a micellar second-order rate constant of 0.21 M" s" . This partition coefficient is higher than the corresponding values for SDS micelles and CTAB micelles given in Table 5.2. This trend is in agreement with literature data, that indicate that Cu(DS)2 micelles are able to solubilize 1.5 times as much benzene as SDS micelles . Most likely this enhanced solubilisation is a result of the higher counterion binding of Cu(DS)2... 

Although anation and aquation rates of vitamin B12 are not affected appreciably by aqueous micelles, the solubilized water in reversed micelles, in contrast, influences the rate and equilibrium constants for the formation and decomposition of glycine, imidazole, and sodium azide adducts of vitamin Bl2 (Fendler et al., 1974). A vitamin B12 molecule is conceivably shielded from the apolar solvent (benzene) by some 300 surfactant molecules. 

FIG. 8.4 Determination of the microenvironment of a molecule (a) a portion of the ultraviolet spectrum of benzene in (1) heptane, (2) water, and (3) 0.4 M sodium dodecyl sulfate and (b) ratio of the intensity of the solvent-induced peak to that of the major peak for benzene versus the index of solvent polarity. The relative dielectric constant is also shown versus the index of polarity. (Redrawn, with permission, from P. Mukerjee, J. R. Cardinal, and N. R. Desai, In Micellization, Solubilization and Microemulsions, Vols. 1 and 2 (K. L. Mittal, Ed.), Plenum, New York, 1976.)... 

Estimate the effective dielectric constant at the surface of the micelle from the fact that max occurs at 286 nm for dodecyl pyridinium iodide micelles in water. In light of the value estimated in Section 8.3 for the dielectric constant in the vicinity of solubilized benzene, does it seem likely that the value of er for bulk water applies in the Stern layer ... 

By comparing with corresponding spectra of ion pairs in different solvents one obtains information on the local environment at the micellar surface73. (The polarity was expressed in terms of a so-called effective dielectric constant). Amphiphiles with a benzene ring also show an UV absorption74-76. UV spectra of solubilized species like benzene, naphthalene and pyrene have been extensively studied and compared with spectra of the compound in reference solvents to provide an estimate of the polarity in the vicinity of the solubilizate. 

Fig. 20.—Critical Micelle Concentration (at Room Temperature), Rate Constant (at 60°), and Maximum Amount of Solubilizing Water (at Room Temperature)asa Function of Dielectric Constant of Medium for the Process of Hydrolysis of Elextrin by Means of Dodecylben-zenesulfonic acid. (1, in hexane 2, in cyclohexane 3, in CCI4 4, in benzene S, in toluene and 6, in o-xylene.)...

We apply the concepts discussed above to design a CSTR that operates at 55 °C for the chlorination of benzene in the hquid phase. It is necessary to account for all three chlorination reactions. Chlorine gas is bubbled through the liquid mixture in the CSTR and it must diffuse across the gas-liquid interface before any of the reactions can occur. For this particular problem, it is reasonable to assume that chlorine is present as a solubilized liquid-phase component, and its molar density in the inlet liquid stream is given as a fraction e of the inlet molar density of pure liquid benzene. In a subsequent example discussed in Chapter 24, a two-phase gas-liquid CSTR analysis is presented which accounts for the realistic fact that benzene enters the reactor in an undiluted liquid stream, and chlorine is actually bubbled through as a gas. It is sufficient to consider that the fraction e = 0.25 remains constant for all simulations. In the first chlorination step, benzene reacts irreversibly with dissolved chlorine to produce monochlorobenzene and hydrogen chloride ... 

FIG. 10 Regulation of the catalytic activity of solubilized enz5unes by variation of the surfactant concentration at a constant degree of hydration in the systems ( ) AOT-water-octane (A) dodecylammonium propionate-water-riiethyl ether/benzene (O) Brij 96-water-cyclohexane ( ) lecithin-water/methanol/pentanol-octane. Dashed lines show levels of corresponding catalytic activities in aqueous solution. (From Ref. 10.)... 

---