Binding of nitrodiphenylamines to reverse micelles of AOT in n-hexane and carbon tetrachloride:: Solvent and substituent effects

The absorption spectra of N-[2-(trifluoromethyl)-4-nitrophenyl]-4-nitroaniline (1), N-[4-nitrophenyl]-4-nitroaniline (2), and N-[2-nitrophenyl]-4-nitroaniline (3) were analyzed in reversed micelles of AOT (sodium 1,4-bis (2-ethylhexyl sulfosuccinate) in n-hexane and carbon tetrachloride. For 1 and 2 the intensity of the band characteristic for the pure solvent decreases as the AOT concentration increases and a new band develops. This new band is attributed to the salute bound to the micelle. These changes allowed us to determine the binding constant (Kb) between these compounds and AOT. Kb at W-0 = [H2O]/[AOT] = 0 in n-hexane varies from 81 for I to 5092 for 2. Although similar trends are observed for carbon tetrachloride, the values of Kb are smaller than those for n-hexane. The possible salute-solvent interactions of these compounds were analyzed by means of Taft and Kamlet's solvatochromic comparison method. The strength of binding is interpreted considering their hydrogen-bond donor ability as well as their solubility in the pure solvents. For 1 Kb decreases as W-0 is increased, while fdr 2 no variation was observed. These effects are discussed in terms of nitrodiphenylamine-water competition for interfacial binding sites. Moreover, the effect of the solute size and the presence of the trifluoromethyl group in 1 are important factors to consider in explaining its binding behavior. The spectra of 3 change very little with AOT concentration and only a slight bathochromic shift is observed. Thus, 3 acts as nonhydrogen bond donor solute, merely sensing a slight change in the polarity of its microenvironment. (C) 1998 Academic Press.