The electronic absorption spectra of a series of nitroanilines: p-nitroaniline (pNA); onitroaniline (oNA); 2.4~dinitroaniline (dNA) and N.N-dimethyl-p-nitroaniline (N,NpNA) have been measured in solutions of sodium 1 ,Cbis (2-ethylhexyl) sulfosuccinate (AOT) in n-hexane at different W=[H20]/[AOT].
On increasing AOT concentration, it is observed that the characteristic absorption band of pNA, oNA and dNA in n-hexane decreases in intensity, and a new band develops at higher wavelength. A net isosbestic point is detected in every case, The spectral changes are interpreted as a consequence of an equilibrium between nitroaniline free and bound to AOT in the micellar system. These changes allowed us to determine the binding constant (Kb) between the nitroanihnes and AOT. The values of Kb vary from 73 Me' for oNA to 3370M" for (WA at W = 0. For N,NpNA only a shift of the absorption band in nhexane is observed. The magnitude of the possible solute-solvent interactions of these compounds was analyzed by means of the Tall and Kamlet's solvatochromic comparison method. Thus, taking into account our studies in pure solvents, the strength of binding is interpreted considering their polarity and hydrogen-bond donor ability as well as their solubility in n-hexane. The binding of oNA to AOT shows a decrease in Kb followed by a plateau value as W is increased. For pNA and dNA, which have large values of Kb, no variation is observed as W is increased. This effect is discussed in terms of a competition between the nitroaniline and water for inter-facial binding sites. In addition the absorption maxima of the free nitroaniline tends toward that on hexane (hypsochromic shift) when increasing the water concentration and mainly for oNA, indicating that water competition for the hydrogen bond acceptor sites of AOT is greater for the weaker bounded nitroaniline. The absorption maxima of the bound nitroanilines shifts bathochromically on water addition as expected for the increasing polarity of the micellar interface. 0 1997 Elsevier Science B.V. 1 Solubilization is the phenomenon of increased solubility of a substance, normally insoluble or slightly soluble in a continuous solvent phase, in the presence of micelles in that phase.' The term was initially coined for aqueous micellar solution but has been generalized