Bromine−AOT Charge-Transfer Complexes and Hydrogen-Bond Donor Ability of Water in AOT−isooctane−H2O Reverse Micelles and Water-in-Oil Microemulsions

Changes, induced by aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT), in the absorption spectrum of bromine, are shown to markedly depend on the nature and the hydration of the AOT-aggregates. A charge-transfer complex (CTC) between bromine and monomeric AOT (λ max = 270 nm, ε 315 = 1.4 × 10 3 cm -1 M -1, and K = 4 × 10 3 M -1 ) is observed in isooctane, in agreement with a very strong electron donor ability of AOT, attributed to its sulfonate headgroup. In “dry” reverse micelles ([AOT] > cmc), the constant for the CTC formation K = 1.3 M -1 is markedly smaller. In aqueous reverse micelles, the observed ε decrease (ε 315 = 700 cm -1 M -1 and K = 1.1 M -1 for W = [H 2 O]/[AOT] = 9) is consistent with a competition between water and bromine in the stabilization of the AOT-head groups. Finally, in water-in-oil (w/o) microemulsions ( W > 12), the complex disappears. The bromine−tribromide ion equilibrium is also investigated in AOT-w/o microemulsions in the presence of bromide ions. The marked dependence of the equilibrium constant for Br 3 - formation on the microemulsion composition ( W = 12, K = 178 M -1; W ≥ 20, K = 60 M -1; bulk water, K = 16 M -1 ) is attributed to a change in the transfer free energy of bromide ion from bulk water to the aqueous pseudophase of the microdroplets (δΔ G tr (Br - ) = −2.6 and −1.3 kcal mol -1 for W = 12 and 20, respectively). The results are discussed in terms of changes in the hydrogen-bond donor ability and polarity of water in AOT−w/o microemulsions.