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, AO T), in the absorption spectrum of bromine, are shown to markedly depend on the nature and the hydration of the AOT-aggregates. A charge-transfer compl ex (CTC) between bromine and monomeric AOT (lambda(max) = 270 nm, epsilon(3 15) = 1 4 x 10(3) cm(-1) M-1, and K = 4 x 103 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 c onstant for the CTC formation K = 1.3 M-1 is markedly smaller. In aqueous r everse micelles, the observed epsilon decrease (epsilon(315) = 700 cm(-1) M -1 and K = 1.1 M-1 for W = [H2O]/[AOT] = 9) is consistent with a competitio n between water and bromine in the stabilization of the AOT-head groups. Fi nally, in water-in-oil (w/o) microemulsions (W > 12), the complex disappear s. The bromine-tribromide ion equilibrium is also investigated in AOT-w/o m icroemulsions in the presence of bromide ions. The marked dependence of the equilibrium constant for Br-3(-) formation on the microemulsion compositio n (W= 12, K = 178 M-1; W greater than or equal to 20, K = 60 M-1; bulk wate r, K = 16 M-l) is attributed to a change in the transfer free energy of bro mide ion from bulk water to the aqueous pseudophase of the microdroplets (d elta Delta G(tr) (Br-) = -2.6 and -1.3 kcal mol(-1) for W = 12 and 20, resp ectively). The results are discussed in terms of changes in the hydrogen-bo nd donor ability and polarity of water in AOT-w/o microemulsions.